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ORGANOFUNCTIONAL SILICA MESOSTRUCTURES WITH
IMPROVED ACCESSIBILITY AND APPLICATIONS AS HEAVY
METAL ION ADSORBENTS

presented by
Xin Sun

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ORGANOFUNCTIONAL SILICA MESOSTRUCTURES WITH IMPROVED
ACCESSIBILITY AND APPLICATIONS AS HEAVY METAL ION ADSORBENTS

By

Xin Sun

A DISSERTATION

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

DOCTOR OF PHILOSOPHY
Department of Chemistry

2008

ABSTRACT

ORGANOFUNCTIONAL SILICA MESOSTRUCTURES WITH IMPROVED
ACCESSIBILITY AND APPLICATIONS AS HEAVY METAL ION ADSORBENTS

By
Xin Sun

Effective approaches to the removal of toxic metals from contaminated water
have involved the use of solid adsorbents, such as activated carbons, ion exchange resins,
and functionalized silica-based materials including silica gels, clays and meSOporous
silicas. The advantages of functionalized mesoporous silicas for environmental
remediation are their high surface areas, well-defined pore size, and the ability to
covalently link organic groups to the framework to allow for the selective adsorption of
specific toxic heavy elements.

The overall goal of this work is to design frmctionalized mesostructured silica
materials for use as heavy metal ion (i.e. Pb“) adsorbents. The approaches investigated
include the incorporation of organosilanes, heteroatoms, and surfactants into the
framework structure. The hydrogen bonded supramolecular assembly of a neutral
alkylarnine surfactant and a nonionic silica sources was used to form wormhole
mesostructures."2 The key objective was to improve the accessibility of the functional
sites within the mesostructured silica framework.

Mesoporous amine-functionalized organosilicas were synthesized via several
routes. A hydrophobic tBoc-protected amine group or a nitrile group was incorporated
into the framework by direct assembly of an organosilane and tetraethylorthosilicate
(TEOS) in the presence of dodecylamine as a surfactant. The amine groups were formed

inside the mesopores of organosilica framework by Boc deprotection or nitrile reduction.

The obtained mesostructure showed a much higher structural order and much higher
accessibility of the amine groups in comparison to derivatives synthesized by the
conventional direct co-condensation of TEOS and an amino-fimctional organosilane. The
resulting mesostructures had a Pb2+ trapping capacity of 0.23-0.48 mmol g".

An attempt also was made to utilize the structure—directing surfactant for the
removal of heavy metal ions from solution. As-made mesophases containing intercalated
tallow amine surfactants showed good affinity towards Pb2+ ions, having a maximum
trapping capacity of 1.2 mmol g". However, it remained a concern that the surfactant was
not irreversibly bound inside the mesopores. This problem was solved by introducing an
epoxide organic moiety into the silica synthesis process in order to immobilize the amine
surfactant inside the mesopores through the formation of covalent bonds. This
modification also reduced the hydrophobic nature of the mesopore environment. A
mesophase containing 5 mol % immobilized dodecylamine surfactant had a Pb2+ trapping
capacity of 0.33 mmol g".

In addition, the alumination of mesostructured silica using lithium aluminum
hydride, in—situ generated sodium aluminate and sodium aluminate solution as aluminum
sources also was investigated. The tetrahedrally coordinated Al sites incorporated into
the mesophases served as the cation exchange sites. The resultant adsorbents showed
excellent Pb“ trapping capacities of 075-1 .0 mmol g".

(1) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Rieker, T. P. J. Am. Chem.
Soc. 1999, 121, 8835-8842.

(2) Kim, Y.; Lee, B.; Yi, J. Sep. Sci. T echnol. 2004, 39, 1427-1442.

Copyright
By
thiSun

2008

ACKNOWLEDGEMENTS

This dissertation would not have been realized without the help, support, guidance
and efforts of a lot of people. First and foremost I want to thank Dr. Tom J. Pinnavaia,
my doctoral advisor, for instilling in me the qualities of being a good scientist and
chemist. I shall never forget his encouragement for independent thinking and inspiring
discussions during the past five years. I also really appreciate his support on my
internships and all his advices for my career launching. He is one of the rare advisors that
students dream that they will find.

Additionally, I would like to thank my group members, former and present, for
their friendships. I am grateful for the rewarding discussions with Dr. Zhaorong Zhang
and Dr. Hui Wang. I also want to thank DK, Christ and Susan for making our office and
lab an attractive and fun place to go everyday.

I must acknowledge as well all the friends I made at Michigan State University.
They made East Lansing feel like a second home to me. I would especially like to thank
Meng Li for her great fi'iendship and help during the past years.

Finally, I would like to thank my family. My parents showed great understanding
and support on my decision of pursuing my Ph.D. study abroad. They have always been
having faith in me. I also want to give my special thanks to my husband, Lingyun, my
best friend I have known for fourteen years. I am deeply grateful for his patience, support

and love.

TABLE OF CONTENTS

LIST OF TABLES ........................................................................................................... viii
LIST OF FIGURES ........................................................................................................... ix
ABBREVIATIONS ......................................................................................................... xiii
Chapter 1
Introduction ......................................................................................................................... 1
1.1 Mesoporous silica molecular sieves background .................................................... 1
1.1.1 Electrostatic interaction pathways .................................................................. 4
1.1.2 Hydrogen-bonding interaction pathways ........................................................ 4
1.2 Organic functionalization of mesoporous silicas .................................................... 5
1.2.1 Incorporation of the organic functionality by grafting ................................... 6
1.2.2 Incorporation of the organic functionality by direct assembly ..................... 10
1.2.3 Advances improving the structural integrity and accessibility of the organic
functionality incorporated through the direct assembly method ................................... 13
1.3 Characterization techniques .................................................................................. 18
1.4 Remediation of heavy metal cations with functionalized mesoporous silicas ...... 25
1.4.1 Heavy metal trapping .................................................................................... 25
1.4.2 Lead trapping ................................................................................................ 29
1.5 Research objectives ............................................................................................... 30
1.6 Reference .............................................................................................................. 32
Chapter 2
Removal of lead species from aqueous solution by surfactant-containing mesophase silica
materials ............................................................................................................................ 40
2.1 Introduction ........................................................................................................... 40
2.2 Experimental methods .......................................................................................... 44
2.2. 1 Reagents ........................................................................................................ 44
2.2.2 Materials synthesis ........................................................................................ 45
2.2.2.1 Tallow amine surfactant intercalated mesophase silicas .......................... 45
2.2.2.2 Epoxide functionalized surfactant intercalated mesophase silicas ........... 46
2.2.3 Pb2+ trapping experiments ............................................................................. 47
2.2.3.1 Pb2+ trapping isotherms for tallow amine surfactant intercalated
mesophase silicas ...................................................................................................... 47
2.2.3.2 Pb2+ trapping experiments for epoxide functionalized amine surfactant
intercalated mesophase silicas .................................................................................. 47
2.2.4 Characterization ............................................................................................ 48
2.3 Results and discussion .......................................................................................... 49
2.3.1 Tallow amine surfactant intercalated mesophase silica ................................ 49
2.3.2 Epoxide firnctionalized dodecylamine containing mesostructured silicas.... 55
2.4 Conclusion ............................................................................................................ 70

vi

2.5 References ............................................................................................................. 72

Chapter 3
Synthesis of mesostructure silicas with more accessible amino groups and their
application as lead (Pb2+) adsorbents ................................................................................ 75
3. 1 Introduction ........................................................................................................... 75
3.2 Experimental methods .......................................................................................... 82
3.2.1 Reagents ........................................................................................................ 82
3.2.2 Material synthesis ......................................................................................... 82
3.2.2.1 Synthesis of HMS-NHtBoc and HMS-NH2 (Boc) materials ................... 82
3.2.2.2 Synthesis of HMS-CN and HMS-NH2 (CN) materials ............................ 84
3.2.3 Pb2+ trapping experiments ............................................................................. 84
3.2.4 Characterization ............................................................................................ 85
3.3 Results and discussion .......................................................................................... 87
3.3.1 Synthesis of mesostructure HMS-NHtBoc and HMS-NHZ (Boc) materials 87
3.3.2 Synthesis of mesostructured HMS-CN and HMS-NH; (CN) materials ..... 100
3.3.3 Adsorption performance ............................................................................. 1 10
3.4 Conclusions ......................................................................................................... 1 14
3.5 References ........................................................................................................... 1 16
Chapter 4
Alumination of siliceous wormhole mesostructure and its application as heavy metal
adsorbents ....................................................................................................................... 119
4.1 Introduction ......................................................................................................... 1 19
4.2 Experimental methods ........................................................................................ 124
4.2.1 Reagents ...................................................................................................... 124
4.2.2 Material synthesis ....................................................................................... 124
4.2.2.1 Alumination of mesoporous silica by LAH and water work-up ............. 124
4.2.2.2 Alumination of mesoporous silica by sodium aluminate ........................ 125
4.2.3 Pb“ trapping experiments ........................................................................... 126
4.2.4 Characterization .......................................................................................... 126
4.3 Results and discussion ........................................................................................ 127
4.3.1 Aluminated HMS-LAH and HMS-LAH-C mesostructures ....................... 127
4.3.2 Aluminated HMS-Al-I, HMS-Al-II, HMS-Al-I-C, and HMS-Al-II—C
mesostructures ............................................................................................................. 135
4.3.3 Lead Adsorption experiment ....................................................................... 143
4.4 Conclusion .......................................................................................................... 147
4.5 References ........................................................................................................... 149

vii

LIST OF TABLES

Table 1.1. Modified mesostructured (SiOz)l.,,(SiO1,5L)x organosilicas for heavy metal ion
trapping. . ........................................................................................................................... 26

Table 2.1. Physical properties of tallow amine templated mesostructured silicas after
calcination at 600°C for 4 hours ....................................................................................... 54

Table 2.2. Physical properties of epoxide functionalized mesostructure silica materials
after solvent extraction x-HIVIS-epoxide-E and calcination x-HMS-epoxide-C ............... 65

Table 2.3. Pb2+ trapping performance of epoxide functionalized, dodecylamine-containing
mesostructured silica materials, denoted x-HMS-epoxide-E ............................................ 69

Table 3.1. Physical properties of Boc protected amine fimctionalized mesostructured
silica (HMS-NHtBoc), the recovered amine functionalized mesostructured silica (HMS-
NH; (Boc)) and conventionally assembled amine functionalized silica (HMS-NH; (conv.))

synthesized with a dodecylamine porogen. ...................................................................... 93

Table 3.2. Physical prOperties of HMS-CN, HMS-NH; (CN) and HMS-NH; (conv.)
organosilicas assembled at 60 °C .................................................................................... 105

Table 3.3 The lead trapping performance of amine functionalized HMS organosilicas
synthesized by the alternative routes and the derivative synthesized by the conventional
co-condensation of TEOS and APTES. .......................................................................... 113

Table 4.1. Summary of the literature work on the alumination of mesostructured silica
materials by post synthesis/ grafting methods ................................................................. 121

Table 4.2. Physical properties of mesostructured silica aluminated by reaction with LAH.
......................................................................................................................................... 133

Table 4.3. Physical properties of aluminated mesostructured silicas using sodium
aluminate prior to and upon calcination .......................................................................... 141

Table 4.4. The lead trapping performance of aluminated HMS mesostructure silica
materials made using LAH and sodium aluminate as the aluminum sources ................. 146

viii

LIST OF FIGURES

Figure 1.1. Formation of mesoporous materials by structure-directing agents through true
(a) liquid-crystal template mechanism, or (b) cooperative liquid crystal template
mechanism.6 ........................................................................................................................ 3

Figure 1.2. Schematic representation of interactions between the silica precursors and the
head group of the surfactant molecules with consideration of the possible synthetic
pathway in basic, acidic and neutral conditions .................................................................. 5

F igurc 1.3. Schematic diagram for the grafting pathway used in the preparation of organic
functionalized silica materials6 ........................................................................................... 7

Figure 1.4. (A) Schematic diagram of the imprint-coating process: first the complexes are
introduced between target metal ions and bifunctional ligands, then the siloxane groups in
the bifunctional ligands are hydrolyzed, and finally covalently coated on the mesopore
surfaces. (B) Schematic representation of the difference between the cavities generated
by conventional coating (left) and imprint coating (right).20 .............................................. 9

Figure 1.5. Single site distribution of amine groups obtained using the bulky trityl
protecting group as a spacer.22 .......................................................................................... 10

Figure 1.6. Schematics of direct-assembly pathway, where R is the firnctional group and
OR’ is a hydrolysable group,24 for the synthesis of an organofunctional mesoporous silica
from co-condensation pathway. ........................................................................................ 11

Figure 1.7. An exaggerated schematic showing the functional organosilane distribution
arising via co-condensation ............................................................................................... 14

Figure 1.8. Schematic representation of the utilization of an anionic organoalkoxysilane
for controlling the functionalization of an organosilica made by direct assembly pathway.
The MCM—41 type mesoporous channels are illustrated by the parallel stripes shown in

the TEM micrograph.56 ..................................................................................................... 16
Figure 1.9. Typical XRD powder patterns for mesostructured silica materials: (A)
hexagonal arrayed MGM-42 and (B) worrnhole HMS‘f’6 ................................................... 21
Figure 1.10. IUPAC classification of adsorption isotherms1 ............................................ 22

Figure 1.11. A typical N2 adsorption-desorption isotherm for calcined mesostructured
HMS silica.“ ..................................................................................................................... 23

ix

Figure 1.12. Schematic illustration of Q" and T m framework silicon sites in an
organosilica framework. Q and T denote the binding of silicon to four and three oxygen
centers, respectively, and n and m indicate number of oxygen that bridge to neighboring
silicon centers .................................................................................................................... 24

Figure 1.13. Distribution of hydrolysis products Pb,,(OH)y in aqueous solution at (a) high
and (b) low concentration.'05 ............................................................................................ 30

Figure 2.1. Schematic illustration of the location of hydrophobic organic molecules in the
surfactant-containing mesophase silica, reproduced from reference 25. C343, C480 and
PAC343 stands for hydrophobic organic molecules coumarin dye 343, coumarin dye 480

and propylarnide coumarin dye 343 .................................................................................. 42
Figure 2.2. Chemical structure of Tallow amine surfactant series ................................... 45
Figure 2.3. Chemical structure of (3-Glycidyloxypropyl) trimethoxysilane .................... 45

Figure 2.4. X-ray powder diffraction of (A) as-made tallow amine surfactant intercalated
mesophase silicas and (B) the counterparts after calcination at 600°C for 4 hours .......... 50

Figure 2.5. (A) N2 adsorption-desorption isotherms and (B) BJH framework pore size
distributions calculated fiom the N2 adsorption branch of the calcined mesostructured
silica materials synthesized with tallow amine surfactants as templates. The isotherms are
offsite vertically by 400 cm3 g'1 for clarity. ...................................................................... 53

Figure 2.6. The Pb2+ trapping isotherms for tallow amine surfactant containing
mesophase silicas. ............................................................................................................. 54

Figure 2.7. (A) The formation of the covalent bonds between the epoxide functional
moiety and the dodecylamine surfactant and (B) a schematic illustration of the
mesostructured silicas after different synthesis steps. ...................................................... 56

Figure 2.8. X-ray powder diffraction patterns of epoxide fimctionalized mesostructured
silicas made with dodecylamine as the surfactant and subsequent solvent extraction, x-
HMS—epoxide-E, where x represents the fraction of silicon centers with an epoxide
functional moiety. ............................................................................................................. 59

Figure 2.9. (A) N2 adsorption-desorption isotherms and (B) pore size distributions
determined from the N2 adsorption isotherm branch for x-HMS-epoxide-E silicas
templated by dodecylamine after extraction with ethanol, where x is the fiaction of
fiamework silicon atoms linked to epoxy groups. ............................................................ 60

Figure 2.10. TEM images for epoxide functionalized, surfactant-containing mesophase
silicas made with dodecylamine as surfactant, denoted x-HMS-Epoxide-E, where the '
theoretical epoxy loading is x=0.05 (A, B) and 0.10 (C, D). ............................................ 61

Figure 2.11. TEM images for epoxide functionalized, surfactant-containing mesophase
silicas made with dodecylamine as surfactant, denoted x-HMS-Epoxide-E, where the
theoretical epoxy loading is x=0.25 (A, B) and 0.50 (C, D). ............................................ 62

Figure 2.12. (A) N2 adsorption-desorption isotherms and (B) pore size distributions
determined from the N2 adsorption isotherm branch for x-HMS-epoxidc-C silicas
templated by dodecylamine after calcination, where x is the fraction of framework silicon
atoms linked to epoxy groups. .......................................................................................... 64

Figure 2.13. Thermal gravimetric analysis (TGA) profiles of 0.50-HMS-epoxide-E before
and after water extraction .................................................................................................. 68

Figure 3.1. Penta-coordinate intermediate formed by the aminopropyl organosilane ...... 77
Figure 3.2. Uncontrolled polymerization of aminopropyl organosilane ........................... 77

Figure 3.3. The possible positions for a hydrophobic mercaptopropyl organic moiety L
group (left image) and a hydrophilic aminopropyl group in an as-made silicate
mesostructure templated by dodecylamine as a surfactant. .............................................. 79

Figure 3.4. Chemical structure of butyloxycarbonylaminopropyltriethoxysilane
(NHtBocTES). .................................................................................................................. 82

Figure 3.5. Transmission electron microscopy (TEM) images of (A) Boo-protected amine
fimctionalized mesostructured silica, 20% HMS-NHtBoc and (B) the recovered amine
functionalized mesostructured material after Boc deprotection, 20% HMS-NH2 (Boc). . 88

Figure 3.6. X-ray diffraction patterns of amino functionalized HMS mesostructured
silicas before and after the deprotection of tBoc groups at different organosilane loadings
of 10 mol % and 20 mol %. .............................................................................................. 91

Figure 3.7. (A) N2 adsorption-desorption isotherms and (B) BJH pore size distributions
for mesoporous amino-functional HMS worrnhole silicas (10 and 20 % NH2 groups)
before and after the deprotection of the tBoc groups ........................................................ 92

Figure 3.8. FT -IR spectra of (A) 20 mol % HMS-NHtBoc before the Boo group
deprotection, (B) 20 mol % HMS-NH2 (Boc) after the deprotection and (C) calcined
HMS containing no organic groups. ................................................................................. 95

Figure 3.9. 13C CP-MAS NMR spectra of the mesostructure organosilicas 20 mol %
HMS-NHtBoc and 20 mol % HMS-NH2 (Boc) ................................................................ 96

Figure 3.10. 29$i MAS NMR spectra of (A) the mesostructured organosilica 20 mol %
HMS-NHtBoc and (B) 20 mol % HMS-NH2 (Boc). ........................................................ 99

xi

Figure 3.11. X-ray diffraction patterns of amino-functionalized HMS mesostructured
silicas before and after the reduction of CN groups to NH2 groups at different
organosilane loadings of 10 mol % and 20 mol %. ........................................................ 101

Figure 3.12. (A) N2 adsorption-desorption isotherms for mesoporous AP-HMS amino-
functional silicas (10 and 20 % AP groups) before and after the reduction of CN groups
to amine groups. (B) provides the BJH framework pore size distributions obtained from
the adsorption branches of the isotherms. ....................................................................... 104

Figure 3.13. Transmission electron microscopy (TEM) images of (A, B) nitrile-
functionalized mesostructure silica material, 10 % HMS-CN and (C, D) the recovered
amine functionalized mesostructure silica material 10 % HMS-NH2 (CN) at low and high
magnification. ................................................................................................................. 106

Figure 3.14. FT-IR spectra of (A) calcined mesostructured silica HMS, (B) 20 % HMS-
CN, and (C) 20% HMS-NH2 after CN reduction. .......................................................... 108

Figure 3.15. '3 C CP MAS NMR spectra of 20% HMS-CN and 20% HMS-NH2 (CN) after
the reduction of the nitrile group. ................................................................................... 109

Figure 4.1. Low angle X-ray diffraction patterns of aluminated HMS made using LAH as
the aluminum source before and after calcination at 600°C. The numbers in parenthesis
provide the overall Si/Al used in the synthesis. .............................................................. 130

Figure 4.2. Wide angle X-ray diffi'action patterns of aluminated HMS made using LAH
as the aluminum source before and after calcination at 600°C. The numbers in parenthesis
provide the overall Si/Al used in the synthesis. .............................................................. 131

Figure 4.3. (A) N2 adsorption-desorption isotherms and (B) BJH framework pore size
distributions for pristine HMS, HMS-LAH and HMS-LAH-C. ..................................... 132

Figure 4.4. ”Al MAS NMR spectra of aluminated HMS-LAH(2.8), (A) before and (B)
after calcination at 600°C for 4 hr. ................................................................................. 134

Figure 4.5. Low angle X-ray diffraction patterns of HMS-Al-I and HMS-Al-II materials
before and after calcination at 600°C. ............................................................................ 138

Figure 4.6. Wide angle X-ray diffraction patterns of HMS-Al-I and HMS-Al-II materials
before and after calcination at 600°C. ............................................................................ 139

Figure 4.7. (A) N2 adsorption—desorption isotherms and (B) BJH framework pore size
distributions for pristine HMS, HMS-Al (filled marker) and HMS-Al-C (opened marker)
synthesized by method 1 (square) and method 11 (triangular). ........................................ 140

