Catalysis by Polymers

Catalysis by Polymers

E. A. Bekturov S. E. Kudaibergenov

WILEY-VCH Verlag GmbH & Co. KGaA

E. A. Bekturov and S. E. Kudaibergenov

Catalysis by Polymers

WILEY-VCH Verlag GmbH & Co. KGaA

This Page Intentionally Left Blank

E. A. Bekturov and S. E. Kudaibergenov Catalysis by Polymers

This Page Intentionally Left Blank

E. A. Bekturov and S. E. Kudaibergenov

Catalysis by Polymers

WILEY-VCH Verlag GmbH & Co. KGaA

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek Die Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at . © 1996 Hüthig & Wepf Verlag, Hüthig GmbH, Heidelberg © 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-29674-3

Preface

Considerable gains have recently been made in the research of polymer catalysis which has emerged from the interaction of the fields of macromolecular, coordination and catalytic chemistry. Using synthetic macromolecules one can create polymeric catalysts which function like enzymes and almost simulate their activity and selectivity. Consequently, polymer catalysis could enable the high-yield manufacture of industrially important products at low-reaction volumes involving minimal energy consumption. Catalytically active metal complexes fixed on polymeric supports can be essential in solving some important problems encountered by the chemical and petrochemical industry. Heterogeneous metal complex catalysts can be regarded as a novel class of catalysts which combine the advantages of both homogeneous and heterogeneous systems and which possess high activity and selectivity, specific character and stability in operation. Hence, this area is a significant field of catalytic chemistry. This monograph intends to acquaint the reader with the basic material available in the field of catalysis. Because this field was previously treated as a marginal area of polymer and catalytic chemistry, the authors mostly cite recent literature sources. It covers the catalytic properties of a broad class of functional polymers and their metalion complexes as well as ionite and heterogeneous (polymer-supported) metal-complex catalysis. The first chapter of the book deals with enzyme-like catalysis by synthetic polymers - catalysis by polymeric acids and bases, amphoteric polyelectrolytes and nonionic polymers. Because coordination compounds of metal ions with macromolecular ligands are interesting with regard to bioinorganic chemistry, this book elucidates some problems involving the catalysis by water-soluble polymer-metal complexes. Ester hydrolysis, hydrogen peroxide decomposition, oxidation of disubstituted phenols, hydroquinones, mercaptoalcohols and other types of reaction are chosen as model processes. A section devoted to interfacial catalysis is also included. The second chapter handles various problems regarding catalysis by ion-exchange resins. Mechanisms of reactions involving acidic and basic resins are discussed. In respect of the wide use of anion-exchange resins as interfacial transfer catalysts, attention is paid to the specific nature of this reaction. Similarities and differences in the catalysis of non-ionic resins and linear polyelectrolytes are shown. The third chapter presents different methods of production of heterogenized homogeneous catalysts from complexes of transition metals and polymer matrices. Different ways of fixing a homogeneous metal complex o n a polymeric support (encapsulation, covalent bond, coordination bond, ion exchange and gel immobilization) were investigated. The catalytic activity of Group VIII metals fixed on different polymeric supports as well as the problems of their control are elaborated. In some cases the working mechanisms of heterogenized catalytic systems are analyzed. Particular attention is paid to the effect of matrix isolation of catalytically active centers

VI

Preface

on the reactivity of heterogenized metal-complex catalysts, longevity and leaching problems as well as to methods of reducing leaching. The effect of the combined properties of a polymer matrix, metal ions and a solvent on catalyst activity and selectivity is depicted. In conclusion, general problems of the catalysis both by functional groups of polymers and by their metal complexes are discussed. An attempt is made to illustrate some characteristic properties of such metal complexes and to outline promising directions of research in this expanding field. E. A . Bekturov S.E. Kudaibergenov

Contents

Preface

.................................... . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 1. Catalytic Properties of Polymers in Solutions

1.1 1.2 1.3 1.4 1.5

.................

1

Hydrolysis of Esters ............................................. Catalysis by Water-Soluble Polymer-Metal Complexes . . . . . . . . . . . . . . . . Oxidation of 2. 6 Disubstituted Phenols ............................. Macromolecular Effects During Oxidation of Thiols . . . . . . . . . . . . . . . . . Interfacial Catalysis ..............................................

1 10 22 31 38

References

.

......................................................

References

.

44

...........................

50

......................................................

64

......................

67

Chapter 2 Catalysis by Ion-Exchange Resins

Chapter 3 Heterogenized Homogeneous Catalysts

3.1 3.2 3.3 3.4 3.5 3.6

V

Catalysts Dispersed on Polymer Support Surfaces . . . . . . . . . . . . . . . . . . . 67 74 Catalytic Properties of Coordination Polymers ...................... 92 Gel-Immobilized Catalytic Systems ................................. Heterogeneous Catalysts Fixed on Ionites ........................... 102 Heterogeneous Metal Complex Catalysis ............................ 110 Decomposition of Water by Polymer-Supported Complex Catalysts . . . . 132

......................................................

136

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References .................................. ........................

145 149

References

Subject Index

.......................................................

