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Nanoporous Layered Silicate Heterostructures Famed By Intra-

Gallery Assemblies of Organic Surfactants

by

Anis Fakhruddin Barodawalla

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry
1997

Abstract

Nanoporous Layered Silicate Heterostructures Formed By

Intra-Gallery Assemblies of Organic Surfactants

by

Anis Fakhruddin Barodawalla

A new templating mechanism which we recently reported1 allows us
to expand the use of lamellar compounds for the design of nanoporous
materials. By appropriate choice of a reaction medium and combinations
of neutral and ionic co-surfactants, new intercalated structures containing
large channels similar to those formed in mesoporous molecular sieves
(MCM-4l)2,3 can be synthesized. Here we present the gallery-templated
synthesis using clays as layered hosts and affording what we call porous
clay heterostructures (PCHs). Our approach is based on the use of
intercalated quaternary ammonium cations (noted Q4”) and neutral amines
into ionic lamellar solids. Into these swollen galleries is added a neutral
inorganic precursor (for example, tetraethylorthosilicate (TEOS)) which
forces the co-surfactants to order in a rod-shaped array of mixed
quaternary ammonium cations and neutral amines. The inorganic structure
self-assembles around this array leading to one-dimensional channels.
Removal of the surfactants by calcination affords mesoporous solids with

surface areas in the range 470-750 mzlg and pore widths in the range 14-

23 A depending on the quaternary ammonium/neutral amine chain length
used. These new porous structures are more stable than large pore
molecular sieves because of the pore structure being confined between 2d
lattices. PCHs provide unique opportunities to improve the chemical and
physical properties of surface active lamellar structures by enhancing the
diffusion in such layered materials. In addition, owing to the
complementary chemical functionalities of the layered and gallery-
templated components, PCHs offer new design strategies for heterogeneous

catalyst systems.

1 Galarneau, A; Barodawalla, A. and Pinnavaia, T. J ., Nature , 374,
529-531 (1995).

2 Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartulli, J. C.; Beck,
J. 8., Nature 359, 710-712 (1992).

3 Beck, J.S.; Vartulli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C.
T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. 111.; Sheppard, E. W.;

McCullen, S. B.; Higgins, J. B.; Schlenker, J. L., J. Am. chem. Soc.
114, 10834-10843 (1992).

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Amine-solvated Q+-clay Templated heterostructure Porous Clay Hetrostructure

ACKNOWLEDGEMENTS

I would like to thank my research advisor Professor Thomas J.
Pinnavaia for supplying patient guidance and support during this course of
study. Special thanks go to Professor Harry A. Eick for his helpful
comments as a second reader and Prof Baker and Prof Blanchard for being
on my committee. I would also like to thank my deepest gratitude and
heart-felt thanks to all the Pinnavaia group members past and present, who
made graduate school an enjoyable learning experience. Prof. Wang, Dr.
M. Chibwe, Wouter de0, Dr. S. Bagshaw deserves special thanks for their
encouragement and friendship they have given me over the years. A
special thanks goes to Dr. A. Galameau for collaborating on this project,
and also making it an enjoyable experience.

Financial support given by the Department of Chemistry Michigan
State University, the National Science Foundation is gratefully
acknowledged and appreciated.

To my family I extend my love and thanks for being so supportive.
I especially want to thank my father, mother, brother, sister, Grandparents,
Aunts and Uncles who have stood behind me always and who have an
unending affection and gratitude.

Most importantly I am grateful to my wife Rumu for all the love and
understanding she has given me over this past year. Her friendship and
patience during this time were instrumental to the completion of this

dissertation.

TABLE OF CONTENTS

Chapter pages
LIST OF TABLES ............................................................
LIST OF FIGURES ...........................................................

CHAPTER 1. Advances in the design of nanoporous molecular

sieves.

A. Introduction

1. Introduction to nanochemistry ..................................................... 1
2. Classification of porous materials ................................................ 5
B. Enlarging the pore sizes of zeolites and related compounds ........... 8

C. Propping layered materials - pores by pillaring
1. Layered solids .......................................................................... 13

D. Mesoporus molecular sieves
1. Assemblies of surfactant molecules as templates - a milestone to the
synthesis of mesoporous molecular sieves ................................... 25

2. Mobils ordered mesoporous M41S materials - A leap into the ultra-

iv

large pore size molecular sieves and the current advances in their
design ...................................................................................... 28

References ................................................................................ 39

Chapter II. A new synthetic strategy for forming

POP”?

nanoporous molecular sieves via intra-gallery
templating of a lamellar host.

Abstract ................................................................................... 48
Introduction ............................................................................. 49
Experimental ............................................................................ 67
Results and Discussions .............................................................. 73
Conclusions ........................................................................... 102
References ............................................................................. 103

Chapter III. Pillaring reaction vs intra-gallery templating

F3

processes in a lamellar solid - A comparative

study.
Abstract ................................................................................. 107
Introduction ........................................................................... 108
Experimental .......................................................................... 1 10
Results and Discussions ............................................................ 111
Reaction stoichiometry ............................................................ 11 1

Pore size dependence on surfactant chain lengths ........................ 118

Effect of surfactant head group size on PCH formation ............... 125
Conclusions ............................................................................ 148
References .............................................................................. 150

Chapter IV. Physico-chemical properties of PCH materials

S”

Abstract ................................................................................. 153
Introduction ........................................................................... 154
Experimental .......................................................................... 155
Results and Discussions ........................................................... 161
Thermochemical alterations in PCH-fluorohectorite ................... 161
Template removal ................................................................... 176
Hydrothermal stability of PCH-rectorite .................................... 181
Enchanced acidity of PCH materials .......................................... 185
Metal ion substitution during PCh synthesis ................................ 187

Covalent grafting of Ti on the hydroxyl groups lining the pore walls

of PCH-fluorohectorite ........................................................... 193
Attempts at PCH formation using low CEC smectites .................. 198
References .............................................................................. 207

vi

LIST OF TABLES

Table Page
11.1 Idealized structural formula for representative 2:1
phyllosilicates. In each formula the parentheses and bracket define metal
ions in tetrahedral and octahedral sites respectively ............. y ................. 54
11.2 Surfactant-intercalated clays .............................................. 74
11.3 The nitrogen BET surface areas of alkali metal clays and
organoclays (Qt-clays) ....................................................................... 75
11.4 X-ray powder diffraction data for various alkylamine solvated
organoclays derivatives ( alkylamine/Q+-clays) .................................... 82
11.5 Properties of PCHs prepared by gallery -templated
synthesis ........................................................................................... 94
11.6 Neutral amine and intercalated silica content of porous clay

heterostructures synthesized from fluorohectorite and vermiculite as
layered host and HDTMA‘l' and neutral alkylamines as co-templates ........ 97

111.1 Gallery heights of reaction products obtained using quaternary
ammonium cation as exchanged-cation and decylamine as co-swelling agent
at different TEOS/amine molar ratios ................................................. 113
111.2 Pore Sizes and Gallery Heights (A) of PCHs Prepared by
Gallery - Templated Synthesis ........................................................... 121
111.3 Gallery heights of intercalates formed by using a quaternary

ammonium cation or a primary ammonium cation as exchanged-cation and
decylamine as co-swelling agent at different 'TEOS/amine molar
ratios ............................................................................................. 127

1V.l Gallery heights, BET surface area, Horvath and Kawazoe pore

sizes, carbon and nitrogen content determined by chemical analysis of
PCH-fluorohectorite derivatives calcined at different

vii

Table Page

temperatures ................................................................................... 174
1V.2 BET surface area, Horvath and Kawazoe pore sizes of PCH-
rectorite after hydrothermal treatment at different temperatures .......... 182

viii

LIST OF FIGURES

Figure Page
1.1 Nanochemistry as compared to nanophysics ......................... 3
1.2 (A) Defination of micro-, meso-, and macropores together

with the representative materials and their pore size distributions. (B)
Schematic X-ray diffractograms of a zeolite, pillared solid and a ultra-
large pore molecular sieve ................................................................... 6

1.3 Pore size of microporus zeolites and other recently developed
molecular sieves ................................................................................. 11

1.4 The approach to mesoporosity via pillaring of layered solids
:(a) In most pillared solids, the interlayer regions contain only
micropores.(b) Mesoporosity is usually connected with a disordered
arrangement of layer packages ............................................................ 17

1.5 Approaches to mesoporosity via pillaring of layered solids :
(a) Attempts to use large elongated ("slim") pillars to create interlayer
mesoporosity may fail because the pillars are inclined w.r.t the layers.(b)
This failure may be prevented by post-intercalating organic substances
that(c) can be burnt off during calcination when stable links between pillars
and the layers are formed. ((1) Another possibility 1s the intercalation of
pillars with bases ................................................................................ 19

1.6 Schematic representation for the intercalation and pillaring of
alkylammonium solvated magadiite ...................................................... 24

1.7 Synthesis mechanism for mesoporous materials derived from
kanemite as proposed by inagaki et al ................................................... 26

1.8 (A) Possible mechanistic pathways for the formation of MCM-
41 :(1) liquid crystal phase initiated and (2) silicate anion initiated (B)
Transmission electron micrographs(a, b and d) and scanning micrographs

(c) of MCM-41 ................................................................................. 29
1.9 Nitrogen adsorption isotherm for MCM-4l (above) and
amorphous silica -BET surface area 306 mzlg (below) .......................... 32

Figure Page

1.10 Representative XRD patterns of calcined MCM-4l (above),
calcined cubic MCM-48 (middle) and as-synthesized unstable lamellar
material (below) ................................................................................ 33
1.11 Schematics of Porous Clay Heterostructure Formation ......... 38
11.1 The formation of composite aluminum oxygen or silicon-
magnesium oxygen layers ................................................................... 50
11.2 Idealized oxygen framework of clay minerals ..................... 52
11.3 Schematic structures of palygorskite and sepiolite ................ 59
11.4 The surfactant orientation in the interlayers of various layer
charge density clay ............................................................................. 62
11.5 The X-ray diffraction patterns of various organoclays
(Q+-clays)

(A) Hexadecyltrimethylammonium-Fluorohectorite, air dried.

(B) Hexadecyltrimethylammonium-Rectorite, air dried.

(C) Hexadecyltrimethylammonium-Vermiculite,air dried.
(D)Dioctadecylldimethylammonium-Fluorohectorite,air dried ...... 76

11.6 (a) X-ray powder diffraction patterns of decylamine solvated
HDTMA+-Fluorohectorite gel with a molar ratio of 20 : 1 respectively.(b)
X-ray powder diffraction patterns of the air dried HDTMA+-
Fluorohectorite/amine. (i) octylamine & (ii) decylamine ....................... 84

11.7 Schematic representation of the exchange reaction of clay with
quaternary ammonium cations (filled head groups) to form Q+-clay with a
paraffin structure. Solvation of Q+-clay with neutral alkylamine (open
head groups) affords a lipid likebilayer structure ................................ 85

11.8 29 Si MAS NMR spectras. (a)Amine solvated air dried
fluoorhectorite. (b)Li+-Fluorohectorite ............................................. ‘2?

11.9 Systematic illustration of the steps involved in porous clay
heterostructure synthesis ................................................................... 88

11.10 The X-ray powder diffraction patterns of air dried templated
heterostructures and calcined PCH's prepared at a molar ratio of 1 : 20 :

Figure Page

150 for the Q+-clay : amine : TEOS. (A) HDTMA+ Fluorohectorite :
decylamine : TEOS, air dried. (B) HDTMA+ Vermiculite : decylamine :

TEOS, air dried. (C) HDTMA+ Rectorite : decylamine : TEOS, air dried.
(D)HDTMA+F1uorohectorite : decylamine : TEOS, calcined at 650°C.
(E)HDTMA+ Vermiculite : decylamine : TEOS, calcined at 650°C.
(F)HDTMA+ Rectorite : decylamine : TEOS, calcined at 650°C ............. 89

11.11 Thermal gravimetric analysis of PCH prepared from
octylamine-solvated HDTMA+-Fluorohectorite showing five steps of lost
weight, the first one attribuate to the desorption of water (5%), the second
and third due to the loss of neutral amine (16.5%), the fourth relative to
the decomposition of the quatemaryammonium cation (14%) and the fifth
related to water losses via condensation of silanol groups to form siloxane
bonds (1.9%) ................................................................................... 92

11.12 Nitrogen adsorption/desorption isotherms and the
corresponding H-K pore size distribution for porous silica-clay
heterostructures (PCHs) prepared by gallery-templated synthesis using
(A)vermiculite, (B) fluorohectorite, & (C) rectorite as layered host.
HDTMAfldecylamine were used as templates. The HIDTMA+-clay : amine
: TEOS reaction stoichiometry was 1 : 20 : 150 ................................... 93

11.13 Schematic representation of the gallery structures
(A) Symmetrical pore structure formation in fluorohectorite and rectorite.
(B) Thicker wall structure of PCH derived from vermiculite ................ 96

11.14 TEM image of a porous silica - vermiculite heterostructure (
dom = 37 .0A ) showing evidence for regularly ordered lamellar structure.
Owing to the turbostratic nature of the intercalate, the galleries pores are
not easily oriented for imaging. TEM images do not indicate the presence
of significant amounts of extragallery silica ......................................... 99

11.14-(ii) TEM image of a porous silica - fluorohectorite heterostructure
( d001 = 32.0A ) showing evidence for regularly ordered lamellar
structure.The TEM image was obtained on a JEOL 100CX using an
accelerating voltage of 120kV and a 20pm objective lens aperture.

(Note : 2.5 cms = 400A) ................................................................. 100
11.15 Scanning electron micrographs at X2200 for (A) HDTMA+-
fluorohectorite air dried (Top). (B) HDTMA+-Vermiculite air dried
(below) .......................................................................................... 101

Figure Page

111.1 (a)X-ray powder diffraction patterns for air dried PCH
formed by reaction of decylamine solvated HDTMA+-Fluorohectorite with
TEOS at HDTMA+-Fluorohectorite : decylamine : TEOS molar ratios of 1
: 20 : X where 10 S X S 200. (b)X-ray powder diffraction patterns of
porous clay heterostructures (PCH) obtained by calcination at 650°C for 4
h. Note that at X 2 100, the caolcined PCH-fluorohectorite exhibits well
ordered c axis spacing of ~32A ......................................................... 112

11. 2 A schematic representation for the mixed layering

mechanism. (a) In smectites with ~9. 6A layer thickness along the c axis. (b)
In interstratified mineral such as rectorite with ~19. 0A layer thickness
along c axis ..................................................................................... 115

111.3 X-ray powder diffraction patterns of calcined PCH reaction
products formed at a different HDTMA+-FH : octylamine : TEOS reaction
stoichiometry :(a) 1 : 20 :150 (b) 1: 5 : 37.5 (c) 1 : 2 : 15.(Q+-clay :
amine : TEOS.) ................................................................................ 117

111.4 X-ray powder diffraction patterns for air dried PCH formed
by reaction. (A) decylamine solvated DTMA+-Fluorohectorite with TEOS
at DTMA+-Fluorohectorite : decylamine : TEOS molar ratios of 1 : 20 :
150. (B) decylamine solvated HDTMA+-Fluorohectorite with TEOS at
HDTMA+-F1uorohectorite : decylamine : TEOS molar ratios of l : 20 :
150 ................................................................................................. 1 19

111.5 Nitrogen adsorption/desorption isotherms and the
corresponding H & K pore size distribution for porous silica-clay
heterostructures (PCHs) prepared by gallery-templated synthesis using (a)
hexylamine, (b) decylamine, & (c) dodecylamine as co-templates. The
HDTMA+-clay : amine : TEOS reaction stoichiometry was 1:20: 150 ..... 120

111.6 (A) XRD pattern of air dried and calcined gallery templated
product formed using amine co-template. (B) XRD pattern of the air dried
and calcined silica-pillared products formed in the absence of a amine co-
template .......................................................................................... 123

111.7 (a) Nitrogen adsorption/desorption isotherms and H&K pore
size analysis for porous silica-clay heterostructures (PCHs) prepared by
gallery-templated synthesis using HDTMA+-Fluorohectorite : decylamine :
TEOS with a reaction stoichiometry was 1 : 20 : 150. (b) Nitrogen
adsorption/desorption isotherms and H&K pore size analysis for silica

Figure Page

intercalated derivative prepared using HDTMA+-Fluorohectorite : TEOS
with a reaction stoichiometry of 1 : 150 .............................................. 124

111.8 (a) X-ray diffraction patterns for the air dried products
formed by the reaction of decylamine solvated HDA+-Fluoorhectorite with
TEOS at HDA+-FH : decylamine : TEOS molar ratio of 1 : 20 : X where
10 S X S 200. (b) X-ray diffraction patterns for the calcined material
formed by the reaction of decylamine solvated HDA+-Fluoorhectorite with
TEOS at HDA+-FH : decylamine : TEOS molar ratio of 1 : 20 : X where

10 S X S 200 ................................................................................... 126

111.9 (A) X-ray diffraction patterns prior to template extraction
using ethanol as solvent. Q+-FH : decylamine : TEOS at a molar ratio of 1:
20 : 150, (DTMA+ as Q+ ion) and P+-FH : decylamine : TEOS at a molar
ratio of l: 20 : 150, (DA+ as P ion). (B) Thermal gravimetric analysis of
decylamine solvated n-decylammonium-Fluorohectorite.(Pn+-FH system)

and PCH prepared from decylamine-solvated DTMA+-
Fluorohectorite.(Qn+-FH system) prior to template removal ................. 130

111.10 (A) X-ray diffraction patterns after template extraction using
ethanol as solvent of Q+-FH : decylamine : TEOS at a molar ratio of 1: 20 :
150, and DTMA+ as Q+ ion and P+-FH : decylamine : TEOS at a molar
ratio of 1: 20 : 150, and DA+ as P ion. (B) Thermal gravimetric analysis
of decylamine solvated n-decylammonium-Fluorohectorite.(Pn+-F H

system) and PCH prepared from decylamine-solvated DTMA+-
Fluorohectorite.(Qn+-FH system) after template removal ..................... 131

111.11 Nitrogen adsorption/desorption isotherms and the
corresponding H & K pore size distribution for porous silica-clay
heterostructures (PCHs) prepared by gallery-templated synthesis using

decylamine, as co-templates and (a) DTMA+ (b) HDTMA+ as gallery
cation The Q+-clay : amine : TEOS reaction stoichiometry was 1 : 20 :
150 ................................................................................................. 133

111.12 Nitrogen adsorption/desorption isotherms and the
corresponding H & K pore size distribution for products formed by the
reaction of (A) Decylamine solvated DA+-F1uorohectorite with TEOS at
DA+-FH : decylamine : TEOS molar ratio of 1 : 20 : 150. (B) Decylamine
solvated HDA+-Fluorohectorite with TEOS at HDA+-FH : decylamine :
TEOS molar ratio of l : 20 : 150 ....................................................... 134

Figure Page

111.13 (A)X-ray powder diffraction patterns for air dried PCH
formed by reaction of decylamine solvated DODDMA+-Fluorohectorite
with TEOS at DODDMA+-Fluorohectorite : decylamine : TEOS molar
ratios of l : 20 : 150. (B) X-ray powder diffraction patterns of air dried
(PCH) obtained by calcination at 650°C for 4 h ................................... 136

111.14 X-ray diffraction powder patterns for DDDMA+-magadiite :
decylamine : TEOS, with a molar ratio of l : 20 : 150, respectively. (A)
DDDMA+-magadiite, (B) As-synthesized, (C) Ethanol-extracted, (D) As-
synthesized, calcined at 650°C, (E) Ethanol-extracted, ion-exchanged....l38

111.15 N2 adsorption/desorption isotherms and the corresponding H-
K pore size distribution for (a) As-synthesized, calcined, (b) Ethanol-
extracted, ion-exchanged material ..................................................... 139

111.16 X-ray diffraction powder patterns for HDTBP+-magadiite :
decylamine : TEOS mixture with 1 : 20 : 150 molar ratio, respectively. (A)
As-synthesized, (B) As-synthesized, calcined at 650°C, (C) Ethanol-
extracted, ion-exchanged, (D) Ethanol-extracted ................................. 141

111.17 Thermogravimetric plot of (a) HDTBP+~magadiite (b)
Ethanol-extracted, ion-exchanged HDTBP+-magadiite : decylamine : TEOS
mixture ........................................................................................... 142

111.18 N 2 adsorption/desorption isotherms and the corresponding H-
K pore size distribution for HDTBP+-magadiite : decylamine : TEOS
mixture at 1 : 20 :150 molar ratio. (A) as-synthesized, calcined (B)
Ethanol-extracted, ion-exchanged ...................................................... 143

111.19 Proposed mechanism for the formation of a PCH by gallery -
templated synthesis: (A) Amine - solvated bilayer structure with a thickness
equivalent to the length of the quaternary cation (filled head groups) and
the neutral amine (open head groups) surfactants; (B) Intercalation of
TEOS by partial displacement of neutral amine ; (C) Templated
heterostructure in which a 2D hydrated silica is organized around micellar

assemblies of 0" and neutral amine; (D) Calcined porous clay
heterostructure with a 2D framework of porous silica intercalated between
the clay layers .................................................................................. 145

IV.1 X-ray diffraction pattern of PCH-Fluorohectorite calcined at

xiv

Figure Page

different temperatures for 4 hours in air. Insert : The corresponding
evolution of the d-spacing ................................................................ 162

IV.2 Infrared spectras of PCH-Fluorohectorite calcined at different
temperatures for 4 h. The samples were prepared as KBr wafer. No
effort was made to remove physisorbed water .................................... 164

1V.3 (3) Infrared spectra of PCH-Fluorohectorite calcined at 20,
100, 150, 250 and 350°C for 4 h. (b) Pure silica MCM-4l mesostructure
calcined at 20, 100, 150, 250 and 350°C temperatures. Spectra were
recorded after removing the adsorbed water under vacuum. Self supported
pellets were used ............................................................................. 166

1V.4 29Si MAS-NMR spectra of PCH-Fluorohectorite calcined at
different temperatures for 4 h. Chemical shifts are relative to TMS ...... 168

IV.5 Schematic of crosslinking between the gallery silica
mesostructure and the layers showing inversion of some silica tetrahedra of
the layer ......................................................................................... 169

1V.6 Representation of the octahedral sheet in fluorohectorite (see
text). Fluorine atoms are represented by white or black circles, depending
whether they are above or below the layer plane ................................ 171

1V.7 19F MAS-NMR spectra of FH-PCH calcined at different
temperatures for 4 h ........................................................................ 172

1V.8 Nitrogen adsorption/desorption isotherm for PCH-
Fluorohectorite calcined at different temperatures ranging from 300 to
550°C. Insert : The correspoding Horvath-Kawazoe pore size distribution
curves of PCH-Fluorohectorite calcined between 300°C to 550°C ......... 175

1V.10 X-ray diffraction pattern of PCH-Fluorohectorite prepared
using HDTMAfldecylamine template and TEOS at a molar ratio of 1 : 20 :
150.(a) As-synthesized PCH-fluorohectorite. (b) Ethanol-extracted PCH-
fluorohectorite.(c) Ethanol-extracted and calcined PCH-fluorohectorite.l77

1V.11 Thermal gravimetric analysis of PCH-fluorohectorite prepared
from decylamine-solvated HDTMA+-Fluorohectorite/TEOS. (a) As-
synthesized PCH-fluorohectorite. (b) Ethanol-extracted PCH-
fluorohectorite ............................................................................... 178

XV

Figure Page

1V.12 Nitrogen adsorption/desorption isotherm for PCH-
Fluorohectorite (a) calcined at 650°C and (b) Ethanol-extracted and
calcined at 650°C Insert : The correspoding Horvath-Kawazoe pore size
distribution curves of PCH-Fluorohectorite ...................................... 180

1V.13 X-ray diffraction pattern of PCH-rectorite prepared using
HDTMAfldecylamine template and TEOS at a molar ratio of 1 : 20 : 150,
calcined at 700°C prior to hydrothermal treatment at different
temperatures. (a) 400°C steaming, (b) 500°C steaming, (c) 600°C steaming
and ((1) 700°C steaming .................................................................... 183

1V.14 Nitrogen adsorption/desorption isotherm for PCH-rectorite (a)
calcined at 700°C and (b) Calcined at 700°C and steamed treated at 600°C
and (c) calcined at 700°C and steam treated at 700°C Insert : The
corresponding Horvath-Kawazoe pore size distribution curves of PCH-
rectorite at different hydrothermal temperatures .............................. 184

1V.15 Infrared spectra of pyridine adsorbed on PCH-vermiculite
calcined at 650°C and silica MCM-41 calcined at 650°C.pyridine-adsorbed
sample was evacuated at 150°C ......................................................... 186

1V.16 X-ray diffraction powder patterns for as-synthesized and
calcined PCH-fluorohectorite prepared using different Ti/Si molar ratios.
(A) Ti/Si molar ratio = 0.001, (B) Ti/Si molar ratio = 0.01, (C) Ti/Si
molar ratio = 0.1 ............................................................................ 189

1V.17 X-ray diffraction powder patterns for as-synthesized and
calcined PCH-fluorohectorite prepared using different Al/Si molar ratios.
(A) Al/Si molar ratio = 0.1, (B) Al/Si molar ratio = 0.01 .................... 192

1V.18 X-ray diffraction powder patterns for freshly calcined and Ti
grafted PCH-fluorohectorite prepared using different Ti loadings. (A)
Freshly calcined PCH-fluorohectorite. (B) Ti-PCH with Ti-alkoxide cone
of ~0.9g/g of PCH-fluorohectorite. (C) Ti-PCH with Ti-alkoxide loading
of ~0.45g/g of PCH-fluorohectorite .................................................... 195

1V.19 N2 adsorption/desorption isotherms and the corresponding H-
K pore size distribution for (A) PCH-fluorohectorite, calcined at 650°C.
(B) Ti-PCH with Ti-alkoxide conc of ~0.45g/g of PCH-fluorohectorite..l96

IV.20 X-ray diffraction powder patterns for alkali metal-ion

xvi

Figure Page

exchanged and their corresponding quaternary ammonium exchanged Q+-
clay.(A) montmorillonite (Arizona). (B) beidellite(Chinese). (C)
montmorillonite (Wyoming) ............................................................. 199

1V.21 X-ray diffraction powder patterns for PCH prepared using
decylamine/HDTMA+ as templates and different low charge density
smectites with a Q+-clay : amine : TEOS of l : 20 : 150 respectively. (A)
as-synthesized. (B) calcined at 500°C for 4 h .................................... 200

IV.22 X-ray diffraction powder patterns for the dialkyl quaternary
ammonium exchanged clays (Q+-clays). (A)DDDMA+-
montmorillonite(Wyoming). (B) DODDMA+-beidellite (Chinese). (C)
DODDMA+-montmorillonite (Wyoming). (D) DDDMA+-beidellite
(Chinese). (Insert) A plot of the basal spacings to cation exchange
capacity/100g of clay. (solid square - DODDMA+, open circle -
DDDMA+) ...................................................................................... 203

IV.23 X-ray diffraction powder patterns for the dialkyl quaternary
ammonium exchanged clays (Q+-clays) in excess amine suspension with a
molar ratio of 1 : 20 between Q+-clay and amine. A)DDDMA+-
montmorillonite(Wyoming). (B) DDDMA+-beidellite (Chinese). (C)
DODDMA+-beidellite (Chinese). (D) DODDMA+-montmorillonite
(Wyoming) ...................................................................................... 204

IV.24 X-ray diffraction powder patterns for PCH prepared using
decylamine/dialkyl quaternary ammonium as templates and different low
charge density smectites with a Q+-clay : amine : TEOS of 1 : 20 : 150
respectively. (A) as-synthesized. (B) calcined at 500°C for 4 h ........... 206

xvii

LIST OF THE ABBREVIATIONS

IUPAC - International Union of Pure and Applied Chemistry.

TEOS - tetraethyl orthosilicate : Si(OC2H5)4.

TEOT - tetraethyl orthotitanate : Ti(OC2H5)4.

TIPOT - teraisopropyl orthotitanate : Ti[OCH(CH3)2]4.

TMAOH - tetramethylammonium hydroxide : (CH3)4NOH.

DDDMA+ - didecyldimethylammonium : (C10H21)2N(CH3)2+.
DODDMA+ - dioctadecyldimethylammonium : (C13H37)2N(CH3)2+

DA+ - decylammonium : (C10H21)NH3+.

DTMA+ - decyltrimethylammonium : (C10H21)N(CH3)3+.

DDA+ - dodecylammonium : (C12H25)NH3+.

DDTMA+ - dodecyltrimethylammonium : (C12H25)N(CH3)3+.

HDA - hexadecylammonium : (C16H33)NH3+.

HDTMA+ - hexadecyltrimethylammonium : (C15H33)N(CH3)3+.