Figure 4.8. (A) 27Al MAS NMR spectra of aluminated mesostructure, HMS-Al-I and (B)
the counterpart after calcination, HMS-Al-I-C. .............................................................. 142

xii

APTMS
APTES
BET
BJH
Boc
CMC
CN
CNTES
CP
CTA
CT AB
DDA
EtOH
FT-IR

HMS

HR-STEM

ABBREVIATIONS

Atomic absorption

Aminoprpyl

Aminopropyltrimethoxysilane

Aminopropyltriethoxysilane

Brunauer—Emmett-Teller

Barrett-Joyner—Halenda

3-tert-butyloxycarbomate

Critical micelle concentration

Butyonitrile

4-(Triethoxysilyl)butyronitrile

Cross polorization

Cetyltrimethylammonium

Cetyltrimethylammonium bromide

Dodecylamine

Ethanol

Fourier transform infrared spectroscopy

Wormhole mesostructured silica synthesized with amine surfactant
using hydrogen bonding interactions under neutral condition

Hi gh-resolution scanning transmission electron microscopy

Neutral silicate precursors, which are obtained under neutral

reaction conditions

xiii

IUPAC
LAH
LCT
MAS

mmol

MPTMS

N"

NHtBoc
NHtBocTES
NMR

P/Po

PEO

PMOs

Cationic silicate precursors, which are obtained under acidic
reaction conditions

Anionic silicate precursors, which are obtained under basic reaction
conditions

International Union of Pure and Applied Chemistry
Lithium aluminum hydride

Liquid crystal templating

Magic angel spinning

Millirnoles

Mercaptopropyl

Mercaptoprypyltrimethoxysilane

Non-ionic surfactants (such as alkyl poly(ethylene oxide))
3~tert-butyloxycarbonylarninopropyl
3-tert-butyloxycarbonylaminopropyltriethoxysilane
Nuclear magnetic resonance

Relative pressure P=pressure Po=saturation pressure
Polyethylene oxide

Periodic mesoporous organosilicates

Parts per million

Phenyltrimethoxysilane

Incompletely condensed silica sites Si(OSi)2(OH)2
Incompletely condensed silica sites Si(OSi)3(OH)1

Completely condensed silica sites Si(OSi)4

xiv

TBOS

TEOS

TEM

TGA

TLCT

VTES

Neutral surfactants (such as alkyl amine surfactants)

Cationic surfactants (such as alkylammonium surfactants)
Anionic surfactant (such as carboxylic salt surfactants)

Specific surface area in m2 g'1 obtained from the linear part of the
adsorption isotherm using the Brunauer-Emmett-Teller equation
Scanning electron microscopy

Sulfonic acid moiety

Functionalized Q2 site RSi(OSi)2(OH)

Functionalized Q3 site RSi(OSi)3

Tetrabutyl orthosilicate

Tetraethyl orthosilicate

Transmission electron microscopy

Thermogravimetric Analysis

True liquid crystal templating

Trimethylbenzene

Vinyltriethoxysilane

XV

Chapter 1

Introduction

1.1 Mesoporous silica molecular sieves background

Porous materials have properties suitable for a variety of applications such as
adsorbents, ion exchangers, catalysts, etc. They possess accessible void space within their
interior structure, as in zeolites and mesoporous silicas. Such porosity is described as
“framework porosity”, whereas the aggregation or intergrowth of small particles often
results in the formation of “textural porosity” between the grains. According to the
classification made by IUPAC,l porous solids can be arranged in three main categories,
depending on their pore size (diameter, d).

1. Microporous materials with pores < 2 nm

2. Mesoporous materials with pores in the intermediate range between 2-50 nm

3. Macroporous materials with pores > 50 nm

The dimensions and accessibility of pores of microporous materials, including
zeolites and related crystalline molecular sieves, are restrained to their sub-nanometer
scale. This limits the application of microporous material systems to small molecules.
During the past decade, an important effort has been focused on obtaining molecular
sieves showing larger pore sizes. The introduction of supramolecular assemblies as
templating agents (liquid-crystalline self-assembled surfactant micellar aggregates, rather
than single surfactant molecules in the case of zeolite templating) permitted a new family
of mesoporous silica and aluminosilicate compounds (M418) to be obtained. These
mesophases were first developed by research groups at Mobil Corp.2’3 Several studies

have investigated the mechanism of MCM-41, a mesostructured amorphous silica. It was

found that two different pathways are involved.“5 In the true liquid crystal templating
(TLCT) pathway, the concentration of the surfactant is so high, much higher than the
critical micelle concentration (CMC), that under the prevailing conditions (temperature,
pH) 3 lyotropic liquid-crystalline phase is formed without requiring the presence of the
precursor inorganic fiamework materials. On the other hand, it is also possible that liquid
crystal templating (LCT) forms as a cooperative self-assembly of the structure directing
agents and the already added inorganic precursors even below the CMC, in which case a
liquid-crystal phase with hexagonal, cubic, or laminar arrangement can develop. The two
mechanism pathways are shown in Figure 1.1.

In either case, an attractive interaction between the template and the silica
precursors is needed to ensure the condensation of a continuous framework without phase
separation taking places. The resulting organic-inorganic mesostructure could be
alternatively viewed as an array of structured micellar rods embedded in a silica matrix.
The removal of the surfactants, usually by calcination method, will produce the open,
mesoporous silica fi'arnework. Generally, there are two types of interactions between the
surfactant head groups and the inorganic precursors, namely, electrostatic interaction and
hydrogen bonding interactions. These two types of interactions lead to different structures

as described in the sections below.

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1.1.1 Electrostatic interaction pathways

Hexagonal MCM-41 is the first example of a mesostructure silica material
synthesized through the electrostatic interaction between the positively charged
ammonium surfactant head group (8+) and the negatively charged silicate precursors (I')
under basic synthetic conditions. Cubic MCM-48 and lamellar MGM-50 were also
synthesized via this S+I' pathway.3

The SBA series7’8 of mesostructures was obtained by similar electrostatic
interactions between the surfactant and inorganic precursor, except that a counter ion
X' to mediate the interaction between the protonated inorganic silica species and
protonated surfactant of the same charge under acidic synthetic conditions, denoted the
S‘IX'I)r pathway.

Although mesoporous metal oxides also have been successfully made through the
electrostatic interactions by anionic surfactant, no pure siliceous mesostructure has been

made using an anionic surfactant via either ST or S'X‘T pathway.

1.1.2 Hydrogen-bonding interaction pathways

Other synthesis routes rely on nonionic or neutral surfactants, where the main
interactions between the template and the inorganic species are H-bonding or dipolar
interactions, giving rise to the so-called neutral path. Tanev and Pinnavaia first
synthesized mesostructured silica termed HMS using neutral primary alkylamine
surfactant micelles (S0) and an unchanged silica species (1°), such as

tetraethylorthosilicate.9 As in the assembly of HMS mesostructures, MSU-X

mesostructures utilize a hydrogen-bonding interaction between an uncharged silica
species ([0) and the non-ionic surfactant (NO).10
The different possible hybrid inorganic-organic interfaces are schematized in

Figure 1.2.

 

 

 

 

 

 

 

Figure 1.2. Schematic representation of interactions between the silica precursors and the
head group of the surfactant molecules with consideration of the possible synthetic
pathway in basic, acidic and neutral conditions

All mesostructured silicas possess extremely high surface areas and easily
accessible, unifomly sized pores. Most importantly, the pore sizes exceeded those
attainable in zeolites and they could be tuned in the nanometer range by choosing an
appropriate surfactant templating system, sometimes augmented by a co-solvent or

swelling agentI '.

1.2 Organic functionalization of mesoporous silicas
All mesostructured silicas possess extremely high surface areas and easily
accessible, unifome sized pores. Most importantly, the pore sizes exceeded those

attainable in zeolites and they could be tuned in the nanometer range by choosing an

appropriate surfactant templating system, sometimes augmented by a co-solvent or
swelling agentl '.

To further expand the use of mesoporous silicas, organic functional groups have
been incorporated into the framework of the structures. By the choice of organic moiety,
mesoporous derivatives can be designed to selectively trap toxic metal ion pollutants
(such as mercury”, arsenic”, and lead”), as supports to immobilize enzymes‘s'm, or as
chemical sensors”. In general, there are two different pathways by which the organic
firnctionality can be incorporated into the mesostructures: l) the subsequent modification
of the pore surface of a purely inorganic silica material (“grafting” pathway), and 2) the

simultaneous co-condensation of corresponding silica and organosilica precursors

(“direct assembly” pathway).

1.2.1 Incorporation of the organic functionality by grafting

Grafting is a method of covalently linking reactive organosilane species with
surface silanol groups (terminal Si-OH groups). A typical synthesis involves two steps.
First, pure inorganic mesostructured silica is synthesized through the methodologies
mentioned before (electrostatic or electrically neutral pathways). Secondly, an
organosilane reagent is allowed to react with the pre—formed mesostructured silica by
condensation reaction, also called silylation, with surface silanol groups on the
framework surface (Fig. 1.3). This also is called the post-synthesis functionalization
method. The organosilanes used here must have hydrolysable group such as alkyloxy or

haloid attached to the organosilicon sites.

 

 

     
    
    

8.x ' \ "

/I\ ‘

ooo (30H

   

Figure 1.3. Schematic diagram for the grafting pathway used in the preparation of organic

functionalized silica materials”

If a high surface coverage of functional groups is desired, it is important to
maintain a large number of surface silanol groups after removal of the surfactants.
Surfactant removal is carried out either by calcination or by appropriate solvent or ion
exchange extraction methods. Calcination promotes condensation of unreacted silanol
groups, and many surface groups are lost at typical calcination temperatures (400-600'C).
The calcined mesoporous silicate can be rehydrated in boiling water and then removing
the excess water by azeotropic distillation, e.g., with toluene or benzene.

Mesoporous materials have been grafted with a variety of organic functional
groups, depending on the properties and applications sought. Grafting with groups that
exhibit little or no reactivity, such as alkyl chain or phenyl group, can be used to tailor the
pore size of the mesoporous solids and increase the surface hydrophobicity which can be
used for organic pollutant removal from aqueous system. Reactive functional groups like

alkyl thiols, alkyl amines, nitriles, etc. also have been grafted to the framework.

In order to have uniform distribution of surface organic groups through the
grafting method, some unique methods have been used. A strategy of imprint coating was
first designed by Dai to have specific ligand position towards the formation of a complex
(Fig. 1.4).20 This coating methodology allows precise control of the stereochemical
arrangement of ligands on the surfaces of mesopores, which in turn optimizes the binding
of a targeted metal ion. Moreover, a well-defined amine-functionalized silica surface
displaying evidence of isolated amine sites was prepared using a molecular patterning
technique?“23 A tritylimine patterning agent, which can be removed afterwards, was used
to spatially position the amine sites a minimum distance fiom each in order to have single

well paced organic functional sites (Fig. 1.5).

Complexes coated
on mesopores

A Complexes in solution

N
H2N H Cu(l:_)i NH2 ...... . A,
N

V\/

Hydrolysis and condensation reactions
of silicon alkoxide groups in aapts

n
‘A Cuali‘JHZ

MNL/NHQ

NH2

NH Cu(||)NH2
/——/— H\//

   

 

Figure 1.4. (A) Schematic diagram of the imprint-coating process: first the complexes are
introduced between target metal ions and bifunctional ligands, then the siloxane groups in
the bifunctional ligands are hydrolyzed, and finally covalently coated on the mesopore
surfaces. (B) Schematic representation of the difference between the cavities generated

by conventional coating (left) and imprint coating (right).20

000 0%

 

{0M0 /0Me
’3'. A
i??”?”?”? ‘i ?”

Figure 1.5. Single site distribution of amine groups obtained using the bulky trityl

protecting group as a spacer.22

The original structure of the mesoporous support is generally maintained after
grafting. However, there are some drawbacks of the grafting pathway. First, due to the
differences in geometrical conformation, the functional groups are not likely to link
completely to the surface. Under certain conditions the O-Si bond to the framework can
break due to hydrolysis processes, causing functionality lost of the post-synthesized
materials. Second, the functional groups may not be uniformly distributed on the surface
of the framework. And if the loading percentage goes higher, the organosilanes may self-
polymerize. Still fiirther, because of the large number of sequential steps, the grafting

pathway is time consuming.

1.2.2 Incorporation of the organic functionality by direct assembly

Another approach to synthesize organically functionalized mesoporous materials

is the direct-assembly pathway, in which silica precursor tetraethyl orthosilicate (TEOS)

10

and organosilane (R’O)3Si-R (where R’ is Et or Me and R is non~hydrolysable organic
group) precursors co-condensate in the presence of structure directing agents leading to
materials with organic residues anchored covalently to the pore walls (Fig. 1.6). The co-
condensation of a silicon tetraalkoxide as a precursor such as TESO, which constructs the
pore wall, is indispensable because the three-connected organosilane itself can not build
an infinite framework. The removal of surfactant by using solvent extraction generates

the mesoporous silicate with surface functionalization.

Ir
R OR' M$2
SI" + SI. surfactant _ ,Q
I ,
R'O/ I'\OR' R'O/ \OR vcatalyst H2O I. .
O

R' (LRnR surfactant removal‘

 

 

Figure 1.6. Schematics of direct-assembly pathway, where R is the functional group and
OR’ is a hydrolysable group,24 for the synthesis of an organofimctional meSOporous silica

from co-condensation pathway.

This one-pot direct assembly method has been commonly used to prepare organic
functionalized mesoporous silica materials through various synthetic conditions. Mann et
al. first used a cationic surfactant as the structure directing agent to incorporate phenyl
and n-alkyl chain into the silicate framework under basic condition, which was the
reaction condition used for the synthesis of MCM related mesoporous silica materials.”’26
It was reported that numerous kinds of organic groups, such as aminopropyl,
mercaptopropyl, glycidyloxypropyl, imidazolidinyl, vinyl, methyl, and ethyl, have been
27-33

successfiilly incorporated into the MGM-41 silicate framework through the ST route.

Acidic conditions were used for the incorporation of organic groups into SBA related

11

hexagonal or cubic structure silica materials through S+X'I+ route.34'39 Compared to basic
conditions, the interactions between the surfactant and the silicate surface under acidic
conditions are weaker. Therefore, the surfactant could be extracted with ethanol, instead
of an acid/alcohol mixture, which is the method used for materials synthesized via S+I'
route, although some structural disordering was observed after this process.

Macquarrie first used the neutral synthetic conditions to incorporate aminopropyl
and cyanoethyl groups and obtained worrnhole structured organic-inorganic hybrid
materials via the SOI0 route (HMS).40 Pinnavaia and Mercier subsequently extended the
incorporation to other functional groups, including vinyl, imidazole, mercaptopropyl,
phenyl, octyl, butyl, etc.’"'45 Neutral N010 assembly strategy was also utilized to
successfully synthesized mercaptopropyl functionalized hybrid materials.”48 Due to the
weak hydrogen bonding interaction between the surfactant and the silicate surface, the
structure directing surfactants can be readily removed by alcohol extraction.

Since the organic functionalities are direct components of the silica matrix, pore
blocking which might happen via the grafting method is not a problem in the direct
assembly method. Furthermore, the organic units are generally more homogeneously
distributed than in materials synthesized with the grafting process. It allows for a greater
loading of the organic group, fewer steps in the overall synthesis, and facile recovery of
the surfactant. Until recently, direct-assembly was limited to organosilanes and TEOS,
since the two are miscible. However, this process has recently been extended to the use of

sodium silicate as the inorganic silica source.”50

12

1.2.3 Advances improving the structural integrity and accessibility of the organic

functionality incorporated through the direct assembly method

The direct assembly method is considered superior to the grafting method in order
to obtain mesostructured silica materials with a high organic functionality loading and a
homogeneous distribution of organic moiety. However, the direct assembly method also
has some disadvantages. In general, the degree of mesostructure ordemess of the products
decreases with increasing concentration of (R’O)3SiR in the reaction mixture and
consequently the content of organic functionalities in the modified silica phases does not
normally exceed 40 mol%. Usually, basic conditions may be favorable to forming
crossed frameworks in a sol-gel derived material, and in turn, ordered mesostructures can
be well maintained even with the addition of three-connected organosilanes. On the
contrary, the cross linkage of silicate species in an acidic solution is easy to be disrupted
by the three connected bonds, leading to the destruction of the final mesostructures. Wei
el al. observed a decease of the surface area and pore volume while increasing the thiol
functional moiety in the SBA-15 mesostructure and concluded that the molar
concentration of mercaptopropyl organosilane should be limited to less than 20% in order
to ,frmctionalize SBA-15 silicas without a significant change of pore structure.5| Chong
also observed a disordered mesostructure caused by the strongly adverse effect of
aminopropyltrimethoxysilane (APTES) on the formation of SBA-15 even at aminopropyl
organosilane loading of 10%.37 The introduction of amine organic moiety in to 3D
worrnhole mesostructure of HMS silica also sacrificed the structural integrity of the

mesophase.“52

l3

Furthermore, increasing the proportion of organosilane in the reaction mixture
favors homo—condensation reactions, which is caused by different hydrolysis and
condensation rates of the structurally different precursors. It is a constant problem in co-
condensation because the homogeneous distribution of different organic functionalities in
the framework cannot be guaranteed. The binding site of the organosilane may, or may
not, be accessible, and some of them in fact may be buried within the walls of the silica.

An exaggerated schematic is shown in Figure 1.7.

 

 

7,. .J’
g = organosilane moiety

Figure 1.7. An exaggerated schematic showing the firnctional organosilane distribution

arising via co-condensation.

Hennans et al. utilized high-resolution scanning transmission electron microscopy
(HR-STEM) technique whereby the accessibility of the organo—functionalized sites can
be determined through directly visualized images.” Several separate simple transition

metal clusters which have specific coordinating ability with different organic groups

14

introduced into the mesostructures were used for this purpose. According to the HR-
STEM images, they concluded that the thiol functional groups were accessible and
mainly located within the channels of the MGM-41 nanoparticles. In contrast, most of the
amino functional sites were unavailable due to embedding within the silica walls during
the co-condensation synthesis. Yokoi et al. compared the results from the elemental
analysis and the argentometric titration of the amino functionalized MGM-41 and
revealed that all the amino moieties incorporated were not present on the surface, but
some of them were in the wall of the hexagonal channels.54 Rosenholrn also studied
amino-functionalized mesoporous SBA-15 silicas prepared by direct assembly and post-
grafting.55 Special focus was put on the accessibility of the introduced function which
was determined by a surface imine formation, followed by UV-vis spectrophotometry.
They observed a decrease not only in the relative but also the absolute number of
accessible amine groups per unit area with an increasing amino orgmrosilane molar
fraction for the co-condensation materials.

The spatial orientation of the organic functional groups and the interactions with
the surfactant in the mesoporous channels will certainly affect the accessibility. This is
illustrated by the carboxylate-, sulfonate- , and tiolated mesostructured MGM-41 silica
made by direct assembly method by Lin’s group.56 The precursors used for these
materials are shown in Figure 1.8. The amount of chemically accessible organic ligands
in -SO3H modified mesoporous silicas is higher than those of —COOH and —SH
functionalized materials, despite a lower loading of organic groups in the former. It could

be attributed to the competition of the anionic species, i.e., Br', silicate, and organosilane

15

during the formation of organosilicas. The least hydrated sulfonate group gave rise to the
highest loading of chemically accessible organic groups.
OMe

OEt (IDEt Si
EtO-SI‘ri-O-Sli-OEt

   
   

II
C S Si(OMe)3 CDSP—TMS
eO/ows/ (V)

9" ,OMe .. 3
Si;OMe— s s Si(OMe)3 SDSP—TMS
OMe 90/5\/\s/ U3

G>s/\/\Si(0Me)3 MP—TMS
Figure 1.8. Schematic representation of the utilization of an anionic organoalkoxysilane

for controlling the functionalization of an organosilica made by direct assembly pathway.
The MCM-41 type mesoporous channels are illustrated by the parallel stripes shown in
the TEM micrograph.56

Several groups have already made some progress on improving the structural
integrity and the accessibility of organic groups introduced by direct assembly methods.
Wang et al found that the TEOS prehydrolysis prior to the addition of amino organosilane
was essential to obtain well-ordered mesoporous materials SBA-15 with amino (i.e.,
monoamine and diamine) ftrnctionality.57'58 The Tatsumi group used anionic surfactant
successfully synthesized functionalized mesostructured silica with the assistance from the
protonated amine functional groups under synthetic conditions.”'60 The 40% amino

functionalized materials synthesized with an anionic surfactant still showed mesoporosity

and a high surface area and pore volume. The results from CHN elemental analysis and
argentometric titration suggested that almost all the aminopropyl moieties were on the
surface in contrast to the MGM-41 type materials synthesized with a cationic surfactant.
Thus, the resultant material showed a higher adsorption capacity for Co2+ cations.61

Voss et al proposed an “all-in-one” approach using silsesquioxane surfactant
precursors for the functionalization of the channel walls with primary amine groups.‘52
The monomer is made by a hydroboration/aminolysis sequence on the base of a
commercial monomer, with the template bound to the functionalization site by
hydroboration and released after silica condensation and arninolysis. This combination
ensures both the placement of the amine groups exclusively along the channel interface
as well as optimal use of the template. The increased accessibility of amine groups was
verified by Cu(II) binding experiment.

