Carbon molecular sieves as catalysts, catalyst supports, and in advanced materials applications

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Carbon molecular sieves as catalysts, catalyst supports, and in advanced materials applications
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xi, 184 leaves : ill. ; 28 cm.
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Grunewald, Gerald C., 1963-
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Molecular sieves   ( lcsh )
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bibliography   ( marcgt )
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Thesis:
Thesis (Ph. D.)--University of Florida, 1989.
Bibliography:
Includes bibliographical references (leaves 172-183).
Statement of Responsibility:
by Gerald C. Grunewald.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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Full Text









CARBON MOLECULAR SIEVES AS CATALYSTS,
CATALYST SUPPORTS, AND IN ADVANCED MATERIALS
APPLICATIONS









BY

GERALD C. GRUNEWALD


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


1989




















TO MY FAMILY FOR THEIR CONSTANT SUPPORT












ACKNOWLEDGEMENTS

There have been a large number of people who have helped to make

my journey through graduate school very enjoyable. The list presented

here is by no means complete. Foremost, I would like to express my

gratitude to my research advisor, Russell Drago, whose support and

guidance have been essential to my education. Besides showing me the

excitement of chemical research, Doc has also contributed significantly

to my knowledge of two-on-two basketball and has taught me the fine art

of eating spaghetti without cutting it. I would equally like to thank

Ruth Drago whose humor and hospitality helped to make Gainesville a home

away from home. Maribel Lisk also deserves special thanks. Her

friendship and assistance, along with her seemingly endless supply of

stationery goods, have been most appreciated.

My coworkers during my years in the Drago group have been a

valuable source of knowledge and advice as well as being great friends.

Former group members Jeff Clark, Andy Griffis, Ken Balkus, Rich Riley,

Pete Doan, Ed and Cindy Getty, and Shannon Davis helped a great deal in

showing me "the ropes." Present group members Larry Chamusco, Ngai

Wong, Bobby Taylor, Tom Cundari, Al Goldstein, Mike Naughton, and Steve

Petrosius have been comrades, compatriots, and cohorts in both

scientific and social endeavors. I would also like to thank my

roommates of the renowned Thunderdome. Mark Hail, Mark Barnes, Don

Ferris, and Steve Showalter have been great friends as well as often

being good cooks and housecleaners. A special thanks is also due to the








Stranger band who provided many a night of great entertainment for the

revelers of the Inorganic and Analytical Divisions.

One other former Drago group member has also had a significant

influence on my career. I would like to thank my future wife, Cindy

Bailey, for her love, support, and advice. Although there are a lot of

things I will miss in Gainesville, the frustration of a long-distance

relationship and the high phone bills will not be among them. Lastly, I

would like to thank my family for their constant prayers and support.

In particular, I want to thank my brother-in-law, George Fisher, who was

a great help in preparing this manuscript.














TABLE OF CONTENTS


ACKNOWLEDGEMENTS .

LIST OF TABLES . .

LIST OF FIGURES .

ABSTRACT . .

CHAPTERS


GENERAL INTRODUCTION .

PREPARATION AND CHARACTERIZE


Introduction .
Experimenatal .

3 CARBON MOLECULAR SIEVES

Introduction .
Experimental .
Results and Discussion

4 CARBON MOLECULAR SIEVES

Introduction .
Experimental .
Results and Discussion

5 CARBON MOLECULAR SIEVES
APPLICATIONS .

Introduction .
Experimental .
Results and Discussion

6 CONCLUSIONS .

REFERENCES . .

BIOGRAPHICAL SKETCH .


AS C





AS C





IN A


TION OF CMS MATERIALS




ANALYSTS .

. . .



CATALYST SUPPORTS .





ADVANCED MATERIALS




. . .

. . .

. . .

. . .


page



vi

vii

x














LIST OF TABLES


Table

2-1.


2-2.

2-3.

2-4.

3-1.


3-2.


3-3.


3-4.

3-5.

3-6.


4-1.

4-2.

4-3.


page


Elemental Analysis of PAN Pyrolysis in
Various Carrier Gases . .

Surface Area of PPAN Samples .

Physical Properties of AX21 . .

Elemental Analysis of AX21 .

Ethyl Benzene Reactivity Over PPAN
Catalysts at 2500 C . .

Ethyl Benzene Reactivity Over PPAN
Catalysts at 3500 C . .

Ethyl Benzene Reactivity Over AX21
and Other Catalysts at 3500 C .

Butene Reactivity Over CMS Catalysts .

Ethanol Reactivity Over CMS Catalysts .

Reactivity of C3 Oxygenated Substrates
Over AX21 Catalysts . .

Commonly Studied Fischer Tropsch Systems

Typical Methanol Oxidation Reaction Routes

Effect of Carrier Gas Cycling on Acetaldehyde
Reactivity Over Ru/PPAN . .


Methanol Reactivity Over Molybdenum Doped Catalysts

Deep Oxidation of CH2CL2 Over Doped AX21 Catalysts

Results of LiC1/PPPO Ion Selective
Electrode Experiments . .


. 13

. 19

. 21

. 22


. 49


. 50


. 54

. 57

. 59


. 62

. 68

. 73


. 110


4-4.

4-5.

5-1.













LIST OF FIGURES


Figure

1-1.

1-2.

2-1.

2-2.

2-3.

2-4.


2-5.

2-6.

2-7.

2-8.

2-9.

2-10.

3-1.

3-2.


3-3.


4-1.

4-2.

4-3.

4-4.

4-5.


Formation and reaction cycle of PPAN .

Formation of PPPO . .

Pyrolysis profile of PPAN . .

Formation of pyridone moieties in PPAN structure

DRIFT spectra of PAN and PPAN . .

Infrared comparison of PPAn samples prepared
under various conditions . .

Infrared spectrum of PPO before pyrolysis .

Infrared spectra of PPO during pyrolysis .

DSC of undoped PPO . .

DSC of Ti(O) doped PPO . .

DSC of LiCl doped PPO . .

DSC of CoC12 doped PPO . .

Schematic diagram of reactor setup .

Proposed mechanism for AX21
dehydrogenation activity . .

Proposed mechanistic routes for
conversion of n-propanol . .

DRIFT spectra of PPAN and Ru3(CO)12/PPAN ..

DRIFT spectrum of unsupported Ru3(CO)12 .

DRIFT spectrum of Ru3(CO)12 . .

GC traces for reactant gas cycling experiments

Mass spe crum of propane peak
showing C incorporation . .


S 4

S 7

. 11

S. 14

. 16


. 17

. 24

. 27

S. 29

. 30

S. 31

. 32

. 46


S. 56


S. 63

80

81

82

S. 91


93









Product distribution of Ru/PPAN versus Ru/A1203

Alkene production in Ru/PPAN system .

ASF plot of PPAN versus A203 .. .

SEM of Ru/A1203 before and after catalysis .

SEM of Ru/PPAN before and after catalysis .


Product distribution of Ru/AX21 versus Ru/A1203

ASF plot of AX21 versus A1203 .. ...

Effect of CO addition to CO2 methanation reaction

Effect of acetaldehyde addition to CO/H2 reaction


Proposed mechanism for product enhanceme
from aldehyde addition .


4-10.

4-11.

4-12.

4-13.

4-14.

4-15.


4-16.

4-17.

4-18.

4-19.

4-20.

4-21.

4-22.

4-23.

4-24.


4-25.


4-26.


4-27.


4-28.


reacti

reacti

reacti


Mo03


. 94

. 96

. 97

. 99


102

103

106

108


nt
. 112

on ... 120

on ... 121

on ... 122

. 125

. 126

. 127

. 128

. 131


. 132


. 133


. 135


. 136


. 137


viii


4-6.

4-7.

4-8.

4-9.


SEM of bulk Mo03 before and after

SEM of Mo03/SiO2 before and after

SEM of MoO3/AX21 before and after

XRD of Mo03/SiO2 before reaction

XRD of Mo03/AX21 before reaction

XRD of Mo03/SiO2 after reaction

XRD of Mo03/AX21 after reaction

XPS of the Mo 3d orbitals of bulk

XPS of the Mo 3d orbitals of
Mo03/AX21 pre-catalyst .

XPS of the Mo 3d orbitals of
Mo03/SiO2 pre-catalyst .

XPS of the Si 2s orbital of
MoO3/SiO2 pre-catalyst .

XPS of the Mo 3d orbitals of
Mo03/AX21 post-catalyst .

XPS of the Mo 3d orbitals of
Mo03/SiO2 post-catalyst .








4-29. XPS of the C Is orbital of
MoO3/SiO2 post-catalyst .... 138

5-1. Schematic of cell design in ion
selective electrode experiments .... .153

5-2. Cyclic voltammogram of Ti/PPPO
system in N2 and 02 . . 156

5-3. Effect of pH on Ti/PPPO 02 reduction system .... 157

5-4. Effect of cathodic and anodic extreme potential
variation on Ti/PPPO 02 reduction system. ... 159

5-5. Effect of scan rate variation on
Ti/PPPO 02 reduction system . .. ... 162













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

CARBON MOLECULAR SIEVES AS CATALYSTS,
CATALYST SUPPORTS, AND IN ADVANCED MATERIALS
APPLICATIONS

By

Gerald C. Grunewald

December 1989

Chairman: Russell S. Drago
Major Department: Chemistry


The ability of activated carbons prepared from both natural and

synthetic precursors to function as molecular adsorbents has been known

for a very long time. Such materials operate efficiently as general

adsorbents because their pore sizes are very large and thus they are

unselective. In addition to their excellent adsorption capabilities,

carbonaceous materials have also been reported to catalyze

dehydrogenation reactions and a great deal of work has been done with

carbon based systems in advanced materials applications. For the most

part, however, typical charcoal and other activated carbon adsorbents

differ widely from one sample preparation to the next, are commonly

poorly characterized, and generally are not considered to be chemically

reactive entities.

In this study, Carbon Molecular Sieves (CMS) materials prepared

from the pyrolysis of polymeric precursors have been investigated as








catalysts, catalyst supports, and in advanced materials applications.

These CMS systems are shown to be active catalysts for the oxidative

dehydrogenation and dehydration of a variety of substrates. In several

cases, the activity of these materials exceeds that of inorganic oxide

based catalysts. Metal doped CMS systems have also been studied in

several different types of reactions and the CMS supports are found to

have inherent advantages over the widely employed inorganic oxides.

Synergistic interactions between dopant metal species and CMS supports

are observed and characterized. The CMS systems have been further

investigated for use as novel electrodes, electrocatalysts, and

structural composites.














CHAPTER 1
GENERAL INTRODUCTION


Inorganic oxides such as alumina and silica have been widely

studied and employed in a host of industrial processes as heterogeneous

catalysts and catalyst supports for the past fifty years.1-3 The

introduction of synthetic zeolites in the early 1960s sparked a further

surge of research into materials science and catalysis.4 In the ensuing

years hundreds of patents, books, and papers have described the

application of zeolites to a plethora of separation processes and shape

selective catalysis systems. Currently a new class of materials has

been prepared which has the potential to equal or surpass the alumino-

silicate compounds in industrial operations. Carbon Molecular Sieves

(CMSs), which are produced from the controlled pyrolysis of polymers or

polymeric precursors, are being actively studied. Initial

investigations have shown these materials to be very effective in gas

separation studies,5-7 pressure swing absorption experiments,8 and in a

variety of catalytic systems.9-11 Despite these encouraging preliminary

results, there have been surprisingly few publications on CMS systems

and the majority of these reports are in the European and Japanese

patent literature.12-17 In discussing CMS's, it is important to note

that the term has been used in the literature to describe a wide variety

of materials. Studies of carbonaceous compounds with sieve-like

properties were reported as early as the late 1930's.1821 Much of this








2

work focused on natural organic substances such as coals and charcoals

which had sorption properties generally related to their porosity. Due

to their amorphous nature, however, the pore sizes of these materials

ranged a great deal between samples. More versatility and

reproducibility in pore dimensions were obtained through the pyrolysis

of synthetic polymeric precursors. Pyrolyzed polyvinylidenechloride was

determined to have slit shaped pores which were efficient in separating

molecules based on steric limitations.22 Branched hydrocarbons such as

isobutane were successfully separated from their straight chain isomers.

Sieves based on pyrolyzed polyfurfuryl alcohol were shown to have

similar separation capacities. These materials were even employed as

catalytic supports in hydrogenation reactions. In separate studies,

Trimm and Cooper23'24 and Schmitt and Walker25 demonstrated that

platinum doped pyrolyzed polyfurfuryl alcohol would selectively

hydrogenate linear over branched alkenes. The catalytic hydrogenation

was due to the metal, but the sieve provided the selectivity. The most

effective polymeric precursor to date has been polyacrylonitrile.