151

Catalysis by Polymers

E. A. Bekturov and S . E. Kudaibergenov Copyright 0 2002 WILEY-VCH Verlag GmbH & Co. KGaA

Chapter 1 Catalytic Properties of Polymers in Solutions

Polymers are increasingly being used as active components in synthesis and catalysis [I - 121. Three basic directions of polymer application are cited as follows: ( 1 ) use of polymers as reagents; (2) application of functional groups of polymers as catalysts; and (3) use of polymers as supports. Catalysis by functional groups of macromolecules has attracted scientists’ attention owing to the rapid development of enzyme catalysis. High catalytic activity of enzymes is attributed to the broad variety of their particular functional groups which enables their involvement in multiple interactions. The main limitation for using polymers in enzyme catalysis is their lack of a diverse functional group. Consequently, complex enzyme functions cannot yet be realized by synthetic analogues. Nevertheless, the molecular nature of proteins has prompted many scientists to focus on catalytic systems based on synthetic polymers as simple model proteins, metalloenzymes and ferments. This chapter presents various examples of enzyme catalysis by polymers including ester hydrolysis, decomposition of hydrogen peroxide, oxidation of disubstituted phenols and hydroquinone, interfacial catalysis and other types of reaction. Because metal ions (Fe, Zn, Cu. Mn, Co, erc.) are often involved as coferments during enzyme catalysis, some examples illustrating their catalytic action are also given. The catalytic activity of polymeric coordination compounds is shown to depend on the strength of the ligand-metal bond.

1.1 Hydrolysis of Esters Research by Overberger [13], Kabanov [14, 151, Klotz [16], Kunitake [I71 and others [18, 191 has contributed much to development of the catalysis by water-soluble polymers. Among numerous synthetic polymer catalysts, imidazole derivatives are cited in particular because they are constituents of hydrolytic enzymes [ l , 20-221. Polyvinylimidazole (PVI) is several times more catalytically effective than imidazole because of its polyfunctional nature. Let us consider three possible mechanisms of cooperative catalysis (see p. 2). Mechanisms A and B represent the general basic catalysis. Cooperative interaction between two neutral imidazole molecules enhances the nucleophilic attack on the substrate. Mechanism C is a variety of the general acid catalysis caused by the nucleophilic attack. To prove the polyfunctional character of the catalysis, let us compare activation parameters of polymers with their low-molecular weight analogues. For instance, for a polymeric catalyst the change in enthalpy ( A H ) is 15.5 kJ/mole, whereas for imidazole this value is 29.4 kJ/mole [23]. Additional entropy is obtained from the formation of a transition-state complex in which catalytic and reactive groups are oriented with respect to each other. Besides, with the transition from a low-molecular

2

Chapter I: Catalytic Properties of Polymers in Solutions A

9

R‘ A

R* n

0

weight catalyst to a high-molecular weight one, the probability of polyfunctional catalysis increases because active groups in a macromolecular coil approach each another. Some scientists believe [24] that a cooperative effect is exhibited starting from the n>8-degree of polymerization. It is suggested [25] that the volume of macromolecular coils is several orders of magnitude higher than that of reactive substrate molecules. Thus, a considerable degree of catalytic activity occurs inside these coils which act as individual microreactors. Therefore, a polymeric catalyst is most effective when a polymer chain is present in the solution as a tightly compressed coil. The effect of PVI conformation on the rate of hydrolysis of the neutral ester, p-nitrophenyl acetate (p-NPA), can be illustrated as follows: whereas the rate of pNPA hydrolysis increases both at high and low concentrations of ethanol in the ethanol-water mixture, a minimum rate of substrate decomposition is observed at 40- 60 vol.070 C2H50H [26]. In aqueous media bifunctional (partially quarternized) polybases containing simultaneously sulfur and nitrogen atoms enhance the hydrolysis of p-nitrophenyl esters at neutral pH. The effectiveness of the polymer catalysts as high as several thousand times with respect to polymer-free solutions buffered at the same pH. In all cases the globular state of polymer catalysts showed outstanding catalytic activtities which were tentatively attributed to the distinct medium formed in water by the core of the globules. It was concluded that the core of the globules acts as a superbasic medium for catalytic reaction [26a]. Compression of the polymer chain at low and high ethanol concentrations is attributed to an increase in hydrophobic interaction as well as to the formation of hydrogen bonds between imidazole groups. A decrease in PVI activity at intermediate solvent concentrations is caused by a decrease in the bifunctional action of imidazole groups owing to expansion. Hydrophobic forces represent one of the most significant kinds of force binding a substrate with a catalyst [27]. Let us consider two cases in which (1) a polymeric catalyst contains hydrophobic binding sites (polysoaps) besides active nucleophyllic centers, and (2) a substrate has long hydrophobic sites. In the former case, an increase in the hydrolysis rate is observed for long-chained esters [28]. Such specificity is at-

1.1 Hydrolysis of Esters

3

-4

2

'"

4

6

Fig. 1.

Dependence of the decarboxylation rate on the average number of carbon atoms [l]

tributed to the structural conformity of binding sites to a long-chained substrate. The catalytic hydrolysis of esters proceeds at a higher rate in the so-called hydrophobic domains. When a long-chain substrate, e.g. 5-nitro-4-dodecanoylbenzoicacid, is employed, the hydrolysis rate in the presence of PVI is several orders of magnitude higher than by the hydrolysis of p-NPA [26, 291. The literature offers many examples of catalysis by water-soluble polymers possessing long alkyl chains. To establish a relationship between the structure of water-soluble polymers and their catalytic effianion by polyvinylciency, decarboxylation of the 6-nitrobenzoxazole-3-carboxylate pyridines quaternized by octyl (3 - 15%), dodecyl (22-33%), octadodecyl (3- 15%) and docosyl (3 - 10%) groups was studied. An increase in the decarboxylation rate is caused by formation of hydrophobic domains resulting from side chain aggregation. One proposal was using the average length of side chains as a measure of the relative contribution of side alkyl groups to the hydrophobicity of polymers. The transition from a polyelectrolyte to polymer micelles is observed at nL = 40 (where n is the number of links in an alkyl side chain). A detailed study of decarboxylation reaction and of the ratio between the polysoap structure and the rate of hydrolysis of hydrophobic esters showed the reaction rate to correlate well with the change of hydrophobicity of the microscopic environment (Fig. 1). For quaternary ammonium salts of poly-4-vinylpyridines (P4VP) containing dodecyl and ethyl groups, the rate increases markedly starting from a 15% content of dodecyl substituents. This increase corresponds precisely with the transition from a polyelectrolyte to polymeric micelles [ l , 341. Furthermore, during the alkaline hydrolysis of N-nitrophenyl-3-indole acetate and N-(indole-3-acryloyl)imidazolein the presence of poly4-vinyl-N-propylpyridinium chloride (PVPPCI), poly-4-vinyl-N-benzylpyridinium chloride (PVBPC1) and a copolymer (CPL) of 4-vinyl-N-benzylpyridiniumchloride with 4-vinyl-N-cetylpyridinium bromide, the reaction decelerates in the following sequence: CPL > PVBPCl> PVPPCl [35]. Association constants of substrates with