Chong et al. studied the effect of 3-aminopropy1triethoxysilane (APTES), 3-
mercaptopropylmethoxysilane (MPTMS), phenyltrimethoxysilane (PTMS),
Vinyltriethoxysilane (VTES), and 4-(triethoxysilyl)butyronitrile (CNTES) on the SBA-15
mesostructure made by the direct assembly method.63 Among the organosilanes studied,
the disruptive effects on the formation of the SBA-15 mesostructure followed the order
VTES< CNTES< PTMS< MPTMS< APTES. Such different effects are interpreted in
terms of their different behaviors under acidic synthetic conditions and in terms of steric
molecular sizes and shapes, which all have a direct impact on the interactions of P123
with silicate species and on micellation of the P123 template. Therefore, a post-

modification of the functionalized mesoporous materials by direct synthesis could be an

alternative to obtaining ordered mesoporous materials with large and/or hydrophilic

l7

functional groups, which often have large disruptive effects on the formation of the
mesostructure.

Mehdi et al. synthesized ordered mesoporous silicas with large-pore diameters
incorporating aminopropyl groups in variable quantity via the co-condensation of
tetraethyl orthosilicate (TEOS) and 3-tertbutyloxycarbonylarninopropyltriethoxysilane
templated with nonionic surfactant P123 under acidic conditions. The deprotection of
amino groups was then quantitatively achieved either by thermal treatment or acid
hydrolysis followed by Et3N treatment. Both routes led to exactly the same materials.
They showed that the fi’ee amino centers are fully accessible, by using the condensation
of the amine function with benzaldehyde.64 The same group of researchers also
synthesized ordered (hexagonal phase) mesoporous silica functionalized with iodopropyl
groups [I(CH2)3] located in the pore channels by the co-condensation method.“ The iodo
group allows easier and faster chemical transformations, giving rise quantitatively to a
variety of mesoporous materials with large and/or hydrophilic functional groups located

within the channel pores that are difficult or impossible to obtain by other routes.

1.3 Characterization techniques

Mesostructured molecular sieves are typically characterized by a battery of
techniques including X-ray diffraction (XRD), N2 adsorption desorption, Transmission
Electron Microscopy (TEM), 298i and 13C Magic Angle Spinning Nuclear Magnetic
Resonance (293i, '3 C MAS NMR), Thermogravimetric Analysis (TGA), Infrared
Spectroscopy (IR), and elemental analysis.

XRD has turned out to be an invaluable tool for the determination of the shape

and, in particular, the spatial distribution of the pores for highly ordered arrays such as in

18

MGM-41 and a more disordered arrangement of pores, for instance in HMS. For almost
all types of mesoporous materials, no single crystal diffraction data have been observed.
Therefore, only powder diffraction raw data are obtained as 1D plots of the coherent
scattering intensity (i.e. counts) versus the scattering angle 20. The characteristic length
scale d describing the pore system and the corresponding diffraction angle 20 are related
by the Bragg equation, s=1/d=(23in0)/X, where it is the wavelength and s is the
corresponding scattering vector. Figure 1.9 shows the typical XRD patterns for MCM-41
and HMS mesostructure silicas. The successive four peaks at low angle indicate the
hexagonal structure of MGM-41, while the single broad diffraction around 20 of 2 degree
suggests the wormhole structure of HMS.

Adsorption of a gas by a porous material is described quantitatively by an
adsorption isotherm, the amount of gas adsorbed by the material at a fixed temperature as
a function of pressure. Gas sorption represents a widely used technique for characterizing
micro- and mesoporous materials and provides porosity parameters such as pore size
distributions, surface areas, and pore volumes. The IUPAC classification of adsorption
isotherms is illustrated in Figure 1.10.1 The six types of isotherms are characteristic of
adsorbents that are microporous (type I), nonporous or macroporous (type II, III, and VI)
or mesoporous (type IV and V). At low P/Po adsorption in micropores takes places, which
is supposed to be a process of volume filling rather than capillary condensation. In a
mesoporous substrate, with increasing values of P/Po, a liquid-like adsorbate fihn is
formed on the pore walls. At a certain pressure, capillary condensation takes place, filling
the mesopores with liquid, which is apparent in isotherms as a pronounced increase in the

adsorbed amount. Figure 1.11 shows the typical N2 adsorption desorption isotherms of

19

mesostructure HMS silica. The hysteresis at relative pressure P/Po of 0.3-0.4 indicates the
existence of mesopores, while the uptake steps at high relative pressure (i.e. P/Po > 0.8)
suggest the pores caused by spaces between inter-particles or called texture pores.

29Si MAS NMR is performed to asses the degree of framework cross-linking and
degree of organic functionalization. Two kinds of resonances bands denoted as Qn and Tm
are normally observed. The former with chemical shifts at about -90 ~ -110 ppm is
assigned to the silicate species which connected with tetrahedrally coordinated silicate
species and surface silanol groups, Q"=Si(OSi),.(OH)4.n. While the latter with chemical
shifts at about ~60 ~ —80 ppm is characteristic of completely cross-linked organosilane,
Tm= RSi(OSi)m(OH)3-m. Figure 1.12 shows the structural nature of different silicon sites.
Quantitative assessment of the firnctional groups content of the mesoporous silica can be
monitored by the means of measuring the integrated areas beneath of each Q" and T"1
band.

Electron microcopies such as TEM and SEM are indispensable tool for the
investigation of mesoporous materials. The biggest advantage of these techniques is that
they deliver an optical image of the samples, providing direct views of the topology and
morphology of the materials.

TGA is typically done to determine if the surfactant is completely removed and to
probe the thermal stability of the structures. TGA and elemental analysis are the primary
methods to investigate the organic moiety loading in the organosilicas. '3 C MAS NMR is
canied in conjunction with FT —IR for the characterization of the incorporation of organic

functional groups in the mesostructures.

20

 

 

 

 

 

 

 

 

 

 

 

 

 

g hkl d(A)
"' 100 39.8
110 22.9 .
a, ' 200 19.8 J
'5 210 14.9
C
3 ’ l
.E >
S o
‘- 3 C
RI
fi'riv' 'T"TI“T'T‘T'VITF'TIF"'T'—""'I'Tv‘
2 4 B 8
Degrees 2-theta
.E‘
(D
C
.9
.E
O A 2 A 4 ‘ 6 8 T 10
Degrees 2-theta

Figure 1.9. Typical XRD. powder patterns for mesostructured silica materials: (A)
hexagonal arrayed MCM-42 and (B) wormhole HMS66

21

 

 

 

III I IV

 

 

 

 

 

 

 

-o
o
.o
I-
o
in
u
<
*E
3 V VI
E
<
Relative pressure ’

Figure 1.10. IUPAC classification of adsorption isotherms1

22

 

1 500

 

1 000

 

A .L A L i A

 
  

Nitrogen volume adsorbed (cc/g)
0|
0
C
An - I I J 9-

 

 

L A l A L L .2

0 0.2 0.4 0.6 0.8 1
PIPO

Figure 1.11. A typical N2 adsorption-desorption isotherm for calcined mesostructured
HMS silica.67

23

02: —Si—
| °\ ./

Si SI
/ I \0/ \OH

\

T3: —Si—
/

O
l \ /R(organic group)
/Si\ /Si
l o \O

\

T2: —Si-—
/

O
I \ ./R(organrc group)

/Si SI
[\0/ \OH

Figure 1.12. Schematic illustration of Q" and Tm framework silicon sites in an

organosilica framework. Q and T denote the binding of silicon to four and three oxygen

centers, respectively, and n and m indicate number of oxygen that bridge to neighboring

silicon centers.

24

1.4 Remediation of heavy metal cations with functionalized mesoporous silicas

1.4.1 Heavy metal trapping

“Heavy metal” is a general collective term applied to the group of metals and
metalloids with an atomic density greater than 6 g cm'3 and includes such elements as Cu,
Cd, Hg, Ni, Pb, Zn, Co, and F C.“69 These metals contaminate the aqueous waste streams
of many industries, such as metal plating facilities, mining operations and tanneries. The
soils surrounding many military bases are also contaminated and pose a risk of metals
groundwater and surface water contamination. Heavy metals are not biodegradable and
tend to accumulate in living organisms, causing various diseases and disorders. Health
agency guidelines set maximum acceptable heavy metal concentration in drinking water
that are typically less than 3 ppm.7°’7' Treatment processes for metals contaminated waste
streams include chemical precipitation, membrane filtration, ion exchange, carbon
adsorption, and coprecipitation/adsorption.72

Organo-functionalized mesoporous materials are good adsorbents for either
organic pollutants or heavy metal ions. If the incorporated organic group is an alkyl chain,
it increases the hydrophobicity of the framework. Thus, this material can adsorb an
organic pollutant which is hydrophobic, but yet the pollutant can have significant
solubility in water from the standpoint of toxicity. With the incorporation of thiol or
amine groups, the modified mesostructured silicas show good affinity to trap heavy metal

ions. The modified mesostructured silicas used for environmental remediation especially

for heavy metal removal are listed in the following table (Table 1.1):

25

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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28

1.4.2 Lead trapping

The toxicity of lead is now well established and all over the world guidelines set
the maximum acceptable concentration of lead in drinking water at very low levels.
Environmental Protection Agency (EPA) requires the lead concentration at consumer taps
lower than 15 ppm.

Although the adsorption of mercury (Hg2+) by organo-functionalized
mesostructured silicas has been studied exclusively in the past decade, there were only
few studies on the removal of lead by mesostructured organosilicas. Yeung et al. showed
thiol and amine functionalized MGM-41 synthesized by grafting method having Pb2+
capacities of 0.2 and 1.25 mmol g”1 respectively.“87 Thiol grafted HMS also showed
trapping capacity towards lead of 0.2 mmol g".97 The incorporation of amine organic
groups into the SBA-15 and MGM-41 mesostructured silicas by direct assembly method
results in the adsorbents for many cations including Cu”, Pb”, Zn2+, Mn2+, F e2+ and Ag”.
The lead trapping capacities for amine functionalized SBA-15 and MGM-41 are 0.34 and
0.13 mmol g", respectively.104

Figure 1.13 shows the distribution of hydrolysis products Pb,-(OH)y in aqueous
solution at different concentrations. The high concentration distribution 0.1 m
corresponds to 2.1><104 ppm, whereas the low concentration distribution 10-5 m
corresponds to 2.1 ppm. Considering the practical condition of lead trapping experiment,
the distribution of hydrolysis products at low concentration is closer to the target

contaminated water conditions. Thus, at pH lower than 6 which is the case for water

containing heavy metal ions, the majority of the species is the Pb2+ cation. Therefore, the

29

design of mesostructured silica adsorbents should be focusing on the trapping mechanism
of cation exchange, or complex formation toward divalent lead ions.

Pbx(OH)y

(a) 0.1m Pbun (b) 10‘5m PM!)
‘W Irvv'vrvrIvvttliVIIIIITIIIUIUIVIIIIIl') ‘00 '

 

80

60

Pb (II) °/o

40

20

 

 

 

 

 

Figure 1.13. Distribution of hydrolysis products Pb,.(OH)y in aqueous solution at (a) high

and (b) low concentration.105

1.5 Research objectives

Porous materials are of interest to material chemists due to their expansive
applicability. Applications of porous materials include use as ion-exchangers,
heterogeneous catalysts, gas adsorbents, and environmental remediation agents. As
introduced before, the formation routes can be electrostatic or hydrogen-bonding
interactions between the surfactant micelles and inorganic silica precursors. The
electrostatic interactions oflen yield mesostructures with long-range hexagonal
morphology. Mesostructures prepared through a hydrogen bonding interaction pathway

have a short range ordered wormhole morphology. The 3D wormhole structure has been

30

shown to have greater accessibility to ligands present in the pores in comparison with 2D
hexagonal structure.”106

The overall goal of this work is to design and synthesize functionalized

mesostructured materials (i.e. mesostructured silica) for potential use in environmental
remediation. The functional moiety could be a conventional organic moiety introduced by
organosilanes, hetero-atoms in the silicate framework, or the less commonly used
surfactant. Hydrogen bonding supramolecular assembly between neutral alkylamine
surfactant and a nonionic silica source will be utilized in order to have a better accessible
wormhole structure morphology. The key objective is to improve the accessibility of the
effective functional sites within the mesostructure silicas. Specifically, this objective will
be achieved by:

1. Design and preparation of surfactant containing mesophase silicas where the
surfactant is chemically stable inside the mesopores and can be utilized as the
binding sites for heavy metals.

2. Design alternative synthesis routes to improve the mesostructure and the
amine accessibility for the amine functionalized mesostructured silica
materials.

3. Preparation of mesostructured aluminosilicate with high percentage of
framework incorporated Al sites for better ion exchange capacity.

4. Examination of the lead adsorption capacity on the surfactant containing
mesophase silicas, amine functionalized mesostructured silicas with improved

amine accessibility and the mesostructured aluminosilicate material with high

percentage of framework incorporated Al sites.

31

1.6 Reference

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39

Chapter 2
Removal of lead species from aqueous solution by surfactant-containing mesophase

silica materials

2.1 Introduction

Until the discovery of the cooperative assembly of surfactant micelles with
silicates by Mobil scientists in 1992, which led to the synthesis of MGM-41 and the
M418 family of mesoporous materials,I the surfactant component were almost
exclusively used as surface and interface modifiers. This discovery sparked more than a
decade of extensive research on surfactant-directed assembly, a process in which
surfactant is used to direct and assemble inorganic building blocks into mesoscopically
ordered structures. Organic components, either on the silicate surface or as part of the
silicate walls, have been incorporated into the siliceous frameworks to further modify the
physical and chemical prOperties of mesoporous silicates. These mesoporous inorganic or

inorganic-organic hybrid materials have been considered for a wide range of applications

5 10,11

in the areas of catalysis,2' environmental remediationf'9 optically active materials,

polymerization science,'2"4 fixation of biological active species,15 and drug delivery.“"18

Up to now researches have mainly focused on the application of surfactant-free
mesoporous inorganic or organofunctionalized derivatives. Except for a few studies, little
attention has been paid to the properties and applications of as-made periodic mesophase
silica materials in which the structure-directing surfactant functions as an organic
modifier of properties. Sugi et al.19 found that as-made, surfactant containing MGM-41

silica exhibits high catalytic activity for the Knoevenagel condensation, being attributable

to basic (SiO)3SiO' sites which occur in large amounts in as-made silica mesophases in

40

order to balance the charge of the cetyltrimetylammmounium (CTA) surfactant in the
pores. The surfactant-retained mesophase materials also have potential as adsorbents of
organic pollutants. The sorption of hydrophobic organic molecules (e.g. chlorophenol)
from aqueous solution into as-made MCM-41 mesophases was first reported in 1998.20

Later on, the as-made surfactant containing MCM-4l mesophase materials were utilized

1

by different groups to remove trichloroethylene and tetracholoroethylene,2 chlorinated

122

pheno benzene,23 toluene and naphthalene“, and organic dyes.25’26 Miyake et al. used

mesophase MCM-41 microspheres synthesized from tetrabutylorthosilicate (TBOS) to

2728

separate phenol from aniline in the column study. Other surfactant-containing

mesophase materials, such as HMS, MGM-48, MCM-SO and FSM-16 systems were also
investigated for the removal of organic compounds from aqueous solution.22’23’29 As in
micelles, nonpolar molecules can be dissolved in the hydrophobic core of the
mesostructure-intercalated surfactant, where the hydrocarbon tails effectively act as a

solvent. The carton in Figure 2.1 shows the possible positions for the hydrophobic

organic molecules in the silica-surfactant mesophase materials.

41

    

Hydrophobic .. , -(
interior 1 1 ‘_ ‘T.

‘\

 

' o
PAC343
. g . o ,. ,

(

 

 

Figure 2.1. Schematic illustration of the location of hydrophobic organic molecules in the
surfactant-containing mesophase silica, reproduced from reference 25. C343, C480 and
PAC343 stands for hydrophobic organic molecules coumarin dye 343, coumarin dye 480

and propylamide coumarin dye 343.

Recently, the as-made materials have been used as adsorbents for heavy metal ion
in aqueous solution. The Sayari group successfully used as-made MCM-41 with a co-
intercalated co-surfactant to expand the pore size of MCM-41 to adsorb C02+, Ni”, Cu2+
ions from aqueous solution using the nitrogen binding site on the intercalated co-
surfactant N,N-dimethyl alkylamine.30 This was the first example using a surfactant-
containing mesophase silica as an adsorbent for heavy metals. Choi et al. later on

observed that the as-made surfactant containing MCM—41 and HMS silica materials are

42

good adsorbents for Pb2+ from aqueous solution, while the calcined pure siliceous
materials afier surfactant removal did not show any trapping capacities.31

The surfactant component of the as-made mesostructure is the primary factor
controlling the performance of as-made mesophase silica materials for the removal of
metal ions from solution. Amine surfactants are good choices since they have nitrogen
with lone electron pairs, therefore having good affinity with heavy metal to form a
complex. The work reported by the Sayari and Choi groups of as-made surfactant-
containing mesophase silica materials used tertiary and primary amines as the binding
sites in the case of co-intercalated surfactant expanded MCM—41 and HMS, respectively.

An important issue regarding the use of surfactant-containing mesophase
materials as metal ion adsorbents is the concern for leaching of the surfactant. Zhao et al.
observed slow alkylammonium ion surfactant leaching from as-made MCM-41 material
under conditions that mimic ground water.21 Similar behavior has been reported for
organic-exchanged smectites32 and surfactant modified zeolites,33 though with a relatively
faster leaching rate. The lower leaching rate for the mesophase may be related to the
structure location of the surfactant in the mesoporous materials (i.e., interior to the Si
framework). The interaction between the surfactant and silicate moiety during the
synthesis process can be electrostatic or hydrogen bonding. No stable interaction, such as
covalent bond, is formed between the surfactant head group and the silica moiety.
Therefore, stabilizing the surfactant inside the mesoporous silica materials remains a
chaflenge.

In this work, alkylamine surfactants containing different numbers of amine groups

were chosen as the templating agents for the silica framework, as well as serving as

43

adsorption sites for heavy metals. The trapping capacity was compared for surfactants
containing different amine groups in order to investigate the adsorption mechanism. Also,
an epoxide functional group that can react with the surfactant amine head group was also
introduced into the mesostructured silica framework. Covalent bond formation between
the surfactant and the silicate framework was expected to immobilize surfactant inside
the framework pores. The resultant mesophases were characterized by XRD, N2
adsorption-desorption isotherms, TEM and TGA analysis, Pb2+ adsorption experiments

were also carried out.

2.2 Experimental methods

2.2.1 Reagents

Tallow mono-, di-, tri- and tetraamine with the structures shown in Figure 2.2
were obtained as free samples from Tomah3 Product, Inc. Dodecylamine (DDA),
tetraethyl orthosilicate (TEOS), (3-Glycidyloxypropyl) trimethoxysilane (Epoxide
organosilane), lead atomic absorption standard solution (~1000 ppm), and lead nitrate
(99.999% purity) were purchased from Aldrich. The structure of the epoxide
organosilane is provided in Figure 2.3. Ethanol was purchased in-house. All the reagents
were used without further purification. Water used in the synthesis was double-

exchanged to remove cations and anions via a Millipore filter apparatus.

44

WWW/NH2

tallow monoamine (N1)

\/\/\/\/\/\/\/\'/\/NH2

tallow diamine (N2)

H
N

 

tallow triamine (N3) lNH
NH2

H
N
tallow tetraamine (N4) 1N

 

I

I

P2

NH2

Figure 2.2. Chemical structure of Tallow amine surfactant series

OCH3

I
l1CO— —sr/“\//\\oa/ \Si7
3 l

OCH3

Figure 2.3. Chemical structure of (3-Glycidyloxypropyl) trimethoxysilane

2.2.2 Materials synthesis

2.2.2.1 Tallow amine surfactant intercalated mesophase silicas
Desired amount of tallow amine surfactant was dissolved in a plastic bottle with

162 g H20 and 46 g EtOH. After obtaining a homogeneous surfactant solution, TEOS

45

was added and the mixture was aged at 60°C for 24 hr. The products were recovered by
filtration and air-dried. The total molar ratio of reagents was Tallow surfactant: TEOS:
H20: EtOH = 0.25: l: 180: 20. The products were denoted Tallow N1, N2, N3, N4
respectively, corresponding to the tallow amine, diamine, triamine and tetraamine used as

surfactants.

2.2.2.2 Epoxide functionalized surfactant intercalated mesophase silicas

Desired amount of dodecylamine surfactant was dissolved in an aqueous solution
containing 25 wt % ethanol in a plastic bottle. After obtaining a homogeneous surfactant
solution, a mixture of TEOS and epoxide organosilane was introduced while stirring. The
mixture was aged at 60°C for 24 hr. The products were recovered by filtration and air-
dried. The total molar ratio of reagents was DDA: Epoxide organosilane: TEOS: H20:
EtOH = 0.25: x: (l-x): 180: 20, where x is the fraction of silicon centers containing the
epoxide organofunctional groups. The non-covalently-bonded surfactant was removed by
solvent extraction at a solid to solvent (EtOH) ratio of l g to 50 mL. The extraction was
repeated once, and the solid was rinsed with ethanol. The final products were obtained by
filtration and air-dried at 80°C. The extracted products were denoted x HMS-epoxide-E,
where x represents the percentage of epoxide functionalized silicon centers and E
indicates that physically adsorbed surfactant was removed by ethanol extraction. To
further study the structural properties of the materials, some of the as made materials
were also calcined at 600°C for 4 hr. to remove all the surfactant and organic moieties.
The calcined products were denoted as x HMS-epoxide-C, where x represents the mole
fraction of epoxide functionalized silicon centers and C indicates the material was

calcined.

