The preparation and properties of pyrolyzed polyacrylonitrile catalyst materials

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Title:
The preparation and properties of pyrolyzed polyacrylonitrile catalyst materials
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ix, 161 leaves : ill. ; 28 cm.
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Clark, Jeffrey Lee, 1954-
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Subjects / Keywords:
Acrylonitrile   ( lcsh )
Catalysts   ( lcsh )
Pyrolysis   ( lcsh )
Chemistry thesis Ph. D
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bibliography   ( marcgt )
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Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Bibliography: leaves 156-160.
Statement of Responsibility:
by Jeffrey Lee Clark.
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Typescript.
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Vita.

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University of Florida
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Full Text


















THE PREPARATION AND PROPERTIES OF PYROLYZED
POLYACRYLONITRILE CATALYST MATERIALS





By

JEFFREY LEE CLARK


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


1987
















DEDICATION




To IC, who almost never lost the faith.














ACKNOWLEDGEMENTS


There are many people who facilitated this work through

actual assistance as well as thoughtful discussion and

encouragement, and although there are too many to mention,

their contribution is not overlooked. I am especially

appreciative to the glass, machine and electronics shops for

their invaluable assistance in constructing and repairing

the equipment used in this study. I would also like to

thank Dr. Willie Hendrickson of the 3M corporation, Ann

Livesey of the U.S. Army, and Ngai Wong of the University of

Florida for their generous assistance in this work. In

addition, I am grateful to the University of Florida, the

U.S. Army, and Geo-Centers Inc. for financial support during

this endeavor. Finally, I am deeply indebted to Dr. Russell

S. Drago for the freedom and support he provided me during

this study.


iii















TABLE OF CONTENTS

Page


ACKNOWLEDGEMENTS.................................... iii

LIST OF TABLES........................................ v

LIST OF FIGURES........................................ vi

ABSTRACT............................................ viii

INTRODUCTION................... ....................... 1

BACKGROUND........................................... 4

EXPERIMENTAL................................... ...... 28

Materials..................................... ....... 28

Preparations............. ............ .......... 29

Methods........................................... 34

RESULTS AND DISCUSSION..................................50

Pyrolysis Studies............................. 50

Basicity Studies................................ 76

Catalytic Results................................ 86

SUMMARY AND FUTURE RECOMMENDATIONS.................. 132

APPENDIX: XPS SPECTRA.............................. 138

REFERENCES............................................ 156

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















LIST OF TABLES


Page

1. Elemental Analysis Results...................... 52

2. Surface Area Results............................ 74

3. CEES Products................................... 98

4. XPS Results; Elemental Ratios................... 108

5. XPS Results; Ionization Energies................. 109

6. Effects of Adsorbed Metal Salts; Series 1....... 114

7. Effects of Adsorbed Metal Salts; Series 2....... 117

8. Miscellaneous Catalyst Results................... 119

9. Glass Spheres Results........................... 122

10. Elemental Analysis Results For Suported
Catalysts....................................... 125

11. Catalytic Results For Supported Catalysts....... 127

12. GC-MS Results for Discolored Products........... 130












LIST OF FIGURES


Page

1. Proposed Pyrolysis Reaction.......................... 6

2. Imine-Nitrone Structure............................. 7

3. Proposed Catalytic Mechanism........................ 11

4. Dehydrogenation of Acridine......................... 13

5. Possible Dehydrogenation Mechanisms................. 14

6. Proposed Structure and Pyrolysis Product for
Polycyanoacetylene.................................. 16

7. Initial Pyrolysis Apparatus......................... 36

8. Modified Pyrolysis Apparatus........................ 38

9. Initial Catalytic Evaluation System.................. 41

10. Oven Design. ........................................ 43

11. Improved Catalytic Evaluation System................ 45

12. Syringe Design....................................... 46

13. Representative Temperature-Time Profiles............. 51

14. Isothermal Thermogravimetric Analysis............... 54

15. Thermogravimetric Analysis, 2.50C/minute............ 56

16. Thermogravimetric Analysis, 5.00C/minute............. 57

17. Thermogravimetric Analysis, 10.00C/minute........... 58

18. Composite of Temperature Programmed
Thermogravimetric Analysis.......................... 59

19. Thermogravimetric Analysis, 0.50C/minute............ 61

20. Thermogravimetric Analysis and Differential
Thermal Analysis .................................... 62








21. Temperature Programmed Differential Scanning
Calorimetry ........................................ 65

22. Differential Scanning Calorimetry, 0.50C/minute..... 66

23. Isothermal Differential Scanning Calorimetry........ 68

24. Plot of Residual Nitrile Content Versus Area
of Exotherm..................... ...................... 70

25. Naptheridine Binding Modes.......................... 79

26. Electron Paramagnetic Resonance Spectra of PPAN
and Co(DMGH)2............................................ 81

27. Electron Paramagnetic Resonance Spectra of PPAN
and Copper (II) Chloride............................. 83

28. Polymerization Mechanisms................................ 94

29. Structures of Nerve Gases and
Dimethyl-methylphosphonate.......................... 103

30. Selectivities of Metal Doped Catalysts............... 133


vii










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


PREPARATION AND PROPERTIES OF PYROLYZED
POLYACRYLONITRILE CATALYST
MATERIALS




By

Jeffrey Lee Clark

December, 1987


Chairman: Russell S. Drago
Major Department: Chemistry


The preparation and properties of pyrolyzed

polyacrylonitrile (PPAN) catalyst materials was

investigated. Specifically, the effects of variations in

the pyrolysis conditions (heating rate, maximum temperature,

atmosphere, etc.) were examined using elemental analysis,

Differential Scanning Calorimetry and Thermogravimetric

Analysis. In addition, the effects of metal additives and

oxide supports on the catalytic activity of PPAN catalysts

were studied using the dehydrogenation of ethanol as a test

reaction. It was found that small amounts of metal

additives were capable of profoundly affecting both the

activity and selectivity of PPAN catalysts. Supporting PPAN

on oxide supports served to increase the surface area by


viii








greater than an order of magnitude. However, the catalytic

activity seemed to be more a reflection of the catalytic

activity of the support used because blank runs using only

the support resulted in very similar selectivity and

activity. In addition, PPAN catalysts were found to

decompose chloroethyl ethylsulfide at temperatures as low as

2000C under both aerobic and anaerobic conditions.
















INTRODUCTION


In the last twenty years, due to the increased

consumption of manmade materials and energy, the area of

catalysis has blossomed into a science in its own right.

Due to its more general application to spectroscopic

techniques, homogeneous catalysis has traditionally been

better understood than heterogeneous catalysis, which is

less amenable to spectroscopic techniques, making study

more difficult. In fact, it has been frequently stated by

industrial catalyst chemists that heterogeneous catalyst

preparation is more an art than a science. However, recent

advances in surface analytical techniques have enabled

chemists to more fully understand the reactions occurring on

the surfaces of heterogeneous catalysts. At present,

though, the high relative cost of these techniques is

frequently prohibitive in many academic environments.

Metals and their complexes and salts have traditionally

been the catalysts of choice industrially. Indeed, one of

the first catalytic chemical plants (excluding biological

"plants" of course) was built by Germany during World War II








2

and utilized a catalyst composed of mixed metal oxides to

convert CO and H2 (syngas) into liquid fuel using the

Fischer Tropsch Process.1 Both the petroleum and polymer

industries initially led research efforts in catalysis. With

the advent of automobile exhaust emission controls, however,

extensive research was dedicated to developing effective

emission control catalysts.2 In many cases, the catalysts

employed in industrial operations consist of small metal

crystallites or complexes supported on the surface of an

inorganic oxide (such as silica gel (SiO2), titanium dioxide

(TiO2) and alumina (A1203)), whose primary function is to

uniformly disperse the active catalyst species. This

provides greater efficiency per unit of metal species by

increasing the catalytic surface area available for chemical

reaction. Due to the high cost of precious and strategic

metals, interest has been directed towards less expensive

alternatives to precious metal catalysts. One area of

investigation has involved the use of heat stable organic

materials. In particular, organic pyropolymers have been

examined as possible catalyst candidates due to their

paramagnetism, semiconducting tendencies and heat stability.

One such polymer, pyrolyzed polyacrylonitrile (PPAN), has

been studied extensively as a catalyst for several

reactions. At present there are very few published reports

on the effects of metal additives or of supporting PPAN

catalysts on inorganic oxide supports. This study involves








3

a systematic investigation into the effects of variations in

the preparation of PPAN catalyst materials.














BACKGROUND


It was reported in 1958 by Burlant and Parsons3 that

upon treatment with thermal or ultraviolet radiation,

polyacrylonitrile (PAN) undergoes a chemical reaction

leading to discoloration and the appearance of an Electron

Paramagnetic Resonance (EPR) signal. Since that time,

research has been carried out on this process for three

general reasons: (1) textile chemists are concerned with

eliminating this phenomenon, which causes premature decay of

PAN containing fabrics; (2) catalyst chemists have been

interested in exploiting the catalytic properties inherent

in pyrolyzed polyacrylonitrile; and (3) materials scientists

have been interested in this process since it is the first

step in the conversion of acrylonitrile to carbon fibers,

which are an increasingly used component of composite

structural materials. Pyrolyzed polyacrylonitrile films

have even been proposed as a cost-effective amorphous

semiconducting material to replace silicon crystals in solar
4
collectors.
The earliest studies on the pyrolysis of

polacrylonitrile were infrared studies in which it was

observed that the carbon-nitrogen triple bond stretch

disappeared, the bands in the spectrum broadened
4










considerably, and a large broad absorption grew in the

region where C=N, C=0 and C=C would appear. However, the

spectrum was too featureless to permit specific assignments.

On the basis of this evidence, it was proposed that the

reactions occurring during pyrolysis and polymerization were

as illustrated in Figure 1.5,6,7 A recent spectroscopic

study on the vacuum pyrolysis of PAN8 concluded that an

intermediate phase existed at pyrolysis temperatures between

200 and 2600C. This material was said to be an intrinsic

semiconductor with an extensively delocalized r electron

system as depicted in Figure lb. They also suggested that

the reaction which occurs in this temperature range could

involve bonding and conjugation between adjacent chains

without interruption of the carbon backbone, due to the

atactic nature of the PAN starting material. This reaction

has also been proposed to occur during the alkaline

degradation of PAN.9 It has also been proposed that

partially oxidized species such as the imine-nitrone

copolymer illustrated in Figure 2 could be present and

contribute to the optical properties of PPAN (not pyrolyzed

under vacuum), based on studies done on synthetic model

compounds which have similar absorptions in the UV-visible

region.10, 11

PPAN exhibits a strong, fairly narrow EPR signal at

about g=2, and this has been used as a probe of the

pyrolysis reaction. The g value increases with increasing











nC H=C H
C HN


POLYMERIZATION


CH
*^/
CH

C


CH2


CH CH
I I


C H


PYROLYSIS


CH


,CH

CH


b. 1
C C

N


CH CH CH2
CH CH2


N


N


PYROLYSIS


CH

C


CH CH CH

C C C


c.