Q+-Clay - organo clays

Qn+-FH - Quaternary ammonium exchanged fluorohectorite.

Pn+-FH - primary ammonium exchanged fluorohectorite.

PCH - porous clay heterostructure.

EPMES - equipotential periodic minimal energy surfaces.

XRD - X-ray diffraction.

TEM - transmission electron microscopy.

SEM - scanning electron microscopy.

TGA - thermogravimetric analysis.

NMR - nuclear magnetic resonance.

SBET - specific surface area in m2/g obtained from the linear part of the
Brunauer - Emmett - Teller equation.

BET - Brunauer - Emmett - Teller.

P/Po - relative pressure. P is the equilibrium pressure of the adsorbate and
P0 is the saturation pressure of the adsorbate at the temperature of the
adsorbent, volume adsorbed is at standard temperature and pressure.

H-K - Horvath - Kawazoe pore size distribution.

AlPO - aluminophosphate molecular sieves.

VAPO - vanadium phosphate molecular sieves.

M41S - broad family of mesoporus silica - based molecular sieves with
lamellar, hexagonal or cubic structures.

MCM-4l - Mobil Composition of Matter number 41 possessing long range
hexagonal order (a member of the M41S family)

MCM-48 - Mobil Composition of Matter number 48 possessing cubic

xviii

symmetry ( a member of the M41S family)

HMS - hexagonal mesoporous molecular sieves.

FID - flame ionization detector.

TS-l - microporous titanium - substituted silica molecular sieve with MFI
topology ( analogous to ZSM-S)

TS-2 - microporus titanium - substituted silica molecular sieve with MEL
topology (analogous to ZSM-l 1)

Al-MCM-41 - aluminum - substituted analog of MCM-4l.

Ti-MCM-4l - titanium - substituted analog of MCM—41.

Ti-PCH - titanium substituted analog of PCH.

Chapter 1

"Advances in the design of nanoporous molecular sieves".

A . Introduction

1. Introduction to nanochemistry.

As stated by Geoffrey Ozinl, nanochemistry, as opposed to
nanophysics, is an emerging sub-discipline of solid-state chemistry that
emphasizes the synthesis rather than the engineering aspects of preparing
little pieces of matter with nanometer sizes in one, two or three
dimensions. Currently there is considerable interest in nanoscale objects,
since they exhibit novel material properties largely as a consequence of
their finite small size. The nanochemist can be considered to work towards
this goal from the atom "up", whereas the nanophysicist tends to operate
from the bulk "down". Building and organizing nanoscale objects under

mild and controlled conditions "one atom at a time instead of
"manipulating" the bulk, should in principle provide a reproducible method
of producing materials that are perfect in size and shape at the atomic level.
A cartoon illustration of this comparison is shown in Figure 1.1 These
little objects can be made of organic, inorganic and/or organometallic
components. Their structure property relationships are designed to yield
new materials with novel electronic, optical, magnetic, transport,
photochemical, electrochemical, catalytic and mechanical behavior. Areas
of application that can be foreseen to benefit from the small size and
organization of nanoscale objects include quantum electronics, nonlinear
optics, photonics, chemoselective sensing, and information storage and
processing.

The chemist prides himself on being able to synthesize perfect

objects having nanometer dimensions. To be able to make nanostructures

that are useful in electronic, optical and information processing systems,

chemists

 

 

 

 

 

Figure 1.1 : Nanochemistry as compared to nanophysicsl.

also have to develop synthetic methods that have the ability to position these
tiny objects in appropriately connected organized arrays. The chemical
alternatives for meeting this challenge and building such nanoscale devices
from scratch involve patterning and templating methods. Nanolithography
is used to spatially define chemically active foundation sites, usually on
planar substrates, upon which subsequent site specific chemical synthesis
allows the growth of nanoscale objects. Templating methods exploit the
perfectly periodic, single size and shape channel, layer and cavity spaces of
crystalline nanoporous host structures for performing host-guest inclusion
chemistry. Both approaches benefit from the principle of synthesis and self
organization in preexisting regions of a planar substrate or a porous solid,
both with restricted dimensions on the nanoscale.

With respect to the template based preparation, organization and
stabilization of nanoscale objects, the nanochemist generally dreams up
chemical synthesis inside the void spaces of nanoporous host materials.
The strategy requires the judicious selection of the host materials in
addition to suitable precursors to the desired guest(s). There now exists a
huge range of hostsz. They can be of an inorganic, organic or
organometallic compositional type, with one dimensional (l-D) tunnel, 2-D
layer and 3-D framework structures. Hosts may be of the insulating,
semiconducting, metallic or superconducting type, or may attain these
properties following inclusion of the chosen guest3. On surveying known
host structures, one finds that channel, interlamellar and cavity dimensions
vary widely in size, separation and perfection, spanning the size range
from barely being able to accommodate the smallest ionic or molecular
guests all the way to channel dimensions of about 5-10000 A, interlamellar
spaces of 3-50 A and cavity diameters of 6-10000 A.

2. Classification of porous materials

In general terms, nanoporous materials are solids with an accessible
open space in the 1.0 - 10.0 nm range. In describing porous materials, the
IUPAC has defined three size domain : micropores, <2 nm; mesopores, 2-
50 nm; and macropores, 50 nm4( see Figure 1.2). Thus, the nanoporous
regime spans the traditional midmicropores to lower-mesopore size range.
Inorganic macroporous solids are well known as porous gels5 or porous
glasses6; these substances generally possess relatively broad pore size
distributions due to their amorphous structures. Their typical applications
are in the field of separation processes, for example in chromatography or
as catalyst supports. Meso and macropores usually are associated with
materials that are either finely divided or structurally highly disordered
(amorphous). That is, meso and macro porosity often are consequences of
the texture of a material. Microporous solids are exemplified by zeolites
and their related compounds. Zeolites are alunrinosilicates with a periodic
three-dimensional framework structure containing voids; related
compounds possess similar structures but differ in chemical composition of
the frame work ( e.g., aluminophosphates). Due to the periodicity of the
structure of these crystalline solids, they exhibit an extremely narrow pore
size distribution. This is important for their size-specific application in
absorption, molecular sieving and shape selective catalysis7'9. More
recently, non classical uses of the host-guest chemistry of zeolites have been
envisaged10'21. The open frame work structures are used as nano-sized
reaction vesselsll'l3 or as hosts for the controlled construction of
assemblies of metal and semiconductor clusters”, 15, of organic

moleculesl5‘18 or for

 

 

 

 

 

 

 

 

 

Micropores Mesopores Macroporec C)
porous glasses
porous gels

 

 

 

 

 

 

 

 

 

   
 

 

 

 

 

 

 

 

 

ULP zeolites “1
pillaredds layered . . . . . n_1
.m a)
zeolites
1 1 1 l 1 l
5 10 50 1m 5w 1W 1 A 1 L 1 A J
Pore diameter (A) 1 10 20 30

Figure 1.2 : (a) Defination of micro-, meso-, and macropores together
with the representative materials and their pore size distributions. (b)
Schematic X-ray diffractograms of a zeolite, pillared solid and a ultr-large
pore molecular sieve1

polymerization reactions leading possibly to molecular wires in the case of
conducting polymer515'19. Exciting applications have been proposed for
these nanocomposite materials, for example in nonlinear opticszo, as
devices in electronic and optical computing and image processing21 or as
pigments.

The outstanding properties of zeolites and related compounds have
made these materials one of the most important subjects of research in
solid state chemistry in the last two decades. The major drawback for their
use has been their limited pore size, which excludes larger molecular
entities from the size specific processes occurring in the voids of these
materials. Since the first applications of zeolites have been established, an
urgent need has been developed for solids with well-defined pore sizes
larger than the ~7A diameter window occurring in X/Y zeolites. Typical
extensions of current uses of zeolites include :

OThe catalytic cracking of heavy oil fractions : Aluminosilicate
zeolite Y is currently being used with an enormous impact in the oil-
refming industry as a cracking catalyst for the "middle distillates" in order
to convert these to gasoline. However the restricted pore size of zeolites
excludes the larger hydrocarbon molecules of the "heavy end of the barrel"
from highly selective intra-pore cracking processes.

OZeolite-like compounds with a pure silicate framework ("zeosils"),
for example silicate, exhibit a high efficiency in selectively sorbing organic
hydrophobic pollutants from waste waters. Increased pore sizes might
extend the use of pure silicate frameworks to larger molecules, for
example to the adsorption of toxic polychlorinated biphenyl's from

chemical products or of traces of herbicide and pesticide molecules from
drinking water.

0More recently, metal porphyrin complexes have been incorporated
into zeolite void structures, and molecular microstructures consisting of
several redox-active organic and inorganic species have been generated on
zeolite host systems1 1, providing immobilized enzyme mimics22 or models
of biochemical electron transfer chain811123. Again, larger pores would
allow a much better fine-tuning of such structures, for example by the
possibility of using bulkier ligands.

The fact that most of the envisaged uses of mesoporous solids are
extensions of applications of zeolites means that the mesoporous regime of
most interest is located at the lower end (pore sizes of 20A to >200 A) of
the IUPAC definition. Compared to the chemistry of microporous solids,
that of mesoporous solids is at a less elaborate stage. Two major paths of
the synthesis of mesoporous solids have developed over the years. Firstly,
there have been numerous attempts to extend the hydrothermal synthesis
procedures used to prepare microporous zeolite-type structures to the
mesoporous regime. Until recently, however, success was very limited, so
that in the meantime other routes to mesopores solids with narrow pore
size distribution had been investigated. The most important of these is the
"pillaring" of layered solids, 1.6. the controlled intercalation of spacers

between the layers of clay minerals

B. Enlarging the Pore Sizes of Zeolites and Related

Compounds

The most obvious approach1 to obtaining controlled mesoporosity is

the extension of zeolite-like three dimensional framework structures to

larger pore sizes. The early work on hydrothermal zeolite synthesis by
Barrer and co-worker324 showed that the inorganic and oganic cations
present in an aluminosilicate gel that is subjected to hydrothermal treatment
play a decisive role in determining the type of three-dimensional
framework formed. Even nowadays, this "template effect" is not
understood completely. Templating agents change the chemistry of the gels
and are occluded in the growing zeolite crystals during hydrothermal
synthesis. Larger organic molecules are thus supposed to act also as "void
fillers" that inhibit the crystallization of thermodynamically more stable
nonporous phases. Quite naturally, it was anticipated that the use of larger
templating agents, especially of larger organic molecules and cations,
would lead to larger pore sizes — from the micro to the mesoporous
regime. Other attempts were directed at changes in the gel chemistry, with
one or both of the components Al and Si being substituted by main-group
elements Be, Ga, Ge, 1?, and As or by transition metals such as Ti, Fe and
Co.

For a longtime, however, these attempts failed to enlarge the pore
sizes of zeolite-like solids. Zeolites X/Y, naturally occurring as the
mineral faujasite and synthesized for the first time in the 1950's, with their
~12A diameter cavity and their ~7A diameter windows, maintained their
position as the largest pore materials in the class of zeolites and related
materials for a long time.

After roughly four decades of zeolite and molecular sieve synthesis,
it appeared that the experimental accessible range of pore sizes had an
intrinsic upper stability limit of the 12 T-atom ring, with a diameter of
about 7-8 13.25. (T refers to apex linked tetrahedral T04 building units,

where T is most commonly Si, Al, P). However, a dramatic turn of events

10

occured in 1988 with the discovery by Davis and co-workers26 of an extra-
large pore molecular sieve based on aluminophosphate, denoted VPI-S,
having an essentially circular 18 T atom ring uni-dimensional channel
structure, with a diameter of about 12-13 A. This breakthrough shattered
the "psychological" 12 T atom ring barrier, and had the effect of
revitalizing synthetic efforts aimed towards extra-large-pore and even
ultralarge-pore materials. Not long after this amazing breakthrough, a
method was discovered in 1990 for the controlled phase transformation of
VPI-5 to a related yet smaller pore 14 T atom ring material, denoted
AlPO-8 27. In 1991, Estermann and co-workers28 announced another
spectacular discovery with the synthesis and structure determination of an
extra-large-pore gallophosphate molecular sieve called cloverite. This
novel material contains a 20 T atom Cloverleaf-shaped entrance window
leading into an impressive 29-30 A diameter supercage. The overall
structure of the cubic unit cell is based on two non-intersecting, three
dimensional, 8 T atom and 20 T atom channel systems. An intriguing
feature of cloverite is the existence of P(OH)OGa(OH) terminal hydroxyl
groups completely covering the inside surface of the supercage and
protruding into the Cloverleaf-shaped pore openings. Zeolite and
molecular sieve frameworks normally contain T04 (T: Si, Al, P)
tetrahedral building units connected by T-O-T bonds, with only a low level
of terminal T(OH) defect groups, whereas cloverite contains large numbers
of terminal T(OH) groups as an integral part of the structure and is
referred to as having "interrupted framewor ". The therrnal stability and
chemical reactivity of cloverite is under intense investigation, as the
properties of the interrupted framework will probably determine the use of

the material for the

1 982 1 988 1991 1 993
Before 1980

O
:1.
<1
>

\

207

15-

Cloverite

 
 
  
 
 

VPI-S

.1
O

Pore Size (A)

 

U1

 

Molecular Sieves

Figure 1.3 : Pore size of microporus zeolites and other recently
developed molecular sieves”.

12

catalytic, adsorption and advanced materials applications. Shortly after
Estermann and co-workers discovered cloverite, Xu and co-worker529
disclosed the synthesis of JDF—20, an aluminophosphate molecular sieve
with an elliptically shaped 20 T atom ring unidimensional channel
structure. Curiously, JDF-20 also contains terminal hydroxyl groups
lining the inside surface of the 20 T atom channel and protruding into the
pore opening, just like cloverite. This raises the interesting question as to -
whether interrupted frameworks are an intrinsic characteristics of such
extra-large-pore molecular sieves, so far represented by cloverite and JDF—
20, and are possibly a kind of extended "giant defect".

A common theme pervading the synthesis of the majority of known
zeolites and molecular sieves is the use of quaternary alkyl ammonium
and/or amine organic additives under hydro or organo or amino thermal
reaction conditions30’31. It is generally agreed that these template moieties
serve one or more structure-directing, space filling and charge-balancing
functions in the self-assembly of, for example, silicate aluminate and
phosphate basic building units. One scientifically appealing way of
thinking about the template-mediated nucleation and growth of zeolite and
molecular sieve materials is in terms of density fluctuations of transient
local order, involving the cationic, anionic and neutral constituents of a
synthesis mixtures under hydrothermal reaction conditions. Equipotential
periodic minimal energy surfaces (EPMESs) having zeolite and molecular
sieves type topologies have recently been proposed to be associated with
these fleeting structure domains in the liquid and/or gel phases”. Proper
use of template on the loci of these EPMESS, the basic building units are
imagined to undergo a spatially constrained condensation polymerization to

form the seed that is responsible for the nucleation of a particular zeolite

13

or molecular sieve structure type. The surfaces of the crystal nuclei
themselves bear their own characteristic EPMESs, which are viewed as
being responsible for the continued growth and crystallization of the
product material.

With this in mind, it is interesting to note that some zeolites and
molecular sieves synthesized to date, based on monomeric, oligomeric and
even polymeric quaternary alkyl ammonium and/or amine templates,
appear to display framework topologies that bear a resemblance to the
shape of the template. Thus spherically shaped templates may lead to
cavity type structures and rod shaped ones, may result in channel type
structures. However, the pore sizes, channel and cavity spaces have been
generally restricted roughly to the molecular diameter of individual
spherically shaped templates or the width of the rod-shaped ones, resulting
in the current pore size maximum of the 20 T atom rings found in

cloverite and JDF-20.( Adapted from an article by Ozin)1.

C. Propping Layered Materials -— Pores by Pillaring.

1 . Lamellar Solids.

Solids with layered structures possess basal planes of atoms that are
tightly bonded within the planes but relatively weakly bonded in the
direction perpendicular to the planes. The asymmetric bonding
interactions translate into greatly different physical properties for the
material in the in-plane and out-of-plane directions. The weakly

interacting region between the stacked units is usually referred to as the

l4

"interlayer" or "gallery" region. When the layers are electrically neutral,
as in graphite or FeOCl, the galleries are empty and the basal planes of the
adjacent layers are in van der Waals contact. Neutral guest molecules often
can be incorporated between the host layers to form regularly intercalated
derivatives. The incorporation of neutral species into the van der Waals
gap typically is accompanied by electron-transfer reaction between the
molecular guest and the layered host”. The free energy change associated
with the electron transfer step provides much of the driving force for the
intercalation reaction.

In several classes of lamellar solids, the layered units carry a net
electrical charge”. These include smectite clays, layered double
hydroxides, and Group 4 metal phosphates. To achieve an electrically
neutral structure, counterions, usually solvated by water or other polar
molecules, occupy the gallery region between the layers. Thus, ionic
lamellar solids qualitatively resemble the conventional intercalation
compounds formed by electron-transfer reactions between neutral guest
and layered host precursors. The difference, however, is that in ionic
lamellar solids, charge separation between gallery ions and the layers is
complete, whereas in conventional intercalates the extent of charge transfer
between guest and layered host is seldom complete. Consequently, ionic
lamellar compounds can justifiably be described as intercalation
compounds, although in practice they are not formed by electron-transfer
reactions. Instead they simply crystallize, complete charge separation
between the gallery species and the host layers being a distinguishing
feature of their structure.

Owing to their nanoscale periodicity, ionic lamellar solids give rise

to very large intracrystalline surface areas of several hundred square

15

meters per gram or more. However, in most cases the gallery surface area
is accessible only to water and other small polar molecules that are capable
of solvating the gallery counterions and the charged layer surfaces.
Removing the solvating molecules by outgassing at elevated temperatures
results in the recollapse of the galleries, especially if the intercalated
counterions are small relative to interstices occupied by the ions on the
gallery surfaces. If the counterions are relatively large, they can function
as molecular props or "pillars" and thereby prevent the galleries from
collapsing completely when the solvating medium is removed35r36. The
gallery space might then be accessible to other small molecules the size of
H20, for example, N2, CO, or NH3. But simply facilitating the adsorption
of small molecules is relatively uninteresting. Ideally, one would like to
tailor the gallery structure on a length scale that would allow the
accommodation of organic and inorganic molecules for molecular assembly
and, perhaps, catalytic chemical conversions. Pillaring reactions of a
lamellar host are an important route to achieving these desired structural
modifications.(adapted from an article by Pinnavaia)93

Compared to the hydrothermal synthesis of zeolite-like compounds,
where the aggregation of matter to a porous material has to start from a
"zero point", i.e., from a solution or gel, the preforrnation of layers in the
precursor compounds of pillared materials37 should facilitate the process
of formation of large pores. In a simplified box of bricks picture,(see
Figure 1.3 a) it is only necessary to choose the right length of the props and
distribute them in an ordered manner between the layers. Real chemistry
is not that simple, of course. There are currently three main types of
layered materials that are used for pillaring, namely smectite clays, layered

double hydroxides, and phosphates and phosphonates of tetravalent metals.

l6

Clays are naturally occurring three-layer sheet silicates. The sheets
are formed of two layers with tetrahedrally coordinated atoms surrounding
a layer with octahedrally coordinated atoms. The whole structure carries a
net negative charge resulting from substitutions in either the tetrahedral
(Al+3, Fe+3 for 81‘”) or the octahedral (Fe+2, Mg+2 for Al+3, Li+ for
Mg+2) layer. This negative charge is compensated by interlayer cations,
which in natural smectite clays are alkali or alkali earth ions. In the case of
smectite clays, the net negative charge of the layer is rather small; the clay
can then easily swell in water. After swelling, the interlayer cations may
be exchanged for larger oligomeric cations, for example the Keggin ion
[A113O4(OH)24(H20)12]+7 ("A113")38 or the zirconyl cation
[Zr(OH)2(H2O)4]4+3, These cations are present in solutions of the metal
ions under appropriate conditions of pH, concentration and temperature.
Subsequent calcination of the exchanged clay at elevated temperatures, e.g.
500°C, dehydrates the pillars and establishes stable links between them and
the layers. Due to the fact that the ion exchange capacity, i.e. the number
of cations necessary for charge compensation, is small, aporous material
with space left between pillars is obtained ( see Figure 1.4 a).

Pore size distributions derived from sorption data of materials
prepared in this way shows that the majority of pores are in the
microporous range <10A, as may be expected due to the rather small
pillars. However, these compounds also exhibit mesopores with a rather
broad size distribution between 20 and 200A. The mesoporosity in these
compounds may be traced back to a disordered arrangement as depicted
schematically in figure 1.4 b : During swelling and ion exchange the clay
delaminates to form small layer packages or single layers. On drying after

the pillaring reaction these layer packages arrange themselves as shown in

figure [.3 b and are stabilized in this configuration by calcination. This

a)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mesoporosity
Figure 1.4 : The approach to mesoporosity via pillaring of layered
solids:(a) In most pillared solids, the interlayer regions contain only
micropores. (b) Mesoporosity is usually connected with a disordered
arrangement of layer packagesloo.

18

leaves open spaces — mesopores —-between them. Another possibility for
the formation of mesopores is bending of the layers. Both types of
mesoporosity, however, are generated in an uncontrolled process and there
is no simple way of designing pore shapes or sharpening the pore size
distribution.

The aim of obtaining controlled interlayer mesoporosity by pillaring
has been tackled by the use of larger pillars39‘41. These attempts include
the DIMOS route, i.e., the direct intercalation of metal oxides sols.
Enlarged interlayer distances were indeed observed in the pillared clays.
However, sol particles are usually spherical (or nearly so), so that, as they
enlarge the height of the interlayer galleries, they also fill up the additional
interlayer space. Large elongated ("slim") pillars tend to incline with
respect to the layer surfaces(Figure 1.5 a). It may be possible to overcome
this problem by the post-intercalation of suitable organic molecules. Their
space requirement might serve to keep the pillars upright (Figure 1.5.b).
During calcination, when stable links are being established between the
layers and the pillars, the organic burns off, leaving behind a pillared solid
with increased interlayer spacing (Figure 1.5 c). Another way to achieve
interlayer mesoporosity requires elaborate control of the shape of pillar,
which is equipped with "bases" so that it cannot adopt an inclined position
(Figure 1.5 (I). So far, this approach seems to be feasible only for
zirconium phosphonates.

Another important class of layered solids consist of layered double
hydroxide(LDHs). The structure of these compounds is based on that of
Mg(Ol-I)2, brucite. In brucite, magnesium ions occupy the octahedral sites
between each second layer of a close-packing of hydroxide ions, thus

giving rise to a layer sequence OH-Mg-OH-OH-Mg-OH in the direction

 

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Figure 1.5 : Approaches to mesoporosity via pillaring of layered solids :
(a) Attempts to use large elongated ("slim") pillars to create interlayer
mesoporosity may fail because the pillars are inclined w.r.t the layers. (b)
This failure may be prevented by post-intercalating organic substances that
(c) can be burnt off during calcination when stable links between pillars
and the layers are formed. (d) Another possibility is the intercalation of

pillars with bases100

20

perpendicular to the layers. Mg(OH)2 layers are neutral but substituting
some of the divalent Mg+2 ions by trivalent ions, e.g. Al+3, gives rise to
positively charged layers. The interlayer gaps are filled by charge
compensating anions (often carbonate and/or hydroxide) and water. A
typical LDH is the mineral hydrotalcite, [Mg5Al2(OH)16]CO3 . 4H20.
LDH can be prepared with many different combination of di and trivalent
cations, e.g., Fe”, Ni“, Zn+2, Cr+3, Fe+3, and the ratio of divalent to
trivalent metal ions can vary in a broad compositional range. The most
important property of LDHs is the fact that they are anion exchangers, and
these compounds have been termed anionic clays42. Most of the inorganic
ion exchange active compounds such as zeolites, clays and others are cation
exchangers“, and thus LDHs play an outstanding role in that regime.
Besides that, they have (potential) applications in catalysis, as sorbents and
as pharmaceuticals, e.g. as antacids. Although the possibility of pillaring
LDI-Is has been demonstrated“, this chemistry is still in its infancy. The
fact that among the oligomeric oxo/hydroxo ions many more anions than
cations are known makes the extension of the pillaring approach from clays
to LDHs very promising. The thermal stability of the hydroxides,
however, is reduced in comparison to that of aluminosilicate clays. This
shows up, for example, in the fact that calcining an LDH under conditions
where clays are stable leads to an amorphous solid; astonishingly enough,
the double hydroxide layer is restored after intercalation of an organic
compound.

The third important class of pillared layered solids is based on the
expansion of the interlayer space of the phosphates Me(HPO4)2 . nH2O of
tetravalent metals (Me) such as Ti, Zr and Sn45e45. In these compounds the
metal ions are octahedrally coordinated by oxygen atoms of the phosphate

21

ions; metal and phosphate ions together the layers of the structure. The
protons of the [HPO4]2' group project into the interlayer region, which is
filled with water. Typical substances capable of intercalating into the
acidic interlayer regions of the layered phosphates are organic amines.
Pillaring agents such as the Keggin ion A113 do not intercalate directly but
enter the gaps only when the layers have been spread apart by pre-
intercalation with amines“. This procedure leads to porous materials with
a 10-30A range. Direct intercalation of pillars is also possible if the pillars
are combined with organic amino functions, as is the case for
octa(aminopropyl)silsesquioxanes (H2NCH2CH2CH2)3[Si3012]. Although
this species fills up the interlayer region, the organic part of the intercalate
can be burned off, thus generating interlayer porosity43.

Another type of pillaring is possible by using organic disphosphonic
acids (HO3P—R-PO3H), which contain two phosphonates groups bonded to
an organic residue“. Pillared layered zirconium phosphonates can be
prepared directly by hydrolysis of zirconium fluorocomplexes in the
presence of the diphosphonic acid. Zirconium phosphonates Zr[O3P-R-
P03] are formed with Zr(PO3)2 layers similar to those occurring in the
phosphate, but with the R group protruding into the interlamellar region
and keeping the layers apart. By using this approach, the interlayer space
is filled by the organic residue due to the fact that there is a 1:1 ratio of Zr
atoms in the layer and the residues "R". Partial replacement of the pillars
"R" by smaller residues R' such as H or CH3, however, should lead to the
formation of pores in the mixed phosphonates Zr(O3P-R-PO3)1-
x(R'PO3)2x. Then the length of the residue "R" determines the height of
the pores, and the ratio of the residues RzR', their width. Due to the ability

of the organic chemist to construct a large variety of diphosphonic acids

22

with different residues "",R this approach seems especially promising with
regard to the aim of a fully controlled design of not only pore size and
shape but also, by introducing special functional groups on the pillars,
intrapore reactivity46.49.50. The construction of pillars, with bases, to
prevent them from being inclined with respect to the layers, also seems to
be feasible. The disadvantage of this class of compounds lies in the reduced
thermal stability of the organic residues, restricting their use to
temperatures below 250°C. Higher thermal stability would be provided by
inorganic pillars. If in the future it becomes possible to influence the
chemistry of inorganic oligomeric species as deliberately as that of organic
molecules, a similar approach could yield thermally stable "tailor-made"
porous materials.

Recently a novel pillaring procedure (Figure 1.6) to convert dense,
layered metal oxides and silicates into high surface area molecular sieves
with large interlayer separations was developed by Landis, et a151. This
procedure is applicable to a wide variety of layered phases and allows for
the engineering of microporous materials with diverse composition and
physical properties. '

The pillaring procedure developed for smectite clays are not
generally applicable to the wide variety of laminar metal oxides that do not
spontaneously delaminate in water. This new approach has been effective
to tailor-make materials with varied but controllable pore sizes from metal
oxides with unique chemistries, _ It was found that pillaring could be
facilitated by utilizing a pre swelling step in which the interlayer is exposed
to organoammonium ions. According to procedures developed earlier,
layered metal oxides52 and silicates53 were intercalated with an aqueous

solution of long-chain organoammonium salt or amine. An organic pillar

23

precursor such as tetraethyl orthosilicate (TEOS) was then absorbed into
the organophilic interlayer region, where it was converted to a metal oxide
pillar. Typically, the organoammonium ion exchanged product was stirred
with excess TEOS for 1-3 days at 25-80°C, filtered and dried. The final
microporous material was produced by calcination for several hours in air
at 538°C, which removes the water, the pre swelling organoammonium
ion, and the organic byproduct from TEOS hydrolysis, affording a silica-
pillared product. This procedure has allowed preparation of porous
products from a wide variety of layered oxides, including alkali titanates,
alkali metal-substituted titanates Ax[MTi]2O4 54 and layered silicates such
as magadiite, Na28i7015, and kenyaite, K28114029.