Sieves composed of pyrolyzed polyacrylonitrile have been established to

have shape selectivities greater than 100:1 for molecules which differ

by as little as 0.2 A in their critical diameter.26 The most recent

development in carbon molecular sieve manufacturing has been the thermal

activation of petroleum coke and coal tar pitch residues to produce

materials with very high surface areas.27,28

The research described in this dissertation concentrates on three

different CMS systems for use as catalysts, catalyst supports, and in

advanced materials applications. Pyrolyzed polyacrylonitrile (PPAN) has










been prepared, characterized, and studied as a catalyst in oxidative

dehydrogenation reactions and as a support in heterogeneous catalytic

systems for synthesis gas (CO and H2) conversion. AX21, a commercially

available CMS, has been characterized and studied as a catalyst in both

dehydration and dehydrogenation reaction schemes and as a synergistic

support in a variety of catalytic transformations. Pyrolyzed poly (2,6-

dimethylparaphenylene) oxide (PPPO) has been prepared, characterized,

and studied in several advanced materials applications as a novel

support for new electrocatalysts and as ion selective electrodes. The

specific characteristics of these materials which make them ideal for

the studies described are discussed below for each of the three CMS

systems.

The interesting structural and physical changes that occur during

the pyrolysis of polyacrylonitrile (PAN) have been studied periodically

for the last thirty years. Burlant and Parsons reported that upon

thermal treatment, PAN undergoes a chemical reaction leading to

discoloration and the appearance of an EPR signal.29 IR studies

attribute these observations to a cyclization and dehydrogenation

process as shown in Figure 1-1.30-37 The formation of this doubly

conjugated ladder structure is believed to give the material its

semiconductor nature.38-40 Due to the ability of the condensed pyridine

ring moiety of PPAN to be hydrogenated (step 4 of Figure 1-1) and

subsequently dehydrogenated via air oxidation (step 5 of Figure 1-1),

PPAN has been studied for a number of catalytic processes. The

dehydration and dehydrogenation of alcohols,41 the isomerization and

dehydrogenation of olefins41,42 and the dehydrogenation of ethyl













H2 H2 H2 H2
-C -C 1 C H C H
C CC C
I I I PAN
C C N C N C
^N N ^N ^N
heat

H1 H2 H12 H2
Hc A_,
C H C H CHC.H/
C C C C

N/ .N N '*NN/ N
heat
H H H H
CCC




Hydrogen source
Hz Hz Hz Hz
CHCHICHC 1H /
C C C C
C C C
N/ N N/ N
I I I
H H H

1 02
H H H H

.C0 C C C
I (I C + H20
N/ -N N N/C


Figure 1-1. Formation and reaction cycle of PPAN.








5

benzene43 have all been studied on various PPAN systems. Metal doped

PPAN catalysts have also been employed for reaction schemes such as the

decomposition of nitrous oxide,44 the epoxidation of ethylene'45 and the

oxidation of cumene and ethyl benzene.46

AX21 is a truly remarkable CMS. It is prepared by a patented

process involving the high temperature pyrolysis of polymeric petroleum

coke in the presence of large amounts of KOH.28 The surface area of

this material, as measured by the standard BET method,18 is in excess of

3000 m2/g. This value underscores the extraordinary nature of the

material. The theoretical maximum for surface area based on a monolayer

of carbon including the area on both sides of the plane is 2620 m2/g.47

Exposure of both sides of every carbon atom is an impossibility for a

solid with any physical integrity. Thus this compound is on the

threshold of sensitivity for surface area measurement techniques and is

among the most porous materials ever synthesized. In addition to the

phenomenal surface area, AX21 also has exceptional adsorption

capabilities. Studies have shown that this material has four to five

times the adsorption capacity of other typical highly adsorbent

carbonaceous compounds.4850 These properties make AX21 an attractive

candidate for catalytic studies since high surface area combined with a

strong affinity for a substrate are key parameters for successful

catalytic systems.

Poly (2,6-dimethylparaphenylene) oxide (PPO) is a very versatile

polymer which is used for a variety of purposes. The high decomposition

temperature, due to the stability of the ether linkage, makes this

material ideal for applications such as heat-exchange fluids and high-








6

temperature lubricants.51'52 The pyrolysis of this polymer cleaves some

of these ether linkages and causes a significant amount of cross-

linking. This process leads to a product having functionalities such as

those shown in Figure 1-2.53 With respect to industrial lubricants and

other similar applications, the formation of this highly crosslinked,

completely insoluble, material is clearly not desirable. This compound

is, however, potentially useful as a template in which a variety of

dopants can be added and the chemistry of the dopant probed while it is

contained in the pyrolyzed polymer matrix. This type of host-guest

interaction has not been investigated previously for dopants such as

metals or ionic salts; however, some polymer blend studies have shown

that PPO can stabilize other polymers in thermal degradation studies.

Polystyrene (PS) has been shown to be thermally stabilized in the

presence of PPO due to a cage-like effect in which the PPO encapsulates

the PS constituent.54

In studying these three particular CMS systems, the research

centers on the novel chemical structure and reactivity of these

materials which impart to them unique properties. The specific

reactions and applications that are studied in this work are chosen in

order to utilize these exceptional features. The focus of the research

is on the treatment of CMS materials as chemical entities. Much of the

work done to date has been in studying the physical nature, e.g. pore

sizes and adsorption capacity, of CMS systems. The work presented here

shows that the chemical reactivity of various CMS materials is quite

versatile and is potentially applicable to a variety of industrial

processes.














L n"3 n




0-

0o




o\
Figure 1-2. Formation of PPPO











Figure 1-2. Formation of PPPO














CHAPTER 2
PREPARATION AND CHARACTERIZATION OF CMS MATERIALS



Introduction


Pyrolyzed Polyacrylonitrile (PPAN)


Although the condensed pyrdine ring structure of PPAN shown in

Figure 1-1 has been invoked by virtually all researchers who have

studied this material, most agree this is an over simplification of a

much more complex structure.32-37 Upon initial observation, the

formation of the conjugated ladder structure appears straightforward.

The nitrile group acts as a nucleophile to attack the adjacent chain.

This is followed by dehydrogenation of the newly formed ring to yield

the proposed structure. In practice, however, the synthetic path to

PPAN has many side routes. PAN is an atactic polymer. The oligomer

shown in Figure 1-lb is drawn as an isotactic fragment. When present in

this configuration, the "zippering" of the backbone to form the cyclized

product can readily occur. Unfortunately, PAN has not been synthesized

with any large degree of stereo regularity on a bulk scale.55,56 Some

work has been done on the electrocatalytic deposition of thin PAN films

on metal electrodes which is proposed to yield isotactic species,

however, the polymer coating formed is only a few monolayers thick.57,58

Clearly, any hope to use these materials as catalysts requires a








9

preparative method that can generate reasonably large quantities. Due

to this lack of isotacticity, the bonding rearrangements that occur

during the thermal process can proceed in different directions and give

rise to a significant amount of crosslinking. In light of the

complexity of this synthetic process, it is easy to understand why there

are many variations among PPAN materials that have been reported.

In this research effort, PPAN materials are prepared for use in a

variety of catalytic studies as both catalysts themselves and as

catalyst supports. A number of different substrates as well as a

variety of dopants are examined. Because of the nature of this work,

which involves continuous comparisons to be made from experiment to

experiment to optimize conditions, it is essential that the PPAN

materials be the same from each preparation. Furthermore, since the

proposed active species for dehydrogenation catalysis is the condensed

pyridine ring structure, the preparation employed should maximize the

content of this active species in the final product. The two most

important parameters to control in order to accomplish these goals are

the pyrolysis procedure and the nature of the carrier gas. Among the

reported PPAN studies, a wide variation in heating cycles and carrier

gas compositions have been employed. A lot of time has been spent in

our laboratory in designing optimal conditions for reproducible PPAN

preparation.

Thermo-Gravimetric Analysis (TGA) and Differential Scanning

Calorimetry (DSC) were carried out by Jeff Clark at the University of

Florida and the results are described in this Ph.D. dissertation.59

This work was used to help design a suitable pyrolysis procedure. In








10

isothermal TGA studies, weight loss occurs sooner and to a greater

extent as heating temperature is increased in successive experiments.

Temperature programmed TGA work shows similar results. Faster heating

rates cause increased weight loss. The DSC studies compliment the TGA

experiments. The onset of weight loss corresponds to a large exotherm.

In isothermal studies, the sharpness of the exotherm increases with

temperature. Temperature program DSC work shows that the size and

specific temperature of the exotherm are directly proportional to scan

rate. The simultaneous weight loss and exotherm are proposed to be due

to the cyclization of the nitrile groups. Further weight loss and a

much smaller exotherm are due to the last dehydrogenation step. It is

clear from these results that the heating process should be done in

stages and that the progression from step to step should be done very

slowly. If the heating rate is too fast, the exothermicity of the

process will cause a runaway reaction leading to large weight loss,

extensive crosslinking and the formation of a very complicated network.

The pyrolysis profile shown in Figure 2-1 is found to yield

samples which are reproducible in their color homogeneity (black), EPR

signal (very intense organic radical with g = 2.01), elemental analysis,

and Infrared spectra. This heating cycle is different from that

employed in most of the reported studies. Several of the literature

preparations invoke simply heating isothermally at 200-2500 C for a few

hours.38'39,46,57 Another common preparation involves a two-step

process where the sample is initially heated isothermally at 2000 C in

an air atmosphere and then subsequently heated at 4000 C in a N2

atmosphere.31-37 It has been our experience that the first of these





































5 10
ELAPSED


15
TIME


) 25
HOURS)


Figure 2-1. Pyrolysis profile of PPAN.


500



400

C)
0
o

W 300
ry

F-
<
S200
LLJ

100
F- 100








12

methods does not fully pyrolyze the sample. The color of the product is

usually brownish and there is still a significant amount of nitrile

functionality observed in the IR of the product. The second method does

yield a homogeneous black product with no nitriles remaining; however,

both elemental analysis and IR show that there is a lot of oxygen

incorporation in the final material.

The presence of oxygen in the end product has proven to be a

ubiquitous observation among PPAN samples prepared under a variety of

conditions. Table 2-1 contains the elemental analysis results of PPAN

samples pyrolyzed in several different carrier gas atmospheres. It is

evident that the product is quite sensitive to oxygen incorporation even

under what should be anaerobic conditions. There is a significant

amount of disagreement in the literature over this observation. Grassie

and McGuchan pyrolyze in a N2 atmosphere and still observe the presence

of oxygen.32-34 They invoke a pyridone type structure such as that

shown in Figure 2-2 to account for this. Coleman and Petcavich observe

the presence of oxygen containing species even when the pyrolysis is

carried out under reduced pressure.35 Rafalko as well as Chung and co-

workers, however, report elemental analyses consisting of over 99% C, H,

and N.36,37 In comparing these various literature results to the

present work, again there are important experimental differences to be

noted. All the literature studies cited use thin films of PAN weighing

about 100 mg. In order to produce enough material for a series of

catalytic experiments from the same batch, it is necessary to use 5-7g

of PAN in the pyrolysis step. This large sample size makes it difficult

to remove all intestitial air which is felt to be the source of the










Elemental Analysis of


13

PAN Pyrolyses in Various Carrier Gases


PYRnI Y~TS ATMOSPHFRF


C % %H %N TOTAL


before pyrolysis

N2

air

NH3

CO


The non C, H, or N constituent is found to be 0.


Table 2-1


SAMPL F


67.10

69.73

65.02

69.99

69.76


5.86

2.95

3.19

3.15

2.70


26.16

21.07

22.64

22.38

21.05


99.12

93.75

90.85

95.47

93.51


,,, F .v...V-... A.I-,PER %C %H %N TOTAL






















HHH
H H H
102
n


H H H H2


Figure 2-2. Formation of pyridone moieties in PPAN structure.








15

incorporated oxygen. Due to the exothermicity of the structural

transformations, it is not advantageous to tightly pack pyrolysis tubes.

When the samples are packed together, the heat generated from the first

cyclization raises the temperature of the system too quickly and causes

undesirable charring. Thus, the PAN powder is layered out in a

horizontal pyrolysis tube.

The presence of the oxygen would suggest that there is a

significant amount of the pyridone moiety in the product prepared in

this work. Infrared studies, however, do not support this. Figure 2-3

shows the complete spectrum overlay of both PAN and PPAN. The key

comparative features are the loss of the C=N stretch at 2240 cm-1 and

the growth of broad peaks from 1600 1200 cm-1. These can be assigned

to the various C=C, C=N, and C-C, C-N functionalities. Figure 2-4 is an

expansion of the essential double bond region. This spectrum is

displayed in the absorbance mode so that it can be directly compared to

the spectrum reported by Coleman and Petcavich.35 The absence of broad

shoulders in the 1700 cm-1 region, which is indicative of C=0 stretches

resulting from pyridone moieties, suggests that there is not an

appreciable amount of pyridone functionality in the samples prepared by

our method. The oxygen present may be involved in chain terminating

hydroxyl species, since there are broad absorptions above 3200 cm-1

After devising a method of preparing reproducible PPAN samples

with a large amount of condensed pyridine ring functionalities, the next

goal was to increase the surface area of the product. Results of

studies employing PPAN materials as catalysts and catalyst supports are

discussed in the following two chapters. One of the key parameters for











































3800. 0 3300.0 2500. 0 2300. 0 1800. 0 1300. 0 800.00
VIAVENUMBERS (CM-1)


Figure 2-3.