CN CN C NC N


Figure 1: Proposed Pyrolysis Reaction


CH
a.
C


CH:












0
t
~NN
OOO..N


Figure 2: Imine-Nitrone Structure








8

thermal treatment time, becoming essentially constant after

about five hours.12 It was learned that the concentration

of unpaired spins per gram (as determined by an EPR

technique) was strongly dependent upon the pyrolysis

conditions as well as the temperature of measurements. In

addition, the number of unpaired spins per gram was

decreased by the presence of air during the measurement.

The important parameters of the pyrolysis seemed to be the

rate and duration of heating in addition to the atmosphere

in which the pyrolysis was being carried out. For example,

a sample pyrolyzed in an ammonia atmosphere contained more

unpaired spins per gram than an identical sample pyrolyzed

in air or nitrogen.13

There have also been studies to determine what volatile

products are formed during the pyrolysis reaction and it was

found that ammonia, hydrogen cyanide, acetonitrile,

acrylonitrile monomer, propionitrile, methacrylonitrile,

isobutylacrylonitrile and vinyl acrylonitrile were formed

during the reactions.14 It was also found that the relative

amount of volatiles produced generally increased with

temperature in the range studied (300-800oC).

Unfortunately, the temperature-time profile of these

experiments consisted of very fast rise times (<1 sec) to

the desired temperature followed by maintenance at that

temperature for 10-20 seconds after which time the products

were analyzed by Gas Chromatography (GC). The temperature-










time profile refers to a plot of the temperature versus

time, which enables one to make better comparisons of the

thermal history of each sample. It has been known for some

time that there is a strong exothermic reaction which occurs

between 200 and 3000C which is capable of generating enough

heat in a bulk sample to cause ignition of the sample15

concomitant with drastic weight losses resulting in a

material which has little catalytic activity. (Several

times during our investigations at the University of Florida

these runaway reactions were observed due to the

catastrophic failure of a temperature controller.) Since

the active catalytic species is presumed to be the product

of the reaction in Figure 1, it is logical to choose the

pyrolysis conditions designed to minimize weight losses

since the cyclization-dehydrogenation reactions depicted in

Figure 1 would result in a theoretical weight loss of less

than 5%.

There have been many reports describing the catalytic

activity of pyrolyzed polyacrylonitrile. PPAN has been

shown to be capable of isomerizing alkenes and

dehydrogenating alcohols,16 decomposing formic acid and

nitric oxide,17,18 dehydrogenating ethylbenzenel9 and

cumene,20 and oxidizing ethylene to ethylene oxide.21

Manassen and coworkers, in a series of publications,

described the dehydrogenation of alkenes and of alcohols and

the isomerization of alkenes using PPAN catalysts. It was








10

found that unlike conventional dehydrogenation catalysts, no

gaseous hydrogen was evolved. Instead, the hydrogen was

thought to be either physically adsorbed or chemically bound

to the surface of the catalyst, and while prolonged heating

at elevated temperatures in a nitrogen atmosphere did not

restore the catalytic activity, a short treatment in air at

temperatures as low as 1400C completely restored the

activity with the concomitant production of water. The

dehydrogenation of cyclohexene to benzene was also observed

to follow a different pathway using PPAN catalysts in that

no disproportionation occurred as with commercial

dehydrogenation catalysts. When cyclohexene was passed over

various types of charcoals or graphite at elevated

temperatures, significant amounts of cyclohexane and other

products were formed while PPAN catalysts produced benzene

exclusively. In addition, as with the alcohols tested, no

gaseous hydrogen was produced and prolonged heat treatment

under a nitrogen atmosphere failed to restore the catalytic

activity while a short treatment in air was found to

completely restore the catalytic activity. It was also

found that dehydrogenation reactions tended to deactivate

these catalysts quicker than isomerization reactions, again

suggesting that the hydrogenation of the catalyst surface is

responsible for the deactivation of the catalyst. These

authors proposed the reaction illustrated in Figure 3 to be

the mechanism for the catalytic activity of PPAN catalysts.






















02 HYDROGEN
DONOR
H



I
H


Figure 3: Proposed Catalytic Mechanism








12

It has been demonstrated by Braude and coworkers22 that

dihydroquinoline can by hydrogenated by acridine while the

reverse reaction does not occur, as shown in Figure 4.

On the basis of these observations it was proposed that

higher annelation of a system of condensed heterocyclic

aromatic rings will result in poor H-donating properties.

It is also known that annularly condensed aromatic molecules

become less and less stable with an increasing number of

rings while the hydrogenated compounds gain in stability,23

which may help to explain why a compound containing the

proposed structure (Figure 1) of PPAN catalysts could be

both a good hydrogen acceptor and a poor donor.

Furthermore, model compounds were synthesized which were

incapable of forming hydroaromatic structures while

retaining aromaticity and were found to be catalytically

inactive.24 In addition, the transformation of a

hydroaromatic structure to an aromatic structure by air

oxidation has been shown to occur with acridine25 and model

compounds of condensed pyridine rings.26

Manassen and coworkers used the reaction of

5-ethyl-5-methyl-1,3-cyclo-hexadiene over PPAN catalysts in

an attempt to elucidate the form of the hydrogen transfer.

The scheme in Figure 5 illustrates the possible products

based on hydride ion abstraction, hydrogen atom abstraction

and proton abstraction. A comparison of the product

distributions obtained by passing
















H

HO
H


NH
N N


Dehydrogenation of Acridine


+


4'


+-


Figure 4:


N N N


N NO
N N












(a) Hydride ion abstraction


Wagner-Meerwein
-H- + shift


o-ethyl-toluene


5-ethyl-5-methyl-
1.3-cyclohexadiene


(b) Hydrogen atom abstractio-



-H $S-fission b +


5-ethyl-5-methyl-
1,3-cyclohexadiene


to I uene


ethyl radical


(c) Proton abstraction


R-fission
-


-H'



5-ethyl-5-methyl-
1,3-cyclohexadiene


ethylbenzene


methyl
carbanion


Figure 5: Possible Dehydrogenation Mechanisms


I








15

5-ethyl-5-methyl-1,3-cyclohexadiene over various catalysts

led the authors to conclude that the behavior of PPAN

catalysts is somewhat intermediate between that of

commercial dehydrogenation catalysts, which abstract H-atoms

and acidic alumina, which is a proton abstractor. Pyrolyzed

polycyanoacetylene, the structure of which is illustrated in

Figure 6, was found to exhibit only H-atom abstraction,

leading to the proposal that the hydride transfer aspect of

PPAN catalysis originates from another structural element

than the proposed structure of the active catalyst, since

both PPAN and pyrolyzed polycyanoacetylene should

theoretically have the same structure. They found support

for this hypothesis by the observation that treating the

PPAN with dimethyl sulfoxide can greatly enhance the

percentage of ortho-ethyl-toluene formed.

It is of interest to note that different preparations of

PPAN catalysts can have very different catalytic activities.

For example, Manassen and coworkers used the following

general preparative method for synthesizing their catalysts.

Polyacrylonitrile was spread out in a thin layer in a draft

oven, slowly heated to 2300C and kept at this temperature

for 12 hours, during which time the color went from white to

brown via yellow. The brown powder was pelleted at 8000

psi, crushed, sieved and then calcined at either 350 or

4500C for 30 minutes, during which time the color went from

brown to black. It was found that catalysts calcined at










CH CH CH CH

C C C C


N
N


N


C

N


PYROLYSIS


CH CH CH CH


C C C C



N N N N


Figure 6: Proposed Structure and Pyrolysis Product for
Polycyanoacetylene








17

4500C underwent decomposition, presumably yielding large

crosslinked structures, which were apparently somewhat

acidic resulting in a high activity for double bond shifts

as well as ethyl shifts. (It was the material calcined at

4500C which was used for the 5-ethyl,5-methyl-l,3-

cyclohexadiene experiment.) PPAN catalysts which were

calcined at 3500C showed much less activity for double bond

or ethyl shifts, which illustrates the importance of the

pyrolysis conditions on the activity of these catalysts.

These results are in agreement with electrical measurements

which have shown that drastic changes occur above 350oC.26

Another theory to account for the catalytic properties

of organic pyropolymers correlated the number of unpaired

spins per gram (as as measured by an EPR technique) to the

catalytic activity for the decomposition of nitrous

oxides.28,29 This theory includes a scaling factor called

the intrinsic activity of the free spins, which presupposes

that all of the free spins are not active in catalysis. The

assumption is that structural rearrangements preceding

graphitization bring about exchange interactions between the

free spins rendering some of them catalytically inactive. A

correlation was developed between the width of the EPR line

and the relaxation time, T1 (as determined by the saturation

technique) and the intrinsic catalytic activity of the free

spins.30 This concept seems somewhat related to the

electronic theory of catalysis on semiconductors, described








18

by Vol'kenstein31 and Hauffe.32 In the electronic

mechanism, the semiconductor catalyst acts as an electron

reservoir by either donating electrons to or accepting

electrons from the substrate in question. This mechanism is

in contrast to that proposed for alumina, in which the

reaction is suggested to occur on Bronsted acid or proton

donating sites through the formation of an adsorbed

carbonium ion. Cutlip and Peters15 examined the kinetics of

dehydration of t-butyl alcohol over PPAN catalysts at 240 to

2800C and applied both single site and dual site models to

describe the kinetics. After a considerable amount of

mathematical manipulation, these authors concluded that it

was impossible, based on their data, to unequivocally

determine the reaction mechanism since a statistical best

fit could not be obtained for any of the models chosen.15

Since the electronic theory of catalysis greatly increases

the possible number of rate limiting steps in the mechanism

(dissociation of adsorbed molecules, transfer of electrons

between adsorbed species and the catalyst), it was only

possible to qualitatively evaluate this mechanism, and it

was proposed that the rate-limiting step would probably be

related to the concentration of free electrons in the

conduction band of the catalyst. This could possibly be

related to what Gallard-Nechtschein and coworkers described

as the intrinsic catalytic activity of the free spins and

its relationship to the catalytic activity of these








19

catalysts,30 thus raising the possibility that there are

unpaired electrons in addition to conduction band electrons.

One problem with reviewing the literature pertaining to

PPAN catalysis is that one is consistently faced with the

prospect of comparing apples to oranges in the sense that

many of the studies reported preparing their catalysts by

different methods, making it impossible to be certain

whether or not the preparations consisted of the same

chemical (or semiconducting, for that matter) species. In

many cases, the catalysts were only characterized by their

method of preparation and their catalytic activity.

Manassen and coworkers24 (as discussed earlier) noted a

difference in catalytic activity, as well as in elemental

analyses (C, H, N), by calcining in nitrogen for 30 minutes

at 3500C instead of 4500C. Another pertinent observation is

the fact that the sum total of the carbon, hydrogen and

nitrogen analyses is usually between 75 and 90%, indicating

that from 10 to 25% of the final catalyst is composed of an

element other than carbon, hydrogen or nitrogen. The most

likely candidate is oxygen since many of the catalyst

preparation methods involve pyrolysis in air for some

period, and none have sought to rigorously exclude oxygen

from the preparations. This gives further evidence that

structures such as the imine-nitrone depicted in Figure 2

may constitute a considerable proportion of PPAN catalysts,








20

as well as possibly being responsible for the Bronsted type

acidity.

