Development and reactivity of novel heterogeneous catalysts for hydrocarbon conversions

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Development and reactivity of novel heterogeneous catalysts for hydrocarbon conversions
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xiii, 161 leaves : ill., photos. ; 29 cm.
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Chemistry thesis Ph. D
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Thesis (Ph. D.)--University of Florida, 1992.
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Includes bibliographical references (leaves 151-160)
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by Steven C. Petrosius.
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Typescript.
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Vita.

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DEVELOPMENT AND REACTIVITY OF NOVEL
HETEROGENEOUS CATALYSTS FOR
HYDROCARBON CONVERSIONS















By

STEVEN C. PETROSIUS


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1992






























This work is dedicated to my wife, Sandra, and my son, Nicholas. It has truly

been their dedication which has made it possible.




























"The known is finite, the unknown infinite; intellectually we stand on an islet

in the midst of an illimitable ocean of inexplicability. Our business in every

generation is to reclaim a little more land."

T. X. Huxley, 1887












ACKNOWLEDGEMENTS


The content of this thesis and the years of study it represents have in no

small way been inspired by a multitude of friends, family and co-workers. Initially,

I would like to express my gratitude to Dr. Russell Drago, my research advisor, for

his helpful criticisms and for sharing his enthusiasm of chemistry. Doc's ideas

about dedication and fairness will stay with me for many years. Appreciation goes

to Ruth Drago for her always generous hospitality and for making UF a "home

away from home" for myself and my family. Maribel Lisk and April Kirch deserve

special thanks for their assistance in cutting through the seemingly endless

amounts of red tape one encounters at UF.

Present Drago group members Mike Naughton, Don Ferris, Steve

Showalter, Doug Patton, John Hage, Chris Chronister, Todd Lafrenz, Mike

Robbins, Robert Beer, Krzysztof Jurczyk, Phil Kaufman and Karen Frank have all

been invaluable sources of ideas as well as good friends. Former group members

Al Goldstein, Tom Cundari, Jerry Grunewald and Bob Taylor deserve recognition

for showing me the ropes and tolerating sometimes stupid questions. The UF

glass shop and machine shop are acknowledged for their technical contributions.

Finally, I would like to thank my family. Mom and Dad P. as well as Mom

and Dad H. have shown exceptional moral support in this endeavor despite the







fact that I cannot always abbreviate the chemical jargon adequately to explain my

work. The two people who deserve my deepest gratitude are my wife, Sandra,

and my son, Nicholas. They have never faltered in their love or support. That

they have endured being 1500 miles from home and living the modest lifestyle of

graduate school is a testament to their dedication which I will cherish always.












TABLE OF CONTENTS



ACKNOWLEDGEMENTS ..................................... iv

LIST OF TABLES ........................................... viii

LIST OF FIGURES ............................................ x

ABSTRACT .............................................. xii

CHAPTERS

1 GENERAL INTRODUCTION ......................... 1



2 REACTIVITY AND STRUCTURAL CHARACTERISTICS
OF A NOVEL SUPPORTED ALUMINUM CHLORIDE
CATALYST ....................................... 6

Background ........................................ 6
Reactivity Studies .............................. 6
Synthetic Studies .............................. 12
Experim ental ...................................... 17
Results and Discussion .................. ........... 24
Reactivity of AICl2(SG)n in Hydrocarbon Conversions 24
Dehydrohalogenation and hydrodehalogenation 25
Friedel-Crafts alkylation ................... 30
Low-temperature isomerization of butane ...... 34
Reactions of n-hexane with AlCl2(SG)n ........ 42
Cracking of hydrocarbon polymers ........... 46
Synthesis Variations ............................ 54
Silica hydration studies . ... ........ 54
Sealed-system catalyst preparation ............ 67
C conclusions ....................................... 74
Reactivity Studies ............................. 74
Synthetic Studies .................. .......... 77











3 POROUS, CARBONACEOUS ADSORBENTS AS
CATALYSTS AND CATALYST SUPPORTS FOR ACIDIC
METAL OXIDES ................................. 79

Background ....................................... 79
Experimental .................................... .. 93
Results and Discussion .............................. 98
Catalytic Combustion Reactivity Studies ............. 98
Kinetic Experiments .......................... 106
Oxidative Dehydrogenation of 1-Butene ............ 120
Catalyst Characterization ....................... 123
X-ray photoelectron spectroscopy ........... 123
Magnetic susceptibility ................... 129
Scanning electron microscopy .............. 133
Porosimetry ........................... 136
Conclusions ...................................... 142



4 SUMMARY ......................... .. ........... 148

Supported Aluminum Chloride ........................ 148
Porous Carbon Adsorbents .......................... 149

REFERENCE LIST .......................................... 151

BIOGRAPHICAL SKETCH ................................... 161












LIST OF TABLES


Table Page

2-1. Product Distributions for the Reaction of 1,2-Dichloroethane Over
Various Metal-Doped Catalysts ................... .. .... 26

2-2. Friedel-Crafts Alkylation with AICI2(SG)n. ..................... .32

2-3. Products from the Reaction of N-Hexane over AlCl2(SG)n. ........ 44

2-4. Product Distribution from the Reaction of N-Hexane Over
AICl2(SG)n, 1-Hexene Promoter Present .......... ............ 45

2-5. Cracking of Polymers Over AICl2(SG)n ...................... 48

2-6. Reactivity of AICl2(SG)n Catalyst Samples Prepared with
Silica in Varying Degrees of Hydration ....................... 59

3-1. Structural Parameters of Carbonaceous Adsorbents ............... 94

3-2. Activity of Various Metal Oxide/Porous Carbon Catalysts in the
Catalytic Combustion of Methylene Chloride. .................. 100

3-3. Activity of Base-Treated Carbon Adsorbents for the Catalytic
Combustion of Methylene Chloride ......... ............... 102

3-4. Values of the Rate Constant for the Catalytic Combustion of
Methylene Chloride at Various Flow Rates ..................... 107

3-5. Reactivity of Cr03/563 in the Catalytic Combustion of Halogenated
Organics. .............................................. 111

3-6. Influence of Water on 1,2-Dichloroethane Reactivity Over Carbon
and Zeolite Catalysts...................................... 113

3-7. Bond Energies for Chlorinated Methane Compounds ............. 115







Table Page

3-8. Deep Oxidation of CCI4 Over Ambersorb 563 ................. 116

3-9. Elemental Analysis of Ambersorb 563 Before and After CC14 Deep
Oxidation. ............................................. 118

3-10. Summary of XPS Data for Chromium Catalysts ................. 128

3-11. Porosimetry of Ambersorb 563 and CrO3/563. ................ 141













LIST OF FIGURES


Figure

1-1. Proposed Structure of AlCl2(SG)n. ...............

2-1. Reaction of A12C16 with Sulfonated Resins. .........

2-2. Structures of Various Hydrocarbon Polymers. .......

2-3. 27Al SS MAS NMR Spectrum of AICl2(SG)n ........

2-4. Equilibrium of Silanol Groups with Siloxane Groups on

2-5. Schematic of Batch Reactor ...................

2-6. Schematic of Flow System. ................... ..

2-7. Dehydrohalogenation of 1,1,1-Trichloroethane Over
RuCI3ZnCl2AICl2(SG)n. ......................

2-8. Cumene Dealkylation. ........................

2-9. Mechanism for the Promotion of Butane Isomerization
1-Butene ..................................

2-10. Isomerization of N-Butane over AlCl2(SG)n. .......

2-11. Sequential Batch Cracking of Poly(ethylene) with
AICl2(SG)n. .................................

2-12. Model of Silica Gel Surface Illustrating the
Dehydration/Rehydration Equilibrium. ............


Silica ....


....by
by
. . .


2-13. Infrared Spectra of Pyridine Adsorbed on AICl2(SG)n Prepared with
Conditioned Silica and Dry Silica............................


Page

. 2

. 7

. 11

. 15

. 16

. 20

. 21


. 29

. 35


. 38

. 39


. 53


. 57


. 63







Figure Page

2-14. SS NMR of AICl2(SG)n Catalysts.
a) Prepared with Dry Silica.
b) Prepared with Conditioned Silica ......................... 65

2-15. Infrared Spectra of Pyridine Adsorbed on AICl2(SG)n Samples
Prepared in CCI4 and in a Sealed System, 72 Hour Reaction ........ 68

2-16. Infrared Spectrum of Pyridine Adsorbed on AlCl2(SG)n Prepared in
Sealed System, 18 Hour Reaction. ........................... 69

2-17. Infrared Spectrum of Pyridine Adsorbed on AICl2(SG)n Prepared in
a Sealed System, Desorbed at 300 C........................... 71

2-18. SS MAS NMR of AlCl2(SG)n Prepared in a Sealed System ......... 73

3-1. Structure of Pyrolyzed Poly(styrene/divinylbenzene) at
Various Temperatures. .................................... 81

3-2. Proposed Surface Structure of Silica-supported
Chromium(VI) Oxide. .................................... 91

3-3. Influence of Air on the Catalytic Activity of CrO3/563
in CH2C12 Deep Oxidation. ............................... 104

3-4. Arrhenius Plot for CH2CI2 Deep Oxidation with Ambersorb 563. .. 109

3-5. Low-Temperature Catalytic Combustion Using a Water Co-Feed. ... 121

3-6. Oxydehydrogenation of 1-Butene with Various Catalysts ........... 124

3-7. XPS Spectrum of CrO3/563 Before Reaction, 0 = 0 ........... 125

3-8. XPS Spectrum of CrO3/563 Before Reaction, 0 = 90 ............ 127

3-9. SEM of Ambersorb 563, 6000 Magnification. ................. 134

3-10. SEM of Ambersorb 563, 30,000 Magnification ................. 135

3-11. SEM of 563/CrO3, 6000 Magnification. ....................... 137

3-12. SEM of 563/CrO3, 30,000 Magnification. ..................... .138













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

DEVELOPMENT AND REACTIVITY OF NOVEL
HETEROGENEOUS CATALYSTS FOR
HYDROCARBON CONVERSIONS

By

Steven C. Petrosius

August, 1992


Chairperson: Dr. Russell S. Drago
Major Department: Chemistry

Preparation of the novel solid acid catalyst AICl2(SG)n has been

accomplished in a sealed tube vapor deposition process using an extended time

period comparable to that for a previously reported CCI4 synthesis. Reactivity

studies and spectroscopic investigations indicate a similar structure for surface acid

sites in the catalysts prepared via the two different methods. It has also been

determined that excessively dry silica is deficient in surface silanol (Si-OH)

moieties such that effective reaction of Al2Cl6 is not achieved. Exposure to humid

air is sufficient to hydrate the silica to a level where the active catalyst can be

prepared.

The activity of AlCl2(SG)n in a number of acid-catalyzed reactions has

been studied. In the alkylation of benzene and toluene, the catalyst was found to







be active in batch reactions but gives poor selectivity at elevated temperature in a

flow reactor. The isomerization of butane and hexane have been examined with

especially successful results obtained for the butane reactant. Cracking of

hydrocarbon polymers as a possible recycling procedure is promising with

AlCl2(SG)n. At 100 C in a batch reactor the activity of the catalyst for cracking

polymers is comparable to that observed with n-hexadecane.

Catalysts comprised of metal oxides dispersed on porous carbons are very

effective in the oxidative decomposition of halogenated hydrocarbons at 250 C in

air. Methylene chloride, 1,2-dichloroethane, 1,2,4-trichlorobenzene,

tetrachloroethylene, tetrachloroethane and carbon tetrachloride are decomposed

to CO2, CO and HCI over the carbon catalysts with high selectivity to HCI for the

chlorinated products. Trends in reactivity with support variation indicate that a

determining factor in the activity of the catalysts appears to be the micropore

volume of the carbon support. The presence of water in the reaction mixture has

no effect on the activity of the carbon catalysts. Reactions comparing CH2CI2

with CD2CI2 indicate a rate dependence on cleavage of a carbon-hydrogen bond.

Kinetic studies in the methylene chloride experiment show a first order

dependence on CH2C12 concentration and an activation energy of 11.0 kcal/mol.












CHAPTER 1
GENERAL INTRODUCTION


Solid acid catalysis is an indispensable tool for modern industrial chemistry.

The ability of heterogeneous acids to activate carbon-hydrogen bonds is a key

aspect to their versatility. In fact, the conversion of crude oil to hydrocarbon fuels

is the biggest use of heterogeneous acid catalysis today; at 1 x 1012 kg/year, crude

oil refining even outpaces sulfuric acid production in volume.1 Solid acids typically

exist in one of three forms: inorganic oxides (SiO2, TiO2, ZrO2, etc.), organic

polymers with acidic functional groups (ion-exchange resins) or acidic metal

compounds deposited on supports, which may be either inert or active themselves.

A novel solid acid has been developed which consists of chemisorbed

aluminum chloride on silica gel, denoted AICl2(SG)n.2-4 Characterization by IR

and NMR spectroscopy indicates the presence of surface tetrahedral aluminum

sites; an example is illustrated in Figure 1-1. This representation is a simplified

model of one possible surface site. It is well established that the catalyst possesses

both Lewis acid sites (Al centers) as well as Br6nsted sites (adjacent -OH groups).

The unique stability observed for the catalyst arises from the condensation of

A12C16 with the surface hydroxyl groups on the silica support. There is a 72 hour

reflux period for the catalyst preparation which allows full reaction of the metal

species, in contrast to typical vapor deposition methods5-11 which use shorter










CI


Cl
All
0


/


0


Iiii,... -


Figure 1-1. Proposed Structure of AIC12(SG)n.









contact times and produce catalysts with very short lifetimes due to desorption of

Al2Cl6 from the support. AIC12(SG)n has been shown to be active in cracking3'4

and dehydrohalogenation/hydrodehalogenation reactions12 under conditions where

typical zeolite or silica/alumina catalysts are inactive.

