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6/01 cJCIRC/DateDuo.pB§—p.15

THE CATALYTIC AND MECHANISTIC PROPERTIES OF
ORGANOCLAYS FOR TRIPHASE CATALYSIS

by
Ton Lee

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Department of Chemistry
1992

ABSTRACT
The Catalytic and Mechanistic Properties of Organoclays for
Triphase Catalysis
by

Ton Lee
Organoclays are smectites that are ion-exchanged by cationic organic
compound such as tetraalkylammonium or -phosphonium ions. For the
use in triphase catalysis, the onium ions are usually the cationic
surfactants which have one long alkyl chain. Differences in layer charge
density result in different orientation of surfactant cations in the clay
interlayers. In laponite, the cationic surfactants adopts monolayer
structures in which the alkyl chains lie parallel to the silicate surface
whereas lateral bilayer structures are formed in hectorite and Wyoming
montmorillonite interlayers. As the layer charge density of the clays
increased as in organo montmorillonite and F-hectorite, a pseudo-
trimolecular structure of the cationic surfactant in Arizona
montmorillonite interlayer and a paraffin-like structure in F-hectorite

gallery were observed.

Organo laponites with a monolayer structure do not stabilize
emulsion mixtures of aqueous and organic solutions, causing them to be
poor triphase catalysts. Organo hectorite with lateral bilayer structure do
stabilize the water-in-oil emulsion and are good triphase catalysts. The
mechanism for this organo hectorite triphase catalysis has been elucidated
by studying the effect of solvent polarity on reactivity and by determining

the dependence of reaction rates on the reactant concentrations. The
relative rates of the alkylation for benzyl bromide and naphthoxide also
provide mechanistic information. The most plausible mechanism is one in
which the organic reactant is first adsorbed at the boundary of the clay
surface and organic solution. Subsequent nucleophilic substitution
reaction occurs at the aqueous-catalyst interface.

Arizona montmorillonite organic derivatives with a pseudo
-trimolecular structure and organo F-hectorite with a paraffin-like
structure can also stabilize emulsion mixtures of aqueous and organic
solution. However, for lipophilic surfactants, such as hexadecyltributyl
phosphonium ion in the high layer charge density clay interlayer, the
surfactants are sometimes ion-exchanged by metal cations and dissolved
into organic solution, causing the organoclays to be poor catalysts for
recycling. Surfactant desorption can be reduced by using non-polar
organic solvents or a hydrophilic nucleophile in the triphase catalysis
system. Hydrophilic surfactants such as hexadecyltrimethyl ammonium
in these high layer charge density clay hosts do not desorb after triphase
catalysis reaction. The desorption of hydrophilic surfactant occurs after
triphase catalysis reaction when a co-surfactant, such as octanol, is present
in the reaction system.

Organoclays exhibit mechanistic phase transfer properties different
from those of polymer supported catalysts. Chemoselectivity and
regioselectivity different from that of the typical triphase catalysts can be

obtained by using organoclays for the triphase catalysis reaction.

To MY PARENTS

ACKNOWLEDGMENTS

I would like to express my appreciation to Professor T. J. Pinnavaia
for his patience, encouragement, guidance and support during the course
of the work and the preparation of this dissertation.

Gratitude is also extended to Professor H. A. Eick for his carefully
correcting the dissertation.

I would also like to thank all my colleagues for fruitful discussions
and their cooperation.

Financial support from National Science Foundation and Department
of Chemistry is also acknowledged.

Finally, I would like to thank my family for their faith and

encouragement which made all of this possible.

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES

CHAPTER I INTRODUCTION

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A. Structure and Properties of Layer Silicates 1
B. Principle of Phase Transfer Catalysis 6
l. Biphase Catalysis 6
2. Triphase Catalysis 13
(a) Nature of triphase catalysis 13
(b) polymer supported catalysts 15
(c) Inorganic-based triphase catalysts -- 20

(d) Poly(ethylene glycol) derivatives as phase transfer
catalysts 23
(e) Organoclay as triphase catalysts 25
C. Research Rationale and Objectives 25
1. Nature of Organoclays as Triphase Catalysts 25
(a) The structure of organoclays 25
(b) Colloidal properties of the organoclays - 29
(c) Mechanism of organoclay triphase catalysis 31

2. Longevity of the Clay Derivatives for Phase Transfer

Catalysis 34
(a) Desorption of interlayer organic compounds 34
(b) Thermal and chemical instability of the organoclay ------- 37
(c) Co-surfactant influence on surfactant desorption ---------- 39

CHAPTER II EXPERIMENTAL

 

 

 

 

 

 

 

A. Material - 43
1. Sodium Hectorite 43
2. Sodium Montrnorillonite 43
3. Laponite R 44
4. Fluorohectorite (F—Hectorite) 44
5. Rectorite 44
6. Organic Reagents 46

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7. Inorganic Reagents 46
8. Synthesis of Organic Precursors 46
Preparation of Organic Clay Derivatives --- 46
1. Organoclays 46
2. Crown Ether Clay Complex 47
Organoclay Triphase Catalysis 47
1. Determination of Cyanation kobs(Equation 5) ==— 47
2. Determination of Cyanation k'obs (Equation 6) when

the Amount of Pentyl Bromide Is Much larger

Potassium Cyanide 48
3. Solvent Effect on Iodination 48
4. O/C Alkylation of Benzyl Bromide and Sodium

Naphthoxide 50
5. Dependence of kobs on the Volume Ratio - 50
6. Catalytic Cyanation in the Absence of Water — 51
L011 ngevities of Organoclay Derivatives 51
l. Recyclability of Organoclays for Cyanation 51
2. Recyclability of Organoclay for Chlorination 51
3. Surfactant Desorption of Organoclays 52
4. Effect of Co-surfactant on Phase Transfer Catalysis

Reactions 55
5. Conversion of n-Bromoalkan-l-ol to N-cyanoalkan—l-ol

by Using Phase Transfer Catalysts 55
6. Conversion of n-Bromoalkane to N-cyanoalkane by Using

Phase Transfer Catalysts 56
7. Recyclability of Crown Ether Clays for Cyanation 56

Application of Organoclay Triphase Catalysis for
Organic Synthesis -57
1. Synthesis of Symmetrical Formaldehyde Acetals

9:59!"

 

 

 

(Equation 10-12) —— 57
Synthesis of Alkyl Bromide by Dehydration of Alkyl

Alcohol in Strongly Acidic Conditions (Equation 13) ---------- 57
Dehydrohalogenation of 2-Bromoethylbenzene

(Equation 14) 57
Oxidation of trans-Stilbene (Equation 15) 58

 

Synthesis of 2,4-Dinitrophenyl Ether (Equation 16) ------------ 58

F. Physical Measurements 59

 

 

 

 

 

 

1. Infrared Spectroscopy — 59
2. Gas Liquid Chromatography 59
3. NMR Spectroscopy 59
4. X-Ray Diffraction 59
5 UV-Vis Spectroscopy 60
6. BET Surface Area Measurement 60

 

CHAPTER III RESULTS AND DISCUSSION

 

 

 

 

 

 

 

 

 

 

 

 

A. The Preparation and Structure of Clay Derivatives - 61
1. The Preparation of Organoclays 61
2. The Structure of Organoclays - 64
3. The Preparation and Structure of Crown Ether Clay
Complexes 72
B. The Catalytic Properties of Organoclays = 75
1. Wettability and Catalytic Capacities of Organoclays—---------- 75
2. Dependence of Catalytic Reactivity on the Bulk Reactant
Concentration of Liquid Phases 83
3. Dependence of the Organoclay Catalytic reactivity on the
Polarity of Organic Solvent 89
4. Dependence of O/C Alkylation on Biphase and
Triphase Catalysts 92
5. The Dependence of Organoclay Catalytic Reactivity on
the Volume of Two Liquid Phases 95
6. Organoclay Catalytic Reaction in the Absence of Water ------- 98
7. Mechanism of the Organoclay Triphase Catalysis 99
C. The Longevity of Organoclays for Triphase Catalysis 104
1. Recyclability of Organoclays as Triphase Catalysts-----------104
2. Surfactant Desorption 111
(21) Influence of aqueous electrolyte solution on t
surfactant desorption of organoclays 111

 

(b) Dependence of the surfactant desorption on the
surfactant structure in the presence of the sodium

 

chloride aqueous solution and toluene 114
(c) Desorption of surfactant from clay hosts in the presence

of aqueous sodium bromide solution and toluene -------- 117
(d) The dependence of surfactant desorption on

electrolyte concentration 118

 

(e) Dependence of the surfactant desorption on the
organic solvent polarity -- 122

 

iii

3. Co-Surfactant Effects for the Triphase Catalysis -124
(a) Influence of co-surfactant on [C15H33NMe3+]

 

 

 

desorption 124
(b) Influence of co-surfactant on biphase and triphase
catalysis 125

(0) Catalytic properties of organoclays and onium ion
surfactant for the cyanation of alkyl bromide and .
bromoalkyl alcohol 130

4. The Catalytic Properties of Crown Ether Clay Complexes-u 133

 

D. The Applications of Organoclays in other Triphase Catalysis

 

 

 

 

 

 

Reactions —135

1. The Synthesis of Symmetrical Formaldehyde Acetals -------- 135

2. Synthesis of Alkyl Bromide 141

3. Dehydrohalogenation of 2-Bromoethylbenzene 144

4. Oxidation of trans-Stilbene 144

5. Synthesis of 2,4-Dinitrophenyl Ether 146
CHAPTER IV CONCLUSIONS 151

REFERENCES 159

 

iv

LIST OF TABLES

TABLE PAGE

Surfactant-intercalated clay Catalyst. 62

 

The BET surface area of sodium clays and the
organoclays in which the surfactant, [C16H33NMe3+],
is intercalated in the interlayers. 65

 

The d-spacings of crown ether-clay complexes derived
from X-Ray diffraction measurement. 73

 

The catalytic activities and the colloidal properties of the
organoclays or the onium ion salts as phase transfer catalysts.----76

Solvent effect for the iodination reaction of pentyl
bromide (Equation 5). 91

 

The product ratios of the O/C alkylation of sodium
naphthoxide with benzyl bromide when organoclays and
onium salts were used as the phase transfer catalysts. 93

 

The catalytic activity of the higher layer charge density
C16H33PBu3+organoclay after multiple reaction cycle for
cyanation and chlorination of pentyl bromide. — 108

 

The desorption of onium ion from the organoclays in the
presence of 2.5N NaCl aqueous solution. 112

 

Desorption of surfactant from the organoclays in the
presence of NaCl aqueous solution and toluene. 1 15

 

10.

11.

12.

13

14.

15.

16.

17.

18.

19.

20.

21.

Desorption of surfactant from the organoclays in the
presence of aqueous 2.5N Hafiz aqueous solution and
toluene. 1 19

 

The desorption of the surfactant under the triphase condition
with other organic solvent. 123

 

Desorption of [C15H33NMe3+] from the organoclays
in the presence of octanol as co-surfactant. 126

 

Influence of octanol and tetradecanol co-surfactants on the
cyanation of pentylbromide by byphase catalysts and triphase
catalysts. 128

 

The cyanation of bromoalkanes and bromoalkanols under
biphase and triphase catalysis condition. 132

 

Chemical yields for the cyanation of benzylbromide to
benzyl cyanide. 134

 

The chemical yields of the acetal synthesis (Equation lO-l2).---139

The chemical shifts in 1H NMR of the starting materials
and the reaction products of the acetal synthesis reaction
(Equation 10-12). 140

 

The synthesis of octyl bromide from octanol by
hydrobromic acid dehydration (Equation 13). 145

 

Synthesis of styrene from 2-bromoethylbenzene under
alkaline solution (Equation 14). — 147

 

The formation of benzoic acid by the oxidation of
trans-stilbene (Equation 15). 148

 

The synthesis of 2,4-dinitrophenyl ether from l-chloro-2,4-
dinitrebenzene and phenolate (Equation 16). 149

 

LIST OF FIGURES

 

Figure Page
1. The structure of a 2:1 dioctahedral alumina silicate mineral. ------- 3
2. The structure of a 2:1 trioctahedral magnesium silicate mineral.---4
3. The structure of rectorite. -7
4. Mechanism of biphase catalysis with a nucleophilic

 

substitution reaction. 10

5. A general process of triphase catalysis for a nucleophilic
substitution reaction. --14

 

6. Linear and cross-linked structures of polystyrene-supported

 

 

 

triphase catalyst. 16
7. The O/C alkylation of benzyl bromide and naphthoxide in

protic or aprotic solvents. 18
8. Mechanism of the polymer-supported triphase catalysis. ---------- 19
9. The synthesis of silica gel supported catalysts. 21

10. (a) The structure of polymer-supported poly(ethylene glycol)
(b) The function of the PEG atom as oxygen chelating sites
for the metal cation. 24

 

11. The surfactant orientation in the interlayers of various charge

vii

12.
13.

14.

15.

16

17.

18.

19.

20

21.

 

density clay. 26

A droplet dispersion of oil-in-water microemulsion. 41

 

The standard curve of the intensity area ratio of
[C5H11CN]/[0.100g C10H22] in GLC analysis versus

[C5H11CN] in 6 mL toluene as the organic solvent. 49
dependence of the UV-Vis absorption of

C151133NMe3+-methyl orange chloroform solution at 418 nm

on concentration of C15H33NMe3Br aqueous solution. ------------ 53

 

Dependence of UV-Vis absorption of
C15H33PBu3+-methyl orange chloroform solution at 418 nm
on concentration of C15H33PBu3Br aqueous solution. ------------- 54

The X-ray diffraction pattems of various organoclays:
(a) L24AA; (b) H24AA; (c) W24AA; (d) A24AA; (e) F24AA.-- 66

The orientations of crown ethers in hectorite and Arizona
montmorillonite interlayers. 74

 

Classification of the colloidal behavior of 0.100 g of organoclay

in liquid mixtures containing 3 mL of 6.25M potassium cyanide
aqueous solution, 6 mL of toluene and 2 mole of

pentylbromide. 82

 

(a) Determination of a kobs from the slope of a plot of
[C5H11Br]/[C5H11Br]o versus the reaction time in the presence

of 0.100 g of C15H33NMe3hectorite.

(b) The determination of a kobs' from the slope of a plot of
[CN']/[CN']0 against the reaction time in the presence of

0.100 g of C15H33PBu3hectorite (26AA). 85

 

(a)Dependence of the observed rate constant (kobs) on nucleophile
concentration for the cyanation reaction at 90°C in the presence

of C15H33PBu3hectorite (H26AA) as the triphase catalyst.
(b)Dependence of the observed rate constant (kobs') on the

organic electrophile concentration for the reaction at 90°C

in the presence of 0.100g C15H33PBu3hectorite as the

triphase catalyst. 87

 

(a) The dependence of kobs on the volume of toluene for the

22.
23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

reaction of pentylbromide with KCN.
(b) The observed rate plotted against the volume of aqueous
solution while the volume of toluene solution was kept

 

 

constant at 2 mL. 96
The emulsion of aqueous and organic solution stabilized by

an amphiphilic organo hectorite. — 100
The mechanism of the triphase catalysis reaction with organo
hectorites as the catalysts. 102

 

The recyclability of C15H33PBu3+hectorite (H26AA) over 10
reaction cycles for the cyanation of pentyl bromide at 90°C.---- 105

The recyclability of C15H33NMe3+hectorite (H24AA) over 10
reaction cycles for the cyanation of pentyl bromide at 90°C.---- 106

The recyclability of C16H33NMe3+F-hectorite (F24AA) over 10
reaction cycles for the cyanation of pentyl bromide at 90°C.---- 109

Dependence of [C15H33PBu3+] desorption from organo
F-hectorite (F26AA) on the concentration of sodium chloride
in an aqueous-toluene emulsion. 120

 

The X-ray diffraction pattem of C15H33PBu3+F-hectorite
(F24AA) reaction products formed by reaction of
aqueous-toluene emulsions at various NaCl concentration. ------ 121

The IR spectra of (a) sodium hectorite, dool=12.1A;
(b) 18C6Hect, d001=15.6A; (c) recycled 18C6Hect after one
cycle of reaction, dool=l3.2A. 136

 

The X-ray diffraction pattem of C15H33NMe3+hectorite (H24AA)
after the catalysis reaction in which the reaction mixture contains
50% NaOH aqueous solution and 10 mL of methylene solution. 142

The X-ray diffraction pattern of sodium hectorite treated with
50% NaOH aqueous solution. 143

 

The 1H NMR spectra of (a) 1-chloro-2,4-dinitrobenzene
(b) 2,4-dinitrophenyl ether. 150

 

CHAPTER I

INTRODUCTION

A. Structure and Properties of Layer Silicates

The layer silicates, hectorite and montmorillonite, described in this
dissertation are smectite minerals which are a class of naturally occurring
minerals. The term 'clay mineral' refers to specific silicates with particle
size less than 2 pm and with definite stoichiometry and crystalline
structure.

Smectites are composed of units made up of two silica tetrahedral
sheets and a central octahedral sheet of magnesia or alumina”. The
silicate tetrahedra are usually oriented so that the three basal oxygen
atoms of each tetrahedron lie on the same plane, while the fourth oxygen
atom defines a second common plane. The octahedral sheet contains a
cation, usually Al or Mg, surrounded by six oxygens in an octahedral
arrangement. The tetrahedral and octahedral sheets are combined so that
the tips of the tetrahedrons of each silicate sheet and the oxygens of each
octahedral sheet form a common layer. Smectite clays are 2:1 layer
minerals which are divided into two structures: dioctahedral aluminum

silicate minerals (Figure 1) and trioctahedral magnesium silicate minerals

2

charge arises predominantly from isomorphous substitution in the
octahedral layer or from the substitution in the tetrahedral layer. Cations
at particular locations of the silicate structure can be replaced by some
other cation with similar ionic radius without changing the structure of
the minerals. If the replacing cation has lower valence, a net negative
charge will be deve10ped. The negative charge is then balanced by the
presence of hydrated metal cations in the interlayer region of the
structure. These hydrated metal cations are usually located adjacent to
the point of anion charge on the basal plane.

' The trioctahedral silicate minerals used in the work are laponite,
hectorite and F-hectorite. The idealized unit cell composition for laponite

is
Lio.36[Lio.36Mgs.64]“(Si8.oo)1V020(0H)4

in which the superscripts (IV) and (VI) refer to the respective cation in
the tetrahedral and octahedral sites. The first "Lio,36" in the formula
designates the exchangeable hydrated lithium in the interlayer region, and
the second one represents the octahedral sheet lithium. Laponite is a
synthetic low-charge smectite.

The idealized unit cell composition of hectorite is

Mo.67[Lio.67Mgs.331(Si8.oo)Ozo(OH. F)4

3

Figure 1 The structure of a 2:1 dioctahedral alumina
silicate mineral.

Interlayer Region
0 Al O O

031 @011

Figure 2 The structure of a 2:1 trioctahedral magnesium
silicate mineral.

1 :5, ; ;
«it: i '

Interlayer Region

WW

()Mg 00

031 (3301-1

5

in which the "M" designates a hydrated monovalent cation.
Fluorohectorite is also a synthetic clay in which the hydroxyl groups are
substituted by fluoride. The idealized unit cell composition of hectorite is

Li1.6[Li1.6Mg4.4l(Sis.oo)020F4.

The dioctahedral silicate minerals used in the dissertation are
montrnorillonites from Arizona and Wyoming. The idealized unit cell

compositions for both montrnorillonites are
M1.161A12.34Mg1.16]Sis.00020(0H)4, and

M0.67[A1333M80.67](Si8.00)020(0H)4, respectively

Rectorite3 as shown in Figure 3 consists of a regular alternation of a
mica-like layer and an expandable layer having a smectite composition.
Rectorite also belongs to the family of regularly interstratified clay
minerals as well as to the family of smectite clays. Its basal spacing is
greater than 19A which is approximately double that of the smectite basal
spacing. The formula of rectorite is

[Nao.72Ko.02Cao.os)(Cao.24Nao.07)l(Al4.ooMgo.02)[Si6.ssAlr.62)022]

The charge on the mineral arises from isomorphous substitution in the
tetrahedral particle sheet of the 2:1 smectite clay layer. The cation
exchange capacity (CEC) of rectorite is 60 milliequivalents per gram.

6

surface charge density of this material is approximately equal that of a
smectite clay in which the CEC is 120 mqu 100g.

Smectite clays can be swelled by adsorption of water or some
organic solvents4‘7. With multiple layers of solvent, the galleries become
liquid-like and accessible for chemical reactions. As the solvent content
increases, the basal spacing of smectite clays also increase. The swelling
capacity of the clay can be reduced by introducing electrolyte to the
solution. The electrolyte reduces of the electric double layer repulsion of
two neighbor silicate layers. Generally, higher layer charge density clays
are less swellable4 than the lower layer charge clays.

Smectite clays and their derivatives have shown catalytic properties
for many reactionsg'“. The acidic nature provides the source of the
catalytic capacity. Both Lewis and Bronsted acidity have been noted, the
former derived from aluminum or iron species located in the crystal. The
Bronsted acidity resulys from dissociation of the interlayer water

molecule coordinated to polarizing interlayer exchangeable cations”.

B. Principles of Phase Transfer Catalysis
1. Biphase Catalysis

Chemists frequently encounter the problem of bringing the reactants
in two mutually immiscible solutions into proximity to attain rapid

reaction rates. The traditional procedure is to

Figure 3. The structure of rectorite.

 

Inter layer Region Expandable Interayer
O M g Q 0 . K+
. Si(Al) © (XI

9

dissolve the reactants in a homogeneous medium. However, a suitable
solvent is not always available and it is usually expensive and difficult to
remove after reactions. Solvents such as THF or DMF sometimes present
environmental problems in large scale operations.

The technique of phase transfer catalysis can permit or accelerate
reactions between ionic compounds and organic, water-insoluble
substrates in solvents with low polarity. The concept of " phase transfer
catalysis" is to transfer the ions, neutral molecules or free radicals from
one phase to another where reaction can occur. It is clear that a phase
transfer catalyst has considerable advantages over conventional
procedures since it eliminates the requirement for expensive anhydrous or
aprotic solvents, improves reaction rates, and lowers the reaction
temperature13'16.

The first type of phase transfer catalyst is the biphase catalyst13’14.
Quaternary ammonium or phosphonium halide is typically used for
typical biphase catalysis. The amphiphilic character of quatemary onium
salts allows them to be wetted in both aqueous solution and organic
solvents with low polarity. Stark and Owens13 first developed the use of
ternary ammonium and phosphonium salts for phase transfer catalysis in
organic synthesis. The mechanism of biphase catalysis was studied by
Stark and Owens14 and Brandstonn”.

A nucleophilic substitution reaction using onium salts as biphase
catalysts13 is represented in Figure 4. An onium cation pairs with an

anionic nucleophile Y' in the aqueous phase and the

10

Figure 4 Mechanism of biphase catalysis with a nucleOphilic
substitution reaction.