A recent spectroscopic study done on silver backed

polyacrylonitrile films also implicated structures such as

the conjugated imine in Figure 1 as products in the UV

degradation of PAN films using light in the 250-400 nm

region.33 (It has been known for some time that UV light

and strong bases bring about similar structural changes as

heat treatment, producing dark colored, intrinsically

paramagnetic solids.) In the UV degradation study, changes

in the silver backed PAN films were monitored using Fourier

transform infrared reflection absorbance (FTIR-RA) under

both oxidative and non-oxidative conditions. Their results

indicated that under oxidative conditions, oxygen-containing

species such as alcohols, carboxylic acids, hydroperoxides

or ethers could be present. Since UV light is capable of

generating ozone, ozonolysis products are also theoretically

possible, although the presence of ozone was not detected.

In addition, the appearance of an N-H stretch and a C=N

stretch, as well as the loss of the carbon nitrogen triple

bond stretch, suggested that cyclization had occurred. The

film became discolored and developed an EPR signal; however,

when the film was redissolved in dimethyl sulfoxide, the EPR

signal disappeared while the color remained. This indicated

that discoloration is a necessary but not sufficient

condition for the presence of paramagnetism. Although








21

analogies can be drawn between the UV and thermal

degradation of PAN, it must be emphasized that there are

distinct differences since thermal degradation generally

results in an insoluble material. For example, only one

hour of heating at 1600C results in a solid of which only

20% can be extracted with dimethylformamide, the solvent of

choice for PAN. This may be attributed to more extensive

crosslinking occurring in the thermal process than in the

photochemical reaction, suggesting that the reactions in the

thermal process are considerably more complex. An

additional complication is the fact that all

polyacrylonitrile is not identical since the polymerization

conditions and polymerization catalysts used will have a

strong effect on the composition of the final product due to

defects and inevitable chain ends which may or may not

consist of polymerization catalyst residues. In studies of

the production of carbon fibers it has been learned that

pre-oxidation (prolonged isothermal heating at low to

moderate temperatures (150-2500C)) minimizes the exothermic

reaction and yields a flameproof material which has a

concentration of approximately 10% oxygen by weight.34

Although most of the catalytic studies of PPAN-based

catalysts had chosen simple, industrially unimportant

reactions in an attempt to establish a relationship between

the catalytic activity and the chemical or electronic

structure of the material, there have been a few studies of








22

industrially important reactions. Degannes and Ruthven18

investigated the oxidative dehydrogenation of ethylbenzene

to styrene (at atmospheric pressure and from 180 to 2800C)

and found that the reaction was zero order in oxygen and

approximately second order in ethylbenzene. The zero order

dependence on oxygen suggests that the reaction rate is

controlled by the rate at which ethylbenzene can be adsorbed

and dehydrogenated. Although most of these experiments were

for kinetic purposes carried out at low conversions under

differential conditions, a limited series of integral

experiments showed that conversions greater than 80% at

3250C could be achieved with no significant by-products.

This is in contrast to commercially available

dehydrogenation catalysts which do not operate oxidatively,

resulting in an endothermic process requiring temperatures

in excess of 5000C to achieve conversions of 50% to styrene,

with benzene and toluene being significant side

products.35,36,37

There have been very few studies in the literature

dealing with two potentially important areas: doping PPAN

catalysts with metals and supporting PPAN catalysts on

inorganic oxide supports. In one study, acrylonitrile was

polymerized using 2,2'-azobis[2-methyl propionitrile (AIBN)

in the presence of silica gel and the resulting material was

pyrolyzed at several different temperatures.20 Metals were

then added to these materials by slurrying with ethanol








23

solutions of the corresponding metal chlorides followed by

filtration and washing with ethanol. X-ray photoemission

spectroscopy (XPS) data were cited to propose copper binding

to nitrogen atoms in the PPAN since it was noted that only

the Cu 2p 3/2 and the N Is binding energies shifted upon

addition of CuCl to the silica supported PPAN samples. The

reactions studied were the oxidations of cumene and

ethylbenzene at 1000C and under 1 atm. of oxygen. For

cumene, the total conversion was 63% with the selectivity

being 63% cumyl alcohol and 28% acetophenone; for

ethylbenzene, the total conversion was 21% with a

selectivity of 87% acetophenone and 13% 1-phenylethanol.20

These results for ethylbenzene are quite different from the

vapor phase results obtained by Degannes and Ruthven19 who

observed styrene as the only product. This apparent

contradiction is not too surprising when one considers the

differences in the catalyst preparation and catalytic

reaction conditions. The silica-supported PPAN catalyst

contained of course Sio2 and was also different in that it

was pyrolyzed for 12 hrs. at 1900C. The previous work by

Degannes and Ruthven had employed a catalyst which had

slowly been heated to 2300C in air, was maintained at that

temperature for 16 hrs., and then was calcined at 4000C in

an atmosphere of nitrogen for 4 hrs. In addition, one study

passed gaseous ethyl- benzene over the catalyst from 180 to










3250C, while the other reaction occurred in the liquid phase

(or at the liquid-solid interface) at 1000C.

The data available in the open literature concerning the

catalytic activity of PPAN are sketchy at best, which is

rather surprising since these few studies suggest that PPAN-

based materials could be promising catalysts for

dehydrogenation as well as oxidation and dehydration.

Perhaps one reason why there are so few reports of PPAN

catalysts is because of the formidable problems associated

with studying a material without being able to employ

common solution techniques such as nuclear magnetic

resonance spectroscopy, UV-vis spectroscopy and infrared

spectroscopy.

It was the goal of this investigation to increase the

growing body of knowledge concerning catalysis using PPAN-

based materials in three general areas: the effects of the

pyrolysis conditions on the catalytic activity of PPAN

catalysts; the effects of metal additives on the catalytic

activity of PPAN catalysts; and the effects of supporting

PPAN catalysts on oxide supports such as silica (SiO2),

titania (TiO2) and alumina (A1203). As in many of the

earlier studies, a simple model reaction was chosen for ease

in handling the analysis. In this case, ethanol was chosen

as the substrate and was shown to be capable of undergoing

dehydrogenations as well as dehydrations.








25

The effects of variations in the pyrolysis conditions

were investigated by several methods. Differential scanning

calorimetry (DSC) and thermogravimetric analysis (TGA) were

carried out on PAN samples both isothermally and in the

temperature programmed mode in an attempt to learn more

about the destructive exotherm and concomitant weight loss

characteristic of the pyrolysis reaction. In addition,

several different atmospheres were used in the pyrolysis

reaction in an attempt to learn whether the pyrolysis

atmosphere affects either the composition or catalytic

activity of these materials. Finally, the effect of

variations in the pyrolysis reaction heating rate on the

composition and catalytic activity of the resulting

materials was investigated.

In the hopes of obtaining useful information about the

effects of metal additives on the catalytic activity of PPAN

catalysts, three general approaches were taken: the

intrinsic basicity of PPAN was studied by titrating with

dilute acid, paramagnetic metal species were deposited on

the surface of PPAN in an attempt to learn more about the

metal environment using electron paramagnetic resonance

spectroscopy (EPR), and the effects of various metal

additives on the catalytic activity of PPAN were examined in

the dehydrogenation of ethanol.

The investigation of the effects of supporting PPAN on

oxide supports was carried out by preparing samples of PPAN








26

supported on silica gel and alumina and then comparing the

catalytic activities of these materials in the

fore-mentioned ethanol reaction with unsupported PPAN

catalysts as well as pure silica gel and alumina. The

surface areas of these materials were also measured in order

to make activity comparisons on a surface area basis. As

stated previously, the goal of this research was to expand

the general knowledge about PPAN based catalytic materials

in the hope that a greater understanding of the

structure-reactivity relationships could be obtained so that

in the future it may be possible for chemists to engineer

low-cost organic catalysts which are tailored to optimize a

specific reaction. Although the work of one graduate

student is not sufficient to achieve these lofty goals, it

is possible to address some apparent inconsistencies in the

literature regarding the selectivity of PPAN catalysts in

the reaction of ethylbenzene and the significance of the

support interactions, if any. The report of silica

supported PPAN complexes by Bai and co-workers20 was very

brief and many important experiments were either not carried

out or not reported. For example, the activities of the

metal-silica-PPAN catalysts were ranked as follows: Cu I >

Cu II > Co II > Mn II. Unfortunately, no comparison was

made between doped and undoped catalyst preparations. In

addition, no blank runs using only silica were attempted,

making it impossible to determine what role (if any) the








27

silica support plays in the catalytic reaction. Although

ethanol was used as the substrate in the present study for

ease in handling, it was felt that the results obtained

would be applicable to the ethylbenzene reaction since both

involved primarily dehydrogenation reactions.

In addition, a limited number of experiments were

carried out to determine the feasibility of using PPAN

catalysts to decompose dimethyl methylphosphonate (DMMP) and

chloroethyl-ethylsulfide (CEES), two compounds of military

interest as simulants for chemical warfare agents.

Preliminary experiments were also carried out to determine

whether PPAN catalysts are photochemically active or active

towards syngas conversion.














EXPERIMENTAL


Materials



N,N-Dimethylformamide was reagent grade, purchased from

Aldrich and used without further purification.

Acrylonitrile was reagent grade, purchased from Aldrich and

used without further purification. AIBN (2,2'-azobis[2-

methyl propionitrile]) was purchased from Eastman Chemicals

and used without further purification. Polyacrylonitrile

was reagent grade, purchased from Aldrich and used without

further purification. Alumina (neutral) was purchased from

Fischer and used as supplied. Titanium dioxide (anatase,

TiO2) was supplied by Baeyer and used without further

purification. Metal complexes were all reagent grade and

used as supplied. Zirconium basic carbonate and zirconium

basic acetate were generously donated by Mr. Brady Crom and

Dr. Tom Wilson, of Zirtech Inc., Gainesville, Florida, and

were used without further purification. Vacuum distilled

chloroethyl ethylsulfide (CEES) was generously provided by

Dr. Yu-Chu Yang of the U. S. Army. Silica gel (SiO2), Grade

62 with a mesh size of 60-200, was provided by Davison and

used without further purification. Ethylbenzene was reagent








29

grade, purchased from Aldrich and distilled before use to

remove traces of toluene and benzene. Dimethyl

methylphosphonate (DMMP) was reagent grade, supplied by the

U.S. Army and used without further purification. Silver (I)

trifluoromethanesulfonate (AgCF3SO3) was purchased from

Aldrich and used without further purification. Bis(2,2'-

bipyridine) ruthenium (II) chloride (Ru(bipy)2Cl2) was

generously provided by Dr. E. Stine and used without further

purification. Glass spheres (8-58 pm, Standard Reference

Material 1003a) were obtained from the National Bureau of

Standards and used without further purification.