46

2.2.3 Pb2+ trapping experiments

2.2.3.1 Pb2+ isotherms for tallow amine surfactant intercalated mesophase silicas

For the tallow amine surfactant-containing samples, 0.1 gram of the dry silica was
dispersed in 100 mL water for 24h at room temperature to pre-wet the samples. Then 100
mL of Pb2+ solution with the desired concentration was added and the solution was kept
stirring another 24h. When the reaction was complete after the 24 hr period, the mixture
was filtered and the filtrate was collected for the atomic absorption analysis. The ratio of
silica to initial Pb2+ solution was 0. lg to 200 mL and the initial lead nitrate concentration
was from 5 to 300 ppm. The difference between the initial and equilibrium lead
concentrations indicates the amount of lead that was trapped on the solid adsorbents. The
lead trapping isotherms were plotted using the equilibrium concentration as the x axis and

the adsorbed Pb2+ on the adsorbents as the y axis.

2.2.3.2 Pb2+ trapping experiments for epoxide functionalized amine surfactant
intercalated mesophase silicas

The Pb“ adsorption experiments were carried out by stirring 0.05 g of epoxide
functionalized amine surfactant intercalated silica in 100 mL of lead nitrate (Pb(NO3)2)
solution at 25°C. The initial Pb2+ concentration for these experiments was 116 ppm in all
cases. The mixtures were stirred for 24 hours and then filtered through a 0.25 pm filter
paper to collect the final filtrate solution.

The amount of Pb2+ in the filtrate after the adsorption experiments was analyzed

by cold vapor atomic absorption spectroscopy (AAS). The difference between the initial

47

and equilibrium lead concentrations indicates the amount of Pb2+ that was trapped on the

solid adsorbents.

2.2.4 Characterization

X-ray diffraction (XRD) patterns were obtained on a Rigaku Rotaflex 200B
diffractometer equipped with Cu Ka X-ray radiation and a curved crystal graphite
monochromator operating at 45 kV and 100 mA.

N2 adsorption-desorption isotherms were obtained at —196 °C on a Micromeretics
Tristar 3000 sorptometer using standard procedures. Samples were outgassed at 150 °C
and 1045 Torr for a minimum of 12 hr prior to analysis. Surface areas were calculated
from the linear part of a BET plot of the nitrogen adsorption data according to IUPAC
recommendations. The Barrett-Joyner-Halenda (BJH) method was used to obtain the pore
size distribution from the adsorption branch of the isotherms.

Transmission electron microscopy (TEM) images were taken on a JEOL 2200FS
microsc0pe with filed emission electron source and an accelerating voltage of 200 keV.
Sample grids of mesoporous silicas were prepared by sonicating the powdered sample in
EtOH for 10 min and evaporating 2 drops of the suspension onto a holey carbon-coated
film supported on 300 mesh copper grids.

The TGA curves were recorded in air on a Cahn TGA System 121
therrnogravimeter using a heating rate of 5 °C/min.

Equilibrium concentrations of Pb“ in solution were measured on a cold vapor
atomic absorption spectroscopy Varian SpectrAA-200 using lead cathode with maximum

working current of 10 mA.

48

2.3 Results and discussion

2.3.1 Tallow amine surfactant intercalated mesophase silica

Tallow amine surfactants have an average of 16 alkyl carbon atoms in the
hydrophobic chain and hydrophilic head groups containing various nitrogen numbers.
Due to the similarity between tallow amine surfactants and dodecylamine, which is used
as the surfactant template for the assembly of HMS silica mesophases, wormhole
mesostructures were expected for the mesophases synthesized using tallow amine
surfactants as structure directing templates. The X-ray powder diffraction patterns of the
surfactant-containing Tallow N1, N2 N3 and N4 materials and their surfactant-free forms
obtained from calcinaiton at 600°C for 4 hours all showed a single reflection at a two
theta value around 1.5-2.0 degree or a d-spacing of 4-6 nm (Figure 2.4), indicating
wormhole structures was successfully made. The intensities of the XRD patterns for the
as-made surfactant containing mesophase materials are weaker than those for the calcined
derivatives. This is because there are two phases in the as-made materials, the surfactant

phase and the silicate phase, resulting in contrast matching of the XRD patterns.

49

 

 

 

    
  
 
 

 

 

 

 

 

 

    
   

 

 

 

 

 

7000 « A
6000 -
monoamine (N1)
5000 1
a . .
g 4000 diamine (N2)
0
0
3°00 triamine (N3)
2000 « tetraamine (N4)
1000 -
o a.._f r x
0 2 4 6 a 10
29 (degrees)
8000 , - --.-_______ h- __,-_.
B
7000-1
6000 , triamine (N3)
5000 4 /tetraamine (N4)
3
5 4000
8
3000 _ /monoamlne (N1)
2000 -
1000 1
l
o 2."-
o 2 4 6 8 10

20 (degrees)

Figure 2.4. X-ray powder diffraction of (A) as-made tallow amine surfactant intercalated

mesophase silicas and (B) the counterparts after calcination at 600°C for 4 hours.

50

The N2 adsorption-desorption isotherms and the BJ H pore size distributions for
the calcined Tallow—N1, N2, N3 and N4 were studied in order to further understand the
structure and porosity properties of the surfactant containing mesophase materials. As
shown in Figure 2.5, the N2 filling steps at relative pressure between 0.4 and 0.6 indicated
the existence of mesoporosity in the calcined Tallow amine materials. The pore size
decreases upon increasing the number of nitrogen atoms in the surfactant head group,
from 3.8 nm for the silica synthesized by using tallow monamine (N 1) as surfactant to 3.0
nm for the material synthesized with tallow tetraamine surfactant (N4). It is well known
that the interaction between the surfactant micelle and the silica moiety involves
hydrogen bonding. Increasing the number of nitrogen atoms in the head group of the
surfactants increases the hydrogen bonding interaction with the inorganic silica moiety.
The pore size decreases with the tighter interaction. The BET surface area remained
relatively consistent while increasing the nitrogen numbers in the surfactants. The
detailed physical properties are summarized in Table 2.1.

Pb2+ adsorption performance was investigated on the as-made surfactant
containing mesophase materials. The trapping isotherm patterns are summarized in
Figure 2.6. The monoamine surfactant containing mesophase material reached a
saturation plateau at an equilibrium concentration of 25 ppm. The maximum lead
trapping capacity of the monoamine surfactant containing mesophase (N 1) was about 0.2
mmol g". The diamine, triamine and tetraamine surfactant containing meSOphase all
share a similar isotherm pattern and have the same maximum trapping capacity of 1.2
mmol g". The trapping isotherm saturation plateau of diamine (N2) and triamine (N3)

surfactant containing mesophases was reached around equilibrium concentration of 40

51

ppm, while the saturation plateau of tetraamine surfactant containing mesophase (N4)
was reached around equilibrium concentration of 80 ppm. The monoamine surfactant
containing mesophase behaves differently from the multiamine surfactant containing
mesophases. This might be due to the chelating complex formation between the
multiamine surfactant and the Pb2+ cation. The monoamine surfactant only has one
nitrogen atom in one surfactant molecule; therefore the formation constant of lead
complex would be relatively low. Because of the certain geometry of the multi nitrogens
in a multiamine surfactant molecule, only certain amount of the lead cations can accept
the long pair of electrons on the nitrogen. Therefore, there is no significant difference
between the lead adsorption isotherms among the diamine, triamine and tetraamine
surfactant containing mesophase materials.

The tallow amine series surfactants were not just utilized as the structure direct
agents, but as the binding sites for heavy metal (e.g. Pb“) as well. The surfactant
containing mesophase materials showed excellent trapping performance, having a
maximum trapping capacity varied from 0.2 to 1.2 mmol g".

During the trapping experiments, however, foaming was noticed while filtering
the adsorbents from the equilibrium filtrate. This suggests the leaking of the surfactant
from the mesophase materials. The mesophase materials could not have good potential
uses in the heavy metal removal application unless the surfactant can be somehow

“fixed” inside the mesopores and will not be leaching out.

52

 

2000

 
   
  
  

:- 1300

U)
n 1600 ' tetraamine (N4)

1400 -

120° triamine (N3)
1000

800 diamine (N2)

Volume N2 adsorbed (cm

 

 

30.5

I
$0.4 -

 

4
D (nm)

Figure 2.5. (A) N2 adsorption-desorption isotherms and (B) BJH framework pore size
distributions calculated from the N2 adsorption branch of the calcined mesostructured
silica materials synthesized with tallow amine surfactants as templates. The isotherms are

offsite vertically by 400 cm3 g'1 for clarity.

53

Table 2.1. Physical properties of tallow amine templated mesostructured silicas after

calcination at 600°C for 4 hours

 

 

 

 

 

 

. . . droo spacing Pore size21 BET surface area Total pore volumeb
Calcrned srlrca 2 _1
("I“) (nm) (In 8) (cm’ a")
Monoarnine (N 1) 4.9 3.8 746 1.58
Diarnine (N2) 4.6 3.4 860 2.15
Triarnine (N3) 5.2 3.4 866 1.18
Tetraamine (N4) 5.2 3.0 809 0.99

 

 

 

 

 

 

aPore size diameter was determined by the BJH model from the adsorption branches of

the isotherms. b Total pore volume was determined at P/Po=0.98.

 

 

 

9 :4 .- .-
a N «h or 00 N
I l I l l

    
 

J

Pb” adsorbed (mol 9")
9 P
on

O)

 

N1

 

 

0 50 100 150 200
equilibrium cone. (ppm)

Figure 2.6. The Pb2+ trapping isotherms for tallow amine surfactant containing

mesophase silicas.

54

2.3.2 Epoxide functionalized dodecylamine containing mesostructured silicas

Arr epoxide moiety was introduced into the dodecylamine surfactant containing
mesophase materials in order to link the dodecylamine surfactant inside the mesopores
through covalent binding. The head group of a dodecylamine surfactant molecule is a
primary amine which contains two active hydrogens. When the epoxide organosilane was
introduced to the surfactant solution, the active hydrogen on the dodecylamine molecule
will open the epoxy ring to form C-N covalent bonds. The epoxy ring will be opened and
a covalent bond will be formed between the nitrogen on the surfactant molecule and the
less substituted carbon on the epoxy ring. If more epoxide is available, the active
hydrogen on the secondary amine formed in the first step reaction will react with another
epoxy ring to form another covalent bond between the less substituted carbon on the
second epoxy ring and the nitrogen on the surfactant molecule. The overall reaction is
described in Figure 2.7A. The reaction stoichiometry used to form the
organofunctionalized surfactant-containing mesophase was DDA: Epoxide organosilane:
TEOS: H2O: EtOH = 0.25: x: (l-x): 180: 20, where x is the fraction of silicon center
linked with epoxide organofunctional groups and x equals to 0.05, 0.10, 0.25 and 0.50.
When x equals to 0.05 or 0.10, only a part of the dodecylamine surfactant molecules are
able to form covalent bonds with epoxy rings, while the remaining non-covalent-bonded
surfactants will be removed after solvent extraction with ethanol. Partially surfactant
filled pores were expected for the extracted materials x-HMS-epoxide-E with x=0.05 or
0.10. Figure 2.7B illustrates the as-made surfactant containing mesophase products, the
ethanol extracted products containing covalently bonded partial surfactant, and the

calcined products which have no surfactant or organic epoxide groups in the pores. If the

55

x value (epoxide moiety) is higher than 0.25, all of the surfactant was expected to form
covalent bonds with the epoxy rings linked to the framework. Thus, no surfactant can be

removed by the solvent extraction.

(A)
OCH,
H3CO— —Si/\/\ 0% + HZNWW

OCH, 1
OCH,

H3CO— _S:i/\/\O O/\(\N N’\/\/\/\/\/\
OCH3

OCH,

HaCO— -S|i/\/\Q om

0C H3 N’\/\/W\/\
OCH,
H,Co—SI/\/\o

OCH, OH

(3)

As-made

extracted

calcined

Figure 2.7. (A) The formation of the covalent bonds between the epoxide functional
moiety and the dodecylamine surfactant and (B) a schematic illustration of the

mesostructured silicas after different synthesis steps.

56

Figure 2.8 shows the X-ray powder diffraction patterns of the extracted epoxide
functionalized surfactant-containing mesophase materials x HMS-epoxide-E. In all cases,
a single diffraction peak was observed at low angle, indicating the formation of a
wormhole mesostructure. The center of the single diffraction peak moves to higher two
theta value, indicating a smaller dIoo spacing, with increasing epoxide moiety loading.
The intensity of the XRD patterns decreases with increasing epoxide loading. With
increasing the epoxide loading, more dodecylamine surfactant was kept inside the
mesopores through the formation of covalent bonds. Stronger hydrogen bonding
interaction between the surfactant micelles and the silicate moiety resulted in a smaller
unit cell size. The intercalation of more surfactant will cause the XRD intensity to
decrease due to increased phase contract matching.

N2 adsorption desorption isotherms and pore size distributions obtained from the
adsorption isotherm branch for the x HMS-epoxide-E silicas are shown in Figure 2.9A
and 2.9B, respectively. Increasing the epoxide functionality fi'om 0 to 0.05 and 0.10
causes the step in the adsorption-desorption isotherms to move from higher to lower
relative pressure, indicating a decrease of the pore size. The decrease in the pore size is
also clearly shown in the pore size distributions in Figure 2.9B. Extracted HMS-E silica
without an epoxide functionality has an average pore size of 2.3 run, while the 0.05-
HMS-epoxide-E and 0.10—HMS-epoxide-E have a pore size of 1.8 nm and 1.6 nm,
respectively. In the case of HMS-E, there is only hydrogen bonding interaction between
the head group of the surfactant micelles and the silicate framework. The interaction is
relatively weak and loose. When the desired amount of epoxide was introduced into the

silicate framework, at least equal amounts of the dodecylamine surfactant molecule will

57

form covalent bonds with the less substituted carbon on the epoxy ring by ring opening
reaction. Therefore, the interaction between the surfactant micelle and the silicate
framework became stronger and caused the pore size of the final product to shrink. The
more epoxide introduced into the framework, the smaller the pore size of the final
mesostructure. Figures 2.10A and 2.103 show the TEM images of 0.05-HMS-epoxide-E
at low and high magnifications. The wormhole structure can be Clearly observed. When
the epoxide functionality is increased to 0.10, the wormhole mesostructure was
maintained according to the TEM images (Fig. 2.10C, D). There is no significant
difference between the TEM images of 0.05- and 0.10-HMS-epoxide-E materials.
x-HMS-epoxide-E derivatives with x=0.25 an 0.50 both showed no porosity
based on the N2 adsorption desorption isotherms (Fig. 2.9). At these epoxy loadings, all
of the dodecylamine surfactants were expected to form covalent bonds with the
functionalized silicate framework, so that free surfactant can be removed by solvent
extraction. The absence of porosity for the x=0.25 and 0.50 HMS-epoxide-E derivatives
was confirmed by the TEM images (Fig. 2.11). Solid spherical particles with diameter of
few hundred nanometers were observed for the 0.25-HMS-epoxide-E material (Fig 2.11A,
B), while the 0.50-HMS-epoxide-E material showed no regular shape for the poreless
particles (Fig. 2.11C, D). When the epoxide fimctionality was increased to 0.50, it is hard
to fully crosslink all the organosilane into the silica framework. Therefore some uneven

contrast was observed in the TEM images of the 0.50-HMS-epoxide-E product.

58

 

14000«-
12000 ~
10000 ~

8000 ~

 

Counts

6000 ~

4000 -

2000 4

 

 

 

 

O 2 4 6 8 10
20 (degrees)

Figure 2.8. X-ray powder diffraction patterns of epoxide functionalized mesostructured
silicas made with dodecylamine as the surfactant and subsequent solvent extraction, x-

HMS-epoxide—E, where x represents the fraction of silicon centers with an epoxide

functional moiety.

59

 

900 ———-—~ ~-
”CD 700 L ‘1
E .
V 600
'0
Q3
'2 500
O .
'0 400
z 300

3 200

me

 

 

 

Vol
-b
O
O O

. 0.6 0.8 1
PIP,
1 ‘ (B)

0.9 - nx=0

0.6 1

 

dVIdD

 

 

 

 

Pore size diameter (nm)

Figure 2.9. (A) N2 adsorption-desorption isotherms and (B) pore size distributions
determined from the N2 adsorption isotherm branch for x-HMS-epoxide-E silicas
templated by dodecylamine after extraction with ethanol, where x is the fraction of

framework silicon atoms linked to epoxy groups.

60

 

Figure 2.10. TEM images for epoxide fimctionalized, surfactant-containing mesophase

silicas made with dodecylamine as surfactant, denoted x-HMS-Epoxide-E, where the

theoretical epoxy loading is x=0.05 (A, B) and 0.10 (C, D).

61

 

Figure 2.11. TEM images for epoxide functionalized, surfactant-containing mesophase

silicas made with dodecylamine as surfactant, denoted x-HMS-Epoxide-E, where the

theoretical epoxy loading is x=0.25 (A, B) and 0.50 (C, D).

62

To further understand the structure and porosity properties of the epoxide
functionalized surfactant containing mesophase materials, calcined derivatives denoted x-
HMS-epoxide-C were also investigated. HMS-epoxide-C materials share a similar trend
as the HMS-epoxide-E materials. N2 adsorption-desorption isotherms showed the
adsorption steps moved to lower relative pressure with increasing degree of epoxide
moiety functionality, indicating a decrease on the pore size of x HMS-cpoxide-C
materials (Fig. 2.12A). The physical properties of each product are summarized in Table
2.2. It is noteworthy that the x HMS-epoxide—C materials always have larger pore size,
pore volume and surface area than the counterparts x I-lMS—epoxide—E materials. Take the

=O.10 epoxide functionalized material for instance, the 0.10-HMS-epoxide-C derivative
has a pore size of 2.2 nm, total pore volume of 1.32 cm3 g", and surface area of 1374 m2
g'1 while the corresponding values for the 0.10-HMS-epoxide-E are 1.6 nm, 0.76 cm3 g",
and 758 m2 g". 0.25- and 0.50-HMS-epoxide-C also showed some porosity, which is
totally different from the counterparts obtained by solvent extraction. For example, 0.25-
HMS-epoxide-C has a pore size of 1.3 nm, pore volume of 0.76 cm3 g", and BET surface
area of 1128 m2 g'1 while the 0.25-HMS-epoxide-E derivative does not have any porosity
at all. This is because during the calcination process, the bonded epoxy and surfactant
were from the framework pores. The previously partially “stuffed” pores were open again,

causing the pore size, pore volume and surface area to increase.

63

 

 

(A)

3

Volume N2 Adsorbed (cm 9'1)
8
O

600 -

0|

0

O
l

 

N (A
O O
O O
l l

 

 

 

 

 

 

 

 

 

0 2 4 6 8 10
Pore size diameter (nm)

Figure 2.12. (A) N2 adsorption-desorption isotherms and (B) pore size distributions
determined from the N2 adsorption isotherm branch for x-HMS-epoxide—C silicas
templated by dodecylamine after calcination, where x is the fraction of framework silicon

atoms linked to epoxy groups.

64

Table 2.2. Physical properties of epoxide fiinctionalized mesostructure silica materials

after solvent extraction x-HMS-epoxide-E and calcination x-HMS—epoxide-C.

 

 

 

 

 

 

 

 

 

 

 

 

materials leO spacing Pore sizea Seer vmlb
(nm) (nm) (m2 g") (cm3 g")
HMS-E 4.80 2.3 862 1.02
0.05-HMS-epoxide-E 4.75 1.8 972 1.18
0.10-HMS—cpoxide-E 4.60 1.6 758 0.76
0.25-HMS-epoxide-E 3.77 N/A 0.1 l 0.09
0.50-HMS-epoxide-E 3.71 N/A 0.04 0.03
HMS-C 4.85 2.7 840 1.04
0.05-HMS—epoxide-C 4.85 2.3 1048 1.28
0.10HMS—epoxide-C 4.41 2.2. 1374 1.32
0.25-HMS-epoxide-C 3.87 1.3 1 128 0.76
0.50—HMS-epoxide-C 3.68 N/A 437 0.29

 

 

 

 

 

aPore size diameter was determined by the BJH model from the adsorption branches of

the isotherms. b Total pore volume was determined at P/Po=0.98.

65

 

Through the formation of a covalent bond between the surfactant and the epoxide
organic moiety in the silica framework, the surfactant was successfully retained inside the
framework pores of HMS-epoxide—E materials. Thermal gravimetric analysis was carried
out on the 0.50-HMS-epoxide-E material before and after the water extraction treatment.
The TGA profile of 0.50-HMS-epoxide-E material afier water extraction overlapped with
the one without water extraction treatment, indicating that the surfactants inside the
framework pores are stable and will not leach out (Fig. 2.13). It was also noted that
foaming phenomenon was not observed during Pb2+ trapping experiments, further
proving that the surfactants are covalently bonded to the silicate framework and are not
leached.