 

 

 

 

 

 

 

 

Figure 1.6 : Schematic representation for the intercalation and pillaring of
alkylammonium solvated magadiitewl.

25

The ability to controllably modify chemical composition as well as
pore size provides the flexibility to tailor catalysts for specialized end uses,

and the catalytic potential of these materials is actively being explored.
D . Mesoprous Molecular Sieves.

1. Assemblies of surfactant molecules as templates - a

milestone to the synthesis of mesoporous molecular sieves.

The considerable synthetic effort toward expanding the uniform
micropore size available in zeolites and molecular sieves met with limited
success until 1992. This was mainly due to the use of single organic
molecules as structure directing agents or templates. Simultaneously, clay
scientists studied the behavior of intercalated assemblies of surfactant
molecules in the galleries of their lamellar host555. This fact had an
important impact on the discovery of mesoporus molecular sieves.
Independently, Yanagisawa et al 55. and Inagaki and co-workers57‘59
prepared mesoporous silicates/aluminosilicates by a quite different strategy,
i.e., by using the layered polysilicate kanemite (idealized formula
NaHSi205.3H20) and a quaternary ammonium surfactant melecule, e.g.,
C16TMAC1, as starting reactants materials. At this time very little is
known about the formation of the mesoporous materials that are derived
from kanerrrite. Although the layers of kanemite consist of single-layered
sheets of SiO4 tetrahedra, the precise structure has not yet been
determined. Lagaly and co-workers have shown that interlayer cations in
layered silicates and other materials, e.g. Na+ in kanemite, can be

exchanged by organic cations with

26

 

 

 

 

 

 

e. '1’.“
c
"9' Route 1
tin-kanemite
Olssolutlon Route 2
Calclnatlon
—+
C “TMA’

   

Figure 1.7 : Synthesis mechanism for mesoporous materials derived from
kanemite as proposed by inagaki et a177.

27

long alkyl chains such as C16T M A +. Because of the
hydrophobic/hydrophilic properties of these long chain organic cations,
their positively charged head groups interact with the negatively charged
inorganic layers resulting in the formation of bimolecular packings of
organic cations between the silicate layers. It is postulated that the long-
chain cations in the organic bilayers are arranged in the so-called "kink-
block" and/or "gauche-block" structures. The "kink" conformation
originates from regular sequences of trans and gauche bonds of the long
alkyl groups. Inagaki et al., 5759 proposed that mesoporous materials are
formed from kanemite and surfactant molecules by route 1 schematically
illustrated in fig *. First, the surfactant molecules pillar the kanemite
layers (may be similar to procedure described by Beneke and Lagaly,53)
after which the flexible silicate layers wind around the exchanged
C15TMA+ ions. The curvature of the silicate layers facilitates the
condensation of silanol groups on the adjacent silicate layers and results in
the formation of a three dimensional, highly ordered mesoporous material
with an hexagonal arrangement of uniformly sized channels, as confirmed
by 29Si MAS NMR and TEM. The silicate-organic complexes were
converted by calcination, to micro- and mesoporous materials with
uniform pore size distribution. The specific surface areas of the calcined
products are 900 mzlg and the pore size can be altered by varying the chain
length of the alkyltrimethylammonium ions used. This route represents an
entirely new way to prepare mesoporous, and possibly microporous,
materials, i.e., by the transformation of a two-dimensional silicate structure

into a three-dimensional network.

28

2. Mobils Ordered Mesoporous M4lS Materials-A leap into

the ultralarge-pore size molecular sieves.

A giant stride forward into the ultralarge pore size range of 15-
100A has very recently been disclosed60 by Mobil researchers in the patent
and open literature, with the creative use of surfactant liquid crystal
templates. These consist of hexagonal or cubic close packed aggregates of
cylindrically shaped micelles, the latter being composed of surfactant
molecules containing hydrophobic alkane chains, hydrophilic
alkylammonium cationic head groups and proximal charge balancing
hydroxyl and/or halide anions. It is believed that under the hydrothermal
conditions of a zeolite or a molecular sieve type synthesis, silicate
aluminate and/or phosphate building units undergo a spatially constrained
condensation-polymerization reaction on a cylindrically shaped EPMAS
associated with the close packed array of cylindrically shaped micelles.
The latter effectively serves as "massive templates" for the creation of
ultralarge cylindrically shaped pore, zeolite and molecular sieve-like
structures. By judiciously selecting the alkyl chain lengths of the
surfactant, the solution chemistry, and the use of auxiliary organic
molecule fillers, the effective diameter of the cylindrical micellar
aggregates can be adjusted to yield ultralarge-pore zeolite like structure
with diameters in the range of 15-100 A.

Members of this family of Mobil materials designated MCM-4l,
were first observed in electron micrographs of products from
hydrothermal reactions of aluminosilicate gels in the presence of
quaternary ammonium surfactants. A typical mobil preparation of MCM-

41 involves an aqueous solution of cationic surfactant, such as

29

observed in the low angle 20 region can be indexed on a hexagonal unit cell
with a = 45 A (2d100N3). Generally, both‘electron and X-ray diffraction
pattern show only a few low order members of the hkO subset of hexagonal
reflections. The BET surface area of the preparation is >1000 m2/gm with
exceptionally high sorption capacities of 2 50 wt% cyclohexane at 40 torr,
49 wt% n-hexane at 40 torr, and 67 wt% benzene at 50 torr. The pore
volume of this sample is 0.79 cm3/gm. The range of pore volumes for
MCM-41 samples is 07-12 cm3/gm*. Figure ?? shows the N2 adsorption
isotherms for this material and for an amorphous mesoporous silica. The
morphology of MCM-4l depends on the synthesis conditions, but it is
possible to obtain relatively large ( 211m) hexagonal prisms of MCM-41.
The nature of the ordering in the walls of MGM-41 - that is, the degree to
which the atoms are precisely ordered is not fully understood. The C/N
molar ratio for as-synthesized C16-based MGM-41 is 19, which is
consistent with the surfactant remaining intact. Another way of altering
the pore diameter of MCM-41 is to add auxiliary hydrocarbons such as
alkylated benzene (for example 1, 3, 5 trimethylbenzene), to the synthesis
mixture“. The incremental addition of 1, 3, 5 trimethylbenzene results in
the concomitant increase of C1100 and the pore diameter. Hexagonal phases
with pore diameter up to 100 A have been characterized.

The microscopy and diffraction results presented above are
strikingly similar to those obtained from lyotropic liquid crystal phases52'
54 which are ordered arrays of surfactant aggregates that occur at specific
amphiphile concentrations55'66. One such phase, the middle or "H1" phase,
produces TEM images57 analogous to those of MCM-4l . The H1 phase is
a hexagonal array of cylindrical micelles in which the hydrophobic

hydrocarbon chains are gathered in the center and the polar groups are

30

 

 

 

 

 

 

 

000~— mAdsorbed
700— --Desorbed
6001-
soot——
M—
3m—
am—
(I)
0’ o 1 1 1 1 1 1 1 1 1
”E 0.0 0.1 0.2 0.3 0.4 0.5 0.5 0.7 0.0 0.9
O
8
a
31,200
3 mmAdsorbed
glpoor- --Desorbed
.3.
9m—
m..-
m—
2001-
07-1 1 1 1 1 1 1 1

 

 

 

L
0.0 0.1 02 0.3 0.4 0.5 0.0 0.7 0.0 0.0
Relative pressure (p/po)

Figure 1.9 : Nitrogen adsorption isotherm for MCM-41 (above) and
amorphous silica -BET surface area 306 m2/g (below)60.

3|

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8 an «Al
' 100 00.0
110 22.0
200 10.0
g 210 10.0
2
.. g o
I;
'§""". "". 0
1111 «N
211 00.0
220 20.0
021 21.7
000 20.:
020 10.1
002 17.0
022 10.0
001 10.0
v ;v ‘ vv; '* I"";" . .0
01:1 all)
g 100 00.2
200 17.0
000 11.0
11' ' '4 ,-- I; 0 10
manual-thou

Figure 1.10 : Representative XRD patterns of calcined MOM-41 (above),
calcined cubic MCM-48 (middle) and as-synthesized unstable
lamellar material (below)60.

32.

arrayed on the surface, in contact with a continuous region of water
surrounding the micelles. The repeat dimensions of the MCM-41 prepared
as described above are consistent with those determined for
hexadecyltrimethylammonium based liquid crystals, where a cylinder to
cylinder repeat distance of 40 A is observed65.

The observed dependence on alkyl chain length and the influence of
auxiliary organic molecules on the resultant inorganic product are also
consistent with two phenomena observed for liquid crystals. The diameter
of hexagonal liquid-crystal phases prepared with anionic surfactants
depends on the alkyl chain lengths of the surfactant *. Organic species may
be solubilized inside the hydrophobic regions of micelles, causing an
increase in nricelle diameter. Reagents soluble in micellar solutions can
increase the porosity of amorphous adsorbents.

These similarities suggest that these mesoporous molecular sieves are
formed by a liquid-crystal templating mechanism. In this mechanism *,
inorganic material occupies the continuous solvent (water) region to create
inorganic walls between the surfactant cylinders. It may be that
encapsulation occurs because anionic aluminosilicate species enter the
solvent region to balance the cationic hydrophillic surfaces of the micelles.
Alternatively, it may be the introduction of the aluminosilicate species
themselves that mediates the hexagonal ordering. Once an ordered array is
established, subsequent thermal processing is used to remove the organic
material and produce a stable molecular sieve. Details of the precise shape
of the pores (e.g. hexagonal, cylindrical) are under intense investigation60.

From the standpoint of a range of basic scientific issues and
technological applications, this discovery of ultra large-pore silicates,

alurninosilicates and aluminophosphates must be considered a landmark in

33

the history of the synthesis of zeolite and molecular sieve type materials.
Presumably one can tune the pore sizes and even dimensionality further by
the appropriate choice of "secondary" chemistry involving the aluminum
sites and / or the surface hydroxyl groups on the inside of the walls of the
cylindrical channels, following removal of the micellar template. These
ultra large-pore materials have tremendous potential for very large
molecule size and shape selective catalysis, gas adsorption and separation,
as well as advanced materials for future nanoscale device applications.

The 6-13 A pore size barrier of known zeolite and molecular sieve
type materials has been dramatically broken by the discovery of the ultra
large-pore materials. The 6-100A range of window and channel spaces
now available enormously expands the kinds of host-guest inclusion
chemistry accessible to the nanochemist. This provides an unprecedented
opportunity to make interesting and significant contributions to the world
of small perfect and organized nanomaterials for various kinds of
nanoscale device applications. The future of the newly emerging field of
nanochemistry, based on these kinds of ultra large-pore materials, looks
extraordinarily bright.

Many results in this research area have been reported, such as
various synthesis and formation mechanisms for the M41S family (MCM-
41, MCM-48, and MCM-SO)68'79. Other examples includes the synthesis
of silica and alumina mesoporous materials with non-ionic polyethene
oxide surfactants30'31, of high aluminum MGM-41 32-34, of mesoporous
materials by using a layered silicate (kanemite)69, layered mesophase
formation“, grafting metallocene complexes“, ion exchange and thermal
stability of MGM-41 37, the synthesis and catalysis of Ti-MCM-41 33-39, V-
MCM-4l 90, B-MCM-41 91-92, Fe-MCM-4l 93, Mn-MCM-4l, Mn-MCM-

34

48 94, V-MCM-48 95 and Ti-MCM-48 96. Some mesostructured transition
element and other main group element oxides have been described. A
generalized synthesis method has been developed for a wide range of
transition and main group element oxide mesostructured materials with
cationic and anionic surfactants, by using acidic and basic media (e. g. SBA-
1, SBA-2, and SBA-3), low concentrations of surfactants, and low reaction
temperatures.

We very recently,97 reported a new route to obtain mesoporous
molecular sieves, through the use of intercalated surfactants for the
templated synthesis of structures within the interlayer spaces (galleries) of
clays. We designated our material as porous clay heterostructures (PCHs).
Our approach to designing porous clay heterostructures is based on the use
of intercalated quaternary ammonium cations and neutral amines as co-
surfactants to direct the interlamellar hydrolysis and condensation
polymerization of neutral inorganic precursor (for example
tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar
solid. (see figure **). As illustrated in typical syntheses that follow, Li+-
fluorohectorite (Li+-FH) was converted to a quaternary ammonium
exchanged form (Q+-FH) by ion exchange with a two fold excess of
aqueous [C15H33N(CH3)3]+C1'. The intercalate was then washed free of
excess surfactant, and air dried. Mixtures of the hydrated Q+-FH (~ 7 H2O
per 020 F4 unit cell), neutral amines and TEOS at different stoichiometric
ratios were allowed to react for 4 h at ambient temperature. Under these
stoichiometric conditions the galleries are swollen by the co-templating
amine and are readily accessible to TEOS. Also, the extra gallery water
concentration is low, as judged by the water content of the amines (~0.4

wt%). Thus the base catalysed hydrolysis of TEOS is much faster in the

35

clay galleries than in solution, minimizing the formation of extra gallery
silica. The resulting intercalates were centrifuged, dried in air to further
promote intragallery TEOS hydrolysis and then calcined at 600°C to
remove the templating surfactants. Two or more orders of (001) X-ray
reflections, indicating layered structures, were observed for all products.
Because of the complementary chemical functionality of the layered and
gallery-templated components and stable pore size distributions, porous
clay heterostructures formed by gallery-templated reactions of layered
silicate clays offer new opportunities for the rational design of

heterogeneous catalyst systems.

36

25953389: >20 25.8.— .m 25955833: woes—n53. Q .mSOuO eoua>_em-o:_E< D

 

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Schematics of Porous Clay Heterostructure Formation 97.

Figure 1.11

37

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Crystals, Vol. 2, 1-23, 1974.

Vartuli, J. C.; Kresge, C. T.; Leonowicz, M. E.; Roth, W. J .;
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Am. Chem. Soc. 1992, 114, 10834.

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Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W.
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P.; Leon, R.; Petroff, P. M.; Schmuth, F.; Stucky, G. D.; Nature,
1994, 368, 317.

Huo, Q.; Margolese, D. 1.; Ciesla, U.; Demuth, D. G.; Feng, P.;
Gier, T. E.; Sieger, P.; Leon, R.; Chmelka, B. F.; Schmuth, F.;
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45

Chapter II

"A New synthetic strategy for forming nanoporous molecular

sieves via intra-gallery templating of a lamellar host."

46

Abstract

The recent discovery by Mobil researchersl- 2 of surfactant-
templated M4lS mesoporous molecular sieves has stimulated interest in
large pore structures for selective catalysis-35 and other materials
applications6. Many of the same ionic surfactants used for mesostructure
synthesis 7’ 3 can be intercalated into a variety of layered structures9» 10.
The assembled nature of the intercalated surfactants raises the possibility of
conducting a templated synthesis of an open framework structure within
the constrained gallery regions of the layered host. The present work
reports the gallery - templated synthesis of a new family of porous clay
heterostructures, denoted PCHs, wherein the layered and gallery
components are regularly stacked on a nanometer length scale. PCH
synthesis is based on the use of intercalated quaternary ammonium cations
and neutral amine surfactants to direct the interlamellar hydrolysis and
condensation polymerization reactions of tetraethylorthosilicate. PCHs
complement conventional zeolites, pillared clays and templated
mesostructures, in part, by exhibiting pore sizes in the supermicropore to
small mesopore range (14 - 22 A). Since the layered and gallery -
templated components of a layered heterostructure can differ in chemical
functionality, PCHs offer new opportunities for the rational design of

heterogeneous catalysts.

47

A Introduction.

In some respects, our approach of porous clay heterostructure is
similar to conventional pillaring reactions of lamellar solids. But, whereas
pillared clays are formed by the insertion of dense, nanoscale aggregates
into the galleries of the layered host, our new intra-gallery templating
process involves in situ assembly of surfactant inorganic precursor
nanostructures, the morphology of which is determined by the collective
energetics of the inorganic and organic species as they assemble together
11_ '

Our approach design strategy for porous clay heterostructures
(PCHs) is based on the use of intercalated quaternary ammonium cations
and neutral amines as co-surfactants to direct the interlamellar hydrolysis
and condensation polymerization of neutral inorganic precursor (for
example, tetraethylorthosilicate, TEOS) within the galleries of 2:1 layered
silicates. High charge density 2:1 smectite clays are well suited layered

hosts.

1. 2 : l Layered Silicate Clays

The hydrous layer silicates commonly known as clay minerals are
part of a larger family called phyllosilicates. The layer silicates considered
in this work contain a continuous two dimensional tetrahedral sheets of
composition M205 (M=tetrahedral cation, generally Si, Al, Fe+3) in which
individual tetrahedra are linked with neighboring tetrahedra by sharing
three corners each (the basal oxygens) to form a hexagonal mesh pattern
(Figure 11.1 a). The fourth tetrahedral corner (the apical oxygen) points in
a direction normal to the sheet and at the same time forms a part of an

immediately adjacent octahedral sheet in which individual octahedra are

48

A!
1:34.

 

Figure 11.1 : The formation of composite aluminum oxygen or silicon-
magnesium oxygen layersl3.

49

linked laterally by sharing octahedral edges (fig 11.1 b). The common
plane of junction between the tetrahedral and the octahedral sheets consists
of the shared apical oxygen and unshared OH groups that lie at the center
of each tetrahedral six-fold ring at the same z-level as the apical oxygen.
Fluorine may substitute for OH in some species. The octahedral cations
normally are Mg”, Al+3, Fe”, and Fe+3. The assemblage formed by
linking one tetrahedral sheet with one octahedral sheet is known as a 1:1
layer. In such layers the uppermost, unshared plane of anions in the
octahedral sheet consists entirely of OH groups. A 2:1 layer links two
tetrahedral sheets with one octahedral sheet. In order to accomplish this
linkage, the upper tetrahedral sheet must be inverted so that its apical
oxygen points down and can be shared with the octahedral sheet below (fig
[1.] C, (1)) 12, 13, 14.

The idealized oxygen framework of a 2:1 clay is shown in figure
11.2. In a unit cell formed from twenty oxygen and four hydroxyl groups,
there are eight tetrahedral sites and six octahedral sites along with four
cavities surrounded by a six-membered oxygen ring on the surface. When
two thirds of the octahedral sites are occupied by. cations, the mineral is
classified as a dioctahedral 2:1 phyllosilicate. A trioctahedral 2:1
phyllosilicate has all the octahedral sites filled with cations. Based on the
magnitude of the layer charge per unit cell, 2:1 phyllosilicates are divided
into five groups; talc-pyrophillite, smectite, vermiculite, mica and brittle
mica (table 11.1)13- 15. The members of each group are distinguished by

the type and location of cations in the oxygen framework.

50

  
  
  

 

A—
‘2
fitnm-VA...

2m”

OF

or

9 OH

       

dllMg

 

Figure 11.2 : Idealized oxygen framework of clay minerals.

In talc, all the terahedral and octahedral sites are filled by Si+4 and
Mg*'2 respectively, and the layers are electrically neutral. In pyrophyllite
two thirds of the octahedral sites are occupied by AH“3 ions leaving a

neutral aluminosilicate layer. Therefore, in crystals of these minerals the

51

layers are coupled through relatively weak dipolar and van der Waals
forces“. In contrast to talc and pyrophyllite, the layers in muscovite and
phlogopite bear a net charge of 2e' per 813020 unit due to a positive charge
deficiency which results from the substitution of Si+4 by Al+3 in
tetrahedral positions. The charge deficiency is balanced by interlayer
potassium ions which are coordinated to the hexagonal arrays of oxygen
atoms at the layer surface13. Among the 2:1 layered phyllosilicates, the
brittle mica group has the highest layer charge, 4e: per 813020 unit cell.
the layer charge in vermiculite arises from the substitution of Al+3 for Si+4
in the tetrahedral layer. Vermiculite has a varying layer charge depending
on the amount of substitution, 1.2-1.8 e' per $13020 unit.

The charge on the layers of smectite is intermediate between 0.4-1.2
e' per Si3020 unit17. To balance the layer charge, layers of hydrated
cations are intercalated between the clay layers. The moderate layer
charge in smectite clays gives physical and chemical properties that are not
found in the end members. In montmorillonite, the most familiar and
common member of the smectite group, the layer charge originates from
the substitution of octahedral AH3 by Mg+2. Hectorite is also negatively
charged with Li+ substituting for Mg+2 in the octahedral sheet. Nontronite
is a negatively charged smectite with Al+3 replacing Si"‘4 in the tetrahedral
sheet and Fe+3 replacing Al+3 in the octahedral sheet. Laponite and
fluorohectorite are synthetic hectorites and they represent two extremes‘in

particle size and layer charge within the smectite group. Fluorohectorite
has

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a particle size of up to about 2000 A with a layer charge of 1.2 e' per
SigOzo unit that originates from substitution of Li+ for Mg+2 in the
octahedral layer while laponite has a particle size of ~200A with a layer
charge of 0.4 e‘ per SigOzo unit as a result of substitution of Li+ for Mg+2
in the octahedral layer. In fluorohectorite all the OH groups are replaced
by F. The layer charge that results from substitution in the octahedral
layers is distributed over the entire oxygen atom framework. These clays
tend to be turbostratic, that is the layers are randomly stacked with respect
to the a-b planes of adjoining layers. The negative charge on the layers of
the tetrahedrally charged smectites is more localized and these derivatives
tend to exhibit greater three dimensional order14a 13.

The sixth group of 2:1 type silicates is sepiolite-palygorskite (Table
11.1). The structures of these minerals can be regarded as containing
ribbons of the 2:1 phyllosilicates structure, one ribbon being linked to the
next by inversion of SiO4 tetrahedra along a set of Si-O-Si bonds. Ribbons
extend parallel to the x-axis, and they have an average width along the Y
axis. The tetrahedral sheet is continuous across ribbons but apices in
adjacent ribbons point in different directions, whereas the octahedral sheet
is discontinuous. Consequence, of this framework is that rectangular
channels run parallel to the x-axis between opposing 2:1 ribbons. As the
octahedral sheet is discontinuous at each inversion of the tetrahedra,
oxygen atoms of the octahedra at the edge of the ribbon are coordinated to
cations of the ribbon side only, and coordination and charge balance are
completed along the channel by W; H20(bound), and a small number of
exchangeable. cations. In addition, a variable amount of zeolite water is

contained in the channels.(figure 11.3) 13.

57

 

 

p :12]:-

 

~—- - cam

-'

PALYGORSKITE

_ b : 25.95 X m

 

 

—-—-c : 13.37 X- --—-

 

g 55% g sepuoure gggég

00:0 0:004 0:",0 °=Si oznaug

Figure 11.3 : Schematic structures of palygorskite and sepiolite19.

58

Smectite clays possess a combination of cation exchange,
intercalation and swelling properties which make them unique. Their
capacity as cation exchangers is fundamental to their intercalation and
swelling properties. This distinguishes smectites from the mica and
pyrophyllite-talc groups of minerals which have little or no ion exchange
capacityzo. Because of the ability of the minerals to imbibe a variety of
cations and neutral molecules, an almost limitless number of intercalates
are possible.

The hydrated cations on the interlamellar surfaces of the native
minerals can be replaced with almost any desired cation by simple ion
exchange methods. Homoionic exchange derivatives are readily achievable
with simple hydrated cations, including hydrated transition metal ions.
With large, complex exchange cations the extent of ion replacement may be
size limited. In hectorite, which typically exhibits a layer charge of 0.6%-
per SigOzo, the cation exchange capacity on an anhydrous basis is 87
milliequivalents per 100 g, about one-fifth the exchange capacity of
sulfonated styrene-divinylbenzene resins. Since the average distance
between exchange equivalents in the mineral is 8.3 A, cations with cross-
sectional diameters greater than this value can fully cover the interlamellar
surfaces before 100 percent exchange is reached. Thus, although the
interlamellar surface is very large (~750 m2/g), the size of the exchanging
ion can be a limiting factor in determining ion loading .

Neutral molecules other than water also can be intercalated between
the silicate layers of smectites. Several binding mechanisms may operate in
the intercalation process The reaction of the hydrated cation functioning as
a Bronsted acid and the intercalant acting as a base is another important

intercalation mechanism. Ammonia, for example, binds as the ammonium

59

ion in Mg+2-montmorillonite.

A variety of experiments have shown that hydrated cations are more
acidic in clay interlayers than in homogeneous aqueous solution 21'25. The
enhanced Bronsted acidity under intercalated conditions is due in part to
the polarizing influence of the cation on the water molecule in the spatially
restricted interlayers. The interlayer acidity is found to increase with
increasing charge to radius ratio of the cation and with decreasing water
content of the interlayers.

The most important property of smectites from the standpoint of
catalyst design is their ability to expand beyond a single molecular layer of
intercalant. The extent of interlayer swelling depends on the nature of the
swelling agent, the exchange cation, the layer charge, and the location of
the layer charge. The Li+ and Na+ exchange forms of the minerals are
particularly susceptible to swelling by water 25’ 27. As the interlayer water
content of Nat-smectites is increased with increasing partial pressure, a
more or less constant interlayer spacing corresponding to monolayer
formation is observed. After this, the spacing jumps abruptly to a value
corresponding to two intercalated water layers. The stepping of the
interlayer spacing is especially well behaved in beidellite and saponite.
Further swelling of the interlayers due to osmotic forces is observed when
minerals are immersed in liquid water.

Alkylammonium derivatives obtained by cation exchange are
distinguished by a great diversity of interlayer structures. Packing density
and type of aggregation depend on the density of the negative layer
charges, the geometry of the surface, and the degree of exchange.

The alkylammonium ion in 2:1 clay minerals (smectites,

vermiculites, micas) lie flat on the silicate surface in mono and bilayers or

60

form paraffin-type arrangements with the chains radiating away from the
surface (fig 11.4). In the pseudo trimolecular arrangement, some chain
ends are shifted above one another, so that the spacing is determined by the
thickness of three alkyl chains. The chains assume the required
conformation by kinking. The interlamellar structure depends on the alkyl
chain length and the packing density which is determined by the layer
charge(see fig 11.4). In some cases, the alkyl chains form paraffin-type
structures rather than pseudo trimolecular layers. The paraffin-type
aggregation allows a better fit of the ammonium groups to the surface
oxygen atoms than do the close-packed chains in pseudo trimolecular

layers.

61

Figure 11.4 : The surfactant orientation in the interlayers of various layer
charge density clay7—7.

62

(a) Monolayer: [Cwl-I33NMe3]+ Laponite
(Cation exchange capacity, 55meq/ 100g)

L' I ' IA

 

 

(a) Lateral Bilayer, [C16H33NMe3]+Hectorite
(Cation exchange capacity, 73meq/100g)

 

 

 

 

 

63

(c) Pseudo Trimolecular Structure:
[C16H33NMC3]+ Montmorillonitc(Arizona)
(Cation exchange capacity, 118meq/100g)

 

 

 

 

 

(d) Paraffin Structure, [C16H33NMe3]+ F-hectorite
(Cation exchange capacity, l40meq/100g)

 

 

 

 

 

 

 

 

(C) Lipid Structure, [(C12H25)2NM62]+ F‘bCCtOTitC
(Cation exchange capacity, l40mcq/lOOg)

 

 

 

 

 

 

(f) [(C‘ZHZShNMeflLaponite
(Cation exchange capacity, SSmeq/lOOg)

\
m E): W, ff if II: )

:‘::_:DJ 1
@WJ ”i

 

 

 

 

 

65

(a) Monolayer: [C16H33NMe3]" Laponite
(Cation exchange capacity, 55mqu 1003)

[ - 14.5A

F ' l

 

  

 

(3) Lateral Bilayer, [C16H33NMejll’Hectorite -
(Cation exchange capacity, 73meq/lOOg)

 

 

 

 

 

66

Sol-gel Processing of Silicates 29.

The sol-gel process involves a solution or sol that undergoes a sol-gel
transition. At that point, the one-phase solution becomes a two-phase solid
solution system due to destabilization, precipitation or supersaturation. The
mechanism is not yet understood, but can be divided into five steps:
hydrolysis, condensation-polymerization, gelation, drying and
densification. The overall chemical reaction can be express in the case of
TEOS as:

Si(0CH2CH3)4 + 2 H20 ----- > Si02 + 4 CH3CH20H

In the hydrolysis reaction of metal alkoxides such as TEOS, water

molecules displace alkoxy (e. g. ethoxy) groups to create silanols:

aSi(0CH2CH3) + H20 ----- > ESiOH + CH3CH20H
The reaction is an electrophilic substitution in presence of acid, but a
nucleophilic substitution in a basic solution. The complete hydrolysis would
produce silicic acid Si(0H)4, but some condensation-polymerization occurs
simultaneously.