The application of AIC12(SG)n to a number of other acid-catalyzed

reactions is presented in Chapter 2. The catalyst has demonstrated activity in

dehydrohalogenation of 1,1,1-trichloroethane, alkylation of benzene and toluene

with a wide range of alkylating agents, room temperature isomerization of n-

butane, cracking and isomerization of n-hexane and finally cracking of

hydrocarbon-based polymer materials. The reactivity profile generated by these

studies is valuable for understanding catalyst structure, such as the existence of

Br6nsted acid sites and the relative acidity of AlCl2(SG)n as compared to other

strong acids.

Structural examinations of AICl2(SG)n are also presented in Chapter 2. A

sealed-system catalyst preparation method was devised to avoid the use of

potentially hazardous chlorinated solvents. Spectroscopic investigations and

reactivity comparisons indicate a virtually identical structure for the sealed-system

and CCl4-prepared catalysts. An important aspect of this synthesis procedure is

the extended time period, greater than 24 hours, that the components are allowed

to react. The hydroxyl content of the silica support is a parameter which was

found to notably influence catalyst structure and reactivity. Examination of

catalysts prepared with silica at varying degrees of hydration has illustrated that







4

the active catalyst can only be prepared with "conditioned" silica; that is, silica with

sufficient quantity of geminal hydroxyl groups (>Si(OH)2). The facile

condensation of these groups under relatively mild conditions has been

investigated in terms of catalyst structure and reactivity.

The AlCl2(SG)n catalyst is an example of a conventional acidic metal

species supported on an inorganic oxide; the preparation itself imparts unique

features on the catalyst structure. Chapter 3 presents the investigations of

catalysts where the supports themselves are novel materials: highly porous

carbonaceous adsorbents prepared by a patented pyrolysis of macroreticular

polymers. These materials are essentially hydrophobic in nature, consisting of an

extended aromatic framework13 which can be modified by various activation

methods to contain a selected level of surface hydrophilic character. The carbons

are studied as both catalysts and catalyst supports. Low-temperature oxidative

decomposition of chlorinated hydrocarbons has been very successful using metal

oxides supported on carbon adsorbents. Complete oxidation of methylene

chloride to carbon monoxide, carbon dioxide and hydrogen chloride is

accomplished at 250 C under an air atmosphere. Other halogenated

hydrocarbons show comparable reactivity in this reaction. One other successful

reaction using the carbon catalysts is the oxidative dehydrogenation of 1-butene.

Carbonaceous materials are known to catalyze oxidative dehydrogenation

reactions, predominantly with ethylbenzene, and in this study the activity of metal

doped carbons surpasses conversions and selectivities of conventional metal oxide catalysts.








5

A number of experiments have been performed to reveal mechanistic

information about the reactions performed with the carbon catalysts. Parameters

such as catalyst acidity, pore structure, surface area, oxygen concentration and

metal dopants are examined with respect to the reactivity of the catalysts in

standard catalytic combustion reactions. Catalyst characterization also reveals

information about how the carbons function as catalysts supports and as catalysts

without metal dopants.












CHAPTER 2
REACTIVITY AND STRUCTURAL CHARACTERISTICS
OF A NOVEL SUPPORTED ALUMINUM CHLORIDE CATALYST


Background


The idea of using a support to modify the reactivity of anhydrous aluminum

chloride was attempted as early as the 1950s by combining A12C16 with oxide

supports.14 This work continued sporadically throughout the 1960s and early

1970s5,8,11,15 but it was not until the OPEC oil embargo of the late 1970's that

vigorous efforts to improve upon existing technology resulted in many new

catalysts and processes being developed based on Al2C16.6'7'9'10'16-22


Reactivity Studies


Fuentes and Gates19 and Fuentes et al.23 have investigated the vapor

deposition of A12C16 onto sulfonated silica and sulfonated poly(styrene-

divinylbenzene) ion exchange resins. Elemental analyses and observation of HCI

evolution indicated that aluminum chloride was chemically bound to the supports,

as shown in Figure 2-1.24 Isomerization of n-hexane and n-butane at

temperatures less than 100 C was accomplished using these "superacid" catalysts.

The preparation of the Gates catalysts and their proposed structures are






















S OO
4 S -OH + A12C16







2 Q ----Al-O-S O H+ + 4 HCI
0 Cl O


Reaction of A12C16 with Sulfonated Resins.


Figure 2-1.









remarkably similar to that for AlCl2(SG)n; it is expected, therefore, that similar

reactivity in hydrocarbon conversion reactions can be demonstrated.

Aluminum chloride is probably best known as a catalyst for the alkylation

of aromatic compounds, the so-called "Friedel-Crafts" reaction.25-27 Most of the

recent developments in this area involve using various metal salts as homogeneous

catalysts. Olah and co-workers, for example, have recently developed boron,

gallium and aluminum trifluoromethanesulfonate (triflate) catalysts for aromatic

alkylation.28 The reported advantages of these triflate catalysts over the chloride

analogs are improved activity and, in particular, decreased volatility. Application

of these materials as heterogeneous catalysts was mentioned as well. It is

apparent that a need exists for more convenient Friedel-Crafts catalysts; solid

acids are a natural choice in this application.

Studies have been done with transition metals supported on inorganic

oxides29 in the alkylation of benzene with aromatic halides, offering fairly

successful results. No mention is made, however, of the use of less reactive

aliphatic halides. The superacid polymer, Nafion-H, has been utilized in these

alkylation reactions30 and offers impressive reactivity even with relatively

unreactive alkenes, but the polymer is cost-prohibitive for use in any large-scale

operations. These examples illustrate the need for more efficient heterogeneous

catalysts in Friedel-Crafts systems. The AICl2(SG)n catalyst has been shown to

have more moderate activity than pure, anhydrous A12C16; therefore, application

of supported aluminum chloride in aromatic alkylations may offer such advantages









as less polyalkylation products (improved selectivity) and the typical assets

accorded to heterogeneous systems such as ease of separation from reaction

mixtures and better stability.24

The initial step of the Friedel-Crafts reaction with alkyl halides generally

entails coordination of the halide to the Lewis acid center to form an ion pair, as

illustrated in equation 2-1:25



R-X + AIC13 <=====> R+AlCIl4 (2-1)



Once this species is formed, the carbocation R+ will normally undergo reaction

with the aromatic reactant, generating a proton which can regenerate the Lewis

acid by formation of HClg).

In the absence of other reactants, the carbonium ion R+ can undergo

deprotonation to produce an olefin.31 Once again, the catalyst can be regenerated

by HCI formation and this reactivity is the basis for acid-catalyzed

dehydrochlorination chemistry, illustrated in equation 2-2:



R-CH2-CH2-X + AIC13 ----> [R-CH+-CH3/AICI4]

----> R-CH=CH2 + AIC13 + HCI (2-2)








The AICl2(SG)n catalyst has been used in the dehydrochlorination of 1,2-

dichloroethane and 1,1,1-trichloroethane. The results will be discussed later in this

chapter.

Cracking of aliphatic, long-chain hydrocarbons with AICl2(SG)n has been

thoroughly explored by Drago and Getty.2b3'4 Exceptional activity and selectivity

to C3 C5 products was observed with n-hexadecane. An interesting similarity

exists between hydrocarbon polymers and long-chain hydrocarbons; namely, the

structures consist mainly of repeating -CH2- units along with -CH3 end groups.

Because the important step in catalytic cracking of paraffins involves hydride

abstraction from a -CH2- unit to produce a carbonium ion, cracking of polymers

should be feasible over acid catalysts much in the same way hydrocarbons are

cracked. Figure 2-2 presents the structures for a selected group of polymers

utilized in cracking experiments with AICI2(SG).n

There has been a limited amount of work done in catalytic polymer

degradation using silica/alumina32-34 and activated carbon catalysts.34'35 These

systems, however, have a number of disadvantages for practical applications. The

temperatures reported in these studies are at minimum 280 C, with temperatures

of 500 600 C generally needed to effect reasonable conversions. One apparatus

used34 consists of a pressurized liquid flow system which is elaborate and can

present difficulties in separating reactants, products and catalyst. These studies

indicate that AlCl2(SG)n should be useful in polymer cracking because the

reported conditions with silica/alumina are the same as for cracking of monomeric










-(CH2CH2)-n


-(CH2-CH=CH-CH2)-n


-(CH2-CH)-


I
CH
HzC H CH3


Poly(ethylene)


Poly(1,4-butadiene)


Poly(4-methyl-l-pentene)







Poly(styrene)


Cellulose Acetate

(R = H, COCH3)


Figure 2-2. Structures of Various Hydrocarbon Polymers.









hydrocarbons. Furthermore, AICl2(SG)n is known to be active at lower

temperatures and pressures where aluminosilicates are inactive.



Synthetic Studies


Schmidt and co-workers have outlined a process for synthesizing and

utilizing an aluminum chloride (on alumina) catalyst for the isomerization and

decyclization of hydrocarbon feeds comprised of C4 to C6 aliphatic

hydrocarbons36 and in some cases solely C6 hydrocarbons.37 Certain aspects of

their catalyst which make it noteworthy to the research presented in this thesis

include the use of an inorganic oxide support (alumina), use of A12C16 as the

acidic component in the level of 2 10 % by weight, need for a chloride source in

the reaction feeds to maintain catalyst activity, deactivation by contact with water

or organic oxygenates and finally deactivation by aromatics. These characteristics

are virtually identical to those observed with AlCl2(SG)n.2-4 The patents which

delineate the Schmidt procedure offer no physical characterization of the catalyst

with which to compare to AIC12(SG)n, so the solid acids cannot be compared

spectroscopically.

Preparation of the Schmidt catalyst entails contacting alumina with

sublimed A12C16 at high temperature for an extended period of time, reported to

be a minimum of 45 minutes at 550 C. This is considerably longer than the

contact times utilized in typical vapor deposition techniques and, as suggested by

the CCI4 studies, may therefore be the distinguishing feature which allows the








catalyst to possess its unique reactivity. Carbon tetrachloride itself is a solvent

with poor solvating ability and has been used extensively in studies of donor-

acceptor interactions to produce results consistent with the gas phase studies.

Therefore, its only function in the synthesis of AlCl2(SG)n could be to provide an

inert medium for the reaction of Al2C16 with the support. By this reasoning, it

should be possible to prepare AICl2(SG)n in the absence of CCI4 if a suitable

method is devised which affords long contact time and uniform contact of A12C16

and silica. A sealed tube reactor containing silica gel and Al2Cl6 heated to just

above the sublimation temperature of Al2C16 and kept at that temperature for a

sufficient time period provides such conditions.

Spectroscopic investigations of surface acidity are useful methods for

obtaining reasonable comparisons between various acid catalysts. One of the most

straightforward methods of examining the types of acid sites and, in some cases,

acid strengths is the infrared spectrum of adsorbed pyridine. Literature reports of

this technique are abundant.38-48 The physical basis for this method arises from

the observation that pyridine bound in a coordinate fashion to Lewis acid sites

exhibits different frequencies than that for pyridinium ion arising from protonation

by Br6nsted sites.38 In addition, the shift of the predominant pyridine frequency

bound to Lewis sites relative to free pyridine can give a qualitative empirical

assessment of acid site strengths. This process has been used to evaluate

AlC]2(SG)n prepared in carbon tetrachloride.3,49 Aluminum chloride-based









catalysts arising from alternate preparations can be contrasted to the literature

results to assess differences in structure and acidity.

Nuclear magnetic resonance (NMR) has been used for many years as a

successful technique in the examination of solution chemistry and since the advent

of the magic angle spinning method, NMR has been applied to the solid state as

well. Of particular interest to examination of AICl2(SG)n, 27A1 solid state magic

angle spinning NMR (SS MAS NMR) has been used with zeolites and amorphous

aluminosilicates to elucidate the structures of these solids.5053 It has been

determined that tetrahedral Al moieties show shifts of between 50 ppm, as

observed for tetrahedral Al centers in zeolite ZSM-5,51 and 105 ppm, as observed

for pure Al2C16.52 Figure 2-3 shows the 27Al spectrum for AlCl2(SG)n prepared

in CCI4 and will serve as the basis for evaluation of catalysts prepared via other

routes.

Methods for pre-treating the silica gel support before catalyst preparation

have the potential to significantly influence the structure and activity of the final

catalyst. The surface structure of silica consists of hydroxyl (silanol, Si-OH) groups

as well as siloxane (Si-O-Si) species. Dehydration of vicinal hydroxyl groups can

result in the formation of siloxane bridges as pictured in Figure 2-4. This figure is

a simplified representation of a three-dimensional silica surface which contains

many different types of silanol environments. Because the process is reversible,

the content of water in the atmosphere directly contacting the silica determines

the relative amounts of both types of surface species. In addition, the

























AL3121 010 JOE


AL'2 CL 6/5102
HAWIk


/'ELLOW MAS


YF= 17596
FPX

1107.7e HZ/iC
:LP
AL9121 .010 JOE

AL2CL6/SIO2/YELLCAJ Nh

1 PULSE SEQUENCE
P2= 13.00 USEC
D5= 900.00 USEC
N4 = 21856
SIZE = 65536
AT = 262.14 MSEC
QPD ON = 4
ABC ON
BUTTERWORTH FILTER ON
DB ATT.= 1
ADC = 8 BITS
AI = 2
SW = +/- 62500.0
DW = 8
RG = 10 USEC
DE- 4 USEC
TL HIGH POWER ON
F2= .000000
BB MODULATION OFF
OF= 440.00
SF= 78.187887
EM= 30.00
PA= 280.0
PB= .1


/Mj


100 50r


SI
0


-50


-100 PPM


27Al SS MAS NMR Spectrum of AICI2(SG)n.