The N contents of the adsorbents were determined by CHN elemental analysis.
The results were listed in Table 2.3. Because all the nitrogen sources are from the
dodecylamine surfactant, the N content indicates the amount of surfactant covalently
bonded to the framework. The N content is proportional to the degree of epoxide
functionality from x=0.05 to 0.25. When the epoxide firnctionality increases from x=0.25
to 0.50, the N content did not double. When there is x=0.25 epoxide organic moiety in
the framework, the ratio of epoxide organosilane to dodecylamine is l to 1 and all of the
surfactant can covalently bind to the epoxide (Figure 2.7A). When increasing the epoxide
moiety to x=0.50, similar amount of surfactant was expected to be covalently bonded to
the framework but with ratio of epoxy ring to dodecylamine 2 to 1.

A single point Pb2+ trapping experiment was carried out on the HMS-epoxide-E
materials. The initial Pb2+ solution concentration was 116 ppm in all the cases. 0.05-

HMS-epoxide-E mesophase material reduced the Pb2+ concentration to 48 ppm and had

66

trapping capacity of 0.33 mmol g". The ratio of N to adsorbed Pb2+ on the 0.05-HMS-
epoxide-B material is 1.5. The nitrogen usage efficiency is relatively high in this case.
Upon increasing the degree of epoxide functionality, the N content increases as well. The
trapping capacity of HMS-epoxide-E material, however, decreased with the increasing
nitrogen content. It might because with the increasing surfactant molecules formed
covalent bonds and stayed inside the pores, the environment became more hydrophobic
in the framework pores. Therefore, it is hard for the hydrophilic lead cation to penetrate
to the nitrogen binding site, causing the overall trapping capacity to decrease. Only a
moderate degree of epoxide functionality provides a sufficient amount of surfactant
inside the framework pores while at the same time maintaining a hydrophilic mesopore

environment.

67

1 10 In, WM..- w

 

 

 

 
  

100 :~
90 {
E 0.50-HMS-epoxide-E
\. 3° g after water extraction
:5» 70 E
Q t
3 :
60 f

40}

 

..
b
30 A r 1 1 LL 1 L 1

5° 0.50-HMS-epoxide-E

 

 

 

 

0 100

200 300 400 500 600 700
Temperature (°C)

Figure 2.13. Thermal gravimetric analysis (TGA) profiles of 0.50-HMS-epoxide-E before

and after water extraction.

68

Table 2.3. Pb2+ trapping performance of epoxide functionalized, dodecylamine—containing

mesostructured silica materials, denoted x-HMS-epoxide-E.

 

 

 

 

 

 

x-HMS-epoxide-E Equilibrium Pb2+ cone. 3 N density b Pb2+ adsorbed N : Pb2+
x= (ppm) (mmol g-l) (mmol g") ratio
0.05 48 0.50 0.33 1.521
0.10 69 0.81 0.23 3.511
0.25 102 2.02 0.07 29:1
0.50 114 2.25 0.01 225:1

 

 

 

 

 

a 1 g of the silica adsorbent was dispersed in 1000 mL lead nitrate solution containing

116 ppm Pb”. b Nitrogen content was calculated from the CHN elemental analysis

carried out in-house.

69

 

2.4 Conclusion

Most of the applications of the mesostructured silicas have focused on the
functionalized surfactant-free mesoporous materials. For the purpose of heavy metal
removal, different organic groups have been grafted into the surfactant-flee silicate
framework in order to achieve specific affinity with various heavy metals. In this work,
an attempt was made to utilize the surfactant for the removal of heavy metal ions from
solution.

The tallow amine surfactant intercalated mesophase silicas showed good affinity
towards heavy metal Pb“. The tallow monoamine surfactant intercalated mesophase (N1)
exhibited a maximum trapping capacity of 0.2 mmol of Pb2+ per gram of adsorbent, while
the tallow diamine, triamine and tetraamine surfactant derivatives had a similar maximum
Pb2+ trapping capacity of 1.2 mmol g". The multiamine surfactant intercalated
mesophases have a higher trapping capacity compared to the monoamine derivative,
suggesting the chelating effect between the amine binding sites and the lead cations. The
leaching rate of the surfactant in the as-made mesophase materials was described as being
very low.2| However, it remained a concern that the surfactant was not irreversibly bound
inside the mesopores.

By introducing the epoxide organic moiety into the silica material synthesis
process using the dodecylamine as the structure directly agents, the surfactant can be
chemically stabilized inside the mesopores through the formation of covalent bonds
between the amine head groups on the surfactant and the carbon on the epoxide ring. The
epoxide functionalized, surfactant-containing mesophase silica HMS-epoxide-E was

successfully synthesized. The surfactant was covalently connected to the silicate

70

framework and would not leach out during the adsorption applications. A wormhole
structure was observed for the surfactant containing mesophases with moderate
functionality of the epoxide moiety (i.e., x=0.05 and 0.10), while further increases in
epoxide loading resulted little or no porosity of the final products due to the surfactant
fully stuffing the mesopores. 0.05-HMS-epoxide—E has a 0.33 mmol g'l Pb2+ trapping
capacity and a high accessibility of the nitrogen. The Pb2+ trapping capacity of the
mesophase materials decreased while increasing degree of epoxide functionality. The
reduction in binding capacity with increased epoxide functionality may be due to a more
hydrophobic pore environment. Therefore, the hydrophilic Pb2+ species is difficult to
penetrate the interface between the surfactant and the silicate, where the nitrogen binding
sites were located. It is suggested that introducing moderate epoxide functionality into the
silica framework keeps the surfactant containing mesophase materials porous and less
hydrophobic. This provides good heavy metal trapping capacity and higher availability of

the surfactant binding site.

71

2.5 References

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

Beck, J. S.; Vartuli, J. C.; Roth, W. J .; Leonowicz, M. E.; Kresge, C. T.; Schmitt,
K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; et al. J. Am. Chem. Soc.
1992, 114, 10834-43.

Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A.
Chem. Commun. 1998, 317-318.

Bossaert, W. D.; De Vos, D. E.; Van Rhijn, W. M.; Bullen, J .; Grobet, P. J .;
Jacobs, P. A. J. Catal. 1999, 182, 156—164.

Yu, W.; Zhang, Z.; Wang, H.; Ge, 2.; Pinnavaia, T. J. Microporous Mesoporous
Mater. 2007, 104, 151-158.

Cao, W.; Zhang, H.; Yuan, Y. Catal. Lett. 2003, 91, 243-246.

Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500-503.

Lesaint, C.; Frebault, F .; Delacote, C.; Lebeau, B.; Marichal, C.; Walcarius, A.;
Patarin, J. Stud. Surf Sci. Catal. 2005, 156, 925-932.

Yoshitake, H.; Yokoi, T.; Tatsumi, T. Chem. Mater. 2003, 15, 1713-1721.

Bibby, A.; Mercier, L. Chem. Mater. 2002, 14, 1591-1597.

Nguyen, T.-Q.; Wu, J .; Doan, V.; Schwartz, B. J .; Tolbert, S. H. Science 2000,
288, 652-656.

Ganschow, M.; Wark, M.; Wohrle, D.; Schulz—Ekloff, G. Angew. Chem, Int. Ed.
2000, 39,161-163.

Kageyama, K.; Tamazawa, J .-I.; Aida, T. Science 1999, 285, 2113-2115.

Zhou, W.; Thomas, J. M.; Shephard, D. S.; Johnson, B. F. G.; Ozkaya, D.;
Maschmeyer, T.; Bell, R. G.; Ge, Q. Science 1998, 280, 705-708.

72

(14)

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Zhang, X.; Guan, R.-F.; Wu, D.-Q.; Chan, K.-Y. J. Mater. Sci. : Mater. Med. 2007,
18, 877-882.

Czuryszkiewicz, T.; Ahvenlammi, J .; Kortesuo, P.; Ahola, M.; Kleitz, F .; Jokinen,
M.; Linden, M.; Rosenholm, J. B. J. Non-Cryst. Solids 2002, 306, 1-10.

Arcos, D.; Ragel, C. V.; Vallet-Regi, M. Biomaterials 2001, 22, 701-708.

Vallet-Regi, M.; Ramila, A.; del Real, R. P.; Perez-Pariente, J. Chem. Mater.
2001, 13, 308—31 1.

Kubota, Y.; Nishizaki, Y.; Sugi, Y. Chem. Lett. 2000, 998-999.

Denoyel, R.; Rey, E. S. Langmuir 1998, 14, 7321-7323.

Zhao, H.; Nagy, K. L.; Waples, J. S.; Vance, G. F. Environ. Sci. T echnol. 2000,
34, 4822-4827.

Abdel-Fattah, T. M.; Bishop, B. J. Environ. Sci. Health, PartA 2004, A39, 2855-
2866.

Vartuli, J. C.; Malek, A.; Roth, W. J .; Kresge, C. T.; McCullen, S. B.
Microporous Mesoporous Mater. 2001, 44-45, 691-695.

Zhao, Y. X.; Ding, M. Y.; Chen, D. P. Anal. Chim. Acta 2005, 542, 193-198.

Yamaguchi, A.; Amino, Y.; Shima, K.; Suzuki, 8.; Yamashita, T.; Teramae, N. J.
Phys. Chem. B 2006, 110, 3910-3916.

Yamaguchi, A.; Watanabe, J .; Mahmoud, M. M.; Fujiwara, R.; Morita, K.;
Yamashita, T.; Amino, Y.; Chen, Y.; Radhakrishnan, L.; Teramae, N. Anal. Chim.
Acta 2006, 556, 157-163.

Miyake, Y.; Yumoto, T.; Kitamura, H.; Sugimoto, T. Phys. Chem. Chem. Phys.
2002, 4, 2680-2684.

73

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Miyake, Y.; Hanaeda, M.; Asada, M. Ind. Eng. Chem. Res. 2007, 46, 8152-8157.

Hashizume, H. J. Environ. Sci. Health, Part A: T oxic/Hazard. Subst. Environ.
Eng. 2004, A39, 2615-2625.

Sayari, A.; Hamoudi, S.; Yang, Y. Chem. Mater. 2005, 17, 212-216.

Choi, H. S.; Lee, D. G.; Cho, G. J.; Lee, C. Y.; Chung, J. S.; Yoo, l.-k.; Shin, E.
W. Hwahak Konghak 2006, 44, 172-178.

Xu, 8.; Boyd, S. A. Environ. Sci. T echnol. 1995, 29, 312-20.

Li, Z.; Roy, S. J .; Zou, Y.; Bowman, R. S. Environ. Sci. T echnol. 1998, 32, 2628-
2632. -

74

Chapter 3
Synthesis of mesostructure silicas with more accessible amino groups and their

application as lead (sz+) adsorbents

3.1 Introduction

Lead poisoning remains one of the most prevalent diseases of environmental
origin in the United States today, particularly affecting young children. The US. Centers
for Disease Control and Prevention currently estimates that 2.2 % of all US. children
aged 1-5 years (434,000 children) have elevated blood lead levels.1 Effective approaches
to remove the toxic metals from contaminated water have involved the use of solid
adsorbents, such as activated carbon, ion exchange resin, functionalized silica-based
materials including silica gels, clays and mesoporous silicas. The advantages of
functionalized mesoporous silicas for environmental remediation are their high surface
areas, well-defined pore size, and the ability to covalently link organic groups to the
framework to allow for selective adsorption of specific toxic heavy elements.

There are generally two methods to prepare mesostructured silicas with organic
groups, namely grafting and direct assembly.2 The grafting of functional groups to the
pore walls of mesoporous molecular sieves can be achieved by the covalent fixation of
hydrolysable moieties (of the type RgsiL, where R = C1 or OCH3 and L = functional
organic group) to the surface Si-OH groups lining the pores of the mesostructures. Due to
the limited number and random distribution of silanol groups on the surface of silicas,
organofunctionalized mesostructured silicas from grafting method often end up having a
low and uncontrollable loading of organic groups on the surface. The direct assembly

method, on the other hand, uses the co-condensation of tetraethyl orthosilicate (TEOS)

75

and functionalized trialkoxysilane R3SiL during the mesostructure synthesis process to
give a better distribution of the organic groups through out the framework and a higher
organic group loading. Therefore, the direct assembly pathway is considered superior to
the grafting method.3

Many organic groups, such as vinyl, amino, thiol, imidazole etc., have been
incorporated into the framework of mesostructured silicas by direct assembly pathway.4'
” Among all the organic groups been incorporated into the framework, mercaptopropyl
and aminopropyl are most popular due to their high affinity towards heavy metals in the
application of environmental remediation. Mercaptopropyl group has been incorporated
into different mesostructures, such as SBA-15, MGM-41 and HMS, trough co-
condensation pathway then applied extensively as adsorbents for heavy metal ions like
Hg(II)4"2'”. Amine functionalized mesostructured silica showed good trapping capacity
for heavy metal ions because of the acid-base interaction between the lone pair on the
nitrogen and metal cation.9"4"5. However the co-condensation reaction of aminopropyl
organosilane with TEOS otten suffers from the strong disruption of the mesostructure,
particularly at organosilane levels above 20 mol %.l6

Macquarrie et al. found that the amine containing silanes consistently give lower
surface area than other kinds of organosilanes when incorporated into HMS silica
mateirals.l7 A low pore volume and surface area were reported for amino functionalized
SBA-15 silicas, as well.'8’l9 Tatsumi et al. also revealed that not all the incorporated
amino moieties were present on the surface; some of them were in the walls of the MCM-
41 mesostructure framework.9 It was suggested that the aminopropyl group of the amino

organosilane binds to the Si atom of the same aminosilane molecule or to a neighboring

76

silicon and forms a penta—coordinate intermediate (as shown in Figure 3.1) that is very
reactive with nucleophiles such as the hydroxyl groups formed after the hydrolysis of
alkoxy groups on the organosilane or TEOS. It is also possible that the penta-coordinate
intermediate is very reactive with water, leading to uncontrolled levels of polymerization
(Figure 3.2).20 Therefore, the direct co—condensation of APTES and TEOS will suffer the

lost of mesostructure, and the accessibility of the amino groups will be low.

 

 

 

H2N ------ S
l"!!’,,
l\ R
R R
Figure 3.1. Penta-coordinate intermediate formed by the aminopropyl organosilane
' T
" H2N ....... Si” H20 : /0\
I uIIIIQCHZCHa /Si Si\
H3CHZCO OCHZCHS _ HaCHzCO \ \ OCHZCH3_m

0011ch3 OCH2CH3

Figure 3.2. Uncontrolled polymerization of aminopropyl organosilane

Chong et al. studied the effect of different organosilanes on the formation of the
mesostructured silica.21 They found that the amine organosilane had the largest disruptive
effects on the formation of the mesostructures. The co-condensation method requires that
the organic group to be sufficiently hydrophobic to enter the core of the micelle and not
too bulky to avoid its penetration. It was suggested that the amine group was very

hydrophilic and this affected the mesostructure formation the most.22 Rosenholm et al.

77

also showed that increasing the content of amine groups in the SBA-15 framework, the
actually accessible amine groups dropped, indicating that most of the amines were buried
in the wall of the silica framework, resulting in a disruptive effect on the formation of the
mesostructure.“5 In the case of a HMS mesostructured silica framework, the introduction
of aminopmpyl compromised the wormhole framework, as verified by the absence of a
diffraction peak at low angle in the X-ray pattern and a low surface area, as obtained
from the nitrogen isothems.23 Figure 3.3 demonstrates the possible positions of a
hydrophilic aminopropyl moiety and a hydrophobic organic moiety (such as the
mercaptopropyl group) within a silicate framework which was synthesized with a
dodecylamine surfactant.

Due to the hydrophilicity of the amine fimctionalized organic moiety and the
hydrogen bonding interaction between the amine organosilane and the silicate framework,
it remained a challenge to synthesize accessible amine groups through co-condensation
methods. Tatsumi proposed the incorporation of amine functional group into the silica
framework through the use of an anionic surfactant during the co-condensation
process.“25 The electrostatic interaction between the positively charged protonated
amino groups in APTES and the negatively charged head groups in the anionic surfactant
is a driving force for the self-assembly of the mesostructured organosilica. The amino
functionalized material synthesized via the use of an anionic surfactant showed higher
accessibility for the aminopropyl moiety in contrast to the MGM-41 derivative

synthesized with a cationic surfactant.26

78

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79

In addition to the assistance provided by an electrostatic interaction between the
positively charged amine moiety and the anionic surfactant, more chemistry can be
utilized to improve the availability of the amine fimctional groups in the silicate
framework. If the organosilane incorporated into the silica framework is hydrophobic, it
will have a less disruptive effect on the formation of the mesostructure. Furthermore, if
the hydrophobic group can be later converted into an amino group, a highly ordered
mesostructure and better accessibility of the amine groups can be achieved.

In the present work, alternative approaches were used for the co-condensation of
TBOS and organosilanes with hydrophobic groups, which later can be converted into
amine groups in order to make them more accessible. First, 3-tert-
butyloxycarbonylaminopropyltriethoxysilane (NHtBocTES) was introduced through co-
condensation with tetraethyl orthosilicate (TEOS) with a dodecylamine surfactant under
neutral pH conditions. The NHtBoc group is more hydrophobic compared to an amine
group, so it is more likely to orient towards the core of the surfactant micelle, thus
forming a better mesostructures. The amine group will be recovered by the subsequent
deprotection of the tBoc group. Using the deprotection of carbamate groups, which are
commonly used as protecting agents for amino groups in synthetic chemistry, earlier
workers successfully incorporated amine groups into bulk silica27 and a SBA-15
mesostructured silica28 systems. The amine groups were recovered upon the removal of
the protecting Boc group by either acidic hydrolysis or thermal treatment. However, the
synthesis condition for SBA-15 is acidic at the very beginning, so the protecting groups

may already be removed partially prior to mesostructure assembly. This work will use

80

dodecylamine surfactant under neutral pH condition in order to keep the Boc protected
amine groups intact during mesostructure synthesis via the co-condensation method.

Secondly, an amine-functionalized mesostructured ‘silica will be synthesized via
the co-condensation of tetraethyl orthosilicate (TEOS) and 4-(Triethoxysilyl)butyronitrile
(CNTES) with surfactant dodecylamine under neutral pH conditions. A nitrile group is
much more hydrophobic compared to an amine group, and could point towards the core
of the surfactant micelle to form more accessible organic groups. It has been successfully
incorporated into the mesostructure by co-condensation method. A CN-functionalized
MGM-41 showed much higher pore volume and surface area than the amine containing
functionalized counterparts.29 CN functionalized SBA-15 also showed good
mesostructure ordemessfm’3| It was suggested that the disruptive effects on the
formation of a mesostructure was much smaller for a nitrile organosilane than an amino
organosilane.” Moreover, the amine group can be formed through a reduction process
after the nitrile been successfully incorporated into the mesostructure framework.

More accessible amine groups were expected to be obtained by these two novel
alternative amine incorporation routes. The resultant materials will be tested for the lead
trapping capacities in the comparison with the HMS-NH2 obtained by conventional direct

co-condensation of TEOS and the amine organosilane APTES.

81

3.2 Experimental methods

3.2.1 Reagents

Dodecylamine (DDA), tetraethyl orthosilicate (TEOS),
aminopropyltriethoxysilane (APTES), 4-(Triethoxysilyl)butyronitrile (CNTES),
concentrated HCl solution, CHZClz, Eth, lead atomic absorption standard solution
(~1000 ppm), and 99.999% lead nitrate were purchased from Aldrich. 3-tert-
butyloxycarbonylaminopropyltriethoxysilane (NHtBocTES) (shown in Figure 3.4) was
purchased from Gelest. Ethanol was purchased in—house. All the reagents were used
without further purification. Water used in the synthesis was double-exchanged to

remove cations and anions via a Millipore filter apparatus.

Figure 3.4. Chemical structure of butyloxycarbonylaminopropyltriethoxysilane
(NHtBocTES).

3.2.2 Material synthesis

3.2.2.1 Synthesis of HMS-NHtBoc and HMS-NH2 (Boc) materials

tBoc protected amine functionalized mesostructured HMS organosilica was
synthesized from tetraethyl orthosilicate (TEOS) and 3-tert-

butyloxycarbonylaminopropyltriethoxysilane (NHtBocTES) using the direct assembly

82

method in the presence of dodecylamine (DDA) as the surfactant. In a typical co-
condensation reaction, the desired amount of DDA was dissolve in ethanol and water co-
solvent to get homogeneous solution. The mixture of TEOS and NHtBocTES was added
to the surfactant solution, followed by aging the mixture at 60 °C for 24 hr. The products
were recovered by filtration and dried in an 80 °C oven overnight. The surfactant
template was removed by solvent extraction using a solid to ethanol ratio of 1 g to 50 mL.
The extraction step was repeated and the final products were dried in the 80°C oven to
remove any excess ethanol. The overall molar composition of the reaction mixture was x:
(l-x): 0.25: 20: 180 NHtBocTES: TEOS: DDA: EtOH: H20, where x represents the mole
fraction of silicon centers containing the Boc protected amine group. The expected
molecular formula for the organosilica products was (Si02)1-x(Si01.5C3H6NHtBoc)x,
where x equals to 0.10 and 0.20. The final products were denoted x HMS-NHtBoc, where
x represents the mol fraction of silicon centers containing the Boc protected amine groups.

For the recovered amine functionalized HMS silica materials, 1.5 g of HMS-
NHtBoc was dispersed in 40 mL 6 M HCl solution in a round bottle flask. The mixture
was heated at 100°C overnight. The solid was filtered, washed with water, and dried in an
80°C oven completely. The dry solid was introduced in 30 mL of Et3N/CH2C12 mixture
(1:5 volume ratio of Et3N to CHzClz) to deprotonate the amine groups. The final product
was recovered by filtration and was dried in an oven overnight. It was denoted x HMS-
NH2 (Boc), where x represents the mol fraction of silicon centers containing the re-

generated amine groups.