DRIFT spectra of PAN and PPAN.


















vacuum pyrolysis


3230

1U2198


)300 O 1CO 2 C00 100
CM'


1l= iaoo 0oo


1610


1280






S1156


3411


N2 pyrolysis


3500.0 3100 0 2700.0 2300, 0 I0o I) 1500.0 1 100. 700. 00


air pyrolysis


JoUU


JuUU


2500 4C 2000


Figure 2-4.


Infrared comparison of PPAN samples prepared under
various conditions.


2222


rr~








18

either of these applications is to have as high a surface area as

possible. The pyrolysis of PAN according to the method described here,

as well as the various literature methods, yields a product with a very


Reported surface areas range between 8-17 m2/g.


PPAN samples prepared in this study

the 9-12 m2/g range. In order to in

pyrolysis agents are investigated.

various dopants have on the surface

is evident that the most success is

and NH4C1. NH4C1 decomposes to NH3

procedure. As the gases volatilize

greatly increase the surface area.

occurs at 3600 C, it is an excellent


are found to have surface areas in

crease these values, a series of co-

Table 2-2 outlines the effects

area of the final PPAN product. It

achieved with a 50-50 mixture of PAN

and HC1 during the pyrolysis

out, they create channels which

Since the decomposition of NH4C1

dopant to employ. The final


dehydrogenation step leading to the fused pyridine ring structure is

proposed to occur between 300-3200 C. Thus the volatization of the

dopant occurs after the desired structure is formed. The surface area

of this material is 50 m2/g which is about an order of magnitude higher

than the undoped PPAN. IR and elemental analysis on the product of this

doped sample verify that all of the dopant is removed by the end of the

pyrolysis procedure. Catalytic studies videe infra) show that this

higher surface area increases the reactivity of the sample.


AX21


The AX21 carbon molecular sieve is purchased from the Anderson

Development Company. The material is made by a direct chemical

activation route in which petroleum coke is reacted with excess KOH at


low surface area.










Surface Area of PPAN Samples


nOPANT


E CAFRUS ARFA m2/a


none N2 pyrolysis

none air pyrolysis


50% NH4C1

50% NaCI

50% sucrose


a) These sample names will be used throughout the rest of this text
when describing particular PPAN material.

b) These pyrolyses are all done in N2. Dopant percentages are
based on weight. The samples are stirred in 300 ml H20 after
pyrolysis to wash out any remaining dopant material.


SAMDI Fa


PPAN 1

PPAN 2

PPAN 3b

PPAN 4b

PPAN 5b


,Zrll Pa ,_, .


Table 2-2








20

5000 C. This produces an intermediate product that is subsequently

pyrolyzed at 9000 C to yield active carbon which contains potassium

salts. These salts are removed by successive water washings. More

detail on the preparation of this material is reported elsewhere.60

Since the AX21 material is commercially available, most of the

characterization studies have been done by the producer. A list of the

more relevant ones to catalytic studies is given in Table 2-3. As was

mentioned in Chapter 1, the reported surface area of this material is

known to have been overestimated. The theoretical maximum for a

carbonaceous material is 2620 m2/g.47 This inflated value, however,

exemplifies the unusual nature of this material.

Transmission Electron Microscopy (TEM) shows this material to be

composed of interwoven carbonaceous lamellae that form a three

dimensional network with molecular size channels throughout.61 These

channels produce pores which give the material its sieving properties.

Unlike standard activated charcoals, the pores of the AX21 material are

considerably smaller and have a very narrow size range. The pore size

for AX21 is 5-7 A, whereas that for typical bone charcoal can range from

twenty to several hundred A.62 Another difference between the AX21

compound and other typical carbonaceous adsorbents is its incredible

adsorption capacity. Studies have shown this material to be far more

adsorbent than other widely used CMS compounds.4850

Outside of the adsorption properties and pore structure, these

materials have proven to be rather difficult to characterize--

particularly from a chemist's point of view. Even elemental analysis, a

seemingly facile technique, became more difficult than anticipated.








21

Table 2-3 Physical Properties of AX21


Surface Area, BET, m2/g

Total Pore Volume, ml/g

Bulk Density, g/ml

Screen Analysis:
passes 100 mesh, wt%
passes 200 mesh, wt%
passes 325 mesh, wt%


2800 3500

1.4 2.0

0.27 0.32


90 99
70 85
55 70








22

Table 2-4 lists the elemental composition of a very dry AX21 sample. To

obtain this data, the sample had to be run at least three separate times

for all the elements and a total of five times for the carbon

quantification. The necessity of the subsequent runs was not that the

sample was heterogeneous, but rather that it didn't always combust

completely. Some of the carbon became refractory, i.e. non-combustible,

and thus led to inaccurate results. The 2.5% elemental composition

which is unaccounted for in Table 2-4 is felt to be due mostly to the

carbon not fully combusting, as opposed to the presence of other

impurities not screened for in the analysis. The experimental support

for this contention comes from Electron Diffraction Spectroscopy (EDS)

which was done in conjunction with Scanning Electron Microscopy (SEM) on

several of the metal doped AX21 samples. The results of these studies

will be discussed in more detail in Chapter 4, but it serves to mention

here that no other elements, outside of the metal dopant and those

listed in Table 2-4, were observed in any of the elemental analysis

scans from EDS.

Other common characterization techniques such as IR and EPR do not

reveal much information. Unlike PPAN, which can be studied quite well

with the DRIFT technique, AX21 displays a completely featureless

spectrum. Since elemental analysis shows in excess of 6% oxygen

content, it is surprising that no hydroxyl, carbonyl, ether, or any

other oxygenated chromophore is observed. IR analysis is even more

unusual when doped AX21 samples are attempted. Figure 2-5 shows the

spectrum overlay of Mo03 alone and MoO3/AX21 in transmittance mode. As

is shown, the spectrum for the latter system appears as an inversion of








23

Table 2-4 Elemental Analysis of AX21



%Carbon 89.23a

%Oxygen 6.18

%Hydrogen 0.00

%Nitrogen 0.01

%Sulfur 0.24

%Ash 1.84b



Note: The cumulative error associated with all these separate
analyses causes the theoretical maximum total to be only 99%.

a) Values varied from 88.6 91.2%.

b) Values varied from 1.29 2.39. Composition is virtually all
potassium.












































S000


WAVENUMBER


Figure 2-5.


Infrared spectrum of PPO before pyrolysis.








25

the dopant. Apparently, the AX21 material absorbs all of the IR energy

via some type of charging mechanism. In all the DRIFT experiments the

sample is mixed with about a hundred fold excess of KBr. Thus, the

total absorption is not due to the opaqueness of the sample. In fact

the reflected beam throughput of the AX21 samples is higher than many of

the PPAN samples. The total absorption of the IR energy is due to the

AX21 material and is not an instrumental artifact.

Since the PPAN samples are highly paramagnetic and AX21 is

prepared from pitch residues, it was expected that the latter material

would also have an intense delocalized organic radical signal in its EPR

spectrum. Initially it was difficult to obtain a spectrum at all

because the samples absorb H20 so readily. When excessively dry,

however, the AX21 material is observed to have no paramagnetic content.

Much characterization work has been done on the metal doped AX21

system. The results of X-ray Diffraction (XRD), X-ray Photoelectron

Spectrocopy (XPS), and Scanning Electron Microscopy (SEM) are discussed

in Chapter 4. For the most part though, these techniques characterize

the dopant and its interaction with the AX21 support more than the AX21

material itself. Several new methods are currently being applied to

these materials which may prove to be quite informative. 129Xe NMR,

which can yield information regarding the chemical nature of the pore

site environment, is just beginning to be investigated by several

research groups.63-67 Although there have not been any studies published

on CMS systems yet, this may prove to be a promising technique in the

future. Titrametric calorimety is another method which can yield

fundamental chemical information on solid materials. Drago and co-








26

workers have studied a Pd/charcoal system and have been able to quantify

the acidic nature of this material.68 The extension of this technique

to CMS materials such as AX21 could provide a wealth of information.


Pvrolvzed Poly (2,6 dimethylparaphenlvene) oxide (PPPO)


As is shown in Figure 1-2, the pyroloysis of poly (2,6-

dimethylparaphenlyene) oxide (PPO) produces a highly crosslinked matrix.

The major experimental evidence to support this proposed structure comes

from Pyrolysis IR studies. Figure 2-6 shows the IR spectrum of PPO at

three different temperatures during its pyrolysis. It is evident that

there is no real change in the general features of the spectrum as the

material is pyrolyzed. The only noticeable variation is a gradual

decrease in the intensity of all the peaks. Concomitant with this

overall decrease in intensity, is the observation of a color change from

white to yellow to eventually black. This darkening of the sample is

the cause of the decrease in intensity. Unlike PPAN, pyrolysis does not

bring about any new functional moieties, but rather just rearrangements

of the ether linkages to produce a highly interwoven, template

structure.

The advanced materials applications discussed in Chapter 5 involve

the entrapment of various metal and ionic species in the cavities of the

PPPO matrix. To verify that the presence of the dopants does not change

the nature of the PPPO structure, Pyrolysis IR studies were done on both

Fe(O)/PPO and LiCI/PPO composites. In both cases, the spectra do not

differ from that of the PPO itself. EPR spectroscopy done on the

undoped material shows a very large delocalized organic radical with a








A



F



D

B C E
\ -
\L,-----^
2 Hrs. Heating 114 C

-1
Peak cm
A 3258
B 1729
C 1605
D 1469
E 1306
F 1189
G 1020
1\ I H 857


4 Hrs. Heating


4.5 Hrs. Heating


2690 C


Infrared spectra of PPO during pyrolysis.


Figure 2-6.








28

g value of 2.01. This is essentially the same as that for the PPAN

material.

Thermo-Gravimetric Analysis (TGA) and Differential Scanning

Calorimetry (DSC) were conducted on the undoped and several of the doped

composites. The TGA shows virtually no weight loss until 3300 C in any

of the samples. As is discussed in the experimental section of this

chapter, the pyrolyses in this study are done isothermally at 2500 C.

This lack of any weight loss supports the proposal that the sample just

crosslinks and does not produce any significant amount of volatiles.

The DSC results are worthy of further discussion. Figure 2-7 shows the

DSC for the PPO material itself. It is evident that there are

essentially no endo- or exothermic processes occurring in the

temperature range studied. A very small endotherm is detected at

2100 C, but this is on the threshold of the instrumental limitations.

Figure 2-8 displays the DSC for a Ti(0)/PPO sample. Again the spectrum

is virtually nondescript save an even smaller endotherm at 2100 C. Thus

the polymer itself and zero valent metal doped composites undergo no

discernable thermochemistry in the temperature range of interest.

Samples containing doped metal salts, however, behave significantly

different. Figure 2-9 shows the DSC for a LiC1/PPO composite. There is

a reasonably large endotherm at 2470 C. The heat absorbed by the system

corresponds to 10.33 J/g. Figure 2-10 displays the DSC for a CoC12/PPO

composite. Like that of the LiCl doped system, there is an appreciable

endotherm. The peak maximum occurs at 2460 C and has a value of

13.55 J/g. Each of these DSC studies were run several times and were

found to be quite reproducible. Slower, faster, or isothermal heating

























































.75

0
-I


J


.375









S180.00


















Figure 2-7.


Temperature (C)


DSC of undoped PPO.























































Temperature IC)


Figure 2-8.


DSC of Ti(O) doped PPO.







































PEAK FROM: 227.768
TO: 258.762
ONSET- 239.185
J/GRAM- 10.3352







1A 247





i
2 i
i Ii


170.00 190.00 210.00 230.00 250.00 270.00
Temoerature (C)


290.00 310.00 330.00


DSC of LiCI doped PPO.


3.75


2.5


Figure 2-9.


















































Temperature (C)


Figure 2-10.


DSC of CoCl2 doped PPO.








33

made little or no difference in the peak height or peak location in any

of the samples. These results suggest that there is some process driven

by entropy occurring with the salt doped samples that is not going on in

the other systems. This is most likely associated with the dissolution

of the ionic dopant in the polymer matrix. The majority of the

endotherm would be a function of the lattice energy of the salt with

some minor contributions possibly coming from structural rearrangements

of the PPO matrix to accommodate ionic species. Since the endotherm is

greater for a divalent ion than a monovalent ion, the is support for the

proposal of lattice energy being an essential component of the process.