Figure 2-3.


S 5 I
}5Q


I I I













/
0OH 0

.S + HO S -1- .....
0 H
0


\ /
0 0

011111 ..... Si 0 --Si .......... 0 + H20

HO 0


Equilibrium of Silanol Groups with Siloxane Groups on Silica.


Figure 2-4.







17
temperature at which a sample of silica gel is dried is a factor in determining the

concentration of surface hydroxyl groups obtained.54 In addition to silanol groups,

SiOH n H20 species exist in uncalcined silica which involves water hydrogen

bonding to surface hydroxyl groups. In the synthesis of AICl2(SG)n, the anhydrous

Al2CI6 can react with isolated SiOH species, geminal silanols (>Si(OH)2 groups)

and water producing a variety of mixed hydroxo, oxo and chloro species.

Understanding these variables is vital for optimizing solid acid acidity and

obtaining reproducibility in catalyst preparation.


Experimental

Reagents. The aluminum chloride used to prepare all catalysts was

purchased from Alfa Chemicals with a purity of 99.997 % (anhydrous), lot number

K17CG. The silica gel support was donated by W. R. Grace Company as Davison

Grade #62. The silica gel has a mesh size of 60-200, surface area of 340 m2/g and

pore volume 1.1 cm3/g. Preparation of the silica support prior to catalyst

preparation consists of activation with 1 M HCI, water washing and drying at 80

C for 72 hours under 1 mm Hg vacuum. It has been determined videe supra)

that following the drying period, adsorption of water at a level of 0.02 g H20 per

gram of dry silica hydrates the support sufficiently for complete reaction of 5.0 g

of Al2CI6 with 10.0 g of silica. This composition produces the most active catalyst

that does not possess unreacted A12C16. The hydration process will be referred to

as conditioning of the silica and is accomplished by exposure of the dry silica to air

(approx. 75 % humidity) for 24 hours. Hydrated silica will refer to silica with







18

excessive water content. Carbon tetrachloride, purchased from Aldrich Chemicals,

was distilled over phosphorus (V) oxide under N2 prior to use. All liquid

hydrocarbon reactants (benzene, n-hexane, n-hexadecane, cumene, 1-hexene) were

purified according to literature methods. Chlorinated organic reactants (1,2-

dichloroethane, 1,1,1-trichloroethane, 2-chloropropane, 2-chloro-2-methylpropane)

were purchased from Aldrich and used as received. Gaseous materials (ethylene,

propylene, 1-butene, n-butane, isobutane, HCI) were purchased from Matheson as

C. P. purity, with the exception of HCI which is technical grade. Poly(1,4-

butadiene) was donated by Dr. Kenneth Wagener. The rubber tire used in the

polymer cracking experiments was supplied by Mr. Todd Lafrenz. All other

reagents and solvents were obtained from Aldrich Chemicals and were used as

purchased unless stated otherwise.

Instrumentation. Gas chromatography analysis for hydrocarbon products

was conducted on a Varian model 940 gas chromatograph equipped with a flame

ionization detector and either a 1/8" x 8' stainless steel Hayesep Q (80-100 mesh)

column or a 1/8" x 6' stainless steel VZ-10 (60-80 mesh) column. Chlorinated

products were analyzed on a Varian 3700 GC with a thermal conductivity detector

and a 1/8" x 9' s.s. Porapak Q (100-120 mesh) column. Aromatic hydrocarbons

were analyzed on a Hewlett Packard model 5890A GC fitted with a Superox II

column (15 m x 0.53 mm I.D. Non-Pakd). Gas chromatography/mass spectroscopy

was performed on a Varian 3400 GC equipped with an 30 m x 0.32 mm RSL-160

column (5 g capillary) interfaced to a Finnegan MAT ITDS 700 mass









spectrometer. A Nicolet 5DXB FTIR spectrometer was used for FTIR analysis.

Magic angle spinning solid-state NMR was performed either by Dr. Gordon

Kennedy at Mobil Research and Development Company on a Bruker AM-500

NMR spectrometer or by Mr. John West on a GE NT-300 superconducting, wide-

bore 300 MHz FTNMR spectrometer operating at 7.02 Tesla. AI(NO3)3 (aq) was

the reference for all 27A1 NMR studies.

Reactors. Reactions utilizing AICl2(SG)n are typically carried out in either

a batch reactor as depicted in Figure 2-5 or a fixed bed flow system depicted in

Figure 2-6, depending on the particular reaction requirements. The batch reactor

consists of a 250 ml Parr pressure bottle with a stainless steel pressure head

(comprised of fill/purge valves, pressure gauge and sample port) and a neoprene

stopper gasket.55

The fixed bed flow system is designed such that gases can be passed over a

catalyst powder at ambient or elevated temperatures. This is accomplished by

using a 10 mm i.d. pyrex tube fitted with a fritted glass disk to support the solid

catalyst while allowing reactant gases to flow through. The reaction tube is heated

by either a commercially available Thermolyne Briskheat flexible electric heat tape

or a resistance oven consisting of nichrome wire wrapped around a 4" pyrex tube

with quartz wool packing material for even heating. The heating element is

controlled by a digital temperature controller model CN-2041 (Omega

Engineering) equipped with a J-type thermocouple. In all cases temperature

varied only 2 C once equilibrated. A 5-inch length of glass beads is placed












SAMPLE PORT


BALL VALVE


PURGE/FILL VALVE







PRESSURE
BOTTLE


PRESSURE
GAUGE


Figure 2-5. Schematic of Batch Reactor.


















SYRINGE PUMP

CARR I ER
GAS

HEAT
TAPE
GLASS BEADS




CATALYST
BED
E FLOW
METER
GLASS FRIT


SAMPLE
PORT


Figure 2-6. Schematic of Flow System.







22
over the catalyst in the reaction tube to ensure proper mixing and heating of the

reactants. Rubber septa attached to the flow line at pre-catalyst and post-catalyst

positions allow for gas sampling for GC analysis. A model FL-320 rotameter

(Omega Engineering) is used to obtain flow control of gas feeds from 0.2 ml/min

up to 50 ml/min. A soap bubble meter in the post-catalyst zone is used to

determine overall flow rates of the gases. Liquid reactants are delivered to the

catalyst zone via a syringe pump which provides a constant feed rate of reactant.

The liquid reactant is vaporized once it contacts the heated catalyst zone. Volatile

liquid reactants are delivered by bubbling the carrier gases through a saturation

bubbler and over the catalyst bed.

Catalyst preparation. The supported aluminum chloride catalyst was

prepared as follows. A 200 ml or 250 ml 3-neck round-bottom flask is thoroughly

dried to remove residual water and equipped with a reflux condenser and a teflon-

coated magnetic stir bar. An anhydrous N2 purge is used to ensure a dry

environment. To the flask is added 100 to 150 ml of CCI4 along with 10.0 g of

silica gel (activated according to previous specifications). To this slurry is added

5.0 g anhydrous A12C16 and the mixture is heated to reflux. After a short time (30

minutes to 2 hours) the slurry changes from colorless to a dark brown color. This

color remains for the entire catalyst preparation period. A minimum of 72 hours

is generally required for full adsorption of A12C16 to occur, as indicated by a

gradual disappearance of Al2C16 crystals in the slurry.








At the end of the 72 hour period, the solid is filtered from the reflux

solution and dried at room temperature under vacuum to obtain a tan solid. No

further treatment of the catalyst is necessary for use in catalytic experiments.

Catalysts which have additional metal promoters are prepared in the same way as

mentioned above, with the exception of refluxing the silica in CC14 with 0.2 g of

the metal chloride promoter for 24 hours prior to A12C16 addition. In the catalysts

where ZnCI2 was used as an additional promoter, an amount equivalent to 3 times

the molar amount of the first promoter is used. The ZnCI2 is added to the slurry

after the first promoter but before the A12Cl6.

Sealed-system catalyst preparation. A sealed-system catalyst preparation

method was devised to provide a vapor phase method for preparing the catalyst.

A 250 ml Parr pressure bottle is dried to remove traces of water and 2.0 g silica

gel (prepared as described previously) along with 1.0 g anhydrous A12C16 is sealed

in the bottle with a neoprene stopper. The pressure bottle is clamped into the

same holder as used for the batch catalytic reactions. No pressure head is used

for this process because HC(g) is generated in the adsorption reaction and would

corrode the stainless steel. Care was taken to determine appropriate amounts of

reagents so that the pressure generated in the bottle from HCI formation would

not exceed 60 psi, the maximum pressure rating for the bottle. The sealed system

is placed in an oil bath in the temperature range 175 190 C to volatilize the

Al2C16. This reaction is allowed to proceed for a minimum of 72 hours after

which time the bottle is cooled and the clamps are slowly loosened to carefully








break the seal and release the pressure arising from the HCI formed in the

reaction. Once the pressure is relieved, the rubber stopper is replaced to inhibit

water contacting the catalyst. A homogeneous-looking gray solid is obtained and

is used in catalytic experiments without further treatment.

Procedure for pyridine adsorption for infrared analysis. A vacuum

desiccator is used for all pyridine adsorption experiments. A sample of

AlCl2(SG)n is placed in the desiccator with a pyridine reservoir and the desiccator

is evacuated to generate a pyridine atmosphere inside. The catalyst is kept in the

pyridine environment at room temperature for 2 to 3 hours, during which time the

catalyst changes from tan to off-white in color. The catalyst is then placed under

vacuum for at least 2 hours to remove excess pyridine. This evacuation step is

conducted at room temperature as well as elevated temperatures (150 and 300 C)

as a way of gauging the strength of pyridine adsorption. Infrared spectra are

taken as a fluorolube mull of the catalyst on KBr plates. No attempt is made to

keep the catalyst dry after pyridine adsorption. The region of 1700 to 1400

cm-1 is used as the fingerprint region for acid site adsorption of pyridine.


Results and Discussion


Reactivity of AICl2(SG)_ in Hydrocarbon Conversions


A vital step in the development of novel heterogeneous acid catalysts is to

determine their applicability in selected test reactions. This process is useful in

assessing the overall strength and types of acid sites in addition to demonstrating








the potential utility of the catalyst in commercial processes. Isomerization of

paraffins, cracking of hydrocarbons and alkylation reactions are among the most

important acid-catalyzed reactions utilized industrially and will be the focus for

reactivity studies with AICl2(SG)n.

Dehydrohalogenation and hydrodehalogenation

One of the reported requirements for cracking with AICl2(SG)n is the

presence of a chloride source, such as CCI4, to maintain catalyst activity during the

experiment. It has been noticed that chloroform and methylene chloride are side

products when using carbon tetrachloride, indicating a hydrodechlorination

(replacement of a chlorine atom with a hydrogen atom) pathway for catalyst

regeneration.2-4,49 Both hydrodechlorination and dehydrochlorination (removal of

HC1 from a chlorinated molecule) are known to be acid catalyzed;56-61 therefore,

the observation of products arising from hydrodechlorination of CCI4 is not

unexpected.

1,2-Dichloroethane is a simple molecule to study for dehydrochlorination:



Cl-CH2CH2-Cl -----> CH2=CH-CI + HCI (2-4)



Conversion of dichloroethane to vinyl chloride is also the largest commercial

dehydrochlorination reaction, although at present the reaction is performed

thermally (450 500 C) without the need for a catalyst.62,63 The goal in studying

this process is to find a catalytic route which can provide better conversion levels








Table 2-1.


26

Product Distributions for the Reaction of 1,2-Dichloroethane Over
Various Metal-Doped Catalysts.


CATALYST T PRODUCTS (MOL/MIN)
(oC)
ETHANE ETHYL VINYL
CHLORIDE CHLORIDE
PdCl2"(SG)nAICl2 150 1.7 x 10-5 1.5 x 10.6 5.0 x 10-7

RhCl3*(SG)nAlCI2 125 2.4 x 10 -5 5.0 x 107 --

RuCl3*(SG)nAlCl2 150 6.0 x 10-7 5.0 x 10-7 2.6 x 10-6

K2PdCI4.(SG)nAIC12 125 1.0 x 10 --- ---

PdCl2*RhCl3-(SG)nAICl2 150 2.0 x 10-5 3.5 x 10-6 1.6 x 10-6

K2PdCIl4RhCI3"(SG)nA1Cl2 150 2.5 x 10-6 1.2 x 10-6 ---

ZnCl2"RhC13*(SG)nAIC12 150 8.8 x 10-6 1.3 x 10-6 2.1 x 10-6

ZnCl2-RuCI3.(SG)nAIC12 125 1.2 x 10-5 5.9 x 10-6 3.3 x 10-6








with respect to the commercial process (50 60 %) while maintaining high

selectivity (95 98 %) to vinyl chloride. It should be mentioned that

hydrodechlorination of 1,2-dichloroethane to ethyl chloride is a side reaction that

can occur when using acid catalysts.

Table 2-1 presents the product distributions for the reaction of 1,2-

dichloroethane (abbreviated 1,2-dce) over a number of metal-promoted variations

of AlCl2(SG)n.12 The products obtained give some indication as to the function of

the metal promoters) added. The feed rate of 1,2-dce is 1.26 x 10-4

moles/minute. The unpromoted AICl2(SG)n catalyst gives primarily 1,1-

dichloroethane, most likely arising from acid-catalyzed Markownikov addition of

HCI to vinyl chloride formed in a dehydrochlorination intermediate step. Metal

promoters can function to modify the acidity of the tetrahedral Al sites on the

catalyst and/or react with HC1 to preclude addition of HCI to the olefin product;

either possibility could account for the absence of 1,1-dichloroethane in the

product stream when using metal-promoted catalysts. The extreme coking

observed with unpromoted AlCl2(SG)n is also inhibited by use of metal promoters.