- Q+ -
Rx(org) 4' Y (aqu) ’RY(org) + X (aqu)

 

 

RY + Q“X‘ Organic

 

 

X' + Q”Y‘ ' Y‘ + Q“X‘ Aqueous

 

Q+ = NR4+; PR4+; NaT-Crown Ether

11

ion pair is extracted into the organic phase. Once the anion has been
transfered into the organic phase, the anion and organic electrOphile RX
undergo nucleophilic substitution and form RY and a new salt, Q+X’.
The new salt then returns to the aqueous phase, where Q+ pairs with a
new anion, Y’, for the next cycle.

If there is one long alkyl chain in the onium ion, the biphase catalyst
may also function as a surfactant. The surfactant molecules form small
aggregates of 10 to 50 organic molecules dispersed in the aqueous phase.
The small aggregations are called micelles, wherein the nonpolar organic
parts of the molecules occupy the intemal volume and the highly polar
group of the surfactant occupy the outer surface”. For micelle-catalyzed
reactions, the positively charged outer surface attracts and concentrates
anions from the bulk aqueous solution into the so-called Gouy layer near
the surface of the micelle. Reaction then occurs at the Gouy layer19‘24.

As comparable volumes of the organic solutions are added to the
aqueous micellar solution, an emulsion mixture of aqueous and organic
solution stabilized by surfactant is formed. If a co-surfactant, which is
typically an alcohol with more than five carbon atoms, is added to the
micellar solution, the emulsion is converted to a transparent
microemulsion25'27. The microemulsion has a smaller droplet size and a
lower surface tension than those of the emulsion27'30. Moreover, a
microemulsion is thermodynamically stable, unlike an emulsion which is
only kinetically stable25'29. As the microemulsion is utilized into the

12

biphase catalysis, a more efficient catalytic reaction can be obtained due
to increased interfacial area between aqueous and organic phases.

f Crown or macrocyclic ethers usually containing the basic unit (-0-
CH2-CH2)n have also been met with interest for phase transfer
catalysis31-35. These are exemplified by l8-crown-6 in which‘18
indicates the number of atoms in the ring and 6 represents the number of
oxygens. Other commercially available crown ethers are dibenzo-18-
crown-6, dicyclohexano-lS-crown-6 and 15-crown-5. A common feature
of all crown and related compounds is a central hole or cavity which can
chelate the substance. The cation complex of general interest for phase
transfer catalysis are those formed with potassium, sodium cations,
hydronium, ammonium, and diazonium.

Complex formation between a crown ether and an anionic
nucleOphile has two important characteristics. The first is the organic
masking of the alkali metal providing an "onium ion" like entity that can
be extracted into organic solvents with the accompanying anion. The
second is that the anion part of the ion pair in the organic solvent is
activated. In a dipolar aprotic solvent, this effect is most notable because
the anion-solvent interactions are weak. Such a system has been
described as involving the "reaction of naked anions".

Since the discovery of phase transfer catalysis, numerous
applications have been described, not only in organic chemistry, but also
in inorganic chemistry“, analytical applications”, electrochemistry33,

13

photochemistry39 and polymer chemistry“0 have utilized phase transfer
catalysis. In organic synthesis , phase transfer catalysis has emerged as a
broadly useful tool for: a) nucleophilic substitution reactions b) alkylation
and condensation c) reaction of dihalocarbenes and‘other carbenes d)
ylide mediated reactions e) oxidations and reductions. In those reactions,
the technique of phase transfer catalysis provides a method which avoids

the use of a polar aprotic solvent and also improves the reaction rate.

2, Triphase Catalyssis

a) Nature of Triphase Catalysis

There is a new type of heterogeneous catalysis called " triphase
catalysis" , in which the catalysts and each member of a pair of reactants
are located in separate phases. Figure 5 illustrates the general features of
a simple reaction of a triphase catalysis system“,

Generally, the reactivity of triphase catalysis is lower than that of
biphase catalysis based on the same equivalent of catalysts being used“.
However, triphase catalysis exhibits the advantage of catalyst recovery
and convenience of workup after catalytic reactions. Moreover, some
triphase catalysts can provide regioselectivity due to the spatial constraint
of the catalyst and the reactants.

Four major types of triphase catalysts have been developed. Three
of these utilize onium ions as the catalytic center. These onium ions are

supported on polymers43-45, inorganic matrices47'50, or layer

14

Figure 5 A general process of triphase catalysis for a
a nucleophilic substitution reaction.

- Catalyst
RX<org> + Y (aqu)

 

RY(org) "' X-(aqU)

 

AQUEOUS ORGANIC

 

RY

 

 

 

 

15

silicates51'54. Another type of the triphase catalysis uses polyethylene
glycol as the catalyst32a55'57.

b)Polymer supported catalysts

Polymer supported catalysts are the first studied and best defined
triphase catalysts. The backbone of the catalysts is polystyrene (Figure
6). Some of the phenyl rings are linked to onium salts. Also, the linear
polymer can be expanded to a two-dimensional or three-dimensional
network by cross linkage of two phenyl groups. In general, the higher the
degree of cross linkage, the lower the diffusion of organic substance into
the catalysts53»59. Tomoi and Ford53o50, Regen and Reese“, and
Montanari et al.,51 have reported extensively on the mechanism and
properties of triphase catalysis. The fundamental mechanism of the
polystyrene-supported catalysts for phase transfer catalysis is verified by
solvent effects45:51’52, O/C alkylation reactions of benzyl bromide and
sodium naphthoxide51:53'55, and the dependence of rate on bulk cyanide
concentrations55. Also, N MR studies of the swellability of the triphase
catalysts in two immiscible solvents“, the rate dependence on cross-
linking of the polystyrene backbone53o59, and the rate dependence on
percentage of onium salts attached to the phenyl groups provide
information on the mechanistic process57.

The location of the catalytic reactions in polymer supported catalysts
is indicated by the following phenomena. a)The polymer-supported
catalysts show higher catalytic capacity in the presence of polar organic
solvents. The dependence of reactivity on the polarity of the organic

solvent increases in the order decane

16.

Figure 6 Linear and cross-linked structures of
polystyrene-supported triphase catalyst.

Backbone

 

/ i // /" 7 ,u Ph
0 C O U U Q
X X
Ph Ph Ph
Ph

x = (CH2),PR3"X‘ or (CH2),,NR3*X'

17

< toluene < o-dichlorobenzeneGlfiZ. This suggests that solvation of the
ionic pair formed by the onium cation and nucleophile anion in the
organic solution dominates the catalytic reaction. b) For the O/C
alkylation reaction of naphthoxide by benzyl bromide (Figure 7)“, the
O-alkylation product is favored in aprotic solvent such as DMF, toluene
etc., and the C-alkylation product is favored in protic solvents such as
water, methanol etc. Polymer-supported catalysts afford mainly O-
alkylation products ( Figure 7)51,53'65. Therefore, the catalytic reaction
for this triphase catalysis occurs mainly in an aprotic, polymer
environment. 0) The reactivity of a polymer supported catalyst is not
sensitive to the bulk concentration of nucleophile“, indicating that the
catalytic reaction does not occur at the aqueous interface of polymer
supported catalysts. Therefore, the nucleophilic substitution reactions
occur at the organic phase, but the anion exchange at the onium cation
site for the next cycle may occur at the aqueous interface.

The properties of polymer-based triphase catalysts suggest the
fundamental kinetic processes6o shown in Figure 8. The first step is the
mass transfer of electrophilic reactant, RX, from bulk solution to the
catalytic surface. The second step is diffusion of the reactant through the
polymer matrix to the active nucleophilic site. Nucleophilic substitution
occurs at the active site in the third step, followed by the diffusion of
products to the surface of the catalyst and mass transfer of the products to
the bulk solution as the fourth step. The ion pair may go back to the
aqueous solution to exchange a new nucleophile for the next reaction

cycle.

18

Figure 7 The O/C alkylation of benzyl bromide and
naphthoxide in protic or aprotic solvents.

m.

In P ti Sol t
In Aprotic Solvent Evita) ven
(DMF)
Intermediate Intermediate
(Energetically Stable) (Stabilized by H-Bond

Formation with Solvent)

CHzPh

OCHZPh
on

O-Alkylation (100%) C-Alkylation (89%)

19

Figure 8 Mechanism of the polymer-supported triphase catalysis.

(3)

4:.
a
“.2
41.
2°
‘82

O .
l “’ 1(4) piii‘é‘“

§§_P+Buay’ g :Rx: EE—P+BU3X. g :RY:

<—— + A ueous
E—P’BuaY' + x _. BuaX' +Y 331111 My

Phase

20

c) Inorganic-based triphase catalysts.

The synthesis of inorganic-based triphase catalysts involves the
anchoring of an alkyltriakoxysilane (either a haloalkyl or
aminoalkyltrialkyoxy silane) to the surface of silica gel or alumina to
afford an alkyl functionalized inorganic matrix (Figure 9)“. Several
types of onium salts can be immobilized by means of various
hydrocarbon chain length containing different functionalization57'48.
Important considerations for effective catalyst design are (i) the porosity
and type of the inorganic support; (ii) the length of spacing of the alkyl
chain; (iii) the chemical structure of the onium salt. Silica gel with a titer
of 0.5 mmole Cl‘lg, is designated as 6OSiC3PBu+Cl'(0.50), where 60Si
indicates a silica gel support with a mean pore diameter of 60A, C3
indicates the length of the alkyl chain supporting the phosphonium salts,
and the value in parenthesis represents the loading in millirnoles of the
onium counterion per gram of catalytic support.

The main difference between inorganic solid and polymer supported
catalysts is the influence of solvent polarity on the triphase catalysis
reaction49'69. In the case of the silica gel triphase catalysis, the activity
increases with decreasing solvent polarity. On the contrary, polystyrene
supported catalysts show higher activity in more polar solvents. The
catalytic reactions are carried out in different microenvironments.

The functionalized inorganic matrices are wettable both by the

aqueous and the organic solution. This property allows these

21

Figure 9 The synthesis of silica gel supported catalysts.

OH

 

HCI 0”
.. OH
Reflux
OH
311103 Gel ' Active Silica Gel
Br(CH2)nSi(OEt)3
O PBU3
\ - CH PBu Br 0
0734 2).. a «— o>s;(CH2),,Br

O o

22

systems to be active in liquid-liquid phase transfer catalysis even without
stirring59, since the inorganic matrix acts as a supply and a pump for the
aqueous phase which contains the nucleophile anions. As the catalytic
reactions proceed very close to the catalyst surface, the exhausted onium

salt can be regenerated easily by means of exchange with the aqueOus ’
solution. The large surface of the support promotes the anion exchange
process70,

The exact structure of these systems during catalytic reactions
remains unknown and not all the questions concerning the deposition of
the aqueous and organic phases into the functionalized pores have been
answered. The adsorption of the organic substrate on the catalyst pores
or surface may play an important role in the mechanism. However, the
diffusion of the organic reactants onto the catalyst surface or pore does
not play an essential role for inorganic material-supported phase transfer
catalysis.

Various applications concerning the use of silica gel or alumina
based triphase catalysts in nucleophilic substitution reactions have been
reported48’49: halogen exchange, synthesis of phenyl ethers and sulfides,
reduction of carbonyl compounds with aqueous sodium borohydride,
synthesis of nitrile, thiocyanate, nucleophilic, and N-alkylphthalimides.
Depending on the reaction conditions, however, the functionalized
inorganic matrices may lose some of their functionalization under the
strong alkaline solution69, and then reactions can actually be catalyzed
both by the phase transfer catalyst and the silica gel or alumina-based
triphase catalysts as well as the hydrolyzed catalytic groups free in

solution.

23

d). Poly(ethylene glycol)derivatives as phase transfer catalysts

The polyethers, specifically poly(ethylene glycol)'s (PEG), are
widely used in phase transfer catalysis. The function of this reagent in
phase transfer catalysis reactions is similar to that of crown ether. PEG's
openly chelate the metal cation (Figure 10)"'2 and carry the nucleophile to
the organic phase to undergo the nucleophilic substitution reaction.
Therefore, PEG may be used in most applications where crown ethers are
currently used. While it is true that the reactivity of PEG's is often less
than that of crown ethers, this may be compensated for by increasing the
concentration of PEG.

Many applications of organic reactions using PEG's as catalysts have
been reviewed by Totten and Clinton”: the Biltz synthesis of phenyltoin;
the synthesis of triaryl phosphate; the N-alkylation of nitrogen
heterocycles; alkyloxymethyl and formation of alkoxypolyethoxymethyl
derivatives of acylanilines; dehydrohalogenation of 2-bromoethylbenzene
by hydroxide; carbonyl reduction; synthesis of phenyl ethers and sulfides;
Williamson ether synthesis; polyether synthesis; aryldiazonium salt
reactions; and oxidation chemistry. However, there is no report about the

recyclability of these catalysts.

24

Figure 10 (a) The structure of polymer-supported
poly(ethylene glycol) (b) The function of
the PEG atoms as oxygen chelating sites for
the metal cation.

(a)

§_._ CH20(CHZCH20),,H

 

O-.~~ O R
"K+

E

o """ 9 ~~~~ OH---OH-
\_/ L/

25

e) Organoclay as triphase catalyst

Organoclays, which are the smectite clays intercalated by ternary
ammonium or other organic cations, have been used as triphase catalysts.
Kadahodayan and Pinnavaia used clay derivatives with metal complexes
such as M(phen)32+/XO42' ion pairs in the interlayer to facilitate the
liquid-liquid phase transfer reactions51. Comelius, Laszlo and P.
Pennetreau utilized montmorillonite derivatives as triphase catalysts for
alkylation of dihalomethane54. More recently, Choudary, et al. used the
acidified clays linked with onium salts to accelerate cyanation
reactions52. Lin, Lee and Pinnavaia contributed extensively to
understanding of the nature and the application of the phase transfer
catalysis by using organoclays as the triphase catalysts53. This work is
continued by focusing on the mechanism, colloidal properties and

longevity of organoclays as triphase catalysts.

C. Research Rationale and Objectives
1. Nature of Organoclays as Triphase Catalysts

a) The structure of organoclays '

The orientation of the cations in smectite clay interlayers depends on
the onium structure and the layer charge density of the layered silicate71'
73. Generally, a good organoclay for triphase catalysis should have an
onium cation with more than one long alkyl chain. This onium ion can
also be used as a surfactant. Lagaly summarized the surfactant
orientations in the galleries of various layer charge density clays (Figure
11)“. For layered silicates with an extremely low layer charge density,

26

Figure 11 The surfactant orientation in the interlayers of
various layer charge density clay.

(a) Monolayer: [C15H33NMe3]+ Laponite
(Cation exchange capacity, 55meq/ 100g)

I WW1 T

 

i 14.5A

 

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

 

 

 

 

 

27

Figure 11(Continued)

(c) Pseudo Trimolecular Structure:
[C15H33NMe3]+ Montrnorillonite(Arizona)
(Cation exchange capacity, ll8meq/100g)

 

 

 

 

 

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

 

 

 

 

 

 

 

28

Figure 1 1(Continued)

(e) Lipid Structure, [(C12H25)2NM62]+ F'hCCtorite
(Cation exchange capacity, 140meq/ 100g)

 

 

 

 

 

 

 

(t) [(C12H25)2NMe2+]Laponite
(Cation exchange capacity, 55meq/ 100g)

\ \
NM MM
‘. WW3 ~~ M 17.5A

 

 

 

 

 

 

 

 

29

the surfactant adopts a monolayer structure with the chain parallel to the
silicate surface (Figure 11a). As the layer charge density of the layer
silicate increases, the ion will form a two layer structure with the chain
parallel to the silicate layer, the so-called bilayer or lateral bilayer
structure (Figure 11b). When the layer charge density of the layered
silicate increases further, a pseudo trimolecular (Figure 11c) or paraffin
structure (Figure 11d) of the surfactant will be adopted in the interlayer of
the smectite clay. If a high layer charge density organoclay contains
surfactants with two long alkyl chains, a lipid-like structure of the clay
can be found (Figure lle). The orientation of the surfactants in the
interlayer can influence the wettability of the organoclay on the aqueous
and organic phase. Also, the efficiency and longevity of the organoclay
as triphase catalyst are affected by the surfactant orientations, as will be
discussed in the later chapter.

b) Colloidal properties of the organoclays

The amphiphilic character of the organoclays is a potentially
important property for efficient triphase catalysis. Sodium clays are only
wettable in aqueous solution or polar aprotic solvents such as methanol.
The purpose of intercalating an onium ion surfactant into the clay
interlayers and extemal surface is to increase the hydrophobicity of the
material. The hydrophilicity or hydrophobicity of the organoclay are
easily judged by observing the wettability of the material. Also, BET
surface area measurements can provide information on the hydrophilic
portion. The surfactant alkyl group will fill the interlayer gallery leaving

no empty space accessible for the nitrogen adsorption.

30

An amphiphilic organoclay can stabilize an emulsion of water and
aprotic organic solution. Therefore, the nucleophile and the organic
electrophile from two immiscible phases can be brought together on the
phase boundary to undergo nucleophilic substitution reaction. Generally,
an organoclay with a monolayer structure is a poor emulsifier for the
water and oil mixture. However, the surfactants adopted in pseudo
trimolecular and paraffin-like structure sometimes desorb from the
reaction condition, making the high charge density organoclays poor
catalysts.

The amphiphilic nature of organoclay surfaces has been applied
already in photochemistry74'77 and as absorbents for environmental
pollutants73i79. Some organoclay surfaces may adsorb neutral
chromophores, such as pyrene dissolved in aqueous solution. The
adsorbed surfactants in clay hosts provide a hydrophobic site for the
adsorption of luminescent organic molecules such as pyrene. In these
colloids a large amount of pyrene excirner, in the form of dirnerized
pyrene, is generated at a pyrene concentration of 5x10'5M, significantly
below that needed to form an excirner in homogeneous solution. Hence,
pyrene excirner formation is a result of localization of pyrene on the
surface of a clay particle, which has the effect of dramatically increasing
the local concentration. The pyrene excirner has an emission spectrum
different from that of the pyrene momomer. Also, the probe molecule
(pyrenylbutyl)trimethylammoniurn bromide (PN+) fluoresces well on the
clay surface containing hexadecyltrimethyl ammonium bromide (CTAB).

31

However, PN+ fluorescence is quenched by dimethylaniline,
nitrobenzene and nitromethane in the CTAB-Clay system. The
hydrophobic clay surface adsorbs the organic quencher and reduces the
life time and the intensity of PN+. This hydrophobic property of the
organoclay has also been utilized for environmental applications. Boyd
and co-workers have used amphiphilic organoclays to eliminate organic
contaminants, such as phenyl derivatives from aqueous solution73i79.
The sorption isotherms of benzene, toluene, ethylbenzene,
propylbenzene, butylbenzene, naphthalene and biphenyl on the CTAB-
clays indicate that sorption occurrs by partitioning interaction with the
C15H33NMe3+-derived phase. In general, increasing the C16H33NMe3+
content and basal spacings with increasing clay charge density increased
the non—ionic organic compound sorption on CTAB-Clays. Increased
sorption of alkylbenzenes by high charge density organoclays can be
attributed to the ability of large basal spacing to accommodate larger
solute molecules.

c) Mechanism of organoclay triphase catalysis

The mechanism of organoclay triphase catalysis are investigated by
(i) observing the influence of organic solvent polarity on catalytic
reactivity (ii) observing the relative O/C alkylation reaction of
naphthoxide and benzyl bromide by using organoclays as catalysts (iii)
studying the dependence of catalytic reactivity on the concentration of
both anionic nucleophiles and organic reactants, and (iv) studying
dependence of catalytic reactivity on the volume ratio of the two

immiscible phases.

32

For polymer supported catalysts the nucleophilic substitution
reaction proceeds in the organic phase. Since polar solvents have good
polymer swelling ability, the catalytic reactivity for the ionic polymer
catalysts increases with solvent polarity. If the swellability of
organoclays by organic solvents plays an important role for triphase
catalysis, the features of organoclay triphase catalysis should be similar
to the catalytic reaction using polymer supported catalysts“. Otherwise,
the nucleophilic substitution reactions may occur in the aqueous phase or
at the boundary of clay-liquid phases.

O/C alkylation (Figure 5) can provide information about whether
nucleophilic substitution proceeds in either aprotic or protic media55963.
An O-alkylation product results from reactions in an aprotic organic
solution, and a C-alkylation product is favored from protic media such as
aqueous solution. The O/C alkylation has been well studied for triphase
catalysis using polymer supported catalysts“. Basically, the reaction
products are predominately O-alkylation when using polymer supported
catalysts, indicating that the nucleophilic substitution reaction occurs in
the aprotic solvent. The organoclay triphase catalysis will be studied by
the same reaction to determine the reaction microenvironment.

The reaction location can also be elucidated by the dependence of

catalytic reactivity of cyanation (Equation 1) on the

C5H11Br + CN' ------ > C5H11CN 4» Br' (1)

bulk concentration of liquid phases. Since the nucleophiles and

33

substrates are not in the same phase, there should be one or two of the
reactants adsorbed on the organoclay where the nucleophilic substitution
reaction may occur. Three different plausible mechanisms can be
described by the three following

Rate = k[organoclay. RX] [CN'](aq) (2)
Rate = k[organoclay. CN'][RX](org). or (3)
Rate = k[organoclay. CN' RX] (4)

equations. Equation 2 can be applied when the organic electrophile is
adsorbed on the organoclay catalyst before reacting with the nucleophile
located in the bulk aqueous solution. Equation 3 is appropriate only
when the nucleophile is adsorbed on the organoclay catalyst and then
contacted with the organic electrophile in bulk organic solution. If
neither Equation 2 nor Equation 3 applies, the mechanism may be
interpreted by Equation 4. If the mechanism for Equation 2 is favored,
the pseudo first order rate constant, kobss can be determined from
Equation 5. The Robs will be equal to k[CN']. That is, the observed

-d[RBr]/dt = kobisBrl (5)
rate constant should be proportional to the bulk nucleophile

concentration, [CN'], when the [CN'] is much larger than the [RX].
However, if the mechanism discussed by Equation 3 is true, another

34

pseudo first order rate constant, kobs’, can be determined from Equation
6. This kobs' will be proportional to the bulk

-d[CN']/dt = kobs'ICN'] (6)

concentration of organic electrophile, [RX], because kobs' is assumed to
be equal to k[RBr]. The dependence of the pseudo first order rate
constant on the bulk concentration of potassium cyanide (Equation 5) has
been studied by Lin”. There is a linear relationship between the kobs
and [CN‘], indicating the nucleophilic substitution reaction occurs at the
catalyst-aqueous boundary. We further investigate the relation between
the reaction rate, kobs'. and [RBr]. A non-linaer relationship between
kobs'. and [RBr] can prove the the reaction is not occuring in organic
phase.