Preparations



Polymerization of Acrylonitrile

Acrylonitrile (100 g were added to 1500 ml of distilled

water in a flask fitted with a reflux condenser and

maintained at 600C in a silicone oil bath under an inert

atmosphere. AIBN (1.00 g) was added to this mixture with

vigorous stirring. The mixture was allowed to stir for 1.0

hr after which another gram of AIBN was added. The

suspension was allowed to stir overnight and then the white

polyacrylonitrile powder was recovered by filtration and

washed with copious amounts of acetone. The resulting

polymer was dried at 500C in a vacuum oven. It was found








30

that vigorous stirring using an overhead stirrer was

required during the reaction to prevent agglomeration of the

polymer into large lumps. In addition, to prevent loss of

the monomer the cooling water was passed through a saltwater

ice bath, using a copper heat exchange coil, before entering

the condenser. Due to its greater density, argon was found

to be superior to nitrogen in preventing the loss of the

highly volatile monomer.



Diphenylglyoximato Cobalt (II) (Co(DPGH))2

This synthesis was carried out under argon in

schlenckware apparatus using the method of Tovrog.38

Methanol was dried by distillation from calcium hydride and

stored over 4 A molecular sieves. Dry methanol (200 ml) dry

methanol was added to a 1 L three-neck round bottom flask

and the oxygen was removed by bubbling argon through the

system. Cobalt (II) acetate (12.45 grams) and

diphenylglyoxime (24.00 grams) were added to the reaction

flask and the mixture was allowed to stir for several hours

until the cobalt (II) acetate dissolved. After several more

hours of stirring, the brown suspension was filtered through

a schlenckware frit yielding a brown solid.

Analyzed: Carbon -- 59.23%, hydrogen -- 4.11%, nitrogen --

9.85%

Calculated for Co(DPGH)2*2H20: Carbon -- 58.99%, hydrogen

-- 4.57%, nitrogen -- 9.77%










PAN on Silica

Two preparations of silica gel supported PAN were

formulated with loadings of approximately 7 and 17% PAN by

weight. The appropriate amount (2.00 grams or 5.00 grams)

of PAN was dissolved in 250 ml of N,N-dimethylformamide.

Silica gel (30 g) was added to the solution and the

suspension was allowed to stir on low heat (900C) for 18

hrs. The suspension was then rotary evaporated to dryness

and subsequently pyrolyzed. While the 7% PAN on silica gel

became an off-white free flowing solid after rotary

evaporation, the 17% PAN on silica gel became caked up and

required grinding in a mortar and pestle before pyrolysis.



PAN on alumina

This sample was prepared as a 7% loading of PAN on

alumina. A solution of 3.66 grams of PAN and 250 ml

dimethylformamide was added to 50.00 grams of alumina and

the suspension was allowed to stir overnight at 900C. The

dimethylformamide was removed by rotary evaporation and as

in the case of the 17% PAN on silica gel, the resulting

solid was caked up and very hard. The material was

pyrolyzed after grinding in a mortar and pestle.










PAN on Titanium Dioxide

The same general procedure was followed to prepare a

solid which was approximately 15% PAN. PAN (3.00 g) was

dissolved in 300 ml of N,N-dimethylformamide to which was

added 20 g of anatase (TiO2) and the resulting suspension

was stirred for 18 hrs. at 900C. The dimethylformamide was

removed by rotary evaporation and the caked up material was

ground in a mortar and pestle and subsequently pyrolyzed.



Metal Incorporation for Catalyst Studies

For the purposes of comparing the effects of various

metal additives on the catalytic activity of PPAN catalysts,

a different method of metal incorporation was used in an

attempt to have equimolar amounts of metal species per gram

of PPAN. Two series of catalysts were prepared using 3.45 x

10-4 moles of metal complex per gram of PPAN. Within each

series of catalysts, the PPAN starting material was all from

the same batch in order to eliminate differences due to

pyrolysis conditions. A weighed amount of metal complex was

dissolved in absolute ethanol and stirred with the

appropriate weight of PPAN to obtain a concentration of 3.45

x 10-4 moles metal/gram of PPAN. This suspension was

stirred overnight without heat, rotary evaporated and then

dried in a vacuum oven at room temperature.








33

Copper-Lithium Catalyst Preparation

PAN (20.00 g) was dissolved in 200 ml of

N,-N-dimethylformamide (DMF) with stirring and then 1.43 g

of LiCl was added. The solution was allowed to stir on low

heat for 18 hrs after which time the DMF was removed by

rotary evaporation. (Before rotary evaporation several

films were prepared by filling a small petrie dish about one

half full of the solution and drying in a vacuum

desiccator.) The resulting hard amber colored chunk of

plastic was ground up in a Waring Blender and then mixed

with 0.40 g of copper powder. The mixture was allowed to

sit with occasional stirring for about two weeks. During

this time, the color of the polymer slowly turned from amber

to green and the copper metal ceased to be observable. This

material was subsequently pyrolyzed as will be described.

Another batch of catalyst was prepared in which the copper

metal was added directly to the PAN-DMF-LiC1 solution and

allowed to stir for about two weeks until all of the copper

was dissolved. The dark brown solution was rotary

evaporated to yield a dark brown plastic which was

subsequently ground up and pyrolyzed. A lithium chloride

catalyst was prepared in the same manner as the copper-

lithium catalyst except that copper was not added in this

case.










Ruthenium Catalyst

A solution of Ru3(CO)12 was prepared by dissolving 0.20

g of Ru3(CO)12 in 200 ml of toluene in a 500 ml erlenmeyer

flask. Powdered PAN (20.00 g, Aldrich) was added to this

solution which was allowed to stir on low heat for two days.

During this time the suspension became green looking. The

material was filtered using a glass frit resulting in a

light green polymer powder. The filtrate was bright orange

and virtually indistinguishable from the original solution.

Upon drying, the polymer powder became somewhat off white in

color. The FT-IR diffuse reflectance spectrum of this

powder indicated that it contained Ru3(CO)12; however the

color of the filtrate indicated that most of the Ru3(CO)12

remained in solution. This material was subsequently

pyrolyzed as will be described.



Methods



Pyrolysis Reaction

The pyrolysis reaction was carried out using a variety

of methods for two general reasons. First, as more was

learned about the pyrolysis reaction, modifications were

made to the apparatus in order to obtain more homogeneous

products. Second, the means of temperature control was

upgraded as funding allowed. The pyrolysis tubes were

constructed of 1.00 in. diameter pyrex tubing with a glass








35

frit at one end and a 24/40 ground glass joint at the other.

The ends were then tapered to about 1/4 inch diameter to

accommodate stopcocks and fittings for tygon hose

connections. Since highly poisonous gases were produced

during these reactions, all operations were carried out in a

fume hood. Using tygon tubing, the mineral oil bubblers

were attached to both ends of the pyrolysis tube to monitor

the carrier gas flow as well as for leak detection. A

schematic diagram of the apparatus is shown in Figure 7.

Initially a Fisher mechanical temperature controller was

used to monitor and control the temperature of the system

which was contained in a commercially available tube furnace

manufactured by Lindberg. A mercury-glass thermometer was

also used as a back-up for monitoring the temperature. It

was subsequently discovered that this temperature controller

had a fluctuation of approximately 500C at any given

set-point. This proved to be unacceptable since it had been

demonstrated in other studies (and born out in this

investigation) that accurate temperature control in the

200-3000C region of the reaction is crucial in controlling

the strong exothermic reaction which occurs in this

temperature region. Failure to control this exothermic

reaction results in a runaway reaction causing the PAN to

char very quickly with a dramatic loss in weight. To

circumvent this problem, an Omega CN-300 digital temperature

controller was purchased. Unfortunately, electrical










GAS IN


MINERALOIL -
BUBBLER




POLYACRYLO NI TRILE





THERMOCOUPLE-





TEMPERATURE
CONTROLLER


EXTERNAL
RE LAY ~-


TYGON TUBE






3E FURNACE




SFRI T


OIL


--- GAS OUT


Figure 7: Initial Pyrolysis Apparatus








37

problems were encountered in that the relay responsible for

turning the oven off and on had a propensity to stick in the

on position causing the oven to heat to 800-900oC, resulting

in damage to the tube furnace and the pyrolysis tube. In

theory, this problem should not have occurred since the tube

furnace was rated to draw only 6 amperes while the

temperature controller was rated for a 10 ampere load. This

problem (which was in all probability caused by power surges

in the line) was alleviated by including a second relay, or

slave relay, rated for 25 amperes in the circuit. In this

configuration, the relay in the temperature controller

served only to switch the slave relay, which directly

controlled the oven. This served to reduce the current

passing through the temperature controller's relay, thus

increasing its lifetime. During the course of these

investigations, several other modifications were made to the

system. Initially, the pyrolysis tube and tube furnace were

positioned horizontally; however this proved to be a problem

since the pyrolysis reaction produced noxious liquids which

tended to settle towards the bottom of the tube and

impregnate the product. It was found that by placing the

apparatus in a vertical configuration, these liquids were

able to drain out of the pyrolysis tube into the tygon

tubing. Figures 7 and 8 schematically illustrate two

different versions of the pyrolysis apparatus positioned

vertically. Glass wool was packed into the ends of the tube












TEMPERATURE


CONTROLLER

GAS IN -


MINERAL OIL


BUBBLER





POLYACRYLONITRILE


THERMOCOUPLE


EXTERNAL RELAY


TUBE FURNACE





FRIT


MINERAL OIL

BUBBLER


GAS OUT


Figure 8: Modified Pyrolysis Apparatus










furnace to prevent heat loss. In addition, it was found

that positioning the control thermocouple outside the

pyrolysis tube resulted in a systematically higher

temperature reading. Therefore the thermocouple was

inserted directly into the PAN near the center of the tube

furnace as shown in Figure 8.

One problem with the Omega CN-300 temperature controller

was that it was manually controlled, which made the exact

duplication of the temperature-time profile rather

difficult. In addition, the temperature was raised

incrementally making the temperature-time profile a step

function. Since one of the goals of this investigation was

to compare catalysts produced using different pyrolysis

conditions, it was imperative that the temperature-time

profiles be highly reproducible. Towards the end of this

study, an Omega CN-2000 programmable temperature controller

was purchased which enabled exact duplication of a given

temperature-time profile. In addition, this controller was

capable of increasing the temperature continuously, thus

eliminating the need to raise the temperature incrementally.

During the course of the pyrolysis experiments, several

aspects of the pyrolysis conditions were varied to determine

the effect on the catalytic activity as well as the

elemental composition. All elemental analyses were

performed by the University of Florida Micro-analytical

Services operated by Melvyn Courtney. One line of








40

investigation involved using the same temperature-time

profile and using ammonia, carbon monoxide, air or nitrogen

as the carrier gas. In addition, variations in both the

rate of increase in temperature and the duration of heating

were examined with respect to the effect that these factors

have on the catalytic activity and elemental composition.



Catalytic Studies

As in the case of the pyrolysis apparatus, the

catalytic evaluation system was modified and upgraded

throughout the course of these investigations. What was

needed was a system capable of operating at atmospheric

pressure in the temperature range from 100 to 3000C. In

addition, a high degree of reproducibility in the reaction

conditions was desirable to enable accurate comparison of

the catalytic activities of different catalyst formulations.