The three products from the reaction over promoted AICI2(SG)n are

ethane, ethyl chloride and vinyl chloride. Ethyl chloride is the

hydrodehalogenation product and ethane can arise from hydrodehalogenation of

the ethyl chloride product. As seen in Table 2-1, PdC12 and RhCI3 are the most

effective hydrodehalogenation promoters which indicates the need to activate H2

in the hydrodechlorination process. RuCI3, which is apparently less effective in








hydrogen activation, gives selectivity to vinyl chloride (70 %) but only < 10 %

conversion of 1,2-dichloroethane. A series of catalysts with two added promoters

was investigated. The overall results show increased conversion of 1,2-dce but

poor selectivity to vinyl chloride.

The persistence of 1,2-dichloroethane to undergo hydrodechlorination

rather than dehydrochlorination led to the use of a different substrate with more

potential for HCI removal. 1,1,1-trichloroethane (tce) was investigated with the

most active catalyst from the dichloroethane studies, ZnCl2-RuCl3'AlCl2(SG).n

1,1,1-trichloroethane is presently converted to 1,1-dichloroethylene

stoichiometrically with a sacrificial base reagent.63-67 A recent report details the

use of CsCI on silica for the catalytic conversion of tce to 1,1-dichloroethylene at

150 300 C in a flow system.68 Conversion was typically -100 % with selectivity

of 80 % at 250 C. Silica was reported to be a vital component in the activity of

the catalyst, so AlCl2(SG)n was used in hope of attaining a system which is active

at lower temperature. Figure 2-7 shows the activity of ZnCl2"RuC13AlCl2(SG)n at

temperatures of 100 to 300 C. A sample of the silica support was also used in

the reaction as a comparison. The reaction was performed over a period of 12

hours (noncontinuous) which is not sufficient for assessment of catalyst longevity.

In addition, because the temperature was changed during the run, the effect of

time on catalyst activity is uncertain. It should be pointed out that

dehydrochlorination was the major pathway here; less than 10 % of the products

observed resulted from hydrodechlorination or any other side reactions. The
























- -*




--- SILICA GEL
- -- Ru/Zn/Al CATALYST


/
/
/
/


100.0



80.0



60.0



40.0



20.0



0.0
0.C


Figure 2-7.


Dehydrohalogenation
RuC13lZnCl2AIC12(SG)n-


of 1,1,1-Trichloroethane Over


. 1 b .6 . 3 '6 .o
P100.0 20E.0 300.0
TEMPERATURE








activity of the metal-promoted catalyst was considerable even at lower

temperatures. Specifically, at 150 200 C the silica showed < 20 % conversion

whereas the promoted AICI2(SG)n catalyst approached 90 % conversion of the

substrate. The selectivity seen in this reaction (90 + %) indicates that the

substrate 1,1,1-trichloroethane is less prone to undergo hydrodechlorination with

A1CI2(SG)n to form dichloroethane than is dichloroethane susceptible to form

ethyl chloride.

Friedel-Crafts alkylation

The alkylation of aromatic hydrocarbons with alkenes and alkyl halides is a

classic acid-catalyzed reaction25-27 and anhydrous aluminum chloride is one of the

most effective catalysts known for this reaction. The search for a heterogeneous

catalyst to perform Friedel-Crafts chemistry is an active area of research, with

catalysts such as Nafion-H,69-72 clay-supported transition metals,29'73'74 graphite-

intercalated Al2Cl672'75 and modified alumina76 reported within the past 15 years.

In the specific case of graphite-intercalated A12C16, the catalyst demonstrated very

good activity initially but over a few hours the activity dropped to zero.72 The two

reasons given for this problem are hydrolysis of A12C16 by water in the reaction

feed and gradual desorption of A12CI6 from the graphite. Clearly, AICl2(SG)n

offers significant improvement over graphite-intercalated aluminum chloride

because A12C16 is chemically bound to the silica and does not desorb even at 250

C. Deactivation by water is an innate problem for catalysts possessing tetrahedral

aluminum active sites and can be deterred by careful drying of the carrier gases.









Table 2-2 presents the results of using AICl2(SG)n in a series of aromatic

alkylation reactions. The activity of the catalyst in both batch and flow systems

has been investigated. In the batch reactions, it appears that the activity is directly

influenced by the stability of the carbonium ion formed from the alkyl halide

reactant. A reactant, denoted R3C-CI can react with a catalyst active site,

depicted as S-O-AICI2 (S = support) to give an ion-pair: S-O-AICI3- R3C+. Alkyl

chlorides in which the Cl atom is connected to a 30 carbon, such as t-butyl chloride

(abbreviated t-BuCl), form very stable 30 carbonium ions upon abstraction of the

chloride;77 hence the relatively high activity of AlCl2(SG)n for alkylation of

benzene and toluene with t-BuCl. If an alkyl chloride is used which gives a 20

carbonium ion, i-propyl chloride (i-PrCl), the activity drops to 10 % conversion

from 50 % with t-BuCl. In this instance, only the strongest acid sites are active for

2 carbonium ion generation. A number of species are formed in this system

which can poison acid sites on AICl2(SG)n, such as olefins, and the strongest acid

sites will be poisoned first. The rate of this competing reaction may determine

how far the reaction progresses toward the alkylaromatic; the faster a carbonium

ion is generated, the better chance it has of reacting with the aromatic reactant to

form the product before catalyst deactivation occurs. It can be speculated that the

low activity of AIC12(SG)n with i-PrCl and benzene can be accounted for by

complete poisoning of the active sites by benzene after only 10 % of the alkyl

chloride has reacted.









Friedel-Crafts Alkylation with AICI2(SG)n.


Conditions


Batch:
Reflux temperature, 50 ml CC14, 11 mmol reactants.

Flow:
100 175 C, 5 30 ml/min N2,
50/50 (mol/mol) reactant feed @ 0.6 ml/hr.


Aromatic Alkyl Conditions Conversion to
Reactant Reactant Alkylaromatic
Products

Toluene t-Butyl Chloride Batch 48 %
Benzene t-Butyl Chloride Batch 54 %
Benzene i-Propyl Chloride Batch 10 %
Benzene 1,2-Dichloroethane Batch 0 %
Benzene 1-Hexene Batch 0 %
Benzene Acetyl Chloride Batch 0 %
Benzene t-Butyl Chloride Flow 0 %
Benzene Propylene Flow 0 %
Benzene i-Propyl Chloride Flow 4 %


Table 2-2.








Protonation of an alkene will also provide carbonium ions in the Friedel-

Crafts reaction. This process, however, appears to be less favorable than chloride

abstraction over AlCl2(SG)n given the lack of activity observed with 1-hexene as

the reactant. Poisoning of the acid site by the olefin may also be involved with the

lack of activity observed in this case. As expected, 1,2-dichloroethane, which

would give a 1 carbonium ion, shows no activity as well. Finally, acetyl chloride

was utilized in an acylation reaction (ArH + RCOCI ----> ArCOR + HC1) with

no products observed. This is not an unexpected result because AIC12(SG)n loses

activity in the presence of oxygen donor groups. Even pure A12C16 is used in

greater than stoichiometric quantity in acylation reactions due to total

complexation of the acyl chloride, which necessitates hydrolysis of the complex to

obtain the ketone product and thereby results in inefficient use of the aluminum

chloride.

Different product selectivity is observed for the flow system as compared to

the batch. Alkylaromatic products are not present in the flow system reaction

with t-butyl chloride at 175 C; propane, isobutane, isopentane and unreacted

benzene are the only significant components of the post-catalyst stream. It can be

proposed that formation of the expected t-butylbenzene product is unfavorable

under the reaction conditions. Dealkylation of alkylaromatics is a known cracking

pathway using acid catalysts; for example, t-butylbenzene is efficiently cracked to

benzene and isobutylene at 280 C over silica/alumina.63 It has been previously

established that AICl2(SG)n is significantly more acidic and more active in







34

cracking than silica/alumina. Therefore, it is reasonable to assume that at 175 C

the t-butyl chloride/benzene reaction equilibrium would lie in favor of the

reactants in the presence of AlCl2(SG)n. The t-butyl cation generated by the acid

is then free to undergo hydride transfer to give isobutane and disproportionate to

give C3 and C5 products. The isopropyl chloride/benzene system also appears to

show similar chemistry as minimal cumene is formed. In this instance, no C4 or

C5 products are seen because of the inability of propyl carbonium ions to undergo

disproportionation.

In order to substantiate the proposal that the alkylaromatic product can

indeed dealkylate under the reaction conditions, the reaction of cumene over

AlCl2(SG)n was performed at the same temperature as the Friedel-Crafts

reactions above and the results are presented in Figure 2-8. Initial conversion of

cumene to benzene and C3 products at 160 C is 100 % but drops rapidly due to

coking or poisoning of the acid sites by coordination of the aromatic reactant.

These results clearly demonstrate that AICl2(SG)n favors dealkylation over

alkylation under the flow-system reaction conditions. As such, the catalyst offers

no improvement over present alkylation processes.

Low-temperature isomerization of butane

The isomerization of n-butane to isobutane is a widely-used commercial

reaction for obtaining the isobutane feedstock utilized in alkylation with 1-butene

for the production of C8 hydrocarbons.78 The highly-branched alkylates are used

in gasoline in order to boost octane ratings. A number of commercial methods for











1(1,.00 \



80.00 -




60.00



40.00




20.00

0.00 -


0.00 100.00 200.00
TIME (MIN


I I I I I I I I I I I I I I I I I i
300.00 400.00
UTES)


Conditions:


6 ml/min N2, saturated with cumene vapors (from bubbler)

160 C

1.0 g catalyst


Cumene Dealkylation.


Figure 2-8.







36

this process are known, such as utilization of a platinum-on-alumina catalyst which

is treated with polyhalides (CCl4, e. g.)79 and the Butamer process involving a

proprietary Pt catalyst.80 There has been a large amount of work done using

Al2C16 for hydrocarbon isomerization. Ono and co-workers have delineated the

use of Al2Cl6 in conjunction with metal salts for the isomerization of n-pentane at

low temperature.18,81-83 Of particular interest is a report by Fuentes et al.

regarding the use of sulfonated silica-supported A12Cl6 and polymer-supported

A12C16 in the isomerization of n-butane.23 It was reported that addition of HCI to

the reactant feed resulted in an increased rate of isomerization. This observation

was attributed to the fact that isomerization requires both Br6nsted and Lewis

acid sites; protons serve to regenerate Br6nsted sites lost during reaction and Cl"

acts to keep the Cl/Al ratio high, thereby maintaining stronger Lewis acid sites.

Isomerization of butane at low temperature (less than 100 C) is important

not only from the standpoint of using less energy to attain high temperatures but

also for maximizing the potential yield of isobutane which is limited by the

isomerization equilibrium. Equation 2-3 expresses the equilibrium temperature

dependence:31



R(ln K) = (2318/T) 4.250 (2-3)



where R is the ideal gas constant in J/mol'K, T is the temperature in Kelvin and K

is the ratio of isobutane to n-butane. The result is that as the temperature







37

increases, the isobutane to n-butane ratio decreases concurrently which lowers the

maximum amount of isobutane which can be produced in an isomerization

procedure.

The addition of small amounts of olefins to the saturated hydrocarbon feed

has been found to have dramatic effects on the levels of isomerization observed.

Early work done by Pines and Wackher84 showed that the addition of even 0.01%

butenes to an n-butane/HCI mixture in the presence of pure A12CI6 at 100 C

changed conversion from < 0.1 % to 11.8 %. As depicted in Figure 2-9,

protonation of the butene results in a carbonium ion, CH3CH2CH+CH3, which

can act as a catalytic agent in the reaction. After rearrangement of the straight-

chain cation to the isobutyl cation, hydride abstraction from an n-butane molecule

can occur to form the isobutane product and regenerate the straight-chain

carbonium ion, thereby continuing the catalytic cycle.

The investigation of n-butane isomerization at various temperatures with

AIC12(SG)n was done in order to determine effects of olefins and HCI on the

activity. Figure 2-10 shows the results for n-butane isomerization over time in the

presence of 4 % 1-butene. The goal was to obtain conditions whereby the

conversion of n-butane is relatively high and stable over extended time periods.

For the case where AICl2(SG)n is used with n-butane and 4 % 1-butene (no HC1),

the conversion is initially very high (ca. 50 %) but drops rapidly to a level of 5 %

after 25 hours. Two reasons for this decline in activity can be proposed. In cases

where catalysts of high acidity are used for hydrocarbon conversions, coking of the
















+ H+


G


+ e"L


Figure 2-9.


Mechanism for the Promotion of Butane Isomerization by
1-Butene.


W


/i















BUTANE ISOMERIZATION
(4 % OLEFIN)


AICI2(SG)n
-A-
PdCI2/AICI2(SG)n
-I-
AICI2(SG)n, HCI
HCI, 50 C
HCI, 50 C


TIME (hr)


Conditions:

25 oC (Unless otherwise noted)

0.70 1.0 g catalyst

Hydrocarbon flow rate: 4 ml/min

HCI flow rate (when used): 0.01 0.05 ml/min


Figure 2-10. Isomerization of N-Butane over AIC12(SG)n.









catalyst (specifically, irreversible adsorption of hydrocarbon fragments which

become highly unsaturated and form a carbonaceous polymer on the catalyst

surface) can result in deactivation of the acid sites. The other explanation is

simple depletion of Br6nsted acid sites during the reaction. Experiments can be

performed to determine the deactivation pathway and thereby formulate a way to

extend the catalyst lifetime.