Experimental


PPAN


Polyacrylontrile samples were purchased from both Aldrich and

Polysciences. Additional samples were prepared by the radical initiated

polymerization of acrylontrile according to literature methods.31 No

significant differences were observed in the PPAN product prepared from

the various PAN sources. The pyrolyses were carried out in a horizontal

tube furnace. Typically 5g of the sample were layered out in a pyrex

tube fitted with a male 24/40 ground glass joint at one end and tapered

to a standard size hose connection spout at the other. Initially a

sintered glass frit was employed at the tapered end, but these were very

difficult to clean and often retained the liquid effluents of the

pyrolysis degradation products. Subsequent runs simply employed a small

glass wool plug at the tapered end. The PAN sample is slowly poured in








34

and layered out over a 3-4 inch space. This sample is usually not in

contact with the glass wool plug. As mentioned previously, tightly

packing the sample into the tube is not advantageous because the

exothermicity of the process causes packed sample to char. Carrier flow

(10 ml/min) is maintained through the pyrolysis tube by fitting a

specially made cap on the end with the ground glass joint. This cap

consists of a 24/40 female joint with 2 septa holes. The carrier gas

line is attached to one end; the thermocouple is placed through the

other. A Type J thermocouple is used with the end of the stick in

contact with the sample bed. The temperature is maintained by an Omega

CN 2000 programmable temperature controller which conducts the pyrolysis

according to the temperature profile in Figure 2-1. A bleach bubbler is

attached to the out gas end in order to deactivate the various Cyano

moieties that are evolved during the pyrolysis procedure.

The various co-pyrolyzed samples discussed in Table 2-2 were

prepared by grinding the appropriately weighted mixtures in a mortar and

pestle and then placing them in the pyrolysis tube as usual. The

samples co-pyroloyzed with NaCl were then stirred in 500 ml of H20

overnight to wash out the salt.

Diffuse Reflectance Infrared Fourier Transform (DRIFT)

spectroscopy was done on a Nicolet 5 DXB FTIR spectrometer fitted with a

Barnes Analytical Diffuse Reflectance unit. Samples were mixed with at

least a hundred fold excess of KBr in order to allow more signal

throughput. The spectra were referenced against a pure KBr background.

Electron Paramagnetic Resonance Spectroscopy was done on a Brukker EPR

operated at room temperature. Elemental Analysis for C, H, and N were








35

performed by the University of Florida Department of Chemistry

Microanalytical service. Analysis for oxygen content as well as routine

checks on the C, H, and N data were done by Desert Analytics

Microanalytical Laboratory of Tuscon, Arizona.


AX21


As mentioned earlier, this sample was purchased from Anderson

Development Company of Adrian, Michigan. The samples were dried in

vacuo at 1000 C for several hours prior to use. The complete elemental

analysis was done by Desert Analytics.


PPPO


The sample was purchased from Polysciences Company. The pyrolysis

was performed isothermally at 2500 C in either the same apparatus as the

PAN samples or in a standard muffle furnace. Since many of the

applications for the material required the use of pellets of the

composites, samples were pressed in a 1 cm die at 10,000 psi on a Carver

Laboratory Press prior to pyrolysis. Composites made with metallic

species such as Fe(0) or Ti(0) were simply mixed by grinding in a mortar

and pestle. sonically doped composites such as LiC1/PPO were prepared

by co-dissolving the appropriate mixtures of the materials (8% by weight

doping was commonly used) in an azeotrope solvent composed of 7% C2H50H

in CHC13. This azeotrope works quite well because the ethanol dissolves

the inorganic salt and the chloroform dissolves the polymer. When the

solution becomes homogeneous (usually after about 15 minutes of

stirring) the azeotrope solvent is rotovapped off. This method allows








36

coprecipitation of the dopant and support species and thus affords

excellent dispersion.

The pyrolosis FTIR work was graciously performed by Professor

Thomas Brill at the University of Delaware. The TGA and DSC studies

were graciously conducted by Professor Michael Babich at the Florida

Institute of Technology.














CHAPTER 3
CARBON MOLECULAR SIEVES AS CATALYSTS


Introduction


This chapter focuses on three different types of substrates and

their reactivity with the CMS catalysts PPAN and AX21. An alkyl

aromatic compound, ethyl benzene, has been studied in the

dehydrogenation reaction to produce styrene. A linear olefin, butene,

has been investigated in dehydrogenation studies to form butadiene.

Lastly, a series of C3 oxygenated compounds -- n- and iso-propanol,

propionaldehyde, and acetone -- have been studied in dehydration and

dehydrogenation reaction schemes. The first two of these catalytic

transformations are large scale industrial processes. In 1987, the

industrial production of styrene was 8,014 million pounds and that of

butadiene was 2,931 million pounds. Estimates for the production in

1989 are even higher; 8,900 and 3,175 million pounds respectively.69

The catalysts typically employed for all of these processes are composed

of inorganic oxides.

The dehydrogenation of ethyl benzene to styrene can be

accomplished by two different catalytic routes. The straightforward

dehydrogenation, shown in equation 3-1below, is used for virtually all

industrial production.70'71










(C6H5)CH2CH3 -> (C6H5)CH=CH2 + H2 AH800K = 124kJ (3-1)


As is shown, this reaction is endothermic and is limited by equilibrium.

In order to achieve acceptable conversions, high temperatures are

required (in excess of 6000 C). Additionally, super-heated steam must

be used as an additive. This provides extra heat needed for the

reaction; reduces the ethyl benzene and hydrogen partial pressures to

maximize yield; and keeps the catalyst clean and active. The catalyst

employed is a promoted iron oxide system.72,73 In a typical reaction

scheme employing this catalyst, conversion is about 50% with over 90%

selectivity to styrene.

In spite of the thermodynamic limitations, there are several

advantages to the straightforward dehydrogenation from an industrial

standpoint. Catalysts lifetimes are very long. Typical industrial

catalysts can last 2-3 years in continuous operation. Catalyst

producers consider a lifetime of least one year as a virtual requirement

for any potential catalyst systems.74 Another advantage to the direct

dehydrogenation is that H2 is obtained as a byproduct. In many

industrial schemes this is used as a fuel to provide some of the heat

required for the reaction.

Despite the long-standing success of the straightforward

dehydrogenation reaction scheme, there has recently been a great deal of

interest in the oxidative dehydrogenation process.75-83 In the presence

of 02 (usually as air), the dehydrogenation is quite exothermic as a

result of the formation of H20 as a byproduct. This reaction is shown

below in equation 3-2.












(C6H5)CH2CH3 + 1/2 02 ---> (C6H5)CH=CH2 + H20 AH0800K= -119 kJ (3-2)


Due to the favorable thermodynamics of this reaction scheme, the process

should be able to operate at lower temperatures and complete conversion

is theoretically possible.

As with the direct dehydrogenation, the systems most often studied

for the oxidative process are based on promoted inorganic oxide

catalysts. One of the major drawbacks of the early systems studied for

this pathway is that selectivities are low. Since the primary use of

styrene is as a monomer for polystyrene production, it is essential that

the selectivity be very high in order that any small amount of side

products can be completely removed. The bismuth molybdenum catalyst,

which is an industry standard for mild oxidations, has a selectivity of

only about 50% in this reaction.78 Recent studies employing other

inorganic oxide systems, however, have shown much improvement with

regards to selectivity. Murakami and co-workers have studied a SnO2

P205 catalyst and found it to have moderate activity--greater than 30%
conversion with greater than 80% selectivity--at 5500 C.78-80 Their

work has particularly focused on the role promoters play in modifying

the acid-base properties of the catalysts. Emig and Hofmann have

reported a Zr02 P205 system which is slightly more active in the same

temperature range -- conversions up to 55% with over 80% selectivity.83

Vrieland has similarly worked with phosphate promoted catalysts and has

recently reported a cerium pyrophosphate system which exhibits 76%

conversion with 90% selectivity at 605 0C.74'81








40

In addition to focusing on the acid-base properties of the system,

the latter two studies also investigated the role of carbonaceous

overlayers on the surface of the catalyst. It was determined that each

of the various systems studied had an induction period during which a

thin uniform layer of carbon built up on the surface. A subsequent

study suggested that the nature of the layer is a circular pattern of

condensed rings.81 In another recent study, Cadus and co-workers

similarly found carbon overlayers present on a sodium doped alumina

catalyst.82 They provide a mechanism to show how the carbonaceous

species, not the inorganic oxide, bring about the catalytic

transformation.

Since the actual catalytic surface is primarily carbon and

carbonaceous substances such as charcoals have been known for a long

time to have some dehydrogenation ability,84'85 it is possible that the

bulk of the catalytic reactivity in such systems is a function of the

carbon. The major role of the inorganic oxide constituents may merely

be to modify the acid-base properties of the carbon. This is the

premise for employing CMS catalysts in the oxidative dehydrogenation of

ethyl benzene to styrene.

As mentioned previously, the dehydrogenation of butene to

butadiene is a widely studied industrial process. The typical catalyst

is a mixture of Bi203 MoO3 which is usually referred to as bismuth

molybdate. This system requires a reaction temperature of at least 5000

C to yield a conversion of 40% with 95% selectivity.86 The process is

virtually always done via an oxidative route as shown by the reaction

below:








41

CH2=CH-CH2-CH3 + 1/2 02 -----> CH2=CH-CH=CH2 + H20 (3-3)



Recently a number of other inorganic oxide based systems have been shown

to be even more active under similar reaction conditions. Lambert and

Germain report that a tin cadmium phosphate catalyst exhibits 88%

conversion with 71% selectivity at 4700 C.87 Herniman and co-workers

have employed a tin antimony mixed oxide catalyst and have observed

comparable activity.88

The mechanism that is commonly invoked for these mixed oxide

catalysts focuses on lattice oxide ions as the active sites where

adsorption and dehydrogenation of the alkene occur.89 The role of the

gaseous oxygen reactant is to reoxidize the catalyst. This is

accomplished by dissociation on the surface to form oxide ions and then

diffusion through the bulk to the active site.90 It is fairly well

established from kinetic studies that the reaction is zero order in

oxygen at oxygen to butene ratios above 0.5.91

In light of the proposed mechanism for inorganic oxide catalysts

which utilizes the oxide species to a significant extent, one would

initially conclude that CMS based catalysts should not have much

reactivity in butene dehydrogenation schemes. Alternatively, if CMS

systems are active, other mechanistic interactions would seem more

plausible. CMS systems have, however, been shown to be active for

butene conversion and an oxide migration type mechanism has been

proposed. Ademodi and co-workers report that a PPAN based system

oxidatively dehydrogenates butene to butadiene to about 35% conversion

at 3500 C with essentially 100% selectivity.92 The mechanism is not








42

fully delineated, but it is proposed to involve adsorption,

dissociation, and migration of oxygen species through the sieve

material. Since AX21 is far more porous than PPAN and since it contains

an appreciable amount of oxide functionalities, this should also be an

interesting system to study for the conversion of butene to butadiene.

The final class of substrates to be investigated in this chapter,

the various alcohol and related oxygenated compounds, are not nearly as

attractive from an industrial standpoint as ethyl benzene or butene.

There are far better ways of producing large scale quantities of

propylene, for example, than dehydrating n- or isopropanol. In fact,

this is opposite of the usual direction such catalytic transformations

are pursued--oxygenated products are typically more desirable. The

study of such reaction sequences is nonetheless beneficial. By

examining a variety of substrates, a better understanding of the

reactivity of CMS catalysts can be obtained. Furthermore, the series of

C3 compounds offers the gamut of organic functionalities: straight-chain

alcohol, branched-chain alcohol, aldehyde, and ketone. An analysis of

the reactivity of these catalysts towards the respective compounds, as

well as the nature of the product distributions, should provide a handle

on the mechanistic interactions that are occurring.

The study of the alcohol substrates also offers a further

opportunity to compare the reactivity of CMS catalysts to inorganic

oxide systems. Much work has been done with various A1203 based

catalysts in alcohol dehydrogenation and dehydration reaction

schemes.93,94 The activity is a function of the Lewis acidity of the

catalyst with the products being formed via carbocation intermediates.








43

Although carbonaceous materials are not usually considered to have much

Lewis acidity, several studies have shown carbon based catalysts to be

active for alcohol substrates with reaction pathways similar to the

inorganic oxide systems. A thorough examination of the reactivity of

these substrates over both types of catalysts should provide a better

understanding of their similarities and differences in catalytic

transformations.