4

Chapter I: Catalytic Properties of Polymers in Solutions

polyelectrolytes found by the kinetic method increase in the order: PVBPCl< PVPPCl< CPL, which indicates the substantial contribution of charge transfer interaction (besides electrostatic and hydrophobic interaction) to the association. The activity of poly-3-N-hexadecyl-1-vinylimidazolium iodide in respect of p-nitrophenyl laurate is somewhat higher compared with p-NPA. Moreover, an increase in the polyionic hydrophobicity causes an increase not only in the constant of polyelectrolyte complexing with the substrate, but also in the hydrolysis rate of esters [27]. The block copolymer of dodecane and a graft copolymer of polyethylene imine (PEI) with 4(5)methylimidazole effectively catalyzes the hydrolysis of activated phenyl esters in aqueous solutions at 26 "C, pH 6 - 9.1, and ,u = 0.02 [36]. Especially in the region of high pH, polymeric catalysts containing isolated nonpolar blocks exhibit increased activity by the hydrolysis of p-NPA and p-nitrophenyl butyrate as opposed to the graft copolymer catalyst, PEI and 4(5)-methylimidazole [37]. The catalytic behavior of the block-copolymer is influenced by non-polar binding of the substrate inside a non-polar polymer fragment. In the hydrolysis of longchain p-nitrophenyl esters (S4- 12), one observes a 20- 100-fold increase in the rate constant of the second-order reaction, which is indicative of the considerable hydrophobic interaction. The contribution of a non-polar block to hydrophobic binding is absent during the hydrolysis of p-nitrophenyl caproate and N-nitrophenyl laurate. In both cases, hydrophobic interaction is decisively influenced by the intermediate formation of a substrate-imidazole complex but not by the non-polar block in the catalyst macromolecule. The character of the dependence of the p-nitrophenyl ester hydrolysis rate on the pH of the medium in the presense of different catalysts implies the absence of any cooperative interaction between imidazole or amine units. Polyethyleneimine (PEI) serves as a convenient model of biologically active polymers because it contains primary, secondary and tertiary atoms of nitrogen in the chain which are capable of undergoing different chemical reactions. Catalytic properties of PEI during hydrolytic reactions of substrates are enhanced by its acylation with fatty acids [38] or alkylation with alkyl halides of different length [39]. The denaturation process caused by a change of environmental conditions (temperature, pH, etc.) is a characteristic property of alkylated PEI which is analogous to protein denaturation. By changing the hydrophobic-hydrophilic balance, one can adjust the PEI conformation and thereby its catalytic properties. The conformation of alkylated PEI greatly depends on the alkyl radical chain [40]. For instance, sedimentation coefficients and intrinsic viscosity values show that linear PEI with C12-Cf8, alkyl substituents forms closepacked globular structures in water similar to globular protein structures [41]. Longchained alkyl groups in the polymer chain provide high local density of nucleophylic groups and create favorable conditions for binding low-molecular weight substances [42]. Thus, by adding aliphatic radicals of different length to PEI macromolecules, one can vary the ability of a polymer to bind different substrates. The kinetics of the hydrolysis of nitrophenyl esters of fatty acids, ranging from acetic to caprylic acid, catalyzed by linear alkyl-substituted PEI, were studied earlier [43]. Butyl (PEI-C,), heptyl (PEI-C,), dodecyl (PEI-C12) and hexadecyl (PEI-C16) groups were chosen as substituents. Examination of the relationship between the binding constant of a substrate and the number of -CH2 units in a polymer group