The way in which the species hydrolyze and polymerize determines
the propagation of the linking units:

.=.Si(0CH2CH3) + ESiOH ----- > ESiOSiE + CH3CH20H
sSiOH + =-.:Si0H ----- > ESiOSiE + H20
Under acidic condition, the first hydrolysis causes the subsequent
hydrolysis of the same unit to be more difficult, so that linear polymer
growth will be favored. Furthermore, acid catalyzed reactions in solutions
containing a low water concentration produce linear polymers, while
solutions containing high water concentration produce cross-linked

polymers or branched clusters. Under basic conditions, subsequent

67

cation exchange capacity is approximately 150 meq./ 100g of air dried clay.

Natural Brazilian vermiculite which contains iron impurities, was
purified by conventional clay purification techniques30- From elemental
analysis the chemical composition of the material is Mgo,7 (Si6,06 A1194)
[A1 043 Mg 4,05 Fe 1.5] 020 (0H)4. The negative charge on the layer
arises from the isomorphous substitution of the silicon layer along with
some substitution in the octahedral layer. The cation exchange capacity is
180 meq./100g of the air dried clay.

Rectorite mineral was from Ba-Tou, China. Particles larger than
20m were removed by suspending the mineral in an aqueous solution for 8
hours. Rectorite consists of a regular alternation of mica-like layers and
expandable layers having the smectite composition. The chemical
composition formula of the material is
{(Nao.72Ko.02Cao.os)(Cao.24Nao.o7)}(Al4.ooMgo.02)[Si6.58Al1.121022.

The negative charge on the layer arises from the isomorphous substitution
on the tetrahedral silica oxygen sheet. The cation exchange capacity is 60
meq./ 100g of airdried clay.

All reagents were obtained commercially and used without further
purification. Hexadecyltrimethylammonium bromide,
dodecyltrimethylammonium bromide, and decyltrimethylammonium
bromide were obtained from Kodak Chemical Co. Neutral alkylamines
with different chain lengths such as hexylamine, decylamine, octylamine,
and dodecylamine, were obtained from Aldrich Chemical Co.

Sodium hydroxide, sodium bicarbonate, and sodium acetate were
purchased from EM Science company. Hydrochloric acid, and sodium

chloride were obtained from Columbus Chemical Industries, Inc. Acetic

68

acid, sulfuric acid were purchased from Mallinckrodt. Sodium citrate was
from J. T. Baker Chemical company. The silica precursor, i.e.
tetraethylorthosilicate of 98% purity, was obtained from Aldrich Chemical

company.

2 . Synthesis.
3. 0rganoclays-(Q+-clays)

An aliquot of 1% aqueous suspension of the alkali metal clay was
added to an aliquot of aqueous solution of known tetraalkyl ammonium
concentration at room temperature. The amount of the onium salts was
abou twice that of the clay cation exchange capacity (CEC). For example,
for Lithium Fluorohectorite with a CEC of 150 meq./ 100 g, an aliquot of
aqueous solution containing 7.90 mole of onium salt was added to the
suspension solution with 100 g clay to ensure that all the interlayer alkali
ions were replaced by onium ions. After 24 hrs of stirring, the products
were purified by repeatedly washing with ethanol to remove excess onium
ion and then resuspended in water. This process was repeated until the
suspension was free of halide ion as tested by AgN03. The pure products
were collected by centrifugation and air dried at room temperature.

Amine solvated organoclays were prepared by mixing the desired
neutral alkylamines to the organoclay (Q+-clay) with a reaction molar ratio
of 20 : 1 between alkylamine : Q+-clay. The mixture was stirred in a
closed beaker for 4 h. The products were analyzed as wet gels or as air

dried samples on a glass plate.

69

b. Templated heterostructures.

A wet amine--solvated organo-clay gel was mixed with
tetraethylorthosilicate (TEOS) at different Q+-clay : Alkylamine : TEOS
molar ratios in the range of 1:2:15 to 1:20:200. The suspensions were
stirred for 4 h before being separated by centrifugation (10 mins at 6000
rev/min.) The solid gel-like material was then air dried on a glass plate at
room temperature. The mass of Q+-clay used for these preparations
ranged from 0.5g (~ 10.0 "‘10'4 mol) to 6.0 g ( ~60.0 * 10'4 mol).

To obtain a porous clay heterostructure, the air dried templated
heterostructure was calcined at 650°C in open air for 4 hrs in a
programmable furnace with a ramp rate of 2°C/min. The sample was then

cooled at a constant cooling rate of 5°C/min to room temperature.

Elemental analysis.

A 50 mg sample was fused with 300 mg (5.5 * 10'3 mol) of lithium
borate for 10 min at 1000°C in a pre-ignited graphite crucible. The
resultant molten glass was transferred to 50 ml of 6% wt nitric acid
solution. This solution was stirred until dissolution of the glass was
complete and then further diluted to 100 ml with deionized water. The
solution was analyzed by inductively coupled plasma atomic emission
spectrometry (ICP), at the Inorganic Laboratory of the Michigan State
University Toxicology Department, using a Jarnell-Ash atom-comp
instrument.

Carbon, hydrogen, and nitrogen analyses were performed at the

70

University of Illinois Microanalysis Laboratory by oxidation of the

elements and chromatographic detection of the gases produced.

b. Physical Measurements.

The basal spacings were determined by X-ray powder diffraction, by
using a Rigaku Rotaflex diffractometer equipped with, a rotating copper
anode and producing K; radiation of 1.5405 A. The current and voltage
used to operate the anode were 45 mA and 100 kV, respectively. The scan
speed was l°/min, and the sample was scanned over a 2-theta range from 1°
to 30°. Peak positions in the angle 2-theta were converted to basal spacing
by using a standard chart. '

Nitrogen adsorption-desorption experiments were performed on an
Omnisorb 360CX sorptometer after outgassing the samples for 12 hours at
the specified temperature of 150°C under vacuum. Helium gas was used
for volumetric calibration. Surface area values and pore size data were
obtained by the BET equation and the t-plot method, respectively.

The Fourier transform infrared spectra were recorded on an IBM
IR44 spectrophotometer, by the KBr pressed pellet technique or by
preparing pressed pellets of the pure material. Some experiments were
conducted, under a flow of helium gas, under reduced pressure, at 150°C.
The sample was cooled m before recording the spectrum.

Solid state 29Si NMR experiments were performed on a Varian 400
VXR solid state NMR spectrometer operated at 79.5 MHz. A Bruker

multinuclear MAS probe equipped with zircon rotors was used for the

7l

measurements. The spectra were obtained by using a 4.6 us 90° pulse
width, a 4 kHz spinning rate, a delay time of 6005 and by accumulating 12
scans, unless otherwise specified.

Scanning electron microscopy images were obtained on a JEOL JSM-
25 microscope at the Michigan State University Center for Electron Optics.

Transmission electron microscopy images and the electron
diffraction patterns were recorded on a JEOL JEM-lOOCX microscope at
the Michigan State University Center for Electron Optics.

Thermogravimetric curves were obtained on a Cahn TG system 121
analyzer. The sample was held at 30°C for 5 min. It was then heated to

700°C at a ramp rate of 5°C/min. The furnace cooling fan was switched
off at 400°C.

72

Results and Discussions.
A. Preparation and structure of organic clay derivatives.

The reaction of an alkali metal ion exchanged smectite in liquid
suspension with stoichiometric amounts of onium salts dissolved in water,
ethanol or a solution containing 1:1 (v/v) waterzethanol results in products
in which the alkali metal ion is displaced by the onium cations. The series
of organoclays described in Table 11.2 were prepared by this method. If
the reaction conditions involved water or ethanol as solvent, displacement
of the alkali metal ion by the onium cations was strongly favored. Nearly
complete cation exchange was achieved by using stoichiometric amounts of
reagents. However, if the ion exchange reaction was carried out in acetone
only partial exchange of the organic occured because of the low dielectric
constant of acetone limits swelling of the silicate layers. Only those
intercalates prepared from exchange reaction in water or ethanol were used
for further studies.
The X-ray d-spacing of organoclays depends on the smectite clay layer
charge densities and on the structures of the surfactant onium ions. Figure
115 lists the representative XRD patterns of organoclays : (a) HDTMA+-
Fluorohectorite, (b) HDTMA+-Rectorite, (c) HDTMA+-Vermiculite and
r (d) DODMA+-Fluorohectorite. High d-spacing organoclays are formed
for smectites with high layer charge densities. For example, the d-spacing
of HDTMA+-Fluorohectorite with a CEC of 150 mqu 100 gms is 28 A.
The BET surface areas are dramatically reduced when alkali metal ions are
replaced by the surfactant cations in the interlayer. The reduction in

surface area is presumed to result from improved

73

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74

stacking of the layers the narrow line widths of the XRD patterns support
this hypothesis.
In Table 11.3 the N2 BET surface areas for the alkali metal clays and

organo clays synthesized from HDTMA+ as the exchange cation.

Table 11.3 : The BET surface area of alkali metal clays and
-8 in whim surfactantflDtTMAds intcaltederan -

 

 

 

Clay Host BET Specific Area
(mzlg)
Alkali Metal Clay l-IDTMA+-Clay#
Fluorohectorite 5.6 3.0 l
Vermiculite 23.0 4.0 I

* The clays were outgassed at 100°C for 24 h under vaccum (10‘6 torr)
prior to surface area measurements.
#I-IDTMA+ C15H33N(CH3)3+

75

Figure 11.5 : The X-ray diffraction patterns of various organoclays
(Qt-clays)
(A) Hexadecyltrimethylammonium-Fluorohectorite, air dried.
(B) Hexadecyltrimethylammonium-Rectorite, air dried.
(C) Hexadecyltrimethylammonium-Vermiculite,air dried.
(D)Dioctadecylldimethylammonium—Fluorohectorite,air dried.

Relative Intensity

76

 

 

28.12A

 

 

14.06A

9.3711

1

l

i6OA ’

(A)

 

l 0
Two Theta (degrees)

15

20

 

77

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15.10A

 

 

30.20A

 

1..

5.2.3:— 2523—

10.0611

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15 20

10
Two Theta (degrees)

Relative Intensity

78

 

 

38.10A

 

19.05A

Two Theta (degrees)

(D)

.__J

 

 

79

Furthermore the particle size of the silicate layers also helps in
improving the layer stacking for the organoclays. From equation 1, the
scattering domain L can be calculated :

L = AKIBcosO (1)
where L is the mean of crystallite dimension in A along a line normal to
the reflection plane, 2. is the wavelength of X-ray, K is a constant near
unity and [3 is the width of a reflection at half-height expressed in radians.
Values of L are listed in Table 11.2. For instance vermiculite derivatives
shows better layer stacking compared to synthetic fluorohectorite.
Crystallinity is dependent on the particle size of the silicate layers. The
surfactant in the clay interlayers does not alter the crystallinity due to the
constant particle size of the silicate layers.

The orientation of linear cationic surfactants in the interlayer of
clays with various layer charge densities have been studied by Lagaly and
Weiss. They observed that the gallery orientation which co-related to basal
spacing was depended on the charge density of the smectite clays. For
instance, with basal spacings of 14 to 18A the surfactants laid flat on the
silicate surface in mono or lateral bilayer orientations. However, with the
increase in basal spacing to ~22A, the surfactants formed a pseudo
trimolecular structure inside the silicate galleries. Further increase in basal
spacing made the chains radiate away from the layers to give a paraffin
type of orientation inside the galleries. Rectorite derivatives containing
HDTMA+ cations are suggested to adopt pseudo trimolecular structures
(Figure 11.5 b) with a gallery height of ~9-10A. The surfactants in organo-
fluorohectorite and organo-vermiculite interlayers are oriented as in
paraffin-like structures (Figure 11.5 c and Figure 11.5 a, resptively).

However, the orientation of the surfactants might differ from the above

8O

structures if the surfactants contain two hydrocarbon chains. For instance,
the organo fluorohectorite with dioctadecyldimethylammonium in the
interlayer, exhibits a lipid-like orientation and a paraffin-like structure
most likely forms on its external surface (figure II.5 d).

As was revealed by the work of Lagaly et.al., amines readily swell
organoclays. The swelling is promoted by the solvatation of the onium
ions by neutral amine molecules and results in an energy gain from van der
Waals interactions, allowing the layers to separate.

The alkylamine/organoclays have been studied under different
conditions by using the alkylamines as swelling agents. The alkylamines
used were, octylamine, and decylamine. The alkylamine/organoclays were
studied in the fully solvated (wetted) state or as air dried compounds. As
the amine-solvated products are unstable intermediates, we have only been
able to record their X-ray diffraction patterns. However, the air dried
compounds formed by evaporation of excess amine at room temperature
have been characterized by both X-ray diffraction and 29Si MAS NMR.

X-ray powder diffraction has been our main analytical tool for
investigating gallery expansion. The data are gathered in Table 11.4

The neutral decylamine/organofluorohectorite pattern in excess
amine suspension is displayed in figure 11.6. The narrow peak at 30.0A for
the decylamine sample is inconsistent with the presence of a swelled

amine/organofluorohectorite. The sharpness of this peak suggests the

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82

presence of an alkylammonium salt formed by reaction of the amine with
atmospheric C02 to form an alkylammonium carbonate. To confirm this
salt formation hypothesis, we first crystallized the alkylammonium chloride
salt by adding concentrated hydrochloric acid to an amine film on a
microscope slide. The X-ray diffraction patterns exhibited a narrow peak
at 31.011 for decylammonium chloride. The proximity of this d-spacing to
that found on the decylamine/organofluorohectorite pattern suggests a
similar packing. On the basis of the amine dimensions (table 11.4), the
ammonium salt crystallizes in a lipid-like double layers. In
organofluorohectorite, no counter-anions are present for alkylamine salt
formation. Furthermore, the salt peak intensity is dependent on the aging
time of the glass slide: the older the preparation, the less intense the narrow
peaks, According to the following equation
RNHzaiq)
H20(g) + C02 <---------> [H2C03] <----==> [RNH3+ . HCO3'](solid)

 

 

in air <--- drying unstable <--- drying bicarbonate ammo salt
As the excess liquid amine evaporates in air, the equilibrium shifts to the
left. So, the bicarbonate ammo salt disappears on aging. These two facts
support the hypothesis of a reaction with the atmosphere.

The relatively broad XRD peaks at 46A for the decylamine solvated
organofluorohectorite is assigned to organofluorohectorite swollen by
decylamine. This arises due to the intercalation of amine into the
organoclay galleries. The gallery height is 36.4A for decylamine solvated
organo fluorohectorite. Furthermore as was stated by Lagaly, alkylamines
are arranged in a lipid-like arrangement within the clay galleries.(see
figure 11.7). By us gallery heights obtained support that hypothesis.

The air dried decylamine/organofluorohectorite exhibits slightly

83

different basal spacings. The crystallinity decreases upon drying by ~50—
lOOA in layer ordering along the c axis. (see Table 11.4).

Van der Waals interactions between the amine chains play a role in

determining the stacking order of the clay layers. The greater the van der

Waals interactions, the better the crystallinity. As the number of carbon

Relative Intensity

Relative Intensity

 

 

 

 

 

 

 

 

 

 

 

 

Two Theta (degrees)

30.00A (A)
46.00A
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(ii) 001 46.0
53" m 002 23.0
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__ (1)
A ! (ii)
J l L1 1 4 l j n 44
1 s 9 13 17 21

Figure 11.6 : (A) X-ray powder diffraction patterns of decylamine
solvated HDTMA+-Fluorohectorite gel with a molar ratio of 20 : 1 (B) X-
ray powder diffraction patterns of the air dried HDTMA+-
Fluorohectorite/amine. (i) octylamine & (ii) decylamine

84

Figure 11.7 : Schematic representation of the exchange reaction of clay
with quaternary ammonium cations (filled head groups) to
form Q+-c1ay with a paraffin structure. Solvation of Q+-c1ay
with neutral alkylamine (open head groups) affords a lipid like
bilayer structure.

85

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86

atoms in the alkylamine chain increases from octylamine to decylamine,

the size of the scattering domain along the layer stacking direction of the
amine/organofluorohectorite increases. For example in HDTMA+-
fluorohectorite/octylamine it is 300A, and for HDTMA+-
fluorohectorite/decylamine it is 325A, however HDTMA+-fluorohectorite
exhibits a scattering domain size of 250A. For the air dried
amine/organofluorohectorite, the loss of neutral amine from the galleries
caused the lowering of the van der Waals interaction. The crystallinity
decreases as compared to that for the amine solvated wet gel. Yet, this
value is still higher than that of air dried organofluoorhectorite intercalate.
These results support the conclusion that, for amine/organoclays, van der
Waals interactions betWeen the chains and specimen crystallinity are
related.

Air dried decylarnine/HDTMA+-fluorohectorite is stable and exhibits
a large basal spacing equal to that of the same clay solvated in liquid amine.
We thus have been able to obtain 29Si nuclear magnetic resonance spectra
and to determine the effect of alkylamine on the layer structure. The air
dried decylamine/l-IDTMA+-fluorohectorite solid state 29Si NMR spectrum
is displayed in Figure 11.8.

The air dried decylamine/HDTMA’r—fluorohectorite exhibits an NMR
spectrum equivalent to the original HDTMA+-fluorohectorite (Figure 11.8).
The resonance characteristic of the silicon environment for the
fluorohectorite layers, i.e., a Si(OSi)3(OMg) Q3 at -93.3 ppm, is identical
to the value for Li+-fluorohectorite signifying that the amine co-template

only separates the layers and does not affect the layer structure.

87

B. Porous clay heterostructure formation.

The PCH materials described herein can be synthesized over a broad
range of layer charge densities (3.82 S x S 1.33, where "x" is the layer
charge per IOOAZ). These materials can be synthesized under ambient
conditions where both the type and amount of the exchange cation and
neutral amine co-surfactants, along with chain length play an important
structure directing influence on the final products. A typical synthesis is
Illustrated in Figure 11.9. An alkali metal ion exchange clay was converted
to a quaternary ammonium exchanged form (Qt-clay) by ion-exchange
with a two fold excess of aqueous [Cal-12,1.” N(CH3)3]+ X' , washed free of
excess surfactant and air dried. Mixtures of hydrated Q+-clay (~8.0 H20
per 020 unit cell), neutral amine and TEOS at molar ratios in the range of
1 : 2 : 15 to 1 : 20 : 200 were allowed to react for 4 h at ambient
temperature. The resultant intercalates were then centrifuged, dried in the
open air to give the as-synthesized templated heterostructures. Removal of
the template by calcination at 650°C in air for 4 h gave the final as-
synthesized porous clay heterostructure (PCHs) derivatives. Figure II.10 (
a, b, c, d, e, & f), illustrates the X-ray powder diffraction patterns for the
air dried templated heterostructures and the porous clay heterostructure
(PCHs), synthesized using rectorite, vermiculite and fluorohectorite as
layered hosts. All diffraction patterns exhibited multiple (001) reflections,
indicating the presence of a layered structure. Furthermore, the
crystallinity of the air dried templated heterostructures was better than
that of the calcined PCH's. Depending on the chain length of the exchange
cation and neutral amine co-template, the gallery height could be varied
from 14.9A (hexylarnine) to 24.0.31 (dodecylamine).

88

 

 

  
 

   
    
 
     
    
 
   
   

   
 

lwt%

0.3 M aqueous solution
Li‘ - Clay

CnHZn+lN(CH3)3 " X '

  

Suspension stirred at 50°C for 24 h
' washed with ethanol / water,
air dried

 

 

Organo- Clay ( 0* - Clay)

 

  

Mixture of Q‘ - Clay : Amines : TEOS
Molar ratio of 1 : 20 z 150

   
   
 

 

 

Porous Clay Heterostructure

Figure 11.9 : Systematic illustrations of the main steps involved in porous
clay heterostructure synthesis.

89

Figure [1.10 : The X-ray powder diffraction patterns of air dried
templated heterostructures and calcined PCH's prepared at a molar
ratio of 1 : 20 : 150 for the Q+-clay : amine : TEOS.

(A) HDTMA+ Fluorohectorite : decylamine : TEOS, air dried.

(B) HDTMA+ Vermiculite : decylamine : TEOS, air dried.

(C) HDTMA+ Rectorite : decylamine : TEOS, air dried.
(D)HDTMA+Fluorohectorite : decylamine : TEOS, calcined at 650°C
(E)HDTMA+ Vermiculite : decylamine : TEOS, calcined at 650°C.

(F)HDTMA+ Rectorite : decylamine : TEOS, calcined at 650°C.

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Thermogravimetric programmed analyses were carried out to study weight
losses up to 900°C under flowing N2. The weight loss curve for PCH
prepared from HDTMA+-fluorohectorite/0ctylamine template is shown in
figure 11.11. The processes expected with increasing temperature in PCH
materials would be :(a) Loss of small amounts of intra-gallery water whose
presence is observed in the infra-red spectrum of the as-synthesized
material. (b) Loss of the neutral amine by sublimation which occurs at a
much lower temperature than the thermal decomposition of the quaternary
ammonium surfactants. (c) Finally the loss of water due to silica
dehydroxylation. All these features of thermal degradation are observed in
PCH materials. The first step of 5% from 25°C to 140°C corresponds to
loss of physisorbed water, the weight loss of 16.5% that occurs until 360°C
and the weight loss of 14% up to 460°C are due the decomposition of the
organic templates. The final weight loss of 1.9% between 460-600°C is
related to water losses via condensation of silanol groups to form siloxane
bonds. Similar thermogravimetric behavior has been observed in MCM-
41.

Nitrogen BET surface areas for PCH materials synthesized using
different Q+ and neutral amine co-surfactants ranged from 470-850 m2/g.

Figure II.12 (A, B & C) illustrates the N2 adsorption/desorption
isotherms for all the PCHs prepared from HDTMAfldecylamine as
templates, with rectorite, vermiculite and fluorohectorite as layered hosts.
The nearly linear portions of the adsorption curves in the partial pressure
region ~0.02-O.25 are indicative of supermicropores (~14-2OA pore
diameter) or small meSOpores (~20-25A). As shown in Figure II.12 (inset)
a Horvath-Kawazoe analysis of the adsorption data yields average pore

sizes of ~21A. On the basis of the widths at half maximum for the pore

93

size distribution curves, the pore structures of PCH materials (~7A peak
width) are similar in uniformity to MGM-41 type mesostructures of
equivalent pore size (~5A peak width) formed by surfactant templating.
Table 11.5 gives the pore sizes of related PCH prepared using other
combination of Q+ and neutral amine co—template. Increasing the chain
length of the amine co-template systematically increases the pore size of the
PCH. That the chain length of the Q+ also is important in the templating

 

100 ______ __

 

 

% Weight
on
o

 

 

70
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6°26 ' 120320320 4261‘ 320620720 320‘ '
Temperature in °C

 

 

 

 

 

 

 

 

 

 

 

Figure [1.11 : Thermal gravimetric analysis of PCH prepared from
octylamine-solvated HDTMA+-Fluorohectorite showing five steps of lost
weight, the first one attributed to desorption of water (5%), the second and
third due to the loss of neutral amine (16.5%), the fourth related to the
decomposition of the quatemaryammonium cation (14%) and the fifth
related to water losses via condensation of silanol groups to form siloxane

bonds (1.9%).

94

Figure II.12 : Nitrogen adsorption/desorption isotherms for porous
silica—clay heterostructures (PCHs) prepared by gallery-
templated synthesis using

(A)vermiculite, (B) fluorohectorite, & (C) rectorite as layered
host. HDTMAfldecylamine were used as templates. The

HDTMA+-clay : amine : TEOS reaction stoichiometry was 1 :
20 : 150. Relative pressure is P/Po, where P is the

equilibrium pressure of the adsorbate and P0 is the saturation

pressure of the adsorbate at the temperature of the adsorbent;
the volume adsorbed is at STP. Insert : The corresponding
Horvath-Kawazoe pore size distribution curves. (dW/dR is the
derivative of the normalized adsorbate (nitrogen) volume
adsorbed with respect to the pore diameter of the adsorbent).
Before measurement each sample was heated overnight at
150°C and 10'6 torr. The isotherms were measured at -196°C
on a Coulter Omnisorp 360CX Sorptometer using standard
continuous adsorption procedures.

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process is evidenced by the PCH prepared using DTMA+ and decylamine as
gallery co-templates. Consistent with a gallery-templated pathway, this
latter surfactant system affords an average pore size of 14A, substantially
smaller than the 21A pore size provided by HDTMA+ with decylamine as
co-template. Included in Table 11.5 are the gallery heights for the calcined
PCHs, the corresponding air dried PCH precursors and the initial amine
solvated Q+-clay.

As illustrated in Table 11.5, the gallery height for the air dried
samples increases with chain length of the alkylamine. On calcination of
the samples at 650°C there is a slight contraction in the gallery height due
to the loss of intragallery water, loss of the template and dehydroxylation
of the silica mesostructure. In the case of fluorohectorite and rectorite (see
Figure 11.13 (a) ) the correlation between gallery height and the H-K pore
size of the calcined material suggests that the pores of the gallery templated
silica are symmetrical and systematically increase with the increase in the
length of the carbon chain. However, in the case of vermiculite (see Figure
11.13 (b) ) the gallery heights observed are much larger than the H-K pore
size, suggesting that the walls of the pores are thicker and contribute
towards the gallery height of the calcined material. However the effective
pore size is very similar to the corresponding fluorohectorite system. Also
elemental analyses (given in table [1.6) of these samples show the average
gallery composition as (~8.0 mol of silica per 020 unit cell) for
fluorohectorite and for rectorite and for vermiculite as (~11.0 mol of silica
per 020 unit cell), as the gallery height increases, the amount of amine and
silica increases, with the final Si/amine ratio around 3.2 ( >3.2 for

hexylamine due to its high volatility and < 3.2 for dodecylamine as it is a

98

A
Templated Fluorohectorite ( )

Heterostructure Porous Fluorohectorite
' Clay Heterostructure

  
 

calcinc, 650°C

 

(B)

Porous Vermiculite

Templated Vermiculite Clay Heterostructure

Heterostructure

    

Figure [1.13 : Schematic representation of the gallery structures
(A) Symmetrical pore structure formation in fluorohectorite
and rectorite
(B) Thicker wall and smaller pore structure of PCH derived
from vermiculite.

99

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solid at room temperature). A similar Si/N ratio = 3.4 was reported for
the bulk templated mesostructure MCM-41 using HDTMA+ as a template.

The relationship between PCH pore size distribution, chain lengths of
the templating surfactants and the reaction stoichiometry support a
templating mechanism analogous to structure-directed M418 mesostructure
syntheses.

Scanning electron micrographs of air dried and calcined porous clay
heterostructures formed from fluorohectorite using HDTMAfldecylamine
as templates are shown in figure ?? B, C respectively. The micrographs
reveal the same platty morphology as the initial organofluorohectorite (see
Figure A). The similarity between the air dried and calcined PCH product
indicates that the templating reaction between the interlayer regions of
smectite clays occurs in a topotactic fashion.

Our efforts to image PCH pores by transmission electron microscopy
(TEM) have met with limited success owing to the turbostratic nature of
the intercalates. For vermiculite as the layered host, the intra-gallery
hydrolysis of TEOS in the presence of I-IDTMA+ and decylamine as co-
template affords a calcined PCH with a gallery height of 33.311. TEM
images of the vermiculite heterostructure as shown in figure ?? reveal clear

evidence for a lamellar morphology and uniformly expanded galleries.

10]

Figure 11.14-(i): TEM image of a oporous silica - vermiculite
heterostructure ( d001 = 37.0A ) showing evidence for

regularly ordered lamellar structure. Owing to the
turbostratic nature of the intercalate, the galleries pores are
not easily oriented for imaging. TEM images do not indicate
the presence of significant amounts of extragallery silica. The
TEM image was obtained on a JEOL lOOCX using an
accelerating voltage of 120kV and a 20pm objective lens
aperture.

(Note : 2.5 cms = 175A).

 

103

Figure 11.14-(ii): TEM image of a porous silica - fluorohectorite
heterostructure ( dom = 32.0A ) showing evidence for
regularly ordered lamellar structure.The TEM image
was obtained on a JEOL IOOCX using an accelerating
voltage of 120kV and a 20um objective lens aperture.

(Note : 2.5 cms = 40013.)

 

105

Figure II.15 : Scanning electron micrographs at X2200 for (A)
HDTMA+-fluorohectorite air dried (Top). (B) HDTMA+-Vermiculite air
dried (below).

IBKU X2280 1239

 

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Figure 11.15-(ii): Scanning electron micrographs for

(A) HDTMA+-fluorohectorite/decylamine/TEOS mixture with
a molar ratio of 1 : 20 : 150 respectively air dried (Top).

(B) HDTMA+-fluorohectorite/decylamine/TEOS mixture with
a molar ratio of 1 :20 :150 air dried, calcined at 650°C for 4 h
in air (Bottom).