Experimental


Reagents


Ethyl benzene was purchased from Fisher Scientific Company and was

purified by vacuum distillation prior to use. Several later

experiments were performed with ethyl benzene samples that were not

vacuum distilled and the results were the same as in the purified

systems. 1-Butene was purchased from Matheson Gas Products as C.P.

grade with 99.0% minimum purity. Samples were checked by gas

chromatography (GC) to verify that there were no detectable impurities

present. Ethanol, 200 proof, was purchased from Florida Distillers

Incorporated and was dried over 4 A molecular sieves prior to use. n-

Propanol, iso-propanol, and propionaldehyde were all purchased from

Eastman Kodak Company as reagent grade and were dried over 4 A molecular

sieves prior to use. Each sample was checked by GC to verify purity.

Reagent grade acetone was purchased from Fisher Scientific and was dried

in the same manner as the other substrates prior to use. Other

chemicals employed as GC standards for product verification--benzene,








44

toluene, butadiene, diethyl ether, acetaldehyde--were reagent grade and

used as received.


Preparation of Catalysts


The PPAN samples were prepared as described in the previous

chapter. AX21 samples were dried in vacuo at 100 o C for at least 8

hours prior to use. Unused portions were stored in a desiccator for

future experiments.. The activated charcoal (AC) material was purchased

from Mallinckrodt Incorporated and was dried in vacuo in the same

fashion as AX21. The AC material has a nominal surface area of 800

m2/g. A1203 was chromatographic grade and was purchased from Davisson.

It was calcined at 3500 C prior to use. The nominal surface area of the

A1203 sample is 360 m2/g. The cerium pyrophosphate catalyst was

synthesized from Ce(S04)2 (Alfa) and H3PO4 (Fisher Scientific) according

to the method of Vrieland.74 The industrial mixed oxide catalyst--93%

Fe203, 5% Cr203, 2% K02 on MgO--was prepared according to the method of

Ghublikian.9 All the oxide samples were purchased from Fisher

Scientific and used without further purification.

Reactor


The reactor is a fixed-bed flow reactor modelled after that

designed and described by Clark.59 For the butene system, the butene

and air reactants are mixed through a two-stage glass bubbler and then

the blended gas stream is fed into the reactor. In all the other

systems, the substrate is delivered as a liquid via a syringe pump. The

pump is also modelled after a design by Clark,59 but additionally is








45

fitted with an adjustable gear box which enables variable flow rates to

be administered. Temperature is monitored by a thermocouple (type J

connected to an Omega Engineering CN 300 temperature controller ) which

is encased in a glass tube that is in contact with the catalyst bed.

Unless otherwise stated, 0.5g of catalyst is used in each experiment.

Catalyst bed height is approximately 30 mm for all CMS systems. A

schematic diagram of the reactor setup is shown in Figure 3-1.

In the liquid feed systems, a six inch layer of glass beads

(Fisher Scientific) is placed on top of the catalyst to facilitate

complete vaporization of the substrate and to allow thorough mixing with

the carrier gas before contact with the catalyst. Liquid products are

condensed in a trap that is maintained at 00 C. Blank experiments in

which the system was set up with the reactor tube filled with glass

beads were run for all substrates. No activity was observed in any of

these blank systems.


Analysis


Routine product analysis is done via gas chromatography. Pre-

catalyst and post-catalyst gas samples, as well as post-catalyst liquid

samples, are periodically examined for each experiment. Typical

injection sizes are 0.5 ml for gas samples and 0.15 ul for liquid

samples. For the ethyl benzene experiments, a Varian 3400 GC fitted

with a 3m Dega S column and a Flame Ionization Detector is employed.

The column temperature is ramped from 75 1500 C at 100/min. Data

analysis is performed with a Varian 4290 recorder/integrator. For

butene and the various oxygenated substrates, a Varian 940 GC fitted






















































Leads
to
temperature controller


----- Carrier gas inlet











---- Tube furnace


-- Reactor tube

Glass beads


Catalyst






--- Thermocouple













---- Septa for gas
Sampling

Gas outlet



---- Dewar filled
ice


Schematic diagram of reactor setup.


Figure 3-1.








47

with a 6 ft. Hayesep Q column and a Flame Ionization Detector was

operated isothermally at 1300 C. Data analysis is performed on a

Hewlett Packard 3390A recorder/integrator. Further product

identification and verification is done by GCMS using a Varian 3400 GC

interfaced with a Finnegan MAT ITDS 700 Mass Spectrometer. The column

employed for this technique is'a 15m DB1 column operated isothermally at

400 C. Quantification for all GC data is done by standard calibration

methods. Reported conversion percentages are based on the difference

between the moles of substrate entering the reactor and the amount

present in the post reactor stream.


Results and Discussion


Ethyl Benzene Substrate

As mentioned in the background section, one of the major

advantages of the oxidative dehydrogenation process is that it can

operate efficiently at lower temperatures than the direct

dehydrogenation scheme. Nevertheless, most of the recently reported

oxydehydrogenation catalysts have been studied in the 4500-6000 C

temperature range.74-83 Since these systems must first form the active

carbonaceous overlayer--a step most likely requiring a significant

amount of heat--it is quite possible that the CMS catalysts can function

at significantly lower temperatures. Another motivation for operating

at reaction temperatures below 4500 C is that this is the final

pyrolysis temperature of the PPAN samples used in this study. As

discussed in the previous chapter, PPAN can be further pyrolyzed up to








48

15000 C to go all the way to the formation of carbon fibers. Our

results have shown, however, that the condensed pyridine ring

structure--which is the proposed catalytic species videe infra)--is

maximized at temperatures up to 4500 C. Reaction temperatures above

4500 C will lead to further structural rearrangements and the formation

of volatile surface species which could lower catalyst activity and

complicate product analysis.

The first series of catalytic experiments were performed at

2500 C. Table 3-1 shows the results for various PPAN systems in the

ethyl benzene oxydehydrogenation reaction. The reported product

analyses are calculated from post-catalyst gas samples according to the

method described in the experimental section of this chapter. It is

evident that the PPAN catalysts have slight to moderate activity with

high selectivity to the styrene product. Also shown in Table 3-1 are

the results for an A1203 sample in this reaction scheme. This

experiment is done in order to compare the reactivity of a typical high

surface area (360 m2/g) inorganic oxide under the particular reaction

conditions employed. As is shown in the table, the oxide sample is not

very active.

The observation of this type of reactivity at such a low

temperature is quite encouraging. Since the above study is performed at

less than half the temperature commonly employed for this reaction, a

series of studies are done at higher temperatures to see if the activity

can be further increased. The results for the PPAN materials in the

oxydehydrogenation of ethyl benzene at 3500 C are shown in Table 3-2.

It is observed that all systems are far more active at this temperature








49

Table 3-1 Ethyl Benzene Reactivity Over PPAN Catalysts at 2500 C


R EIRRAC GAS


air

N2
air

N2
air

N2

air


OISREVNOC% Na


5.3

0.1

6.1

0.1

10.2

0.8

0.8


%SELECTIVITYb


85.0

48.9

81.3

50.1

86.7

43.2

60.0


CONDITIONS:


Substrate delivery--0.2 ml/hr. Carrier flow--5 ml/min.
Reaction time--20 hrs for air runs, 4 hrs for N2 runs.


a) non-styrene products are CO2, benzene, toluene, and benzaldehyde.

b) percentage of all products that are styrene.


SAMPI F


PPAN1

PPAN1

PPAN2

PPAN2

PPAN3

PPAN3

Al203


SAMPLE CARRIER GAS








50

Ethyl Benzene Reactivity Over PPAN Catalysts at 3500 C


(N2 carrier)


N OISREVNOC%


11.6

14.6

22.3

2.5

1.6


YTIVITCELES%


90.5

76.0

95.6

80.0

62.5


CONDITIONS: Same as those in Table 3-1.


Table 3-2


SAMPI F


PPAN1

PPAN2

PPAN3

PPAN3

A1203


IZAMD I 1: Lrv~------








51

than at 2500 C. Since 3500 C is still far below the reaction

temperature of other reported catalyst systems, all further catalytic

studies for this reaction are done at this temperature.

Several observations can be made form the data in Table 3-2. It

is clear that surface area is a key factor to the level of activity.

PPAN3 which was co-pyrolyzed with NH4C1 and thus has a significantly

higher surface area than the undoped PPAN1 sample, is twice as active

for the conversion of ethyl benzene to styrene. This is not at all

surprising since a higher surface area allows more substrate/catalyst

interaction. Such a direct effect between surface area and activity has

been shown for a wide variety of heterogeneous catalyst systems.

It is also evident from the data in Tables 3-1 and 3-2 that this

catalytic process is occurring via an oxidative route. When the system

is operated in the absence of 02 (present in the air carrier flow) the

activity is only slight and it subsides within a couple of hours. The

system can be cycled back and forth, however, between active stage (air

carrier) and inactive stages (N2 carrier). Upon return to an air flow,

reactivity is returned virtually immediately. This series of

experiments was followed through five complete cycles and no loss in

activity or selectivity was observed. The mechanism typically invoked

for dehydrogenation processes over PPAN catalysts involves hydrogen

abstraction from the substrate producing a hydrogenated PPAN moiety.

The catalytic cycle is completed by reaction with 02 to yield H20 and

the original catalyst structure. This reaction scheme was previously

shown in Figure 1-1. The inability of the PPAN to be dehydrogenated is








52

the reason suggested for the eventual loss of activity in the N2 carrier

experiments.

Despite the fact that this mechanism has been widely proposed to

account for the catalytic activity of PPAN samples,31'34'41-43 it should

be recalled that PPAN1 and PPAN3 also contain 8.5% by weight oxygen and

oxygenated carbonaceous residues have been proposed as active catalysts

for this reaction. PPAN2 which is partially prepared in air according

to the method of Degannes and Ruthven,43 has an appreciably higher

oxygen content -- 13%. Furthermore the DRIFT results discussed in the

previous chapter show that some of this oxygen content is present in

carbonyl functionalities not unlike the polynapthoquinone moieties

proposed by Cadus82 and Emig and Hoffman.83 In light of this, one might

expect PPAN2 samples to have a higher reactivity than the other PPAN

samples. This, however, is not the case. PPAN2 is only slightly more

active than PPAN1, less active than PPAN3, and has a lower selectivity

than either of the N2-carrier preparations. It is concluded from this

then, that the majority of dehydrogenation activity in the PPAN samples

reported here takes place by the typical mechanism depicted in

Figure 1-1. Any participation by oxygenated moieties is considered to

be minimal.

The direct relationship between higher surface area and higher

activity was the motivation for investigating the commercially available

CMS system prepared from coal pitch residues. AX21 possesses one of the

highest surface areas ever measured -- >2500 m2/g. Additionally, the

novel structure and the oxygen content of this material make it a very

close analogue to the carbonaceous overlayer species present on the








53

inorganic oxide catalysts discussed previously. The results of these

commercially available CMS materials, as well as the data for samples of

the industrial Fe203/Cr203 catalyst and Vrieland's cerium pyrophosphate

system are shown in Table 3-3.

It is evident that under the experimental conditions of this

study, the two literature-preparation samples are not very active. This

is not unexpected since neither of these systems is reported to have

much activity at 3500 C. AX21, on the other hand, is remarkably active.

Conversion of 80% with greater than 90% selectivity is observed in a

single pass. Like the PPAN systems, this is an oxidative process which

can be cycled. When run in N2 carrier flow, the activity drops

immediately. When air is returned, the formation of styrene is readily

observed. This system is also active over a long period of time. An

extended run was followed for 120 hours in continuous operation and no

appreciable loss in conversion or selectivity is seen. The sample can

even be removed from the system for several days and activity is re-

established when the reactor is set up again. Pre-catalyst and post-

catalyst weight is further monitored in order to determine if the sample

itself is being oxidized under the reaction conditions. No appreciable

catalyst weight loss is observed in any of the AX21 runs described here

for the oxydehydrogenation of ethyl benzene.

Clearly, a significant amount of this increased reactivity is due

to the extraordinary surface area of the AX21 material. This alone,

however, can not explain the overwhelming activity. The activated

charcoal (AC) system has a surface area of 800 m2/g, and yet its

activity is only slightly greater than PPAN3, the "high surface area"








54

Table 3-3 Ethyl Benzene Reactivity Over AX21 and Other Catalysts at
350 C



SAMPLE %CONVERSION %SELECTIVITY

AX21 80.0 90.1

A.C.a 26.0 84.2

CeP207b 6.7 92.5

Fe/Cr/K/Mgc 0.9 45.1

AX21 (N2 flow) 5.2 40.4



CONDITIONS: Same as those in Table 3-1.



a) Activated Charcoal

b) Prepared according to reference 74.

c) Prepared according to reference 95.