1.1 Hydrolysis of Esters

5

showed this relationship to be linear, straight lines for different esters intersecting the abscissa axis at the n = 2 point. This indicates that at n = 2, the constant of substrate binding by a polymer is zero. The very high reactivity of alkyl-substituted PEI can be explained by substrate binding occurring at the expense of hydrophobic interactions and by the decrease in pK [44]. Substrate binding involving a change in macromolecular structure cannot be realized when the hydrophobicity of groups in the interacting polymer and substrate fails to compensate for interaction the loss in conformation entropy. The formation of a hydrophobic environment of active centers reduces their pK by several units. One finds that the dissociation constants of amino groups of high-molecular weight catalysts containing alkyl radicals of different length gradually decreases according to the following sequence: PEI, PEI-C7, PEI-C,6. The same sequence is observed for the change of the ester hydrolysis rate as a function of medium pH. Such a relationship between the acid-base properties of amino groups and the ester hydrolysis rate was recognized earlier [3 1,441. Apparently, the hydrophobic interaction between hydrocarbon groups reduces the degree of hydration of amino groups inside a macromolecular micelle. The hydrolysis of p-nitrophenyl esters in the presence of alkylated polyallylamines was studied elsewhere [45]. The catalytic activity of polyallylamine during N alkylation increases with the frequency the longer alkyl chain occurs in a particular polymer. N octylated polymer exhibits maximum activity in the hydrolysis of p-nitrophenyl hexanoate. The structure-reactivity relationship for polyamine derivatives in activated ester hydrolysis was previously established [46]. Polyvinylamine (PVA), linear (LPEI) and branched (41 Vo branching) polyethylene imine (BPEI) as well as their dodecyl- and imidazole-substituted derivatives with an approximate and equal degree of substitution (1 6-20070) were applied as catalysts. The compounds p-NPA and 4-acetoxy-3-nitrobenzoic acid (ANBA) as well as some of their homologues were used as substrates. At an excessive catalyst concentration relative to the substrate concentration, reactions proceeded at pseudo first order. In each series of polymers, the reaction rate constant was increased considerably by substitution of dodecyl (hydrophobic site) by imidazolyl (catalytic center) and when a charged substrate (electrostatic effect) was employed. At an equal degree of substitution, the catalytic activity increased in the following order: LPEI < PVA < BPEI. By applying the Cosover-Mike method it was found that the polarities of the LPEI and BPEI microenvironments are almost identical. Substitution of ethyl or 2-diethylaminoethyl side groups in LPEI (40% substitution) slightly changes the catalytic activity of LPEI. This implies that primary amino groups in BPEI side chains are responsible for high catalytic activity as opposed to that of LPEI and PVA. At substrate concentrations exceeding those of the catalyst, reactions proceed by the Michaelis-Menten mechanism. Comparison of complexing and hydrolysis rate constants of the systems under study is thus possible. In the case of BPEI, AH’ = 6.9 J/mol enthalpy and AS’ = - 196 J/mole entropy of activation were determined for p-NPA hydrolysis over a 293 - 323 K range of temperatures. Besides, the low enthalpy and entropy values of complexing indicate weak hydrophobic effects.

Chapter 1: Catalytic Properties of Polymers in Solutions

6

In order to investigate the effect that unneutralized atactic poly(methacry1ic acid) (at-PMAA) has on the rate of a neutral hydrolysis reaction, several acyl-activated esters (1) and a series of l-acyl-l,2,4-triazols (2) were chosen as subtrates [47]: H

R = H or alkyl

R , = alkyl or aryl R,= H or aryl

(1I

(21

The kinetic probes 1 and 2 were selected because the hydrolysis of substrates is characterized by pH-independent rates between pH = 2-6. This is also the pH region in which at-PMAA undergoes a compact-coil to random-coil transition. By changing the hydrophobicity of substrates through variation of R , R , and R2 in 1 and 2, it is also possible to elucidate the role of hydrophobic microdomains inside PMAA. It is well-known that at-PMMA maintains a compact-coil conformation below pH = 4.5 and an extended-coil conformation above pH = 5.5. The conformational transition takes place in the intermediate pH region. Thus, the kinetic effects will reflect the influence of the compact coil of PMAA. Changes in the pseudo-first-order rate constants (kobsd)for the neutral hydrolysis of p-methoxyphenyl dichloroacetate (1 a) andp-methoxyphenyl2,2-dichloropropionate(1 b) at 298 K as a function of the at-PMAA concentration are shown in Fig. 2. For both reactions, considerable rate reductions are observed with increasing concentrations of at-PMAA effected by hydrophobic binding of the substrates to the hydrophobic microdomain within the compact-coil conformation of unneutralized at-PMAA. It is interesting to follow the effect of the stereoregularity of PMAA on the rate of hydrolysis of 1 a (Fig. 3): a more pronounced hydrolytic rate in the presence of syn-

I v)

\

300

0

n v)

To

200

Ln

0 7

100

0

1

2 [at-PMAA] /(g-dl-’)

Fig. 2 . Pseudo-first-order rate constants (kobsd)for the neutral hydrolysis of 1 a ( 1 ) and 1 b ( 2 ) in aqueous solutions of at-PMAA at 298 K [47]

1.1 Hydrolysis of Esters

7

300

2 3

200

100

4

Fig. 3. Pseudo-first-order rate constants (kobsd)for the neutral hydrolysis of 1a in water in the presence of PVP and isobutyric acid ( l ) , methacrylic acid (2), PAA (3), at-PMAA (4) and st-PMAA ( 5 ) at 298K and pH of cu. 3 [47]

diotactic PMAA (st-PMAA), as compared with at-PMAA, PAA (polyacrylic acid) and PVP (polyvinylpyridines), can be explained by the fact that the syndiotactic sequencing of the methyl side-chains renders PMAA in a more compact conformation. This accounts for an increased hydrophobic binding of the substrate to the hydrophobic microdomains. Kinetic studies of atactic PAA and PMAA as well as of acrylic acid - methacrylic acid copolymers (AA-MAA) containing 34 (CP-I), 54 (CP-2), 75 (CP-3) and 88 (CP-4) mole percent of MAA, respectively, were carried out during the hydrolysis of 1 -benzoyl-3-phenyl- 1,2,4-triazole. The rates decrease when the MAA content of the copolymers increases. It is likely that by incrementing the MAA content in the copolymer, hydrophobic microdomains are formed, and the copolymer behaves like PMAA. The influence of protein denaturants (such as alcohols, urea and urea derivatives, and quaternary ammonium salts) on the stability of the compact-coil conformation of PMAA is also described. These protein denaturants, when added to aqueous solutions of PMAA, considerably limit the rate of the neutral hydrolysis of l-benzoyl-