It has been shown previously49 that addition of 2 % of a noble metal

promoter (Rh, Ru, Pd, Pt) will decrease the rate of coking and prolong catalyst

activity by hydrogenating the carbonaceous deposits on the catalyst surface. In

view of this, a catalyst was prepared by reacting PdCI2 with silica in CCI4 and then

treating the PdC12/silica with A12C16 to make a noble-metal promoted solid acid

catalyst. As seen in Figure 2-10, no apparent advantage is observed in this

experiment as the PdCI2 promoted catalyst shows the same decline in activity over

the same time period. This observation, along with the fact that the catalyst did

not darken in color as is usual in cases where coking occurs, leads to the

conclusion that coking is not involved in catalyst deactivation. It should be

mentioned that the addition of an organic chloride, CC14, to the gas stream which

is reportedly useful in maintaining activity in cracking reactions also does not

change the rate of catalyst deactivation.

To test the proposed pathway of Brinsted site depletion, HCI(g) at 10 % by

volume was added to the gas stream. The results in this case are notably

different, with activity maintained at 25 to 30 % even after 20 hours with no









decline. Based on this data, it can be said that protonic acidity is crucial to

maintaining catalytic activity in butane isomerization when olefins are present.

The selectivity to isobutane in all of the butane isomerization experiments

mentioned thus far meets or exceeds 85 %. The other products obtained are

propane and isopentane arising from a dimerization/cracking pathway. This

cracking side-reaction is temperature dependent, with more cracking observed as

the temperature is increased. The efficiency of butane isomerization at elevated

temperature is therefore limited by the selectivity to the desired isobutane

product. Our supported aluminum chloride catalyst has been used at elevated

temperature, with the optimal temperature apparently being 50 C. The data for

this experiment is also presented in Figure 2-10. A conversion of 45 % can be

maintained for a minimum of 30 hours with 85 % selectivity to isobutane. The

theoretical limit according to equation 2-3 is 58 % conversion at 50 C.

Therefore, the activity of the catalyst is approaching the maximum that can be

obtained with any system. If the reaction is performed at 100 OC, initial

conversion of n-butane is approximately 100 % but selectivity to isobutane is only

65 %. After 20 hours of reaction at 100 C the conversion drops to only 40 %

and the catalyst is black, which is a sign that coking is deactivating the catalyst at

100 C.

The blank reaction using HC1, 1-butene and n-butane (no solid catalyst)

results in no observed production of isobutane which proves that AlCl2(SG)n is

required in this reaction. Another blank reaction where 1-butene is omitted from









the reaction feed results in only 3 % conversion to isobutane with a very rapid

decline to 0 % after 3 hours. In the absence of olefins, the only pathway for the

formation of carbonium ions from n-butane is hydride abstraction which requires

very strong Lewis acid sites. The initial isobutane formation demonstrates the

presence of strong Lewis sites on AlCI2(SG)n but their rapid deactivation offers

little potential for utilizing such sites in this system. It is well known in the

literature24'31 that olefin protonation requires less energy than hydride abstraction

and the above results simply follow that trend.



Reactions of n-hexane with AICl2(SG)n

The conversion of n-butane over solid acids can be considered a rather

simple reaction, with possible products consisting only of isobutane and cracking

products propane and isopentane. N-hexane, in contrast, offers a more

complicated reaction chemistry due to its tendency to undergo cracking more

easily than butane in addition to skeletal isomerization. The researchers in Gates'

laboratory85,86 have examined the reaction of n-hexane (with 0.1 % 1-hexene

promoter) over sulfonic-acid supported A12C16 at 85 C. The initial product

distribution consisted of -15 % branched C6 products and -65 % cracking

products including isobutane (42 %), isopentane (21 %) and n-pentane (2 %).

The remaining 20 % of the product stream was unreacted n-hexane for a total

conversion of 80 %. After 2 hours the conversion had dropped to 12 %.







43

Table 2-3 lists the product distribution from the reaction of n-hexane over

AICI2(SG)n at 100 C in a hydrogen atmosphere (no HCI or olefin present). The

initially high conversion of 99 % drops to -40 % in less than one hour, a result

which is identical to that observed with n-butane: rapid deactivation of the catalyst

in the absence of HCI and olefin promoter. Table 2-4 presents the results for the

same reaction with the addition of 1-hexene to the reactant mixture and 10 %

HCI to the gas stream. The retention of 90 % conversion after 6 hours of

reaction shows that once again the Brinsted activity of AICl2(SG)n is important

for hydrocarbon skeletal isomerization.

The selectivity to branched hexane isomers is poor in the flow system

experiments due to the predominance of cracking at the temperatures used. As

mentioned above, cracking requires more energetic conditions than isomerization

given that cracking requires net cleavage of C C bonds but isomerization can

proceed via stabilized 3-center intermediates which do not require the energy

necessary for full bond breaking.87 In order to obtain better selectivity to the

branched C6 products, then, a lower temperature must be used. The flow system

typically used in our hydrocarbon conversion reactions cannot be used for hexane

conversions at lower temperature because the reactant must be in the vapor state.

Liquid-phase batch reactions are the only alternative for lower-temperature

systems; however, the solvation effects of the solvent result in less efficient catalyst

utilization. Nonetheless, the reaction of hexane with AICl2(SG)n in a batch

reactor was studied.









Products from the Reaction of N-Hexane over AICl2(SG),.


PRODUCT MOLE % MOLE %
t = 10 MINUTES t = 45 MINUTES
Methane 0.08 ---
Ethane 0.02 ---
Propane 7.4 2.6
Propylene 0.4 ---
Isobutane 56.4 21.6
N-Butane 8.6 0.6
Isopentane 17.6 11.0
N-Pentane 4.2 0.6
2,2-Dimethylbutane 2.3 0.8
2-Methylpentane & 2.0 3.3
3-Methylpentane
N-Hexane 1.0 59.6


Conditions:
100 C, 1.0 g catalyst, hexane feed 0.6 ml/hr,
3 ml/min H2 flow, CCI4 bubbler in pre-catalyst stream.


Table 2-3.










Product Distribution from the Reaction of
AICI2(SG)n, 1-Hexene Promoter Present.


N-Hexane Over


PRODUCT MOLE % MOLE % MOLE %
t =5 MINUTES t= 4 HOURS t= 6 HOURS
Methane 0.4 0.3 0.8
Propane 13.1 11.7 12.4
Propylene 0.8 --- ---
Isobutane 40.1 35.3 37.4
N-Butane 10.6 1.8 1.7
Isopentane 22.4 35.0 33.4
N-Pentane 6.3 1.5 ---
2,2-Dimethylbutane 3.0 1.7 1.3
2-Methylpentane & 2.7 5.4 1.5
3-Methylpentane
N-Hexane 0.4 7.5 11.6

Conditions:
1.0 g catalyst, 100 C, 1 % 1-hexene in liquid feed @ 0.6 ml/hr,
3 ml/min H2 flow, 0.2 0.4 ml/min HCI flow (no CCI4 bubbler)


Table 2-4.






46

A temperature of 25 C was used for the batch reaction. The catalyst was

slurried with 10 ml of 0.1 % 1-hexene in n-hexane and 30 ml CCl4 under 27 psig

H2. There was a barely detectable amount of isobutane present in the reactor

after 18 hours, corresponding to <<1 % conversion of the reactant. Interestingly,

there was no hexene observed in the product mix. This observation can best be

explained by irreversible coordination of the olefin to the acid sites on the catalyst;

the 2.3 x 103 moles of Al sites on the catalyst are more than enough to form 1:1

complexes with all of the 8.0 x 10-4 moles of 1-hexene used in the experiment.

At higher temperatures, the batch reactor does not offer much

improvement. For a reaction performed at 65 C (no hexene), only 2.5 %

conversion of the hexane was observed with minimal branched C6 products

obtained. It is apparent from these results that hexane highly favors cracking

reactivity with AICl2(SG)n, based in part on the level of catalyst acidity and

propensity of hexane to undergo cracking over acid catalysts.

Cracking of hydrocarbon polymers

Disposal of plastics and rubber is arguably the biggest problem facing

polymer manufacturers today. Recycling efforts are being made in many

communities but the recovered plastics are essentially utilized as filler material,

such as incorporation of used poly(ethylene terephthalate) in soda bottles and

used automobile tires in asphalt. Increasing attention has been given to utilizing

spent polymers as a chemical resource,88 with the ultimate goal of recovering the

starting monomer via some depolymerization method.








Hexadecane cracking with AICl2(SG)n occurs very readily at 100 C.3,4

Reactions between AICI2(SG)n and polyolefins are expected to be similar based

on the obvious structural similarities. The unique aspects of polymers which pose

additional complications are the solid physical state and the presence of additives

which are not present in petroleum feedstocks.

Despite the possible drawbacks, some recent literature work with acidic

aluminum compounds as catalysts for polymer degradation illustrates the potential

for utilizing AICl2(SG)n in this type of reaction. A catalytic study using

ethylaluminum chloride (Et-AICI2) to crack poly(isobutylene) in solution89 has

resulted in decreased molecular weight of the polymer and formation of branched

structures, both of which indicate a carbonium ion mechanism is occurring in the

polymer degradation. Examination of the acids MAIC14"H20 (M = Li, Na, K) in

cracking of poly(ethylene), poly(propylene), poly(isobutylene) and butyl rubber

demonstrates a dependence on molecular weight, presence of double bonds and

extent of branching for catalyst activity.90 These results again show a cationic

pathway for polymer decomposition, as in the ease of protonation of double bonds

to generate carbocations, for example. A recent report with exceptional

significance to our work with AlCl2(SG)n is the catalytic decomposition of butyl

rubber with pure AlBr3.91 No high molecular weight products were observed, the

polymer unsaturation was reported to increase (due to hydride abstraction by

AlBr3) and a cationic mechanism was proposed for the reaction.










Cracking of Polymers Over AICI2(SG).n


a Conditions:


100 C, 1.0 g catalyst, 2.0 g polymer,
25 ml CCI4, 25 psig H2 (unless noted otherwise).


b Nitrogen used in place of hydrogen.

c Reaction at 175 C.

d Presented for comparison purposes.


POLYMERa MOLES ISOBUTANE/MOLE Al
SITES
Poly(ethylene) 0.20
Poly(ethylene)b 0.04
Poly(4-methyl-l-pentene) 0.28
Poly(butadiene) 0.08
Poly(styrene) < 0.0001
Rubber Tire 0.05
Cellulose Acetate < 0.0001
PE Shopping Bag 0.03
PE Shopping BagC 0.07
N-Hexadecaned 0.24


Table 2-5.








The results for polymer cracking with AICl2(SG)n are presented in Table

2-5. The first entry in the table gives the conversion for cracking poly(ethylene)

(PE) and the results are very encouraging. The product distribution with PE is

identical to that obtained from typical monomeric hydrocarbons: isobutane,

isopentane and propane are the major products in a 5:2:1 ratio with minor

products n-butane and n-pentane. The amount of isobutane produced per Al site

on the catalyst provides a pseudo-turnover number (PTON) and is used as a

gauge to compare polymer reactivity. The value of 0.20 for the PTON with PE is

within the experimental error of 0.02 for the result we obtain with n-

hexadecane, 0.24. The identical product distributions and similar conversions

suggest that the same carbonium ion mechanism is at work for both monomeric

and polymeric cracking with AICl2(SG)n. Hydrogen appears to be an integral part

of the reaction scheme according to the second entry in the table where N2 is

utilized as the atmosphere and a large difference in activity is observed; only

about 1/5th of the activity is seen in this case. It is not known whether hydrogen is

actually consumed in the reaction or is involved in an as yet undetermined non-

consumptive pathway.

To investigate further the involvement of hydrogen in the cracking scheme,

a version of the catalyst containing 2 % PdC12 promoter was used in PE cracking.

The ability of palladium to activate hydrogen was found to have an unexpected

effect on the product distribution, with the amount of isobutane formed at the

exact same level as the unpromoted catalyst (PTON = 0.20) but the amount of






50

methane and ethane in the product mix increased sharply. The ratio of methane

to ethane to isobutane in the product mix was 1.3 : 1.2 : 1. The C1 and C2

products are proposed to arise from a hydrogenation of surface coke or

dehydrohalogenation of CCI4 or a combination of the two. These results imply

that the mechanism of polymer cracking involves utilizing hydrogen via a different

pathway than that employed by PdCl2.

The branched polymer poly(4-methyl-l-pentene) gives higher activity, as

expected, due to its branched structure. The presence of 30 carbons on the

polymer (refer to Figure 2-2 for the structure) allows for easier hydride

abstraction, which is proposed to be the pathway for generation of the carbonium

ion intermediates from saturated hydrocarbons.

It was proposed in the previous section on hexane isomerization with 1-

hexene promoter that the olefin can coordinate to the acid sites and thereby

inhibit the reaction; this effect is also observed in the reaction of poly(butadiene)

where the activity drops by a factor of -2.5 as compared to PE with the presence

of an unsaturated functionality on the hydrocarbon chain. Poly(styrene) is another

example of an unreactive polymer. The lack of reactivity observed in this instance

can be postulated to be due to the observation that the polymer did not melt

under experimental conditions or the presence of aromatic groups which may have

a poisoning influence on the acid sites.

Carbon-carbon double bonds are considered to be weak bases due to the

electron-donating ability of the pi bond and it has been shown that olefins can









have a detrimental effect on the activity of AICI2(SG)n. It is expected that

stronger bases like oxygen or nitrogen donors will show an even stronger impact

on the reactivity of the catalyst; indeed, water can completely deactivate

AlCl2(SG)n by coordination of the oxygen lone pairs to the Al sites. Cellulose

acetate, by virtue of possessing 5 oxygen donors per repeat unit, exhibits this

problem as evidenced by its lack of reactivity in the cracking reaction.