The initial screening studies employed a catalyst evaluation

system as depicted in Figure 9. A carrier gas (usually

nitrogen or air, both unpurified) flowed through a bubbler

containing neat substrate and the resulting gas stream was

assumed to be saturated in substrate. The reactor tubes

were approximately 1 cm I.D. with a sintered glass frit in

the center to support the catalyst. The heating system

consisted of a homemade tube furnace controlled by a

variable AC power supply and monitored with a mercury

thermometer located near the center of the tube furnace










GAS IN-


ETHANOL
BUBBLER


GC SAMPLE-



VARIAC
CONTROLLED
TUBE FURNACE


CATALYST'






THERMOMETER


REACTOR
TUBE




-- GLASS BEADS






GC SAMPLE


OUT


MINERALOIL
BUBBLER


Figure 9: Initial Catalytic Evaluation System








42

outside the reactor tube. The tube furnaces were

constructed as in Figure 10 using pyrex or vycor tubing and

heavy gauge nichrome wire. The following substrates were

used in this system to determine if PPAN catalysts had any

activity towards them: CEES, DMMP, ethanol, methanol, CO/H2

(syngas), ethylbenzene, norbornene and 2-propanol. In the

case of syngas, the gases were introduced with a system of

two bubblers whidh enabled one to vary the ratio of carbon

monoxide and hydrogen. The system contained enough volume

for adequate mixing and the resulting mixture was passed

through an additional bubbler (corresponding to the

substrate bubbler in Figure 9) which was filled with mineral

oil and used to monitor the gas flow. The mineral oil

bubbler at the end of the system was used to monitor the

effluent flow as well as to aid in the detection of leaks in

the system. In the reactions involving CEES and DMMP,

Clorox bleach was used in the last bubbler to hydrolyze the

potentially harmful starting materials and reaction

products.

Although a system such as that illustrated in Figure 9

was useful for determining whether a particular catalyst had

any catalytic activity towards a certain substrate, there

were several inherent problems in the system which prevented

accurate catalyst activity comparisons. The two main

problems with this system were the inability to accurately

control and monitor the temperature and rate of influent

























HEAT RESISTANT
TAPE ----






POWER
IN








PYREX OR
VYCOR TUBE


GLASS

' WOOL







NICHROME
-WI RE



PYREX

TUBE


Figure 10: Oven Design








44

supply. In addition, the amount of substrate that was

supplied by the bubbler was so small that it generally took

more than a week of continuous running to pass 1 ml of

substrate through the system. Figure 11 is a schematic

drawing of a catalytic evaluation system which was designed

to eliminate these difficulties. To overcome the

temperature control problem, a thermocouple well was built

into the reactor tube allowing accurate measurement of the

temperature near the catalyst bed. The K thermocouple was

connected to a digital, time-proportioning control, solid

state temperature controller (Omega CN 300) which enabled

the temperature to remain at + 20 C of the set point. It

was found that the temperature as measured by the

thermocouple in the thermocouple well was consistently

5-100C cooler than the temperature measured by the

thermometer outside the reactor tube. In addition, a

mechanical syringe was designed and constructed as

illustrated in Figure 12 in order to both stabilize and

increase the feed rate. Generally a 5 ml Hamilton Gas-tight

syringe was employed and the mechanical syringe, operating

with a 10 RPM motor, delivered 4.6 ml in approximately 7

hrs, making the Weight Hourly Space Velocity (WHSV) equal to

approximately 0.6 hour-1. The WHSV denotes the ratio of the

mass flow rate of feed to the mass of the catalyst used as

defined by the following equation: WHSV= pV/W where p is

the density of the feed, W is the weight of the catalyst and











GAS IN


TUBE FURNACE


THERMOCOUPLE


EXTERNAL -H----i I $ht


M MINERAL OIL
BUBBLER

SEPTUM

TYGON TUBE





GLASS BEADS
CATALYST

FRI T

h-GAS OUT


RELAY
TEMPERATURE MINERAL 01 L
CONTROLLER BUBBLE R


DRY ICE/ACETON
BATH


LIQUID
TRAP


Figure 11: Improved Catalytic Evaluation System



























































Figure 12: Syringe Design








47

V is the characteristic volumetric flow rate of the fluid.

The space time is defined as the reciprocal of the space

velocity. This corresponds to a very fast flow rate, almost

as high as used in many efficient, industrial processes.

Therefore the conversions obtained using this system (Figure

11) tended to be much lower than in the earlier catalytic

set-up illustrated in Figure 9.



Basicity Studies

Two types of basicity studies were attempted in this

investigation: the direct titration with dilute HC1 of

slurries of PPAN in distilled water, and the determination

of metal complex uptake from slurries of PPAN in aqueous

solutions of several metal complexes using UV-vis or EPR

spectroscopy. In the direct titration method, a weighed

sample of PPAN was slurried in a 100 ml erlenmeyer flask

containing about 50 ml of distilled water (Millipore

Nanopure Water System) and titrated with a 0.01 molar

solution of HC1, using a Fisher pH meter to monitor the pH.

Since the titrations were often lengthy, the buret tip and

pH electrode were sealed to the flask using parafilm to

prevent evaporation. In the metal binding studies,

solutions of metal complexes were slurried with PPAN

preparations for about 24 hours and then washed and dried

under nitrogen. The dried PPAN was subsequently examined

in the EPR for evidence of metal binding. In another set of








48

experiments, a solution with a known absorbance was slurried

with PPAN and then filtered and washed. The filtrate and

washings were combined and reduced by blowing nitrogen to

the original volume of solution. The absorbance of this

solution was then checked by UV-vis and compared to the

absorbance of the original solution.



Photolysis Reactions

A Hanovia medium pressure mercury lamp was used as the

light source for photolysis reactions. Suspensions of PPAN

in various substrates were placed in quartz or pyrex

reaction vessels and stirred using a magnetic stirrer.

Photolysis products were analyzed by gas chromatography.



Thermal Analysis

Differential Scanning Calorimetry (DSC) and

Thermogravimetric Analysis (TGA) were performed by Ann

Livesey of the U.S. Army on a Dupont 9900 DSC and a Kahn

TGA. These analyses were carried out using commercially

available PAN supplied by Aldrich Chemical Company. In

addition, TGA and DSC results were obtained at the

University of Florida using a Perkin Elmer Series 7 Thermal

Analysis System.










Surface Area Measurements

A Micromeritics Digisorb 2600 was used to measure the

surface areas of many of the catalysts. In addition,

surface areas were also obtained courtesy of Dr. Willie

Hendrickson, 3M Corporation. The measurements were based on

the BET method. On analyses performed on the Digisorb 2600,

the samples were degassed for 12 hours at 900C, after which

time the samples were weighed. By weighing after degassing,

the contribution due to adsorbed water was minimized. The

sample weights were entered into the computer and the

analysis, calculations and report printout were performed

automatically by the instrument.



Gas Chromatography-Mass Spectrometry Analysis

GC-MS Analysis of the catalytic reaction products from

several substrates was performed by Dr. Dennis Rohrbaugh of

the U.S. Army.














RESULTS AND DISCUSSION


Pyrolysis Studies

One area of investigation in this study was the effects

that different pyrolysis conditions had upon the chemical

and catalytic properties of PPAN. Variations were made in

the temperature-time profile of the pyrolysis reaction as

well as the atmosphere under which the pyrolysis was carried

out in order to determine what effects these factors had

upon the elemental analysis and catalytic activity of these

preparations. Table 1 contains the elemental analysis

results obtained by varying the atmosphere under which the

pyrolysis was carried out. The last two entries differ from

the other samples in that they were pyrolyzed using a

programmable temperature controller. In addition, the

temperature programs for these two samples were identical;

therefore, the only difference between these two

preparations was the atmosphere under which the pyrolysis

was carried out. Figure 13 gives some representative

temperature-time profiles for some of the preparations

listed in Table 1. It should be noted that with the

exception of sample number 5, all of the manually controlled

pyrolysis reactions employed approximately the same


















(9,10)


0 1 2 3 4 5 6 7 8 9 10 11 12
TIME (DAYS)


Representative Temperature-Time Profiles


Figure 13:






















o o 0
o un o
Cr) t. .tz.


l LO 0 tp t *.-4 -H-
0 (O 0 0 z
.4 I 4 4 0 0
-H -H *il *H *( l *H-H
0 z r. r. A$A


0 0
0 0
o o






0
Cn


O *-
o -


x 0


0U
W4


r- Efl
0 0
m


'4
&




0
E-4




O


r-H (MN n I n % co O


oD 0

01





* *0
N( M





e o



O 0
H 0




* N

nO %0


O \




H 3)


C/
zl-


r-i
UO
0








0)0



0
H




U)







a)l
S0)




MH-


* cw








53

temperature-time profile within the limits of experimental

error. Although at first glance one sees no apparent

relationships between the different preparations, several

general conclusions can be drawn. Reactions carried out in

an air atmosphere tend to have lower total percentages of

carbon, hydrogen and nitrogen than reactions carried out

under atmospheres of nitrogen, ammonia or carbon monoxide.

Presumably, this is due to increased oxygen incorporation

into the products of air pyrolyzed samples. In addition,

air pyrolyzed samples tend to have a lower percentage of

carbon than samples pyrolyzed in nitrogen, carbon monoxide

or ammonia. This effect appears more dramatic when one

looks at the C:N ratios of less than 3:1 for air pyrolyzed

samples while samples pyrolyzed in other atmospheres have

C:N ratios of greater than 3:1. Another general observation

is that all other factors being equal (temperature-time

profile, atmosphere), higher pyrolysis temperatures seem to

result in lower percentages of hydrogen in the final

product. These results are in general agreement with

reported results.3



Thermogravimetric Analysis Results

The TGA results also indicated that the pyrolysis

conditions are very important in determining the composition

of the final PPAN product. Figure 14 shows the effects of

isothermally heating at 280, 290 and 3000C on the TGA














Sample: POLYACRYLONITRILE
Size: 10.32 mgT GA
Method: RAMP/ISOTR 280 FOR 60
Comment: PT PAN / N2 100ML/MIN
105

100 --
II sot
Isot
95 i




85 -------- Isot

- 80

75 -

70

65

60
0 5 10 15 20 25 30 35 4
Time (min)


Isothermal Thermogravimetric Analysis


Figure 14:








55

thermograms of PAN. The method used was to ramp the

temperature rapidly (20oC/minute) to the given temperature

and then monitor the weight loss as a function of time. It

can be seen that the onset times for weight loss were

shorter and that the total weight loss was greater for

higher isothermal temperatures. Figures 15, 16, and 17 are

the results of temperature programmed TGA scans for

temperature ramping at 2.50C/minute, 5.00C/minute and

100C/minute, respectively. The typical sample size was

approximately 10.0 mg and the analyses were carried out in a

platinum boat under nitrogen flowing at 100 ml/minute.