3-phenyl-l,2,4-triazole. It would be more useful to use different combinations of functional groups of polymers as efficient catalysts [48]. For instance, copolymers of vinylimidazole and vinylphenol (VI-VP), and of vinylimidazole and vinyl alcohol (VI-VA) are more effective than imidazole and PVI for the decomposition of neutral, positively or negatively charged esters [23, 49, 501. Apparently the increase in VI-VP copolymer activity during p-NPA hydrolysis is attributed to the bifunctional action of imidazole groups and phenolate ions. The mechanism of catalytic action was unambiguously established for water-soluble polymers containing hydroxamate and vinylimidazole groups [5 11. Different catalytic properties are exhibited by 1-vinyl-3-methylimidazolium iodide-vinyl iodide-vinylphenol copolymers with respect alcohol and 1 -vinyl-3-methylimidazolium to different esters [52]. Copolymers containing quaternized imidazoline groups effectively catalyze the hydrolysis of negatively charged esters of 4-acetoxy-3-nitrobenzoic acid (ANBA) and 4-acetoxy-3-nitrobenzosulfonate (ANBS). The catalytic action of

8

Chapter I: Catalytic Properties of Polymers in Solutions

vinylimidazole-vinylpyrrolidone [53] and vinylimidazole-vinylpyrrolidone-acrylamide copolymers [54] was observed during ester hydrolysis. Phenylimidazole-vinylpyrrolidone (PI-VP), phenylimidazole-acrylic acid (PI-AA) and phenylimidazole-methacrylic acid (PI-MAA) copolymers were also used as polymeric catalysts [55]. The analysis of kinetic data shows that extension of acyl groups in a substrate molecule by seven methylene units leads to an 100-fold increase in the substrate-binding ability of PI-VP, 50 fold for that of PI-MAA, and 4-12 fold for that of PI-AA copolymers. These results agree well with the following sequence of the change of copolymer hydrophobicity: VP > MAA > AA. The alternating copolymer of vinylimidazole and maleic acid, VI-MA, and the statistical copolymer of vinylimidazole and acrylic acid, VIAA, with an almost equal ratio of carboxyl and imidazole groups, catalyze the hydrolysis of 3-acetoxy-N-trimethylanilinium iodide (ATMAI), ANBA and p-NPA [56]. Depending on the type of substrate, the catalytic activity of the alternating copolymer decreases in the sequence: ANBA >p-NPA > ATMAI, whereas that of the statistical polymer decreases in the order: ATMAI >p-NPA >ANBA. The catalytic activity of these copolymers is attributed to the difference in pK of the COOH groups. Catalytic centers in VI-MA are represented by the respective free imidazole groups, and the reaction rate is augmented by the action of carboxylate anions. With regard to VI-AA, it is controlled mainly by the electrostatic interaction between the substrate and carboxylate anions. By critical analysis of ATMAI, p-NPA and ANBA hydrolysis in the presence of PVI and PAA under identical conditions, it was found that the catalytic activity of PVI decreases in the sequence: ANBA >p-NPA > ATMAI, analogous to the catalytic activity of VI-MA. PAA, however, did not catalyze the reaction. The kinetics of 2,4-dinitrophenyl-acetate hydrolysis catalyzed by polymers containing imidazole, carboxylic acid, oxidation groups and their complexes with surfactants, such as 1-cetylpyridinium chloride and cetylundecyldimethylammonium bromide, was determined by spectrophotometry [57]. Catalytic rate constants of the secondorder-rate increase with a rise in the surfactant concentration until they reach a plateau at a polymer/surfactant ratio of 1 : 6. Anionic surfactant does not accelerate the polymer-catalyzed hydrolysis. The catalytic mechanism of a polymer/surfactant complex enables the penetration of the substrate into a pseudophase of a soluble complex. This leads to an increase of the ester concentration in the neighbourhood of a polymer imidazole fragment and accelerates the process. Such a pseudophase promotes the protonation of imidazole rings. The hydrolysis of ATMAI catalyzed by VI-AA copolymer in ethanol at pH 9.0 was studied further [58]. The strongest catalytic effect was exhibited by a copolymer containing 42-50 mole 9'0 of imidazole units. This high catalytic activity was attributed to VI-AA-VI sequences. It was, moreover, found that the electrostatic attraction of positively charged ester to carboxylate anions determines the copolymer's selectivity for ATMAI. An attempt was made to find a correlation between the catalytic efficiency and the succession of monomer units in a copolymer. Theoretical calculations showed that in the copolymer containing 42- 50 mole 9'0 of imidazole groups, these groups are just isolated from each other. After substitution of acrylic acid with vinylsulfonic acid, however, the hydrolytic activity of a copolymer markedly decreased, owing to the relative ease of sulfonic acid ionization as compared with