Unfortunately, the sensitivity of AICl2(SG)n towards bases presents a severe

limitation for application of the catalyst in polymer remediation. Investigations of

catalyst reactivity toward commercial plastic and rubber, therefore, will

concentrate on materials which are devoid of basic functional groups.

The final entries in Table 2-5 illustrate the use of commercial polymer

products in cracking to useful end-products. Utilization of a PE shopping bag

results in only a small amount of isobutane under the reaction conditions. Two

reasons can be postulated for this: first, the bag is constructed from high-density

(highly crosslinked) PE which did not form a melt as did the low-density PE

mentioned earlier, and second, the bags are designed for bacterial degradation in

landfills by the addition of starch, which contains oxygen functionalities. The

extent of starch addition is unknown but may not be excessive; the use of higher

temperature to assure polymer melting can determine if the presence of starch is

the deactivation factor in this instance. Cracking of the PE bag at 175 C is

presented in Table 2-5 and the resulting low activity indicates that the starch

additives may indeed inhibit reaction to a certain extent. The polymer did form a









melt under the reaction conditions but the activity only increased very slightly,

which shows that the determining factor in the reactivity is not simply contact of

the polymer with the catalyst but also involves additives which can poison the

catalyst.

A sample of vulcanized rubber tire provides a PTON of 0.05 despite the

fact that no melting of the polymer occurs at experimental conditions. To account

for this observation, it can be proposed that either a solid/solid reaction occurred

between the catalyst and tire or unidentified organic tire additives were extracted

from the rubber matrix by the solvent and were the actual reactants in the

cracking reaction. A Soxhlet extraction performed with CCl4 on a sample of the

tire resulted in a brownish-colored liquid phase which evaporated down to a black

sludge. Cracking of the tire after extraction gave a PTON of 0.016 ( 0.004) as

compared to the untreated tire which showed 0.050 ( 0.010). Utilization of the

sludge extract showed a PTON of 0.006 ( 0.002), indicating that the tire extract

does indeed provide cracking products. However, based on the reactivity still

observed with the extracted tire a solid/solid reaction may be occurring.

The longevity of AlCl2(SG)n in the cracking of polyolefins is a parameter

which is difficult to assess in a batch reactor but needs to be addressed

nonetheless. The cracking of PE in the batch reactor is observed to give

consistent PTON results for each replicant experiment. The similarity in the

product distribution for each replicate is an indication that an equilibrium exists

which determines the maximum amount of products formed for a single batch run.





















AICI2(SG)n, 100 C, 28 psig H2


I .

1 '


- 0.8


D 0.6
E
b 0.4

9 n-


SE2 3 4 5 6
SEGMENT NUMBER


Figure 2-11. Sequential
AICl2(SG)n.


7 8


Batch Cracking


of Poly(ethylene)


with









Figure 2-11 shows a PE cracking experiment where the reactor was purged at

elevated temperature after each 18 hour segment; the effect of this is to remove

the products and attain greater conversion. The high activity observed in the

initial two segments can be attributed to the stronger acid sites on the catalyst.

Coking of these sites and their consequential deactivation reduces the efficiency of

the catalyst, as seen in comparison of segment two to segment three. The

remaining acid sites, being less acidic, do not coke as rapidly so there is a more

gradual decline after each segment. This observation correlates well to results

obtained for cracking in a flow system4 where equilibrium is not attained because

the carrier gas constantly removes the reaction products. After the 9 segments,

the total PTON was 0.80 (- 0.10), which is roughly four times more than obtained

in a one-segment experiment. These results demonstrate that the batch polymer

cracking is an equilibrium reaction which, if perturbed, presents a method for

efficient utilization of the acid catalyst.


Synthesis Variations

Silica hydration studies

The surface of silica has been the subject of many investigations with the

goal of gaining an understanding of heterogeneous catalyst preparation. As early

as 1940 it was found that silica possesses both silanol groups, Si-OH, and siloxane

groups, Si-O-Si, on the surface of a bulk solid that consists of Si04 tetrahedra.92

It is generally accepted that SiOH groups can be condensed reversibly to siloxane

groups, as depicted in Figure 2-4. This phenomenon has a distinct influence in the






55

preparation of AICI2(SG)n because it has been determined that reaction of A12C16

with Si-OH groups is the factor which provides stability to the resulting surface

aluminum sites.

A review of silica gel surface studies has been presented by Hockey,93 who

describes synthetic procedures and the influence of temperature on silica gel

hydroxyl group content. Gel-type silicas are generally prepared by the acid

hydrolysis of reactive silicon compounds, such as SiCI4 or Si(OR)4, to obtain

polymerized orthosilicic acid, Si(OH)4, which is concentrated to form the

gelatinous solid material. These gels typically possess some short-range crystalline

order but as a whole are amorphous, due to incomplete condensation of silanol

functionalities and imperfect ordering during the polymerization process.

For silica which has not been subjected to calcination (200 C or higher)

the structure can be described as an imperfect semi-crystalline lattice with defects

arising from uncondensed proximal SiOH groups. Peri and Hensley94 have

proposed that these lattice defects consist of neighboring geminal hydroxyl groups

(two hydroxyls attached to one silicon atom). At ambient temperature, the

adjacent silicons are constrained in the gel lattice so that condensation does not

occur. Mild heating (to about 100 C) is enough to allow the silicons to form the

siloxane bridge. The strain on the lattice induced by this reaction is the driving

force for the reverse reaction at room temperature in the presence of water. If

the silica is taken to high temperature, >200 C, then the surface loses more

hydroxyl functionalities and becomes more ordered. This structure reordering at







56

high temperature can result in alleviation of the ring strain mentioned above and

accounts for the hydrophobicity of silicas which are treated at elevated

temperatures.

Maciel and Sindorf54 have derived a surface model of the

dehydration/rehydration process, shown in Figure 2-12. The geminal silanols

condense with either a geminal neighbor to form two single hydroxyls on vicinal

silicon atoms, or a neighboring single silanol to form a new hydroxyl of different

orientation. It should be clear from Figure 2-12 that the relationship between the

(111) plane and the (100) plane changes dramatically upon even mild heating.

This model is certainly an idealized picture of silica gel; Peri and Hensley94

propose that unheated silica possesses only geminal silanol groups and would have

a much more random structure than Figure 2-12 presents.

The interaction of A12C16 with the silica surface can take place in a number

of ways. Based on the stoichiometry of HCI evolution during the synthesis, the

conceptually simplest interaction is reaction of an AICI3 unit with a lone SiOH

moiety to form SiO-AICI2, giving rise to the active catalyst. Recently revised HCI

quantification has shown that the reported one mole of HCI formed for each

AIC13 used is actually closer to 1.5, indicating that SiO-AICI2 is not a wholly

accurate representation of the adsorbed aluminum sites. If reaction of A12C16 was

favored at single silanol sites, as present on the (111) plane in Figure 2-12, it is

expected that dehydration of the silica should not be a determining factor in

catalyst preparation because siloxane formation between single silanol groups is

























+ H20 H20


* = OH Group


Figure 2-12. Model of Silica Gel Surface Illustrating the
Dehydration/Rehydration Equilibrium.








unfavorable at temperatures less than 400 C due to the major reordering that

must occur to accommodate the resulting geometry.

Interaction of aluminum chloride with geminal silanol sites would be

notably different than for single silanols. A value of 1.5 moles HCI evolved for

each A1CI3 unit indicates that there are at least two SiO-Al bonds for an

appreciable quantity of acid sites. The orientation of the geminal hydroxyls

pictured in Figure 2-12 appears to be more favorable for formation of two SiO-Al

bonds per Al site as compared to the single silanol groups. Furthermore,

assuming that there is a predominance of geminal sites on the surface, as

proposed by Peri and Hensley, it is likely that A12C16 would react with those sites

as opposed to single silanol sites. If interaction between A12C16 and geminal

silanol groups is the preferred reaction, then it is expected that mild dehydration

of the silica, which results in condensation of these sites, would have a major

influence on the catalyst preparation. In addition, rehydration of the silica by

limited exposure to water would restore the geminal sites and return the silica to

its active state.

The effect of silica hydration on the preparation of AlCl2(SG)n is perhaps

best illustrated by comparing the activity of catalysts prepared with "dry" silica

(limited quantity of geminal silanol groups), hydrated silica (excess adsorbed H20)

and conditioned silica (optimum content of geminal sites). Table 2-6 illustrates

the activity comparison for the cracking of n-hexadecane in a batch system with

the catalysts prepared using the various silica samples. Silica sample A was









Reactivity of AICI2(SG)n Catalyst
Silica in Varying Degrees of Hydration.


Samples Prepared with


SAMPLE COLOR MOLES RELATIVE DETAILS
HCI CRACKING
PROD./A] ACTIVITY
A Brown 0.7 0.16 C1 C3 in
large
quantity
B Yellow 1.3-1.7 1.0 standard
R+
distribution

C White 2.2 < 10-5 minimal
products
observed


Silica Pre-treatment:

Sample A: Activate with 1 M HCI, dry at 80 *C for 72 hours
under vacuum.

Sample B: Same as A but exposed to atmospheric water for
24 hours.

Sample C: Activate with 1 M HCI, dry at 40 C for 24 hours.


Table 2-6.







60

activated with 1 M HCI and dried at 80 C for 72 hours, as reported in references

3,4 and 49. This sample was used directly from the drying oven in the catalyst

synthesis procedure. The method gives a brown catalyst intermixed with white

particles, indicating less than complete reaction of the A12Cl6 with the silica

sample. The reactivity of this catalyst in hexadecane cracking shows isobutane

produced in a small amount with additional products of Cl to C3 hydrocarbons.

Unsupported Al2Cl6 will crack hydrocarbons itself but coordinates very easily to

trace olefins which are produced and rapidly deactivates,49 so the minimal activity

seen with this catalyst is not unexpected.

Silica possessing a high adsorbed water content is also unfavorable for

AICl2(SG)n preparation. By altering the silica drying procedure to a temperature

of 40 C for 24 hours, the silica retains enough water to hydrate a majority of the

Al sites to a less acidic octahedral environment, as evidenced by the white color

and excessive HCI formation. With 2.2 moles of HCI formed per Al site,

hydrolysis is approaching the limit of 3 for full hydrolysis of Al-Cl bonds. It is

therefore not surprising that the catalyst shows no activity in cracking, as the much

lower acidity of octahedral Al vs. tetrahedral Al is not sufficient for carbonium ion

generation.

It is evident that relatively small variations in silica hydration produce very

different materials when reacted with Al2C16. The active version of the catalyst is

prepared with silica which is allowed to absorb water from the atmosphere for a

minimum of 24 hours. During this time period, the mass of the silica gel increases








by 0.02 g ( 0.001 g) per gram of dry silica, or 28 mmol water per gram silica.

This corresponds to 56 mmol of SiOH groups formed if each molecule of water

hydrolyses one Si-O-Si linkage. This is much higher than the estimated 10 to 20

mmol of silanol moieties proposed as the typical concentration of these groups in

uncalcined silica. This implies an excess of water is physisorbed on the support

but it is not known whether this excess water plays a role in the formation of

active catalytic sites. The profound influence of mild dehydration conditions on

the catalyst preparation leads to the conclusion that A12C16 is interacting primarily

with geminal silanol groups on the silica, as they are the only sites which are

altered at the mild dehydrating conditions used in the silica pre-treatment.

Infrared examination of adsorbed pyridine. In order to obtain a qualitative

assessment of the different acid sites for AlCl2(SG)n prepared with silica at

various stages of hydration, the infrared spectrum of adsorbed pyridine can be

used. The catalyst samples studied in this method are the active catalyst

prepared with conditioned silica and the sample prepared with excessively dry

silica. The two samples are degassed under vacuum at 200 oC for 3 or more

hours, contacted with pyridine vapors for a minimum of 3 hours and finally

degassed a second time at various temperatures to remove excess and weakly

bound pyridine. Parry38 has done a thorough assignment of the pyridine bands on

acidic surfaces and conclusions can be made based on these assignments. A band

at -1540 cm-1 involves C-N+-H bending in the vibration which justifies its use as

a fingerprint for Br6nsted acidity. At 1445 cm-1 a band occurs which is indicative








of Lewis-bound pyridine and can be used as a gauge for the presence of Lewis

acid sites. It should be mentioned that there is also a contribution from hydrogen-

bonded pyridine in the 1445 cm-1 band which can present problems when

assessing Lewis acidity such that other factors should be considered before using

this peak for fingerprinting purposes. A third band is present at 1485 cm-1 but

occurs for both Lewis and Br6nsted acidity; conclusions can not be made directly

using this particular peak.

Figure 2-13 presents the absorbance IR spectra for the two catalysts made

with dry silica and conditioned silica. In both cases, there are significant amounts

of both Br6nsted and Lewis sites on the catalysts but the patterns for the three

fingerprint peaks are sufficiently different that qualitative judgements can be made

as to respective amounts of each acid site type. The Bronsted peak at 1540 cm-1

for the active catalyst (made with conditioned silica) has an overall intensity close

to that seen for the Lewis band at 1445 cm'1; relative to the sloping baseline the

Bronsted peak is more intense the Lewis band. This three band pattern is very

reproducible for catalytically active AICl2(SG)n. In the case where the catalyst is

prepared with dry silica, the Br6nsted band is much less intense than the

Lewis/hydrogen bonding peak. Two explanations for the difference in the two

spectra can be proposed. Assuming the validity of the proposed catalyst structure

as shown in Figure 1-1, it is apparent that Br6nsted acidity for AICl2(SG)n arises

from coordination of the oxygen lone pairs of a silanol group to a vicinal Al site

which thereby activates the SiO-H bond for proton donation. As the dry silica









63



(IJ
SACl2(SG)n Pyridine Adsorbed, Conditioned Silica Prep.