These scans indicate that the weight remains essentially

constant until at least 280oC; however, Figure 18, which is

a composite of Figures 15, 16, and 17, exhibits somewhat

anomalous behavior. Specifically, the scan corresponding to

a program rate of 50C/minute seems to exhibit rather

uncharacteristic behavior. For the purposes of catalyst

preparation, 5000C is probably the maximum temperature of

interest and at that temperature one would expect the 50

C/minute scan to fall between the scans corresponding to 100

C/minute and 2.50C/minute. Instead, the scan at 50C/min

exhibits a smaller weight loss than either of the other two

scans. It is most likely that either an instrument

malfunction or an experimental error was the cause of this

discrepancy. If one looks at Figure 16, it can be seen from

the derivative curve that this sample is gaining weight from
















Sample:
Size:
Method:
Comment:
110






90i



-










50
3











-i


TGA


106.62min
27..15'C
99.64%


173. 11min
4434.62*C


-- -----------------------
350.70min


0 100 200 300 400 500
Temperature (*C)


600 700 800


Thermogravimetric Analysis, 2.50C/minute


POLYACRYLONITRILE
13.12 mg
PAN 900 0 2.5
PT PAN / N2 100ML/MIN


279.70*C


- 19


- 17


- 1-











- 713


-5
1.







-


900


Figure 15:




















Sample: POLYACRYLONITRILE
Size: 10.05 mg
Method: PAN 900 @ 5
Comment: PT PAN N2 100 ML/MIN


i t]11


100-
so-

90-
s-
80-

70-

60-


288.46'C
99.29%


334.62'C


298. 87'C


II


I I


TGA


K.


La





I -1

(_3

-os


854. 30C


200 300 400 500 600
Temperature (*C)


700 800 900


Thermogravimetric Analysis, 5.00C/minute


(1


I


Figure 16:



















POLYACRYLONITRILE
10.02 eg
PAN 900 a 10
PT PAN / N2 100MI


TGA


L/MIN


Sample:
Size:
Method:
Comment:
1QO-


S 100o 200 300 400 500
Temperature (OC


-1---- i'-2
700 800 900
General VI.OJ DuPont 9900


Thermogravimetric Analysis, 10.00C/minute


-30



- 26


- 22=






- 14J

o-

- 10



-6


-2


-80


S70C


29. imin
2958.08C
99.34%


305.61"C


37.59min
382.69*C


.40min


39.93%


- ---_--'


Figure 17:























POLYACRYLONITRILE
10.02 mg
PAN 900 @ 10
PT PAN / N2 100ML/MIN


File: A:PAN.13

T GA Operator: ABL
Run date: 01/13/85 13:42


= 10 deg/min



5 deg/min




-------- 2.5 deg/min


1O +-


200 300 400 500
Temperature (*C)


700 00o
General VI.OJ


900
DuPont 9900


Figure 18:


Composite of Temperature Programmed
Thermogravimetric Analysis


Sample:
Size:
Method:
Comment:
110-


100-


90-


80








60

about 5000C to about 7000C. Since this is not apparent in

Figures 15 and 17, it is doubtful that the increase in

weight is due to nitrogen incorporation from the atmosphere.

Most likely, the atmosphere in the TGA chamber was

contaminated with oxygen, which was responsible for the

increase in weight. Unfortunately, it was impossible to

repeat these experiments to determine the cause of the

error. The data obtained before 3250C appear to be

reliable, however, and several important conclusions can be

drawn about the pyrolysis reaction.

As earlier studies have indicated, the reaction begins

at higher temperatures when faster heating rates are used.

This has been attributed to the induction period for the

exothermic polymerization of the nitrile groups39,40,41

which can reach explosive speeds resulting in the

destruction of the polymer chain.42 Figure 19 contains the

TGA (obtained at the University of Florida) corresponding to

a temperature program rate of 0.50C/minute and the results

qualitatively agree with the scans done at faster heating

rates. The onset temperature for weight loss is

considerably less than in the scans shown in Figure 18. In

addition, the percentage weight loss is also less than for

the other runs; however, since this run was carried out

using a different instrument, the absolute percentages

should not be taken too literally since the instruments may

not have been identically calibrated. Figure 20, from some


















Samoine WeghtA 0.742 mg

polyocryionttrile


104.0

102.0



98. 0

96.0

94.0

92.0

90.0O

B8.0

86. 0

84.0

2. 0

80.0


PERKIN-ELMER
7 Series Thermal Analysis System


225.0 250.0 275.0 300.0


35. 0 350.0 375. 0 400.0 425. 0

Temperature (C)


Figure 19: Thermogravimetric Analysis, 0.5oC/minute








62















680 yN



-40





20 I I


2p 390
temperature OC



























Figure 20: Thermogravimetric Analysis and
Differential Thermal Analysis








63

work by Grassie and McGuchan, shows Differential Thermal

Analysis (DTA) curves for polyacrylonitrile heated for

100C/minute in air, nitrogen and vacuum.43 It can be seen

from Figure 20 that air pyrolysis results in greater weight

loss than nitrogen pyrolysis, while the greater weight loss

for the vacuum pyrolysis can be attributed to the ease of

volatilizing high-boiling fractions. In addition, it can be

seen that the onset temperature for weight loss is lowest

for vacuum and lower for air than for nitrogen. This would

seem to suggest that there are processes occurring in the

air atmosphere at lower temperatures than in the nitrogen

atmosphere. Most likely, the lower temperature reactions

are due to oxygen incorporation reactions not occurring

during the nitrogen pyrolysis. The Differential Thermal

Analysis (DTA) curve in Figure 20 is also interesting in

that it is tilted due to the fact that the sample is giving

off so much heat that the system becomes hotter than the

programmed rate of temperature rise (the DTA instrument

measures the difference in temperature between a reference

and a sample when both are heated under identical

conditions). It should be noted that these results have

severe implications for the pyrolysis of bulk samples of PAN

due to the exothermicity of the nitrile polymerization

reaction. The destructive reaction could occur at

relatively low temperatures if the heat of polymerization is

not dissipated rapidly enough to prevent the interior of the








64

polymer from reaching the critical temperature.42 Since the

catalyst preparation method used in this study employed a

pyrex tube containing about 15 grams of PAN, heat

dissipation was potentially a serious problem.



Differential Scanning Calorimetry Results

The DSC results also suggested that there is some sort

of an induction period associated with the pyrolysis

reaction. Figure 21 shows the effects of heating rate on

the exotherms. As in the temperature programmed TGA

analysis, the onset temperatures increase with the heating

rate. If there were no induction period, one would expect

the onset temperature to be lower with increasing heating

rate. A comparison between the onset temperatures for the

DSC and TGA results of temperature programming at 2.5, 5.0,

and 100C per minute demonstrates that there is fairly good

correlation between the onset temperatures for weight loss

and the onset temperatures for the exotherm, implying that

the reaction resulting in weight loss is an exothermic

reaction. The shapes of the exotherms in Figure 21 are

quite similar but the curves differ in the onset

temperatures and the amount of heat given off. Figure 22

contains the DSC results for a heating rate of 0.50C/minute,

and again the results qualitatively agree with those for

faster heating rates in that the onset temperature for heat

loss is less while the amount of heat loss seems to be








65












30
POLYACRYLONITRILE AT 20=-
WTi 0. 00 mg
SCAN RATE: 20.00 deg/min
---- = PULYACRYLONITRILE AT 10
A
T --- POLYACRYLONITRILE AT 5 /

-----= POLYACRYLONITRILE AT 2.5


S15

-0- - ----











187 207 227 247 267 287 307 317

TEMPERATURE (0C) DSC
86/01/08 TIME: 14:03












Figure 21: Temperature Programmed Differential
Scanning Calorimetry



















PERKIN-ELMER
7 Series Thermal Analysis System



14.0- Resulta Pack 5, 7/87 S.21 AM

13.0 an

:2.0- T1 234.751 *C

.1.0- T2 248.081 *:
Pook 242.541 *C
10.0 A-rea -910.547 nJ
.- Daltoa H -505.592 J/g
Height -2.220 MW
S .0 O- neat 239.946 'C
7. 0

- 6.0-
LL
5.0-
4- 4.0
w_


Temperature (C)


Differential Scanning Calorimetry, 0.50C/minute


Figure 22:








67

reduced with respect to the results obtained at faster

heating rates. Again, direct comparisons should not be

taken too literally due to the fact that two different

instruments were used to obtain these results. In general,

it seems that faster heating rates result in a greater

amount of heat being given off, correlating with the TGA

results which indicated that more weight was lost with

faster heating rates. It is also interesting to note that

there seems to be an endothermic process occurring at higher

temperatures than the exothermic process. In the exothermic

process, the amount of heat absorbed also seems to slowly

increase with increasing temperature, although this could be

an instrumental artifact. Figure 23 shows the effects of

isothermally heating at 240, 245, 250, 255, and 2600C. The

samples were heated at the rate of 10C per minute until

reaching the desired temperature and then maintained at that

temperature for a period of time. The isothermal DSC

results qualitatively agree with the isothermal TGA results

in that lower isothermal temperatures result in longer onset

times. Also, the amount of heat given off increases with

increasing temperature, in agreement with TGA results which

showed that higher isothermal temperatures resulted in

greater weight loss. In addition, the shape of the exotherm

varied with isothermal temperature in that higher isothermal

temperatures resulted in larger, sharper exotherms. As in

the temperature programmed DSC runs, there seems to be an








































247 257 267 277 287 297 307 317 327

TEMPERATURE (C) DOC


Isothermal Differential Scanning Calorimetry


Figure 23:








69

endothermic process occurring after the exothermic process

and the amount of heat absorbed increases with increasing

isothermal temperature. Again it is not known at this time

whether this is an instrumental artifact. From the thermal

analysis results, several conclusions about the pyrolysis

reactions can be drawn. The reactions occurring during the

pyrolysis are dominated by a strongly exothermic reaction

concomitant with a dramatic loss in weight, preceded by an

induction period. The onset times for weight loss and heat

loss are strongly dependent upon the rate of heating and the

absolute temperature. In addition, there appear to be some

inconsistencies in treating this pyrolysis reaction as

merely a nitrile polymerization reaction since the dramatic

weight losses observed in this study suggest that a

considerable amount of chain destruction is taking place.

Earlier studies on a commercial acrylic fiber containing

methyl acrylate and acrylonitrile found a high correlation

between the nitrile content and the heat evolved during the

pyrolysis.44 The amount of unreacted nitrile groups was

determined by infrared analysis before and after pyrolysis

(Figure 24). The results of these analyses suggest that the

exothermic reaction is associated with the disappearance of

the nitrile groups, although it seems like an

oversimplification to attribute this reaction to a simple

nitrile polymerization reaction in the case of a

polyacrylonitrile homopolymer. In a copolymer containing























I I I


10








40
2C c




2Ccc
I I I


- -


20 40 50 80 100 120 140 160


Exotherm


(cal/gram)


Figure 24:


Plot of Residual Nitrile Content Versus
Area of Exotherm


Residual


I I








71

relatively unreactive polymer subunits interspersed with the

nitrile containing subunits, it might be possible to view

the pyrolysis as a simple nitrile polymerization reaction.