The rate dependence on the volume ratio of organic and aqueous
phase also provides information about the emulsion structure and the
wettability of organoclays in the two liquid solutions. If the three phases,
organoclay, aqueous and organic solution, adopt oil-in-water emulsion, a
milky emulsion and excess of organic solution will be observed.
Likewise, an water-in-oil emulsion will perform a milky emulsion and
excess aqueous phase when the volume of the aqueous solution is more
than the maximum content of aqueous droplets in the emulsion phase.
This emulsion structure can be observed by changing the volume ratio of
the two liquid phases. In a water-in—oil emulsion system, organoclays are
mainly suspended in organic solution and aqueous droplets are immersed

in the aqueous solution. As the volume of the aqueous solution increases,

35

a redundant aqueous solution appears and does not participate in the
catalytic nucleophilic substitution reaction. That is, the reaction rate will
not be influenced by the volume of bulk aqueous phase if the an excess
quantity of aqueous solution appears. However, increasing the volume of
organic solution will decrease the organoclay concentration in the organic
suspension for the water-in-oil emulsion. This will result in low
efficiency for the adsorption of organic electrolyte and catalytic reactivity
in this water-in-oil emulsion system.
2. Longevity of the Clay Derivatives for Phase Transfer Catalysis.

a) Desorption of interlayer organic compounds

The most advantageous property of triphase catalysts is their
potential recyclability. Polymer and inorganic matrices covalently link
the onium salts so the catalyst structure will be maintained after the
catalytic reactions. However, the polymer supported catalysts have yet to
find industrial applications because of their diffusion limitations of
mechanism and chemical instability30o31. Several inorganic supports
suffer the same general disadvantage, namely, low reactivity on structural
instability under reaction conditions. For example, silica gel as a support
is destroyed by alkaline solution59. The decrease in organoclay
recyclability for triphase catalysis is potentially limited by the ion
exchange of the surfactants in the clay hosts with the counter cations of
the nucleophile. The dissociated surfactants may dissolve into the
organic phase, so the surfactant desorption may be dependent on the
polarity of the organic solvent, the surfactant hydrophobicity and the
affinity of the layered silicate for the surfactant.

36

Generally, polar organic solvents exhibit higher solubility for
surfactants. Consequently, the surfactant may dissociate from the clay
hosts to join the organic phase. To reduce the surfactant desorption,
non-polar organic solvents are suggested for use in triphase catalysis. We
expect organoclays suspended in non-polar organic solvents to exhibit
good recyclability. Also, the organoclay triphase catalysis reactivity
should be greater in non-polar solvents than polar solvents.

The desorption or ion exchange properties of an organoclay under
triphase reactions should depend on the nature of the onium ion itself.
For example, the carbon-rich surfactant, hexadecyltributyl phosphonium,
may desorb from F-hectorite but hexadecyltrimethyl ammonium may not
desorb from the clay host during the triphase catalysis reactions. The
three bulky butyl groups shield the positively charged phosphonium
leading to a surfactant with more organic than ionic character. Therefore,
the ion pair of the carbon-rich surfactant and nucleophile should have
good solubility in organic solvents, and this should favor desorption of
the surfactant into the organic phase. Since the charge radius ratio of the
positively charged nitrogen in hexadecyltrimethyl ammonium is large and
this cation is only shielded by three methyl and one long chain alkyl
groups, leaving the surfactant more ionic than organic in nature. The
solubility of the surfactant derivative should be poor in organic solvents.

The attraction between the layered silicate and the surfactant should
also depend on the layer charge density of the silicate host. The charge
density of the host will determine the orientation of the organic cation in
the gallery. Higher layer charge density clays should give paraffin
structures and this should result in more surfactant desorption from the

37

layer sheet. However, surfactant desorption is not expected on
organoclays with low layer charge densities causing these materials to be
good triphase catalysts in terms of recyclability.

Behaving differently from intercalated surfactants, cationic crown
ether complexes can also pillar the clay interlayer“. However, croWn
ether complex are poor candidates for triphase catalyst. The X-ray
diffractogram diffraction data suggest thst a crown ether clay complex
after a triphase catalysis reaction undergoes complex desorption by ion
exchange. However, the recycled clay derivatives show IR absorptions
of characteristic of the crown ether and reactivity for the catalytic
reaction. Thus crown ethers may remain partially bonded to the extemal
surface of the layered silicate instead of being intercalated in the gallery.

b) Thermal and chemical instability of the organoclay

Natural layered silicates are sometimes destroyed by strongly acidic
or alkaline solutions, a potential limitation for their use in catalytic
reactions which involve acidic or basic reagents. Surfactants based on
the silicate surface can change the chemical reactivity of the smectite clay
hosts. Structure changes due an acid or base reaction can easily be
observed from the X-Ray diffraction measurement. It is very surprising
that some of the organoclay derivatives with medium layer charge density
such as organo hectorite are not affected by alkaline solution. This
resistance to strong base is important and valuable since many
nucleophilic substitution reactions employ strong bases such as cyanide

or hydroxide as nucleophiles. In contrast, high layer charge density clay

38

derivatives do not seem to tolerate strong alkaline solutions since
surfactant desorption produces an unprotected layered silicate surface.
For triphase catalysis systems in strongly acidic environments all of the
clays and their organoclays are expected to decompose at elevated
temperature. However, typical phase transfer catalysis condition seldom
utilizes strong acid reactants.

The layer charge density of dioctahedral smectite clays (Figure 1)
with small metal cations in the interlayer causes them to be thermally
unstable at temperature above 250°C. An interlayer small cation such as
lithium or sodium may migrate into the empty octahedral site at elevated
temperature, resulting in lower layer charge density. Since there is no
octahedral empty site in trioctahedral smectite clays (Figure 2), the layer
charge density of the layered silicate is not influenced by the thermal
treatment. Under triphase catalysis conditions, the aqueous phase
typically contains metal cations which may affect the layer charge density
of the dioctahedral smectite clays33’34 and cause surfactant desorption
from the clay host. Thus, clay layer charge density reduction is a
potential mechanism for surfactant desorption and the influence of
thermal treatment on the the reduction of layer charge density is a
potential disadvantage for dioctahedral organoclays as phase transfer
catalysts. However, almost all phase transfer catalysis reactions proceed
at temperatures below 250°C so this effect should not be significant in
organoclay triphase catalysis.

39

Alkyl ammonium surfactants in clay interlayers may be thermally
unstable under alkaline solution reaction conditions Quaternary
hydroxides, for instance, are known to undergo Hofrnann degration35-37.
Thermal decomposition of the ammonium surfactants occurs at elevated
temperatures. [SH-elimination of the alkyl ammonium is the major cause
for decomposition to olefin and amine compounds. The alkyl ammonium
surfactants in the clay interlayer may be decomposed under reaction
condition. The Hoffman degration reaction could be reduced if the alkyl
ammoniums do not contain B-hydrogen. Fisk at Dow Chemical
Company employs N-alkylarylpyridinium salts as the surfactant adsorbed
on the metal surface for corrosion inhibition, since the surfactants can
tolerate higher temperature treatment due to the absence of a B-hydrogen.

c) Co-surfactant influence on surfactant desorption

Hexadecyltrimethyl ammonium bromide (CTAB) is a typical
cationic surfactant that forms an emulsion of two immiscible liquid
phases. Adding an alcohol co-surfactant causes microemulsion
formation. When this same surfactant is intercalated in layered silicate
hosts for a triphase catalysis reaction, it is not expected to desorb even in
the presence of concentrated electrolyte owing to the low solubility of the
surfactant-anion pair in organic and aqueous solutions. Dobias found that
adsorption of hexadecyl pyridinium chloride on minerals such as quartz
decreases dramatically when the surfactant concentration is higher than
the critical micelle concentration (CMC)33. It was suggested that
surfactant in the form of a three dimensional micelle is more stable than a
two-dimensional film adsorbed on the mineral surface. It is of interest to

characterize the Istability of an organo smectite clay when a

4o

thermodynamically stable microemulsion is formed in the presence of a
co-surfactant such as an alcohol with more than five carbon atoms.

Microemulsions are dispersions of oil and water made with
surfactant and co-surfactant molecules2535. Either type of dispersion,
oil-in-water or water-in-oil (Figure 12) is possible. The droplet size are
very small, typically 100A, about 100 times smaller than typical emulsion
droplet sizes27923. Also, the interfacial tension of the fluid is lower27-
2939, leading to larger contact surface between two immiscible liquids90.
Moreover, microemulsions are thennodynamically stable, compared to
emulsions25,25. Because the smaller surface tension results in smaller
particle dispersion and an entropy increase. The negative entropy may
compensate the surface energy to minimize the free energy. The co-
surfactant plays an important role in partitioning between two
neighboring polar groups of cationic surfactants to reduce the surface
charge density on the surfactant-aqueous boundary9oi91.

Microemulsions are very useful in industry. Microemulsions have
been used to improve oil recovery when oil prices reached levels where
tertiary recovery methods became profitable”. .

Nowadays, microemulsion application is focused on solar energy
conversion, liquid-liquid extraction, detergency and lubrication. Besides
these applications, microemulsions can enhance the catalytic reactivity
for micelle catalysis or biphase transfer catalysis. Introducing the co-
surfactant to the oil\surfactant\water reaction mixture improves the
catalytic reactivity of the biphase catalysis, because the smaller
dispersion droplet leads to greater interfacial area for the catalytic

reactions.

Figure 12 A droplet dispersion of oil-in-water microemulsion.

42

On the other hand, co-surfactant may facilitate desorption of a
cationic surfactant such as hexadecyltrimethyl ammonium from some
layered silicate surfaces by forming a more thermodynamically stable
microemulsion containing the co-surfactant, the two immiscible liquids
and the ammonium surfactant. The recyclability of the organo clays
would then be reduced if the reaction mixture contains the co-surfactant.
Alternatively the co-surfactant could improve dispersion of the clay

particles in the triphase reaction and thus enhance reactivity.

CHAPTER II

EXPERIMENTAL

A. Material.
1. Sodium Hectorite

Naturally occurring California sodium hectorite (Bl-26) with a
particle size of <2 um was obtained from Source Clay Mineral
Depository, University of Missouri, in the pre-centrifuged and spray dried
form. The mineral was purified by removing carbonates using pH5
acetate buffer solution and eliminating iron oxides by employing sodium
hydrosulfate93i94.

The idealized anhydrous unit-cell formula of hectorite is
Nao,67[Lio,57Mg5,33(Si3,oo)020(OH,F)4], and the experimentally
determined cation exchange capacity is about 73 meq/100g95 of the air
dried clay.

43

44

2. Sodium montrnorillonites

Two naturally occurring sodium montrnorillonites from Wyoming
and Arizona with a particle size of <2 um were also obtained from Source
Clay Mineral Depository, University of Missouri, in the pre-centrifuged
and spray dried form. The purification method for the montmorillonite is
the same as that stated above for hectorite.

The idealized anhydrous unit-cell formula of montmorillonite
(Wyoming) is Nao.7o[Mgo.7oAls.30(Sis.oo)02o(0H)4l. and the
experimentally determined cation exchange capacity is about 75
mqu 100g of air dried clay95. The unit-cell of montmorillonite (Arizona)
is Na1.16[Mg1.16Al2.s4(Si8.oo)02o(0H)4l, with a cation exchange
capacity of 118 meq/l 00g of air dried clay.

3. Laponite R

Synthetic laponite was obtained from Laporte Company, in England
and was used without further purification. The unit-cell of this material is
Lio,36[Lio,35Mg5,54(Si3,oo)Ozo(OH)4], with the cation exchange capacity
of 55 meq./100g95 of air dried clay.

4. Fluorohectorite (F-hectorite)

In this synthetic hectorite the octahedral lattice hydroxyl groups have
been replaced by fluoride ions. The unit-cell formula of F-hectorite is
L11,5o[Li1,soMg4,4o(Si3,oo)OzoF4]. The particle size of the material is
larger than 2pm and the cation exchange capacity is approximately
l40meq./100g95 of air dried clay.

45

5. Rectorite

This mineral was from Ba-Tou, China3. The particles larger than
2pm were removed by suspending the mineral in an aqueous solution for
8 hours. Rectorite consists of a regular alternation of mica-like layers and
expandable layers having the smectite composition. The chemical
composition formula of the material is
l(Nao.72Ko.02Cao.os)(Cao.24Nao.m)l(Al4.ooMgo.02)[Sio.58A11.121022.
The negative charge on the layer arises from isomorphous substitution on
the tetrahedral silica oxygen sheet. The cation exchange capacity is 60
meq./100g of air dried clay3.

6. Organic Reagents

All reagents were obtained commercially and used without further
purification. Hexadecyltributhyl phosphonium bromide;
hexadecyltrimethyl ammonium bromide; tetrabutyl ammonium bromide;
and didodecyldirnethyl ammonium bromide were obtained from Chemical
Dynamics Company.

Pentyl bromide, pentyl cyanide, decane, dodecane, benzyl bromide,
1,2-dibromo-l-phenylethane, trans-stilbene, n-octanol, n-decanol, n-
tetradecanol, 1,8-octandiol, 1,10—decandiol, 6-bromohexanol, 8-
bromooctanol, benzaldehyde, l-chloro-2,4-dinitrobenzene, l8-crown-6,
dicyclohexa-chrown-6, and 2-bromoethylbenzene were obtained from
Aldrich Chemical Corp. Toluene, methylene chloride, and
dibromomethane were purchased from Mallinckrodt. Chloroform,
methylene chloride, o-dichlorobenzene, butanol and naphthol were
obtained from J. T. Baker Chemical company. Methyl orange, benzyl
alcohol and isopentyl alcohol were purchased from Ficher-Scientific

Company.

7. Inorganic Reagents

Sodium hydroxide, sodium bromide, sodium bicarbonate,
hydrobromic acid and sodium acetate were purchased from EM Science
company. Potassium cyanide was obtained from Fisher Scientific
company. Hydrochloric acid, magnesium sulfate and sodium chloride
were available from Columbus Chemical Industries Inc. Acetic acid,
sulfuric acid and sodium hydrosulfate were purchased from Mallinckrodt.
Sodium citrate was obtained from I. T. Baker Chemical company.
8. Synthesis of organic precursors

BrClonoOH97 was prepared by refluxing the mixture of 30 mmole
of HOC10H200H and 20 mL of 48% HBr at 115°C in an oil bath for 5.5
hours followed by chromatographic separation using first hexane and then
1:1 hexanezether as eluent. The purity of the BrC10H200H was 45%.
The remaining 55% of the products consisted of HOClonoOH and
BrCronoBr-

Sodium naphthoxide was prepared by mixing 0.20 mole of NaOH in
20 mL aqueous solution and 0.21 mole of 2-naphthol in 100 mL
methanolic solution and allowing the solvent to evaporate under reduced
pressure at room temperature. The yield of pure sodium naphthoxide was
75.2%.

47

B. Preparation of Organic Clay Derivatives
l. Organoclays

An aliquot of 1 % aqueous suspension of the sodium clay was added
to an aliquot of aqueous solution of known amount of tetraalkyl
ammonium or phosphonium at the room temperature. The amount of the
onium salts was twice on the clay cation exchange capacity (CEC). For
example, for sodium hectorite with a CEC of 73 meq/ 100g, an aliquot of
aqueous solution containing 146 mole of onium salt was added to the
suspension solution with 100 g clay to ensure that all the interalyer
sodium cations were replaced by onium ions. After 24 hours of stirring,
the products were purified by repetitively washing with ethanol to remove
excess onium salt and then, resuspended in water until free of halide ion
as tested by AgNO3. The pure products were collected by centrifugation
and air dried at room temperature.
2. Crown Ether Clay Complex

To sodium clay suspensions in methanol was added 2 CBC
equivalent of l8-crown—6 or cis—dicyclohexal8-crown-6. These mixtures
were stirred for 24 hours at room temperature. The excess crown ethers

were removed by continuously washing with methanol.

C. Organoclay Triphase Catalysis
1. Determination of Cyanation kobs (Equation 5)

To a Coming culture tube was added 0.100 g of organoclay catalyst
containing 0.06 mole of onium ion, an aliquot of 3 mL 6.67 M
potassium cyanide aqueous solution, 6 mL toluene solution containing 2
mole of pentyl bromideand the internal standard, 0.15 g decane, the tube
was sealed and stined vigorously in a 90°C oil bath. Aliquots of 5 to 10

48

microliter of the organic solutions were withdrawn at 20 to 30 minutes
interval and diluted in 0.5 ml of ether for GLC analysis. The conversion
of pentyl bromide to pentyl cyanide was calculated from the ratio changes
of pentyl bromide to decane. Pseudo first order rate constants were
calculated with a least square program. Generally, the total reaction time
was 1.5 to 3.5 hours.
2. Determination of Cyanation k'obs (Equation 6) when the Amount
of Pentyl Bromide Is Much Larger than Potassium Cyanide

To a Corning culture tube, was added to a solution of 0.130g of
potassium cyanide in 3 mL of water, 0.100 g of organoclay, and 5 mole
of pentyl bromide in 6 mL toluene solution containing exactly 0.100 g
decane. The same procedure for determining different k'obs was applied
to pentyl bromide concentrations in toluene equal to 0.83M, 1.67 M, 2.50
M, and 3.33 M. Reactions carried out in a 90°C oil bath were monitored
by withdrawing 5 to 10 microliter samples from the organic phase at 20 to
30 minute intervals. It was assumed that the loss of cyanide ion was equal
to the formation of pentyl cyanide. A standard GLC curve was made by
area integration of a standard solution containing 0.2 to 2 mole versus
0.100 g decane. The plot of mole of pentyl cyanide against the ratio of
pentyl cyanide to decane is shown in Figure 13. The chemical yields of
pentyl cyanide were determined by calculating the integration ratio of
pentyl cyanide to decane from the GLC data and interpolating the ratio
value in Fig 13 to find the pentyl cyanide concentration in the reaction

mumm.

Intensity Ratio of [CSHuCNMOJOOg C1011”)

49

Figure 13 The standard curve of the intensity area ratio of
[CSHnCN]/[0.100 g C10H22] in GLC analysis versus
[CSHuCN] in 6 mL toluene as the organic solvent.

1.6

 

 

 

 

 

I ' I ' I ' U I
0.0 0.4 0.0 1.2 1.6 2.0

[CsHuCN], mmole

50

3. Solvent Effect on Iodination

To a Coming culture tube, 0.100 g of hexadecyltributyl phosphonium
hectorite (H26AA) (equivalent of 0.06 mole of hexadecyltrimethyl
phosphonium) was added, then 8 mole of potassium iodide in 3 mL
water followed by 6 mL of desired organic solvent containing 2 mole of
pentyl bromide. The deserved organic solvents could be decane, toluene,
or o-dichlorobenzene. An internal standard, 0.15 g decane or dodecane,
was added to the mixture, and the tube was sealed and stirred vigorously
at 900C in an oil bath. Aliquots of 5 to 10 microliter of the organic
solution were withdrawn at intervals of 20 to 30 minutes and diluted in 0.5
mL of ether for quantitative GLC analysis. The conversion of pentyl
bromide to pentyl cyanide was calculated from the ratio of pentyl bromide
to internal standard. Pseudo first order (rate constants were calculated with
a least square program.

4. O/C Alkylation of Benzyl Bromide and Sodium Naphthoxide.

A deserved amount of catalyst was added to a mixture of 0.5g (3
mole) of sodium naphthoxide in 5 mL water and 0.34 g (2 mole) of
benzyl bromide in 5 mL of organic solution. The mixture was stirred at
room temperature for 4 hours, then the organic phase was separated and
the organic solvent was evaporated under reduced pressure. The ratio of
O-alkylation and C-alkylation product (Figure 7) was determined by
integration of the methylene proton absorption resonances in the 1H NMR
spectra. The methylene proton resonances in the 1H NMR spectrum for
C-alkylation and O-alkylation products occur at 4.48 ppm and 5.12 ppm,
respectively.

5. Dependence of kobs on the Volume Ratio.

51

Aliquots of 6.25 M potassium cyanide aqueous solution, 0.33M
pentyl bromide in toluene, and 0.100 g catalyst were stirred in a 90°C oil
bath. The conversion of pentyl bromide to pentyl cyanide was calculated
from the chromatographic ratio of pentyl bromide to the internal standard
decane in GLC analysis.

6. Catalytic Cyanation in the Absence of Water

A 6 mL quantity of toluene solution containing 2 mole of pentyl
bromide, 20 mole potassium cyanide and 0.100 catalyst H26AA were
stined at 900C in an oil bath. The conversion of pentyl bromide to pentyl
cyanide was calculated from the chromatographic ratio of pentyl bromide
to decane in the GLC analysis. The total reaction time was generally 1.5

to 3.5 hours.

D. Longevities of Organoclay Derivatives
l. Recyclabilities of Organoclays for Cyanation

Aliquots containing 20 mole of potassium cyanide in 3 mL water
and 2 mmole of pentyl bromide in 6 mL organic solution were added to
0.100 g of the organoclay catalyst. The organic solvent was either toluene
or decane. The mixture was sealed in a Corning tube and stirred at 90°C
in an oil bath. After the catalytic reaction was finished and the observed
rate constant (kobs) was determined, the organoclay catalyst was filtered
and then washed with 10 ml of water and ethanol. The catalytic reaction
was then repeated using the recycled catalyst.

52

2. Recyclabilities of Organoclays for Chlorination

Aliquots of 4 mL 2.5N NaCl aqueous solution and 4 mL of a toluene
solution containing 2 mole of pentyl bromide and an intemal standard of
0.15 g decane were added to 0.100 g of the organoclay catalyst. The
mixture was stirred at 90°C for 8 hours. The conversion was determined
from the change in the integral GLC intensity for pentyl bromide and the
intemal standard. The catalyst was filtered and washed with 10 mL fresh
water and ethanol. The same catalytic reaction was repeated using the
recycled catalyst.
3 Surfactant Desorption of Organoclays

The desorption of surfactants from clay hosts was determined by
measuring the concentration of cationic surfactants in bulk liquid phase
after ion-exchange reactions. The desorbed cationic surfactant
concentration in liquid solution was determined by the method developed
by Wang and Langley93. Mixtures containing 0.03g organoclay, 4 mL
aqueous solution with desired electrolyte concentrations and 4 mL of
organic solvents were stirred at 90°C or room temperature for the desired
time. After the ion-exchange reactions were finished, 1 mL of both
organic and aqueous solutions were transfered to a 50 mL buffer solution
containing citric acid and 1.0 mg methyl orange. If the desorption of
onium ion was very high, the quantity of methyl orange was doubled or
increased further to ensure an excess of indictor. The aqueous solutions
were extracted using 25 mL of chloroform. Standard curves of UV-Vis
absorptions in chloroform solution with a 10 mm length cell against
known surfactant concentrations (Figures 14 and 15) were determined.
The absorption for the surfactant-dye ion pairs in chloroform were

measured at 418 run. If absorbancies were over the limit of the

53

Figure 14 Dependence of the UV-Vis absorption of C14133NMe3Brt
methyl orange chlorofonn solution at 418 nm on
concentration of C161133NMe3Br aqueous solution.