SB+L


CB L






LO
a.





rn




'-1700 1666 1i32 1i98a Ie6- 130 1496 1462 142a lis9
WAVENUMBER



0 AIC12(SG)n Pyridine Adsorbed, Dry Silica Prep.







,a' B+L
in L

0
m








rn
E3L










oi

-1o00 1666 1632 1t9a 1e64 13O 1496 1462 1426 1







Figure 2-13. Infrared Spectra of Pyridine Adsorbed on AIC12(SG)n
Prepared with Conditioned Silica and Dry Silica.








would have a decreased concentration of silanol moieties, the occurrence of a

silanol group near an aluminum center is less probable; the result of this is that a

greater quantity of silanol groups are available to hydrogen bond with pyridine

which would show up as an increased intensity of the 1445 cm-1 band. This

explanation also accounts for the decreased intensity of the Br6nsted peak. The

other explanation, which may not be mutually exclusive of the first, is that the

large amount of unreacted Al2Cl6 present in the dry silica catalyst sample

coordinates pyridine via a Lewis interaction and therefore increases the intensity

of the 1445 cm-1 peak.

Solid State Magic Angle Spinning NMR. One of the most useful methods

for elucidation of heterogeneous catalyst structure is solid state magic angle

spinning nuclear magnetic resonance (abbreviated SS MAS NMR). The 27Al SS

MAS NMR spectrum (300 MHz) for AlCl2(SG)n prepared in CCI4 with

conditioned silica is presented in Figure 2-3. The peak at 65 ppm is assigned to a

Si-O-AICl2 moiety, which is consistent with the value of 62.8 ppm obtained for

aluminum chlorohydrate (Cl/OH = 2.5).95

Figure 2-14(a) gives the spectrum for the corresponding catalyst prepared

with dry silica. The spectrum for the conditioned silica catalyst is presented as

figure 2-14(b) as a comparison. The conditioned silica spectrum has been sized to

match the scale depicted in the figure. A peak at 0 ppm in Figure 2-14(a)

indicates the presence of octahedral aluminum. A very prominent feature of this

spectrum is the sharp signal at 101 ppm, which coincides with the value of 105



























b) Prepared with Conditioned Silica.
















a) Prepared with Dry Silica.


J 1 I I I I I I 1 I I I I I 0 I
100 50 I I
100 50 0 -50 -100


6 (ppm)


Figure 2-14. SS NMR of AlCI2(SG)n Catalysts.
a) Prepared with Dry Silica.
b) Prepared with Conditioned Silica.







66

ppm reported for anhydrous A]2C6.52 This observation along with the increased

intensity for the IR band of Lewis-bound pyridine shows that a substantial amount

of A12Cl6 has not reacted with the support. The best evidence of unreacted

A12C6 lies in the observation of desorption of the aluminum chloride at 200 C

under flowing N2; at least 18 % of the A12C16 used to prepare the catalyst desorbs

off at this temperature. This result is in stark contrast to that obtained with the

catalyst prepared from conditioned silica which shows no desorption of A12Cl6

even at 250 to 275 C.

The lack of major features in the 40 80 ppm range for the dry silica

catalyst in Figure 2-14(a) is also quite different than for the spectrum in Figure

2-14(b). This is an indication that the catalytic activity for hydrocarbon conversion

reactions is due to the Al species appearing at 65 ppm. The broad peak at 30

ppm in Figure 2-14(a) may be due to a species similar to the one giving rise to the

signal at 15 ppm in Figure 2-14(b), proposed to be octahedral aluminum with

chloride and oxygen-donor ligands.

The overall result of the silica hydration studies presented here is to

demonstrate that minor variations in the silica preparation can have a dramatic

effect on the properties of the resulting catalyst. Based on literature studies

investigating the dehydration/rehydration of silica gel, the presence of geminal

silanol groups appears to be critical for obtaining complete reaction of A12C16 with

the silica surface and generating active catalytic sites. Reproducibility in catalyst

preparation is obtained by optimizing the hydration of the silica support.









Sealed-system catalyst preparation

Preparation of AlCl2(SG)n in a sealed system was performed to test the

proposal that CCI4 functions as an unreactive medium. The carcinogenic

properties and toxicity of chlorinated solvents96 provide the motivation for

developing alternate methods of catalyst preparation which avoid the use of CCI4

or other potentially hazardous chlorinated solvents. As mentioned in the

background section, vapor deposition methods have been utilized 5-11 for

preparation of supported aluminum chloride but presumably due to short contact

times Al2Cl6 does not form exceptionally stable catalysts with inorganic oxides

under standard vapor deposition conditions.

One variation on this vapor deposition technique has arisen which does

offer some improvement. The method devised by Schmidt and co-workers36'37

involves contacting gaseous A12C16 with alumina for an extended time period

relative to fast-flow vapor deposition techniques. The Schmidt catalyst has

reported characteristics quite similar to the Drago/Getty AICl2(SG)n catalyst and

serves as a precedent for catalyst preparation using long-term vapor deposition

methods.

What appears to be a key aspect of the CC14 synthesis procedure is the

amount of time the catalyst components are allowed to react. The sealed-system

reaction was run for a minimum of 72 hours in order to be consistent with the

CC14 synthesis. Other syntheses using shorter time periods were also performed

to investigate this variable.











ALCL(SG)N. CCLk PREP EVAC 8 RT


ALCL2(SGIN S/S PREP EVAC B RT


WAVENUMBER


Figure 2-15. Infrared Spectra of Pyridine Adsorbed on AIC12(SG)n Samples
Prepared in CC14 and in a Sealed System, 72 Hour Reaction.




















ALCL2 SG)N S/S PYR ADS


Peak List


1441.1 cm-1


1485.8 cm-1


1538.1 cm-1


(Lewis, H-bonding)


(Lewis & Br6nsted)


(Br6nsted)


Figure 2-16. Infrared Spectrum of Pyridine Adsorbed on AIC12(SG)n
Prepared in Sealed System, 18 Hour Reaction.


CO
Lad
0



to3
CO


CD
Ln)


U
ZO



CD
4,





to


CP)
(0
-1







70

Figure 2-15 shows the infrared spectrum of pyridine adsorbed on a catalyst

made in CCI4 and the catalyst made in a sealed system over 72 hours. The peak

at 1540 cm"1 is assigned to pyridinium ion produced at Brinsted acid sites,

labeled (B). The peak at 1445 cm-1 is due to Lewis-bound pyridine with some

surface hydrogen-bound pyridine peak overlap. This peak is labeled (L). A band

at 1490 cm-1 exists for both Lewis and Br6nsted sites (L + B), therefore it is

not conclusive as a fingerprint signal. The overall pattern for the various sites is

identical for the catalyst prepared in CC14 and in the sealed system which implies

a similar distribution of acid sites for both samples. This point becomes more

clear after examining Figure 2-16 which shows the IR spectrum for pyridine

adsorbed on a catalyst prepared in the sealed system for only 18 hours as opposed

to the normal 72 hour period. In this spectrum, there is a significantly different

pattern observed, with the Lewis band more intense in relation to the Br6nsted

peak. This is what would be expected for a case where excess Al2C16 is present to

bind the pyridine preferentially over the weaker silica Bronsted acid sites.

Establishing the strength of the Lewis acid site is also possible by IR

spectroscopy. Figure 2-17 presents the spectrum of a catalyst exposed to pyridine

and evacuated at 300 C. This procedure ensures that all the pyridine is removed

with the exception of the most tightly bound pyridine. The signal at 1456 cm-1 can

be assigned to Lewis-bound pyridine. This is a shift of 18 cm-1 from the free

pyridine stretch at 1438 cm" and is indicative of a very strong Lewis acid

interaction. The shift of the Lewis band as reported for the CCl4-prepared





























a)











w |
Z.3
M 3N
O
U)
CD
3o




CD






0 103 1~69 1635 1601 1t67 1t33 1499 1465 1431 1 97
WAVENUMBER














Figure 2-17. Infrared Spectrum of Pyridine Adsorbed on A1C12(SG)n
Prepared in a Sealed System, Desorbed at 300 'C.









catalyst4 is identical to the above result, indicating very similar acid strength for

the Lewis sites on the catalysts prepared by two different techniques.

The solid state MAS NMR spectrum for AIC12(SG)n prepared in a sealed

system is presented in Figure 2-18. Comparing this spectrum to that of the

catalyst prepared in CCI4 as shown in Figure 2-3, a number of similarities can be

pointed out. The signal at 65 ppm is very strong for both samples and both

samples exhibit a shoulder at 50 ppm, which could be a spinning side-band. The

peaks are attributed to tetrahedral Al sites. The spectrum in Figure 2-18 also

exhibits a peak at 0 ppm which can be assigned to octahedral aluminum. The

signal at 20 ppm in Figure 2-18 is attributed to an octahedral Al site with both

chloride and hydroxo ligands, also similar to the catalyst prepared in CCl4.

Since the spectroscopic results showed a distinct similarity in acid site

distribution, the sealed-system (s/s) catalyst was used in the cracking of n-

hexadecane in a batch reaction in order to make a reactivity comparison to the

CCl4-prepared catalyst. At 100 C under 30 psig hydrogen, 1.0 g of the s/s catalyst

gave an isobutane production of 0.22 ( 0.02) moles isobutane/mole Al sites as

compared to AlCl2(SG)n prepared in CCl4 which showed 0.24 ( 0.02) moles

isobutane/mole Al sites. It is apparent that both catalysts give good activity in

cracking reactions and are indistinguishable within the error limits for the

experiment. It can be said at this point that the strong acid sites on the catalyst

are the same or very similar in structure and reactivity when the catalyst is

prepared in carbon tetrachloride or via prolonged vapor deposition. The





















AL,912Q. 011 WEST
AL' CL 6/5 102/CREl
MiA5 4.5 k1H-2, 27HA.
HAWA


DEC


K


YF= 10433
>PX
FROM 176.31 TO
1085.66 HZ/CM
)LP
AL9120 .011 WEST
AL2CL6/SI02/GREY
MAS 4.5 KHZ. 274.4 DEG
HAWK
1 PULSE SEQUENCE
P2= 13.00 USEC
05= 900.00 USEC
NA = 21856
SIZE 65536
AT = 262.14 MSEC
QPD ON = 4
ABC ON
BUTTERWORTH FILTER ON
DB ATT.= 1
ADC = 8 BITS
AI = 3
SW = +/- 62500.0
DW = 8
RG = 10 USEC
DE= 4 USEC
TL HIGH POWER ON
F2= .000000
BB MODULATION OFF
OF= 739.61
SF= 78.187887
EM= 20.00
PH. 287.0
PB- .1


15 'f. -50 -10, I
15Q 1QQ 50 0 -50 100o


-101.34




























PPM


Figure 2-18. SS MAS NMR of AICI2(SG), Prepared in a Sealed System.


'Nov.. 1, v


"









similarity in reactivity for the two catalyst samples further supports the proposal

that the NMR resonance at 65 ppm, which is common to the spectra of both

samples, is the active site for catalysis.


Conclusions


The goal of the project delineated in this chapter is to examine the

reactivity of AICl2(SG)n for various hydrocarbon conversions as well as to develop

an alternative preparation for the catalyst and to determine how silica hydration

affects catalyst structure and reactivity.


Reactivity Studies


Dehydrohalogenation and hydrodehalogenation of 1,2-dichloroethane was

studied with various metal-promoted variations of AICl2(SG)n. The activity of

AlCl2(SG)n was found to increase markedly with the presence of noble metal

promoters but these promoted catalysts offered mixtures of hydrodehalogenated

and dehydrohalogenated products with poor selectivity to the vinyl chloride target

product, presumably due to the propensity of the catalysts to promote

hydrodehalogenation. Trichloroethane offered better results, with 90 %

conversion of the reactant with 90 + % selectivity to the dehydrohalogenated

product. Comparison to a silica blank indicated that the reaction was catalytically

and not thermally induced.









Alkylation of aromatic substrates with alky] halides and olefins did not

produce the expected results but offered insight into the mechanism nonetheless.

In batch reactions performed in CC14, it was determined that alkylated products

are obtained readily from 30 halides (t-BuCl, e.g.), less readily from 2 halides and

not at all with 1 halides. This reflects the tendency of the catalyst to form the

most stable carbonium ions from the corresponding halides. Catalyst poisoning by

the aromatic reactant is proposed as a competing pathway which limits cation

formation to only the more reactive alkyl halides. Olefins show no reactivity in

batch reactions since prolonged contact of the olefin with the catalyst favors rt-

coordination of the olefin to the Lewis sites over protonation of the olefin to form

the reactive carbonium ion intermediate. Acyl chlorides show the expected lack of

reactivity due to the coordinative nature of the oxygen donor group which is

known to deter hydrocarbon activation activity for AICl2(SG).n

Alkylation in a flow system at more extreme conditions shows that

AIC12(SG)n is too vigorous, with t-BuCI reacting to give products arising from

oligomerization and cracking of the alkyl halide. Evidence also shows that the

alkylaromatic product can dealkylate under the experimental conditions as

evidenced by reaction of cumene over AICl2(SG)n to give C3 products and

benzene. The cumene cracking activity signifies that formation of alkylaromatic

products in the flow system is unfavorable, therefore limiting the use of

AICl2(SG)n to low-temperature batch-type reactions.








The supported aluminum chloride catalyst was found to be quite active in

the isomerization of n-butane, even at room temperature, if the correct additives

are used to initiate and maintain activity. Pure n-butane gave limited reactivity;

addition of 4 % 1-butene to the feed resulted in a significant jump to 50 %

conversion. A mechanism involving proton addition to the olefin is proposed

based on this result. Addition of HCI to the flow is required to maintain activity,

presumably by regenerating the Brinsted sites as they are depleted during the

reaction.