55

PPAN material (50 m2/g). The microscopic structure of AX21, which is

composed of carbonaceous lamellae, gives this material an exceptional

adsorption capability. Toxicological studies have shown that these

materials adsorb considerably more substrate than other highly adsorbent

carbonaceous materials.48-50 This affinity for substrate, together with

a surface chemical structure which is especially suited for this type of

catalytic transformation (due to the oxygen content) surely contributes

to the outstanding reactivity of AX21 in this reaction. The specific

mechanism for the AX21 catalyst is most likely analogous to that

proposed by both the Cadus82 and Emig83 groups. Shown in Figure 3-2,

this reaction scheme invokes a concerted hydrogen abstraction from the

substrate by the oxygen functionalities of the carbonaceous overlayer

formed from coke deposition on an inorganic oxide surface.


Butene Substrate


Although there is not near as much evidence on the role of

carbonaceous materials in the dehydrogenation of butene as there is for

ethyl benzene, there have been several reports of CMS type materials

being used in this reaction scheme. Table 3-4 shows the results for the

dehydrogenation of butene to butadiene over a variety of catalyst

materials. It is evident that only the AX21 sample has any appreciable

activity under the reaction conditions employed. The A1203 sample is

essentially unreactive and as before, this is not unusual at these low

reaction temperatures (2500 C). It is surprising though, that the PPAN

systems are not more active in this reaction scheme since Ademodi and

























styrene nin



condensation -3H2


a~K


+02
surface
oxidation


HHC-C0


I 0 1 0



"coke'



H20 -- 1/2 02
H~c


Proposed mechanism for AX21 dehydrogenation activity.


Figure 3-2.










Table 3-4 Butene Reactivity Over CMS Catalysts


N OISREVNOC%


1.0

1.2


11.0

<1.0


%SELECTIVITY


84.0

83.3

91.8


CONDITIONS:


Reaction temperature--2500 C.
Carrier feed--3 ml/min.


Substrate feed--2 ml/min.


Percentage of products which is butadiene. Other products are C1 -
C3 hydrocarbons.


SAMPI F


PPAN1

PPAN3

AX21

AI203


SAMPLE---








58

co-workers reported a PPAN system that has 20% conversion at 2500 C92

The Ademodi catalyst is prepared in a slightly different manner and the

authors do not present any characterization studies. Another difficulty

is that some of the reactor data, such as catalyst bed height, is not

given. As discussed in Chapter 2, there are some problems with

comparing PPAN samples from one author to the next.

The results for AX21 show that this material has moderate activity

with excellent selectivity to butadiene. The conversion of 11% appears

low compared to the reactivity of this catalyst towards ethyl benzene,

however, other studies have shown carbonaceous materials to have less

reactivity in this system than in ethyl benzene systems. Emig and

Hoffman report considerably lower reactivity for their carbonized

zirconium phosphate system in butadiene conversion as opposed to ethyl

benzene.83


Oxygenated Substrates


In order to extend the study of the catalytic reactivity of CMS

materials, a series of oxygenated substrates are investigated. Initial

work is done with ethanol. The dehydrogenation of ethanol to

acetaldehyde is well characterized on inorganic oxide catalysts.96'97

The results for the reactivity of this substrate over CMS catalysts at

2300 C is shown in Table 3-5. Again the results for an alumina run are

also included to directly compare the reactivity of CMS materials to

commonly employed inorganic oxides. At 2300 C, the A1203 sample is

found to have very little activity. The PPAN catalyst (PPAN3, the high

surface area sample) is considerably more active. Overall conversion is











Ethanol Reactivity Over CMS Catalysts


SAMPLE CARRIER ACETALDEHYDEE


A1203

PPAN3

AX21

AX21


%ETHYL %DIETHYk %OVERALL
ACETATE ETHER CONVERSION


air


air


CONDITIONS:




*


Reaction temperature--2300 C.
ml/hr.Carrier flow--5 ml/min.
air runs, 4 hrs for N2 run.


Substrate flow--0.2
Reaction time--20 hrs for


Percentage of stated product among all products.


__


Table 3-5








60

20% with the major products being acetaldehyde and ethyl acetate. The

former product is the result of the direct dehydrogenation of ethanol.

The latter product must occur via a secondary condensation reaction in

which an acetaldehyde fragment reacts with an unconverted ethanol

molecule. As with the other substrates studied in this chapter, the key

mechanistic step involved here is hydrogen abstraction. The extensive

work in this area by Iwasawa proposes that the alcohol adsorbs

dissociatively to form a bound alkoxide.96 This is followed by hydrogen

abstraction to produce the dehydrogenated product. PPAN has been shown

previously to be efficient at hydrogen abstraction with alcohol

substrates.31,41,97 The nature of the product profile, acetaldehyde

being the major product, shows that the catalyst preferentially

abstracts a hydrogen from the methylene carbon.

As might be expected from the ethyl benzene and butene systems,

AX21 is far more active than the PPAN system. Overall conversion is 70%

with a product profile very similar to the PPAN catalyst. The major

products are again acetaldehyde and ethyl acetate. The similarity in

the selectivities between PPAN and AX21 towards ethanol conversion

suggest that the two materials operate through similar mechanisms. The

significantly greater activity of AX21 over PPAN, however, exemplifies

how much stronger a hydrogen abstraction catalyst is the former CMS

moiety.

The ethanol results are enlightening in that some reactivity

information is obtained, but since there is really only one possible

hydrogen abstraction site (it would be highly unlikely for any

abstraction to occur at the methyl hydrogens) not much mechanistic








61

information can be learned. For this reason a series of C3 substrates

are examined. The straight-chain alcohol, n-propanol, the branched-

chain alcohol, isopropanol, the aldehyde, propionaldehyde, and the

ketone, acetone, are each studied for their reactivity with the AX21

catalyst.

The results for the C3 substrates are shown in Table 3-6. The

reaction temperature (2300 C), catalyst weight (0.5g), carrier flow

(5ml/min), and substrate flow (0.2 ml/min) were all the same as for the

ethanol experiment. It is evident that the alcohols were quite

susceptible to both dehydration and dehydrogenation. The aldehyde

substrate was equally reactive, however, the products are significantly

different--they are all C2 species. Acetone is virtually unreactive as

a substrate. Analogous to the ethanol system, the reactivity for the C3

substrates is dependent on the nature of the carrier flow. When N2 is

used in place of air, product formation ceases.

For n- and iso-propanol, the major product is propylene. Proposed

mechanistic pathways to this and the other products observed for the

former substrate are shown in Figure 3-3. The latter substrate reacts

via similar routes. As is shown, the initial coordination of the

propanol occurs via the donor oxygen to a Lewis acid site on the

catalyst surface. This mode of coordination is often invoked in alcohol

oxidation processes.96'98 The formation of propylene is the result of

hydrogen abstraction from the secondary carbon (labeled C2 in

Figure 3-3) by a surface oxygen species. This step is drawn in Figure

3-3 as a hydride abstraction which produces a carbocation. The

formation of the alkene is shown to occur in a subsequent step. It is









62

Reactivity of C3 Oxygenated Substrates Over AX21 Catalyst


PRnnPl CT~


L LAREVO CONV ERS IONb


n-propanol



iso-propanol



propanal




acetone


Table 3-6




SUBSTRATE


CONDITIONS:


Reaction temperature--2300 C.
Substrate flow--0.2 ml/hr.


Carrier flow--5 ml/min.


Percentage of stated product among all products.

Other products include small amounts of ethers and esters.


propene
propanal


propene
acetone


acetaldehyde
ethanol
ethylene


46



48




< 1


PpnnHC TS











+ 0


/H H
C C CH
/ H H
0 O
1 1


C1 Abstraction


H/ /
0 0 0O

\\\\^\\\\^^ ~ ~ ~ \ Iu I, ''i u Ji~ im I


0 /


\ 02 Abstraction


/H H
C C C CH3
H ?'H 3
0 0


H H
0C+ C- CH3
HO 0 HO







CH3CH2C(0)H


H H


HO 0 / OH






CHCH
CH3OH=CH2


Figure 3-3.


Proposed mechanistic routes for conversion of
n-propanol.


H
/
0


1 1


-.,..~...--~..


CH3CH2CH20H










quite possible that a concerted process which incorporates both of these

stages is occurring. This is analogous to the proposed mechanistic

pathway in the ethyl benzene system. The formation of propionaldehyde

must occur through abstraction from the functionalized carbon (labeled

C1 in Figure 3-3). The fact that a secondary carbocation is more stable

than a primary one most likely accounts for the predominance of the

alkene product. The observation of a small amount of di-n-propylether

shows that coupling reactions can also occur. It should additionally be

noted that there is a small amount (<2%) of acetaldehyde and ethylene

produced in the n-propanol system. These products are formed via

secondary reactions of the propionaldehyde species videe infra). This

shows that the catalyst is able to readsorb and cause further reaction

of the primary products.

As is shown in Table 3-6, the propionaldehyde substrate yields all

C2 products. This seemingly unusual result can best be explained by a

hydrogen atom abstraction mechanism. Abstraction of the aldehydic

hydrogen atom gives rise to and RCO' species. This then decarbonylates

to produce the C2 fragment, R', which on further reaction accounts for

the various products observed. Again this is most likely a concerted

process since attempts to detect a radical intermediate have proved

unsuccessful.

These results demonstrate that the AX21 material can function in

either hydride or hydrogen atom abstraction reactions depending on the

nature of the substrate. Hydrogen atom abstraction occurs rather easily

on an aldehydic species such as propionaldehyde; whereas, hydride

abstraction is more facile on alcohol substrates. In the case of








65

acetone, neither hydride or hydrogen atom abstraction should occur

readily and as expected, this substrate is found to be unreactive.














CHAPTER 4
CARBON MOLECULAR SIEVES AS CATALYST SUPPORTS


Introduction


This chapter focuses on the study of CMS materials as supports for

three different types of heterogeneous catalytic systems. In the first,

PPAN and AX21 have been doped with ruthenium species and have been

investigated in the Fischer-Tropsch reaction. A great deal of work has

been done on this system employing inorganic oxide supports. Virtually

all of these catalysts yield broad product distributions. The goal of

this study is to devise a system which will be active towards the

conversion of synthesis gas (CO and H2) with high selectivity to small

molecular weight products. The second catalyst system studied employs

Mo03 doped AX21 for the oxidation of methanol. As discussed in the

previous chapter, AX21 itself is active towards alcohol substrates.

Methanol, however, is one of the most difficult alcohols to

catalytically transform. The objective of this study is to combine a

catalytically active metal species with a similarly active support to

produce a system which can display a synergism between its constituents.

Lastly, molybdenum and tungsten doped AX21 systems have been

investigated in the deep oxidation of chlorinated hydrocarbons. There

is a growing interest in developing catalytic systems which can fully

oxidize various organic pollutants, e.g. CH2C12, to compounds that are








67

easily disposed like CO2 and HC1. The goal of this study is to combine

the exceptional adsorption capabilities of the support with the

oxidative catalytic ability of the metal dopant. In each of these

applications, the specific structure and properties of the CMS materials

offer unique advantages towards accomplishing the prescribed goals.

Fischer and Tropsch first reported the catalytic conversion of

synthesis gas to higher hydrocarbon products in 1926.99 During the more

than 60 years since that time, there has been a great deal of work

conducted in the field. Traditionally this reaction has been performed

under rather severe conditions--pressures of 100-200 atmospheres and

temperatures in excess of 4000 C. Throughout the study of this process,

though, there has been a wide range of conditions employed. Table 4-1

lists some of the different systems that have been investigated.

Included in Table 4-1 are the conditions for the SASOL plant in South

Africa which is the only presently operating industrial facility that

uses the Fischer-Tropsch reaction for the production of hydrocarbons.

It is evident that many of the transition metals show some

activity for CO hydrogenation. In considering a industrially viable

system, however, overall activity is not the most significant parameter.

Selectivity to a certain desirable range of products is of paramount

importance. Virtually all Fischer-Tropsch systems eventually give rise

to a Schulz-Flory product distribution.100,101 This model, which was

originally derived to explain polymerization sequences, can be described

by the equations below:

Wn = n (1-a)2 an-1 (4-1)

log Wn/n = n loga + log [(1-a)2/a] (4-2)








68

Table 4-1 Commonly Studied Fischer Tropsch Systems



Typical Metal Dopants

Ru, Rh, Os, Fe, Co



Common Supports

Si02, A1203, MgO, Ti02



Typical Reaction Conditions

Pressure: 100 200 atm

Temperature: 300 4000 C


Reported Ranges

Pressure:

Temperatu



SASOL Plant

Catalyst:

Pressure:

Temperature:


1 2000 atm

re: 150 4500 C





promoted Fe203

25 atm

220 3400 C








69

The variables in these equations are defined as follows: n is the carbon

number of a particular polymer unit or, in the case of Fischer-Tropsch

systems, it is the number of carbons in a particular product; Wn is the

weight fraction of that product among all the products; and a is the

probability of chain growth. This parameter is a function of the degree

of polymerization and rates of propagation and termination.