 

0.7

0.6 e

0.5 e

0.4 -

0.3 -

Absorption

0.2 4

0.1 -

 

 

 

 

0.0 . . . . .
0.0 1.0 2.0 3.0 4.0

[CroHaaNM‘BsBrL 118/ml

Various concentrations of C16H33NMe3Br in aqueous solution were
added to excess amount of methyl orange. The ion pairs of the cationic
surfactant and anionic dye were extracted into 25 mL of chloroform.
The absorptions of the chloroform solutions in a 10 mm length cell
were determined bya UV-Visible spectrophotometer at the

wavelength 418 nm.

54

Dependence of the UV-Vis absorption of CIJI33PBU3BT+-
methyl orange chloroform solution at 418 nm on

concentration of C16H33PBu3Br aqueous solution.

Figure 15

 

Absorption

 

 

0.0 I l I I ' I '
0.0 1.0 2.0 3.0 4 0

[C16H33PB‘1331‘L 118/“11

Various concentrations of C161133PBu3Br in aqueous solution were
added to excess amount of methyl orange. The ion pairs of the cationic
surfactant and anionic dye were extracted into 25 mL of chloroform.
The absorptions of the chloroform solutions in a 10 mm length cell
were determined by a UV-Visible spectrophotometer at the

wavelength 418 run.

 

55

instrumental measurement, the solution was diluted by adding more
chloroform until the absorption was within the standard curve. According
to Beer's Law, the concentration of the surfactant-dye adducts were
proportional to absorption. Thus, desorption of surfactants from clay
hosts to liquid phases were derived by measuring the UV-Vis absorption
and finding the relative surfactant concentration from the standard curve.
4. Effect of Co-surfactant on Phase Transfer Catalysis Reactions.

To a Coming culture tube was added 2 mole potassium cyanide in
3 mL of water, 0.100g of clay supported catalyst (equivalent to 0.060
mole of onium salt), and then 1.511 g (20 mole) of pentyl bromide and
the desired amount of co-surfactants in 6 mL of toluene. Subsequently
0.100 g of decane, the internal standard, was added to the mixture, the
tube sealed and vigorously stined at 90° for 4 hours. The conversions of
pentyl bromide were determined from the GLC integrated intensity of
pentyl cyanide relative to the decane integral intensity. A standard GLC
curve for the integration ratio of pentyl cyanide to decane is determined
(Figure 13) to quantify the amount of pentyl cyanide in the reaction
mixtures. '
5. Conversion of n-Bromoalkan-l-ol to n-Cyanoalkan-l-ol by Using
Phase Transfer Catalysts

To a Corning culture tube containing 0.100 g of the organoclay
catalyst (equivalent to 0.06 mole of C15H33NMe3Br) was added on
aqueous solution containing 10 mole potassium cyanide in 3 mL of
water, and then 2 mole of n-bromoalkan-l-ol in 6 mL of toluene. After
the internal standard, decane, was added to the mixture, the tube was
sealed and stirred vigorously at 90°C in an oil bath for 4 hours. The

56

conversion of n-bromoalkan-l -ol was determined from the integrated
intensities of n-bromoalkan- 1 -01 to decane peaks by GLC.
6. Conversion of n-Bromoalkane to n-Cyanoalkane Using Phase

Transfer Catalysts

To a Coming culture tube was added an aliquot solution containing
10 mole potassium cyanide in 3 mL of water, 0.100 g of the organoclay
catalyst (equivalent to 0.06 mmole of C15H33NMe3Br) and 2 mole of n-
bromoalkane in 6 mL of toluene. After 0.15 g of decane, the internal
standard, was added to the mixture, the tube was sealed and stirred
vigorously at 900C in an oil bath for 4 hours. The conversion of n-
bromoalkan-l-ol was determined from the ratio change of the integrated
intensities of n—bromoalkan-l-ol to decane peaks by GLC analysis.
7. Recyclability of Crown Ether Clays for Cyanation

A toluene solution containing 2 mmole of benzyl bromide and 0.15 g
ofdecane as the internal standard was added to 0.100 g ofthe crown ether
clay and 5 mmole of potassium cyanide in 1 mL of water. THis mixture
was placed in a Coming tube which was then sealed and stirred at 90°C in
an oil bath for 12 hours. After the catalytic reaction was stopped, the
conversion was determined by GLC. The solid material was filtered and
washed using 10 mL of ethanol. The same catalytic reaction was repeated
by using the recycbd material.

57

E. Application of Organoclay Triphase Catalysis to Organic

Synthesis
1. Synthesis of Symmetrical Formaldehyde Acetals (Equation 10-12)

Mixtures of 10 mL methylene chloride solution containing 10 mole
of the desired alcohol, 10 g of dibromomethane and 10 mL of 50% NaOH
aqueous solution were added to 0.2 g of the desired organoclay catalyst.
The mixtures were refluxed with stirring at 90°C for 12 hours. After
reaction, the liquid phases were easily filtered. The organic layer was
dried over MgSO4 and the organic residue was obtained by evaporating
the volatile organic solvent under reduced pressure. The structure and the
yield of the product were determined by 1H NMR spectra.

2. Synthesis of Alkyl Bromide by Dehydration of Alkyl Alcohol in

Strongly Acidic Conditions (Equation 13)

To a Coming tube was added a mixture of 2.5 mole of octanol, 0.1
g of decane, 1 mL 47% HBr and 0.1 g of organoclay. The mixture was
stirred at 90°C in an oil bath for 11 hours. The yield was determined by
GLC.

For the reaction with diluted reactants, 1 mL of water and 1 mL of
toluene were added to the mixture indicated above. This reaction was also
carried out at 90°C in an oil bath for 11 hours.

3. Dehydrohalogenation of 2-Bromoethylbenzene (Equation 14)

A mixture of benzene containing 0.2 g of decane as the internal
standard, 4 mole of 2-bromoethylbenzene, 2 mL 50% NaOH aqueous,
and 0.1 g organoclay were introduced to a Corning tube which was sealed
and heated at an elevated temperature. Yields were determined by GLC.

58

4. Oxidation of trans-Stilbene (Equation 15)

A mixture of 3 mole of trans-stilbene in 10 mL benzene, 6 mole
of KMn04 in 6 mL water and 0.200g organoclay catalyst, were refluxed
with stirring at the room temperature for 8 hours. After filtering the
reaction mixture, the acidic product in the organic layer was extraction
with 20% NaOH aqueous solution. The aqueous layer was combined with
the alkaline washing solution. The basic aqueous solution was acidified
by hydrochloric acid, followed by extracted with 40 mL of methylene
chloride. The pure benzoic acid was obtained by evaporating the organic
solvent under reduced pressure and the yields were determined by
weighing the product.

5. Synthesis of 2,4-Dinitrophenyl Ether (Equation 16)

In a round bottomed flask a mixture of 5 mmole of 1-chloro-2,4-
dinitrobenzene and 6 mole of phenol in 8 mL of benzene, 4 mL 1.5 N
NaOH aqueous solution and 0.100 g of the organoclay catalyst were
stirred at room temperature for 6 hours. After the organic layer was
washed with deionized water, the product residue was obtained by
evaporating the organic solvent under reduced presure. The purity of the
organic residue was determined by 1H NMR spectra 1
F. Physical Measurements
1. Infrared Spectroscopy

Infrared spectra were recorded using an IBM Single Beam FI‘ IR44
model spectrophotometer. Liquid spectra were obtained by using 0.1 mm
NaCl cells and solid spectra were recorded by mixing the samples with

KBr and pressing them into disks.

59

2. Gas Liquid Chromatography

All product mixtures together with solvent and internal standard were
analde by gas liquid chromatography either on a model 5880 Hewlett-
Packard instrument filled with a flame ionization detector and a 25 m x
0.25 mm cross-linked dirnethylsilicone capillary column or on a model
5890 Hewlett-Packard chromatograph with a flame ionization detector
and a capillary 60 m x 0.25 mm column. Products were identified by
comparison of GLC retention times with those of authentic samples. The
percentage yield of products was determined by integration of the
“absorption peaks of the starting reagents and the internal standard.
3. NMR Spectroscopy

Proton and carbon-13 nuclear magnetic resonance spectra were
recorded on a Bruker WM-250 or Gemini-300 MHz spectrometer.
Chemical shifts were usually measured relative to tetramethylsilane as
intemal standard and reported in units of ppm.
4. X-Ray Diffraction

X-ray d001 basal spacing were determined for oriented film samples
with either a Philips X-ray or a Rigaku X-ray diffractometer using Cu-Ka
radiation with wavelength equals to 1.5405 A. The fihn specimens were
prepared by allowing an aqueous suspension of the samples to evaporate
on a microscope glass and monitoring the diffraction over a 2-theta range
from 1° to 40°. Peak positions in the angle 2 theta were converted to d-
spacing with a standard chart.

5. UV-Vis spectroscopy

UV-Vis spectra were recorded by a 9430 UV-Visible IBM
Spectrophotometer. Sample concentrations were determined by
. absorption measurements. Sample and reference solutions were contained
in 10 mm quartz cells.
6. BET Surface Area Measurement

The BET surface area were recorded by a QUANTASORB Jr.
Sorption Analyzer. The specific surface area of the samples was
determined by measuring sorption of nitrogen on the sample at liquid

nitrogen temperature.

CHAPTER III

RESULTS AND DISCUSSION

A. The Preparation and Structures of Clay Derivatives
1. The Preparation of Organoclays

The reaction of sodium smectite in liquid suspension with
stoichiometric amounts of onium salts dissolved in water, ethanol or
acetone results in products in which the sodium ion is displaced by
onium cations. A series of organoclays shown in Table l were prepared
using this method. The first letter designates the clay source or type,
e.g. "A" is Arizona montmorillonite and "F" represents the clay host,
fluorohectorite. The number designates the onium ion in the clay
interlayer and the reaction solvents are represented by the last two
letters. For example, "AA" of A26AA means that the organoclay is
synthesized in aqueous solution. Likewise, H24AB is synthesized in
ethanol and H24AC is generated in acetone, and the number "26"

designates the onium ion, C16H33PBu3+.

61

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64

If the reaction condition involved water or ethanol as solvent,
displacement of the sodium cation by onium cations was strongly
favored. Nearly complete cation exchange was achieved by using
stoichiometric amounts of reagents. However, the ion exchange
reaction could not be completed in acetone because of the low acetone
dielectric constant which limits swelling of the silicate layer. The
organoclay (H24AC) is not completely ion-exchanged, so its d-spacing
of 140A is smaller than those of products H24AA and H24AB with
values of 17.8A.

'2. The Structure of Organoclays.

The X-ray d-spacings of organoclays depend on the smectite clay
layer charge densities and on the structure of the surfactant onium ions.
Figure 16 lists the representative XRD patterns of organoclays: (a).
L24AA; (b) H24AA; (c) W24AA; (d) A24AA; (e) F24AA. High d-
spacing organoclays are formed for smectites with high layer charge
densities. For example, the d-spacing of C15H33NMe3+laponite
(L24AA) with the CBC of 55meq/100g is 14.5A, but the d-spacing of
C15H33NMe3+F-hectorite (F24AA) with the CEC 140meq/100g is
27.2A. The BET-Surface areas are dramatically reduced when sodium
is replaced by surfactants in the interlayer due to the absence of space
accessible for nitrogen adsorption in the interlayers (Table 2). The
surfactant hydrocarbons are packed in the clay gallery and more of the
cationic surfactants are loaded in higher layer charge density clays to

balance the negative charge of the silicate layers.

65

Table 2 The BET surface area of sodium clays and organoclays
in which the surfactant, [C16H33NMe3+], is intercalated

 

 

 

in the clay interlayers.
Clay Host BET Specific Area
(mzlg)
Sodium Clay C16H33NMe3+ clay
Laponite 413 160
Hectorite 108 10.1
Arizona
Montrnorillonite 104 4.2
F-hectorite 4.2 3.5

 

66

Figure 16 The X-Ray diffraction pattern of various organoclays.
(a) L24AA; (b) H24AA; (c) W24AA; (d) A24AA; (e) F24AA.

 

650 [
520 ‘r
390 i (a) L24AA ((C16H33NMe3I-Laponite)
14.5 A
260 "
1h I! ‘ I
f. l |; ‘
ii ,,
130
4
0 ‘ *‘0 - - . . -J .2. AILEM'DUI‘LML flu - ml.
5 10 15 20
2 THETA

(Continued)

67

Figure 16 (continued)

17.8A
2630 it

.1 ‘
2104 p (b) H24AA ((C16H33NMe3I-Hectorite)

 

1578

 

1052

 

 

526 ‘*

.% V ‘ ¢ - -' 1 .

 

 

0 ‘WWTWWWW
5 10 15 2° 25
2 THETA

(Continued)

Figure 16 (continued)

3500

2780

2060

1340 "

620

'99

17.8A

(c) W24AA (C15H33NMe3I-montmorillonite (Wyoming))

 

 

 

 

 

sssss Jtrrrrirrrfi'llIIfill "1!T§!1ITiIIPItIIIfI'm
5 10 15 20 25
2 THETA

(Continued)

omm\m-tzo

2773

2218

1664

1109

555

Figure 16 (continued)

(d) A24AA(Ct5H33NMe3*-montmorillonite(Arizona))

 

 

 

 

 

 

r L _r' J_ L r j r 1
rrrrrlrrrrlrrr—TfrfirrrrTr'rrrrrIrrrrrrrritttiTlltfifi

5 10 15 20 25
2 THETA

(Continued)

70

Figure 16 (continued)

3816

3053

2290

1526

783

27.2.4

.. . (e) F24AA (c,6H33NMe3t-F-hectorite)

 

 

 

13.5 A

 

 

AL. #A— .L_ A-‘
VVVVV I'VV'rUT'r'VU'U'U'VU'IU'U'U' 'U'UrUi'U'll'

5 10 15 20 25
2 THETA

(Continued)

71

The synthetic F-hectorite derivatives show better C-axis stacking
order than the analogous natural smectite and synthetic laponite
derivatives. Laponite derivatives show the poorest crystal size as
determined by XRD (Figure 16a). The first order reflection of L24AA
is much broader than the those of the other organoclays (Figure 16).
From Equation 7, the scattering domain L can be calculatedlz

L = AK/Bcose (7)

L is the mean of crystallite dimension in A along a line normal to the
reflection plane, A is the wavelength of X-ray, K is a constant near unity
and B is the width of a reflection at half-height expressed in radians.
Values of L are listed in Table 2. Crystallinity is dependent on the
particle size of the silicate layers. The surfactant in the clay interlayers
can not alter the crystallinity due to the constant particle size of silicate
layers. Large particle smectite clays afford small hydrophilic edge
surface area and this edge surface area can not be coated by cationic
surfactants. Therefore, small particle size smectite clays such as
laponite are difficult to convert to an amphiphilic material by
intercalation of cationic surfactants.

The orientations of linear cationic surfactants in the interlayer of
clays with various layer charge densities have been studied by Lagaly
and Weiss71r72. Laponite intercalated by surfactants with only one long
chain alkyl group (L24AA and L26AA) orient in the gallery with a
monolayer arrangement parallel to the surface of the silicate layer
(Figure 7a) The external surface of the clay particle should be half-
covered by surfactants. The surfactant orientation on the layered
silicate surface is significant because it can influence the hydrophobicity

of the material. For hectorite and Wyoming montmorillonite

72

derivatives, (H24AA, H26AA W24AA and W26AA), two surfactant
hydrocarbon chains lie parallel to the silicate surfaces of the clay
interlayers adopting a lateral bilayer structure (Figure 7b). The
external silicate layer of these materials are covered with a monolayer
of surfactant so that the surface wettability is totally changed. As the
clay layer charge densities increase, pseudo-trimolecular and paraffin-
like surfactant structures are obtained. Arizona montmorillonite and
rectorite derivatives containing C15 chains (A24AA, A26AA, R24AA
and R26AA) are suggested to adopt pseudo trimolecular structures
(Figure 7c), and the surfactants in organo-F-hectorite (F24AA and
F26AA) interlayers are oriented as in paraff'm-like structures (Figure
7d). However, the orientation of the surfactants might differ from the
above structures if the surfactants contain two long hydrocarbon chains.
For instance, the organo laponite (L30AA) with didodecyldimethyl
ammonium in the interlayer, the surfactant exhibits a bilayer structure
rather than a monolayer structure (Figure 7e). Likewise, a lipid-like
surfactant orientation is found in the gallery of F-hectorite (F30AA)
and a paraffm-like structure most likely forms on its external surface
(Figure. 7f).
3. The Preparation and Structure of Crown Ether Clay Complexes
Two crown ethers, l8-crown-6 and dicyclohexano-l8crown-6,
have been employed as pillars in hectorite and Arizona montmorillonite.
The complexes were synthesized by mixing the sodium clay and two
times the CEC of crown ether in methanol suspension. The d-spacing

are dependent on the smectite clay layer charge density and the structure

73

Table 3 The d-spacings of crown ether-clay complexes derived

from X-Ray diffraction measurement

 

 

Clay-Crown Clay Host Crown Ether , d-spacing

18C6Hect Na-Hectorite l8-Crown—6 15.6A

18C6Mont Na-Mont. l8-Crown-6 17.4A
(Arizona)

Hexl8C6Hect Na-Hectorite Dicyclohexanol8-Crown-6 17 .8A

Hexl8C6Mont Na-Mont. Dicyclohexanol8-Crown-6 19.0A

(Arizona)

 

74

Figure 17 The orientations of crown ethers in hectorite and
Arizona montmorillonite interlayers.
These figures are adapted from E. Ruiz-Hitzky and
H. Casal, NATO ASI Series: Chemical Reaction in
Organic and Inorganic Constrained Systems, 213 (1986)

(a) 18-Crown-6 Hectorite

 

 

 

 

 

 

 

(b) CyclohexalS-Crown-6 Hectorite and
18-Crown-6 Montrnorillonite (Arizona)

 

 

 

 

619619659 w)

 

 

 

 

(c) Cyclohexal8-Crown-6 Montrnorillonite (Arizona)

 

 

 

 

 

 

 

 

75

of the crown ether (Table 3). The orientations of crown ethers in
complex clay interlayers have been investigated by Ruiz-Hitzky and
Casa132. The clay supported crown ether complex, sodium 18C6Hect,
has an interlayer distance (gallery height) of close to 4.5 A, which
corresponds to the thickness of the crown ether assuming a planar
arrangement of ligand parallel to the silicate surface (Figure 17a). In
sodium exchange forms of Hexl8C6Hect and 18C6Mont, with d-
spacings of 15.6A and 17.4A, respectively, a distortion of the crown
ethers occurs in order to fill the space of the clay gallery (Figure 17b).
The crown ether, dicyclohexano-18-crown-6, in Arizona
montmorillonite exhibits a gallery height of 19.0A. The complex
probably adopts a tilted orientation to fit the interlayer distance (17c).

B. The Catalytic Properties of Organoclays
l. Wettabitity and Catalytic Capacities of Organoclays

The cyanation of pentyl bromide (Equation 1) is employed as the
probe reaction in which the

C5H11Br + CN' -------- > C5H11CN + Br‘ ' (1)

nucleophile, CN-, was held in ten fold equivalent excess over the
organic substrate, CsHllBr, so that the observed rate constant (kobs)
could be measured and calculated by Equation 5.

-d[CsH1 lBr] /dt = koblesH 1 131'] (5)
Organo clays that facilitate formation of water and organic emulsions
were usually good phase transfer catalysts due to the efficient mixing of
the two immiscible liquid phases.

The organo laponites (L24AA and L26AA) containing
C115H33NMe3+ and C15H33PBu3+, respectively, always show poor

76

Table 4 The catalytic activitiesa and the colloidal properties of
the organoclays and the onium salts as phase transfer
catalysts.

 

Catalyst Wetting of the clay 105xkobs, see1

under reaction conditions

 

L24AA water 0.31
H24AA emulsion 2.93
H24AB emulsion 3.07
H24AC water 0.25
W24AA emulsion 3.64
A24AA emulsion 4.07
F24AA emulsion 4.09
R24AA Paste in 1.40
toluene
L26AA water 1.10
H26AA emulsion 12.9
W26AA emulsion 27.2
A26AA ion desorption 54.6
F26AA ion desorption 56.5
R26AA Paste in 8.20
toluene

 

Continued

77

Table 4 (Continued)

 

Catalyst Wetting of the clay 105xkobs, see-1
under reaction conditions

 

L30AA partial emulsion 1.96
H30AA emulsion 2.92
W30AA emulsion 4.45
A30AA emulsion 8.39
H18AA water 2.01
C1¢5H33NMe3Brb emulsion 2.43
C15H33PBu3Brb toluene 62.2
NMeaBrb toluene 15.6

 

3. Reaction and wetting conditions: 2.0 mmole pentyl bromide in 6
mL toluene; 20.0 mmole potassium cyanide in 3 mL water; 0.100g
organoclay; 90°C.

b Biphase catalyst: The amount of C15H33NMe3Br, C15H33PBu3Br
and NMe4Br are 0.60, 0.55 and 0.61 mole respectively.

78

catalytic activities with kobs' of 0.31 x10“5 and 1.10x10'5 sec-1,
respectively, in triphase catalysis (Table 4), owing to inability to form
the emulsion. The hydrophobicity of the material can not be effectively
enhanced because of the large edge surface area versus the lamellar
surface area of the clay. In Table 2, laponite derivatives are seen to
exhibit larger surface areas than the other organoclays, resulting from
the small particle size of the larnella laponite particles. The surface area
receives significant contribution from the particle edge, which can not
be covered by the alkyl chains of the surfactants. The incomplete
coverage of the basal laponite surface by surfactant (Figure 7a) also
results in poor amphiphilic properties. When the interlayer surfactant
in hexadecyltrimethyl laponite was replaced by didodecyldimethyl
ammonium( L30AA), kobs increased from 0.3lx10-5 to 1.96x10-5 sec-1,
but the improvement was not satisfactory. For the lattice onium ions,
the surfactant chains adopt a lateral bilayer structure in the clay
interlayer and a monolayer structure on the external clay surface
(Figure 7e), so that the extemal surface is totally covered by surfactant
alkyl chains.