83

3.2.2.2 Synthesis of HMS-CN and HMS-NH2 (CN) materials

A nitrile functionalized mesostructured HMS organosilica was synthesized from
tetraethyl orthosilicate (TEOS) and 4-(Triethoxysilyl)butyronitrile (CNTES) using the
direct assembly method in the presence of dodecylamine (DDA) as the surfactant. The
methods used for the synthesis were analogous to those provided in section 3.2.2.1 for the
synthesis of HMS-NHtBoc. The resultant material was denoted x HMS-CN, where x =
10% or 20%, denoting the percentage of framework silicon centers that are organo-
functionalized.

For the amine functionalized HMS silica materials, CN-HMS was dispersed in
THF solution containing lithium aluminum hydride (LAH) as the reducing agents. The
reaction was run at room temperature under N2 protection for 4 hr followed by a HCl
solution work-up. A Bth/CHZCIZ mixture was used to remove the acidic proton on the
protonated amine group. The final product was denoted x HMS-NH2 (CN) indicating the
amine groups were obtained by reduction of the nitrile groups, where x equals to 10 or 20
mol%.

A conventional direct co-condensation of TEOS and aminopropyl (APTES) was
also carried out at x equals to 20mol % for comparison purposes. The product was

denoted 20 % HMS-NH2 (conv.).

3.2.3 Pb“ trapping experiments

The adsorption experiment was carried out for metal lead by stirring 0.1 g of
functionalized silica material in 100 mL of lead nitrate (PbCN03)2) solution at 25°C. The

initial concentration for these experiments was 100 ppm in all cases. Mixtures were

84

stirred for 24 hours and then filtered through a 0.25 um filter paper to collect the final
filtrate solution.

The amount of lead in the filtrate after adsorption experiments was analyzed by
cold vapor atomic absorption spectroscopy (AAS). And the difference between the initial
and equilibrium lead concentrations indicates the amount of lead that was trapped on the

solid adsorbents.

3.2.4 Characterization

X-ray diffraction (XRD) patterns were obtained on a Rigaku Rotaflex 200B
diffractometer equipped with Cu K, X-ray radiation and a curved crystal graphite
monochromator operating at 45 kV and 100 mA.

N2 adsorption-desorption isotherms were obtained at — 196 °C on a Micromeretics
Tristar 3000 sorptometer using standard procedures. Samples were outgassed at 150 °C
and 10 *5 Torr for a minimum of 12 hr. prior to analysis. Surface areas were calculated
from the linear part of a BET plot of the nitrogen adsorption data according to IUPAC
recommendations. The Barrett-Joyner—Halenda (BJH) method was used to obtain the pore
size distribution from the adsorption branch of the isotherms.

29Si MAS NMR and ‘3 C CP MAS NMR spectra were recorded on a Varian 400
solid state NMR spectrometer with a Zirconia rotor at a spinning frequency of 4 kHz. 29Si
MAS NMR spectra were obtained at 79.5 MHz under single-pulse mode and a pulse
delay of 400 seconds. '3 C CP MAS NMR spectra were recorded at 100.5 MHz under

cross polarization mode with a pulse delay of 2 seconds and contact time of 1.3 ms.

85

Fourier transform infrared spectroscopy was used to identify the functional groups
in the samples. Spectra were recorded with a Mattson Galaxy series 3000 equipment
between 400 and 4000 cm'1 using KBr pellets.

Transmission electron microscopy (TEM) images were taken on a JEOL 2200FS
microscope with filed emission electron source and an accelerating voltage of 200 keV.
Sample grids of mesoporous silicas were prepared by sonicating the powdered sample in
EtOH for 10 min and evaporating 2 drops of the suspension onto a holey carbon-coated
film supported on 300 mesh copper grids.

Equilibrium concentrations of Pb2+ in solution were measured on a cold vapor
atomic absorption spectroscopy Varian SpectrAA-200 using lead cathode with maximum

working current of 10 mA.

86

3.3 Results and discussion

3.3.1 Synthesis of mesostructure HMS-NHtBoc and HMS-NH2 (Boc) materials

The butyloxycarbonyl-protected amine group (—NHtBoc) has hydrophobic
properties and can be converted into amino groups through Boc group deprotection.2728
Also the Boo group is relatively large and has a size similar to a silica tetrahedron.
Therefore, it is less likely to be buried in the walls of the silica framework and to form a
better mesostructure by direct assembly method in comparison with the direct co-
condensation of TEOS and APTES. Accordingly, 10 and 20 mol % of
butyloxycarbonylaminopropyltriethoxysilane (NHtBocTES) was incorporated into the
mesostructured silica by co-condensation reaction with TEOS in the presence of
dodecylamine as surfactant at a reaction temperature of 60 °C. The resulting
mesostructure was expected to survive the Boc deprotection process.

As is evident from the T EM images (Fig.3.5A), a wormhole mesostructure was
successfully synthesized by the co-condensation of a Boc protected amine moiety and
TEOS. The product after Boc deprotection still retained the wormhole structure according
to Figure 3.53. The pore size in the recovered amine functionalized mesostructured HMS
silica appeared somewhat larger than that of the Boc protected amine functionalized

derivative based on the TEM images. The accurate pore sizes will be determined from the

N2 adsorption-desorption data.

87

 

Figure 3.5. Transmission electron microscopy (TEM) images of (A) Boc-protected amine
functionalized mesostructured silica, 20% HMS-NHtBoc and (B) the recovered amine
fimctionalized mesostructured material after Boc deprotection, 20% HMS-NH2 (Boc).

88

Figure 3.6 provides the powder XRD patterns for the NHtBoc functionalized
HMS silicas and the amine derivatives after the removal of the protecting groups. The
products containing 10 mol or 20 mol % NHtBoc groups showed a single X-ray
diffraction at 20 around 2.0°, indicating the presence of a wormhole framework (Fig. 3.6).
After the Boc group deprotection process, the HMS-NH2 (Boc) materials also showed a
single diffraction at low angle without any broadening of the peaks, suggesting that the
wormhole mesostructure was well maintained. The d spacing of the functionalized
mesostructure silica calculated from the 20 values remained the same, 4.4 nm for lOmol
% functionality and 4.7 nm for 20 mol % functionality samples, prior to and upon the
removal of the tBoc protecting groups, indicating that the deprotection process did not
affect the overall framework order. The XRD patterns agree with the observation in the
TEM images.

Figure 3.7 reports the N2 adsorption-desorption isotherms and Barrett-Joyner-
Halenda (BJH) framework pore size distributions of the HMS-NHtBoc and HMS-NH2
(Boc) mesostructured organosilicas. Table 3.1 lists the d spacing, surface area, average
framework pore size and pore volume for each product. The 10 and 20% HMS-NHtBoc
derivatives and the 20% HMS-NH2 (CN) products showed steep N2 uptake in the P/Po
region of 0.2, suggesting the pore size for these three materials are more towards the
micropore range. The 10% HMS—NH2 (Boc) from showed steep N2 uptake in the P/Po
region of 0.3-0.4, suggesting the pore size of this material to be close to the mesopore
range. The pore size for 10 and 20% HMS-NHtBoc materials are 1.2 and 1.5 nm, while
values of 1.5 and 1.8 nm are obtained for 10 and 20% HMS-NH2 (Boc), respectively. The

pore size decreased from 1.5 nm to 1.2 nm, while increasing the NHtBoc loading from

89

10% to 20%. This is because the introduction of the hydrophobic NHtBoc groups will
occupy space inside the mesopores, causing the pore size to decrease. HMS-NH2 (Boc)
materials have a pore size 0.3 nm larger than the HMS-NHtBoc materials for both 10 and
20 mol % amine loadings due to the removal of the bulky tert-Boc groups. As the
functional group loading is increased, the surface area and pore volume decreases. This is
a common result for directly assembled functionalized mesostructured materials, due to
pore congestion by the organic moieties. Moreover, the total volume and BET surface
area of HMS-NH2 (Boc) were comparable to the counterparts before the Boc deprotection
process. Take 20 % functionalized HMS materials, for example. The 20 % HMS-NHtBoc
derivative has a pore volume of 0.97 cm3 g'I and a surface area of 573 m2 g'I while values
for the 20% HMS-NH2 (Boc) are 0.95 cm3 g'l and 567 m2 g", respectively.

The physical properties of the 20 mol % HMS—NH2 (conv.) which was
synthesized by the conventional direct co-condensation of TEOS and APTES directly are
also listed in Table 3.1. This derivative showed no X-ray diffraction and a very broad
pore size distribution. The pore volume and surface area were only 0.60 cm3 g‘1 and 99
m2 g". While for the 20mol % HMS-NH2 (Boc) synthesized by the Boc protection
pathway, the material showed d spacing of 4.7 nm, a pore size diameter of 1.2 nm, a BET

surface area of 573 m2 g"1 and a total pore volume of 0.97 cm3 g".

90

14000

 

    
 

 

 

 

 

12000 -
10% HMS-NH2 (Boc)
10000 _
3 8000 4
S

8 0000 4 10% HMS-NHtBOC
20% HMS-NHtBoc

4000 ~

20% HMS-NH2 (Boc)

2000 a

0 t .

 

 

O 1 2 3 4 5 6 7 8 9 10
29 (degrees)

Figure 3.6. X-ray diffraction patterns of amino functionalized HMS mesostructured

silicas before and alter the deprotection of tBoc groups at different organosilane loadings
of 10 mol % and 20 mol %.

91

800

700

600

500

400

300

200

Volume N2 Adsorbed (cm3 9'1)

100

    

 

(A) 1 0% HMS-NHtBoc

.._.

  

, 20% HMS-NHtBoc
20% HMS-NH2 (Boc)

 
  

 

 

 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

PIPO
E (B)
; / 10% HMS-NHtBoc

/ 20% HMS-NHtBoc

3 10% HMS-NH2 (Boc)

 

 

 

  

 

0 2 4 6 8

Pore Diameter (nm)

10

Figure 3.7. (A) N2 adsorption-desorption isotherms and (B) BJH pore size distributions

for mesoporous amino-functional HMS wormhole silicas (10 and 20 % NH2 groups)

before and afier the deprotection of the tBoc groups.

92

Table 3.1. Physical properties of Boc protected amine functionalized mesostructured
silica (HMS-NHtBoc), the recovered amine functionalized mesostructured silica (HMS-
NH2 (Boc)) and conventionally assembled amine functionalized silica (HMS-NH2 (conv.))

synthesized with a dodecylamine porogen.

 

 

 

 

 

 

 

 

 

 

 

d spacing Pore size a Total volume b BET surface

(Hm) (um) (cm3 g") area (m2 3")
10%HMS-NHtBoc 4.4 1.5 1.13 948
10%HMS-NH2 (Boc) 4.4 1.8 1.04 713
20%HMS-NHtBoc 4.7 1.2 0.97 573
20%HMS-NH2 (Boc) 4.7 1.5 0.95 567
20%HMS-NH2 (conv.) no peak 2.2 0.60 99

 

 

aPore size diameter was determined from the BJH model and the adsorption branches of

the isotherms. b Total pore volume was determined at P/Po=0.98.

93

Figure 3.8 shows the complete conversion from —NHtBoc groups to ~NH2 groups,
as judged by FT-IR spectroscopic study. In Figure 3.8A, the absorbance around 1700 cm'

I is from the carbon-oxygen stretching frequency for the —C=O in the Boc group. The

absorbances around 2987 and 2950 cm'1

are the characteristic peaks for the OH
stretching modes in the t-Butyl groups of the Boc group, and the 1370 cm”I absorbance is
the OH bending from the t-Butyl groups of the Boc groups. After Boc removal, the FT-
IR spectrum of the amino functionalized mesostructured silica, denoted HMS-NH2 (Boc),
showed no absorbance characteristic from Boc groups, indicating that the removal of Boc
group was successful and complete (Fig. 3.83). The calcined mesostructured HMS silica
is also shown in Fig. 3.8C for comparison. The spectrum in Fig. 3.83 is very similar to
the spectrum in Fig. 3.8C except for absorbance around 2950 cm'1 indicating the alkane
C-H stretching, and the 1540 cm'l band indicating the N-H bending frequency.

The 13C cross polarization MAS NMR spectrum for HMS-NHtBoc showed the
presence of a carbonyl group (-C=O) around 154 ppm, methyl groups around 24 ppm and
the tertiary carbon around 75 ppm from tert-Butyl groups, indicating that the —NHtBoc
fimctional groups were successfully incorporated into the silica material (Fig. 3.9). After
the deprotection of tBoc group, only the alkyl carbons from aminopropyl groups were
observed, further proving that the removal of Boc is complete and the conversion to NH2
is successful. The resonance at 54 ppm is attributed to the methylene carbon of an ethoxy
group most likely originating from the ethanol used to remove the surfactant. The

resonance from methyl carbon of the ethoxy groups which should be around 18 ppm was

not distinguishable due to other peaks present in that region.

94

 

1‘ * " ’ ‘ . ""1
I N-H bendi\n0 C-H bendmg from t-butyl

  

C-H stretch from t-butyl
C=O stretch

‘\/ V“ /"\

K1

triliLilliliiJliiiluiiilirLiliililrLLrlihLl

4000 3600 3200 2800 2400 2000 11600 1200 800 400
wavenumber (cm' )

 

 

 

Transmittance %

 

 

 

 

 

 

Figure 3.8. FT-IR spectra of (A) 20 mol % HMS-NHtBoc before the Boc group
deprotection, (B) 20 mol % HMS-NH2 (Boc) afier the deprotection and (C) calcined

HMS containing no organic groups.

95

 

 

5 2
4 * ** 3 L\1\_

 

 

 

 

 

2 3 2 1
E M
200 150 100 50 0 -50
chemical shift (ppm)

Figure 3.9. 13C CP-MAS NMR spectra of the mesostructure organosilicas 20 mol %
HMS-NHtBoc and 20 mol % HMS-NH2 (Boc).

* Side bands in the MAS NMR spectra.
** These two peaks are attributed to residual either ethoxy groups from the solvent

ethanol used for extraction of the surfactant.

96

The HMS-NHtBoc and HMS-NH2 derivatives showed no significant difference
on the 29Si MAS NMR spectra, both having three peaks prior to and after the removal of
the Boo groups (Figure 3.10). The one around -110 ppm is the Q4 band which indicates
the complete crosslink of SiO4 units. The -100 ppm peak is Q3, demonstrating the silica
center with three oxygens bridging to another tetrahedral silica center and a silanol group.
The T3 band around -70 ppm indicates the silicon centers which have been functionalized
by organic groups (existence of Si-C bond). The 20 mol % HMS-NHtBoc product
showed 13% organo-functionalization based on the integral intensity of the area beneath
the T3 band. The relative intensity remained the same after the removal of tBoc group for
the 20 mol % HMS-NH2 (Boc) material. The similarity in 29Si MAS NMR spectra for the
HMS-NHtBoc and HMS-NH2 materials suggested that the silica framework was not
changed during the removal of Boc protecting groups.

As expected, the Boc-protected, amine-functionalized mesostructured HMS silica
was successfully synthesized due to the hydrophobicity of the bulky tBoc groups.
Moreover, the mesostructure was retained after the removal of Boc protecting groups and
the amine functionalized mesostructured synthesized by this alternative route showed a
better quality mesostructure in comparison with the derivative synthesized by the
conventional co-condensation of TEOS and APTES. The accessibility of the recovered
amine groups will be investigated by heavy metal ion trapping experiments and reported
later this chapter.

The Boc protected amine moiety has only been incorporated previously into SBA-
15 mesostructured silica system as reported in the literature.28 This work was repeated at

20% organic moiety loading to obtain 20% SBA-NHtBoc and 20% SBA-NH2 (Boc)

97

materials. Neither material showed characteristic peaks for the hexagonal arrayed
mesostructure in the XRD patterns. This might because the synthesis conditions for SBA-
15 co-condensation are very acidic, though not as strongly acidic as the acidic hydrolysis
conditions used to deprotect the Boo groups. Nevertheless, the Boo protected amine
organic groups do not appear to completely stable during the direct assembly process.
Thus, it will have unprotected amine groups, same as the direct co-condensation of TEOS
and APTES. Therefore, the Boc protecting groups may not function as expected in the
SBA-15 system. The accessibility of the amine groups on the 20% SBA-NH2 (Boc)

materials will be studied in the lead trapping experiment described below.

98

 

 

Hr‘g'r'1’m‘4—t—L‘ ‘ t"*""“‘r—t—"‘—‘_‘”t L * . %J—* L ‘ i ' ‘ t ‘ * *144‘4-4—1
0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200
chemical shift (ppm)

Figure 3.10. ”Si MAS NMR spectra of (A) the mesostructured organosilica 20 mol %
HMS-NHtBoc and (B) 20 mol % HMS-NH2 (Boc).

99

3.3.2 Synthesis of mesostructured HMS-CN and HMS-NH2 (CN) materials

The hydrophobic nitrile group has been previously incorporated into a silica
mesostructure by the co-condensation method. CN-functionalized MCM-4l showed a
much higher pore volume and surface area than the amine-containing functionalized
counterparts.29 CN—fimctionalized SBA-15 also showed good mesostructure order.6’3°'3| It
was suggested that the disruptive effects on the formation of the mesostructure was much
smaller for the nitrile organosilane than the amino organosilane.21 Moreover, the CN
group has the potential to be converted to an amine groups through reduction process
after been successfully incorporated into the mesostructure fi'amework. Accordingly, 4-
(triethoxysilyl)butyronit1ile (CNTES) was co-condensed with TEOS to make an organo-
functionalized mesostructure silica, denoted HMS-CN. Then, lithium aluminum hydride
(LAH) was used to reduce the nitrile group to an amine groups. This alternative route was
chosen due to the difficulty to make amino-fimctionalized mesostructure by the direct
assembly pathway.

X-Ray diffraction patterns were measured for 10 and 20 mol % HMS-CN
mesostructure silicas and the counterparts obtained after the nitrile reduction to HMS-
NH2 (CN) materials. In all cases, a single reflection was observed and centered at 20 of
2° (Fig. 3.11), indicating the successful formation of a wormhole HMS mesostructure
with nitrile functionalization and a well retained mesostructure after nitrile reduction.
However, the intensity of the reflection decreased after the nitrile reduction. The d
spacing for 10% HMS-CN and HMS-NH2 (CN) are both 4.5 nm, and the value for the 20
% functionality forms are both 4.2 nm. This indicates that the long term ordering of the

mesostructures was maintained after the reduction from nitrile to amine groups.

100

 

 

 
 
   
 
    

 

 

 

 

1
14000—4
10% HMS-CN
12000»4
1”” “ 20% HMS-CN
'2’ 8000 ~
3 20% HMS-NH2 (CN)
U
6000 «
10% HMS-NH2 (CN)
4000 -
2000 4
o 4-4 1 l 1 ---__
012 3 4 5 6 7 s 910

20 (degrees)

Figure 3.11. X-ray diffraction patterns of amino-functionalized HMS mesostructured
silicas before and after the reduction of CN groups to NH2 groups at different

organosilane loadings of 10 mol % and 20 mol %.

101

The results of the N2 adsorption-desorption isotherms and the pore size
distributions obtained from the adsorption branch are shown in Figure 3.12. The physical
properties of the organofunctionalized mesostructured silicas before and after the nitrile
reduction are summarized in Table 3.2. The N2 isotherms for the HMS-CN materials
showed an obvious step in the region of relative pressure of 0.2-0.4, indicating that the
mesostructure was successfully synthesized. The pore size of HMS-CN decreased from
2.0 nm to 1.6 nm upon increasing the nitrile loading from 10 to 20 mol %. The pore
volume and surface area also decreased with increased nitrile loading. Interestingly, after
the nitrile reduction process the mesostructure was retained. For instance, the 20 % HMS-
NH2 (CN) mesostructured silica showed-pore size of 1.6 nm, a pore volume of 0.70 cm3
g'l and a surface are of 818 m2 g], which are compatible to the values for 20 % HMS-CN
before the nitrile reduction process. The pore size did not change significantly after the
nitrile reduction; this might be due to the fact that the total length of the organic group
did not change much from butyronitrile to butyl amine conversion. The nitrogen content
based on elemental analysis decreased only slightly after the nitrile reduction reaction,
suggesting that the majority of the functional group is still incorporated in the silica
framework. It is noteworthy that the 20 mol %HMS-NH2 (conv.), which was made by the
conventional co-condensation of TEOS and APTES directly, showed no X-ray diffiaction,
a broad pore size distribution, a pore volume of 0.60 cm3 g'1 and a surface area of 99 m2
g". In contrast, the 20 mol % HMS-NH2 (CN) synthesized by the nitrile reduction
pathway showed a single X-ray reflection around 20 = 20° (4.2 nm), a narrow pore size
distribution with a maximum peak of 1.6 nm, a total pore volume of 0.70 cm3 g'1 and

BET surface area of 818 ng'l.

102

The TEM images of functionalized HMS materials before and after the nitrile
reduction both showed wormhole structure (Fig. 3.13). This further proves that the
structure of functionalized material is retained after the reduction process. The TEM
images of amine functionalized HMS silica after nitrile reduction showed less order for
the wormhole structure compared with the counterpart before the reduction, which is in
agreement with the reduced intensity for the X-ray diffi'action pattern and the pore size

distribution profile.