1.1 Hydrolysis of Esters

9

that of carboxylic groups. To determine the spatial effect of cooperative functional groups, catalytic activity of 4(5)-vinylimidazole-acrylamide and N-acrylhistamineacrylamide copolymers was compared during the hydrolysis of 1-acetylbenzotriazole. The distribution of the determined sequences showed that alternation of monomers in 4(5)-vinylimidazole-acrylamide copolymer coincides well with the curve calculated from the copolymerization constants, rl and r2. These results support the fact that the ester hydrolysis by the copolymer is realized through the short-range interaction of imidazole-imidazole pairs, and the cooperative interaction between remote groups on the chain occurs in N-acrylhistamine-acrylamidecopolymer. Shimidzu et al. [60] studied the bifunctional catalysis of imidazole and carboxyl groups and the tri-functional catalysis of imidazole, carboxyl and hydroxyl units in the esterolysis reaction of ATMAI and p-NPA. Whereas splitting the cationic ester obeys Michaelis-Menten kinetics, splitting p-NPA obeys the kinetics of second-order reaction. The ability of copolymers to bind a substrate reaches a maximum at 30 mole 070 content of imidazole groups. It was determined that carboxyl residues not only break down imidazole sequences in the chain but also reduce the nucleophilicity of a polymeric catalyst. Synthetic polyampholytes obtained by polymerization of N,N-dimethylaminoethyl methacrylate and methacrylic acid exhibit enzyme-like activity with respect to urea [61]. The copolymer catalytic activity increases at 24-68 mole Vo acid content units and decreases at 84 mole Yo. This is probably caused by the conformation change of ampholyte macromolecules as a function of the medium pH. Earlier [61], a comparison was made of the catalytic decomposition of p-NPA in the presence of the following equimolar polyampholytes: monoethanolamine vinyl ether hydrochloride and acrylic acid (MEAVEHC-AA); styrene and N,N-dimethylaminopropylmonoamide of maleic acid (St-DMAPMAMA); 2,5-dimethyl-4-vinylethynylpiperidol-4 and acrylic acid (DMVEP-AA); 2-methyl-5-vinylpyridine and acrylic acid (2M5VP-AA); and I-vinylimidazole and acrylic acid (IVI-AA). The catalytic activity of the copolymers decreased in the following order: MEAVEHC-AA > St-DMAPMAMA > DMVEP-AA > 2M5VP-AA > 1VI-AA. The same order was observed for the change in basicity of amino groups. Furthermore, the p-NPA decomposition rate greatly depended on the 1-VI-AA copolymer composition (Fig. 4). A maximum value of k,,, corresponded to the equimolar composition; this is in good agreement with previously obtained data [58, 591. For the polyampholytic copolymer series 1VI-AA, 1 VI-MAA and 1 VI-MA, respectively, the k,,, values were found to be 4.4; 0.6; and -2.2 rnole-'.min-'. Apparently, this was attributed to differences in the dissociation groups [56]. Catalytic properties of polypropyleneglycine (PPG), polyethylenalanine (PEA), polystyrene-N-(2-~arboxybutylarnine)(PBA) were studied by the hydrolysis of p-NPA and ANBA at different medium p H values (Table 1). Compared with low-molecular weight model compounds such as ethylenediamine acetic acid (EDAA), aminobutyric acid and pentylamine, the hydrolysis rate is somewhat higher in the presence of polyampholytes due to the high local density of the macromolecular functional groups. Here it is also possible to follow the effect of hydrophobicity of amphoteric macromolecules on the rate of the process: PBA exhibits a stronger catalyzing effect than the other test catalysts.

Chapter I: Catalytic Properties of Polymers in Solutions

10

0

40

80 mol-%

I-VI

Fig. 4. Influence of composition of 1VI-AA copolymer on the rate constant of p-NPA hydrolymol/l; [lVI-AA] = 5 . 10-4mol/l; T = 298 K [62] sis; @-NPA]= 5 .

Table 1. Solvolysis of substrates by polyampholytes (k,,,, l/mole.min) PH

8.09 9.08 10.58

PEA

PPG

PNPA

ANBA

PNPA

ANBA

PBA PNPA

0.28 1 .I6 47.00

0.70 1.67 50.00

0.74 2.57 52.00

1.88 3.87 70.00

27.8 52.0 866.0

EDAA PNPA

ANBA

-

19.0

0.05 16.00

1.2 Catalysis by Water-Soluble Polymer-Metal Complexes Coordination compounds of different metal ions with macromolecular ligands are particularly interesting because iron, copper, cobalt, zinc, and other ions play an important role in enzyme reactions [64-671. The most likely function of metal ions in enzymatic reactions is to capture and move a substrate closer to an active catalyst center through formation of a ternary catalyst-metal ion-substrate complex [68]. Homogeneous polymer-metal complexes show especially high activity and specificity. Catalytic processes in solution have been examined with conventional physicochemical methods leading to an understanding of their working mechanisms. This, in turn, has made it possible to use a number of discovered regularities for better understanding the mechanisms of heterogeneous catalysis. It has been shown [69] that the action of biocatalysts can be modeled by using synthetic polymers through preparation of ternary polycarboxylate-metal ion-low-molecular weight ester complexes:

1.2 Catalysis by Water-Soluble Polymer-Metal Complexes

I

11

COO-

The metal ion effects the formation of the ester-metal coordination bond with the subsequent nucleophilic attack of a carbonyl group by an acetate ion. The rate acceleration of poly(acry1ic acid) (PAA) and poly(methacry1ic acid) (PMAA) complexes with C u 2 + , Ni2+, Co2+ and Zn2+ ions occurring in the reaction of 2,4-dinitrophenylisonicotinate is considerably higher than that of a monomeric complex. The order of the rate of increase of ester decomposition follows the sequence: Cu2+ >Ni2+ >Cu2+ > Z n 2 + , and it correlates well with rate constants of these metals complexing with polyacids. The catalytic activity of copper(I1) ion complexes with polyvinylpyridines during oxidation of ascorbic acid runs similarly [70,71]. The analysis of kinetic curves of ascorbic acid oxidation in the presence of catalysts (copper(I1) ions, copper-polymer complexes, copper-low-molecular ligand) shows that a polymeric additive augments the catalytic activity of copper ions greatly. Polymers, depending on their particular character, increase the effectiveness of copper ions by 200- 1500 fold. However, the activity of polymer-metal complexes is 300 times lower than that of enzyme-ascorbatoxidase complexes. Let us consider some examples of the catalytic action of water-soluble polymer complexes with transition metal ions during reactions of hydrogen peroxide such as decomposition, oxidation, hydrogenation, etc. Hydrogen peroxide decomposition aptly illustrates the enzyme-like action of polymer-metal complexes. Such complexes often exhibit higher activity than low-molecular weight analogues. For instance, poly-4-vinylpyridine (P4VP) complexes with Co-dimethylglyoxime can serve as a model of vitamin B I 2 [ 7 2 ] : -CH2- C y -

ICOX IDHI. PLVP 1 H3C- C =N

0

. . *

H - 0

where X is CI or CN -, and DH is dimethylglyoxime. The catalytic activity of monomeric and polymeric complexes decreases in the > CoCN(DH))2P4VP > CoCN(DH)2 V C~CI(DH)~pyridine P order: C O C I ( D H ) ~ P ~ > pyridine, and depends on the nature of the X-ligand.