As the clay layer charge density increases and the surface area of
the organo clay decreases, the triphase catalytic activity of an
organoclay increases (Table 4). Hectorite derivatives, H24AA and
H26AA, with kobs values of 2.93x10'5 and 12.9x10-5 seC'1 respectively,
facilitate the catalytic reactions more readily than laponite derivatives,
L24AA and L26AA, with kobs values of 0.3lx10-5 and 1.10x10-5 sec-l
respectively. For H24AA in which hexadecyltrimethyl ammonium is in
the interlayer cation the triphase catalysis rate constant (2.93x10‘5sec-1)
for cyanation is slightly larger than that kobs for the micellar biphase

79

catalysis when using the same equivalent surfactant, hexadecyltrimethyl
ammonium bromide with kobs of 2.43x10‘5 3e0'1. The organo hectorite
H26AA with hexadecyltributyl phosphonium as the interlayer onium ion
(kobs = 12.9x10'5 seC'l) does not perform any better as a triphase
catalyst than the corresponding biphase catalyst (kobs = 62.3x10‘5 sec-1).
However, the recyclability and the greater convenience of workup after
catalytic reaction more than compensates for the disadvantage of a
lower reaction rate. The hectorite derivative with tetrabutyl ammonium
in the interlayer (H18AA) is not a good triphase catalyst (kobs =
2.01x10-5 seC'l) owing to the instability of this clay to a toluene/water
emulsion although tetrabutyl ammonium bromide is a powerful biphase
catalyst (kobs = 1.56x10'4se0'1). This lack of amphiphilic character
results from the short alkyl chain in the onium ion and the incomplete
coverage of the external hectorite basal surface leading to poor
enhancement on the material hydrophobicity. Therefore, the contact
area of the two immiscible liquids partitioned by H18AA is much
smaller than the emulsion stabilized by the high d-spacing organo
hectorite, H24AA and H26AA.

The solvent used for intercalation of the cationic surfactant into the
hectorite interlayers also plays an important role in determining the
extent of onium ion exchange and, hence, the colloidal and catalytic
properties for the organo hectorite. Water and ethanol have high
dielectric constants and produce good swellability for the clay silicate
layers. This facilitates the interlayer exchange of the metal cations by
the cationic surfactants. The d-spacings of organo hectorites, H24AA
and H24AB, prepared in water and ethanol, respectively, exhibit no
difference in d-spacing (17.8A) (Table 1). Also, the catalytic capacities

80

for the two materials are comparable. However, the C15H33NMe3+
surfactant can not be completely ion-exchanged into the hectorite
interlayer using acetone (14.0A) as the solvent for the intercalation
reaction and the d-spacing for the material synthesized in acetone is
lower than that of H24AA and H24AB. Therefore, the catalytic
capacity of H24AC with kobs 0.25le5 see1 is very poor. The silicate
layer does not swell very well in low dielectric organic solvents.

The triphase catalytic reactivities of the organo montrnorillonites
and F-hectorite, A24AA and F24AA interlayered by hexadecyltrimethyl
ammonium are also high (kobs = 4.07x10'5 and 565le5 sec-1
respectively). These clays have high layer charge densities. However,
F26AA and A26AA do not stabilize emulsion formation even though
they show unusually high kobs values. In this case, however, the
surfactant is desorbed from the clay hosts. In fact, the observed
reaction occurs by biphase catalysis. That is, A26AA and F26AA
catalysis occurs by the desorbed and soluble surfactant instead of by
solid organoclays. .

Although rectorite derivatives, R24AA and R26AA, are also
amphiphilic materials, they only form a paste-like gel with the water
phase of the triphase catalyst system instead of forming a milk-like
emulsion. A paste-like gel retards dynamical stirring during reaction.
Thus the catalytic activities for the two rectorite derivatives with kobs
1.40x10-5 (R24AA) and 8.20le5 sec-1 (R26AA) respectively, are not
as good as those of the other high layer charge density layered silicate
derivatives. The thickness of the rectorite layer (19011) is twice that of
smectite layer (9.6A) because rectorite layer stacking consists of a mica
and an expandable smectite sheet3. Rectorite particles with a thickness

81

double that of the corresponding smectite particle may have a lower
suspension extent in liquid solution than the smectite particle.
Therefore, the rectorite particle aggregated and a paste formed.

The colloidal properties of organoclays for triphase catalysis
reactions as listed in Table 1 can be classified into three categories, as
shown in Figure 18. The first class of organoclays, designated Type A
clays, is amphiphilic and stabilizes emulsions of 3 mL 2.5 N NaCl
aqueous and 6 mL toluene solutions (Figure 18a). The density of
unsolvated organoclays should be greater than the density of water and
toluene. However, when toluene is adsorbed by the amphiphilic
organoclay, the density of the organoclays become intermediate between
the density of water and toluene. After the amphiphilic
clay/water/toluene mixtures are centrifuged, the solid organoclays are
located between the aqueous and organic phases (Figure 18a). The
second type of the organoclays, designated by Type B in Figure 18b,
does not support emulsions of aqueous and organic solutions. Organo
F-hectorite undergoes surfactant desorption and organo laponites
become suspended in the water liquid phase. Toluene is not adsorbed on
the hydrophilic organo laponite or ion exchanged sodium F-hectorite
formed by the surfactant desorption. Upon centrifugation, the Type B
clay materials collect at the bottom of the two liquid phases shown in
Figure 18b. Type C organoclay is unique in rectorite derivatives. A
paste is formed in the organic phase which retards dynamic stirring
(Figure 18c). Since Type C materials also absorb toluene, these
organoclays will be located at the boundary of the aqueous and organic
solutions after the mixtures of organo rectorite, toluene and aqueous

phases are centrifuged.

82

Figure 18 Classification of the colloidal behavior of 0.100 g of organoclay
in liquid mixtures containing 3 mL of 6.25M potassium cyanide
aqueous solution, 6 mL of toluene and 2 mole of pentylbromide.

1101A 12129.9
H24AA:I-126AA;H30AA; gal-2mm , R24AA; R26AA
W24AA; W26AA; W30AA;

A24AA; A30AA; F24AA

a—Organic Layer j—(hganic Layer ~:L-(Jrganic Layer

 

 

 

VJ—Aqueous Layer * __Aqueous Layer Nil—Aqueous Layer
Organoclay EId—‘Organoclay V—- Organoclay
Before Stirring Before Stining Before Stirring

U U U

 

 

 

 

 

—Milky Emulsion —Immiscible Liquids ‘_ P33“
v _Aqueous Layer
Dims“ sumn‘ ' a Drum stirring mm 3 stirring
I Organic Layer Organic Layer
—Milky Emulsion __PaS‘c
U SClay-Water Aqueous Layer
uspensron
After standing for 1 hour After standing for 1 hour After standing for 1 hour
U U U
_1— Organic Layer 4— Organic Layer — Organic Layer
07833001” Aqueous La er Organoclay
Aqueous Layer :J_.. Organoclayy Aqueous Layer

 

After Centrifugation After Centrifugation After Centrifugation

83

2 . Dependence of Catalytic Reactivity on the Bulk Reactant

Concentration of Liquid Phases

Under triphase reaction condition, the nucleophile and organic
substrate are segregated into separate phases. Thus, one or both of the
reactants must be adsorbed on the organoclay to interact with each other
either at the solid liquid interface or within the solid phase. As
described in the Introductory Chapter three different triphase process
are possible and are characterized by the three following equations.

Rate = k[organo clay- RX] [CN'](aq) (2) ,
Rate = k[organo clay- CN'][RX](org). or (3)
Rate = k[organoclay- CN' RX] (4)

Equation 2 describes the catalytic reaction occurring at the clay-
aqueous interface. If the equation holds for the pseudo first order
condition, i.e., [CN'] >> [RX], a pseudo first order rate constant, is
designated kobs. is equal to k[CN']. The observed constant, kobs. was
calculated from Equation 5 in which the [RX] value was traced by GLC

at different reaction time during the biphase reaction.

leXl/dt = kobisXl (5)
then. lanXI/[RXIO = 'kobst
Figure 19a is an example of determining the -kobs value by finding the
slope of ln[RX]/[RX]0 against the reaction time, t. The observed rate
constant, Robs. is proportional to the bulk cyanide concentration if the

triphase reaction occurs at catalyst aqueous interface.

84

For this assumption, four aliquots of cyanide solution with 5, 10,
15 and 20 mole in 3 mL aqueous solution versus 2 mole of pentyl
bromide in 6 mL of toluene were prepared. According to Lin's
investigation”, kobs is linear to [CN‘] (Figure 20a), indicating that
equation 2 is an appropriate description of the reaction rate under
pseudo first order rate condition and the catalytic reaction are suggested
to occur at the catalyst-aqueous boundary. For polymer supported
phase transfer catalysts, the catalytic reactivity is insensitive to the bulk
concentration of cyanide“. Organoclay triphase catalysis reactions are
carried out in different reaction environment from the polymer
supported triphase catalysis.

If the condition for organoclay phase transfer catalysis are first
order in electrophile (Equation 5) by varying pentyl bromide from 5 to
20 mole in 6 mL organic phase which keeping cyanide constant at 2
mole in 3 milliliter of aqueous solution, the kobs' was not linear with
the concentration of [RX] (Figure 20b). This pesudo orer rate constant,
k'obSr is derived from Equation 6.

d[CN']/dt = kobs'lCN'l (6)

Then. Ln([CN'l/[CN']o) = kobs't

0r. Ln((2-[C5H11CN])/2) = kobs't
where the [C5H11CN] designates the miniequivalent of C5H11CN.

85

Figure 19 (a) Determination of a k0.” from the slope of a plot
of [CanBr]/[C5HnBr]° versus reaction time ill the
presence of 0.100 g C16H33NMe3hectorite.

 

-0.20 -

Ln([C5HuBr]/[C5HuBr]o)

 

 

 

-0.40 ....,.
0 5000

 

Time, sec

The initial concentration of CN' was 20 mole in 3 mL aqueous solution and
[CsHuBr] was 2 mole in 6 mL toluene solution. The equivalent of the CN'
was ten times that of CsHuBr', that was recognized that the nucleophile, CN',
was far more than electroophile, CsHuBr. Thus, the observed rate constant
was first order in [CsHuBr] and the kinetic equation was represented by
d[C5HuBr]/dt = -kob,[C5HuBr]. ThereforeJtob. could be derived from
ln([C5HuBr]/[C5H11Br]o) = -kobst.

Ln{(0.67-[05H11Br])/067}

86

Figure 19 (Continued)

(b) Determination of a kob,’ from the slope of a plot
of [CN']/[CN]0 against reaction time in the presence
Of 0.100g C16H33PBU3hCCtOI'itC (H26AA) .

 

 
   

   

 

   

 

 

0.00
-Slope = kob; = 6.271110'5sec'l
-0.20 -
-0.40 -
70-60 ' r ' I ‘ I ' l
0 2000 4000 6000 8000
Time, sec

The initial concentration of CN' was 2 mole in 3 mL aqueous solution

and [CsHuBr] was 10 mole in 6 mL toluene solution. The equivalent

of the CanBr was five times that of CN'. Thus, the reaction was first

order in the concentration of the nucleophile [CN'] and the kinetic equation
was represented by d[CN']/dt = kom't. Then the observed rate constant (kobs')
could be derived from In( [CN']/ [CN']0) = «out. The depletion of CN'

was equal to the production of CSHHCN, which could be analyzed by GLC.
The equivalent of CN' was equal to (2 - CsHuCN).

87

Figure 20 (a) Dependence of the observed rate constant (Robs)
on nucleophile concentration for the cyanation reaction
(Equation 1) at 90°C in the presence of 0.100 g .
C15H33PBu3hectorite (H26AA) as the triphase catalyst.

 

losntomcec“)

 

 

 

r I 1

r
0 2 4 6

Concentration(Mole/Liter)

The pseudo order rate constant kg.” was determined under conditions when
the reactant is first order in electrophile; i.e., the nucleophile concentraction

was much larger than the organic electrophile concentration, which was kept
at 0.33M in toluene.

88

Figure 20 (continued)

(b)Dependence of the observed rate constant (kobS') on the organic

electrophile concentration for the reaction at 90°C in the presence
of 0.100 g C16H33PBu3hectorite as the triphase catalyst.

 

12

 

 

 

 

Pentyl Bromide Concentration (Mole/Liter)

The pseudo first order rate constant kob,‘ was determined under conditions
when the reaction is first order in nucleophile. That is, the electrophile
concentration was far larger than the nucleOphile concentration, which was
kept at only 0.67 M in aqueous solution.

89

Figure 19b is an example of detennining kobs' from the slope of a
plot of ln((0.67-[C5H11CN])/0.67) against reaction time, t. This result
demonstrates that the mechanism represented by Equation 2 does not
hold. This behavior is explained by a model in which the organic
electrophile is initially adsorbed on the boundary of the clay and the
organic solution and then the clay is transferred to the aqueous phase to
react with the nucleophile in the aqueous solution. The catalytic reaction
rate depends on the nucleophile bulk concentration and the surface
concentration of the organic electrophile on the organoclay (Equation
'2). The surface concentration of adsorbed pentyl bromide on the clay
may be increased by increasing the bulk electrophile concentration, but
the adsorption relation is not linear. Consequently, the kobs' curve in
Figure 19b is not a straight line.

3 . Dependence of Organoclay Catalytic Reactivity on the .

Polarity of the Organic Solvent

The solvent polarity dependence of organoclay triphase catalysis
can also afford information on the catalytic reaction mechanism. Lin
has investigated the influence of polarity on the cyanation reaction by
using C15H33PBu3+hectorite as the catalyst”. Organic solvents of
lower polarity result in higher catalytic reactivity for the organoclay
triphase catalysis. In contrast, Montanari, et al.51452 using polystyrene-
supported phosphonium bromide as a triphase catalyst, found that
higher polarity organic solvents led to higher iodination reaction rates.
The organo hectorite, H26AA, when used for iodination (Equation 7),
exhibits a reactivity dependence on the solvent polarity which is
opposite that derived for polymer supported catalysts. The data in
Table 5 indicate that with H26AA as the triphase catalyst solvents of low

90

polarity produce high reactivity for the iodination of pentyl bromide.

If the onium ion is supported in
C5H11Br + I' ----- > C5H11I + 31" (7)

inorganic matrices such as 8102 or A1203, the reactivity of cyanation
increases with decreasing polarity of the organic solvents47r49. The
dependence of solvent polarity on the inorganic based triphase catalysts
parallels that of organoclays. Because of the good swellability of the
polymer-based catalysts in polar organic solvents, anionic nucleophiles
are carried more easily to the ionic center of the polymer.
Consequently, nucleophilic substitution reactions proceed easily in the
polar organic medium. While organoclays swellability increases with
organic solvent polarities, catalytic reactivity under the triphase
catalysis does not improve. This is because the reaction site for an
organoclay triphase catalysis is not located in the organic medium.
Organoclays as catalysts for nucleophilic substitution reactions require
pre-adsorption of the organic electrophiles on the organoclay surfaces.
Highly polar organic solvents exhibit strong affinity for organic
reactants. Also, adsorption of organic electrophiles on the organoclay is
retarded by competitive adsorption of the polar organic solvent. Thus,
the surface concentration of organic reactants will be lowered by
increasing the polarity of the organic solvent and the catalytic reactivity
of organo clays for triphase catalysis will decrease.

91

 

 

Table 5 Solvent effect on kobs for the iodonation reaction of
pentyl bromide (Equation 8) under condition with
pseudo first order in nucleophile

Catalyst Solvent 104kobs. sec-1

H26AA o-Dichlorobenzene 0.81

H26AA Toluene 1.29

H26AA Decarle 3.42

 

Reaction condition: 8 mole of potassium iodide; 2 mole of pentyl
bromide; 0.15 g decane as internal standard; 3 ml water and 6 mL
organic solvent; 0.100g of H26AA; 90°C

92

4. Dependence of O/C Alkylation on Biphase and Triphase

Catalysts

There are two possible products for the alkylation of naphthoxide
by benzyl bromide55o58 as shown in Figure 7. In protic solvents such as
water, trifluoroacetic acid, methanol or ethanol, the C-alkylation
product is predominant. C-alkylation is favored in protic solvents
because the enolate of the C-alkylation precursor has an carbonyl group
as shown in Figure 7 which can be stabilized by hydrogen bonding
between the solvent and the intermediate anionic complex, whereas the
negative charge is not favored to locate on the low electroaffmitive
carbon. On the other hand, the O-alkylation product is favored in
aprotic solvents, such as toluene, benzene, DMF, DMSO and
dichloromethane. The enolate resonance structure of the O-alkylation
precursor is energetically stable because the negative charge is located
on the high electroaff'mitive oxygen. Since the aprotic solvent can not
form a hydrogen bond with the hydroxy group of the potential
intermediate, the intermediate favors the energetically stable form
which will result in the O-alkylation product. The ratio of the two
alkylation products in a triphase catalysis system can provide
information about the location of the reaction.

As shown in Table 6, the major products for the alkylation of
naphthoxide by benzyl bromide with organo hectorites (H26AA and
H24AA) as catalysts are C-alkylated derivatives. This suggests that the
catalytic reaction occurs in the aqueous environment. In contrast to

organoclays, nucleophilic substitution reactions for polymer supported

93

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94

catalysts proceed in organic instead of aqueous medium51.53v64. For
alkylations with C16H33NMe3Br and C15H33PBu3Br as the biphase
catalysts, O-alkylation process dominates the reactions (Table 6), since
the biphase catalyst carries the nucleophile from the aqueous phase to
the organic phase where the nucleophilic substitution reaction occurs.
The O-alkylation percentage of the biphase catalysis is sensitive to
organic solvent polarity. The ion pair of the O-alkylation intermediate
and onium ion in polar organic solvents such as methylene chloride
shows greater solubility than in less polar solvents such as benzene. The
results is a higher O-alkylation percentage in methylene chloride (72%)
than in benzene (60%) when C15H33NMe3Br is the biphase catalyst.
However, for organo clay triphase catalysis, the O/C alkylation ratios
are not significantly influenced by the polarity of organic solvent as
determined by C15H33NMe3+hectorite (H24AA) as triphase catalyst.
The O-alkylation percentages using H24AA as catalyst are 21% and
22% in benzene and methylene, respectively. For
C15H33PBu3+hectorite (H26AA) as the triphase catalyst, the 0-
alkylation yield is high when reacted in methylene chloride (49%) but
low when in benzene (18%).

Organoclays afford predominate by C-alkylation. This result is
very unusual. Organoclays provide a unique selectivity for reaction
products which can only be rationalized if the reaction occurs at the

clay-aqueous interface.

9S

5 . The Dependence of Organoclay Catalytic Reactivity on the

Volume of the two Liquid Phases

A stirred mixture of the organoclay, aqueous and organic solutions
forms a water in oil emulsion. Organo hectorite is homogeneously
distributed in the emulsion and the water droplets are incorporated into
the organic phase. When the volume of water is more than needed for
an emulsion, a clear aqueous phase appears in the triphase system. A
typical emulsion of 6 mL of organic liquid, 3 mL aqueous solution and
0.1g organo hectorite formed by stirring is milky white in appearance
and no excess organic phase or aqueous phase is evidenced. A small
excess organic liquid segregates from the emulsion when the mixture is
allowed to remain for more than five minutes. As the volume of
aqueous phase is increased higher than 3 mL and the volume of organic
solution phase is decreased lower than 6 mL, a clear aqueous solution
phase can be observed during stirring. The excess aqueous solution can
not be incorporated into the emulsion of supported organo hectorite in
the organic phase.

Since the triphase system is a water-in-oil emulsion, a part of the
aqueous phase does not participate the emulsion system. The activity of
an organoclay triphase catalysis is only influenced by the volume of
organic solution and not affected by the volume of aqueous solution
(Figure 21). The catalytic activity decreases when the volume of the
organic phase is increased while the concentration of reactants, as the
total volume of aqueous and organic solution and the amount of
organoclay catalyst, H26AA were kept constant (Figure 21a). Based on

the same amount of organoclay catalyst, increasing the volume of the

96

Figure 21 (a) The dependence of km on the volume of toluene
for the reaction of pentylbromide with KCN.

 

104%, Sec. 1
i

 

 

 

1 r u f u

2 3 >4 . s I 6
Volume of Organic Phase, m1.

Reaction was carried out under a condition that was pseudo first
order in electrophile. The total volume of organic and aqueous
phase was 9 mL. The concentration of cyanide in aqueous solution
was 6.25 M and the pentyl bromide concenu'ation was 0.33 M in
toluene.

97

Figure 21 (Continued)

(b) The observed rate plotted against the volume of aqueous
solution while the volume of toluene solution was kept constant

 

 

 

 

 

at 2 mL.
4.5
4.0 «
q—t .
.0 3.5 ‘ * l l F
0
U)
‘T 3.0 -
O
—1 q
E 2.5 .
8 1
o 2.04
U
o
‘5 1.5 «
ad
1
E 1.0 '4
-° 0.5 4
O
0.0 f I ' r V I ' I
a 4 5 e 7

Volume of Auqeous Phase, mL

Reaction was carried out under a condition that was pseudo first
order in electrophile. The concentration of cyanide in aqueous solution
was 6.25 M and the pentyl bromide concentration was 0.33 M in toluene.

98

organic phase will decrease the suspended catalyst concentration in the
organic solution. Thus, a low efficiency of adsorbing organic reactants
on organoclay results in low catalytic reactivity when the volume of the
organic phase is increased. However, when the volume of organic phase
is kept constant, the catalytic reactivity is not apparently changed by
changing the volume of aqueous solution (Figure 21b). Since only a
certain amount of aqueous solution can be incorporated into the
emulsion, the excess aqueous phase which can not be incorporated into
the emulsion does not participate in the organoclay triphase catalysis.
Therefore, the reactivity is not influenced by the volume of aqueous
solution if the concentration of the nucleophile, CN‘, is kept constant.
6. Organoclay Catalytic Reaction in the Absence of Water.
Nucleophilic substitution reactions are not catalyzed by the
organoclay H26AA in the absence of water. For the cyanation of pentyl
bromide, potassium cyanide can not be transferred by the organoclay
from the solid phase into the toluene solution to react with the organic
reactant. Potassium cyanide is hydrophilic, but it can be suspended in
the organic solution. However, a phase transfer catalysis reaction is
known to occur in the absence of water when a polymer supported
catalyst or an onium salt is employed as the phase transfer catalyst62,99.
A polymer supported catalyst can transfer the nucleophile by ion-
exchanging the anion on the polymer-attached onium group. This
onium-nucleophile ionic pair is accessible to the electrophile in organic
solution for nucleophilic substitution reaction. An organoclay does not
have an ionic center for ion-exchange reaction with a nucleophile.
Instead, the organoclay facilitates the nucleophilic substitution by

forming an emulsion mixture of aqueous and organic solution. The

99

nucleophile can not be transferred from the solid state into the
organoclay. Therefore, if the nucleophile can not be dissolved in the
liquid solution of the phase transfer catalysis reaction, catalytic reaction
can not proceed. Consequently, water is required for catalytic
nucleophilic substitution reactions in the presence of organoclay.
7. Mechanism of the Organoclay Triphase Catalysis

The assembly of an organoclay emulsion is the decisive factor for
the efficient triphase catalysis. Levine and Spence and co-
workersloo,101 have investigated emulsions of organic and aqueous
solution stabilized by fine clay particles containing adsorbed surfactant.
In our organoclay triphase catalysis reaction system, the organoclays act
as an emulsifier to minimize the particle size of the dispersion droplet
(Figure 22). At the aqueous interface, the onium surfactant orient
vertically on the surface to explore the hydrophilic silicate surface and
increase hydrophilic interactionloz. At the organic liquid catalyst
boundary, the surfactant may orient horizontally on the silicate surface
to shield the polar clay surface and maintain the hydrophobicity of the
material surface. For the purpose of forming an emulsion, the organo
clay should have a high layer charge density and be interlayered by an
onium ion with at least one long alkyl chain. Therefore, the inherent
hydrophilic layered silicate will be modified to an amphiphilic material.
Hectorite derivatives (H24AA and H26AA) with long chain alkyl

ammonium can form emulsions and are good triphase catalysts.