103

 

 

100 - 10% HMS-NH2 (cu)

 

o JAILIIJLIIIJLIIIALIILLII

0 0.2 0 4 0 6 0.8 1

   
   

 

 

0.70 'PIP '
{(8) . °
0.60 _ 10/oHMS-CN
0.50 ~
1
0.40 ~ 0
g i // 20/oHMS-CN
g030
' /20% HMS-NH2 (CN)
0.20
0.10 i
0.00 '

 

012345678910
Pore Diameter (nm)

Figure 3.12. (A) N2 adsorption-desorption isotherms for mesoporous AP-HMS amino-
functional silicas (10 and 20 % AP groups) before and after the reduction of CN groups
to amine groups. (B) provides the BJH framework pore size distributions obtained from

the adsorption branches of the isotherms.

104

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Figure 3.13. Transmission electron microscopy (TEM) images of (A, B) nitrile-

functionalized mesostructure silica material, 10 % HMS-CN and (C, D) the recovered
amine functionalized mesostructure silica material 10 % HMS-NH2 (CN) at low and high

magnification.

106

Figure 3.14 reports the FT-IR spectra before and after nitrile reduction. In Figure
3.14B, the absorbance around 2260 cm"1 is attributed to the carbon nitrogen stretching on
the —CEN group. After the nitrile reduction process, the FT-IR spectrum of the amino
functionalized mesostructured silica (Fig. 3.14C) shows a very weak absorbance from —
CEN groups, indicating that the nitrile reduction process was successful and nearly
complete. The pure calcined HMS silica is also shown (Fig. 3.14A) for comparison. The
spectrum of the amino-functionalized mesostructured HMS-NH2 (CN) (Fig. 3.14C) was
very similar to the spectrum in Fig. 3.14A except for the absorbance around 2950 cm",
indicating the alkane C-H stretching from the butyl amine group still exists after the
reduction.

'3 C cross polarization MAS NMR spectroscopy was used to analyze the organic
groups incorporated onto the silica framework (Fig. 3.15). For the 13C CP MAS NMR
spectrum of the 20 mol % HMS-CN sample, the resonance at 118 ppm was attributed to
the carbon in —CEN group. The small peaks at 160 ppm, 80 ppm, and 40 ppm are the side
bands of the triple-bonded carbon resonance. After the reduction of the nitrile group, the
CEN carbon peak disappeared. The new peak at chemical shift of 38 ppm is attributed to
the carbon adjacent to the newly formed amine group, indicating the successful
conversion. For both 13C CP NMR spectra, there are two peaks around 58 ppm and 18
ppm, respectively. These two peaks are attributed to carbon species due to either ethoxy
groups from either incomplete hydrolysis of TEOS or, more likely, residual ethanol

adsorbed in the surfactant removal process.

107

The complete conversion from nitrile to amine group was proven by the
disappearances of the C—=~N stretching at 2260 cm”I in the FT-IR spectrum (Fig. 3.14) and

the nitrile carbon resonance at 118 ppm in the 13C CP MAS NMR spectrum (Fig. 3.15).

 

A

V .

I‘LLL1114JJLLLAIIL11LI

MW

A

Transmittance (%)

 

 

 

 

1 1+; 1 11111 4

3900 3400 2900 2400 1900 1400 900 400
wavenumber (cm")

Figure 3.14. FT-IR spectra of (A) calcined mesostructured silica HMS, (B) 20 % HMS-
CN, and (C) 20% HMS-NH2 after CN reduction.

108

 

 

200 150 100 50 0 -50
chemical shift (ppm)

Figure 3.15. '3 C CP MAS NMR spectra of 20% HMS-CN and 20% HMS-NH2 (CN) after
the reduction of the nitrile group.

* Side bands in the MAS NMR spectra.

** These two peaks are attributed to residual ethanol.

109

3.3.3 Adsorption performance

Pb2+ adsorption on pure HMS silica material is negligible at the 100 ppm level, so
the lead adsorption observed for the modified HMS samples should be exclusively
attributed to the presence of organic functional groups anchored to the silica walls. The
accessibility of the amine groups on the functionalized mesostructured HMS
organosilicas synthesized by alternative routes, namely Boc protection and nitrile
reduction, were investigated by the heavy metal ion (i.e. Pb“) adsorption performance.
The HMS-NH2 (conv.) material synthesized by conventional co—condensation of TEOS
and APTES was also studied for comparison. The results are shown in Table 3.3.

The NHtBoc functionalized HMS mesostructured silica materials (10 mol and 20
mol % HMS-NHtBoc) showed no trapping capacities for lead at all. After the
deprotection of tBoc groups, the amine groups were accessible by pointing towards the
pore and the recovered amine-functionalized HMS silica materials showed good
adsorption performance. The 10 mol % HMS-NH2 (Boc) reduced the lead content from
100 ppm to 30 ppm, having a trapping capacity of 0.34 mmol g". The 20 mol % HMS-
NH2 (Boc) reduced the lead content from 100 ppm to 0.85 ppm, having a trapping
capacity of 0.48 mmol g". The latter derivative did not double the capacity of the 10 mol
% counterpart even though the nitrogen content was double that of 10 mol% HMS-NH2
(Boc). This is because the equilibrium concentration after the adsorption by 20mol %
HMS-NH2 (Boc) was already 0.85 ppm, so the material was not saturated and did not
show the maximum trapping capacity. Since the pure HMS silica material and NHtBoc

functionalized HMS material showed no trapping capacities under the same experimental

110

condition, this firrther proves that the conversion from NHtBoc to NH2 group was
successful.

The HMS-CN mesostructured silica does not have significant trapping capacities.
The counterpart obtained after nitrile reduction, HMS-NH2 (CN), was studied for the
trapping of heavy metal ions (i.e. Pb2+). The trapping capacity increased slightly from
0.23 mmol g" for 10 % HMS-NH2 (CN) to 0.26 mmol g" for 20 % HMS-NH2 (CN),
even though the nitrogen content in those materials increased significantly from 1.13
mmol g'1 to 1.83 mmol g". The HMS-NH2 (CN) organosilica showed a lower adsorption
capacity compared with HMS-NH2 (Boc), due to the lower nitrogen content in the
adsorbent. This reduction in nitrogen content might be caused by the cleavage of organic
moiety during the relative strong reduction condition.

Conventional 20% HMS-NH2 (conv.) also was tested for the lead trapping
experiment. This mesophase has a trapping capacity of 0.17 mmol g", while the
derivatives synthesized by the alternative routes have trapping capacities of 0.48 mmol g'
l for the Boc protection method and 0.26 mmol g'1 for the nitrile reduction method.

The nitrogen to adsorbed lead ratio was calculated for the studied materials to
quantify the accessibility of the amino groups. The N/Pb ratio for 20% HMS-NH2 (conv.)
is 14, while the ratios for 20% HMS-NH2 (Boc) and 20% HMS-NH2 (CN) are 4.9 and 7.0,
respectively. It is clear that the availability of nitrogen is improved by using the amine
group protection and nitrile reduction method as alternative synthesis routes.

The Coniu group has made the effort on SBA-15 silica materials by using the Boc
protection method.28 A problem might arise for the SBA-15 system, because the synthesis

conditions for SBA-15 are acidic, and the deprotection of Boc group is also acidic. Thus,

lll

the amino group may be partially deprotected during the SBA-15 synthesis and the amine
groups may not be accessible again. The HMS system, however, only requires neutral
synthesis condition, allowing the NHth moiety to remain intact during the co-
condensation step. The 20 mol % SBA-15 (Boc) derivative was synthesized and then
tested the adsorption performance. This sample showed a trapping capacity of 0.18 mmol
g'I with a nitrogen to adsorbed lead ratio of 11, which is similar to the value of 14

observed for 20mol % HMS-NH2 (conv.).

112

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113

3.4 Conclusions

Among the various functional groups incorporated into the mesostructure silicas,
the amine group is of greatest interest because of its potential applications in the area of
heavy metal ion adsorption, enzyme trapping, and catalysis. Due to the hydrophilicity and
hydrogen bonding interaction between the amine group and silica surface, it is hard to
incorporate the amino-functional organosilane (APTES) through co-condensation with
TEOS.

In the present work, mesoporous amine-functionalized HMS organosilicas have
been successfully synthesized via alternative routes. A hydrophobic tBoc protected amine
group and nitrile group have been successfully incorporated by direct assembly with
TEOS in the presence of dodecylamine as a surfactant. The mesostructure obtained
showed a much higher structural order in comparison to derivatives synthesized by the
conventional direct co-condensation of TEOS and APTES. The amine groups were
recovered later on inside the mesopores of organosilica fi'amework. Acidic hydrolysis
was used to remove the tBoc protecting group, while lithium aluminum hydride (LAH)
was used to reduce the nitrile group back to amine group. The improved structures of the
amine functionalized organosilica materials were well retained during the removal of
tBoc groups and the reduction of nitrile groups.

Pb2+ trapping experiments were carried out on the resultant amine-functionalized
mesostructured HMS organosilicas in order to investigate the accessibility of the amine
groups. Both HMS-NH2 (Boc) and HMS-NH2 (CN) materials showed good trapping
capacities towards lead species from aqueous solution, having lead trapping capacities of

0.23-0.48 mmol g". The N/Pb ratio was calculated on the lead containing adsorbent to

114

quantify the availability of the amine groups. The amine functionalized mesostructured
HMS organosilicas synthesized by the alternative routes showed a low N/Pb ratio of 3.7-
7.0 in contrast to the N/Pb ratio of 14 for the derivative synthesized by the direct co-
condensation of TEOS and APTES. It suggests that the amine groups obtained by the
alternative routes were much less buried in the framework and the accessibility of the
amine group is much higher. Therefore, the resultant materials can be served as good

heavy metal ion adsorbents.

115

3.5 References

(1) Centers for Disease Control and Prevention Morbidity and Mortality Weekly
Report 2000, 49, 1133-1137

(2) Hoffmann, F .; Cornelius, M.; Morell, J .; Froeba, M. Angew. Chem. Int. Ed. 2006,
45, 3216-3251.

(3) Lim, M. H.; Stein, A. Chem. Mater. 1999, II, 3285-3295.
(4) Brown, J .; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 69-70.

(5) Kang, T.; Park, Y.; Choi, K.; Lee, J. S.; Yi, J. J. Mater. Chem. 2004, 14, 1043-
1049.

(6) Yang, C.-m.; Wang, Y.; Zibrowius, B.; Schueth, F. Phys. Chem. Chem. Phys.
2004, 6, 2461-2467.

(7) Zhang, X.; Zhang, F.; Chan, K.-Y. Scripta Mater. 2004, 51, 343-347.

(8) Wang, Y.; Zibrowius, B.; Yang, C.-m.; Spliethoff, B.; Schueth, F. Chem.
Commun. 2004, 46-47.

(9) Yokoi, T.; Yoshitake, H.; Tatsumi, T. J. Mater. Chem. 2004, I 4, 951-957.
(10) Mori, Y.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 2173-2178.

(11) Shah, J .; Kim, S.-S.; Pinnavaia, T. J. Chem. Commun. 2004, 572-573.
(12) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500-503.

(13) Lesaint, C.; Frebault, F .; Delacote, C.; Lebeau, B.; Marichal, C.; Walcarius, A.;
Patarin, J. Stud. Surf Sci. Catal. 2005, 156, 925-932.

(14) Etienne, M.; Sayen, S.; Lebeau, B.; Walcarius, A. Stud. Surf Sci. Catal. 2002,
141, 615-622.

116

(15) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Chem. Mater. 2003, 15, 1713-1721.
(16) Rosenholm, J. M.; Linden, M. Chem. Mater. 2007, 19, 5023-5034.

(17) Macquarrie, D. J .; Jackson, D. B.; Mdoe, J. E. 6.; Clark, J. H. NewJ. Chem. 1999,
23, 539-544.

(18) Wang, X.; Lin, K. S. K.; Chan, J. C. C.; Cheng, S. J. Phys. Chem. B 2005, 109,
1763-1769.

(19) Wang, X.; Chan, J. C. C.; Tseng, Y.-H.; Cheng, S. Microporous Mesoporous
Mater. 2006, 95, 57-65.

(20) Kanan, S. M.; Tze, W. T. Y.; Tripp, C. P. Langmuir 2002, 18, 6623-6627.

(21) Chong, A. S. M.; Zhao, X. S.; Kustedjo, A. T.; Qiao, S. Z. Microporous and
Mesoporous Mater. 2004, 72, 33-42.

(22) Kim, Y.; Lee, B.; Yi, J. Sep. Sci. T echnol. 2004, 39, 1427-1442.

(23) Bois, L.; Bonhomme, A.; Ribes, A.; Pais, B.; Raffin, G.; Tessier, F. Colloids Surf,
A 2003, 221, 221-230.

(24) Yokoi, T.; Yoshitake, H.; Tatsumi, T. Chem. Mater. 2003, 15, 4536-4538.

(25) Che, S.; Garcia-Bennett, A. E.; Yokoi, T.; Sakamoto, K.; Kunieda, H.; Terasaki,
0.; Tatsumi, T. Nat. Mater. 2003, 2, 801-805.

(26) Yokoi, T .; Yoshitake, H.; Yamada, T.; Kubota, Y.; Tatsumi, T. J. Mater. Chem.
2006, 16, 1125-1135.

(27) Bass, J. D.; Katz, A. Chem. Mater. 2003, 15, 2757-2763.

(28) Mehdi, A.; Reye, C.; Brandes, S.; Guilard, R.; Corriu, R. J. P. New J. Chem. 2005,
29, 965-968.

117

(29) Huh, 8.; Wiench, J. W.; Yoo, J.-C.; Pruski, M.; Lin, V. S. Y. Chem. Mater. 2003,
15, 4247-4256.

(30) Yang, C.-M.; Zibrowius, B.; Schueth, F. Chem. Commun. 2003, 1772-1773.

(31) Fiorilli, S.; Onida, B.; Bonelli, B.; Garrone, E. J. Phys. Chem. B 2005, 109,
16725-16729.

118

Chapter 4
Alumination of siliceous wormhole mesostructure and its application as heavy metal

adsorbents

4.1 Introduction

Microporous aluminosilicate molecular sieves known as zeolites contain regular
arrays of uniformly sized channels and admit molecules below a certain critical size into
their internal space which makes them useful as heterogeneous catalysts and adsorbents.
The discovery of M418 family of mesoporous materials with uniformly sized
unidimensional pore structure has widened the applications because of its much higher
surface area and larger pore size."2 Ion-exchange, catalytic and adsorptive properties of
such molecular sieve materials originate from acid sites which were modified by
framework substitution. Modification is usually achieved by incorporation of hetero-
atoms, particularly A1 atoms, into the otherwise electrically neutral siliceous framework
which consequently would have no acid sites.3 Therefore, a lot of effort has been devoted
into the insertion of aluminum sites to the pure mesostructured silicas. Typically
aluminum is incorporated into the framework of the silicates by either direct assembly or
by post-synthesis methods.

The direct assembly method often requires specialized synthesis conditions
depending on the respective structures of the mesoporous materials, and suffers from an
undesirable decease in the structure integn'ty.4’5 Therefore it is difficult to directly prepare
well ordered aluminosilicate mesostructured materials with low Si/Al ratio. Moreover, it
was observed that when compared to zeolites, the concentration of the acid sites on the

direct assembled mesostructured aluminosilicate materials is low even at high A1

119

loadingf”7 It was suggested that the low acidity may be related to the mechanism of
formation of the aluminosilicate and to the position occupied by 4—coordinate Al in the
framework.8 A large portion of the Al incorporated in the mesostructured aluminosilicate
may be buried in the inorganic framework and is therefore less efficient at generating
acid sites.

On the other hand, the incorporation of Al onto preformed mesostructures by post
synthesis means is reported to be advantageous with respect to structure ordering and
physical stability.9 Previous studies have shown that aluminum can be incorporated into
siliceous MGM-41 materials via post synthesis procedures by using various aluminum

sources such as sublimated A1C13 aqueous solution,10 AlCl3 non-aqueous solution,l 1’12

13-15

aluminum isoalkyoxide non-aqueous solution, trimethylaluminum in non-aqueous

16.l7

solution, aluminum isopropoxide in supercritical fluids,l8 aqueous aluminum

19'2', and aqueous sodium aluminate solutions”22 etc. Alumination of SBA-15

chlorhydrate,
by a post synthesis method has been investigated using different aqueous and nonaqueous
aluminum sources.”26 The alumination of wormhole mesostructured silica has been
explored by Robert Mokaya and William Jones by using Al(OPr)3 in hexane solution as

aluminum source.13 All the references for the alumination of mesostructured silica

materials by post synthesis/grafting methods are listed in Table 4.1.

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122

Post synthesis methods usually generate two kinds of Al sites, 4-cordinate
tetrahedron site and 6-cordinate octahedron site. Only the Td sites are incorporated into
the framework and can generate acid sites. It is noteworthy that among all the literature
studies, the aluminosilicates with mainly all tetrahedron Al sites were all synthesized by
using aluminum aqueous solution as the aluminum sources.9’22'24 However, the parent
mesostructures that have been investigated are limited to only MGM-41 and SBA-15
silica materials. During the post synthesis/grafting process, the Al is first in contact with
the outer surface of the host silica before penetrating into the internal pore channel
system. Thus, the uniform distribution of Al into the internal pore channels may be
hindered by the rather long and one dimensional pore channels of the mesoporous MCM-
41 and SBA-15 silicas.18 A wormhole mesostructure silica material, however, has three
dimensional pore channel connections and a smaller domain size. Therefore it is
considered to be easier to grafi Al uniformly into the host silica.

In this present work the wormhole mesostructure HMS was utilized as the host
silica material, and lithium aluminum hydride and sodium aluminate were used as the
aluminum source for the post synthesis grafting. The efficiency of these methods for
aluminum incorporation and their effect on the pore structure were investigated by a
combination of characterization techniques including N2 adsorption-desorption, powder
X-ray diffraction, and 27A1 magic angle spinning solid state NMR. The ion exchange
capacity of the aluminated HMS was evaluated by their performance on Pb2+ trapping

experiments.

123

4.2 Experimental methods

4.2.1 Reagents

Dodecylamine (DDA), tetraethoxy orthosilicate (TEOS), lead atomic absorption
standard solution (~1000 ppm), 1.0 M lithium aluminum hydride (LAH) in
tetrahydrofuran solution, sodium aluminate (NazAloz), aluminum chloride (A1C13),
sodium hydroxide (NaOH), and 99.999% lead nitrate were purchased from Aldrich.
Ethanol was purchased in-house. All the reagents were used without fiirther purification.
Water used in the synthesis was double-exchanged to remove cations and anions via a

Millipore filter apparatus.

4.2.2 Material synthesis

4.2.2.1 Alumination of mesoporous silica by LAH and water work-up

The parent mesoporous HMS material was synthesized by using dodecylamine
(DDA) as the surfactant under neutral pH condition. The desired amount of DDA was
dissolved in a plastic bottle with 162 g H20 and 46 g EtOH. After obtaining a
homogeneous surfactant solution, TEOS was added and the mixture was aged at 60°C for
24 hr. The products were recovered by filtration and air-dried followed by calcination at
600°C for 4 hr. The total molar ratio of reagents was DDA: TEOS: H20: EtOH = 0.25: l:
180:20.

A 0.5—g quantity of the calcined parent HMS mesostructured silica was added t
the desired amount of 1M LAH in THF. The mixture was stirred under N2 flow for 1 hr.,
followed by water work-up. Enough water was added in the work-up to consume all the

LAH in the reaction flask. The product was filtered, washed with water exclusively and

124

then dried in air. The product was denoted HMS-LAH (x), where x represents the silica
to aluminum ratio. The HMS-LAH (x) was then calcined at 600°C in air for 4hr. The
final product was named HMS-LAH-C (x), where C represents the calcination process.

Two ratios of silica to aluminum were carried out, namely 2.8 and 1.4, respectively.

4.2.2.2 Alumination of mesoporous silica by sodium aluminate

Two methods were investigated using sodium aluminate as the aluminum sources.
Method I involves the preparation of fresh sodium aluminate by the reaction of Ale and
NaOH. Method II involved the dissolution of purchased sodium aluminate in water
directly.

Method 1. (AlCl3 + NaOH): a 0.5-g quantity of the calcined parent mesostructured silica
was introduced into a desired amount of 0.1M AlCl3 solution with stirring. Then, 0.1M
NaOH solution was added drop wise to the mixture. The molar ratio of AlCl3 to NaOH
was 1 to 4. The mixture was mixed for 1hr. The product was recovered by filtration,
washed by water and then dried in air completely. It was denoted HMS-Al-I. The HMS-
Al-I was then calcined at 600°C in air for 4hr. The final product was named HMS-Al-I-C.
The Si/Al ratio was 2.8.

Method II. (NaAlOz): a 0.5-g quantity of the calcined parent mesostructure silica was
introduced into the desired amount of 0.1M NaAlOz solution with stirring. The mixture
was mixed for 1hr. The product was recovered by filtration, washed by water and then
dried in air completely. It was named HMS-Al-II. The HMS-Al-II was then calcined at

600°C in air. The final product was named HMS-Al-II-C. The Si/Al ratio was 2.8.