IBKU X4480 1244 1.63 05094

ffiiu X548 124a 13.6U 05094

 

109

Conclusions.

The porous clay heterostructure syntheses set forth here represents a
novel strategy for obtaining nanoporous molecular sieves. Because of the
complementary chemical functionality of the layered and gallery-templated
components, and the stable pore size distributions in the super micropore to
small mesopore range (~14-25A), these porous heterostructures offer new
oppurtunities for the rational design of heterogeneous catalyst systems.
Also PCHs bridge a potentially important pore size region between
microporous zeolites and pillared clays (<10A) and M418 mesostructures
(>20A). Heavy crude oils, biological molecules ( for example
metalloprophyrins) and other large macromolecular species can be

accommodated in PCH galleries for shape selective processing or catalysis.

110

References :

95”.“?

11.

12.

13.

Kresge, C. T.; Leonowicz, M. B.; Roth, W. J .; Vartulli, J. C.; Beck,
J. 8., Nature 359, 710-712 (1992).

Beck, J.S.; Vartulli, J. C.; Roth, W. J.; Leonowicz, M. B.; Kresge, C.
T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.;
McCullen, S. B.; Higgins, J. B.; Schlenker, J. L., J. Am. Chem. Soc,
114, 10834-10843 (1992).

Tanev, P. T., Chibwe, M., & Pinnavaia, T. J. Nature, 368, 321-323
1994.

Reddy, M. K., Moudrakovski, I. & Sayari, A. J. CXhem. Soc. Chem.
Commun, 1059-1060, 1994.

Corma, A. , Navarro, M. T.& Perez Pariente, J. J. Chem. Soc.
Chem. Commun. 147-148, 1994.

Bein, T. J.& Wu, C.-G. Science , 264, 1757-1758, 1994.

Huo, Q. et al. Nature, 368, 317-321, 1994.

Huo, Q. et al. Chem. Mater. 6, 1176-1191, 1994.

Lagaly, G. Solid State Ionics , 22, 43-51, 1986.

Vaia, R. A., Teukolsky, R. K. & Giannelis, E. P. Chem. Mater. 6,
1017-1022, 1994.

Galarneau, A.G.; Barodawalla, A.F.; Pinnavaia, T.J.; Nature, 374,
529, 1995.

Wells, A.F. "Structural Inorganic Chemistry" 4th Ed. 1975
Clarendon Press Oxford.

(a) Bailey, S.W. "Crystal Structure of Clay Minerals and their X-ray
Identification ", Brinley, G.W. and Brown, G. Eds. Mineralogical
Society London 1985. (b) Liebau, F. "Structural Chemistry of

14.

15.

16.

17.

18.

23,

20.

21.
22.
23.
24.
25.
26.

27.
28.

29.

ll]

Silicates " Springer-Verlag Berlin Heidel Berg 1985, 93.

Farmer, V.C. "Infra-red Spectra of Minerals ", Farmer, V.C. Eds.
Mineralogical Society London 1974, 331.

Dixon, J .B. "Minerals in Soil Environments ", Dinauer, R.C. Eds
1977, 357.

Giese, R.F. Jr. Clay Miner. 1975, 23, 165.

Beson, G.; Mitsud, C.T.; Mering, J. Clays and Clay Miner. 1974,
22, 339.

Suquet, H.; Calle, de la C.; Pezerat, H. Clays and Clay Miner. 1975,
1.19. Jones, B.F.; Galan, E. "Reviews in Mineralogy " , Bailey S.W.
Eds. 1988, 19, 631.

Micas can be "weathered" to vermiculites, which do exhibit
intercalation properties [K. Norrish, Proc. Int Clay Conf. Madrid 1,
393 (1968).

Raman, K.V. and Mortland, M.M. ibid. 16, 393, 1968.

Mortland, M.M.; Phys Chem. 67, 248, 1963.

Russell, J.D. Trans. Faraday Soc. 61, 2284, 1965.

Fripiat, J.J. ACS Symp. Ser, 34, 261, 1976.

Conard, J. ibid., p.85.

MacEwan, D.M. and Wilson, M.J. in Crystal Structures of Clay
Minerals and Their X-ray Identification, Brindley, G.W. and Brown,
G. Eds. (Mineralogical Society, London, (1980), chap. 3.

Glaeser, R. and Mering, CR. Acad. Sci. Ser. D 267, 463 1968.

Lee, T. Ph.D. thesis dissertation title "The Catalytic and Mechanistic
Properties of Organoclays for Triphase Catalysis". Michigan State
University, 1992.

Klein, L.C. Annual Review of Materials Science, 15, 227, 1987.

112

30. Pinnavaia et at Michigan State University. 1996.

31.
32.
33.

34.

Lagaly, G. Solid State Ionics, 22,43-51, 1986.

Spartan molecular modelling software, Michigan State University.
Brunauer, S.; Emmett, P. & Teller, E. J. “Am. Chem. Soc., 1938,

60, 309.

Horvath, G. & Kawazoe, K. J. J. Chem. Eng. Jap., 1983, 16, 470-
475.

113

Chapter III

"Pillaring reaction vs intra-gallery templating processes in a

lamellar solid - A comparative study."

114

Abstract

Our approach to designing porous clay heterostructures (PCHs)1 is
based on the use of intercalated quaternary ammonium cations and neutral
amines as co-surfactants, to direct the interlamellar hydrolysis and
condensation polymerization of a neutral inorganic precursor such as
TEOS within the galleries of smectite clays. Removal of the surfactant by
calcination then leaves a mesoporous solid with thermally stable pores in
the supermicropore to small mesopore range (~14-25A) depending on the
chain length of surfactants. This concept of gallery templated synthesis
gives a new dimension to the old traditional approach of pillaring lamellar
solids. Here we describe2 the fundamental differences that distinguishes an
intragallery assembly from a conventional pillaring reaction, namely the
effects of the clay layer charge density, surfactant head group size and
chain lengths, critical TEOS : amine molar ratio, and the dependence of the
pore size on the amount of silica in the reaction. All these factors control
PCH formation clearly proving that intragallery templating is involved as

opposed to pillaring.

115

Introduction.

This study investigates the fundamental differences between a
pillaring reaction and a new templated synthesis for 2:1 clays which lead to
the formation of nanoporous silica structures within the interlayer regions
of claysl. A great amount of interest has been expressed for all of these
large-pore materials with controlled pore size for selective catalysis,
molecular sieving, molecular adsorption studies and other materials
applications(3'9). This concept of templating within the constrained gallery
regions of a layered solid gives a new dimension to the traditional approach
of pillaring layered materials.

Historically, clays (1955) and other layered metal oxides were
pillared with polar organic molecules“). Also many surfactants such as
tetraalkylammonium have been introduced by cation exchange between the
layers of smectite clays and hydrous layered silicates such as kanemite;
magadiite; kenyaite, etc“, 12. The use of these organic intercalated
materials has been limited due to their low temperature stability; However
thermostable microporous materials have been obtained by pillaring
lamellar solids such as clays (1960), silicates, titanates, and phosphates with
cationic inorganic precursor such as polyoxocations of aluminum,
zirconium with or without the use of a preswelling reagent such as n-
alkylaminesl3: 14. The pore size of these pillared materials was determined
by the lateral separation of the pillars which in turn was controlled by the
charge density of the layers. The pore sizes were typically in the
microporous regime (<10A).

A more recent pillaring procedure”, 16’ 17 (1989) involved the use

of a neutral pillaring precursor such as tetraethylorthosilicate (TEOS)

116

adsorbed into the organophilic gallery regions of n-alkylammonium
exchanged silicates (magadiites; kenyaite); titanates (NazTi3O7); or
perovskites (KCaNb3Olo). Removal of the organics by calcination leads to
stable microporous solids.

Our new approach to the design of porous clay heterostructures
(PCHs) is based on the use of intercalated quaternary ammonium cations
and neutral amines as co-surfactants to direct the interlamellar hydrolysis
and condensation polymerization of a neutral inorganic precursor such as
TEOS within the galleries of smectite clays. Removal of the surfactant by
calcination then leaves a mesoporous solid with thermally stable pores in
the supermicropore to small mesopore range (~14-25A), depending on the
chain length of both surfactants. In some respects, our approach is similar
to conventional pillaring reactions of lamellar solids. But in contrast to
pillared clays which are formed by the insertion of dense, nanoscale
aggregates into the galleries of the layered host, our intra-gallery
templating process involves m assembly of surfactant inorganic
precursor nanostructures, the morphology of which will be determined by
the collective energetics of the inorganic and organic species as they
assemble together.

Here we report a systematic study of the salient features that
differentiate an intra-gallery templating process from a conventional
pillaring reaction by using high charge density smectite such as

fluorohectorite.

117

Experimental.
Materials

Synthetic Li+-fluorohectorite, abbreviated Li+-Fluorohectorite, with
an anhydrous unit cell formula of Li1,12{[Li1,12 Mg4,33](SigOzo)}F4 was
obtained as a gift from Dow Corning, Inc.

Quaternary ammonium surfactant compounds
([(CnH2n+1)(CH3)3N]X, X = C1 or Br) and n = 10 and 16 obtained from
Kodak chemicals were used without further purification. Neutral
alkylamines with different chain lengths, i.e., C6H13NH2 to C12H25NH2
were obtained from Aldrich Chem. Co. and used without further

purification.

Physical Measurements.

Powder XRD analysis was effected with a Rigaku rotoflex
diffractometer by using Cu Ka radiation. The gallery height was obtained
by subtracting the thickness of the clay unit layer (9.6 A fora 2:1 smectite
and 19.0 A for a interstratified clay like rectorite).

The chain length of the amines and quaternary ammonium ion were
calculated by the Spartan-MSU modelling program 13.

Nitrogen adsorption / desorption isotherms were determined on a
Coulter Omnisorb 360 CX sorptometer at liquid N2 temperatures by using
ultra high-purity N2 and He as adsorbate and carrier gas, respectively. All

samples were outgassed at 150°C under vacuum overnight. Surface areas

118

were determined using the BET19 equation. The method of Horvath and
Kawazoe20 was used to determine the pore diameters of the product.

TGA curves were obtained on a Cahn-121 type thermal analyser by
using nitrogen as a purge gas at a heating rate of 10°C/min.

Elemental analysis was carried out by inductively coupled plasma
emission spectroscopy at the University of Illinois Elemental Analysis

Laboratory.

Results and Discussions.

Reaction Stoichiometry.

In bulk templated mesostructure synthesis, a critical TEOS/surfactant
molar ratio is required for the formation of the final silica templated
mesostructure. In PCH synthesis, the amount of cationic surfactant
intercalated into the layered silicate host is controlled by the layer charge
density. However, the TEOS/amine molar ratio is critical for the
successful formation of the final intra-gallery templated silica
nanostructure, as illustrated below.

The products obtained by the reaction of the alkylamine/organoclay
gel with TEOS at 25°C for 4 h was dependant on the reaction
stoichiometry. (see table III.1) Depending on the molar ratio of TEOS to
amine, products with different basal spacings were obtained. Figure 111.1
(A) illustrates the X-ray diffraction patterns for the uncalcined intercalates
isolated from reaction mixtures contain HDTMA+-fluorohectorite and 0.5,
2.5, 5.0, 7.5, and 10.0 moles of TEOS / mole of decylamine. These

patterns indicate basal reflections corresponding to spacings of 38.0A when

119

Figure 111.1 : (a)X—ray powder diffraction patterns for air dried PCH
formed by reaction of decylamine solvated HDTMA+-
Fluorohectorite with TEOS at HDTMA+-Fluorohectorite :
decylamine : TEOS molar ratios of 1 : 20 : X where 10 S X S
200.

(b) X-ray powder diffraction patterns of porous clay
heterostructures (PCH) obtained by calcination at 650°C for 4

h. Note that at X 2 100, the calcined PCH-fluorohectorite
exhibits well ordered c axis spacing of ~32A.

120

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HDTMAfldecylamine is used as a template. Since the layer thickness of
smectite is ~9.6A, these basal spacings corresponds to a gallery height of
28.4A for fluorohectorite. The gallery heights decreases from the initial
gallery height of 36.4A observed for the decylamine solvated
HDTMAflfluorohectorite precursor gel. This means that the gallery
shrinks by ~10A when decylamine is partly replaced by TEOS molecules.

Figure [11.1 B illustrates the X-ray diffraction patterns for the
products obtained after calcination at 650°C in air for 4 hrs. These
patterns indicate a further decrease in gallery heights for all the
derivatives. In fact, the diffraction patterns for the products obtained
below 5.0 moles of TEOS / mole of amine show the presence of two
phases, namely the surfactant templated porous clay heterostructure with a
broad basal spacing of ~32.0A and a second phase of silica intercalated clay
derivative with a basal spacing of 12.2A. The formation of this mixed
phase system can be explained by the mixed layering mechanism (see
Figure 111.2 a, b) which is operative below the critical TEOS / amine molar
ratio ~ 7.5. .

On increasing the concentration of TEOS in the reaction mixture, the
neutral alkylamines in conjunction with the intercalated quaternary
ammonium ions impart a structure which is more accessible to TEOS and
allows the formation of a reactive intermediate. Some of the neutral
amines in the intermediate are displaced from the galleries by TEOS.
Subsequent hydrolysis of the gallery TEOS affords a hydrous silica
templated around a monolayer of micellar quaternary ammonium and
neutral amine assemblies. The final calcination step removes the template
and completes the dehydroxylation and cross-linking of the gallery

assembled silica structure. Two or more

123

(A)

 

Figure III.2 : A schematic representation for the mixed random layering
mechanism.
(A) In smectites with ~9.6A layer thickness along the c axis.
(B) In interstratified mineral such as rectorite with ~19.0
layer thickness along c axis.

124

orders of (001) reflections indicate the presence of a layered structure with
good ordering along the c-axis.

This set of experiments also demonstrates that the formation of a
porous clay heterostructure is highly dependent on the critical TEOS /
amine reaction stoichiometry. The optimum product formation was
achieved at a reaction stoichiometry of 1:20: 150 of organoclay(Q+-clay) :
amine : TEOS, respectively. This critical TEOS : amine molar ratio is an
important feature that differentiates a gallery templated process from a
conventional pillaring reaction of lamellar solids.

In a pillaring mechanism formation of dense laterally spaced silica
aggregates is dependent on the clay : silica ratio. Such dependence is not
observed for the pore size of products obtained in PCH synthesis. The
same average pore size and gallery silica composition (~8.0 mol silica per
020 F4 unit cell) are observed for PCH products synthesized at HDTMA+-
fluorohectorite : amine : TEOS reaction stoichiometries of 1 : 2 : 15; 1 : S
: 37.5; and 1 : 20 : 150. However the molar ratio of 1 : 20 : 150 was found
to be the most convenient of all.

Figure 1113 illustrates the X-ray powder diffraction patterns of
HDTMA+-fluorohectorite : octylamine : TEOS products synthesized at
different molar ratios but with a constant TEOS : amine reaction
stoichiometry. The air dried intercalates gave similar diffraction patterns
with multiple (001) reflections indicating a highly ordered structure.
Calcination at 650°C in air reduced the basal spacing by ~6A; however, all
the samples exhibited similar gallery heights.

Elemental analysis of the samples gave an average gallery silica
composition of ~8.0 mol of silica per O20 F4 unit cell. The N2
adsorption/desorption isotherms were virtually identical and the H&K pore

125

analysis gave an average pore size of ~18.0A.

 

28.00A

Relative Intensity

 

 

 

 

 

10 15 20
Two Theta (degrees)

Figure [11.3 : X-ray powder diffraction patterns of calcined PCH
reaction products formed at a different
HDTMA+-fluorohectorite : octylamine : TEOS reaction
stoichiometry :

(a) 1 : 20 :150

(b) 1: 5 : 37 .5

(c) l : 2 : 15.

(Q+-clay : amine : TEOS.)

126

Pore size dependence on surfactant chain lengths

Both the quaternary ammonium ion and neutral amine co-surfactant
as well as surfactant chain length play an important structure directing role
in the syntheses of PCH materials. Figure 111.4 A and B illustrate the X—
ray diffraction patterns from HDTMA+-Fluorohectorite/decylamine/TEOS
and DTMA+-Fluorohectorite/decylaminel'l‘EOS systems. Both diffraction
patterns exhibit multiple (001) orders of Bragg's reflections indicating a
well-ordered structure along the c axis. On calcination of the samples at
650°C there is a slight contraction in the gallery height due to the loss of
intragallery water, loss of the template and dehydroxylation of the gallery
silica surface. It is important to note that different gallery heights are
obtained for different surfactant chain lengths, indicating that the chain
lengths of the templates is important in controlling the final gallery heights.
This point was further elaborated when surface area measurement were
performed on the calcined materials.

Figure 111.5 A, B and C illustrate the N2 adsorption-desorption
isotherms for the PCH's prepared from HDTMA+-Fluorohectorite in the
presence of hexylamine, decylamine and dodecylamine, respectively as co-
templates. The nearly linear portions of the adsorption curves in the
partial pressure region ~0.02 - 0.25 are indicative of supermicropores
(~14-20A diameter) or small mesopores (~20-25A). As shown in Figure
III.5(insert), a Horvath Kawazoe analysis of the adsorption data yields
average pore sizes of 15A for hexylamine, 21.03. for decyalmine, and 23A
for dodecylamine indicating that the neutral amine co-surfactant chain
lengths plays an important structure directing role in the synthesis of the

gallery intercalated silica.

127

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 111.4 : X-ray powder diffraction patterns for air dried PCH
formed by reaction (A) decylamine solvated DTMA+-Fluorohectorite with
TEOS at DTMA+-F'luorohectorite : decylamine : TEOS molar ratios of l :
20 : 150. (B) decylamine solvated HDTMA+-Fluorohectorite with TEOS
at HDTMA+-Fluorohectorite : decylamine :TEOS molar ratios of 1:20:150.

Figure HI.5 : Nitrogen adsorption/desorption isotherms and the
corresponding H & K pore size distribution for porous silica-clay
heterostructures (PCHs) prepared by gallery-templated synthesis
using (a) hexylamine, (b) decylamine, & (c) dodecylamine as co-
templates. The HDTMA+—clay : amine : TEOS reaction
stoichiometry was 1 : 20 : 150. Relative pressure is P/Po, where P is

the equilibrium pressure of the adsorbate and P0 is the saturation

pressure of the adsorbate at the temperature of the adsorbent; the
volume adsorbed is at STP. Insert : The corresponding Horvath-
Kawazoe pore size distribution curves. (dW/dR is the derivative of t
he normalized adsorbate (nitrogen) volume adsorbed with respect to
the pore diameter of the adsorbent). Before measurement each
sample was heated overnight at 150°C and 10*6 torr. The isotherms
were measured at -196°C on a Coulter Omnisorp 360CX
Sorptometer using standard continuous adsorption procedures.

Volume Adsorbed (cm3 / g) at STP
W
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129

Horvath - Kawazoe Pore size distribution
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I31

Table 111.2 gives the pore sizes of related PCH's prepared from other
combinations of quaternary ammonium and neutral amines co-templates.
Increasing the chain length of the amine co-template systematically
increases the pore size of the PCH. The fact that the Q" chain length is an
additional important parameter in the templating process is evidenced by
the PCH materials prepared with DTMA+ and decylamine as gallery co-
templates. Consistent with the gallery-templated pathway, this latter
surfactant system affords an average pore size of 14A, substantially smaller
than the 21A pore size provided by the HDTMA+ and decylamine
templates. Included in Table III.2 are the gallery heights for the calcined
PCHs, the corresponding air dried PCH precursors and the initial amine
solvated organo fluorohectorite.

Besides serving as co-templates in PCH synthesis, alkylamines
improve the intragallery hydrolysis-condensation reaction of TEOS
through base catalysis and thereby facilitate the formation of PCH. In
addition, the alkylamines enhance the crystallinity of the final product.

Figure [11.6 A and 13 illustrates the X-ray powder diffraction
patterns of products formed by reaction of HDTMA+-fluorohectorite and
TEOS intercalates in the presence and absence of amines. The air dried
samples exhibit multiple (001) orders of Bragg's reflections indicating a
well-ordered structure along the c axis. However, on calcination at 650°C
in air for 4 h, the sample synthesized in the presence of amines showed
multiple (001) reflections, indicating a layered structure with gallery
heigths of ~21A, but the material synthesized in the absence of amine
collapsed to a basal spacing of 12.2A.

132

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

38.01A (A)
m
H
‘5
5
E 32.50A
II!
0
O:
H
.2
g as-synthesized
calcined at 650°C
1 l i I i J i L l 1 . . 1 l
5 10 15 20
Two Theta (degrees)
32.08A (B)
>.
H
‘8
s:
3
fl
y—i
0
.2
‘5 12.30A
'3
a:
calcined at 650°C
16.04A .-
10.69A as-synthesized

 

 

10 15 20
Two Theta (degrees)

é
Figure 111.6 :(A) XRD pattern of air dried and calcined gallery
templated products formed with an amine co-template. (B) XRD pattern of

the air dried and calcined silica-pillared products formed in the absence of
an amine co-template.

133

 

400

 

350

300

 

 

 

 

 

25° F 1 1.5 2 2.5 3
Pore diameter (nm)

200 ' (B)

150

 

Volume Adsorbed (cm’lgm)

 

 

 

100 (A)
50 _
1 . 1 l i I L I l . 1 L 1 l 1 I 1 i l
00 0.2 0.4 0.6 0.8 1
Relative Pressure (P/Po)

Figure [11.7 : (A) Nitrogen adsorption/desorption isotherms and H & K
pore size analysis for porous silica-clay heterostructures (PCHs) prepared
by gallery-templated synthesis using HDTMA+-Fluorohectorite .
decylamine : TEOS with a reaction stoichiometry was 1 : 20 : 150. (B)
Nitrogen adsorption/desorption isotherms and H & K pore size analysis for
a silica intercalated derivative prepared by using I-IDTMA+-Fluorohectorite
: TEOS with a reaction stoichiometry of l : 150.

134

Figure [11.7 A and B illustrate the Nz-adsorption/desorption
isotherms for the calcined samples. HDTMA+-Fluorohectorite/TEOS has
an isotherm
that lacks the sharp inflection in the low partial pressure region as
compared to the one that is synthesized in the presence of amines. As
shown in figure III.7 (insert) a Horvath and Kawazoe analysis of the
adsorption curves indicates pores around 21A for the sample synthesized in
the presence of amines, while the amine deprived material lacks any useful
pore size information.

Effect of surfactant head group size on PCH formation.

Earlier we discussed the effects of templating surfactant chain
lengths on the pore sizes of PCHs. We shall now consider the role of the
quaternary ammonium surfactant head group size on quantitative PCH
formation.

A series of organoclay derivatives containing CnH2n+1N(CH3)3+-
Fluorohectorite (denoted as Qn+-FH) and CnH2n+1NH3+-F1uorohectorite
(denoted as Pn+-FH) were prepared. The difference between the two
amphiphiles lies in the effective cross-sectional area of the head groups.
The NH3+ head group has a cross sectional area of ~10A2 and the
N(CH3)3+ head group has a cross-sectional area of about ~24A2.

Mixtures containing Qn+-FH : Decylamine : TEOS and Pn+-FH :
Decylamine : TEOS at molar ratio of 1:20:10; 1:20:50; 1:20:100; 1:20:150;
and 1:20:200 were stirred for 4 h, centrifuged and air dried. Diffraction

patterns for the air dried samples of Qn+-FH and Pn+-FH are similar.

Figure 111.] and 111.8 illustrate the X-ray diffraction patterns for
Qn+-FH and Pn+-FH surfactant combinations, both as air dried and as

calcined products. For the air dried HDTMA+-FH : Decylamine :

135

Figure III.8 : (a) X-ray diffraction patterns for the air dried products
formed by the reaction of decylamine solvated
HDA+-Fluoorhectorite with TEOS at HDA+-FH : decylamine :
TEOS molar ratio of l : 20 : X where 10 S X S 200.

(b) X-ray diffraction patterns for the calcined material
formed by the reaction of decylamine solvated

HDA+-Fluoorhectorite with TEOS at HDA+-FH : decylamine :
TEOS molar ratio of 1 : 20 : X where 10 S X S 200.

I36

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137

Table III.3 : Gallery heights of intercalates formed by using a
quaternary ammonium cation or a primary ammonium cation as
exchanged-cation and decylamine as co-swelling agent at different
TEOS/amine molar ratios.

 

 

Clay Exchange Gallery Gallery TEOS : 3

cation height height of neutral amine

of air dried calcined molar ratio .

sample (A) sample (A) I
; Fluorohectorite DTMA+ 27.4 2.7 0.5
' 24.4 2.7 5.0
24.4 13.9 7.5

23.4 14.4 10 ;

DDTMA+ 29.4 2.7 0.5

26.4 16.4 5.0 1

26.4 18 7 .5 I

26.4 18 10 l

I-IDTMA+ 28.4 2.5 0.5 3
28.4 20.0 5.0
28.4 21.0 7.5
:¥ 28.4 21.4 10
:l Fluorohectorite DA+ 27.4 3 0.5
' 24.4 3 5.0
, 24.4 3 7.5
' 23.4 3.4 10
; DDA+ 29.4 2.8 0.5
i 26.4 2.8 5.0
26.4 2.8 7.5
E 26.0 2.9 10
3 HDA+ 30.4 2.5 0.5
27.4 2.5 5.0

26.4 2.5 7.5 .

26.4 2.5 10 J

L.___._,_._.__...__.___.___ _ _ _— __ _...___ _ — -— ._—__ _ .— _._~_

*Gallery egt "- spcan - layer W — DTA+L
C10H21N+(CH3)3. DDTMA“ = C12H25N+(CH3)3. HDTMA+ = C16H33N+(CH3)3. DA+
= C10H21NH3+, DDA+ = C12H25NH3+’ HDA+ = C16H33NH3+.

138

TEOS system, the gallery height remains fixed at 28A for all reaction
stoichiometries. This gallery height is suggestive of a lipid like structure
which consists of a single hexadecyltrimethylammonium and neutral
decylamine surfactant molecule (see figure 11.7). For the air dried HDA+-
FH : Decylamine : TEOS, the XRD data suggests a similar orientation of
the hexadecylammonium and decylamine molecules. At the shorter
surfactant chain systems a similar correlation in gallery heights between
Qn+ and Pn+ is observed.

On calcination of the samples at 650°C for 4h in air, the HDTMA+-
FH : decylamine : TEOS system with molar ratios of l : 20 : 10 and 1 : 20
: 50 both show diffraction patterns due to the formation of two phases,
namely the surfactant templated PCH with a 21A gallery height and a silica
intercalated derivative with a basal spacing of 12.6A. Increasing the TEOS
molar ratio in the synthesis favors the formation of surfactant templated
PCH derivative. It can be concluded that PCH formation is not quantitative
unless the TEOS : amine ratio is > 7.5

When the Pn+-FH surfactant is used, the diffraction patterns of all
calcined materials consist of a predominantly silica-intercalated derivative
with a basal spacing of 12.6A, which leads to the conclusion that the
primary ammonium / neutral amine template combination does not form a
gallery nanostructure.

Table [113 lists the gallery heights of related materials prepared
using other combination of Qn+ and Pn+ surfactants.

On performing solvent extraction of the samples by using ethanol as
a solvent the role of the quaternary ammonium surfactant head group size
on PCH formation becomes crystal clear.