Equation 4-2 shows that a linear function should be obtained from

a plot of log Wn/n vs n. Such a plot is called an Anderson-Schulz-Flory

(ASF) graph.102-104 The slope of the line is related to a which in

Fischer-Tropsch terminology describes the chain growth of the reaction

process. As the a value increases, so does the chain length of the

hydrocarbons formed. This produces a broader product distribution.

Hydrocarbons up to a chain length of C30 are not unusual in many active

Fischer-Tropsch systems. Since Equation 4-2 is derived from a polymer

model, there are certain deviations from linearity when it is applied to

the Fischer-Tropsch process. CI values always fall above the line and

typically are not considered part of the data set. The model is

designed for the linking of methylene fragments such as would occur in a

growing polymer chain. Since CH4 is formed by direct hydrogenation of a

surface carbide fragment, its production is not related to the polymer

model. Many experimental systems also have C2 and C3 values which

deviate somewhat from linearity. Above C4 though, most systems fit ASF

plots very well.

In the aftermath of the oil crisis in the mid 1970's, there was an

explosion of interest in alternatives to petroleum based routes for

hydrocarbon production. Much effort was focused on Fischer-Tropsch








70

systems for the formation of gasoline range products.71,105,106 Many

systems, most of which employed Group VIII metals supported on high

surface area inorganic oxides, were reported to be highly active.

Despite the research enthusiasm, however, the production of gasoline

range products via CO hydrogenation never became significantly cost

competitive with petroleum pathways. It now seems apparent that, at

least for the foreseeable future, the only realistic market for the

Fischer-Tropsch process is in the selective production of small

molecular weight hydrocarbons and oxygenated products which can be used

for the production of specialty chemicals. It would be particularly

desirable to make alkenes such as ethylene and propylene by this route.

The problem with designing a system to selectively produce the

products mentioned above has already been pointed out. Most CO

hydrogenation systems exhibit a broad product distribution with limited

selectivity towards the low molecular weight compounds. As mentioned

previously, there is a great deal of similarity in the reactivity of the

many metal species studied. The mechanism most often invoked for low

selectivity is related much more to the oxide support than it is to the

metal dopant.107-110 Initially, inorganic oxides such as silica and

alumina provide excellent dispersion of the metal species. Since there

is no great chemical attraction between the oxide or hydroxyl nature of

the support and the metal dopant, however, the metals tend to migrate on

the surface of the support during the course of the reaction. This

migration produces a significant growth in metal crystallite size. Such

a phenomenon causes only a slight decrease in activity, but it has a

detrimental effect on the selectivity. The sintering of metal species








71

leads to very broad product distributions.108-110 To devise a Fischer-

Tropsch system for the selective production of light hydrocarbon

products then, it seems prudent to focus on the choice of a proper

support as opposed to the design of new metal species. Prospective

supports should have the ability to strongly interact with small metal

clusters and through this interaction, prevent the formation of large

metal crystallites.

This criteria was the motivating factor for employing PPAN and

AX21 as supports in heterogeneous Fischer-Tropsch systems. The

condensed pyridine ring functionality of PPAN should provide a much

stronger interaction in an acid-base sense than the oxide and hydroxyl

species of inorganic oxides. AX21 should likewise be able to inhibit

cluster formation, but for a completely different reason. The pores and

cavities associated with the structure of this material should provide

isolation of small metal clusters in much the same way as alumino-

silicate zeolite materials have been employed.111-114 Both of these

support systems have the additional advantage of being catalysts

themselves. As was suggested earlier, olefin products are more desired

than paraffins. The dehydrogenation ability of these CMS materials,

such as that discussed in the previous chapter enables the possibility

of synergistic interactions between the metal dopant and the

carbonaceous support.

In addition to the design of Fischer-Tropsch systems with improved

selectivity, mechanistic studies of the basic Fischer-Tropsch reaction

and systems active for CO2 reduction have also been investigated with

CMS supports. In the former work, acetaldehyde is used in a co-feed








72

with syn gas. By following the fate of this additive, some very

interesting information is obtained regarding the fundamental reaction

steps of the Fischer-Tropsch process. Lastly, these systems are shown

to have appreciable activity towards CO2 reduction. Studies conducted

with CO/CO2 co-feeds further demonstrate how product selectivity can be

influenced.

The oxidation of methanol is one of the most widely studied

industrial catalytic processes. It is well documented that there are

two main reaction pathways for this substrate.115 The predominance of

one over the other depends mostly on reaction temperature. In the so-

called low-temperature scheme, the major products are methyl format

(MF) dimethoxymethane (DMM) and dimethyl ether (DME). In the high-

temperature systems, formaldehyde can be produced with high selectivity.

This latter process is performed on a major industrial scale.

Formaldehyde is projected to be the fifth largest produced organic

chemical in 1989.69 The relevant reactions and conditions of these two

processes are shown in Table 4-2.

Although the primary focus in methanol oxidation is on the high

temperature production of formaldehyde, there is a growing interest in

low temperature systems which can selectively produce methyl format.

This product is formed through a dimerization of formaldehyde units in

the so-called Tischenko reaction.116-118 Methyl format has been shown

in a number or recent studies to be a versatile intermediate for the

synthesis of such high volume products as acetic acid,119-121 dimethyl

formamide,122 and ethylene gylcol.123










Typical Methanol Oxidation Reaction Routes


1. Low Temperature Process


2 CH3OH


3 CH3OH +



CONDITIONS:


+ 02


o02


------> CH3OC(O)H +


CH3OCH2OCH3 +


2 H20

2 H20


Reaction temperature--200-2400 C
Reported catalysts--Mo03, Cu2Cr2O4
23% conversion
86% selectivity (MF)


2. High Temperature Processes


CH30H +


02 ------>


CH20


+ H20


CONDITIONS: Reaction temperature--600-8000 C
Reported catalysts--FeMoO4 or Ag
80-90% conversion


Table 4-2










In light of the reactivity of AX21 toward alcohol substrates which

was discussed in the previous chapter, this catalyst is employed for

methanol oxidation studies. Since a great deal of work had already been

conducted in the low temperature regime, initial studies are done at

2300 C. The goal is to design a system which will exhibit higher

reactivity with appreciable selectivity to the desired product--under

these conditions, methyl format. Higher reaction temperatures are also

studied in the attempt to produce formaldehyde. Both the undoped AX21

and a variety of Mo/AX21 systems are found to be active for the

conversion of methanol. The doped systems are found to be especially

interesting due to an apparent synergism between the metal and the

support. This phenomenon is unique in that it is not observed for

inorganic oxide supported systems. A series of surface science

techniques have been conducted on the Mo doped AX21 catalysts as well as

similarly prepared Si02 supported systems. Scanning Electron Microscopy

(SEM), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy

(XPS) have revealed some insight into the nature of the doped metal

species and how it changes during the course of the reaction.

The last section of this chapter focuses on the deep oxidation of

chlorinated hydrocarbons. The industrial production of these materials

for use as solvents, pesticides and a host of other applications has

been rising continuously over the past ten years. Due to the widespread

use of these materials, there is a growing environmental pollution

problem associated with their improper and inefficient disposal.124

These compounds prove to be particularly difficult to dispose of because

they do not burn efficiently in typical waste incineration plants. This








75

problem is further complicated by the fact that the by-products formed

from incomplete incineration are often more hazardous than the original

starting material. In light of this, there is a great deal of interest

in catalytic systems which can fully degrade chlorinated hydrocarbons to

much more easily handled materials such as CO2 and HC1. The systems

most often studied for such reactivity involve transitional metal oxides

doped with a variety of constituents.125 Recently an electrolytic

oxidation system was reported to be efficient for a variety of

chlorinated hydrocarbon species including aromatic systems.126

Several molybdenum and tungsten doped AX21 systems are studied for

their reactivity towards CH2C12. This substrate is chosen for two

reasons. First, its conversion should be relatively easy to follow.

Secondly, it is a relevant system to study, since CH2C12 is by far the

largest industrial chlorinated hydrocarbon pollutant.124 The use of the

AX21 support should offer several advantages over unsupported systems or

those employing inorganic oxide supports. Much of the work discussed so

far has shown that AX21 offers a significantly higher and more stable

degree of dispersion than inorganic supports. This increased surface

area of the active metal species should improve the reactivity of this

system in much the same way as it does for other catalytic reaction

schemes. Additionally, the extraordinary adsorption capability of the

AX21 support can greatly enhance the reactivity of the system. This

should especially be true in "real-world" operations where the organic

pollutant is only present in small concentrations. A lot of work is

being done in employing carbonaceous materials for ground water

purification.47 The work described in this section thus combines the








76

two current methods for toxic waste clean up. The CMS support is

employed to more efficiently adsorb the specific pollutant from the

waste stream and the metal dopants are present for the deep catalytic

oxidation of these materials.


Experimental


Reagents

Synthesis Gas Conversion

The reactant gases were delivered directly from their respective

cylinders and were not further purified. The carbon monoxide (Matheson

Gas Products) used was Coleman Purity grade with a minimum purity of

99.5%. The carbon dioxide (Matheson) sample was Coleman Instrumental

grade having a minimum purity of 99.99%. The hydrogen (Liquid Air

Corp.) supply was prepurified grade with a minimum of 99.9%. The

ruthenium metal dopings were prepared from both RuC13 (Alfa) and

Ru3(CO)12 (Strem Chemical Company). The acetaldehyde (Fisher

Scientific) reactant was dried over 4A molecular sieves prior to use and

was stored in the chemical refrigerator. Identification of products via

gas chromatography was aided by the use of Scotty Analyzed Gases (Scott

Specialty Gases Company). Three different samples were employed: Can

Mix 1 containing C1 C6 linear paraffins; Can Mix 3 containing C2 C6

normal olefins plus acetylene; and Can Mix 32 containing CI C4 linear

paraffins plus isobutane. Isotopic labelling studies were conducted

using a 25% 13C labelled CO sample (Isotec Incorporated, Dayton Ohio).








77

The methanol reactant (Eastman Kodak Company) was spectro grade

and was dried over 4A molecular sieves prior to use. The absence of

impurities was verified by gas chromatography. The air supply (Liquid

Air) was U.S.P. breathing quality. The various metal dopants employed

were reagent grade and were used without further purification. Mo03 and

Na2MoO4 were purchased from Aldrich Chemical Company. The ammonium

molybdate sample was purchased from Mallinkcrodt under the name of Acid

Molybdic. This commonly used reagent is nominally (NH4)2MoO4 but is

essentially 85% Mo03

with added NH40H. The standards for product identification--Methyl

format (Fisher), dimenthoxymethane (Chem Service Company, West Chester,

Pennsylvania), and dimethyl ether (Matheson)--were reagent grade and

used without further purification.

Deep oxidation of chlorinated hydrocarbons

Methylene chloride (Mallinkcrodt) was reagent grade and was used

without further purification. The absence of impurities was verified by

gas chromatography The various metal dopants were reagent grade. The

molydenum species and the air supply were the same as those in the

methanol oxidation studies. All the tungsten samples -- WO3, Na2WO4,

(NH4)2W04 were purchased from Aldrich. The molecular formula for the

last species is not completely accurate. The supplier provided

elemental analysis data which reported the W content to be 66.9% by

weight. The simplified formula given above corresponds to 64.79%

tungsten. The value give by the supplier was used when calculating

molar quantities for catalyst preparation.










Preparation of Catalysts


The preparation of the PPAN and AX21 support materials was

described in Chapter 2. The AX21 sample was dried for eight hours in

vacuo at 1000 C prior to use. This is done in order to obtain an

accurate weight of the sample. The ruthenium doped catalysts for

synthesis gas conversion were prepared by combining 0.05g of Ru3(CO)12

or RuC13 with 2.45g of PPAN or AX21. For either dopant, this loading

corresponds to approximately 1% by weight Ru metal--0.98% in the case of

RuC13 and 0.96% for Ru3(CO)12. For the Ru3(CO)12 doped samples, the

mixed powders were refluxed in 60m1 of hexane (a mixture of isomers) for

a minimum of six hours. The term reflux is not totally accurate since

only the Ru3(CO)12 dissolves. This heating and mixing process, however,

serves to better disperse the metal cluster onto the support. For the

RuC13 doped samples, a similar reflux step is employed using H20 as the

solvent. After the heating and mixing process is completed, the

respective solvents are removed by rotary evaporation. The catalysts

are then dried in vacuo at 600 C (1000 C for the H20 systems) for eight

hours. A Ru3(CO)12 doped A1203 sample used in comparative catalytic

studies was prepared analogous to that of the PPAN and AX21 supported

systems.

In addition to the solvent deposition route described above, doped

PPAN samples were also prepared by mixing the Ru species and PAN

together before pyrolysis and firing the combined mixture. This

technique did not prove to be very successful though, since none of the

catalysts prepared in this manner exhibited any reactivity. It is








79

believed that this method is unproductive because the metal dopant gets

buried in the interior of the support matrix and is thus not readily

accessible to reactant gases.