Chapter 1: Catalytic Properties of Polymers in Solutions

12

Copolymer complexes of mono- and dialkyl esters of itaconic acid modified by ethylene imines, with CoC12 and CuC12, have been obtained with the following structure [73,74]. 0

O\\

I I

I CH2

CH2

I

\H,C/

/OCH3

C

$.\CH2411

C

C

oH

\OCH,

od

\ONHCH~H, I.

2cr

The reactivity of polymeric catalysts has been compared with that of cobalt(I1) trans-dichloro-bis-ethylenediamine. At a fixed cobalt concentration the rate of H202 decomposition was found to be considerably higher for polymer-supported catalysts.

0

0

0

0

H

-C

I 0

0

I

1.2 Catalysis by Water-Soluble Polymer-Metal Complexes

3

13

7

--

Fig. 5 . Influence of addition of mixed complexes of Cu(II), Fe(II1) and polyvinyl alcohol (PVA) on the rate of hydrogen peroxide decomposition; [complex] = 2 . 1 0 - ~mol/I; [H202]= 4.10-* mol/l; pH = 10.5; T = 298 K. (1) - complex; PVA-Fe(II1); (2) - mixed complex; (3) - rate difference for 2 and 1 [2]

\

> 4 I

0 c

Ir

0 10'. [Fe3+]

12

/ [ PVA]

In addition, Fe3+ complexes with low- and high-molecular weight amines (ethylenediamine, triethylenetetramine, butylamine, poly(ethy1eneimine)) were tested as catalase models [75]. The examined polymer-metal catalysts were nearly as effective as the catalase models. Pshezhetskii et al. [76] studied the mechanism of hydrogen peroxide decomposition by PAA-Fe3+ complex in the presence of diethylenetriamine as a cofactor (see the lower scheme on p. 12). Another research project, development of a mixed complex which can be applied as a catalyst of H 2 0 2 decomposition, was synthesized through simultaneous incorporation of Fe3' and Cu2+ ions into PVA polymer chains [74]. This complex is much more effective than a PVA-Fez+ complex (Fig. 5). Considering that the PVACu2+ complex has a planar structure and that PVA-Fe3+ has an octahedral one, one can assume that the mixed complex has a distorted structure. It is particularly the stability of this complex that provides the required fixed conformation of polymer chains for catalysis. In addition, gasometry was employed to study the kinetics of alkaline H202 decomposition with polymeric cobalt phthalocyanine at 25"-75 "C [78]. At low H202 concentrations this catalytic process can be described by the equation for firstorder reactions. The activation energy of this reaction at different catalyst concentrations on the support varies from 54.1 -64.9 kJ/mole. Upon prolonged exposure of the catalyst to an electrolyte in the presence of oxygen, the former is deactivated by oxidation. Furthermore, the catalase activity of complex compounds of polyacryloyllupinine (PAL) with Co2+ and Fe3+ ions was studied during H 2 0 2 decomposition [79]. The rate of substrate decomposition in the presence of complexes was found to depend on the pH of the medium and reach a maximum over the range 6.0-7.2, i.e. in the region where functional groups involved in complexing are deprotonized. Complexes of a series of polyampholytes with transition metal ions (Cu2+, Co2+, Mn2+, Ag') were used as catalysts of H 2 0 2 decomposition [62, 80, 811. Fig.6

Chapter I: Catalytic Properties of Polymers in Solutions

14

I

I

I

I

40

0

80 t/min

Fig. 6. Kinetic curves of hydrogen peroxide decomposition in the presence of complexes: [lVIAA]/[Mt"'] = 1: 1; (1) - Mn(I1); (2) - [Ag(I)]; (3) - Co(I1); (4)- Cu(I1); [Cu(II)] = 5.10-5, [Co(II)] = 1* w4,[Mn(II)] = Ag(1) = 5. 10-4mol/l; [H202]= 5. mol/l; pH 8.5; b = 0.1; T = 298 K [84]

0

G

8

12

16

[PA1/[Mtn+]

Fig. 7. Influence of the polyampholyte/metal ratio on the starting rate of H,O, decomposition in the presence of complexes: St-DMAPMAMA/Cu(II) (I), 2MSVP-AA/Co(II) (2), IVIAA/Cu(II) (3); 2MSVP-AA/Cu(II) (4). [Cu(II)] = 5-10-5, [Co(II)] = 1.10-4, [HZ02]= 5. mol/l; pH 8.5; p = 0.1; T = 298 K [85]