Figure 111.9 and 111.10 illustrates the X-ray diffraction patterns for

139

the DTMA+-FH : decylamine : TEOS and DA+-FH : decylamine : TEOS
respectively. It is observed that for the air dried sample prior to any
solvent extraction, the diffraction patterns exhibit nearly similar gallery
height and multiple (001) reflections, indicating a layered structure.
However upon ethanol extraction of the samples interesting changes are
observed. The diffraction pattern for the DA+-FH : decylamine : TEOS
indicates major structural changes, the basal spacing reduces to ~18A from
the initial value of ~33A and thermogravimetric programmed analysis
curve ( see figures III.9 &10) showed a single step weight loss between
100°C to 450°C, a weight loss of ~20% was registered due to the thermal
decomposition of the organic template. Elemental analysis of the sample
showed ~0.8 decylamine per 020 F4 unit cell and ~1.2 mol silica per 020
F4 unit cell. These results along with a sharp decrease in gallery height
clearly show the absence of any silica templated nanostructure before
calcination. In DTMA+-FH : decylamine : TEOS the gallery height
reduces by ~9A after solvent extraction and thermogravimetric
programmed analysis curve (see fig) exhibited a two step weight loss
between 100°C to 450°C, the initial step is due to thermal decomposition of
the neutral amine co-template, and the second weight loss at a slightly
higher temperature is due to the decomposition of the quaternary
ammonium surfactants. Elemental analysis of the sample shows '~1.2
decylamine per 020 F4 unit cell and ~7.1 mol of silica per 020 F4 unit cell.
These results along with a gallery height of ~15A after solvent washing
clearly indicate the formation of a silica templated nanostructure prior to
any thermal treatment and are in agreement with the previous X-ray

diffraction data, suggesting a critical structure directing

140

 

     
     
  

 

Airdried
(001) N(CH3)3+ head group N143“ head group
>. (001) 33.311 (001) 32A
“
2 (002) 16.66A (002) 16A
3 A
.5: (003) 11A (003) 10.7
0
>
E (002) 003
3 ( ) W
a:
NH3+ head group
I I I J 4 L Li 1 L ‘fi : l . ‘

 

 

 

 

8 '1'2 '16 20 '54
Two Theta (degrees)

 

TGA lPlot - Before ethanol extraction
100 .
. Ramp rate=5°Clmm

     
 

9 s
9 o
8 s
3 ° N(CH3)3+ head group

75

 

Percent Weigth Loss

 

100 200 300 400 500 600

Temperature (°C)

Figure III.9 : (A) X-ray diffraction patterns prior to template extraction
Q+-FH : decylamine : TEOS at a molar ratio of 1: 20 : 150, (DTMA+ as
Q+ ion) and P+-FH : decylamine : TEOS at a molar ratio of l: 20 : 150,
(DA+ as P+ ion). (B) Thermal gravimetric analysis of decylamine solvated
n-decylammonium-Fluorohectorite.(Pn+-FH system) and PCH prepared

from decylamine-solvated DTMA+-Fluorohectorite.(Qn+-FH system).

141

After ethanol extraction

 

 

 

 

 

 

 

 
  
  
  

 

 

 

18.50A
24.00A
>.
.‘.:.’
v:
E
o
H
fl
_
q, _ NH; head group
>
0:
.9;
o
a:
12.00A +
N(CH3)3 head grouv 4.5A
- 2‘ “:‘:..4h"
.LLJ.41.84.ll.nel
5 10 15 20
Two Theta (degrees)
TGA Plot - Afler ethanol extraction
1001
95 I
+-
g Pn FH
4.
i
E 15% weiAh loss
8
A. 85 .
Qn+'FH 19% wt igth loss
80 l l L 1
100 200 300 400 500 600

Temperature

Figure 111.10 : (A) X-ray diffraction patterns after template extraction
of Q+-FH : decylamine : TEOS at a molar ratio of 1: 20 : 150, and
DTMA+ as Q+ ion and P+-FH : decylamine : TEOS at a molar ratio of 1:
20 : 150, and DA+ as P ion. (B) Thermal gravimetric analysis of
decylamine solvated n-decylammonium-Fluorohectorite.(Pn+-FH system)

and PCH prepared from decylamine-solvated DTMA+-Fluorohectorite.

142

role of the quaternary ammonium surfactant primarily due to the larger
head group size.

Surface area measurements and pore structure analysis on calcined
samples were performed to further elaborate the differences between the
two surfactant system.

Figure III.11 illustrates the N2 adsorption/desorption isotherms for
the Qn+-FH : decylamine : TEOS surfactant system. The presence of the
linear portion in the low P/Po region is suggestive of the presence of pores
between (~14-25A); which is a characteristic feature for all PCH materials.
Figure 111.11 A & B (insert) shows the H and K analysis of the adsorption
data which gives average pore size ~15A and 21A for the
DTMAfldecylamine and HDTMA+ldecylamine templates respectively.
Nitrogen BET surface areas were in the range of 470750ng for two
different surfactant chain lengths.

Figure [II.12 A and B illustrate the N2 adsorption isotherms for P114”-
FH : decylamine : TEOS system. The adsorption curves illustrates
virtually non-porosity in the low partial pressure region and hence lack the
charateristic feature of a nanoporous adsorbent. Figure 111.12 (insert)
shows the H-K pore analysis of the adsorption curve. The differential of
the N2 adsorbed with respect to the pore diameter of the adsorbent clearly
lacks any useful pore size evaluation.

Table [11.4 illustrates the BET surface area, H-K pore size and t-plot
analyses of related samples prepared by using other combinations of Qn+

and Pn+ surfactants.

143

 

 

600 _

H-K pore size distribution

soo _

 

400 _

 

 

 

l 1.5 2 2.5 3

Pore diameter (nm)

 

300 _

200

 

Volume adsorbed (cm’lg) at STP

100_

 

 

L 4 l l L l l I 1 i l l l I l L

00 0.2 0.4 0.6 0.8 l

 

Relative Pressure (PIPQ)
Figure III.11 : Nitrogen adsorption/desorption Isotherrns and H-K pore
size analysis for porous silica-clay heterostructures (PCHs) preparedby
gallery-templated synthesis using: (A) HDTMA+-fluorohectorite
decylamine : TEOS with a reaction stoichiometry was 1 : 20 : 150,
respectively. (B) DTMA+-fluorohectorite : decylamine : TEOS with a
reaction stoichiometry of 1 : 20 : 150, respectively.

144

 

 

 

 

 

350
H-K pore size distribution
300 ’
a:
E
B
_ '5
250
(A)
200 7 (B)
llJlllllllJlLlllLll
l 1.5 2 2.5 3

.. Pore diameter (nm)

 

(A)

Volume adsorbed (cm’lg) at STP
3 5‘.
a O

50

 

 

 

L 1 1 L 1 1 P 1 1 1 r l 1 1 1 l 1

0 0 . 2 0 . 4 0 . 6 0 . 8 1
Relative Pressure (P/Pfi)

 

Figure [II.12 : Nitrogen adsorption/desorption isotherms and H-K pore
size analysis for
(A) HDA+-fluorohectorite : decylamine : TEOS with a
reaction stoichiometry was 1 : 20 : 150 respectively.
(B) DA+-fluorohectorite : decylamine : TEOS with a
reaction stoichiometry of 1 : 20 : 150 respectively.

145

Table 111.4 :Horvath and Kawazoe (H-K) pore size, BET surface area,
and t-plot analysis of materials prepared using quaternary Q,{*’ and
primary Pn'l’ ammonium as exchanged-cation in fluorohectorite with
decylamine as co-solvent. Molar ratio of Q,{*’ or Pn+-FH : decylamine :

TEOS of 1 : 20 : 150; and materials were calcined at 650°C.

BET t-plot analyses

Ammonium pore size surface
(A) area ST

(mzlg)
14 560 620
<9 280 280

17 660

330 350
800 760

 

DTMA“ = C10H21N+(CH3)3. DDTMA+ = C12H25N+(CH3)3. HDTMA+ =
C15H33N+(CH3)3. DA+ = C10H21NH3+, DDA‘l' = C12H25NH3+’ HDA+ =
C16H33NH3+.

To further emphasize the effect of the quaternary ammonium
surfactants head group size on PCH formation, we carried out reactions
with intercalated cations of larger head group cross-sectional areas. With
P(C4H9)3+ replacing the N(CH3)3+, the difference in the effective cross-
sectional area is ~80A2- When we used the larger head groups such as the
HDTBP+ ion (Hexadecyltributylphosphonium), no intra-gallery assembly

process occurs, as a result of which there is no gallery nanostructure

I46

formation. Both X-ray diffraction data and surface area measurements
confirm this observation. Furthermore if long chain dialkyl quaternary

ammonium intercalated ions are used, no PCH formation is observed

 

 

 

 

 

 

39.45A
>.
3:
U)
E
o
H
.S'
As-synthesized
2 fl 1‘— w i 4
0;
3
a
a:
12'7A Calcined at 650°C
le-J'llJ—Iejklimt
5 15 20

10
Two Theta (degrees)

Figure 111.13 : (A)X-ray powder diffraction patterns for air dried PCH
formed by reaction of decylamine solvated DODDMA+-Fluorohectorite
with TEOS at DODDMA+-Fluorohectorite : decylamine : TEOS molar
ratios of 1 : 20 : 150. (B) X-ray powder diffraction patterns of air dried
(PCH) obtained by calcination at 650°C for 4 h.

Figure 111.13 illustrates the X-ray powder diffraction patterns for

147

the as-synthesized and calcined DODDMA+—fluorohectorite : decylamine :
TEOS , with l : 20 : 150 molar ratios, respectively (DODDMA+->
Dioctadecyldimethylammonium). The air dried sample shows a substantial
amount of gallery height indicating lipid-like structure formation of the
surfactants within the galleries. However, on calcination of the as-
synthesized sample at 650°C for 4 h, a collapsed silica-intercalated
derivative with a basal spacing of ~12.7A results. This observation clearly
show that the packing density of the organic templates is crucial for the
successful formation of porous clay heterostructures. It just works out
that the monoalkyl long chain quaternary ammonium ion along with the
neutral amine, co-surfactants in conjunction with the neutral silicate species
allows for the formation of intra-gallery micellar units. N2-Adsorption-
desorption measurements of the calcined sample confirm the presence of a
collapsed structure, with a BET surface area of ~150 m2/gm.

Unfortunately attempts at PCH formation in magadiite by using long
chain quaternary ammonium surfactants and neutral amines as co-
surfactants failed; no intra-gallery templated products formed. However
when the monoalkyl quaternary ammonium exchanged were replaced by
dialkyl long chain quaternary ammonium exchanged ions and neutral
amines were used as co-templates, gallery structure was observed after
calcination.

Figure III.14 illustrates the X-ray diffraction powder patterns for as-
synthesized, ethanol extracted, ion-exchanged and air dried, calcined
DDDMA+-magadiite : decylamine : TEOS, with a reaction molar ratio of 1
: 20 : 150, respectively (DDDMA+—) Didecyldimethylammonium ion) The
as-synthesized sample shows a gallery height of ~21A. Calcination of the
sample to 500°C in air for 4 h, yielded no reduction in gallery height.

I48

 

40.10A

 

 

 

 

 

 

 

 

 

 

5'
"2'

3 (E)

= -—

~
3

'5' (D)
5
Ta

°‘ __ (C)

12.6A
k 1 (B)
_ (A)
1 P l 1 1 1 1 l 1 1 1 1 I
l 0 l 5 2 0

Two Theta (degrees)

Figure 111.14 : X-ray diffraction powder patterns for DDDMa+-
magadiite : decylamine : TEOS, with a molar ratio of 1 : 20 : 150,
respectively. (A) DDDMA+-magadiite, (B) As-synthesized, (C) Ethanol-
extracted, (D) As-synthesized, calcined at 650°C, (E) Ethanol-extracted,
ion-exchanged.

149

 

 

      
 

 

 

 

 

 

 

 

P
- H & K pore size distribution
600 '
’ 1:
fl ' 2
m " 3 (A)
3500: 1; (B)
3 ..
n I
E D
3
4001- IIJAALJAILJl—LAIA
'8 L- 1 1.5 . 2 2.5 3
f )- Pore diameter (nm)
8 D
c D
a
E300 l:
3 1'
G 1-
> .
200 '
l
100
l 1 1_1 l 1 1 1 J 1 1 1 l 1 1 1

 

 

. 4 0 . 6 0 . 8
Re‘1ative Pressure (P/Po)

Figure III.15 :N2 adsorption/desorption isotherms and the

corresponding H-K pore size distributionfor (a) As-synthesized, calcined,
(b) Ethanol-extracted, ion-exchanged material.

150

Ethanol extraction of the as-synthesized sample shows an increase in
gallery height from ~21A to 26A. Ion exchange with 0.12N HCl solution
for 24 h results in the complete removal of the template and shows a
gallery height of ~22A. That the template was removed completely was
checked by thermogravimetric analysis of the ion-exchanged sample.
Figure 111.15 illustrates the N2 adsorption-desorption
isotherms and the corresponding H-K pore size distributions for the as-
synthesized, calcined and the template extracted, ion-exchanged samples.
The adsorption isotherms for both samples show a small step in the low
partial pressure region between 0.02 to 0.25 P/Po, indicating the presence
of gallery nanopores; however the H-K pore size distribution curve show
maxima at different pore diameters. For the as-synthesized, calcined
material the average pore size was ~17A, whereas for the ethanol extracted
and ion exchanged material the average pore size was ~21131. These results
are in agreement with the X-ray diffraction data. The BET surface areas
for both materials were ~450 m2/gm.
Use of larger head group intercalated ions such as P(C4H9)3+ instead of
N(CH3)3+ in magadiite did not negatively affect gallery nanostructure
formation. Figure 111.16 shows X—ray diffraction patterns for the as-
synthesized, ethanol extracted, ion—exchanged, and air dried,calcined
HDTBP+-magadiite : decylamine : TEOS mixtures with 1 : 20 : 150 molar
ratio, respectively (HDTBP+ —> Hexadecyltributylphosphonium ion). The
as-synthesized sample shows a gallery height of ~25A. On calcination of
the sample to 500°C in air for 4 h, a gallery height of ~26A was obtained.
Ethanol extraction of the as-synthesized material yields a gallery height of
~27A. Ion-exchange with 0.12N HCl solution results in complete removal

of the template and a gallery height of ~29A is obtained.

lSl

 

38.0015.

 

 

 

 

 

 

 

 

>3
H
'a
r:
3
1: 40.00A
fl
H
3
32
37.00 (C)
360 A
(B)
(A)
1 1 1 l 1 = 1 1 I 1 1 L 1 L 1 1 1 #1
5 10 15 20

Two Theta (degrees)

Figure 111.16 : X-ray diffraction powder patterns for HDTBP+-
magadiite : decylamine : TEOS mixture with 1 : 2O : 150 molar ratio,
respectively. (A) As-synthesized, (B) As-synthesized, calcined at 650°C,
(C) Ethanol-extracted, ion-exchanged, (D) Ethanol—extracted.

l52

 

100 _

0
00

¢
6
I

%tage Weight loss
Q Q
O 01

 

NI
VI

 

 

-
7o 114111111L1111I111111111l141111411L1111

100 150 200 250 300 350 400 450 500
Temperature (°C)

 

Figure 111.17 : Thermogravimetric plot of (a) HDTBP+—magadiite (b)
Ethanol-extracted, ion-exchanged HDTBP+-magadiite : decylamine : TEOS
mixture.

Thermogravimetric analysis of the HDTBP+-magadiite and the
HDTBP+-magadiite/decylamine/TEOS ethanol-extracted, ion-exchanged
material is shown in Figure 111.17. It is observed that the weight loss curve
for the HDTBP‘F-magadiite shows beyond 100°C, a single step at around
380°C, which is due to the decomposition of the 11me ions. In the
ethanol extracted and ion-exchanged material a total weight loss of < 3%
beyond 100°C is observed which clearly indicates the complete removal of
the organic templates from the structure. N2 adsorption-desorption

measurements were performed on the as-synthesized, calcined and

153

completely template-removed but not calcinated materials.

 

500

 

    
  

H & K pore size distribution

400

 

 

 

300

200

Volume adsorbed (cm’lg) at STP

 

100

 

[1111111L111

0 0.2 l 0.4 0.6 0.8 1
Relative Pressure (P/P.)

 

Figure 111.18 :N2 adsorption/desorption isotherms and the
corresponding H-K pore size distribution for HDTBP+-magadiite :

decylamine : TEOS mixture at l : 20 :150 molar ratio. (A) as-synthesized,
calcined (B) Ethanol-extracted, ion-exchanged.

l54

Figure 111.18 illustrates the N2 adsorption-desorption isotherms and
the corresponding H-K pore size distribution for the as-synthesized,
calcined and completely template removed materials. The adsorption
isotherm for both materials shows the presence of a small step in the low
P/Po region, indicating the presence of nanopores; however a BET surface
area of ~600 m2/g was obtained for the templated extracted materials and
the as-synthesized, calcined sample gave a BET surface area of ~530 m2/g.
H-K pore analysis shows a similar pore size is present in both samples.

Thus, these few preliminary intra-gallery reactions in magadiite
suggest that the surface OH groups have an effect on the rate of hydrolysis
and condensation of the silicate species, and also suggest that the optimum
surfactant packing requirements are different from those of the
corresponding smectite clay PCH formations.

Furthermore, earlier studies have reported the intragallery
hydrolysis of TEOS for primary alkylammonium exchanged forms of
magadiites in both the presence and absence of primary amines. However,
these silica intercalated magadiites have been described as pillared
structures. For instance, octylamine solvated octylammonium magadiite as
a reaction precursor affords silica intercalates with 9-15A gallery heights
depending on the magadiite : TEOS ratio. The average pore size of these
derivatives (~6A) is substantially smaller than the gallery heights, a feature
characteristic of metal oxide pillared clays. Also, the observed pore size is
smaller than expected (~12A) for a micelle-templated nanostructure. The
amine solvated galleries of primary ammonium-ion exchanged forms of
magadiite are thus accessible to TEOS for microporous pillaring, but not
for nanoporous templating.

Gallery templated synthesis of PCH materials is not limited to

155

fluorohectorite. We have also prepared PCH derivatives of vermiculite
and rectorite by gallery templated TEOS hydrolysis in the presence of
quaternary ammonium surfactants with neutral amines as co-surfactants.
Each of these PCH derivatives had an average pore size equal (within
experimental uncertainity) to the pore size observed for the corresponding
fluorohectorite system. These results further support intragallery
templating and unequivocally rule out pillaring. If the intra-gallery
hydrolysis of TEOS simply involved the formation of dense silica pillars
between the quaternary ions; the pore size should decrease with increasing
charge density on the host layers. For the layered hosts investigated in the
present work, the layer charge per 100A2 increases in the order rectorite
(1.33) < fluorohectorite(2.5) < vermiculite(3.82). Yet the observed pore
sizes are invariant over the entire layer-charge range. This result
precludes pillaring, but supports templating by micellar units comprised of
neutral amines and quaternary ammonium ions in the appropriate layer

charge-compensating ratio.

Mechanistic considerations.

As outlined in figure 111.19, we propose that PCH pore structures are
determined solely by intrinsic structure-directing interactions between the
intra-gallery sufactants and the inorganic precursor. The initial amine-
solvated organoclay (Q+-clay) is assigned a lipid-like structure (fig 111.19
A). This structure is substantiated by gallery heights that are close to the
values expected based on the chain lengths of quaternary ammonium
surfactants and the neutral amine (see table 111.2). Upon introduction of
TEOS, some of the neutral amine is displaced from the galleries and a

reactive intermediate is formed (figure 198). Also the extra-gallery water

156

concentration is low, as judged by the water content of the amine (~0.4
wt%). Thus, the base-catalysed hydrolysis of TEOS is much faster in the
clay galleries than in solution, and this minimizes the formation of extra-
gallery silica. Subsequent hydrolysis of the gallery TEOS affords a
hydrous silica templated around a monolayer of micellar Q+ and neutral
amine assemblies. Surfactant assemblies formed exclusively from Q+ or
neutral amines are precluded by the fact that both templating components
are needed for the formation of a gallery nanostructure. Spherical micelles
in van der Waals contact or, more likely (by analogy to the templating of
hexagonal mesostructures,such as, MGM-41 ), rod-like micelles are
formed, as shown in cross-section in Figure III.19C. These latter
assemblies are structurally analogous to the layered silica-surfactant
intercalates that have been postulated to form as intermediates in the
synthesis of MCM-41, with the important exception that the layers in the
case of PCHs consist of preformed anionic 2:1 layered silicate. The final
calcination step (Fig III.19D) removes the template and completes the

dehydroxylation and cross-linking of the gallery-assembled silica structure.

157

Figure III.19 : Proposed mechanism for the formation of a PCH by
gallery - templated synthesis: (A) Amine - solvated bilayer
structure with a thickness equivalent to the length of the
quaternary cation (filled head groups) and the neutral amine
(open head groups) surfactants; (B) Intercalation of TEOS by
partial displacement of neutral amine ; (C) Templated
heterostructure in which a 2D hydrated silica is organized
around micellar assemblies of Q+ and neutral amine; (D)
Calcined porous clay heterostructure with a 2D framework of
porous silica intercalated between the clay layers.

158

.22 y/

A. Amine-solvated thlay B. TEOS-Intercalated intermediate

 

    

C. Templated Heterostructure

1 calcine, 650°C

 

159

Conclusions.

Porous clay heterostructures are synthesized by using intercalated
quaternary ammonium surfactants and neutral amines as co-surfactants to
direct the inter-lamellar hydrolysis of a neutral silicate precursor such as
TEOS within the galleries of an ionic lamellar solid. The nature and
amount of the exchange cation and neutral amine co-template, along with
its chain length, are critical to the formation of PCH materials. In fact the
relationship between PCH pore size distribution, surfactant chain length
and reaction stoichiometry supports a templating mechanism analogous to
that of the structure-directed M418 mesostructure synthesis. The
fundamental differences between pillaring a layered solid and the gallery
templation process are listed below.

(1) A critical value of the molar ratio of amine : TEOS is essential
for the formation of a gallery nanostructure in porous clay heterostructure
synthesis, whereas in a conventional pillaring process such a dependence on
reaction stoichiometry is not critical for the quantitative formation of dense
metal oxide aggregates inside the galleries of an ionic lamellar solid.

(2) A conventional pillaring process is characterized by the
dependence of the pore size on the total cation exchange capacity of the
layered host with an increase in charge density decreasing the pore size of
the final pillared solid. However, while in the PCH synthesis the layer
charge per 100.312 for the host layers investigated increased from 1.33 for
rectorite to 3.82 in vermiculite, the observed pore size remained invariant.

(3) The pore size of the calcined PCH does not change when
changing the clay : TEOS ratio is changed, suggestive of an intra-gallery
templating process, whereas in a pillaring reaction the pore size decreases

with an increase in the amount of TEOS in the reaction mixture.

160

(4) A systematic increase in PCH pore size with an increase in the
chain length of the templating surfactants precludes pillaring, but supports
templating by micellar units comprised of neutral amines and quaternary
ammonium ions in the appropriate layer charge-compensating ratio.

(5) Finally, the head group size of the exchange cation on
quantitative PCH formation strongly implies important intra-gallery
assembly interactions between the exchange cation, the neutral amine co-
template and the inorganic silicate precursor, suggesting a gallery

templating reaction.

161

References :

10.

11.
12.

13.
14.

Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J ., Nature, 374,
529-531, 1995.

Barodawalla, A.; Galarneau, A.; Pinnavaia, T.J.; manuscript in

' preparation, 1996.

Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartulli, J. C.; Beck,
J. 8., Nature 359, 710-712 1992.

Beck, J.S.; Vartulli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C.
T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.;
McCullen, S. B.; Higgins, J. B.; Schlenker, J. L., J. Am. chem. Soc.,
114, 10834-10843 1992.

Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J ., Nature, 368,

32221-323, 1994.

Tanev, P. T. and Pinnavaia, T. J., Science, 267, 865-867, 1995.
Reddy, M. K., Moudrakovski, 1.; Sayari, A. J. Chem. Soc. Chem.
Commun. 1059-1060, 1994.

Corma, A. , Navarro, M. T.; Pérez Pariente, J. J. Chem. Soc.

Chem. Commun. 147-148 1994.

Bein, T. J.; Wu, C.-G. Science , 264, 1757-1758 1994.

Barrer, R. M.; MacLeod, D. M., Trans. Faraday Soc., 51, 1290,
1955.

Lagaly, G. Solid State Ionics, 22,43-51, 1986.

Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P., Chem. Mater.,

6, 1017-1022, 1994.

Pinnavaia, T. J ., Science, 220, 365, 1980.

Clearfield, A.; in M. L. Occeli and H. E. Robinsoon (Eds),

15.

16.

17.
18.

19.

20.

162

Expanded Clays and other Microporous Solid. Van Nostrand
Reinhold, New York, 1992, p. 245.

U. S. Pat. Nos 4,859,648, 1989., 4,728,439, 1988., 4,929,587.
1990.; Can. Pat. No. 1,252,432, 1989.

Landis, M. B.; Aufdembrink, B. A.; Chu, P.; Johnson, I. D.; Kirker,
G. W. & Rubin, M. K. J. Am. Chem. Soc. , 1991, 113, 3189.
Dailey, J. S.; Pinnavaia, T. J. Chem. Mater., 4, 855-863, 1992.
Spartan program modelization, Michigan State University,
Department of Chemistry.

Brunauer, S.; Emmett, P.; Teller, E. J. Am. Chem. Soc., 1938,
60, 309.

Horvath, G.; Kawazoe, K. J. J. Chem. Eng. Jap., 1983, 16, 470-
475.

163

Chapter IV

Physico-chemical properties of PCH materials.

164

Abstract

Thermochemical studies were carried out on porous clay
heterostructure (PCH) materials that were prepared by using
hexadecyltrimethylammonium / decylamine as templates , TEOS as the
silica precursor, and fluorohectorite as the layered host. The gallery
heights, surface areas, pore structures, and the degree of crosslinking of
the gallery silica mesostructures were studied by using X-ray powder
diffraction, N2 adsorption-desorption, 29S1 MAS NMR, l9F MAS NMR,
TGA, & elemental analysis.

PCH-fluorohectorite prepared by gallery templating gave surface
area in the range of 555 - 790 m2/g and stable pore sizes between 19 to
23A at temperatures ranging from 200 to 550°C. 19F NMR shows the
depletion of the structural fluorine at calcination temperature of around
350°C and 2981 MAS NMR shows evidence for the crosslinking of the
gallery silica nanostructure to the clay layers. Partial removal of the
template prior to calcination results in decomposition of the gallery
nanostructure. PCH-rectorite exhibits hydrothermal stabilities up to 700°C
for 2 to 3 hrs, and pyridine chemisorption studies on PCHs shows that
silica-interlayered PCHs are intrinsically acidic, whereas pure silica
mesostructure possess little or no acidity. In addition metal ion substitution
in the gallery framework structure was also attempted and PCH formation

using low charge density smectites was attempted.

165

Introduction.

Because of growing environmental concerns, there is interest in
replacing tradition catalysts, such as aluminium trichloride, sulfuric acid
and hydrofluoric acid, with recyclable solid catalysts. Pillared interlayered
clays (PILC) have been extensively studied and are very attractive as solid
acid catalystsl’z. These solids exhibit a tunable acidity, regular
microporosity and relatively high thermal stability. Montmorillonite,
hectorite, beidellite and saponite pillared by A113 hydroxy cations have
been the most studied. These materials revealed a substantial activity for
petroleum cracking and for fine chemical synthesis via alkylation and
isomerization. Recently Butruille et al. reported an improvement in the
acid catalytic properties of alumina-pillared fluorohectorite3’4. The
presence of fluorine in the layer structure enhanced the Bronsted acidity of
the pillared clay at outgassing temperatures below 350°C. This catalyst
exhibited significant Bronsted acidity, which is unusual for smectite clays.

We recently reported a new templating route which allows us to
expand the use of lamellar compounds for the design of nanoporous
materials. By appropriate choice of a reaction medium and combinations
of neutral and ionic co-surfactants, new intercalated structures containing
large channels similar to those formed in mesoporous molecular sieves
(MCM-4l) can be synthesized5. We call these materials porous clay
heterostructures (PCHs). PCHs provide unique oppurtunities to improve
the chemical and physical properties of surface active lamellar structures
by enhancing diffusion. Here we report some important thermochemical
alterations that occur in PCH-Fluorohectorite, formed with

HDTMA+ldecylamine as the templating surfactants. Because the pore

166

structure is confined in a two-dimensional lattice, these new porous clay
heterostructures are more stable than large pore molecular sieves such as
MCM-4l. Hydrothermal studies on PCH-rectorite showed steam stability
of the gallery nanostructure for 2 to 3 h, at tempeartures up to 650°C. In
addition, pyridine chemisorption properties of PCHs showed that due to the
presence of protons which are necessary to balance the clay layer charge.
Silica-intercalated PCHs are capable of transferring protons to organic
substrates adsorbed in the gallery nanopores thereby making them
intrinsically acidic, whereas pure silica mesostructures possess little or no

acidity.

Experimental

Materials

Synthetic lithium fluorohectorite (Li-FH) with the chemical
composition Li1,12{[Li1,12 Mg4,33](SigOzo)}F4.xH20 was obtained as a
gift from Dow Corning, Inc.. Li-Fluorohectorite has a 2:1 layered
structure consisting of two layers of silica tetrahedra sandwiching a central
layer of magnesium octahedra. Lithium ions replace some of the
magnesium ions in the octahedral sheet, generating a net negative charge.
This charge is then balanced by the Li+ ions inside the gallery. These latter
Li+ ions are solvated by water and are ion exchangeable, the cation
exchange capacity (CEC) is 150 mqu 100g.

The quaternary ammonium surfactant [(C16H33)(CH3)3N]+Br was
obtained from Kodak chemicals. Decylamine and TEOS were purchased
from Aldrich Chemical Co.

167

Porous clay fluorohectorite heterostructure synthesis.