100

Figure 22 The emulsion of aqueous and organic solutions stabilized
by an amphiphilic organo hectorite.

Membrane-like
Organrc
Pl Organo clay
86 Assembly

 

 

 

 

 

E OrganicPhase g
E WWWJ
I fi JV
'WLWWW, WWW
: WWWMMM

 

 

:Sondqvvemgm'" ‘

 

E
l

 

 

 

 

32
~35
M'
«we

101

However, not all organoclays are efficient triphase catalysts. Hectorite
derivative, H18AA, with tetrabutyl ammonium in the interlayer, fails to
form an emulsion and is a poor triphase catalyst. The onium ion in this
case fails to shield the clay surface for wetting by the organic phase.
Laponite derivatives do not form emulsions due to the low layer charge
density of the layer silicate, which limits adsorption of onium ion on the
silicate surface and the degree of hydrophobicity. Also, the inherently
small particle size of laponite affords a large edge surface area which i
can not be covered by the hydrophobic surfactant tail, and this also
contributes to the hydrophilicity of the surface.

The mechanism of efficient triphase catalysis by organo hectorites
(H24AA and H26AA) as triphase catalysts is elucidated by the solvent
effect and the dependence on reactant concentrations. Also, the
experiment on O/C alkylation and the study of the volume ratio of the
two liquid phases provides mechanistic information for the organoclay
triphase catalysis reaction. All these results suggest that the reaction
occurs at the clay-aqueous liquid boundary. Figure 23 shows the
proposed mechanism for nucleophilic substitution reaction by organo
hectorites as triphase catalysts. An organo hectorite particle which
stabilizes the organic-aqueous emulsion is a part of the assembled
organo clays. They behave as emulsifiers, or better, as membranes
between the aqueous and organic phases. The catalyst swellability in

organic solvents is not a significant factor for catalytic reactivity.

102

Figure 23 The mechanism of the triphase catalysis reaction
with organo hectorites as the catalysts.

Binding of RX occurs at the clay-organic interface.

Organic Solution F®Q~3W WCH“ Mar

IJW

 

 

Clay Membrane | -

Who
Br
Aqueous Solution «3%: Q
CH3

Nucleophilic attack occurs at the clay-aqueous interface.

 

Hl

 

103

However, the adsorption of organic electrophile on the organo clay
surfaces and the concentration of anionic nucleophile in aqueous
solution are the two most important factors for the catalytic reactivity
of nucleophilic substitution reaction.

Asorption of the organic molecule is occurring at the organic-
aqueous interface. Solvents of low polarity produce high adsorption of
the organic electrophile. Thus, the surface concentration of organic
reactant is high and the catalytic reaction is more efficient when non-
polar organic solvents are used in the triphase catalysis system. The
nucleophilic substitution reaction occurs at the boundary of the catalyst
and the aqueous solution. The nucleophile in the aqueous solution is
directly reacting with the organic electrophile which has been adsorbed
on the organoclay surface, since the reaction rate is proportional to the

bulk concentration of the nucleophile in the aqueous solution.

104

C. The Longevity of Organoclays for Triphase Catalysis
1. Recyclability of Organoclays as Triphase Catalysts

The structural stability and catalytic recyclability of organoclays
for TPC are dependent on the layer charge densities of the silicate layer
hosts, and on the solubilities of the ion-pair formed between nucleophile
anions and surfactant cations in the organic solvents. In order to study
stability and catalytic recyclability, organoclays containing either
C15H33NMe3+ or C15H33PBu3+ as the intercalated cations were
employed as the catalysts.

The hectorite derivative, H26AA, containing the interlayer
C15H33PBu3+ cation shows excellent recyclability for triphase catalysis
(Figure 24). Another hectorite derivative, H24AA, with C15H33NMe3+
gallery cation shows fair catalytic recyclability (Figure 25) for the
probe cyanation reaction (Equation 1). After 10 cycles of cyanation
reaction, in which the total reaction time was 30 hours and the reaction
temperature 90°C, both of the hectorite derivatives retained their d-
spacings, as judged by the XRD. The loss of catalytic activity after use
for vigorous cyanation reaction was approximately 10% and 55% for
H26AA and H24AA respectively. The catalysts were recycled by simple
filtration and resuspension in fresh reaction mixture. This recycle
procedure could result in incomplete recovery of the solid catalyst and
an apparent loss of reactivity. The loss of the catalytic reactivity of
H26AA is most likely due to incomplete catalyst recovery by the
filtration procedure. However, the 55% loss in activity for H24AA

Figure 24

loskobs, Sec-l

O-‘NQ‘hUIO’VGC

105

The recyclability of C16H33PBu3‘hectorite (H26AA)
over 10 reaction cycles for the cyanation of pentyl
bromide at 90°C.

Reaction conditions for each cycle were the same as
those described in Figure 19a.

 

_s
(n

1

_0 _L
Q «ht
l n I

l

_0 .0
d N
111

A

 

_5
o
l

lllllll+llllllll

A ll

 

 

 

'l

O
'0
fi
$1
m
.0
0

Number of catalytic cycles

106

Figure 25 The recyclability of C16H33NMe3+hectorite (H24AA)

Rate, 105 x 10,500“)

over 10 reaction cycles for the cyanation of pentyl
bromide at 90°C.

Reaction conditions for each cycle were the same as
those described in Figure 19a.

 

4.0 ‘

 

1.0-l

 

 

 

0.0 V V I W V I V V I V V ' V V I ff I V Y I V U I i r' I V V V

T
012 3 4 5 6 7 a 91011
Number of catalytic cycle

107

after 10 reaction cycles can not be due to loss of catalyst by the
filtration.

Quaternary ammonium ions may undergo Hoffmann elimination,
the result of which is production of trialkyl amines and alkenes in
strongly alkaline solution“. For example, the interlayer surfactant,
C16H33NMe3+, which contains b-hydrogens may decompose into an
amine and alkene in the basic solution at elevated temperature35‘37.

The structural stability of the extremely high layer charge density
onium ion F-hectorite derivative F26AA, in which the surfactant adopts
'a paraffin-like structure, is almost negligible. More than 98% of the
catalytic capacity is lost after one reaction cycle (Table 7). The X-ray
pattern shows the catalyst is transformed to the potassium form of F-
hectorite, an indication that the surfactant does not remain bound to the
clay host. For C15H33NMe3+-F-hectorite F24AA which adopts a
paraffm-like structure in the interlayer, the catalytic recyclability is
satisfactory (Figure 26) but the solid catalyst is converted to a mixture
of potassium F-hectorite and an interstratified form of F-hectorite in
which the gallery are alternately occupied by potassium and surfactant
cations. This regularly interstratified form of F-hectorite will be
discussed in the later section. The difference in recyclability between
F26AA and F24AA is due to the nature of the surfactants themselves.
The carbon-rich surfactant, C16H33PBu3+ dissolves into the liquid phase
but the other surfactant, C1¢5H33NMe3+ remains intercalated in the
silicate host. This desorption mechanism will be discussed in more

detail in the section D.3.

108

Table 7 The catalytic activity of the high layer charge density
C15H33PBu3+-Clay after multiple reaction cycle for

cyanation and chlorination of pentyl bromidea.

 

Organoclay Organic Nucleophile Pseudo First Order Rate

 

 

Solvent Constant (105 x kobs. See'l)
Reaction Run
1 2 3

W26AA Toluene CN‘ 27.2 10.7 4.33
A26AA Toluene CN' 54.6 9.44 2.1 1
F26AA Toluene CN' 56.5 1.91 0.98
A26AA Decane CN' 71.8 49.6

F26AA Decane CN‘ 46.9 4.96

A26AA Toluene CI' 24.3 %b 22.7%b

 

a. Reaction Conditions: 2 mole pentyl bromide; 20 mole

potassium cyanide or 7.5 mmole sodium chloride; 3 mL

aqueous solution; 6 mL organic solution; reaction temperature

90°C.

b The reactivity is determined by the chemical yield of the

chlorination product after 8 hour reaction.

109

Figure 26 The recyclability of C16H33NMe3+-F-hectorite (F24AA)

Rate, 105xkob,(sec'l)

over 10 reaction cycles for the cyanation of pentyl
bromide at 90°C.

Reaction conditions for each cycle were the same as
those described in Figure 19a.

 

 

 

 

 

5
4-'
3.
l
.1 I-
.I I
211 ' '
.

+
1-
Ottrvrrrrr' l *r'r

0 2 4 6 8 10 12

Number of catalytic cycles

110

The two montmorillonite derivatives, W26AA and A26AA, which
contain C15H33NMe3+ and C15H33PBu3+ onium ions respectively, are
between F-hectorite and hectorite supported catalysts in structural
stability and catalytic recyclability (Table 7). Although the Wyoming
montmorillonite derivative, W26AA, shows the same lateral bilayer
surfactant structure as the hectorite derivative, H26AA, W26AA does
not exhibit catalytic recyclability as good as that of H26AA. For the
Arizona montmorillonite derivative (A26AA) which adopts a pseudo
trimolecular structure, the catalytic recyclability for pentyl bromide
cyanation is also poor (Table 7). The surfactant (C15H33PBu+) appears
to desorb from the montnorillonite host after the phase transfer catalysis
reaction. The extent of desorption of C1¢5H331PBu3+ from the clay hosts
is correlated with the clay layer charge density.

' When the reaction conditions are changed, the recyclability
properties of organoclays for PTC are also changed. The recycled
C15H33PBu3+-Montmorillonite(Arizona) (A26AA) after pentyl bromide
cyanation is a poor catalyst for the next cycle of the same reaction if
toluene is employed as the organic solvent. However, if a non-polar
solvent such as decane is used on a reaction solvent, the catalytic
stability for cyanation can be greatly improved. The poor solubility of
the surfactant-cyanide ion pair in the non-polar organic solvent
undoubtedly contributes to the stability of the intercalate. This low
solubility of the ion pair in the non-polar organic solvent can limit the
desorption of the surfactant from the clay hosts. Also, the recyclability

111

of A26AA for catalytic chlorination is much better than for the
cyanation reaction with the same solvent. The surfactant-chloride ion
pair is not as soluble as surfactant-cyanide pair in the toluene solution.
Therefore, the instability of the organoclay structure and the loss of
catalytic recyclability is due to the desorption of the surfactants by ion
pairing with the nucleophiles in the organic solvent.

2. Surfactant Desorption

The desorption of onium ion surfactants from clay interlayers is
the main cause of the loss of catalytic capacity for organoclay catalysts.
This desorption process has been investigated by treating the
organoclays under various condition and analyzing the concentration of
the desorbed surfactants dissolved in liquid solution.

(a) Influence of aqueous electrolyte solution on surfactant

desorption of organoclays.

Onium ion surfactants are expected to remain adsorbed on the clay
hosts in the presence of concentrated electrolyte aqueous solution
without an organic solvent. The affinity between the surfactant-anion
pair and water is less than the electrostatic force between the surfactant
and silicate layer. The results of the surfactant desorption for different
organoclays in 2.5 N sodium chloride solution is shown in Table 8. The
amount of surfactant desorption was determined by spectrometric
analysis of the surfactant in liquid solution”. The cationic surfactants

in the aqueous phase were extracted into the chloroform layer in the

112

Table 8. The desorption of onium ion from the organoclays in the presence

of 2.5N NaCl aqueous solutiona.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organoclay onium ion emp. Original d-Spacing Change of
Desorption 0C d-Spacing after Surfactant
treatment Orientation
with 2.5N after
NaCl treatment
with N aCl
C161133NM¢3+Laponit¢ < 0.3% 250 14.51: 14.5: None
<Lz4AA) -
4AA <0.5% 900 14.5A 1435 None
C 16H33NMC3+HCCL < 0.5% 250 I 7.83 figA None
(HZ4AA) k
H24AA <05? 900 1 . T781} None
C 16H33NMC3+Mont(W) < 0.5% 250 1 . WA None
%4AA <0.5% 900 18.3X MA None
C15H33NM03+Mont(A) < 0.5? 250 39.4A littersuatifi'a'
£A24AA) - -
AA <0.5% 900 1.8A 39.415 Intersttntifia
C15H33NMe3+F-Hect. <o.5%| 250 TIT. 323A Intetstratified
24AA) 0
AA 031% 900 W. 3TH} 10000110
C léH33PBu3+Laponit¢ < 0. 5% 250 16.0A 17.-2A WOW
(L26AA) A -
D6AA < 0.5% 900 16.015 17:31} None
C16H33PBU3+HOCL < 0.5% 250 19.“ ESA N000
(H26AA) A I
H26AA < 0.5% 900 19.6A 19.5A None
C 16H33PBIJ3+MOITI(W) < 0.5% 250 195A 19.55 NOITC
(W26AA) L
“W26AA <057. 900 195A 19. 5A None
C15H33PBu3+Mont(A) <0.5% 250 783A None
A26 ) I
W <0.5% 900 77.63 78.15 None
C16I-I33PBu3‘l'F-Hect, 0.66% 250 28.0A “IDA InterstratifiEd
6AA) A -
6AA 0. 900 .0A 40.0A Interstrauf' red

 

 

 

(Continued)

Table 8 (Continued)

113

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organoclay onium ion ll'eTnp. Original d-Spacing Change or
Desorption 0c d-Spacing after Surfactant
treatment Orientation
with 2.5N after
NaCl treatment
with NaCl
(ClezshNMez+I,apon <0.5% 250 7771—". T7311 None
L30AA A A
(C12H25)2NMe2+Hect. < 0.5% 250 20.1A 18.8A None
9130M) A -
(C 12H25)2N Mez+Mont(W) «1.5% 250 20.1.6. 19.9A None
W30AA A
(C12H25)2NMe2+Mont(A) 45.5% 250 30.23 10.511 None
A30AA A
(C12H25)2NMe2+F-Hect. <05? 250 33.311 Intersuafified
F30AA A A
N Bu4+Hect. <03?) 250 IHA 14.5A None
(HISAA) -
NBu4+F-Hect. < 0.5% 250 14.5A 14.5A Onium ion
(F 18AA) was removed
from gallery

 

 

a. Aliquot of 4 ml 2.5N NaCl aqueous solution and 0.030 g

organoclay were stirred in 90°C for 3 hours or at room

temperature for 15 hours. The onium ion desorption was

determined by analyzing the onium ion concentration in the liquid

phase.

b. The interlayer onium ion was ion-exchanged by sodium ion but no
onium ion was found in liquid solution.

114

presence of anionic methyl orange as an indicator. The UV-Visible
absorption of the organic cation-organic anion pairs is shifted to 418 nm
in chloroform solution103JO4. According to Beer's Law, the maximum
absorption at 418 nm is proportional to the concentration of the cationic
surfactant-dye pairs in the chloroform solution. As shown in Table 8,
the onium ion surfactants do not desorb from clay hosts in the absence
of water.

(b)Dependence of the surfactant desorption on the surfactant
structure in the presence of the sodium chloride aqueous solution
and toluene

The onium ion surfactants do not desorb from clay host in the
absence of organic solutions but they may desorb when both an aqueous
electrolyte and a polar organic solvent are present. The solubilities of
the surfactant-anion pairs in the organic liquid phase and the clay layer
charge density are important factors in determining surfactant
desorption from the clay hosts as expressed as the following overall

reaction.

s" + NaClm) ——- Nn+ + form,” (9)

 

where the horizontal bars represent the intercalated species and 8+ is the
onium ion surfactant.

The desorption of low lipophilic surfactants containing one long
and three short alkyl chain, such as C15H33NMe3+, is always negligible

(Table 9), regardless of the clay layer charge density. Although the

115

Table 9 Desorption of surfactant from organoclays in the presence of

NaCl aqueous solution and toluene“.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0 _anoc y omumron Temp. 0‘66?! - pacmg oi angeo
Desorption DC d-Spacing after being Surfactant
treated with Orientation
2.5N NaCl after being
and treated with
toluene NaCl and
Toluene
c16n33NM¢3+Laponiw < 0.5% 250 145? mm None
(L24AA) A A
mAA < 0.5% 14.51! 14.413. None
C16H33NMC3+HeCL < 0.3% 250 ”SK TSJA None
AA)
%M < 0.5% TBA—7 WA None
C16H33NM63+Mont(W) <0.5% 50 ISSX TEDA None
24AA) A
AA < 0.5% poo 113T— 'mA None ‘
C16H33NMe3+Mont(A) < 0.5? 250 ELSA 37.4A Interstratit'ia1
A24AA) A -
AA 853% 900 . "TS-DA Intersn-atiBFd'
C16H33NMe3+F-Hect. < 0.5% . P50 27.73 TiEA Interstratifia-
AA) A
% m JgLoo 33;? 333A Inmate?
C16H33NMc3+Recmfim <03?» [:50 . WA Intersnatifiea'
4AA) A A
AA <o.5% poo 31.0A 48.5A KW
C16H33PBu3+Laponitc < 6.5% [250 6.0A TIIA None
) A A
“g3; < 0.5% poo 16.05 ITOA None
C16H33PBUB+HOCL < 0.5% F50 I9.“ . A N011:
9126M)
H26AA < 03% poo 7572A None

 

(Continued)

116

Table 9 (Continued)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organoclay oniumion fimp. Original d-Spacing Change of
Desorption DC d—Spacing after being Surfactant
treated with Orientation
2.5N NaCl after being
and treated with
Toluene 2.5 NaCl
and
Toluene
C16H33PBu3+Mont(W) < 0.5% 250 19.5A 8.8A None
< 0.5% 900 19.55 18.578 None
WA < 03% 90° 193A 18.21} None
C16H33PBu3+Mont(A) 378% I250 "333A Inter-stratum
(A26AA)
m 1 . p00 . 76.01} fitterstratifia-
C16H33P3u3+F-Hect. 88.2% [250 EDT—"1773A Na+Exchange
(F26AA) A
F26AA 83.7% poo TiSA Na+Exchan e
C15H33PBu3+Rectorite 278% [250 '3TOA terstrati
6AA)
gm . poo '53:; - Intersu'TufiEd'
(C12H25)2NMe2+Lapon < 03% [250 137A None
L30AA -
(C12H25)2NMe2+I-Iect. < 0.5% [250 0.118 19.6.4 None
(H30AA)
(C12st)2NMe2+Mont < 63% p50 20.17! WA FEM
(Wyoming)
W30AA
(c121125)2NM¢2+M0m, <0.5% 250 30.23 ”3037 None
(Arizona)
A30AA A
(C12H25)2NM¢2+F-H¢ct. < . 1250 302$ 17.-5A KW
F30AA
NBu4+Hece < 075% p.50 1431!; WA None
(HISAA) -
NBu4+F-Hect. < 0.3% 2,50 115‘ 12.6A Onium ion
(F 18AA) was removed
1’10"“ 52.11%

 

 

a. Aliquot of 4 mL 2.5N NaCl aqueous solution, 4 mL toluene and 0.030 g
organoclay were stirred in 90°C for 3 hours or at room temperature for

15 hours.

b. Only the diffraction peak of Na F—hectorite was found in XRD pattern
but no onium ion could be observed in liquid solutions.

117

surfactant re-orients in the F-hectorite and Arizona montmorillonite to
form a regular ordered mixed ion species, surfactant desorption does
not occur. However, highly lipophilic surfactants, such as
C15H33PBu3+, in the interlayers of F-hectorite, will almost totally
desorb by Na+ exchange and dissolve into the toluene phase by ion pair
formation. For laponite, hectorite and Wyoming montmorillonite
derivatives (L26AA, H26AA and W26AA, respectively) in which the
surfactants adopt monolayer or bilayer structures, desorption of all
surfactants is negligible (Table 9) even in the presence of toluene and
aqueous sodium chloride solution. Moreover, low layer charge density
organoclay structures are not altered by the electrolyte and organic
solvent treatment. .

For the Arizona montmorillonite and rectorite derivatives (A26AA
and R26AA) in which the surfactants adopt pseudo trimolecular
structures in the clay interlayers, the surfactants partially desorb from
the clay hosts (Table 9). There are 2.8% and 3.8% of C15H33PBu3+
desorbed from Arizona montmorillonite and rectorite, respectively, at
room temperature. The desorption increases to 12.3% and 8.6% for
A26AA and R26AA respectively as the temperature is elevated to 90°C
(Table 9). The reaction products in each of these systems are the
mixture of Na+-exchange and interstratified forms of the clays.

(c)Desorption of surfactant from clay hosts in the presence of

aqueous sodiumjzmmide solution and toluene

The anion in the electrolyte solution can also influence surfactant
desorption from the organoclay in a water-organic liquid emulsion. As
the hard and hydrophilic chloride ion is substituted by the softer and
more lipophilic bromide anion, more of [C15H33PBu3+] desorbs from

118

the organoclays with pseudo-trimolecular structures due to the
improved solubilities of the surfactant-anion pairs in the organic
solution. Comparing the two surfactant-ion pairs, [C15H33PBu3+Br-]
and [C15H33PBu3+Cl'], we expect the former ion to have a higher
solubility in toluene. Consequently, the desorption of the surfactants
from both A26AA and R26AA increases when sodium bromide is
substituted for sodium chloride (Table 9 and 10). However, when the
sodium chloride was substituted by by sodium bromide in the aqueous
solution does not increase the desorption of hexadecyltributyl
phosphonium from the F-hectorite derivative (F26AA), but it even
dramatically reduces the desorption percentage.

(d)The dependence of surfactant desorption on 91953132133;

concentration.

The desorption of hexadecyltributyl phosphonium (C15H33PBu3+)
from F-hectorite increases with increasing electrolyte concentration.
Figure 29 shows a plot of surfactant desorption versus the concentration
of aqueous solution in the presence toluene and F26AA at room
temperature. .

The XRD patterns also show the changes that occur in the 001
reflection peaks upon increasing the sodium chloride concentration
(Figure 28). While the concentration is increased, the 28A and 14A
reflections, which are the 001 and 002 reflections of F26AA, become
weaker and a new peak at 12.5A is present due to formation of sodium
F-hectorite. As the sodium chloride concentration is decreased below
0.5N, peaks in the range from 42A to 49A are observed; these are due
to the interstratified F-hectorite derivatives with the ordered stacking of

119

Table 10 Desorption of surfactant from the organoclay in the presence of
aqueous 2.5N N aBr and toluene solutions“.