If one calculates the theoretical weight loss associated

with the reaction in Figure 1, the proposed catalyst

formation reaction, the result is about 5% weight loss

principally due to hydrogen loss during cyclization and

aromatization. This does not account for the fate of

inevitable chain ends, defects and polymerization catalyst

residues which are present to various extents depending upon

individual sample preparation techniques. The weight losses

obtained for isothermal runs at 280, 290, and 3000C were

about 25, 30, and 35%, respectively (Figure 18), after 1

hour of heating at these temperatures. These percentages

seem far greater than what one would expect from the nitrile

polymerization reactions, even with the contributions from

the reactions undergone by the defects and impurities

previously mentioned. Furthermore, the nitrile

polymerization reaction should theoretically produce no

nitrogen containing volatiles; however, these were observed

in this as well as in all other studies. Although it is

conceivable that the nitrogen could have originated from

atmospheric sources, this is unlikely since the nitrile

moiety is far more reactive than the dinitrogen molecule.

In conclusion, while these and other results indicate that

the disappearance of the nitrile functional group is








72

intimately related to the exothermic reactions) occurring

during the pyrolysis, it is unlikely that a simple

intramolecular nitrile polymerization-aromatization is the

only reaction occurring to a significant extent. The

polymers used in this and other studies were atactic

polymers, making the likelihood of extensive intramolecular

cyclization more remote than in an isotactic polymer. In

reality one must view the nitrile groups as being randomly

oriented about the polymer backbone. This conformation

would increase the probability of reactions between adjacent

chains resulting in extensive crosslinking. The observation

that polyacrylonitrile becomes harder and more brittle after

heating supports this interpretation. Although the thermal

analysis studies, which employed about 10 mg of sample, may

not be directly applicable to the bulk pyrolysis of 15 to 20

grams of material, the results indicate the there may be a

serious design flaw in the pyrolysis apparatus used in these

studies in that the dissipation of the heat produced in the

exothermic reaction is crucial in preventing the destructive

runaway reaction. The apparatus employed about 20 grams of

PAN in a 1 inch diameter tube and had typical gas flow rates

of less than 100 ml/minute. The combination of rather

densely packed material and relatively slow gas flow rate

may have resulted in a heat dissipation problem. Although

the pyrolysis apparatus probably was not producing the best

catalysts possible, it proved to be adequate for the








73

purposes of this study. Some catalytic material was

prepared in our controlled temperature pyrolysis since

little or no activity was observed for materials heated too

quickly.



Surface Area Results

The results of the BET surface area determinations are

contained in Table 2. Since these results were determined

using different instruments and conditions (degas time,

degas temperature, etc.), these factors should be taken into

account when comparing the data. Several conclusions may be

drawn from these results. In general, the surface areas of

the pure PPAN materials are quite low, on the order of 0-9

square meters per gram. These results are considerably

lower than those reported by other workers who have obtained

values of 19.0-19.2 m2/gram,19 17.7 m2/gram15 and 18

m2/gram.16 These discrepancies can in part be explained

when one observes that the surface area of the PPANCuLi

catalyst increases after passing 15 ml of ethanol over two

grams of catalyst. In addition, it was observed that the

products collected from the catalytic reaction frequently

were amber colored, indicating that some of the waxes and

oils produced in the pyrolysis reaction had impregnated the











Table 2: Surface Area Results


Max BET Surface
Temp(C) Area (M2/g)


C H
(%) (%)


N
(%)


Comments


PPAN on
Silicaa

PPAN on
Silicaa

Silicaa

PAN

PPANa

PPANa

Aluminaa

PPAN on
aluminaa

PPANb

PPANb

PPAN Cu-Lib

PPAN Cu-Lib


400


400


400

400


400


400

450

500


260 (272)c


243 (263)C


(332)c

(286)C

5 (6)c

7 (9)c

153c

134 (134)c


>lc

4C

>1c

1c


high load


2.55 0.34 0.66 low load


0.20

67.12

65.02

69.39

0.03

3.55


59.47

69.25

60.20


0.32

5.82

3.19

3.78

0.24

0.52


2.40

3.00

4.07


0.43

26.17

22.64

21.59

0.00

0.73


23.21

20.95

18.26


air pyr.

nit. pyr.



nit. pyr.


air pyr.

nit. pyr.

fresh

spent


aThese samples were all pyrolyzed using an identical program
on an Omega CN 2000 temperature controller.

bThese samples were pyrolyzed using an Omega CN 300 manual
temperature controller.

cThese surface areas were generously provided by Dr. W.
Hendrickson of the 3M corporation while the other numbers were
obtained using a Micromeritics Digisorb 2600 surface area
analyzer.

dAfter pyrolysis.

eAfter passing 15 ml of ethanol over 2.00 g of this material
at 3000C.


Sample








75

catalyst, possibly filling up some of the pores and reducing

the surface area. The passage of ethanol through the

material may serve to wash out these residues and increase

the surface area. Granted, the surface areas of these two

samples are very small and possibly within the error limits

for this instrument. However, both samples were run on the

same instrument and although the absolute difference in the

"spent" and "fresh" catalyst samples is relatively small,

the surface areas differ by a factor of five. Degannes and

Ruthven19 reported that the surface areas of fresh and used

catalysts remained identical within the limits of

experimental error. The studies which reported surface

areas in the neighborhood of 20 m2/gram all used a similar

pyrolysis method in that they spread the PAN thinly on trays

and carried out the reaction in a draft oven. This

pyrolysis method is probably superior to the method used in

this investigation for two reasons. First, there is a

greater capacity for heat dissipation in a draft oven;

second, with the PAN spread in a thin layer is less of a

propensity for the pores on the surface to become clogged

with low molecular weight residues from the pyrolysis

reaction, thus lowering the surface area.

The surface area results for the silica and alumina

supported PPAN catalyst preparations were approximately 250

and 130 m2/gram, respectively, indicating that the surface

areas of these supports were not significantly altered by








76

the addition of up to 17% PAN by weight. These surface

areas were more than an order of magnitude greater than the

largest of the unsupported PPAN catalyst preparations, and

these results will be discussed later with respect to

catalytic activity comparisons between different

preparations.



Basicity Studies



Titration Results

Several attempts were made to determine the number of

basic sites on the surface of PPAN catalysts by directly

titrating with dilute HC1. The PPAN samples were either

used as is after the pyrolysis reaction or slurried with

concentrated sodium hydroxide, filtered and then washed with

distilled water. For a number of reasons it was impossible

to obtain quantitative data from this titration method. For

some unknown reason, the pH meter would not stabilize on a

reading in suspensions of PPAN materials. The meter

readings would continually randomly jump around when

immersed in PPAN suspensions but would stabilize immediately

upon being immersed in a solution not containing PPAN. In

addition, the meter would drift up as much as 4-5 pH units

over a period of several hours, making the time at which the

reading was taken very important. Throughout the titration,

the calibration of the pH meter was checked and found to be








77

accurate. Therefore, the instability in the readings was

caused by the PPAN itself, suggesting that some unknown

reaction was occurring at the electrode and/or on the

surface of PPAN. These results are in general agreement

with the only other titration study which demonstrated that

long-time exposure to dilute acid will result in some acid

uptake, indicating a generally basic nature.36 Although, as

stated previously, no quantitative results could be obtained

from this titration study, it is possible to make some

general observations about the basic properties of PPAN

materials. Although PPAN is essentially insoluble in all

acids, bases and solvents, it is by no means inert, as

evidenced by the instability of the pH electrode and the

slow uptake of acid. In addition, PPAN samples pyrolyzed in

air seem capable of absorbing more acid than samples

pyrolyzed in a nitrogen atmosphere.



Metal Binding Studies

As in the case of the titration studies, the results of

the metal binding studies were somewhat inconclusive in that

no direct evidence of metal binding was ever established.

The general method used was to add an EPR active metal

complex to PPAN by adsorption from solution and then look

for nitrogen hyperfine in the EPR. It was assumed that if

the polypyridine type structure in Figure 1 is the basic

site, the metal species would bind to the nitrogen and the








78

unpaired electron on the metal ion would interact with the

nuclear spin of the nitrogen (3/2), resulting in a nuclear

hyperfine interaction. Copper and cobalt were chosen as the

metal species since nitrogen hyperfine is known to be

readily observable for both at liquid nitrogen temperatures.

There have recently been several studies on the use of 1,8-

napthyridine based ligands in the formation of metal

complexes and these studies indicate that the napthyridine

moiety can accommodate several bonding modes, as shown in

Figure 25. Unlike bipyridines, the nitrogens in 1,8-

napthyridines are rather close together, resulting in a

significantly reduced "bite angle", thus making

napthyridines much poorer candidates for bidentate ligands.

In addition, its steric bulk can make the napthyridine

moiety a poor candidate for a monodentate ligand in certain

instances. There are numerous examples in the literature of

napthyridine based ligands forming complexes with many of

the metals in the periodic table, including the lanthanides

and the rare earth metals.45-50 The three general bonding

modes for these are depicted in Figure 25. These materials

are frequently highly colored due to the presence of a metal

to ligand charge transfer band, and have also been known to

exhibit fluxional behavior.51,52









a.
I
M






N N
M





C. 00)
N /N
I/
M


Naptheridine Binding Modes


Figure 25:








80

Diphenylglyoximato cobalt (II) (Co(DPGH)2

This compound was prepared and stored in a nitrogen

atmosphere. The EPR samples were prepared in a glove box

using a 50:50 toluene:methylene chloride mixture which had

been exposed to three freeze-pump-thaw cycles to remove

dissolved oxygen. A saturated solution of Co(DPGH)2 was

slurried with a small amount of PPAN, the mixture was

filtered and EPR samples were prepared from the filtrate and

the solid material. The EPR spectra of solid samples of

Co(DPGH)2 and PPAN after being exposed to Co(DPGH)2 in

solution are given in Figure 26. The PPAN sample was run at

a higher sensitivity to detect any small changes in the

spectrum upon addition of the cobalt complex. The spectra

demonstrate that PPAN has little or no effect upon the EPR

spectra for Co(DPGH)2, other than the superimposition of the

narrow signal at about g=2 which is characteristic of PPAN.

Both the lineshape and g value of the cobalt complex remain

unchanged upon addition of PPAN. It should be noted that

solutions (50:50 toluene:methylene chloride mixture) of

Co(DMGH)2 and its pyridine adduct were prepared and the EPR

spectra were consistent with earlier published results which

showed the typical eight line cobalt spectrum with five line

nitrogen hyperfine in the case of the pyridine adduct

(Co(DPGH)2-2Pyridine).38 These results suggest that the

electronic structure of the cobalt is not perturbed upon

addition of Co(DPGH)2, implying that the Co(DPGH)2 present












3300 GAUSS


I




1-200


T= 298 OC



PPAN+Co(DMGH)2


Co(DMGH)2


Figure 26: Electron Paramagnetic Resonance Spectra of
PPAN and Co(DMGH)2








82

in the sample is merely physically adsorbed and not

chemically bound to the surface. Since pyridine binds to

this complex, the results show that PPAN is a poorer base

than pyridine, although at this time it is not possible to

discern whether the chemical structure, electronic structure

or steric bulk is responsible for PPAN's poor binding

ability. Since Co(DPGH)2 is a square planar complex with a

nearly planar ligand, the steric requirements for axial

adduct formation are not particularly stringent. This

suggests that the steric bulk of the PPAN is probably not

the over-riding cause for the poor binding ability of PPAN

materials.