To a 1 wt % suspension of Li+—FH was added a 0.3 M aqueous
solution of [(C16H33)(CH3)3N]Br in two-fold excess of the CEC value of
the clay. The suspension was stirred for 24 h at 50°C to ensure complete
ion exchange. Next, the resulting solid was centrifuged, washed repeatedly
with ethanol to remove the excess surfactant and then resuspended and
washed with water until free of halide ions. The pure product was
collected by centrifugation and air-dried at room temperature. The
resulting [(C16H33)(CH3)3N]+-fluorohectorite henceforth is noted
HDTMA+-Fluorohectorite.

Decylamine was added to HDTMA+-fluorohectorite in the molar
proportion HDTMA+-F1uorohectorite : decylamine = 1 : 20, and the
resulting suspension was stirred for 30 minutes. Tetraethylorthosilicate
was then added to achieve a final molar ratio of HDTMA+-FH : decylamine
: TEOS = l : 20 : 150. The mixture was vigorously stirred for 4 hours at
room temperature in a closed container. The reaction products were
recovered by centrifugation and air-dried in the open atmosphere.

As-synthesized PCH-fluorohectorite was then calcined at the desired
temperature in the range 200°C to 650°C for 4 h. i.e, ramped at the
corresponding temperature, then held at that temperature for 4 h.

Porous clay rectorite heterostructure synthesis.
To a 1 wt % suspension of alkali-metal-Rectorite was added a 0.3 M

aqueous solution of [(C15H33)(CH3)3N]Br in two-fold excess of the CBC

value of the clay. The suspension was stirred for 24 h at 50°C to ensure

168

complete ion exchange. Next, the resulting solid was centrifuged, washed
repeatedly with ethanol to remove the excess surfactant and then
resuspended and washed with water until free of halide ions. The pure
product was collected by centrifugation and air-dried at room temperature.
The resulting [(C16H33)(CH3)3N]+-rectorite henceforth is noted HDTMA+-
Rectorite.

Decylamine was added to HDTMA+-Rectorite in the molar
proportion HDTMA+-rectorite : decylamine = l : 20, and the resulting
suspension was stirred for 30 minutes. Tetraethylorthosilicate was then
added to achieve a final molar ratio of HDTMA+-Rectorite : decylamine :
TEOS = 1 : 20 : 150. The mixture was vigorously stirred for 4 hours at
room temperature in a closed container. The reaction products were
recovered by centrifugation and air-dried in the open atmosphere.

As-synthesized PCH-rectorite was calcined at 700°C and then
exposed to steam at different temperatures in the range of 400°C to 700°C
for 2 to 3 hrs.

Steaming Procedure and apparatus

Steam treatment was performed by using a iron steaming tube.
Samples placed in a tubular iron crucible were inserted in the steaming
vessel. It was then heated to the required temperature by using a
programmed heating coil. Water was boiled in a 1000 ml two neck round
bottom flask to produce steam which was introduced through one end of

the tube and collected at the other end in an ice trap.

169

Porous clay vermiculite-(aldrich) heterostructure synthesis

An inexpensive source of vermiculite, from Aldrich Chemical
packaging material, was used for pyridine chemisorption studies. The
flaky packaging material was first ground into a fine powder in a electric
grinder and then sieved through a 300 mesh mechanical sieve. The fine
powder obtained after sieving was saturated with Mg+2 ions by refluxing it
in a 2 M MgC12 solution for 48 hrs. The hot solution was cooled and
repeatedly washed with distilled water till free of excess Cl' ions. The
Mg+2-vermiculite was next ion-exchanged for HDTMA+ ions by using a
0.1N HDTMA+Br' solution with a final 1 wt% clay suspension. The
HDTMA+-vermiculite was washed with water and then with ethanol to
remove excess ion pairs from the clay structure. The pure product was
collected by centrifugation and air-dried at room temperature.

Decylamine was added to HDTMA+-vermiculite in the molar
proportion HDTMA+-vermiculite : decylamine = l : 20, and the resulting
suspension was stirred for 30 minutes. Tetraethylorthosilicate was then
added to achieve a final molar ratio of HDTMA+-vermiculite : decylamine
: TEOS = l : 20 : 150. The mixture was vigorously stirred for 4 hours at
room temperature in a closed container. The reaction products were

recovered by centrifugation and air-dried.

MCM-41 synthesis

MCM-41 was prepared following the synthetic procedures of Beck et
al. A 46.7 mmole specimen of a 29 wt% hexadecyltrimethylammonium

chloride solution was combined with 10 wt% TMA-silicate solution (41.5

170

mmoles) and 96.8 mmoles of HiSil 233 amorphous silica under stirring.
The reaction mixture was loaded in an autoclave and heated under
moderate stirring at 100°C for 36 hrs. The resulting solid was recovered
by filteration, washed with deionized water and dried in air at room
temperature. To remove the organic species occluded in the pores of the
as-synthesized MCM-41 samples were calcined in air at 660°C for 4 hrs at
the rate of 2°C/rnin.

Template removal.

Partial template removal was achieved by solvent extraction. The
solvent extraction was performed by using a soxhlet extractor with ethanol
as the extracting agent. Typically ~1g of the as-synthesized PCH-
fluorohectorite was extracted for ~24 h and the wet product was then

placed in the oven at 70°C for ~1 h and dried in open air.
Physical Measurements.

Powder XRD analysis was carried out with a Rigaku rotoflex
diffractometer using Cu-Ka radiation. The products were prepared as
powdered samples for X-ray analysis. The gallery height was obtained by
subtracting 9.6 A, the thickness of the clay unit layer, from the XRD (001)
basal spacing of the intercalated clays.

Nitrogen adsorption / desorption isotherms were determined on a
Omnisorb 360 CX (Coulter) sorptometer at liquid N2 temperatures by

using ultrahigh-purity N2 and He as adsorbate and carrier gas, respectively.

All samples were outgassed at 150°C under vacuum overnight. Surface

171

areas were determined with the BET6 equation. The method of Horvath
and Kawazoe7 was used to determine the pore diameters of the product.

FTIR spectra of PCH samples in the range 400-4000 cm'1 were
recorded by using a Nicolet IR/42 spectrometer. The samples, were
prepared as KBr wafers. FTIR spectra in the range 4000-2500 cm'1 and
FI'IR spectra for the adsorption of pyridine were obtained on an IBM IR44
spectrometer. Self supported pellets of sample (15 mg) were used. The
heating rate was 5°C/min and the compounds were calcined for 2 hours at
the choosen temperature under vacuum. The pyridine adsorption at 150°C
was performed by outgassing the pellets for 2 h at this temperature prior to
admitting pyridine. The pyridine was then removed by evacuation and by
heating the sample at 150°C for 2 H. All spectra were recorded at 150°C.

29Si MAS (magic angle spinning) N MR spectra were performed on a
Varian VXR 400 spectrometer. A Bruker probe was used to spin the
samples at 4 kHz. For the 29Si spectrum, a pulse duration of 9 us and a
delay time of 870 s allowed full relaxation of the Si nucleus. An external
reference of talc (8 = -98.1 ppm relative to tetramethylsilane) was used to
determined chemical shift values.

19F MAS-NMR spectra were obtained using a Doty probe with 35 s
delay time and a spinning rate of 7 kHz. The 19F chemical shifts are
relative to hexafluorobenzene (8(CFC13) = 164.9 ppm relative to
hexafluorobenzene).

Elemental analysis were carried out by inductively coupled plasma
emission spectroscopy at the University of Illinois Elemental Analysis

Laboratory.

172

Results and Discussions.
A. Thermochemical Alterations in PCH-fluorohectorite.

Figure IV.1 illustrates the X-ray powder diffraction patterns of
PCH-fluorohectorite formed by calcination at different temperatures for 4
hrs in air. All patterns exhibit three or more orders of (001) reflection
along with the presence of the clay (021) in-plane reflection at 196° 2-
theta (dozl = 4.5A). The dependence of d-spacing values on the calcination
temperature is summarized in the insert in figure IV.1.

It is observed that on calcining the material up to 200°C, the basal
spacing decreases from the initial value of 41.2A to 38.5.31. This decrease
in d-spacing results from the loss organic template molecules from the

structure

173

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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N
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U E.) g) 9 E) o
o 8 O O o O
N N M m V:- in 1
W
4 Fl
3
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63 h
5 DD
u:.a .1 %
Ira-El v
U
ugh ‘ 3
° 2
q
.:2 ‘29
a
.323 ' 3
M, t-
1 an
.:E i
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l l I l .
N 6 Q ‘ v N
V V M ("I n M J
100
(y) p
- )6
d
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\ -
p4

 

 

Kirsuarul 3.1931321

Figure IV.1 : X-ray diffraction pattern of PCH-Fluorohectorite calcined
at different temperatures for 4 hours in air. Insert : The corresponding
evolution of the d-spacing.

174

and from gallery silica dehydroxylation. On further heating the sample
between 200°C and 350°C, no additional change in d-spacing is observed.
However, on calcinating it above 350°C to 650°C the basal spacing is
reduced further by 4A up to 650°C calcination temperature. This
reduction in gallery height is further explained by IR spectroscopy, 29Si
MAS NMR and 19F MAS NMR data.

The oxide framework of the clay layers gives rise to several characteristic
vibrations in the mid-infrared region. For fluorohectorite, bands at 990
and 1111 cm'1 are characteristic of symmetric and asymmetric Si-O
stretching modes, respectively, and a band at 470 cm'1 is indicative of an
O-Si-O bending mode. The vibrations at 990 and 1111 cm'1 relating to
the stretching of the Si-O-Mg linkage are structure sensitive. The shoulder
at 530 cm'1 corresponds to the Mg-O stretching mode. The metal-
fluorine stretching vibration occurs at around 718 cm‘l.

As shown in Figure IV.2, all of these characteristic fluorohectorite
vibrations are observed for as-synthesized PCH-fluorohectorite.
Additional broad vibrational bands due to the Si-O stretching modes of the
gallery silica mesostructure are observed at ca. 1067 cm'1 (which is
overlapping with the clay layer Si-O stretching modes), a shoulder at 1200
cm'l, and two weak vibrations at 957 and 800 cm'l. These vibrations are
similar to those observed in mesoporous MCM-4l9 as well as hydrated
silica or fumed silica Cab-O-Sil which have linked SiO4 tetrahedra.10 The
vibrations due to the structure-directing amine and alkylammonium cation
surfactants are revealed by the bands at 2855 and 2924 cm'1 for the
symmetric and asymmetric stretching CH2 modes and at 1468 cm'1 for
CH2 scissoring mode. These surfactant vibrations disappear completely at

a calcination temperature of 300°C.

175

 

t——

 

 

M
(II
>
O
1

—_

a”.

 

500°C
450°C '

350° .
300° 1 I
250°

20°C

8?

 

Absorbance

 

 

 

 

 

 

1111I111111111111111111111111[11111

4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers (cm'l)

Figure IV.2 : Infrared spectras of PCH-Fluorohectorite calcined at
different temperatures for 4 h. The samples were prepared as KBr wafer.
No effort was made to remove physisorbed water.

176

Upon calcination (350°C) of the structure we observe a shift in the
symmetric and asymmetric Si-O stretching modes. The vibration at 990
cm“1 shifts to 1017 cm'1 and the one at 1111 cm'1 shifts to 1076 cm‘l. The
metal-fluorine stretching vibration gradually decreases in intensity and
shifts from 718 cm'1 to 703 cm'l. These shifts parallel the structural
changes in the clay layer structure that we observed in the NMR study, i.e.
hydrolysis of the layer by loss of fluorine and its replacement by hydroxyl
groups. At 450°C the asymmetric and symmetric Si-O stretching modes of
the clay layer merge into one broad stretching vibration at 1040 cm'l, also
indicative of some important changes in the structure at this temperature.

The OH stretching mode region in as-synthesized FH-PCH is masked
by the adsorption of molecular water at 3430 cm'1 and hydrogen bonds so
created. This region is observable for the as-synthesized and calcined
samples after dehydrating overnight under vacuum at room temperature
(Figure 1V.3 a). A vibration at 3696 cm'l, corresponding to the O-H
stretching of the Si-OH groups of the gallery silica mesostructure, is
retained until 150°C. The high energy shift upon removal of surfactant
shows that the silanol groups are hydrogen-bonded to the primary amine.
Calcining PCH-fluorohectorite at 250°C removes most of the remaining
organics and causes the Si-OH vibration to shift to higher frequency,
leaving a strong sharp band at 3740 cm'1 due to isolated Si-OH groups.
This Si-OH band at ~3740 cm'1 has been previously assigned to a weakly
acidic hydroxyll 1, capable of just hydrogen-bonding to basic adsorbates.12

It is noteworthy that a similar weak acidic hydroxyl (band at 3740
cm‘l) also is observed at 250°C in mesoporous MCM-41 (Figure 1V.3 b).
Yet, for PCH-fluorohectorite calcined at 250°C and 350°C two new O-H
stretching modes also appear in the region between 3450 and 3700 cm'l,

177

 

 

(a) PCH-fluorohectorite

--3300

 

.. 3362

 

 

 

Absorbance

 

 

—

1 1 1 l 1 1 1 1 1 1 1 I 1 1 1 l 1 1 1 1 1 1 1 l 1 1 1 1 1
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumbers (cm'l)

 

 

 

 

 

 

 

 

 

 

 

(b)MCM-41
8
= _100°C
,2 -5
‘5 E
in
1: I
<
350°C
_‘r’\./\__
111111111 1.1111111111111111.
4000 3500 3000 2500

Wavenumbers (cm' 1)

Figure 1V.3 : (a) Infrared spectra of PCH-Fluorohectorite calcined at
20, 100, 150, 250 and 350°C for 4 h. (b) Pure silica MCM-4l
mesostructure calcined at 20, 100, 150, 250 and 350°C temperatures.
Spectra were recorded after removing the adsorbed water under vacuum.
Self supported pellets were used.

178

frequencies which are not observed in mesoporous MCM—4l. Such low
frequency O-H stretching modes in aluminosilicates are usually attributed
to medium to strong Bronsted acid sites. For siliceous Y-zeolites two
vibrations were reported13 with a low frequency (LF) band at ca.
3560 cm‘1 due to the OH groups pointed in the B-cages and a high
frequency (HF) band at ca. 3630 cm'1 corresponding to hydroxyls
vibrating in the supercages. In PCH-fluorohectorite there is no aluminium
in the structure, so we might explain these vibrations as originating from
Mg-OH-Li groups (due to the hydrolysis of the layer) in very strong
interactions with Si-OH groups of the gallery silica mesostructure. These
bands might also be due to Mg-OH-Si groups formed by protonated Mg-O-
Si groups of the layer by protons formed in the decomposition of the
quaternary alkylammonium cations. Thus, three types of acidic hydroxyls
are detected by infrared in PCH-fluorohectorite at 350°C calcination
temperature. At 650°C the intensity of these vibrations decreases
dramatically due to the condensation of silanol groups giving Si-O-Si bonds
and dehydration of the clay layer.

29Si MAS NMR Spectroscopy

The thermochemical alterations occurring for the calcined PCH-
fluorohectorite derivatives were studied by 29S1 MAS-NMR spectroscopy
(see Figure 1V.4). As-synthesized air-dried PCH-fluorohectorite shows
three silica resonances, the first being characteristic of the silicon
environment of the fluorohectorite layers, i.e., a Si(OSi)3(OMg) Q3 site at
-93.3 ppm. The higher field peaks are due to the gallery-templated silica
framework. The gallery silica mesostructure consists of (SiO)3SiOH
groups (Q3 at -103 ppm) and of completely crosslinked Si(OSi)4 units (Q4
at -110 ppm).

179

 

 

>1
«H
’53
G
3
.E
Q)
.2

E ,1 450°C

53 / 350°C

o-oc

250°C

200°C

A 20°C

I I j l l lJ l l l l l I l l

-70 -80 -90 -100 -110 -120 -130 -140
1’1"“

 

 

 

Figure 1V.4 : 29Si MAS-NMR spectra of PCH-Fluorohectorite calcined at
different temperatures for 4 h. Chemical shifts are relative to TMS.

180

The Q3 resonance of the fluorohectorite layers shifts progressively
from -93.3 ppm to -96.6 ppm upon calcination at temperature to 350°C,
which suggests a widening of the Si-O-Si bond angle.14 The line width of
the resonance remains unchanged from 20°C to 250°C, but it starts
increasing upon further calcination. This increase in the line width may
indicate a dispersion of isotropic chemical shifts due to structural disorder
or to unaveraged dipolar interactions of 29Si nuclei with other N MR active
nuclei, particularly 1H. The resonances due to the gallery silica
mesostructure are not greatly affected in this range of temperature

calcination.

Host layer

tetrahedra

Gallery silica
mesostructure

 

Figure IV.5 : Schematic of crosslinking between the gallery silica
mesostructure and the layers showing inversion of some silica
tetrahedra of the layer.

181

Further interesting changes in the 2981 MAS NMR spectrum take
place for PCH-fluorohectorite calcined at 450°C to 550°C. The line width
of the Q3 peak due to the clay layer gradually increases and finally gives
rise to a second resonance at higher field (~99 ppm) with almost equivalent
intensity. A new Q4-like environment is created for almost half of the
silica of the clay layer. This can be explained by two mechanisms, either
by the dehydroxylation of the clay layers or by a formation of Si-O-Si
linkages between the silicate sheet and the silica tetrahedra of the
intragallery nanostructure during condensation, which is the favored
explanation in our case. In fact Plee15 et al. and Pinnavaial6 et al. suggest
the formation of Si-O-Al linkages between the pillars and the layers in
alumina-pillared clays. This hypothesis is also supported in our system by
the 4 A decrease in the basal-spacing over the temperature range of 450-
550°C. In addition, the Q3 resonance of the gallery silica mesostructure is
transformed into a Q4 structure owing to the silanol groups condensation.
As shown in Figure IV.5, we proposed a model for layer cross-linking in
PCH, suggesting that some silicon tetrahedra in the clay layers are first

inverted before linking to the gallery silica mesostructure.
l9F MAS NMR Spectroscopy

To better understand the structural changes occuring upon
calcination, an effort was also made to examine the structural fluorine of
the clay layers using 19F MAS NMR spectroscopy. The 19F MAS NMR
spectrum of a natural fluorine-containing hectorite has been reported by
Huve17 et al. to exhibit two resonances, and this has been confirmed and

explained by Butruille4 et al. for alumina-pillared fluorohectorite. Two

182

non-equivalent environments at —10 and —16 ppm have been observed with
relative intensities that are determined by the composition of the clay
layers. In fluorohectorite all tetrahedral sites are occupied by silicon and
the octahedral sites are occupied by Mg2+ and Li+. The fluorine atoms
bridge three octahedral metal positions, which they referred to as
"octahedral triads". Each octahedral triad consists of one Ml-type
octahedron in which the fluorine atoms are arranged at trans positions and

two M2-type octahedra

 

 

Figure 1V.6 : Representation of the octahedral sheet in fluorohectorite
(see text). Fluorine atoms are represented by white or black circles,
depending whether they are above or below the layer plane.

with the fluorines at cis positions (see Figure 1V.6). The upfield line at
-16 ppm is assigned to the fluorine atoms in electrically charged triads
with

183

Gov unauauonfiuh.
a? cum :3 can can :2 cu

 

06661

 

 

 

fiuuu&-ddfiudqddud11.1-1ddddqd c

5523—.
n:

mm. on- hm.

 

2-

 

J 1 1 - i I I - I I

 

 

Figure 1V.7 : 19F MAS-NMR spectra of FH-PCH calcined at different

temperatures.

 

 

184

Li+ in a Ml site and two Mg2+ ions in M2 sites. The downfield line at
-10 ppm is attributed to the fluorines of neutral triads with M1 and M2
sites filled by Mg2+. The relative intensities of 19F resonances depend on
layer charge density. In fluorohectorite, the low-field/high-field peak
intensity ratio was 0.90, as expected for a OzoF4 unit cell with a net
negative charge of 1.12.

The air-dried PCH-fluorohectorite exhibits the same two 19F
resonances as described above (see Figure 1V.7). Upon increasing the
calcination temperature from 250°C to 450°C, the intensity of the high-
field resonance of FH-PCH decreases to ~25% of its initial value. Similar
results were obtained by Butruille4 et al. for the alumina-pillared
fluorohectorite. This loss of structural fluorine was attributed to
hydrolysis of lattice fluorine. A plausible pathway for the hydrolysis of
the fluorine in electrically charged triads is the interlayer transport of
water to the basal hexagonal cavities of the tetrahedral sheet, below which
the fluorine atoms are located, the hydrolysis being catalyzed by protons,
coming from the degradation of the quaternary ammonium cations for
PCH-fluorohectorite. We also note that a similar hydrolysis occurs for the
hexadecyltrimethyl ammonium exchanged form of fluorohectorite
(HDTMA+-Fluorohectorite), while no hydrolysis is observed for the Lil’-

exchanged form of fluorohectorite (LR-Fluorohectorite).
BET surface area measurements and pore size distribution.
Nitrogen adsorption-desorption experiments were performed on FH-

PCH calcined at different temperatures (see Figure IV.8 a & b). The

surface areas were calculated by using the BET equation for multilayer

185

surface coverage by the adsorbate molecule. The method of Horvath and
Kawazoe was adapted to determine the pore size distribution of the
products after calcination at 300, 350, 450, 500 and 550°C. The surface
area and pore size distribution for the samples are summarized in Table
IV.1. The BET surface area for the FH-PCH progressively increases with
the calcination temperature up to 550°C. For FH-PCH-300 the BET
surface area is 570 m2/g while PCH-fluorohectorite calcined at 500°C
exhibits a BET surface area of 750 m2/g. The pore size for the calcined
product also

Table IV.1 : Gallery heights, BET surface area, Horvath and
Kawazoe pore sizes, carbon and nitrogen content determined by chemical
analysis of PCH-fluorohectorite derivatives calcined at different

temperatures.

Calcination Gallery SBEI‘ Pore Carbon Nitrogen

 

Temperature Heights (m2/g) diameter per

I (°C) (A) (A) (020 unit (020 unit
cell)

300 18.9 1 1.6

19.6 8.4

22.3 6.0

21.7 0.9

 

 

the fluorohectorite clay layers

  

* - lery eight rs 0e e as e observed -ray o—asspacrng minus '1. ‘ c ess

186

 

 

 

 

 

 

 

 

 

 

 

700
600 ' a:
' '6
\
B
'5
500:
5: 1....W11
"’ ’ 3 (D)
H
a
’63
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3 (C)
‘5
0
1G
I-
8 300
3 ’ (B)
0
E
2
§
200 ’ (A)
1
1
100
7 1 1 ' l 1 1 1 l 1 1 A I 1 1 1_ l 1 1 1
0o 0.2 0.4 0.6 0.8 1

Relative Pressure (P/Po)

Figure IV.8 : Nitrogen adsorption/desorption isotherm for PCH-
Fluorohectorite calcined at different temperatures ranging from 300 to
550°C. Insert : The correspoding Horvath-Kawazoe pore size distribution
curves of PCH-Fluorohectorite calcined between 300°C to 550°C.

187

increases with the calcination temperature, from 19 to 23 A for FH-PCH-
300 and FH-PCH-SSO, respectively. This increase in pore size and surface
area is explained by the more efficient removal of carbon residues from

the pore structure with calcination temperature (see Table IV.1).

Template Removal.

Figure 1V.10 shows the X-ray powder diffraction patterns of as-
synthesized PCH-fluorohectorite, ethanol extracted, ethanol-extracted,
calcined PCH-fluorohectorite prepared by using HDTMAfldecylamine as
template and TEOS as neutral silica precursor, at a reaction stoichiometry
of 1 : 20 : 150 with HDTMA+-Fluorohectorite : decylamine : TEOS
respectively.

The XRD data for the as-synthesized PCH-fluorohectorite shows
multiple (001) reflection which are preserved even after ethanol extraction
of the material, indicating that the structure is preserved. However after
calcination of the ethanol-extracted sample, a significant broadening of the
corresponding X-ray powder diffraction pattern was observed. It is very
important to note that calcination resulted in almost complete
decomposition of the structure. This indicates that prior to calcination,
partial removal of the template by ethanol extraction results in incomplete
crosslinking of the gallery nanostructure; leading to collapse of the gallery.

Figure IV.11 shows the thermo gravimetric analysis of the as-
synthesized, ethanol-extracted, and ethanol-extracted, calcined PCH-
fluorohectorite prepared from HDTMAfldecylamine and TEOS. Several

weight loss features for the as-synthesized PCH-fluorohectorite materials

188

 

   

 

Relative Intensity

 

 

 

 

 

(a)

' (b)
M (C)
1|11l|1|||||11L111111

5 10 15 20

Two Theta (degrees)

Figure 1V.10 : X-ray diffraction pattern of PCH-Fluorohectorite
prepared using HDTMAfldecylamine template and TEOS at a molar ratio
of 1 : 20 : 150.

(a) As-synthesized PCH-fluorohectorite.

(b) Ethanol-extracted PCH-fluorohectorite.

(c) Ethanol-extracted and calcined PCH-fluorohectorite.

189

can be observed. The TGA curve shows a five step weight loss for the as-
synthesized PCH. The first step of 1.5% from 25°C to 140°C corresponds
to loss of physisorbed water, the weight loss of 16.5% that occurs until
360°C and the weight loss of 14% up to 460°C are due the decomposition

of the organic templates.

 

Q
G

(3)

Weight loss (%)
co on
O U!

\l
UI

Q
Q

(A)
6 5

 

I‘ll-lllllllelelllllLllIIJIIIIJIALLA

60 ‘
100 150 200 250 300 350 400 450 500
Temperature (°C)

Figure IV.ll : Thermal gravimetric analysis of PCH-fluorohectorite
prepared from decylamine-solvated HDTMA+-
Fluorohectorite/'1' EOS.
(A) As-synthesized PCH-fluorohectorite
(B) Ethanol-extracted PCH-fluorohectorite.

190

However, in contrast the TGA curve of the ethanol-extracted PCH-
fluorohectorite ( as shown in figure 1V.11 b) clearly differs from that of
the as-synthesized material in the temperature region between 160°C to
500°C. The final weight loss of 1.9% between 460-600°C is related to
water losses via condensation of silanol groups to form siloxane bonds.
The similar kind of weight loss curve has been observed in MGM-41. The
weight loss curve for the as-synthesized PCH-fluorohectorite shows a two
step weight loss for the decomposition of the organic template between
160°C to 500°C. The first step is most probably due to decomposition of
the neutral amine co-template from the structure and the second weight loss
at ~360°C is due to decomposition of the cationic surfactants. The TGA
profile for the ethanol-extracted PCH shows a plateau up to a temperature
of ~350°C, which indicates the absence of any neutral amine in the
structure. However due to the presence of the charge balancing surfactant
the second weight loss arises at ~ 360°C due to the thermal decomposition
of the cationic surfactants in the clay galleries. Figure 1V.12 illustrates the
N2 adsorption/desorption isotherms and the corresponding H-K pore size
distribution curves for the as-synthesized, calcined PCH-fluorohectorite
and the ethanol-extracted, calcined material. Both, isotherm and pore size
distribution curves differ markedly from each other. The calcined PCH-
fluorohectorite adsorption isotherm shows an adsorption behaviour
characteristic for a nanoporous adsorbent, with a linear portion in the
adsorption curve at a partial pressure region of 0.02-0.25, also indicating
the presence of nanopores. Indeed, the H-K pore analysis with a pore size

maxima at ~21A confirms the presence of these pores.

l9]

 

 

400

H & K pore size distribution

350

dW/dR

300

 

 

 

 

 

l 1 . 5 2 2 . 5 3
Pore diameter (nm)

250

200

150

Volume adsorbed (cm’lg) at STP

 

 

I l I I I I I I I I I I I I I I I I I I I I I l I I I I I I I

 

100

50

 

 

l l J I 4 L L 1 l L L

 

0 o 0.2 0.4 0.6 0.8 1
Relative Pressure (P/Po)

Figure 1V.12 : Nitrogen adsorption/desorption isotherm for PCH-
Fluorohectorite (a) calcined at 650°C and (b) Ethanol-extracted and
calcined at 650°C Insert: The correspoding Horvath- Kawazoe pore size
distribution curves of PCH-Fluorohectorite.

The ethanol-extracted PCH-fluorohectorite shows a type II isotherm where
the majority of the porosity is due to the presence of textural pores and not
due to uniform crystallographic intra-gallery pores. The H-K pore size
distribution confirms the absence of gallery nanopores.

In conclusion it can be stated that calcination is probably the best
approach to template removal for PCH materials. The partial removal of
template prior to calcination results in the decomposition of the gallery
structure. The X-ray diffraction data and TGA analyses clearly show that
ethanol extraction for PCH materials does not lead to highly ordered and
thermally stable heterostructures. In other words; the presence of the
template during calcination helps in the cross-linking of the gallery

nanostructur e.