 

d-Spacing Change of

Organoclay Onium Ion Temp Original

Desorption 0c d-Spacing after being Onium ion
treated with Orientation
NaBr and
Toluene
cldi33NM¢3+Lapom < 0.5% 25 14.511 14.711 None
(LZ4AA)
C151-l33NMe3+Hect. < 0.5% 25 17.8A 17.7A None
(1124M)
clén33NM¢3+Mom < 0.5% 25 18.3A 163A None
(Wyoming)
(W24AA)
clén33NM¢3+MonL < 0.5% 25 215A 38.4A None
(Arizona)
(A24AA)
C161~133NM¢3+EH¢cL < 0.5% 25 27.2.4 475A Interstratified
. (F24AA) .
C151133NMe3+RecL < 0.5% 25 310A 39.4A Intersu'aufied
(R24AA)
clw33p3u3+upom < 0.5% 25 16.0A 17.1A None
(L26AA)
c14133p3u3+nm < 0.5% 25 19.6A 18.8A None
(1126M)
C16H33pBu3+MonL < 0.5% 25 19.511 19.3)! None
(Wyoming)
c161433pgu3+M0m 42.5% 25 27.6A 37.9A Interstratified
(Arizona)
(A26AA)
Clw33p3u3+nnm 48.7% 25 28.0A 34.7A(w) Na-Exchange
(P26AA) 12.5A(s)b (Not Complcm)
C16H33p3u3+RecL 44.4% 25 32.7A 26.6A Interstatified
(R26AA)

 

a The experimental conditions were the same as described in Table 9,
except the nucleophile was changed to bromide.

b. The weak diffraction peak at 34.7A results from organo
F-hectorite and that at 12.5A results Na-F-hectorite.

120

Figure 27 Dependence of [C16H33PBuf] desorption from
organo F-hectorite (F26AA) on the concentration of
sodium chloride in an aqueous-toluene emulsion.

 

 

 

 

 

100
23
'1:
8
t) 30..
0
3.
LL
5
an 60"
+
m
=
E I
53
% 40‘
C)
“-4 1
O
3
3 20-
e?

o". ' I I I I I I l ' I
0.0 0.2 0.4 0.6 0.8 1.0

[NaCl], mole/liter

121

Figure 28 The X-ray diffraction patterns of C16H33PBu31-F-Hectorite
(F25AA) reaction products formed by reaction of aqueous
solution-toluene emulsions at various NaCl concentration.

(125A, Na F-hectorite)

1 .ON

 

 

 

0.5N

 

0.4N

 

0.3N

 

 

 

 

I.

 

122

the Na-F-hectorite and [C15H33PBu3+Br]-F-hectorite layers. When the
concentration reaches 1.0N, all of the interlayer surfactant is dissolved
into the toluene phase and the only diffraction peak is that at 12.5A
which results from Na+-F-hectorite formed after the ion-exchange
reaction. If the desorption is due to the one-by-one ion exchange of
surfactant to sodium ion, all the surfactants would be substituted at the
concentration of 0.02N. However, less than 1% of the surfactant is
exchanged (Figure 27) by the sodium cation at the sodium chloride
concentration at which the sodium molarity equals that of the interlayer
surfactant. Therefore, the desorption of the onium ion surfactant,
C15H33PBu3+ is related to the electrolyte concentration instead of the
total amount of metal cation.

(e) Dependence of the surfactant desorption on organic solvent

9.01mi!!-

The desorption of surfactants from organoclays under TPC
reaction conditions can be prevented by using a non-polar organic
solvent to form the emulsion. If the organic solvent is decane instead of
toluene, desorption of hexadecyltributyl phosphonium form A26AA
drops from 12.3% to less than 0.5% at 90°C. Also, the desorption of
the same surfactant from the F-hectorite derivative (F26AA) decreases
from 83% in toluene to 8.78% in decane at 90°C (Table 11). Non-
polar decane not only exhibits good catalytic reactivity, but also
promotes catalyst longevity by inhibiting ion exchange of the
organoclay structure.

123

Table 11. The desorption of the surfactant under the triphase

condition with other organic solventsa.

 

 

 

 

 

 

 

 

 

 

 

 

 

organoclay Surfactant Temp. Organic solvent
Desorption °C

C16H33PBu3+F-Hect. 1.2% 25° Decane

(F26AA)

F26AA 8.8% 90° Decane

C16H33PBu3+MonL <O.5% 25° Decane

(Arizona)

(F26AA)

A26AA <0.5% 90° Decane

C16H33NMe3+F-Hect. <O.5% 25° o-Dichlorobenzene
24AA)

F24AA <0.5% 90° o-Dichlorobenzene

C16H33NMe3+Mont <0.5% 25° o-Dichlorobenzene

(Arizona)

(A24AA)

A24AA <0.5 % 90° o-Dichlorobenzene

 

a. A 4 mL of aliquot of 2.5N NaCl aqueous solution, 4 mL organic
solvent and 0.030 g organoclay were stirred at 90°C for 3 hours
or at room temperature for 15 hours.

 

124

A surfactant of low lipophilic character such as hexadecyltrimethyl
ammonium, does not desorb from the clay host even in the presence of a
highly polar organic solvents such as o-dichlorobenzene (Table 11).
The attractive force between a clay layer and [C15H33NMe3+] is still
stronger than the affinity between the [C15H33NMe3+Br'] ion pair and
o-dichlorobenzene.

3. Co-Surfactant Effects on Phase Transfer Catalysis

(a)Influence of co-surfactant on [C15H33NMe3+] desorption

For smectite clay derivatives intercalated by hexadecyltrimethyl
ammonium ion C16H33NMe3+, the surfactant always remains bound to
the clay hosts instead of being desorbed into the liquid solution. The
cationic surfactant is almost insoluble in toluene, but can be dissolved in
aqueous solution in the form of a micelle if the surfactant concentration
is larger than the critical micelle concentration (CMC). Dobias38 found
that the adsorption of hexadecyl pyridinium chloride on natural
minerals such as quartz decreases tremendously when the surfactant
concentration was higher than the CMC. It was suggested that the
surfactant is more stable as a three-dimensional emulsion mixture than
on a two-dimension quartz surface. However, a similar surfactant,
hexadecyltrimethyl ammonium, in the smectite clays does not desorb
when the surfactant concentration is more than the CMC owing to the
strong electrostatic affinity between the clay and the surfactant.
However, when an alcohol co-surfactant with more than 5 carbon atoms
is introduced into this system, the surfactant may dissociate to form a
more thermodynamically stable microemulsion which is composed of
the surfactant, co-surfactant, water and the organic phase (Figure 12).

This co-surfactant effect on the surfactant desorption is especially

125

significant for clays with high layer charge density. Table 12 lists for
[C15H33NMe3+] the desorption from clays of various layer charge
density that results by adding different concentrations of the co-
surfactant, octanol. The desorption of [C15H33NMe3+] increases with
increasing co-surfactant concentration in the microemulsion system.

The difference between "macroemulsion" and "microemulsion" is
that the later includes a co-surfactant. The function of the co-
surfactants is to separate the positive charge on the polar portion of
cationic surfactants23n91( Figure 12). Therefore, the electric double
layer charge density on the interface of the surfactant and aqueous phase
is lowered, leading to reduction of the interfacial tension of the water
and oil phase25527.23.29.3039’90491.105. Generally, the microemulsion
dr0plet sizes are 10 to 100 fold smaller than those of the emulsion27:23.
In addition, unlike an emulsion which is only kinetically stable, a
microemulsion is not only kinetically stable, but also thermodynamically
stable27:29.

(b) Influence of coosurfactants on biphase and triphase

catalysis. .

Nucleophilic substitution reactions which use the cationic
surfactant, C16H33NMegBr, as a biphase catalyst are strongly influenced
by the addition of co-surfactant. Brown and Fendler et
a113.20.22v23»24,10° have utilized cationic surfactants that form micellar
aggregates to catalyze nucleophilic substitution reactions. When a co-
surfactant is added to a reaction mixture containing a cationic
surfactant, toluene and an aqueous solution, a small droplet-size
microemulsion is formed. Nucleophilic substitutions proceed more

readily in the microemulsion mixture. However, the co-surfactant,

Table 12. Desorption of [C16H33NMe3]+ from organoclays in the

126

presence of octanol as co-surfactanta.

 

Percentage Desorption of Surfactant from Organoclay

 

mmole mmole mmole

2

3

 

 

mole of 1
Octanol

Organo

clayb

L24AA 0.4%
H24AA 0.5%
.W24AA 0.2%
A24AA 0.2%
F24AA 0.4%

0.4%
0.4%
0.3%
0.2%
1.6%

0.7%
0.4%
0.4%
0.9%
5.3%

4

mole

 

0.7%
0.9%
0.9%
2.0%
6.6%

5

mole

 

1.1%
1.1%
1.7%
3.3%
9.1%

 

7 19
mole mmole
2.5% 12.8%
2.2% 8.2%
2.5% 15.0%
5.4% 44.2%
17.3% 57.2%

 

a. A 4 mL aliquot 2.5N NaCl aqueous solution and 4 mL toluene

solution with the indicated amount of octanol was stirred at room

temperature for 18 hours.

b. L24AA: C15H33NMe3+Laponite;
H24AA: C16H33NMe3+Hectorite

W24AA: C15H33NMe3+Montmorillonite (Wyoming);
A24AA: C15H33NMe3+Montmorillonite (Arizona);
F24AA: C15H33NMe3+F-Hectorite.

127

octanol, does not alter the colloidal properties of the emulsion formed
by organoclay aggregates.

For biphase catalysis reactions which use cationic surfactants as
catalysts, the reaction site is assumed to be near the Gouy layer of the
micelle. Upon addition of co-surfactants, a smaller micelle droplet of
microemulsion is formed, leading to greater efficiency for the catalytic
reaction, since the concentration between the two immiscible liquid
phases is increased. Table 13 shows the cyanation reactivity for various
concentrations of octanol as the co-surfactant when C15H33NMe3Br or
an organoclay are the phase transfer catalysts. These results indicate
cyanation reactivity increases with increasing co-surfactant
concentration for the biphase C15H33NMe3Br catalyst. For cyanation of
pentylbromide catalyzed by an organoclay, the effect of co-surfactant on
reactivity is dependent on the layer charge density of the organoclay.

For organoclay triphase catalysis, the co-surfactant does not change
the structure of the organoclay assemblies under triphase conditions.
The co-surfactants can merely be adsorbed on the catalyst surface, just
as other organic materials are. Consequently, the co-surfactant has little
or no influence on the catalytic reaction rate under triphase conditions
(Table 13). However, if the onium ion desorbs from the clay, then
biphase catalysis reactions will occur and the co-surfactant will increase
the catalytic reactivity. Onium ion desorption is especially significant
for those organoclays with high layer charge densities, such a F-
hectorite and Arizona montmorillonite. From the data in Tables 12 and
13, the increase in the catalytic reactivity caused by adding the co-

surfactants is correlated with the desorption of the surfactant from the

128

 

 

 

 

Table 13 Influence of octanol and tetradecanol co-surfactants
on the cyanation of pentylbromide by biphase
catalysts and triphase catalystsa.
Chemical conversion of pentyl
bromide to pentyl cyanidea
Mmole of 0 1 2 3 5 7
Co-Surfactant
Catalyst°/
Co-Surfactant
C15H33NMe3+Brb 23.3% 27.7% 30.8% 33.7% 46.5% 55.9%
fl‘etradecanol
L24AA 11.0% 12.8% 13.1% 11.6% 12.1% 11.7%
fl‘etradecanol
H24AA 22.4% 18.5% 18.2% 19.7% 19.6% 18.9%
fl‘etradecanol
A24AA 27.7% 30.5% 31.7% 33.0% 33.9% 34.8%
[Tetradecanol
F24AA 43.0% 42.5% 43.5% 46.2% 54.7%
[1' etradecanol

 

(Continued)

129

Table 13 (Continued)

 

Chemical conversion of pentyl

 

 

bromide to pentyl cyanidea
Mmole of 0 1 2 3 5 7
Co-Surfactant
Catalyst°/
Co-Surfactant

 

C151-133NMe3“'Brb 23.3% 28.4% 30.4% 36.7% 46.1% 52.1%
[Octanol

L24AA 11.0% 10.4% 9.5% 8.9% 9.1% 10.5%
/Octanol
H24AA 22.4% 19.7% 18.8% 22.6% 20.3% 20.8%
IOctanol
A24AA 27.7% 28.2% 27.0% 35.3% 34.9% 37.0%
/Octanol
F24AA 43.0% 41.5% 43.4% 42.9% 49.3% 56.9%
/Octanol

 

a. Reaction conditions: 3 mL aqueous solutions containing 2 mole
potassium cyanide, 6 mL toluene solution containing 10 mole of
pentylbromide and indicated amounts of co-surfactants; 0.100 g
organoclays were stirred in 90°C oil bath for 4 hours .
b. The surfactant, C16H33NMe3Br.was used as a micellar biphase
catalyst. The amount of surfactant used in this experiment was
equal to the amount of interlayer surfactant contained in
0.100 g H24AA.
c. L24AA: C15H33NMe3+Laponite; H24AA:C15H33NMe3+Hectorite
A24AA: C15H33NMe3+Montmorillonite (Arizona);
F24AA: C16H33NMe3+F-Hectorite

130

clay. The organo laponite (L24AA), organo hectorite (H24AA) and
organo Wyoming montmorillonite (W 24AA) with lower layer charge
densities are not influenced by less than 7 mole of co-surfactant. The
organo F-hectorite, F24AA, behaves almost the same as a biphase
catalyst, and the Arizona montmorillonite derivative, A24AA, is
moderately influenced by the co-surfactants. The high layer charge
density organoclays result in surfactant desorption when the co-
surfactant is added and the desorbed cationic surfactants undergo
biphase catalysis reaction.

(c) Catalytic properties of organoclays and surfactants for the

cyanation of alkyl bromide and bromoalkyl alcohol.

Organic reactants that have co-surfactant character such as 8-
bromooctan-l-ol are much more reactive for cyanation under biphase
reaction conditions than the analogous n-bromoalkanes. Under triphase
catalysis reaction conditions with organo hectorites as the catalysts, the
cyanation rates for bromoalkanols and bromoalkanes are comparable
(Table 14).

The activation energy for cyanation of. a bromoalkanol and
bromoalkane are similar, provided the reaction conditions are not
different. The relative reaction rates for the two organic electrophiles
are determined from the concentrations of the organic electrophile on
the clay surface (Figure 23). For organo hectorite triphase catalysis
reaction, reaction occurs at the catalyst-aqueous interface and the
concentration discrepancy between bromoalkanol and bromoalkane on
the clay surface is very small. Consequently, the cyanation rates for

these two reactants are comparable.

131

Under the biphase catalysis reaction conditions with
C15H33NMe3Br as the catalyst, the co-surfactant reactants can .form a
microemulsion with the cationic surfactant, and the aqueous and organic
solution. The co-surfactants are positioned at the reactive site, which is
at the interface of the surfactant in the aqueous phase (Figure 12).
Moreover, the reactive site for biphase micellar catalysis is also at the
micelle and liquid interface1341941074103. Consequently, the co-
surfactant reactants, bromoalkanol's, have higher surface concentration
in the region where reaction occurs. Since bromoalkane does not have a
hydroxyl group, the organic molecules are mainly located in the toluene
phase, which is not close to the liquid and micelle interface, causing this
reactant to have a low concentration in the region where reaction
occurs. Therefore, the cyanation reactivity of a bromoalkane is much
lower than the reactivity of a bromoalkyl alcohol under biphase
micellar catalysis reaction conditions with C16H33NMe3Br as the
catalyst.

The results in Table 14 also provide information about some clay
derivatives in which the surfactants desorb during the phase transfer
catalysis reaction. The F-hectorite derivative, F24AA, exhibits
behavior similar to that of biphase catalysts, due to the similar
discrepancy in catalytic reactivity between bromoalkane and
bromoalkanol. The surfactant in the organo F-hectorite, F24AA, can
potentially desorb from the silicate layer to carry out the reaction under
biphase catalysis condition. Organo clays with low charge densities do
not have the problem of surfactant desorption, so the reactivities

between bromoalkane and bromoalkyl alcohol are comparable.

132

Table 14. The cyanation of bromoalkanes and bromoalkanols under
biphase and triphase catalysis conditionsa.

 

Catalysts Conyemjonflc)

C3H17Br° BngHlépo
None 0 4
C15H33NMC3BI' 17 50
H24AA 21 27
A24AA 27 49
F24AA 23% 63

C10H2113r B“31011200H
None 0 0
C16H33NMC3BI' 14 65
H24AA 21 27
A24AA 25 26
F24AA 21 62

 

a. Reaction condition: 3 mL aqueous solution containing 10
mole potassium cyanide; 6 mL of toluene solution containing 3
mole of bromo compound; 0.100g organoclays or 0.060 mole

of C16H33NMe3Br were stirred in 90°C oil bath for 4 hours.

133

4. The Catalytic Properties of Crown Ether Clay Complexes

Crown ethers are well known biphase catalysts415»16:32109.110.
These molecules can be pillared into the clay interlayer82 and this pillar
clay can also facilitate the phase transfer catalysis reaction. However,
crown ethers supported in smectite clay interlayers desorb from the clay
host during the phase transfer catalysis reactions. Chemical yields for
benzyl bromide cyanation with clay crown ether complexes are shown
in Table 15. The chemical yields for cyanation with the clay crown
ether complexes are about twice those obtained in the absence of catalyst
under the same conditions. The crown ethers in the clay interlayer are
replaced by metal cations after the catalytic reactions, but the crown
ethers are not completely dissolved in the liquid solutions. The
recovered solid catalysts after one cycle do not have the same
diffraction pattern as the original clay crown ether complexes (Table
15), but they exhibit the IR absorption of the crown ether vibration
(Figure 29c). Figure 29a and 29b are the IR spectra of sodium
hectorite and 18C6Hect respectively. The IR absorptions at 1355, 1470
and 2900cm'1 are due to the crown ether vibration. Both the recycled
and original catalysts exhibit these absorptions.

The recovered solid catalyst still possesses good catalytic capacity
for cyanation (Table 15), although the structure of the clay crown ether
complex is changed after the catalytic reaction. While the catalytic
reactions employ the same amount of crown ether for phase transfer
catalysis, no insoluble crown ether can be isolated from liquid solutions.

The crown ethers are believed to adsorb onto the clay matrices after the

134

Table 15 Chemical yields for the cyanation of benzylbromide
to benzyl cyanidea.

 

Catalyst dam-Spacing (A) Chemical Yieldb (%)

 

Fresh Recycled lst Cycle 2nd Cycle
Catalyst Catalyst

 

18-crown-6 _ _ 71° _
18C6Hect 15.6 13.2 58 53
18C6Mont 17.4 13.1 69 68
Dicyclohexo _ _ 76° —
18-Crown-6

Hex18C6Hect 17.8 13.5 56 55
Hex18C6Mont 19.0 14.1 74 52
None 45

 

a. Reaction condition: 2 mole of benzyl bromide in 6 mL of
toluene; 2.5 mole of potassium cyanide in 1 mL of water; 0.100 g
clay supported catalyst or 0.060 mole crown ether; Reaction
temperature 90°C; Reaction time 12 hours

b. The yields were determined by GLC.

c. No catalyst can be recovered after the catalysis reaction.

135

catalytic reaction. The smectite clays does not change the crown ether
biphase catalysis mechanism, but the clay do act as an adsorbent for the

expensive crown ether after the nucleophilic substitution reaction.

D. The Applications of Organoclays in Other Triphase Catalysis

Reactions
1. The Synthesis of Symmetrical Formaldehyde Acetals

Many formaldehyde acetals can be synthesized by using
organoclays as triphase catalysts (Equation 10, 11 and 12). The
hectorite supported catalysts, H24AA, H26AA and the Arizona
montmorillonite supported catalysts, A24AA and A26AA, are employed
as catalysts for nucleophilic substitution reactions. Comeleis et
al.54’111’112 have used Tixogel VP, a montmorillonite organic
derivative, as the phase transfer catalyst to synthesize formaldehyde
acetal derivatives.

The catalytic reactions proceeded by refluxing the mixture of the
alcohol, dihalomethane and 50% sodium hydroxide aqueous solution at
90°C in an oil bath for 12 hours. The chemical structures and yields
were obtained by 1H and 13C NMR. The chemical yields of all the
formaldehyde acetals are shown in Table 16 and the N MR spectra data
for the acetal products are listed in Table 17. The yields of
formaldehyde benzyl acetal are almost 100% by using all the indicated
catalysts. The yield of the blank reaction is only 12% at the same
condition but in the absence of the triphase catalysts. The chemical
yields for the synthesis of formaldehyde n-butyl acetal and

formaldehyde iso-pcntyl acetal (Equation 11 and 12) are low,

136

Figure 29 The IR spectra of (a) sodium hectorite, d001=12.1A
(b) 18C6Hect, d001=15.6A and (c) recycled 18C6Hect
after one cycle of reaction, d001=13.2A. The absorptions
at 1355,1470 and 2900 cm" are due to a crown ether
vibration.

 

(a)

(b)

 

 

 

 

r l’ r I v I v r r l u r

4000 3550 3100 2850 2200 1750 1300 850 400

Wavenumber, cm'l

137

apparently due to the low acidity of the aliphatic alcohols. Reactions
represented by Equation 11 and 12 can not proceed without a phase
transfer catalyst.

All of these catalytic reactions employed a 50% alkaline solution,
which is highly destructive toward the layer silicates. Comeleis et
al.54.111.112 did not investigate the thermal stability of the organoclay,
Tixogel VP, after vigorous catalytic reactions. In fact, the
montmorillonite equivalent of Tixogel VP, in which the onium ion is a
dialkyldimethylammonium, undergoes surfactant desorption under
reaction conditions. Thus much of the reaction observed by Comeleis
et al. was due to biphase reaction. However we found that the hectorite
derivatives, H24AA and H26AA, can be recovered by filtration and
recycled many times for the same catalytic reactions (Table 16). The
XRD pattern (Figure 30) of the recycled H24AA shows that the
organoclay structure is unchanged after catalytic reactions.
Surprisingly, sodium hectorite is destroyed after treatment with 50%
sodium hydroxide solution even at room temperature, because the
original d001 reflection peak of Na-hectorite disappears (Figure 31).
Therefore, the surfactants in organo hectorite not only increase the
hydrophobicity of the material but also protect the silicate layer from
attack by the alkaline solution. The Arizona montmorillonite
derivatives, A24AA and A26AA, do not have the advantage of resisting
the alkaline solution. Trace amount of solid material could be
recovered by centrifuging the catalytic reaction mixture after the

catalytic reactions. The catalysts, A24AA or A26AA, are decomposed

138

Organoclay
cu,on+CH Br CH c1 ._ 0c Dew. 10
22, 2250%Naon °"’ "’ ()

Organoclay

€411,011 + CHzBr7/CH2C12 , C,H,0CH,0C,H,

 

50% NaOH

Organoclay
(CH ) CH(CH OH-l- CH Br CH C1 ———>
3 2 2)2 2 2/ 2 250% NaOH

C8H17OH + HBr(47%) O’gmday

 

*CanB'

Organoclay Q
c Bf ’ 01-3611,
.011, Hz 50% NaOH

 

 

(‘iibt—EL=Ern<lilb» :::::::fay -- {‘ii’)—cxx>-
.. ..