125

4.2.3 Pb2+ trapping experiments

The Pb2+ adsorption experiment was carried out by stirring 0.05 g of aluminated
silica material in 100 mL of lead nitrate (Pb(NO3)2) solution at 25°C. The initial
concentration of Pb2+ for these experiments was 100 ppm in all cases. The mixtures were
stirred for 24 hours and then filtered through a 0.25 pm filter paper to collect the final
filtrate solution.

The amount of lead in the filtrate after the adsorption experiments was analyzed
by cold vapor atomic absorption spectroscopy (AAS). The difference between the initial
and equilibrium lead concentrations indicated the amount of lead that was trapped in the

solid adsorbents.

4.2.4 Characterization

X-ray diffraction (XRD) patterns were obtained on a Rigaku Rotaflex 200B
diffractometer equipped with Cu Kn X-ray radiation and a curved crystal graphite
monochromator operating at 45 kV and 100 mA.

N2 adsorption-desorption isotherms were obtained at —196 °C on a Micromeretics
Tristar 3000 sorptometer using standard procedures. Samples were outgassed at 150 °C
and 1045 Torr for a minimum of 12 hr prior to analysis. Surface areas were calculated
from the linear part of a BET plot of the nitrogen adsorption data according to IUPAC
recommendations. The Barrett-Joyner-Halenda (BJ H) method was used to obtain the pore
size distribution from the adsorption branch of the isotherms.

MAS NMR spectra were recorded on a Varian 400 solid state NMR spectrometer
with a zirconia rotor at a spinning frequency of 4 kHz. 27Al MAS NMR spectra were

recorded at 104.2 MHz under single-pulse mode and a pulse delay of 0.5 second.

126

Equilibrium concentrations of Pb2+ in solution were measured on a cold vapor
atomic absorption spectroscopy Varian SpectrAA-ZOO using lead cathode with maximum

working current of 10 mA.
4.3 Results and discussion

4.3.1 Aluminated HMS-LAH and HMS-LAH-C mesostructures

Aluminum was introduced into the silica framework by the in—situ hydrolysis of
water work-up of lithium aluminum hydride (LAH). The product of LAH and water is H2,
Al(OH)3 and LiOH, which can further react to produce LiAlOz. Usually, for reduction
reactions carried out by LAH, the excess LAH is worked up using acid to remove any
precipitate of Al(OH);. A water work-up was used in this work to keep the aluminum in
the framework of mesostructured HMS silica materials.

The X-ray diffraction patterns suggest the lost of mesostructure ordering upon
LAH alumination in comparison with the pristine HMS mesostructure silica. A broad
diffraction was observed for HMS-LAH materials, after calcination the HMS-LAH-C
barely showed an undistinguishable peak (Fig. 4.1). Wide angle X-ray diffraction patterns
show new peaks at 20 of 20, 30, 36 and 64 degree for the HMS-LAH, and only a broad
peak around 20 of 66° for the calcined counterparts compared to pristine HMS which
only shows a broad diffraction at a 20 of 23° (Fig. 4.2). The introduction of aluminum
into the mesostructured silica caused the lost of long range wormhole ordering for the
material, as verified by the low angel X-ray diffraction patterns. The new wide-angle
XRD peaks indicates the formation of aluminum hydroxide with some degree of
crystallinity.33 The crystalline structure, however, was not stable and vanished afier the

calcination.

127

Figure 4.3 reports the N2 adsorption—desorption isotherms (Fig. 4.3A) and
Barrett-Joyner-Halenda (BJ H) framework pore size distributions (Fig. 4.38) for
aluminated HMS-LAH silicas and the counterparts after calcination. Table 4.2
summarizes the d spacing, pore size, total pore volume and BET surface area for each
product. The isotherms for aluminated HMS materials before and after calcination all
showed steps at a relative pressure of 0.35-0.45, indicating the presence of mesopores.
The surface area and pore volume decrease upon increasing the Al content in the HMS-
LAH. The calcined HMS-LAH-C showed a pore volume and surface area comparable to
the counterpart before calcination, but with an increased pore size. For instance, the
HMS-LAH (1.4) has a lower surface area (387 m2 g") and pore volume (0.76 cm3 g")
compared with the pristine HMS (surface area of 941 m2 g'1 and pore volume of 1.12 cm3
g"). The HMS-LAH-C (1.4) has a surface area (256 m2 g") and pore volume (0.64 cm3 g'
l) comparable to HMS-LAH, but the pore size increased from 2.4 nm (HMS-LAH) to 2.7
nm afier calcination. The pore volume and surface area decreased upon increasing the A]
content in the initial alumination synthesis mixture. The alumination reaction was only
carried out at room temperature. Thus, the Almost likely is located on the surface rather
than penetrated inside the walls of the inorganic silica framework. The additional layer of
framework Al and the sorption of non-framework Al resulted in the decrease of pore
volume and surface area.

The 27Al MAS NMR spectra were recorded for HMS-LAH and HMS-LAH-C in
order to identify the coordination geometry of the Al centers incorporated into the silica
framework and follow the change in the environment of the Al after calcination.

Generally, there are two types of Al sites in aluminated silica materials, tetrahedrally

128

coordinated (framework) Al sites with a chemical shift around 50 ppm and octahedrally
coordinated (non-framework) Al sites with a resonance at a chemical shifts around 0
ppm.30’33 The 27A1 MAS NMR spectrum of HMS-LAH shows two peaks (Fig. 4.4A). The
resonance at 6.8 ppm, indicates the presence of Al in octahedral sites at 54% abundance.
The resonance at 55 ppm indicates tetrahedral A1 at 46 % abundance. After calcination,
the 27Al MAS NMR spectrum of HMS-LAH-C showed two peaks at the same chemical
shifts (6.8 ppm and 55 ppm) (Fig. 4.4B). Interestingly, after calcination, more tetrahedral
Al was incorporated into the framework while less non-framework octahedral A1
remained in the pores. The abundances of tetrahedral Al and octahedral Al in the calcined
form are 83% and 17%, respectively. The pore size was observed an increase by 0.3 nm
from HMS-LAH to HMS-LAH-C. This suggests that calcination induced the
condensation of hydroxyl groups, resulting in the more incorporation of tetrahedral Al

into the framework at the expense of octahedral A1.

129

 

 

HMS-LAH-C (2.8)

 

 

counts

HMS-LAH (2.8)
HMS-LAH-C (1.4)

 

HMS-LAH (1 .4)

 

 

pristine HMS

 

 

26 (degrees)

Figure 4.1. Low angle X-ray diffraction patterns of aluminated HMS made using LAH as
the aluminum source before and after calcination at 600°C. The numbers in parenthesis

provide the overall Si/Al used in the synthesis.

130

 

HMS-LAH-C (2.8)

 

HMS-LAH (2.8)

 

(I)
C
3
8 , HMS-LAH-C (1 .4)
HMS-LAH (1 .4)
pristine HMS

i,-_l_4_4___L__i__l___l-_J._r,illLlilliJlllLLLllJLlllll

1O 20 30 40 50 60 7O 80
29 (degrees)

 

Figure 4.2. Wide angle X-ray diffraction patterns of aluminated HMS made using LAH
as the aluminum source before and after calcination at 600°C. The numbers in parenthesis

provide the overall Si/Al used in the synthesis.

131

800

 

§

Oi

O

O
.L

0|
O
O

0)
O
O
A

N
O
O

 

Volume N2 Adsorbed (cm3 9'1)
:5
O
O

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O
O

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.. HMS-LAH-C (1 .4)
111111111illLLlJlllilLJlLLLJlilllll l1LLJl4llllll

 

(A)

  
    
 

 

 

 

 

 

  

 

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
PIPO
0.7 —~ (B)
0.6 r pristine HMS
0.5
00.4 ‘1‘
.2 .
> HMS-LAH (2.8)
”0.3 --
HMS-LAH-C (2.8)
0.2 2~
HMS-LAH(1.4)

 

 

  
 

HMS-LAH-C (1.4)

  

 

 

 

Figure 4.3. (A) N2 adsorption—desorption isotherms and (B) BJH framework pore size
distributions for pristine HMS (o), HMS-LAH (I) and HMS-LAH-C (Cl).

2 4 6 8
Pore Diameter (nm)

132

Table 4.2. Physical properties of mesostructured silica aluminated by reaction with LAH.

 

 

 

 

 

 

pore size a Sign Vtot b

(um) (“12 g") (cm3 s")
Pristine HMS 2.4 941 1.12
HMS-LAH (2.8) 2.3 597 0.85
HMS-LAH-C (2.8) 2.7 . 426 0.81
HMS-LAH (1 .4) 2.4 387 0.76
HMS-LAH-C (1 .4) 2.7 256 0.64

 

 

 

 

 

 

a Pore size diameter was determined by the BJH model from the adsorption branches of

the isotherms. b Total pore volume was determined at P/Po=0.98.

133

 

 

(A) i

 

 

 

(B)

 

he+~e-~+L—t—t~t—H~he—t+t—e—L ._+_._-L_._n._+_.__..s_._n.r_. . t . t w . . +1 . t—L--t~4-4— we
500 400 300 200 100 o -100 -200 -300 400 -500
chemical shift (ppm)

Figure 4.4. ”Al MAS NMR spectra of aluminated HMS-LAH (2.8), (A) before and (B)
after calcination at 600°C for 4 hr.

134

4.3.2 Aluminated HMS-Al-I, HMS-Al-II, HMS-Al-I-C, and HMS-AI—II-C

mesostructures

Sodium aluminate has been used as the aluminum source for the alumination fo
mesostructured silica.”922’24’30‘32 In this present work, two methods were used to
incorporate sodium aluminate into the wormhole HMS mesostructure framework.
Method I used AlCl3 solution followed by addition of NaOH solution to make fresh
NaAlO2, while method 11 used a NaAlO2 solution directly. The amount of NaOH used
was four times as the amount of AlCl3, thus the following reaction was expected:

AlCl3 + 4NaOH ——> NaAlO2 + 2H2O + 3NaCl
Therefore, method I and method H both used NaAlO2 as the A] source for alumination.
Only one Si/Al ratio (i.e. 2.8) was investigated.

The X-ray diffraction patterns of aluminated HMS materials by both methods
before and after calcination showed no distinguishable diffraction peak at low angle,
indicating the loss of mesostructure upon aluminum incorporation (Fig. 4.5). The X-ray
patterns at wide angle (Fig. 4.6) showed no peak besides the Si-O short range ordering
diffraction around 20 of 23°, suggesting that all the aluminum is in the amorphous phase.

Figure 4.7 reports the N2 adsorption—desorption isotherms (Fig. 4.7A) and
Barrett-Joyner-Halenda (BJH) framework pore size distributions (Fig. 4.7B) for
aluminated HMS-LAH silicas and the counterparts after calcination. Interestingly, the
HMS-Al and HMS-Al-C prepared by both methods showed nearly identical results,
respectively. The overlapping isotherms and pore size distributions for the aluminated
mesostructure silica synthesized by method I and II suggests that the physical properties

of the resulting materials have no obvious differences. The surface area and pore volume

135

decreased after the incorporation of aluminum compared to the HMS parent silica
material. The pore size of HMS-Al materials (2.4 nm) did not change compared to the
parent HMS silica materials, while the pore size after calcination increased to 2.5-2.6 nm
for HMS-Al-C materials. This might be due to the rearrangement of the incorporated Al
during calcination at elevated temperature. Physical properties of the products are list in

Table 4.3.

The Si/Al ratios obtained for the aluminated mesostructured silica by method I
and method 11 were about 7 .0, which is higher than the value of 2.8 for the composition
of initial reaction mixture. This suggests that only a portion of the starting Al was
retained in the solid after modification. It is clear that the incorporation of aluminum
sacrifices the structure and the surface area and pore volume of the resulting materials.

27Al MAS NMR spectra showed the presence of only tetrahedrally coordinated Al
species in the aluminated silica solid by using the sodium aluminate as the aluminum
source before and after calcination (Fig. 4.8), suggesting that all the incorporated A1 are
in the tetrahedron framework sites. This observation agrees with previous studies of the
alumination of SBA-15,24 SBA-16,32 and MGM-4122 using NaAlO2 solution as the
aluminum sources. In these studies, the 27Al MAS NMR profiles also showed a main
resonance with a chemical shift of 50 ppm, indicating that main coordination
environment for aluminum to be tetrahedral.

NaAlO2 solution has also been used as A1 source for alumination for Al

Containing material Al-MSU-S.30’3' In these works, the authors claimed that the presence
0f cetyltrimethylarnmonium bromide (CTAB) was critical to maintain the framework

mesostructure. In this present work, no CTAB was used to maintain the mesostructure,

136

yet features typical of a mesostructure were observed for the nitrogen isotherms, even
though the XRD results do not support the presence of an ordered mesostructure. The
retention of mesopores, without mesopore order, may be a consequence of the ambient

condition used to carry out the alumination reactions.

137

 

 

 

 

 

HMS-Al-ll-C
HMS-AI-ll
HMS-Al-l-C
HMS-AI-I
pristine HMS

counts

 

 

 

 

 

 

i. _a_._-.‘___.i. .J__ l r r-.. l 1 1 _L_._l_.*l_.__L_.._L___L___I.. J 1 1 _r _l awn—1i

0 2 4 6 8 10
20 (degrees)

 

Figure 4.5. Low angle X-ray diffraction patterns of HMS-Al-I and HMS-Al-II materials

before and after calcination at 600°C.

138

‘_ HMS-Al-II-C

 
 
 
 

 
  

 

HMS-Al-ll

   

 

 

.9

C

3

O

0
HMS-Al-l-C
HMS-AI-l

pristine HMS

 

 

10 20 30 40 50 60 70 80
29 (degrees)

Figure 4.6. Wide angle X-ray diffraction patterns of HMS-Al-I and HMS-Al-II materials

before and after calcination at 600°C.

139

800

 

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0')

mg
a or
o o
o o
J_

03
O
O

N

O

O
l

600 —t

 

Volume N2 Adsorbed (c

O
O

  
 
 

(A) pristine HMS

 
 
   

HMS-Al-l

   
   

HMS-Al-I-C
HMS-AI-II-C

 

rrrrl rrrrrrr lrrrrl rrrrrrrrr lrrrrlrrrrlrrrrlrrrr

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

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0.6 L
0.5 .-

0.4 _

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0.2 ~—

0.1 +

 

FIFO
(3)
pristine HMS
HMS-Al-l

  
  
  
 

HMS-AI-II

 

 

HMS-AI-I-C
HMS-AI-ll-C

 

 

 

Figure 4.7.

distributions for pristine HMS (0), HMS-Al (filled marker) and HMS-Al-C (opened
marker) synthesized by method I (square) and method 11 (triangular).

 

4 6
Pore Diameter (nm)

(A) N2 adsorption—desorption isotherms and (B) BJH framework pore size

140

Table 4.3. Physical properties of aluminated mesostructured silicas using sodium

aluminate prior to and upon calcination.

 

 

 

 

 

 

 

pore size a SBET th b Si/Al ratio e
(um) ("12 g") (cm’ s") '
Pristine HMS 2.4 941 1.12 00
HMS-Al-I 2.4 642 0.81 7.2
HMS-Al-I-C 2.5 513 0.69 7.3
HMS-Al-II 2.4 638 0.77 7.0
HMS-Al-II-C 2.6 512 0.71 6.9

 

 

 

 

 

 

aPore size diameter was determined by the BJH model from the adsorption branches of

the isotherms. b Total pore volume was determined at P/Po=0.98. c Calculated from the

ICP results.

141

 

 

 

ern LrJLlrLrJ‘errrlrrrrlrrrrlmrrr1rrrrlrrrrIrrrrl

500 400 300 200 100 o -100 -200 -300 -400 -500
chemical shift (ppm)

Figure 4.8. (A) 27Al MAS NMR spectra of aluminated mesostructure, HMS-Al-I and (B)
the counterpart after calcination, HMS-Al-I-C.

142

4.3.3 Lead Adsorption experiment

Calcined HMS does not have a binding capacity towards Pb2+ from aqueous
solution. So any trapping site was generated through the proceSs of alumination.
Aluminated HMS mesostructured materials made by using LAH and sodium aluminate as
aluminum sources were evaluated for the lead adsorption performance. The results were
listed in Table 4.4.

Aluminated HMS materials, made from LAH, whether calcinated or not, had Pb2+
trapping capacities of about 0.96 mmol g", leaving the equilibrium concentration of lead
near the detection limit of the AA instrument (which is 0.5 ppm). The HMS-LAH
materials with different initial Si/Al ratio showed similar trapping capacities. This might
be because the materials were not saturated with Pb2+ cations. If a higher initial lead
concentration is used, the difference in trapping capacities for different Si/Al ratios may
be observed.

Aluminated mesostructure silica materials from sodium aluminate by method II
showed better lead adsorption capacity (0.90 mmol g'l) in comparison with the
derivatives obtained by method I (0.75 mmol g"), even though the physical properties of
the materials made by two methods did not show any significant difference. Aluminated
silica materials synthesized by using sodium aluminate solution as the aluminum sources
showed relatively lower adsorption capacities in comparison to the derivatives
aluminated by LAH. This is due to the high Si/Al ratio (lower aluminum content) in the
former materials.

A yellow flame was observed during the AA analysis of the Pb2+ solution that

came in contact with the HMS-Al-I and HMS-Al-I adsorbents. This suggests the

143

existence of sodium cation in the filtrate. No characteristic flame color was observed for
filtrate after contact with HMS-LAH and HMS-LAH-C materials because the counter
ions in these cases are Li+. Therefore, it is suggested that the trapping property was base
on an ion exchange mechanism.

In the aluminated materials, one aluminum tetrahedron site was electrostatically
matched by one Li’r or Na+ counter ions. Each Pb2+ adsorbed on the adsorbent will
replace two of the Li+ or Na)r counter ions during the trapping experiments. Therefore the
maximum trapping capacities of an adsorbent would be half amount of the accessible
tetrahedron Al sites within the material.

For the HMS-LAH (2.8) material, 46% of the Al is in the Td sites according to the
27Al MAS NMR date. The maximum trapping capacity for lead divalent cation is about
0.9 mmol g", which agrees with the 0.96 mmol g’l trapping capacity result observed.
HMS-LAH-C (2.8) material has a theoretical maximum trapping capacity of 1.8 mmol of
lead per gram of the adsorbent, while the trapping capacity is 0.96 mmol g". There are
two possible explanations. First, the material is not totally saturated yet because the
equilibrium concentration is near the detection limit of the AA instrument. Secondly, part
of the tetrahedral Al sites in the HMS-LAH-C (2.8) are generated during the calcination
process through the possible co-condensation of hydroxyl groups on the aluminum sites.
The newly formed Al layer will make the first original Td Al layer not accessible
anymore. Therefore the trapping capacity may not increase with the increasing tetrahedral
Al sitesl.

The theoretical lead capacities of aluminated HMS materials made using sodium

aluminate as Al sources are compatible with the observed trapping capacities. For

144

example, the sodium aluminate incorporated HMS-Al-II-C material after calcination
shows a trapping capacity of 0.90 mmol g", while the theoretical maximum trapping
capacity calculated from tetrahedral Al amount is 0.95 mmol g". It also further proves the
ion exchange mechanism for heavy metal trapping and suggests that all the Td Al sites in
the aluminated HMS by using sodium aluminate are available. In comparison to the low
accessibility of aluminum site in MCM-4134 and SBA-1524 systems, the wormhole 3D

structured showed an advantage with respect to the adsorption application.

145

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146

4.4 Conclusion

Wormhole mesostructured HMS silica was synthesized and aluminated using
different aluminum sources, namely lithium aluminum hydride (LAH), in-situ sodium
aluminate (NaAlO2) (method I) and sodium aluminate solution (method II). The
incorporation of the aluminum into the framework by all the methods caused some
structure disorder and sacrificed surface area and pore volume of the parent materials,
though some mesopores were retained after alumination.

The almninated HMS silica material by LAH showed some crystallinity according
to the X-ray diffraction patterns at wide angle range. However, the low degree of
crystallinity vanished after calcination. LAH aluminated HMS showed both tetrahedrally
coordinated (framework) and octahedrally coordinated (non-framework) Al sites, and the
fraction of framework Al site increased upon calcination. This is the first example of the
alumination of a mesostructure silica material by using LAH as aluminum source. The
resultant adsorbents showed excellent heavy metal ion (i.e. Pb”) trapping capacities,
which is nearly 1 mmol g'l. It was suggested that all the Td Al sites in the HMS-LAH
materials are accessible, while only half of the Td Al sites in the HMS-LAH-C materials
are accessible.

The aluminated HMS silica materials made using sodium almninate by both
method I and 11 showed nearly identical results. These materials only have tetrahedrally
coordinated (framework) Al sites, indicating the efficiency of alumination by using the
aqueous aluminum sources. The aluminated materials showed good trapping capacities
towards lead in aqueous solution, 0.75-0.90 mmol g". It was suggested that ion exchange

is the mechanism for heavy metal adsorption and that all the tetrahedral Al sites are

147

available ion exchange sites. The 3D wormhole structure of HMS material is superior to
the hexagonal arrayed channel 1D MGM-41 and SBA-15 systems with regards to the
alumination of mesostructured silicas for heavy metal ion removal.

The aluminated materials presented in this work have very high percentage of
tetrahedral coordinated framework Al in the aluminosilicate framework. It also proved
that the accessibility of the Td Al Sites is excellent. Besides applications as metal ion

adsorbents, they also have the potential to serve as the acidic sites in catalytic reactions.

148

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151

   

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