B. Hydrothermal stability of PCH-rectorites.

The mineral rectorite has been known to be a good cracking catalyst;
it exhibits high hydrothermal stability. Hence our rationale in using
rectorite for the synthesis of porous clay hetrostructures was to obtain a
nanoporous cracking catalyst with high hydrothermal stability that would

enable shape-selective processing of heavy crude oils.
X-ray powder diffraction.
Figure 1V.13 illustrates the X-ray powder diffraction patterns of

PCH-rectorite exposed to steam at different tempeartures. It is observed

that PCH-rectorite shows high steam stability by maintaining its initial

193

gallery height up to 650°C. However, the structure collapses at 700°C
hydrothermal temperature.

BET surface area measurement and pore size distribution.

Nitrogen adsorption-desorption experiments were performed on
PCH-rectorite exposed to steam at different temperatures (see figure
IV.14). The surface areas were calculated by using the BET equation for
multilayer surface coverage by the adsorbate molecules. The method of
Horvath Kawazoe was adopted to determine the pore size distribution of
the products after hydrothermal treatments. The surface areas and pore
size distribution for the samples are summarized in table IV.2. The BET
surface area and H-K pore size for the PCH-rectorite is unaltered up to a
temperature of 600°C, however steam treatment of the sample beyond
600°C results in the decrease of the BET surface area and loss of the
uniformly stable pore size of 2151. These results are consistent with the

X-ray diffraction results.

Table IV.2 : BET surface area, Horvath and Kawazoe pore sizes of

PCH-rectorite after h drothermal treatment at different temperatures.

1

 

 

Hydrothermal Gallery SBEI‘ H&K
Temperature Heights (m2 / g) pore
(°C) (A) size (A)
400 2 l 410 2 l
500 20 400 20 3
600 20 400 20 E
700 1 l 130 - I

bhudefediaS—mherveflr "721?"-9'190131
thickness of the rectorite clay layers.

I94

 

 

 

 

 

 

 

 

Unsteamed (Calcined at 700°C)
43.1}.
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H
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fl
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E
y—
0
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a
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0 38A
a
(B)
C
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II —_~
LllLlLllllLlLllellllj[ll141111111
l 2 3 4 5 6 7 8

Two Theta (degree)

Figure IV.13 : X-ray diffraction pattern of PCH-rectorite prepared
using HDTMA+ldecylamine template and TEOS at a molar ratio of 1 : 20 :
150, calcined at 700°C prior to hydrothermal treatment at different
temperatures. (A) 400°C steaming, (B) 500°C steaming, (C) 600°C
steaming and (D) 700°C steaming.

195

Nitrogen Adsorption - Desorption Isotherms

3&7”-

MM

2%

fl”

RM

Volume adsorbed (cm3 / gm)

 

100-

 

 

 

0 a2 a4 a6 as I
Relative Pressure (P / P0 )

Figure IV.14 : Nitrogen adsorption/desorption isotherm for PCH-
rectorite (A) calcined at 700°C and (B) Calcined at 700°C and steamed
treated at 600°C and (C) calcined at 700°C and steam treated at 700°C
Insert : The corresponding Horvath-Kawazoe pore size distribution curves
of PCH-rectorite at different hydrothermal temperatures.

196

C. Enhanced acidity of porous clay heterostructures.

In order to more cogently demonstrate the acidity of calcined PCH
compositions; we investigated the pyridine chemisorption properties of
these materials. The chemisorption of pyridine at 150°C as monitored by
FTIR is shown in figure IV.15. PCH-vermiculite calcined at 650°C in air
for 4 hrs and then outgassed at 350°C under vaccum overnight reveals two
new hydroxyl vibrations at 3627 and 3573 cm'1 in addition to a strong
3740 cm"1 band due to isolated Si-OH stretching vibrations from the
gallery silica mesostructure. After exposure several pyridine ring
stretching vibrations in the mid infrared region were observed at 1447,
1491, 1547, 1614 and 1639 cm-1 for PCH-vermiculite. These bands
revealed the presence of both Bronsted bound pyridine and Lewis and
hydrogen bonded pyridine. However in the silica MCM-41, sample the two
main bands of pyridine ring stretching observed were at 1445 cm'.1 and
1597 cm'l, which corresponds to hydrogen bonded pyridine only. There
was no evidence for a band at 1540 cm“1 which would indicate Bronsted
acidity and the 1440 cm‘1 band shift is not large enough to indicate
appreciable Lewis acidity. In the silica MCM-4l the decrease in the
intensity of the 3740 cm‘1 Si-OH vibration upon pyridine adsorption
confirm the presence of hydrogen bonded pyridine and the disappearance
of almost all the pyridine stretch bands by 150°C degassing temperature
indicates that the pure silica mesostructure posesses virtually little or no
acidity. In PCH-vermiculite, the presence of several pyridine stretching
vibrations in the 1400-1700 cm'1 region confirms the presence of both
Bronsted and Lewis acidity, and also after degassing at 300°C, the retention

of the pyridine stretches indicates the strength of these acid sites.

197

Figure IV.15 : Infrared spectra of pyridine adsorbed on PCH-vermiculite
calcined at 650°C and silica MCM-41 calcined at 650°C.pyridine-adsorbed
sample was evacuated at 150°C.

198

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Metal ion substitution during synthesis of PCH materials

The isomorphous substitution of Si by Ti during the synthesis of
zeolites with MFI and MEL structures has generated a new family of
titanium containing zeolites named as TS-l and TS-2, respectively. These
zeolites are able to perform the selective oxidation of organic compounds
such as alkanes, alkenes, and alcohols by H202, The range of organic
compounds that can be oxidized is greatly limited, however by the
relatively small pore size (about 6 nm) of the host framework. The recent
discovery of large pore silica-based molecular sieves by Kresge et al. has
broken the previous pore size barrier of zeolites and opened up new
avenues for selective catalysis. There have been recent reports in the
literature on the synthesis of silica-based molecular sieves, partly
substituted by titanium, aluminum, chromium etc. and a range of other
metal ions. Here we report initial attempts at forming a silica-templated
nanostructure partly substituted with aluminum, and/or titanium metal ions
inside the galleries of a 2:1 layered silicate. While the results obtained do
not support significant metal ion substitution, even at low metal ion

concentration some interesting structural changes are observed.

Ti-ion substitution.

To a 1 wt % suspension of Li+-Fluorohectorite was added a 0.3 M
aqueous solution of [(C16H33)(CH3)3N]Br in two-fold excess of the CEC
value of the clay. The suspension was stirred for 24 h at 50°C to ensure
complete ion exchange. Next, the resulting solid was centrifuged, washed

repeatedly with ethanol to remove the excess surfactant and then

200

resuspended and washed with water until free of halide ions. The pure
product was collected by centrifugation and air-dried at room temperature.
The resulting [(C16H33)(CH3)3N]+-fluorohectorite henceforth is noted
HDTMA+-Fluorohectorite.

Decylamine was added to HDTMA+-fluorohectorite in the molar
proportion HDTMA+-Fluorohectorite : decylamine = 1 : 20, and the
resulting suspension was stirred for 30 minutes. In a separate reaction
vessel, a mixture of tetraethylorthosilicate and titanium iso-propoxide was
mixed at Ti/Si molar ratios that ranged from 0.001 to 0.1. These mixtures
were stirred for 10 minutes in a closed container and then added to the
amine-solvated HDTMA+-fluorohectorite to achieve a final molar ratio of
HDTMA+-fluorohectorite : decylamine : mixed metal alkoxide = 1 : 20 :
150, respectively. The mixture was stirred for 30 minutes at room
temperature in a closed container. The reaction products were recovered
by centrifugation and air-dried. The as-synthesized Ti substituted PCH
derivatives were then ramped to 650°C for 4 h, ata ramp rate of 2°C/min
and heated at that temperature for 4 h.

Figure IV.16 illustrates the X-ray powder diffraction patterns for
both the as-synthesized and calcined PCH-fluorohectorite prepared using
different Ti/Si molar ratios. The as-synthesized materials exhibits multiple
(001) reflections indicating the presence of a layered structure, however the
gallery heights for the as-synthesized Ti substituted derivatives are
substantially smaller than the initial pure silica-templated PCH-
fluorohectorites. It must be noted here that all the Ti substituted materials
require at least 14 days of drying time and the materials obtained after

drying were hard and difficult to grind.

201

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure IV.16 : X-ray diffraction powder patterns for as-synthesized and
calcined PCH-fluorohectorite prepared using different Ti/Si molar ratios.
(A) Ti/Si molar ratio = 0.001, (B) Ti/Si molar ratio = 0.01, (C) Ti/Si
molar ratio = 0.1.

202

 

 

 

 

 

 

 

 

 

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msuatul 9.1133193

Figure IV.17 : X-ray diffraction powder patterns for as-synthesized and
calcined PCH-fluorohectorite prepared using different AI/Si molar ratios.
(A) Al/Si molar ratio = 0.1, (B) Al/Si molar ratio = 0.01.

203

Covalent grafting of Ti on the hydroxyl groups lining the

pore walls of PCH-fluorohectorite.

The recent advances in mesoporous molecular sieves 3'12 have been
at the forefront of research in the field of nanoporous materials design.
The high surface areas (ranging from 600 m2/g to 1200 m2/g) and uniform
pore structures (pore diameters ranging from 20 to 100A) associated with
them, have made these materials promising catalysts or catalyst supports.
They are highly suitable for reactions involving molecules with large
kinetic diameters. One route to obtaining such catalysts has been through
the grafting of organic units on to the walls of MCM-41. Recently, heavy
metal-ion adsorbents have been developed based on the covalent grafting of
chelating functionalities to the pore wall hydroxyls of a mesoporous
molecular sieve.

Here we report the covalent grafting of Ti on the inner lining of the
intra-gallery nanostructure of porous PCH-fluorohectorite. We hoped that
such a synthetic approach could allow for isomorphous grafting of Ti
metal-ions in the structure of PCH and thereby generate a highly efficient
bifunctional catalyst.

Synthesis.

To a 1 wt % suspension of Li+-Fluorohectorite was added a 0.3 M
aqueous solution of [(C16H33)(CH3)3N]Br in two-fold excess of the CEC
value of the clay. The suspension was stirred for 24 h at 50°C to ensure
complete ion exchange. Next, the resulting solid was centrifuged, washed

repeatedly with ethanol to remove the excess surfactant and then

204

resuspended and washed with water until free of halide ions. The pure
product was collected by centrifugation and air-dried at room temperature.
The resulting [(C15H33)(CH3)3N]+-fluorohectorite henceforth is noted
HDTMA+-Fluorohectorite.

Decylamine was added to HDTMA+-fluorohectorite in the molar
proportion HDTMA+-Fluorohectorite : decylamine = 1 : 20, and the
resulting suspension was stirred for 30 minutes. Tetraethylorthosilicate
was then added to achieve a final molar ratio of HDTMA+-FH : decylamine
: T EOS = 1 : 20 : 150. The mixture was vigorously stirred for 4 hours at
room temperature in a closed container. The reaction products were
recovered by centrifugation and air-dried in the open atmosphere. As-
synthesized PCH-fluorohectorite was then calcined at 650°C for 4 hours
using a ramp rate of 2°C/min. 1.0 g of freshly calcined material was then
dried at 110°C under 10'3 torr vaccum and refluxed with titanium iso-
propoxide (ranging from 0.45 g to 1.0 g of Ti-alkoxide / 1.0.g of PCH-
fluorohectorite) in 25 ml of dry toluene for 24 hrs. The resulting material
has covalently grafted titanium on the hydroxyl groups that line the PCH
pore walls. The functionalized product (hereafter denoted as Ti-PCH) was
filtered, washed several times with toluene followed by ethanol and then
dried at 75°C for 1 h.

Figure IV.18 illustrates the X-ray powder diffraction patterns for
the freshly calcined PCH-fluorohectorite, and shows multiple (001)
reflections and a gallery height of ~22.0A. Upon Ti grafting the basal
spacing of the material changes by <2.0A; a slight decrease in the intensity
of the (001) peaks is observed. This decrease in crystallinity is probably
due to some perturbation of the structure by the grafting reaction, however

the intra-gallery nanostructure remains intact

205

Calcination of the as-synthesized materials at 650°C for 4 h gave
powder diffraction patterns with a weak reflection corresponding to a basal
spacing of ~12.7A. This reflection is characteristic of a silica-intercalated
derivative with no intra-gallery nanostructure. BET surface areas for the

materials ranged between 100 to 150 mZ/gm.
Al-ion substitution.

To a 1 wt % suspension of Li+-F1uorohectorite was added a 0.3 M
aqueous solution of [(C16H33)(CH3)3N]Br in two-fold excess of the CEC
value of the clay. The suspension was stirred for 24 h at 50°C to ensure
complete ion exchange. Next, the resulting solid was centrifuged, washed
repeatedly with ethanol to remove the excess surfactant and then
resuspended and washed with water until free of halide ions. The pure
product was collected by centrifugation and air-dried at room temperature.
The resulting [(C16H33)(CH3)3N]+-fluorohectorite henceforth is noted
HDTMA+-Fluorohectorite.

Decylamine was added to HDTMA+-fluorohectorite in the molar
proportion HDTMA+-Fluorohectorite : decylamine = 1 : 20, and the
resulting suspension was stirred for 30 minutes. In a separate reaction
vessel, mixture of tetraethylorhosilicate and aluminum tributoxide was
mixed at Al/Si molar ratios that ranged from 0.01 to 0.1. These mixtures
were stirred for 10 minutes in a closed container and then added to the
amine-solvated HDTMA+-fluorohectorite to achieve a final molar ratio of
HDTMA+-fluorohectorite : decylamine : mixed metal alkoxide = l : 20 :
150, respectively. The mixture was stirred for 30 minutes at room

temperature in a closed container. The reaction products were recovered

206

by centrifugation and air-dried. The as-synthesized Al substituted PCH
derivatives were then ramped to 650°C for 4 h, at a ramp rate of 2°C/min
and heated at that temperature for 4 h.

Figure IV.17 illustrates the X-ray powder diffraction patterns for
both the as-synthesized and calcined PCH-fluorohectorite prepared using
different A1/Si molar ratios. The as-synthesized materials exhibit multiple
(001) reflections indicating the presence of a layered structure, however the
gallery heights for the as-synthesized Al substituted derivatives are
substantially smaller than the initial pure silica-templated PCH-
fluorohectorites. It must be noted here that all the Alsubstituted materials
require at least 10 to 14 days of drying time and the materials obtained
after drying were hard and difficult to grind.

Calcination of the as-synthesized materials at 650°C for 4 h gave
powder diffraction patterns with a weak reflection corresponding to a basal
spacing of ~12.7A. This reflection is characteristic of a silica-intercalated
derivative with no intra-gallery nanostructure. BET surface areas for the
materials ranged between 100 to 150 m2/gm.

The above results suggest that the direct incorporation of metal ions
inside the gallery framework structure leads to the formation of a collapsed
mixed metal oxide derivative with no intra-gallery porosity. This can be
explain due to the difference in the rate of hydrolysis of TEOS and Ti-iso
propoxide or Al-tributoxide or some kind of chemical reaction with the

basic alkylamines.

207

 

600

 

500
(A)

dW/dR

k (B)

v

n l 1 I n L I A n n 1 l 1 1 n
l l . 5 2 2 . 5 3
Pore diameter (nm)

 

 

 

 

 

 

Volume adsorbed (cm3/g) at STP

 

 

 

300
200 B/
fl/r
100
0 1 1 1 l 1 1 1 l 1 1 1 1 1 1 1 l 1 1 1
0 0.2 0.4 0.6 0.8 1

Two Theta (degrees)

Figure IV.19 : N2 adsorption/desorption isotherms and the
corresponding H-K pore size distribution for (A) PCH-fluorohectorite,
calcined at 650°C. (B) Ti-PCH with Ti-alkoxide cone of ~0.45g/g of PCH-
fluorohectorite.

208

It is important to note that the diffraction patterns of Ti-PCH at higher Ti-
alkoxide concentration are similar to those prepared at Ti-
alkoxide=0.45g/g of PCH-fluorohectorite.

Figure IV.19 illustrates the N2 adsorption/desorption and the
corresponding H-K pore size distributions for the calcined PCH-
fluorohectorite and Ti-PCH prepared at a Ti loading of 0.45g/1 g of PCH-
fluorohectorite. The BET surface area for PCH-fluorohectorite was 850
m2/g and decreased to 790 m2/g for Ti-PCH. Here again the BET surface
areas remained constant at higher Ti loadings. The H-K pore size
distribution gave pore diameters of 21A and 18A for the calcined PCH-
fluorohectorite and Ti-PCH, respectively. These observations suggests the
presence of a significant amount of grafted titanium species attached to the
pore walls of PCH, causing constriction of the intra-gallery pore channels,
yet conserving the porosity of the grafted materials.

Catalytic evaluation of Ti-grafted PCH materials could be one an

areas for future investigation.

209

Porous clay heterostructure formation using low charge
density smectites.

Attempts were made to effect PCH syntheses with low charge density
smectites such as Wyoming Montmorillonite, Chinese Beidellite and
Arizona Montmorillonite.

Figure IV.20 illustrates the X-ray powder diffraction patterns for
the alkali metal-ion, and the corresponding quaternary ammonium
exchanged (Q+-clay). The XRD patterns for all Na+-clays show a
diffraction peak with a basal spacing of ~12.1A, which on exchanging with
the HDTMA+ ion increases to a higher value depending on the exchange
capacity of the clay. Mixtures of hydrated HDTMA+-clay, decylamine and
TEOS at the molar ratio of 1 : 20 : 150, respectively, were allowed to react
for 4 h at ambient temperature. The resultant intercalates were then
centrifuged, and dried in open air to give the as-synthesized products.
Removal of the organics by calcination at 500°C in air for 4 h gave the
final calcined material. Figure IV.21 illustrates the X-ray diffraction
patterns for the air dried and the calcined materials synthesized from
Wyoming montmorillonite, Chinese beidellite and Arizona
montmorillonite. The diffraction patterns for the air dried PCH-beidellite
showed a broad (001) reflection with a gallery height of ~20A. Both PCH-
montmorillonite samples exhibited gallery heights of ~9A, significantly
smaller than observed for the beidellite system. The gallery heights of the
as-synthesized PCH materials from low charge density smectites was found
to be significantly smaller than those of the corresponding high charge
density smectite such as fluorohectorite. Calcination of the air dried
sample at 650°C yields materials with a collapsed structure and a basal

spacing of ~12.7A.

210

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

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msuagul axyepx

Figure IV.20 : X-ray diffraction powder patterns for alkali metal-ion
exchanged and their corresponding quaternary ammonium exchanged Q+-
clay.(A) montmorillonite (Arizona). (B) beidellite(Chinese). (C)
montmorillonite (Wyoming).

211

 

31.513.

 

 

Relative Intensity

 

 

 

 

 

 

 

15.7521
30.00.31
(A)
(B)
(C)
l 1 1 1 1 l L 1 1 1 l 1 1 1 1 l
5 10 15 20

Two Theta (degrees)

Figure IV.18 : X-ray diffraction powder patterns for freshly calcined
and Ti grafted PCH-fluorohectorite prepared using different Ti loadings.
(A) Freshly calcined PCH-fluorohectorite. (B) Ti-PCH with Ti-alkoxide
conc of ~0.9g/g of PCH-fluorohectorite. (C) Ti-PCH with Ti-alkoxide
loading of ~0.45g/g of PCH-fluorohectorite.

212

These results suggests that an appropriate layer charge-compensating
ratio is critical for the successful formation of a gallery nanostructure. In
montmorillonites and beidellites, due to the low cation exchange capacity,
the amount of quaternary ammonium ions intercalated inside the smectite is
lower than that in the high charge density fluorohectorite system. In
addition, natural clays exhibit charge heterogeneity which results in a non-
uniform distribution of the intercalated ions. Thus the template
concentration per cross-sectional area fluctuates across the layers,
hindering formation of the co-operative micellar assembly process.

We attempted to overcome the shortcomings of low charge density
smectites by using the dialkyl quaternary ammonium ion and neutral
decylamine co-surfactants. We assumed that the dialkyl long chains would
occupy more space in the clay galleries and thereby stabilize the organic
nanotemplate, which would then enhance gallery nanostructure formation.
However the results clearly suggest an absence of any gallery structure.

Two dialkylchain lengths were used for PCH synthesis, namely
Didecyldimethylammonium (denoted as DDDMA+) and
dioctadecyldimethylammonium (denoted as DODDMA+). Decylamine was
used as co-template and TEOS as the source of a neutral inorganic
precursor. To a 1 wt % suspension of Na+—clay was added a 0.3 M
ethanolic solution of dialkyldimethylammonium bromide in two-fold excess
of the CEC value of the clay. The suspension was stirred for 24 h at 50°C
to ensure complete ion exchange. Next, the resulting solid was centrifuged,
washed repeatedly with ethanol to remove the excess surfactant and then
resuspended and washed with water until free of halide ions. The pure
product was collected by centrifugation and air-dried at room temperature.

The resulting organo+-clay henceforth is noted Q+-clay. Figure IV.22

213

illustrates the X-ray powder diffraction pattern for the dialkyl quaternary
ammonium exchanged (Q+-clay) product. Both the DDDMA+ and
DODDMA+-montmorillonite exhibit multiple (001) reflections, indicative
of a well-ordered structure along the c-axis. The gallery heights for the
DDDMA+-montmorillonite is ~10.0A, indicating a paraffin type of
orientation of the quaternary ammonium cations. In DODDMA+-
montmorillonite, a gallery height of ~18A is obtained with an simiar
orientation of the DODDMA+ onium cations. In DDDMA+-beidellite the
gallery height of ~15A, indicates a paraffin type of orientaion, but with a
much larger tilt angle of the alkyl chains from the silicate layers. In
DODDMA+-beidellite the gallery height of ~28A obtained indicates a lipid-
like structure of the organic quaternary ammonium ions inside the clay
galleries. Figure IV.22 b displays the relationship between basal spacing
and cation exchange capacity per 100 g of these materials. The neutral
decylamine/Q+-clay powder patterns in excess amine suspension are
displayed in Figure IV.23. The narrow peaks at 30.0131 for the decylamine
samples is indicative of the presence of alkylammonium salts, formed by
the reaction of the amine with atmospheric C02, to form an
alkylammonium carbonate. The relatively broad XRD peaks at 42A for
the decylamine solvated organoclay is assigned to Q+—clay swollen by
decylamine. This reflection results from intercalation of amine into the
organoclay galleries. Notice that similar gallery heights are obtained for
both montmorillonite and beidellite inspite of the different orientation of

the intercalated quaternary ammonium cations prior to amine solvation.

214

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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37.00/11
18.00131
(B)
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1 1 1 1 l 1 1 1 1 1 1 1 1 1 l 1 1 4 1
1 6 ll 16 21

Two Theta (degrees)

Figure IV.22 : X-ray diffraction powder patterns for the dialkyl
quaternary ammonium exchanged clays (Q+-clays). (A)DDDMA+-
montmorillonite(Wyoming). (B) DODDMA+-beidellite (Chinese). (C)
DODDMA+-montmorillonite (Wyoming). (D) DDDMA+-beidellite
(Chinese).(Insert) A plot of the basal spacings to cation exchange
capacity/100g of clay.

215

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

42. A
>:
H
'51
1:
4)
H
1::
1—1
0
.2
E 21.00131

4)
a

(D)

(C)

(B)

J (A)

1111111411111J1111l
5 10 15 20

Two Theta (degrees)

Figure IV.23 : X-ray diffraction powder patterns for the dialkyl
quaternary ammonium exchanged clays (Q+-clays) in excess amine
suspension with a molar ratio of 1 : 20 between Q+-clay and amine.
A)DDDMA+-montmorillonite(Wyoming). (B) DDDMA+-beidellite
(Chinese). (C) DODDMA+-beidellite (Chinese). (D) DODDMA+-
montmorillonite (Wyoming).

216

 

 

 

 

 

 

 

 

(A)
18.00A
E‘
g montmorillonite (Arizona)
‘5 30.0011
~
0
.2
‘5
'6
a:
montmorillonite (Wyoming)
beidellite (Chinese)
1 1 1 1 1 L 1 1 1 1 1 1 1 1 1 1
10 20
Two Theta (degrees)
5‘
'5
:1
8
=
fl
0
.2.
fl
a
T»
a:

 

 

 

1 o 1
Two Theta (degrees)

Figure IV.21 : X-ray diffraction powder patterns for PCH prepared
using decylamine/HDTMA+ as templates and different low charge density
smectites with a Q+-clay : amine : TEOS of 1 : 20 : 150 respectively. (A)
as-synthesized. (B) calcined at 500°C for 4 h.

217

Mixtures of Q+-clay : decylamine : TEOS at molar ratio of l : 20 :
150, respectively, were allowed to react for 4 h at ambient tempearture.
The resultant intercalates were then centrifuged, dried in open air to give
the as-synthesized products. Removal of the organics by calcination at
500°C in air for 4 h gave the final calcined material.

Figure IV.24 illustrates the X-ray powder diffraction patterns for
the as-synthesized and freshly calcined materials obtained by using
montmorillonite(Wyoming) and beidellite(Chinese) as layered host and
dialkyldimethylammonium+/decylamine for templating. The diffraction
pattern for the air dried PCH-montmorillonite (DDDMA+/decylamine as
templates) exhibits a single broad diffraction peak with a gallery height of
~9.0A. With DODDMAfldecylamine as template a gallery height of ~21A
is obtained. Significantly higher gallery heights for the corresponding
quaternary ammonium chain lengths are obtained with beidellite as the
layered host. For instance, in PCH-beidellite synthesized by using
DODDMAfldecylamine as templates a gallery height of ~29A is obtained.

The X-ray powder diffraction pattern for the corresponding
materials calcined at 500°C is displayed in Figure V.24 b. All the XRD
patterns show a single weak diffraction peak at 12.7131, corresponding to a
collapsed silica-intercalated phase. These results do not support an intra-

gallery templated nanostructure.

 

(A)

30.0011

38A
PCH-DDDMN-beidellite

 

PCH-DODDMAi-montmorillonite

 

PCH-DODDMA+ -beidellite

Relative Intensity

 

 

 

 

l A A L A l A A A L l A

ll 16 A A 21
Two Theta (degrees)

 

on
a

 

(B)

 

 

Relative Intensity
b
' .53
. g.
1
)
L, P
M
>.

-
l

 

A I l I A 1- 1 ‘ .

 

9 l 3
Two Theta (degrees)

Figure IV.24 : X-ray diffraction powder patterns for PCH prepared
using decylamine/dialkyl quaternary ammonium as templates and different
low charge density smectites with a Q+-clay : amine : TEOS of 1 : 20 : 150
respectively. (A) as-synthesized. (B) calcined at 500°C for 4 h.

219

References :

Figueras, F., Catal. Rev.-Sci. Eng. 1988 30 (3 ), 457.
Izumi, Y.; Urabe, K.; Onaka, M., Zeolite, Clay, and Heteropoly Acid
in Organic Reactions VCH Publishers, 1992, 49—98.

3. Butruille, J .-R. and Pinnavaia, T. J ., Catal. Today, 1989, 62, 3221.

10.

11.

12.

13.

14.

15.

Butruille, J .-R.; Michot L. J. and Pinnavaia, T. J. J. Catal. 1993,139,
664.

. Galarneau, A.; Barodawalla, A. and Pinnavaia, T. J ., Nature 1995,

3 74, 529.
Brunauer, S. ; Emmett, P.; Teller, E., J. Am. Chem. Soc. 1938,
60, 309.
Horvath, G.; Kawazoe, K.J., J. Chem. Engng Jap. 1983, I6, 470.
Farmer, V. C., "The Infrared Spectra Minerals"1974 Ed. V. C.
Farmer, mineralogical society (London).
Chen, C-Y; Li, H-X and Davis, M. E., Microp. Mater., 1993, 2, 17.
"The Sadtler infrared spectra Handbook of minerals and clays" ed.
John Ferraro, 1982, Sadtler Research laboratories. I
Makarova, M. A.; Garforth, A.; Zholobenko, V. L.; Dwyer, J .; Earl,
G. J. and Rawlence, D., Stud. Surf. Sci. Catal., 1994, 84, 365.
(a) Connell, G. and Dumesic, J. A. Journal of Catal., 1987, 105,
285-298. (b) Parry, E. P. Journal of Catal. 1963, 2, 371-379.
Khabtou, S.; Chevreau, T.; Lavalley, J. C., Microp. Man, 1994
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Engelhardt, G.; Michel, D., "High resolution solid-state NMR of
silicates and zeolites " 1987 Ed. John Wiley & sons.
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220

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Chem. Soc. ,1985, 107, 7222.

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