0
Or anocla
15% NaOH 0

N0: N0,

 

(11)

CH2[O(CH2)2CH(CH3)2]2(12)

(13)

(14)

(15)

(16)

139

Table 16. The chemical yields of the acetal synthesis
(Equation 10-12)3.

 

 

Entry R Group Catalyst Chemical Yield (%)
1 Benzyl H24AA >95
2 Benzyl H26AA >95
3 Benzyl A24AA >95
4 Benzyl A26AA >95
5 Benzyl C15H33NMe3Br >95
6 Benzyl C15H33PBu3Br >95
,7 Benzyl No 18
8 Benzyl Recycled H24AA >95
9 Benzyl Recycled H24AA >95
10 Butyl H24AA 37

1 l Butyl H26AA 50
12 Butyl A24AA 63
13 Butyl A26AA 58
14 Butyl C15H33NMe3Br 55
15 Butyl C15H33PBu3Br 56
16 Butyl No 0
17 Butyl Recycled H24AA 35
18 Butyl recycled H26AA 49
19 iso-Pentyl H24AA 32
20 iso-Pentyl H26AA 28
21 ' iso-Pentyl A24AA 58
22 iso-Pentyl A26AA 38
23 iso-Pentyl C15H33NMe3Br 36
24 iso-Pentyl C16H33PBu3Br 33
25 iso-Pentyl No 0
26 iso-Pentyl Recycled H24AA 30
27 Iso-Pentyl recycled H26AA 24

 

a. Reaction condition: 10 mole of alcohol; 103 CHzBrz; 10g

CH2C12; 10 mL 50% NaOH aqueous solution; reaction
temperature, 90°C; reaction time 12 hours.

Table 17 The chemical shifts in the 1H NMR of the starting
materials and the reaction products of the acetal

synthesis reaction (Equation 10-12).

 

 

Chemical Chemical shift (ppm)

C6H5CH20CH20CH2C6H5 W
4.87 ppm (s, 2H)
7.3-7.4 ppm (m, 10H)

C4H90CH20C4H9 0.86-0.94 ppm (t, 6H)

[(CH3)2CH(CH2)20]2CH2

C5H5CH20H

C4H90H

(CH3)2CH(CH2)20H

1.3-1.4 ppm (m, 4H)
1.45-1.55 ppm (m, 4H)

4.62 ppm (s, 2H)
0.90-0.92 ppm ((1, 12H)
1.40-1.51 ppm (q, 4H)
1.60-1.80ppm (m, 4H)

- a
4.62 ppm (s, 2H)

a

7.3-7.4 ppm (m, 5H)

0.86-0.94 ppm (t, 3H)
1.3-1.4 ppm (m, 2H)

1.45-1.55 ppm (m. 2H)
W

0.90-0.92 ppm ((1, 6)
1.40-1.51 ppm (q, 2H)

1.60-1.80ppm (m, 2H)
W

 

a. The underline marks are the absorption peaks used for measuring the
integration ratio of starting materials and reaction products.

141

and converted to smaller particles suspended in the aqueous phase. The
trace amounts of recycled Arizona montmorillonite derivatives, A24AA
and A26AA, do not retain their original structure following reaction, as
evidenced by X-Ray diffraction.

As is mentioned in this chapter, the surfactants in the high layer
charge density smectite clay group, such as Arizona montmorillonite,
will dissociate from the interlayer if the clay derivatives are suspended
in electrolyte solutions. Since the surfactants do not coat the surface of
the silicate layer, hydroxide ion can destroy the unprotected surface and
decompoe the mineral. The surfactant in a relatively low layer charge
density hectorite will not dissociate so the silicate layer surface is always
protected by the surfactant.

2. Synthesis of Alkyl Bromide

The long chain alkyl bromide can be synthesized by refluxing a
mixture of alkyl alcohol in concentrated hydrobromic acid. Montanari
et al.113 have utilized hexadecyltributyl phosphonium bromide as a
phase transfer catalyst to facilitate this reaction. The organo clay
(H26AA) is employed as the phase transfer catalyst for the alkyl
bromide synthesis (Equation 13). The catalytic reaction conversions are
better than the conversion for the blank reaction (Table 18). However,
the organo clay is totally decomposed by the strong acid and the
recycled solid material does not show any catalytic activity. The XRD
pattern of the recycled material is amorphous, with the original d001
absorption peak of H26AA absent. The hectorite supported catalysts can
tolerate alkaline solution but are destroyed by strongly acidic solution.
Fortunately, most phase transfer catalysis reactions are carried out

under neutral or basic condition.

142

Figure 30 The X-ray diffraction pattern of C16H33NMefhectorite
(H24AA) after the catalysis reaction in which the reaction
mixture contains 50% NaOH aqueous solution and 10 mL of

methylene chloride solution.
The peaks at 19A arises from H24AA. The 10A, 4 7.A and
2.4A peaks belong to the reactants.

1000
19A

4711

10A

 

400 (1 2AA

 

 

 

 

 

 

Figure 31

143

The X-ray diffraction pattern of sodium hectorite treated
with 50% NaOH aqueous solution. No reflection peak at
12.5A ccould be found. The reflection peaks at 4.7A and
2.4A are due sodium hydroxide.

 

 

4.7A

 

2.4.4

 

 

 

144

3. Dehydrohalogenation of 2-Bromoethylbenzene

Styrene, which is a very important industrial chemical for
polymerization, can be generated by treating 2-bromoethylbenzene in
strong alkaline aqueous solution using triphase catalysts such as
polyethylene glycol or organoclay (Equation 14). The chemical yields
of the reaction are determined by the GLC. Commercial styrene from
Aldrich Chemical Company shows the same retention time in the GLC
as the product obtained in the catalytic reaction. The chemical yield of
the styrene in the reaction catalyzed by H26AA and H24AA are shown
1 in Table 19. No styrene is formed in the absence of organo clay as
triphase catalyst. The organo hectorite preserves its original layer
structure after this catalytic reaction.
4 . Oxidation of trans-Stilbene

Organo clays can facilitate the oxidation of trans-stilbene under the
triphase catalysis condition (Equation 15). The product, benzoic acid, is
identified by. its melting point and NMR spectrum. Table 20 shows that
the chemical yields of the oxidation using organoclays as catalysts are
much better than the blank reaction without a phase transfer catalyst.
However, the organo clay catalysts are difficult to recycle due to the
large amount of manganese dioxide precipitate mixed-in with the

catalyst.

145

Table 18 The synthesis of octyl bromide from octanol by
hydrobromic acid dehydrationa (Equation 13).

 

 

Catalyst Concentration Reaction Chemical yieldb
of HBr (%) Time

C16H33PBu3+ 47% 11 Hours 79%

Hectorite

(H24AA)

None 47% 11 Hours 37%

C15H33PBu3+ 23% ' 11 Hours 0%

Hectorite

(H24AA)

None 23% 11 Hours 0%

 

a. Reaction Condition: 1 mL of 47% HBr or 2 mL or 23% of HBr;
2.5 mmole octanol; 0.060 mole catalyst; Reaction temperature
90°C.

b. The chemical yields were detennined by GLC.

146

5 . Synthesis of 2,4-Dinitrophenyl Ether

Organo clay triphase catalyst can also be employed for the synthesis
of 2,4-dinitrophenyl ether by treating l-chloro-2,4-dinitrobenzene and
phenol in 1.5N NaOH solution (Equation 16). The chemical yield of the
nucleophilic substitution reaction is almost 100% (Table 21). The
reaction product and chemical yield were determined by 1H NMR and
13C NMR (Figure 32). Tundo and Venturello47 have used silica gel
supported onium ions as the triphase catalysts to synthesize 2,4-
dinitrophenyl ether. However, the reaction occurs in strongly basic
solution. Since organo hectorite is stable in alkaline solution, this
hectorite derivative is a more suitable triphase catalyst for this synthesis

reaction.

147

Table 19 Synthesis of styrene from 2-bromoethy1benzene under

 

 

alkaline solutiona (Equation 14).
Catalyst Reaction Time temperature Yieldb doo1(A)¢
C16H33PBu3+ 13 hours 65°C 73% 21
-Hectorite
(H26AA)
C15H33PBu3+ 13 hours 65°C 70% 18
-Hectorite
(H24AA)
C15H33PBu3+ 10 hours 75°C 76% 21
-Hectorite
(H26AA)
C151133PBu3+ 14 hours 75°C 83% 21
-Hectorite
(H26AA)
None 13 hours 65°C 9%

 

a. Reaction Condition: 4 mole 2-bromoethylbenzene; 0.300g
Decane (Internal Standard); 4 mL Benzene; 0.100g Organoclay;
3 mL 50% NaOH aqueous solution.

b. The chemical yields were determined by GLC.

c. The d-spacing of the recovered catalyst.

148

Table 20 The formation of benzoic acid by the oxidation of
trans-stilbenea (Equation 15).

 

 

Catalyst Reaction Time Yield (%)b
C15H33PBu3+Hectorite 10 Hours 78
(H26AA)

None 10 Hours 63
C16H33PBu3+Hectorite 18 Hours 94
(H26AA)

None 18 Hours 81

 

a. Reaction Condition: 0.200 g H26AA; 3 mole trans-Stilbene; 6
mole KMnO4; 10 mL Benzene; 6 mL Water.

b. The yields were obtained by weighing the isolated benzoic acid after
the reactions.

149

Table 21 The synthesis of 2,4-dinitrophenyl ether from
l-chloro-2,4-dinitrobenzene and phenolatea

 

 

(Equation 16).
Catalyst Reaction Time Chemical Yieldb (%)
C1611133PBu3+ 4 Hours >95
Hectorite (H26AA)
C15H33PBu3+Br 4 Hours >95
None 4 Hours 0

 

a. Reaction Condition: 6 mole phenol and 1-chloro-2,4-
dinitrobenzene in 5 mL benzene; 6 mole sodium hydroxide in 5
mL of aqueous solution; 0.060 mole catalyst; reaction temperature
25°C.

b Yields were determined by the 1H NMR integration area of
starting material and reaction product (Figure 34).

The chemical product, 2,4-dinitrophenyl ether, was also
characterized by its melting point (68-7l°C).

150

Figure 32 The 1H NMR spectra of (a) 1-chloro-2,4-dinitrobenzene
and (b) 2,4-dinitrophenyl ether.

(a)

\07”

1 gag?
.ft/
*1

 

 

 

 

 

 

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4.70
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urlIIYr.tYII[ VVVVVVVVV rVYIYll'VVITI'IlYYYUI IIIIIIII IIIUVIIIVITIY'Y[IIIIIVY'Y'IVYYIVVYV.YYYVIV'V'] VVVVVV “I!
R” 80 84 8d 3) 7.8 76 7.4 7.;.' 709914 0H

CHAPTER IV
CONCLUSIONS

Organoclays provide different properties in triphase catalysis from
those of typical polymer supported triphase catalysts. The mechanistic
property of the clays in which the onium salts are immobilized on
polystyrene is similar to that of onium ion biphase catalysis. In this
mechanisml3'14’1‘5'17'61'62'l 14 the anionic nucleophile is paired with an
onium cation in the aqueous phase followed by extraction into the organic
phase to undergo the nucle0philic substitution reaction. With the polymer
supported catalyst the reactant must undergo mass transfer to the catalyst
surface, diffuse to the active site which is close to the region of the onium
ion, proceed with chemical reaction, and the reaction product must leave
the site. Any or all of these fundamental steps may limit the catalytic
reactivity.

Organo hectorite does not remove the nucleophile from the aqueous
phase to the organic phase. The clay organic derivative provides an
amphiphilic property which can stabilize a water in oil emulsion. Because
the nucleophilic substitution reaction can only occur at the liquid-liquid

interface in the absence of a phase transfer catalyst, formation of an

151

152

emulsion can dramatically increase the interfacial area of the oil and water
phase resulting in a higher probability for chemical reaction. In organo
hectorite triphase catalysis, only the organic reactant must undergo the
process of mass transfer and diffuse to the active site on the catalyst
surface. The nucleophile can freely transfer to the active site without the
two limiting steps. This is evidenced by the linear relationship between
the reaction rates and nucleophile concentration in bulk aqueous phase
(Figure 19a). However, the stronger solvation of the nucleophile in
aqueous solution results in a low nucle0philic ability for the anion. The
activation energy of reactions catalyzed by organo hectorites is
theoretically equal to that of the reaction proceeding in aqueous solution.
The function of the organo hectorite is to increase reactant
concentration in the active site. The nucleophilic substitution reaction
catalyzed by polymer supported catalyst is carried out in the organic
phase61’63'64’115. The function of the polystyrene supported catalyst is
(a) to lower the activation energy of the reaction pathway and (b) to
provide a phase with a high effective concentration of the potential
reactantllé:1 17. The first function, which can be achieved by changing
the chemical reaction environment from the aqueous to the organic phase,
results in enhanced nucleophilic ability of the anionic nucleophile. The
second can be fulfilled by increasing the lipophilic property of the
polymer supported catalyst. The catalytic ability of polystyrene supported
and free onium salts sometimes depend on the extraction coefficient of the
nucleophile between the aqueous phase and the organic phase15'17.
Hydrophobic anions such as SCN- and I- have high extraction
coefficients. However, high hydrophilic nucleophiles such as hydroxide

or chloride which have low extraction coefficients in these two liquid

153

solutions mediated by phase transfer catalysts, lead to a low effective
concentration of the nucleophile at the reaction site.

The typical phase transfer catalysts used for reactions that employ the
hydroxide ion as the nucleophile are expensive crown ether, cryptates or
derivatives of poly(ethylene glycol)31'42'55'57'118. These analogues can
hydrogen bond with the hydroxide, and then be extracted into the organic
phase. Organo hectorite is also a good triphase catalyst for reactions in
this strongly basic system. The hydrophilic anion does not have to be
transferred from the aqueous to the organic phase so the reaction is not
limited by the extraction coefficient. The organic solvents in triphase
catalysis systems also influence catalytic reactivity. In a triphase catalysis
reaction with a polystyrene supported catalyst, use of non-polar organic

46:61. The extraction

solvents results in low catalytic reactivity
coefficients of anionic nucleophiles are low in non-polar organic solvents.
In addition, the low swellability of the ion pair, onium cation and anionic
nucleophile, in the non-polar organic solvent leads to difficulty with mass
transfer and diffusion of reactants to the active site. Use of inexpensive
and non-toxic non-polar organic solvents such as decane in organoclay
triphase catalysis reactions can improve the chemical reactivity relative to
that of more expensive polar organic solvents, such as toluene and o-
dichlorobenzene. The low affinity of the non-polar organic solvent and
the organic reactant results in ease of mass transfer of the organic
electrophile from the bulk organic phase to the organoclay surface. Then
the catalytic reactivity is high because of the high concentration of the
organic reactant on the organoclay surface. Although an organoclay
triphase catalyst does not significantly reduce the activation energy of the

chemical reaction, the independence of the extraction coefficient of the

154

anionic nucle0phi1e provides the advantage that a larger variety of
nucleophiles and non-polar organic solvents can be appropriately used in
the triphase catalysis system. Another kind of triphase catalyst is that in
which the onium ions are supported in inorganic matrices such as silica
gel or a1umina47'48. Chemical reactions using these inorganic-based
materials as triphase catalysts are believed to occur at the liquid-catalyst
interface49'50. The catalytic reactivity of the inorganic-based material
also increases with decreasing the polarity of organic solvent47'49’69. The
advantage of these materials are their physical strength and the lack of
swelling119. Smectite clays also have strong rigiditylzo. Organoclays
swell in organic solvents. A high polar organic solvent, such as o-
dichlorobenzene, dramatically swells organo hectorite. However, a high
polar organic solvent strongly solvates the organic electrophile and limits
mass transfer of the organic molecule with the result of poor catalytic
activity.

The swellability of polystyrene supported onium salts in organic
solvents plays an important role in the catalytic reactivity of the triphase
catalysis reaction. All other factors being equal, reaction rates decrease
with increased cross-linking whenever rate is limited by intraparticle
diffusion 58:59:64’67’119. The more highly cross-linked the catalyst, the
lower is the diffusion of reactants to the active sites. However, the most
common 1% and 2% cross-linked resin in solvent-swollen form is too
gelatinous to be recovered by filtration1 19. Although catalytic activity is
most studied in phase transfer catalysis, recyclability is an aspect in
triphase catalysis that is sometimes the most important. Good polymer
supported catalysts with great swellability in organic solvent may be
difficult to recycle. High cross-linked polystyrene supported catalysts

155

more easily recovered but are lower in catalytic activity than those of low
cross-linked polymers. Organoclay is easily recovered by filtration or
centrifugation after the chemical reaction. The main disadvantage of
organoclay for recycling is that some of the onium ions in the clay
interlayer may desorb from clay hosts with high layer charge densities.
Because the bond between the clay layer and the onium ion guests is
electrostatic, metal cations from the aqueous phase may substitute for the
onium ions. However, hectorite as the host overcomes the problem of
onium ion desorption. The interlayer onium ions in this medium layer
charge density clay host are not ion-exchanged by metal cations from the
aqueous solution. This makes the hectorite derivative a good triphase
catalyst in terms of longevity.

The chemical instability of the triphase catalyst under reaction
conditions also results in poor recyclability. Ammonium and
phosphonium salts may be destroyed during the reaction. This is
especially true for quaternary ammonium salts, which undergo Hoffmann
elimination in strongly alkaline solution and produce trialkylamine and
alkene products”. Most triphase catalysts have encountered this
problemfior121 and so does organoclay. The catalytic activity of
hexadecyltrimethyl ammonium hectorite gradually decays in each
cyanation reaction. Before 50% of the catalytic activity of the organo
hectorite is lost, more than 100 turnovers of the chemical reaction have
been achieved. This organo hectorite is still valuable in this triphase
catalysis cyanation reaction. Quaternary phosphonium is generally more
stable than quaternary ammonium under reaction conditions”. Thus
hexadecyltributyl phosphonium hectorite can preserve its catalytic

capacity better than the quaternary ammonium hectorite. More than 300

156

turnovers of the reaction have been achieved while the hectorite supported
phosphonium still maintains 85 % of the catalytic activity. The 15 % loss
of reactivity is believed to caused by loss of the solid catalyst during
reaction work-up. The hectorite matrix can sometimes reduce the
chemical decomposition of the interlayer guests. For example,
triphenylbenzyl phosphonium and benzaldehyde in hydroxide solution
will undergo the Witting reaction to form trans-stilbenelzz. When the
triphenylbenzyl phosphonium ion was intercalated into the hectorite
interlayer, the reaction could not be observed under the same reaction
Conditions. Therefore, hectorite can more or less prevent the interlayer
guest from participating in the chemical reaction. Silica gel and alumina
are unstable in hydroxide solution and this may limit the use of the
inorganic-based catalysts for some chemical reaction. Silica gel is even
unstable in cyanide solutionl 19. Sodium smectite clays including
hectorite are also unstable in strongly basic solutions. However, as the
interlayer sodium cations of hectorite are ion-exchanged by cationic
surfactants, the material is durable in this vigorous reaction environment.
This organo hectorite can be recovered after use in a triphase catalysis
reaction with hydroxide as a reagent and the recycled catalyst is still
active.

Generally, the reactivity of triphase catalysis is less than that of
biphase catalysis or homogeneous reaction with different polar aprotic
compounds as solvents. The catalytic functional groups anchored on the
macromolecule supports can not move as freely as the analog dissolved in
liquid solution. Hexadecyltributyl phosphonium ion performs catalytic
activity five times more than that of the similar phosphonium group

supported on polystyrene61 . This phosphonium ion also shows four times

157

more catalytic activity than the organo hectorite which contains the same
equivalent of phosphonium in the hectorite interlayer. An exceptional
situation is that in which the hexadecyltrimethyl ammonium hectorite is
more catalytically active than the free surfactant. This surfactant proceeds
by micellar catalysislsrz‘uzg”124 which is slower than the typical biphase
catalysis that uses the onium ions with four bulky alkyl groups attached on
the nitrogen or phosphine. The organo hectorite undergoes a different
mechanistic process and results in a faster catalytic reaction. Polar aprotic
solvents such as DMF or THF are used typically for homogeneous
reactions. These solvents have strong solvation for metal cations but
weak affinity for anionic nucleophiles. They can reduce the shielding of
electrons in anions and strengthen the nucleophilicity of the anionic
reactants. However, these kinds of chemicals are usually expensive and
difficult to remove after reaction and may present environmental problems
in large scale operation.

Triphase catalysts not only have the advantage of catalyst recycling,
but sometimes provide various selectivities for chemical reaction,
re giochemistry, or even stereochernistry. Polystyrene supported onium
salts proceed by a mechanistic process similar to that of biphase catalysis.
This may result in analogous products to those obtained by using these
two materials as. the phase transfer catalysts. For example, the O-
alkylation product (Figure 7) predominates in the alkylation reaction of
benzyl bromide and naphthoxide when ether polymer supported or free
onium salts are used as catalysts since the alkylation reaction“64 occurs
at the organic environment. With organo hectorite the reactions proceed
in the aqueous instead of the organic phase, so the C-alkylation product

predominates in this catalytic alkylation reaction. Also, when the cationic

158

surfactant hexadecyltrimethyl ammonium bromide is used as the micellar
catalyst for the cyanization of n-bromoalkanol or l-bromoalkane, the
former reactant is much more reactive than the latter. However, as the
cationic surfactant is intercalated into the hectorite interlayer and used as
the triphase catalyst for the same cyanization reactions, the reactivities of
both reactants are comparable. The reactant, n-bromoalkanol can change
the micelle structure in micellar catalysis but l-bromoalkane can not.
However, neither of the two reactants can alter the mechanistic structure
of organo hectorite triphase catalysis. Clay supported or free chiral
ammonium such as the (-)-benzquuininium ion can be also used as chiral
catalysts for asymmetric synthesis reactions. Very interestingly, these two
materials always tend to produce the predominate enantiomeric products
with opposite configuration. In borohydride reduction reactions of
alkylphenylketone125'126, epoxidation of chalcone127 and Michael thio-
addition of 2-cyclohexenone128'129, the major enantiomeric products that
results from the tWo chiral catalysts have opposite configurations. The
orientations of the onium ion are different on hectorite and in liquid
solution, leading to a different spatial constraint and resulting in opposite
configurational enantiomers. Moreover, clay-supported and free chiral
onium ions proceed, via asymmetric synthesis reactions in two different
liquid phases. Organic hectorite proceeds by phase transfer catalysis
reactions in aqueous environment but free onium ions carry out the
analogous reactions in the organic phase. The difference of reaction
environment may also provide a different selectivity in the asymmetric

synthesis.

10.
11.

l2.

13.

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