Cupric chloride (CuCl2)

Cupric chloride (0.20 g) was added to a strongly basic

suspension of PPAN in distilled water and the resulting

mixture was allowed to stir for several hours, after which

time the mixture was filtered and the PPAN'CuC12 washed with

copious amounts of distilled water. The EPR spectra of this

material as well as that of solid CuCl2 are presented in

Figure 27. The EPR of PPAN'CuC12 consists of the usual PPAN

signal at about g=2, with another signal appearing at

slightly greater than g=2 as well as a much smaller but

discernable signal at about g=2.5. After washing with 0.1

molar HC1 and copious amounts of distilled water the signal

remained unchanged. Since a strongly basic solution could





















CuC12 (s)


PPAN + CuC12


--I0 0 G*


T= 2980C


t0
3300
G A US


Figure 27: Electron Paramagnetic Resonance Spectra of
PPAN and Copper (II) Chloride








84

result in the formation of Cu(OH)2 on the surface of the

PPAN, it was hoped that washing with hydrochloric acid would

result in the protonation of any remaining basic sites and

the formation of chloride salts on the surface of the PPAN.

A comparison of the EPR spectra of solid cupric chloride and

PPAN-CuC12 reveals that the PPAN'CuCl2 spectrum is not a

simple addition of the spectra for CuCl2(s) and PPAN as in

the case of Co(DMGH)2. Although the characteristic PPAN

signal appears unchanged, the copper signal in PPAN'CuCl2 is

markedly different from that of CuCl2(s), implying that

there is some sort of electronic interaction between the

PPAN and copper's unpaired electron, unlike the case of

Co(DMGH)2. Although the g values and lineshapes of the

copper signals are quite different, there is no evidence of

a nitrogen hyperfine interaction. Bai and coworkers20

prepared silica gel supported PPAN materials and studied

their physical and catalytic properties. Their work

presented XPS results for CuCl, PPAN on silica gel and PPAN

on silica gel with added CuCl, and they concluded that the

copper was coordinately bound to nitrogen based on

differences in the Cu 2p 3/2 and N ls binding energies upon

the adsorption of CuCl onto silica supported PPAN.20

Different preparative methods or the presence of silica gel

makes direct comparison of these results impossible. As

mentioned previously, the nitrogens in PPAN may not be

capable of producing a nitrogen hyperfine interaction, thus








85

making the EPR method unsuitable for determining whether the

copper is bound to nitrogen. In any case, the fact that the

copper signal in PPAN'CuCl2 was significantly different from

that of CuC12, coupled with the fact that repeated washing

of the PPAN with HC1 and distilled water failed to alter or

reduce the intensity of the signal, implies that the copper

was indeed coordinated to the surface of the PPAN, although

it was not possible to determine whether the copper was

bound to nitrogen, carbon, or oxygen.



Bis (2.2'-bipyridine) ruthenium (II)

trifluoromethanesulfonate (Ru(bipy)2(CF3SO3)2)

This compound was prepared in situ by dissolving 0.11 g

of AgCF3SO3 in an ethanol solution containing 0.10 grams of

Ru(bipy)2Cl2, heating the reddish solution to boiling to

coagulate the AgCl, and finally filtering and washing with

absolute ethanol. The deep red filtrate was presumed to

contain Ru(bipy)2(CF3SO3)2. The UV-visible spectrum of a

solution made in a volumetric flask was recorded before and

after the addition of a large excess of PPAN (5.10 grams) It

was found that there were no appreciable absorptivity

changes in the UV-visible spectra, neither in the wavelength

maxima or intensities of the absorptions, indicating that

there was negligible adsorption or binding of

Ru(bipy)2(CF3SO3)2 to the surface of the PPAN. This

particular ruthenium complex was chosen for several reasons.










Being a very poor ligand and hence a good leaving group, the

trifluoromethanesulfonate anion seemed like a good candidate

for being displaced by PPAN, which had not proved to be a

particularly good ligand in previous experiments. In

addition, it was hoped that the presence of two bidentate

2,2'bipyridines bound to the ruthenium would facilitate (by

the "chelate effect") the binding of PPAN as a bidentate

ligand, thus lessening the steric strain associated with the

large PPAN structures. This strategy proved to be

ineffective since no adsorption of ruthenium complex was

detected.



Catalytic Results



The catalytic studies fall into two general categories:

screening reactions were carried out to determine whether

PPAN catalysts were active towards various substrates, and a

specific reaction, that of ethanol over PPAN catalysts, was

used as a model reaction to study the effects of pyrolysis

conditions, metal dopants and oxide supports on the

catalytic activity of PPAN.










Screening Reactions



Isopropyl alcohol and ethylbenzene

Some preliminary experiments with early catalyst

preparations were undertaken to verify the catalytic

activity reported in previous studies. Manassen and

Wallach15 had reported the catalytic activity of PPAN

materials towards isopropyl alcohol. Therefore, the vapor

was passed over PPAN at 1500C and although the results were

not quantified, acetone was detected by gas chromatography.

As a preliminary probe of the photocatalytic capabilities of

PPAN, suspensions of PPAN in isopropyl alcohol were

irradiated with visible and ultraviolet light using a medium

pressure mercury vapor lamp with pyrex and quartz reaction

vessels, respectively. The results, when compared to the

appropriate blank runs (no PPAN present), indicated that

PPAN had no appreciable photocatalytic activity towards

isopropyl alcohol.

The same series of experiments were carried out using

ethylbenzene as the substrate and the results were analogous

to those obtained for isopropyl alcohol. In the vapor

phase, ethylbenzene was converted to styrene, as previously

reported by Degannes and Ruthven,19 and no photocatalytic

reaction was observed to occur. The results of these

experiments served to verify that although the measured

surface areas were somewhat smaller, the catalytic








88

activities of the materials prepared in this laboratory were

comparable to those of earlier published reports.



Norbornene oxidation

It has been previously reported that PPAN is an active

dehydrogenation catalyst, however there have been very few

studies in which PPAN has been tested with respect to its

oxidative capabilities. Towards this end norbornene was

used as a gaseous substrate at 1400C and atmospheric

pressure, as well as at 500C and 35 psi oxygen in a

suspension containing 1 gram of norbornene and 1 gram of

PPAN in 20 ml of acetonitrile. In both cases, there were no

detectable changes in composition under the reaction

conditions as measured by gas chromatography using a

carbowax column and a flame ionization detector. There are

several possible products which could conceivably be formed

from norbornene, namely norbornene oxide, norbornadiene, and

numerous products resulting from the fragmentation of the

bicyclic ring system. Norbornadiene would result from the

dehydrogenation of the carbon atoms symmetrically related by

a mirror plane to the doubly bonded carbons in norbornene.

Since PPAN has been shown to dehydrogenate cumene and

ethylbenzene, which both have a bulky aromatic system

conjugated to the bond being dehydrogenated, it was felt

that norbornene might be a good candidate to test whether

the existence of a r-allyl type system is necessary for








89

dehydrogenation to occur. On the other hand, one must

remember that norbornene, due to its bicyclic structure, has

a far greater steric problem than cumene or ethylbenzene.

If steric factors are of the most significance, norbornene's

lack of reactivity is readily explained since all of the

previously proposed structures for PPAN catalysts are quite

large and bulky, which would render them less suitable for

interaction with a sterically hindered substrate such as

norbornene. The conversion of norbornene to norbornene

oxide has been accomplished using ruthenium (II)

phenanthroline catalysts.53 The fact that norbornene is

stable in the presence of PPAN implies that either

norbornene is too sterically hindered to react with PPAN, or

PPAN is not a particularly good oxidation catalyst or both.

These rationalizations are supported by the available

literature reports since the use of PPAN as an oxidation

catalyst has been reported only twice (in one case, silica

and metal salts were also present, while the other report

employed PAN for the purpose of silver crystallite

deposition on the surface of oxide supports21) and the

polymeric structure of PPAN has been fairly well

characterized as being a large extensively crosslinked

heterocyclic structure.










Syngas conversion

The hydrogenation of carbon monoxide with dihydrogen is

an industrially important reaction since mixtures of

hydrogen and carbon monoxide, called syngas, are a major

by-product of the petroleum mining and refining industry.

Unlike carbon dioxide, carbon monoxide is fairly reactive

due to its valence deficient electronic structure, as

evidenced by its tendency to form numerous metal-carbonyl

complexes. The catalytic mechanism of PPAN has been

described in the literature as one in which the catalyst

dehydrogenates a substrate and subsequently becomes reduced.

The catalyst is oxidized to its original state by a short

treatment in oxygen at reduced temperatures and has been

shown to produce water. Therefore, if carbon monoxide were

to participate in this catalytic cycle, it would have to

replace the oxidant since a product further reduced would be

undesirable except for emission control applications. By

using a mixture of hydrogen and carbon monoxide it was hoped

that the PPAN could adsorb the hydrogen molecules and then

catalytically reduce the carbon monoxide, completing the

catalytic cycle. A 1:2 mixture of carbon monoxide and

hydrogen was passed over a nitrogen pyrolyzed sample of PPAN

at temperatures up to 3000C and the only product observed

using gas chromatography was a small methane impurity which

was contained in the original carbon monoxide. Since

ruthenium is frequently used as a Fischer-Tropsch catalyst,








91

samples of ruthenium doped PPAN were prepared by refluxing

PPAN in a methanol or ethanol solution of RuC13*3H2O for 24

hours. The mixture was then filtered and washed with

methanol or ethanol and dried under a stream of nitrogen.

When a mixture of carbon monoxide and hydrogen was passed

over the catalyst prepared in methanol using the same

catalytic conditions as before, dimethyl ether was the only

observable product, whereas the catalyst prepared in ethanol

produced only acetaldehyde, diethyl ether and ethyl acetate,

as determined by GC analysis. These results indicated that

the only reactions occurring involved the adsorbed solvent

molecules since the same results were obtained using

nitrogen as the carrier gas instead of syngas. The fact

that dihydrogen is not active towards PPAN catalysts is not

too surprising since Manassen and coworkersl6 observed that

gaseous hydrogen was never produced during the catalytic

reaction or after prolonged heating in an inert atmosphere.

Their work indicated that hydrogen binds PPAN as a species

intermediate between a hydrogen atom and a hydride ion.15

Since the residual ethanol solvent produced mostly

acetaldehyde, it is reasonable to assume that the surface of

PPAN must be becoming reduced or hydrogenated. This is

substantiated by the fact that a short air treatment would

restore the original catalytic activity with the concomitant

production of water. Therefore, ethanol and carbon monoxide

were passed over PPAN simultaneously to determine whether