The presence of the Ru3(CO)12 cluster on the PPAN and A1203

supports can be verified via DRIFT Spectroscopy. Figure 4-1 shows the

IR spectra of PPAN before and after being doped with Ru3(CO)12. The

metal carbonyl stretching frequencies can be observed in the 2000-2200

cm-1 region. Figures 4-2 and 4-3 display the metal carbonyl stretching

frequency region for unsupported Ru3(CO)12 and Ru3(CO)12/PPAN. It is

evident that the peaks for the supported system are shifted to lower

wavenumbers indicating a bonding interaction with the support. There

has been a great deal of work in relating the position of the peaks to

the type of the metal/support interaction and to the nature of the metal

species.127-130 The Ru/A1203 system, for example, has been well

characterized by IR assignments. The IR spectra for the Ru/PPAN system,

however, are not suitable for this type of detailed analysis. The peaks

are very broad and due to the high level of noise inherently associated

with PPAN IR spectra, inferences drawn from exact peak locations can not

be conclusive.

The various molydenum and tungsten doped samples were prepared by

the same solvent deposition method. In all cases, deionized H20 was

used. For the Mo03/AX21 sample, 0.5 g of the metal oxide was mixed with

2.5g of dry support. This loading corresponds to 3.47 x 10-3 moles of

Mo and 11% by weight metal content. This level of loading was chosen

because most literature work on support Mo03 systems is in the 8-12%

range.131-133 In order to maintain a degree of consistency between the











































I I I I
4000.0 3400.0 2800.0 2200.0 1600.0 1000.0 400
WAVENUMBERS (CM-1)


Figure 4-1.


DRIFT spectra of PPAN and Ru3(CO)12/PPAN.









81
























I I -I ---I
2200.0 2100.0 2000.0 1900.0 1800
WAVENUMBERS (CM-1)






X- 1933. 8 Y 84. 541
X- 1949 6 Y= 72.091
X= 1964. 4 Y= 69. 826
X= 1980. 9 Y= 55 733
X= 1987 7 Y= 50. 699
X= 2001.0 Y= 44 523
X- 2019 6 Y= 43 876
X= 2028.9 Y= 42 688
X= 2041. I Y= 45.250
X= 2062. Y= 45. 146
X= 2066. 2 Y= 45. 591
X= 2119.6 Y= 84 696


DRIFT spectrum of unsupported Ru3(CO)12.


Figure 4-2.






































I r --1-- ---- ------
2200.0 2100.0 2000.0 1900.0 1800
WAVENUMBERS (CM-i)





X= 1960 2 Y= 13 192
X= 1985. Y= 12 927
X= 1994 8 Y= 13 093
X= 2016. 3 Y= 13 207
X= 2022.5 Y= 13 041
X= 2044.5 Y= 13 020
X- 2052. 4 Y= 12 901


DRIFT spectrum of Ru3(CO)12/PPAN.


Figure 4-3.








83

various metal dopant precursors, this molar value was used for all the

other Mo and W species. For the Na2MoO4 sample, for example,

3.47 x 10-3 moles of Mo corresponds to 0.71g, so this amount is combined

with 2.45g of AX21.


Reactors


The reactor for all of the studies conducted in this chapter is a

fixed bed flow reactor of the same type as that employed in the

experiments of the previous chapter. Unless otherwise stated, 0.5g of

catalyst is used in all experiments. For the synthesis gas conversion

experiments, gas delivery was metered via a three stage silicon oil

bubbler fitted with rotoflow valves. The design of this device has been

described elsewhere.134,135 The top two stages were always used for H2

and CO flow respectively. The bottom could be used for N2 purging, CO2

reactant flow, or for HC1 reactant flow. When the latter species was

employed, concentrated sulfuric acid was used in place of the silicon

oil. The total flow rate of gas reactants was normally maintained at

4ml/min. In most syn gas experiments the ratio of H2/CO was 3:1.

Several studies were done with higher CO levels up to a H/2CO ratio of

1:3.

For the acetaldehyde co-feed experiments, an additional bubbler

was fitted directly to the top of the reactor tube just outside the

oven. Connection to the reactor tube was afforded via a ground glass

joint. The bubbler was filled with 3m1 of acetaldehyde. In order to

slow the rate of substrate delivery, the bubbler is placed in a Dewar

and the temperature is maintained at 00 C. The quantity of the aldehyde








84

going into the reactor can be monitored chromatographically through a

pre-gas septum.

Reaction temperature for all syn gas systems is 2200 C. This

temperature is monitored by a thermocouple encased in a glass tube in

contact with the catalyst bed. The design of the oven and the other

specifications of temperature control are the same as those in the

experiments described in the previous chapter.

In the methanol oxidation studies, the substrate is delivered via

the syringe pump described previously. The flow rate is maintained at

0.2ml/hr. Carrier gas flow, either N2 or air, is monitored by a Cole

Parmer variable flow controller. The flow rate is set at 5ml/min. In a

typical experimental cycle, the initial heating stage is done in N2 flow

with the syringe pump turned off. After reaction temperature is

reached, 2300 C unless otherwise stated, post gas samples are taken to

verify that no thermal decomposition products are present. At this

point carrier flow is switched to air. Post gas samples analyzed by gas

chromatography show that it takes about two hours for the N2 flow to be

completely purged. After the air carrier flow is determined to have the

proper N2/02 ratio, the syringe pump is turned on. The post reactor

effluent is collected in a trap which is maintained at 00 C.

The air injections prior to substrate delivery also serve to

detect any CO or CO2 being produced by oxidation of the CMS material

itself. No CO is observed in any of the systems. Trace levels of CO2

are observed at 2300 C. Usually, the CO2 peak is not even large enough

to be integrated and thus is too small to be accurately quantified. At

high temperature, however, this formation of CO2 from the oxidation of








85

the CMS material is greatly increased. These observations are discussed

in depth later in this chapter.

The experimental design for the deep oxidation studies is very

similar to the other systems previously discussed. The substrate is

delivered via the pre-reactor bubbler attached directly to the top of

the reactor tube. The bubbler is filled with 5 ml of CH2C12. As in the

acetaldehyde experiment, the delivery is slowed by cooling the bubbler

in an ice bath. The carrier flow rate is also reduced so that only a

small amount of CH2C12 is being delivered. The flow rate employed is

less than 1 ml/min. The post reactor effluent is condensed in a trap

which contains 10 ml of deionized H20. The purpose of this quenching in

H20 is to dissolve the HC1 that is generated from the deep oxidation.

The amount of acid formed can be quantified by titration with NaOH.

Periodic gas sampling of the post effluent also establishes the activity

of the system since CO2 formation can be observed.

For all three of the heterogeneous catalytic systems discussed in

this chapter, bed height and catalyst weight are measured before and

after each experiment. Very little loss in either bed height or weight

is observed in systems run at temperature below 2500 C. Above these

temperatures, however, appreciable degradation of the CMS material

occurs. This will be discussed further later in this chapter.


Analysis

Routine product analysis for all the systems discussed is done by

Gas Chromatography. Both Flame Ionization Detection, suitable for

hydrocarbon analysis, and Thermal Conductivity Detection, efficient for








86

N2, 02, CO, CO2 and HC1 analysis, are employed. The former was

conducted on a Varian 940 GC fitted with a Hayesep Q column. The latter

technique was conducted on a Hewlett Packard 5700A GC employing a

Hayesep DB column. Quantification of data was done by a Hewlett Packard

3390A integrator/recorder. Samples are taken in both the gas phase at

the exit of the reactor oven and from the liquid phase after the

effluent has been condensed in the cold trap. Further product

identification was offered through the use of GCIR and GCMS techniques.

The latter method was already described in the preceding chapter. The

former was accomplished with a Hewlett Packard 5890A GC using a DB 130

column in conjunction with a Nicolet 5 DXB FTIR.

Some of the products formed in the systems discussed here are not

amenable to GC detection and quantification. In light of this, a couple

of titration methods have been employed. To quantify formaldehyde

produced in the high temperature methanol oxidation studies, the sulfite

titration method of Walker is practiced.136 The reaction involved for

the titration is shown below:


CH20 + Na2SO3 + H20 -----> NaOH + CH2(NaSO3)OH. (4-3)


The method is done by adding a few drops of phenolphthalein

indicator (0.5g phenolphethalein in 50m1 ethanol) to a 1.0m solution of

Na2SO3 (12.6g Na2SO3 in 100ml D.I. H20). The solution turns magenta

because SO32- (aq) is a base. The color is returned to clear with the

addition of several drops of a standard 1.0 N HC1 titrant solution. The

next step is to add a weighed amount of the reaction solution to be








87

titrated. The flask is shaken to allow complete mixing. The solution

turns magenta again to indicate that base is present. The quantity of

NaOH(aq) generated is determined by titration with the HC1 solution.

The % formaldehyde present in the sample can be readily calculated from

the equation below:


% formaldehyde = ml Acid titer x Normality of acid x 3.003 (4-4)
1 x Weight of sample



The constant, 3.003, comes from the fact that one ml of 1.ON acid is

equivalent of 0.03003g of CH2O.

As mentioned above, the quantification of HC1 produced in the deep

oxidation studies is also determined titrametrically. A standard 1.0 M

NaOH solution is used with a phenolphthalein indicator. The number of

moles of HC1 produced is equivalent to twice the number of moles of

CH2C12 oxidized according to the balance equation shown below:

CH2C12 + 02 ---> 2HCL + CO2 (4-5)


Characterization of the catalytic materials is conducted by a variety of

spectroscopic techniques. DRIFT Spectroscopy is performed as was

previously described in Chapter 2. Scanning Electron Microscopy is

performed with a Jeol JSM 35C electron microscope using an acceleration

voltage of 25 kV. Samples are initially examined at 300X in order to

insure the regions to be studies at higher magnification are

representative of the whole. Micrographs are taken at 6000X and

40,00X. The author would like to acknowledge Richard Krockett of the








88

University of Florida's Major Analytical Instrumentation Center for his

assistance with the SEM studies. X-ray Diffraction Spectroscopy (XRD)

was run on a General Electric 3000 instrument using Cu a radiation. The

author acknowledges the research group of Dr. T.E. Mallouk at the

University of Texas at Austin for conducting the XRD studies. X-ray

Photoelectron Spectroscopy (XPS) was run on a Kratos 9000 instrument.

Samples were mounted on aluminum tape and were evacuated for eight hours

prior to scanning. The author acknowledges Dr. Vaneica Young and Mr.

Michael Clay of the University of Florida for their assistance in

performing these XPS studies. For each of the surface techniques

mentioned above, SEM, XRD, and XPS, the exact same samples were used.


Results and Discussion


Synthesis Gas Conversion


As mentioned in the introduction to this chapter, most of the

transition metals exhibit some activity towards CO hydrogenation.

Ruthenium was chosen as the metal constituent in this study since it has

been shown to be very active for both CO101'127'129'137-146 and CO2

reductions.147-150 Initially, work was conducted with RuC13 as the

metal precursor. For most synthesis gas conversion systems, especially

those operating at ambient pressures, the active metal species is zero

valent. Thus, this Ru species needs to be reduced. Such a practice is

quite common--metal halides have been widely studied as initial catalyst

sources. The reduction step is typically done in H2 flow at elevated

temperatures. For this study, however, RuCl3 did not prove to be a








89

successful precursor. The temperatures required to fully reduce the

metal species (500-6000 C) exceeded the pyrolysis temperature of PPAN

and caused further structural changes in the support.

In light of the difficulties associated with RuC13, Ru3(CO)12 was

selected as the metal dopant to be used in all the CO and CO2

hydrogenation studies. There are several additional advantages to the

choice of this species. The fact that the carbonyl functionality can be

easily observed in the IR makes it very easy to verify that the cluster

has initially been supported. Since both PPAN and AX21 are black, a

simple visual inspection, such as can be done with A1203 or SiO2

supports, (which are bright yellow when Ru3(CO)12 is supported on them),

can not be used to insure the success of the solvent deposition.

Another advantage to the use of Ru3(CO)12 has been shown by recent

studies which suggest that the carbonyl precursor is superior to metal

halide species because the breakdown of the cluster leads to smaller

metal crystallite sizes.151 The only disadvantage to the use of

Ru3(CO)12 is that one must be sure that the observed hydrocarbon

products are not solely from the hydrogenation of the carbonyl groups of

the cluster. To preclude any confusion, the supported Ru catalysts are

subjected to a H2 pre-treatment at reaction conditions for several

hours. During this time methane is observed as a product from the

reduction of the cluster. When this reaction subsides, the CO/H2

reactant stream is added and the Fischer-Tropsch activity videe infra)

is immediately observed. It has been shown in subsequent studies that

the long term activity and overall product distribution are the same

whether or not the H2 pre-treatment step is done.