The chemistry of n-hexane conversion over AICl2(SG)n was found to be

somewhat more complex than that seen with butane due to the introduction of

cracking as a competing reaction. The presence of branched C6 isomers

comprised only 15 % of the products observed under conditions necessary for

hexane activation in the flow system. The increased activity and longevity if

AIC12(SG)n in the presence of olefins and HCI showed that the mechanism for

hydrocarbon activation is similar to that with n-butane; the variation in product

distributions results from different reactivity of the respective carbonium ion

intermediates.

Polyolefin reactants were used as extended-chain analogs to simple

paraffins in cracking reactions for possible application in waste polymer

remediation. Poly(ethylene) in a melt state afforded similar results to hexadecane

used previously in terms of the moles of isobutane produced per aluminum site on

the catalyst. A highly branched polymer, poly(4-methyl-l-pentene), gave the









expected higher activity resulting from the presence of 20 and 30 carbons.

Poly(styrene) and cellulose acetate were unreactive with AlCl2(SG)n for reasons

mentioned previously; namely, coordination of donor groups such as aromatic Tr-

systems or oxygen functionalities serve to poison the acid sites on the catalyst

surface. An important result for possible real-world application of polymer

recycling arose from the observation that commercial polymeric materials such as

rubber tire and plastic poly(ethylene) shopping bags give reasonable cracking

reactivity despite the presence of stabilizers and other industrial additives.


Synthetic Studies


A prolonged vapor deposition method, where sublimed A12C16 is contacted

with silica for an extended period of time (3 to 5 days), was found to result in a

catalyst with similar activity in hexadecane cracking to the catalyst synthesized in

CCl4. Infrared spectroscopy on adsorbed pyridine showed identical acid site

distribution and identical acid strength for the Lewis sites as determined by the

shift of 18 cm-1 for pyridine adsorbed on a Lewis site versus free pyridine. Solid-

state NMR experiments revealed the presence of a species which has a resonance

at 65 ppm in both the CC14-prepared catalyst version and the sealed-system

catalyst version. The consistency in this signal coupled with the consistency in

cracking activity strongly suggests that the species at 65 ppm is the active catalytic

site; based on HCI evolution this is probably a SiO-AICl2 or (SiO)2AICI species.






78
Finally, the preparation of AICl2(SG)n was investigated from the standpoint

of silica hydration. The extent to which silica gel possesses silanol groups was

found to have a significant influence on the binding of A12C16 to the silica surface.

Infrared examination of adsorbed pyridine and solid state MAS NMR indicated

the presence of unreacted A12C16 as well as drastically different acid sites present

on the catalyst prepared with excessively dry silica. Comparing the activity of the

catalysts prepared with conditioned and dehydrated silica it was shown that

catalytic activity results only when A12C16 is condensed with SiOH groups to form

the catalytically active moieties.












CHAPTER 3
POROUS, CARBONACEOUS ADSORBENTS AS CATALYSTS
AND CATALYST SUPPORTS FOR ACIDIC METAL OXIDES


Background


The application of porous, carbonaceous adsorbents to heterogeneous

catalysis is an area which, although known for some time, is receiving increased

attention in recent years. In the early 1970s, Trimm and Cooper97'98 and Schmidt

and Walker,99'100 in independent studies, reported the use of platinum-modified

porous carbons for olefin hydrogenation. In their investigations, shape selectivity

to the linear isomer of a mixture of linear and branched hydrocarbons was

observed; this was an indication of the molecular sieving capability of the carbons

as analogous to that seen with zeolites. More recently, Foley101 has studied the

modification of porous carbons with inorganic oxides through a number of

different preparative routes for application in catalysis. Pyrolysis (heating to 300 -

1200 C in an inert atmosphere) of successive layers of polymer material on the

surface of a metal oxide particle or pyrolysis of a physical mixture of the two

components are two ways of producing a carbon surface over an oxide material.

Adsorption of the oxide (or a precursor compound) on a carbon surface is a way

to modify a carbon with a catalytic oxide component. The adsorption of oxides on









carbon surfaces is the preferred way to synthesize catalysts from commercially

available carbon supports.

A widely used method for preparation of porous carbons, also known as

carbon molecular sieves (CMS), is the pyrolysis of macroreticular (macroporous)

polymers. The polymers themselves are designed to have high macropore volume

to facilitate transport of adsorbates in ion-exchange applications.102'103 During

the pyrolysis procedure the polymer loses hydrogen and becomes essentially a

carbon skeleton of the parent polymer, thereby retaining the macropore

characteristics of the macroreticular polymer in carbon form. Figure 3-1 presents

a proposed scheme for the pyrolysis of poly(styrene/divinylbenzene) which depicts

the resulting carbon structure at various temperature levels. The physical strain

on the polymer as it undergoes pyrolysis causes the formation of fissures in the

carbon matrix. These fissures can be regarded as a separation of two graphite

layers, leaving behind slit-like pores of molecular dimensions, or micropores, which

impart the molecular sieving properties to the carbon.101 Volatilization of small

molecules out of the matrix is also reported to contribute to micropore formation.

The end product of these processes is a highly porous carbon material with high

surface area. This high surface area along with the unique pore structure and

hydrophobic surface allows for exceptional adsorption of organic molecules which

can be utilized as an asset in the application of porous carbons to heterogeneous

catalysis.












300 oC


5000C
N2


0O0

T OO
000
00


Figure 3-1. Structure of Pyrolyzed Poly(styrene/divinylbenzene) at
Various Temperatures.


'


O

81OOR 0
iOOO
~ (T









There have been studies done on the influence of pyrolysis temperature

and duration on the pore structure of polymer-derived carbons. Lafaytis et al.104

have determined that pyrolysis of poly(furfuryl alcohol) at 800 to 900 C produces

the maximum surface area for the final carbon and that higher temperatures can

decrease the micropore volume. Furthermore, longer pyrolysis times were found

to decrease the overall dimensions of the micropores, as examined by diffusion of

CO2 and n-butane into the carbon pores monitored by gas uptake. This

examination demonstrates that microporous carbons can be tailored for

predetermined applications so as to optimize the effectiveness of the CMS for

adsorption. This tailoring of carbon pore structure has already been shown to be

applicable in the development of carbon adsorbents for groundwater

purification.13

The carbon materials which are used in the catalytic and structural

examinations presented in this chapter are derived from poly(vinylaromatic)

polymers. Specifically, the pyrolysis of divinylbenzene-crosslinked poly(styrene)

ion-exchange resins is the synthetic procedure utilized. The ion-exchange resins

possess sulfonate groups which are proposed to stabilize the polymer during

pyrolysis and also result in additional micropore structure as the sulfur oxides

volatilize off of the surface.13'105'106

The presence of slit-like pores is not unique to the divinylbenzene-based

carbons used in this work. The phenomenon was first observed with pyrolyzed

poly(vinylidene chloride) in 1965 where normal butane was found to adsorb








preferentially into the pores of the carbon from a mixture of normal butane and

isobutane.107 In addition to preferentially adsorbing molecules by size, polymer

carbons have been demonstrated to separate molecules by polarity. In a study of

adsorption capacity for water and dichloromethane, it was determined that a series

of commercial carbons, Carboxen CMS, had a greatly increased adsorption

capacity for dichloromethane as compared to water.108 The difference was

attributed to the hydrophobic surface of the carbon showing preference for

hydrophobic molecules. In addition, it was also observed that the extent of

chemical activation of the CMS can influence the surface chemistry and hence the

relative hydrophobicity of the surface by the introduction of oxygen and/or

nitrogen groups. The more activated CMS samples showed a higher affinity for

water adsorption. From these studies, it is apparent that polymer carbons can be

used as two-function adsorbents; molecules are able to be separated based on size

and on hydrophobicity so as to provide remarkable selectivity to target adsorbates.

By careful choice of pyrolysis conditions to determine micropore dimensions and

by chemical activation to control the surface chemistry, CMS materials are unique

in their ability to be tailored for specific applications.

The actual chemical structure of surface species on activated carbons is a

subject of much controversy. The transient nature of these moieties, in particular

oxygen-containing species, makes identification difficult even for relatively rapid

spectroscopic techniques. It should be mentioned, however, that certain chemical

reactivity of substituent groups on the carbon surface has been empirically







84

examined and acidity was found to be a prevalent characteristic. Reaction of basic

and acidic dyes was examined on various carbon samples and it was found that the

basic dyes showed a much larger extent of reaction as compared to the acidic

dyes.109 The data for surface reactivity of the dyes was correlated with NaOH

titration data to establish that acidic groups on the surface are indeed present and

are involved in the chemistry observed with activated carbon.

Given the control that can be obtained over the structure of CMS

materials, it is somewhat surprising that there is a dearth of research in the

literature pertaining to utilization of polymer carbons as catalysts and catalyst

supports. The capacity of carbons to function catalytically has been known only

for about 15 years, as delineated in a study of ethylbenzene oxidative

dehydrogenation using alumina.110 The activity of the alumina catalyst had been

found to be influenced by the coke buildup on the catalyst surface, which in itself

is a direct function of the alumina acidity. Further studies of the coke activityll1

show a correlation between the paramagnetism of the coke to the catalytic activity

observed. Vrieland and Menon112 have recently presented a brief review on the

role of carbon in the oxidative dehydrogenation of ethylbenzene with the ultimate

conclusion that the mechanism of carbon catalysis is as yet undetermined.

Literature reports continue to arise on this question, however, and there may be a

better understanding of catalysis with carbon materials in the near future. For

example, a recent examination of the oxydehydrogenation of ethylbenzene over

alumina supports the proposition that carbon oxide species are responsible for the









activity.113 A number of different techniques were utilized in the study, such as

ammonia desorption, X-ray Photoelectron Spectroscopy, Electron Paramagnetic

Resonance and Secondary Ion Mass Spectrometry. By process of elimination, the

authors were able to narrow the active species down to the oxygen-containing

paramagnetic quinone/hydroquinone and aroxyl/phenol groups.

In addition to examination of surface coke as a carbonaceous catalytic

agent, there have been studies done on pyrolyzed polymers as active carbon

catalysts. The oxidative dehydrogenation of ethylbenzene over a commercial

CMS, Anderson AX21, was studied by Grunewald and Drago.114 The high

surface area CMS was determined to be one of the most active catalysts known

for the oxidative conversion of ethylbenzene to styrene, with 80 % conversion of

the ethylbenzene obtained at 350 C and selectivity to styrene of 90 %. The same

authors also examined the oxidation of alcohols over AX21115 in terms of the

reaction mechanism. The products obtained in the alcohol oxidations

demonstrated that CMS can function via hydride abstraction arising from Lewis

acidity or in hydrogen atom abstraction, presumably from paramagnetic sites. The

reaction mechanism, therefore, is apparently dictated by the reactivity

requirements of the particular substrate.

Grunewald and Drago have also examined the Fischer-Tropsch synthesis of

hydrocarbons from hydrogen and carbon monoxide over carbon catalysts.116 The

product distribution obtained with carbon-supported ruthenium carbonyl species

was found to have a significantly increased selectivity to C2 through C5







hydrocarbons, with no products higher than C5 observed. This result was

attributed to the ability of the carbon to disperse metal species, thereby limiting

agglomeration and formation of metal crystallites. Scanning electron microscopy

supports this proposal115'117 as no formation of metal crystallites was detected on

the carbon in contrast to the analogous alumina or silica catalysts which show

metal aggregation. This dispersion and anchoring of metal species can have a

major influence on catalyst stability and offers another advantage for the

utilization of CMS in heterogeneous catalysis.

One application of carbon-based catalysts which has been devised117 is the

catalytic combustion of halogenated organic compounds. Halogenated

hydrocarbons have widespread application as solvents in the chemical industry due

to their relative inertness and solvating capacity for organic compounds.

Investigations regarding the toxicity and carcinogenic properties of halogenated

organic have raised industry awareness on the matter of proper disposal of these

hazardous materials.118 Incineration is presently the preferred method but

temperatures exceeding 1000 K are required for the process to obtain complete

decomposition119,120 and dioxin is often a byproduct. In cases of low

concentration of contaminant in the gas phase, the process becomes exceedingly

inefficient as the entire sample must be heated to the combustion temperature.

The cost of the fuel alone can reach about 40 % of the total operating cost for a

typical incinerator,121 so development of low-temperature processes for







halogenated waste disposal can offer significant improvement over present

methods.

Spivey122 presents an exhaustive review (to 1987) of low-temperature

oxidative decomposition catalysts for application in environmental remediation.

The majority of the catalysts can be put into two categories: transition metal

oxides (either unsupported or adsorbed onto an inorganic oxide support) and

supported noble metals. The noble metal catalysts (Pd, Pt, Rh, Ru) are poor

choices for oxidation of halogenated hydrocarbons because of the high expense

involved and the poisoning of the catalysts by Cl2 and HCI produced in the

reaction.123'124 A number of patents have been issued for the process of

destroying halogenated hydrocarbons with metal oxide catalysts125-137 but

optimum temperatures for these systems are usually >300 C and the halogenated

hydrocarbon concentration is typically limited to less than 10,000 ppm.

The purpose of the studies performed in this chapter is to examine CMS

materials as catalysts for the catalytic combustion of halogenated hydrocarbons.

Examination of the reaction mechanism may provide further insight as to how

CMS function as catalysts and catalyst supports.

Characterization of the carbon catalysts is necessary to understand the

reactivity of the catalysts and eventually the mechanisms by which the catalysts

operate. Due to certain properties inherent to the carbon material, carbon

molecular sieves are difficult to characterize. Diffuse reflectance infrared fourier-

transform spectroscopy (DRIFTS) and solid state 13C nuclear magnetic resonance