1.2 Catalysis by Water-Soluble Polymer-Metal Complexes

15

presents the dependence of the degree of H 2 0 2 decomposition on reaction time in the presence of complexes of equimolar polyampholyte based on 1-vinylimidazole and acrylic acid (1 VI-AA) with several transition metal ions. In the absence of metal ions the polyampholyte itself does not exhibit the catalytic effect but in the presence of metal ions the polyampholyte possesses higher catalytic activity than respective low-molecular-weight aquo- and hydroxocomplexes. The activity of complexes of IVI-AA/Mt"+ (where Mt"' are metal ions) increases in the order Cu2+ >Co2+> M n 2 + > A g + . It was also shown that the stability of lVI-AA/Mt"+ complexes changes in the same order [81]. From the ratio of polyampholyte and metal ion concentrations at which a maximum reaction rate is observed one can ascertain the composition of catalytically active complexes [82]. The effect of [polyampholyte]/[metal ion] composition on Vo at a constant metal ion concentration for several polyampholyte-metal systems is illustrated in Fig. 7. For a majority of the systems the ratio is not higher than 3. These results support the contention that catalytic activity can occur only in the presence of free sites in the coordination sphere of a metal ion (831. An exception to this is the styrene-N,N-dimethylaminopropylmonoamideof maleic acid/copper(II) complex for which a maximum rate of H 2 0 2 decomposition was found at [polyampholyte]/ [Cu2+]= 16: 1 and pH = 8.5. Because the isoelectric state of the polyampholyte is attained at pH 6.4 it is unlikely that the compression of the macromolecule coil has affected the complex composition. Apparently it is the presence of hydrophobic styrene units in the copolymer that affects the reaction rate. The effect of the initial substrate concentration on the initial reaction rate of mol/l range. -2hydrogen peroxide decomposition was studied over 1 * For Cu2+ complexes in the Vo-lg [Sl0 coordinates, the curve has the shape of an asymmetrical bell (Fig. 8, curve 1). This is characteristic of reactions involving enzymes whenever intermediate enzyme-substrate complexes of ES and ES2-type form in the following reaction scheme:

E+S

KS

ES

k,,,

E+P

wherein E denotes a catalyst; S, a substrate; and P, a reaction product. Here the activity of the ternary complex, ES2, is lower than that of the double complex, ES: O
The values of kinetic parameters evaluated from this equation are summarized in Table 2.

16

Chapter I: Catalytic Properties of Polymers in Solutions

- 2,2

-1,4

19[H202], Fig. 8. Dependence of starting rate of hydrogen peroxide decomposition on the substrate concentration in the presence of complexes: 1VI-AA/Cu(II) (1) and 2MSVP-AA/Co(II) (2) at polyampholyte/metal molar ratio of 1 : 1; [Cu(II)] = 5.10-5, [Co(II)] = 1 . 10-4mol/l; pH 8.5; T = 298 K [62]

Table 2. Kinetic parameters of polyampholyte-Cu(I1) complexes in the hydrogen peroxide decomposition reaction ([PA]/[Cu2+]= 1 : 1, pH 8.5) Copolymer

k,,,, min-'

Ks*10-3, mol/l

KI.lO-*, mol/l

p

E,,,, kJ/mol

St-DMAPMAMA 1 VI-AA 2M5VP-AA MEAVEHC-MAA

50.0 18.2 8.9 4.4

2.36 2.10 2.87 3.05

2.0 1.43 1.11 1.05

0.06 0.12 0.18 0.17

21.77 24.28 46.89 36.84

The dependence of Vo on [H202] for PA/Co(II) complexes is linear (Fig. 8, curve 2). Kinetic curves, [H2O2Ir- 7 are described by the first-order equation. Rate constants found at: pH 8.5; T = 298 K [PA]/[Co(II)] = 1 : 1; [PA] = 1 * mole/l, assume the following values: 4.75. 4.37. 2.31 * 2.19. for 2MSVP-AA, MEAVEHC-MAA, 1VI-AA, St-DMAPMAMA respectively. An increase in the pH of the medium leads to an increase in the rate of hydrogen peroxide decomposition (Fig. 9). The effect that the pH has on the reaction rate is attributed to several processes occurring in the catalytic system. On the one hand, the change in the concentration of catalytically active centres occurs through the rear-

1.2 Catalysis by Water-SolublePolymer-Metal Complexes

17

w

+ .-

)r

v)

c

a,

+

4

.C

a,

> ._

0 3

S

7

9

11

PH Fig.9. Dependence of the relative intensity of ESR signals (1) and starting rate of H202 decomposition (2) on pH of medium for 2MSVP-AA/Cu complexes [84]

rangement of [PA]/[Mt"+] centers and, on the other hand, through the dissociation of hydrogen peroxide molecules (pK = 11.6) [81]. At p H > 8 , the involvement of - 0 O H peroxo-anions in the reaction can be observed. The three complexes CU(COO)~,CU(COO)~(N~) and C U ~ ( O H ) ~ ( N have ) ~ been shown to be present in the 2MSVP-AA/Cu(II) system [84, 851. The first complex forms in the region of 4 < pH < 6, the second at 6 < p H < 10, and the third at p H 2 10. In the acidic region, the concentration of active peroxoanions is low. The complex with CU(COO)~does, therefore, not markedly affect H202 decomposition. The second complex exhibits low catalytic activity because all the coordination sites with a Cu(I1) ion are occupied by macromolecular ligands. Only a binuclear complex, which in the absence of peroxoanions is stabilized by hydroxyl anions, is capable of showing high catalytic activity. The involvement of this complex in hydrogen peroxide decomposition has been confirmed by the change of the intensity of the EPR signal displayand the ing the concentration of the catalytically inactive complex, CU(COO)~(N)~, initial reaction rate as a function of medium pH. The high catalytic activity of binuclear-type complexes caused by the high electron-acceptor activity of Cu2+ - Cu2+ pairs, has been observed for copper-containing enzymes [86] and some polymer-metal complexes [87]. When hydrogen peroxide is added to a solution of PA-Cu" and PA-Co2+ systems, the UV spectra of the solutions demonstrate wide absorption bands with I,,, = 360 and 380 nm, respectively. This implies the formation of peroxocomplexes in the systems [83]. With a rise in pH, the extinction increases at the given wavelengths. The direct relationship between the reaction rate and the concentration is indicative of a concentration dependence in the reaction. The mechanism of catalysis of Hz02 decomposition is presented by the following scheme:

Chapter I: Catalytic Properties of Polymers in Solutions

18

H202 P H + +HOOP ,

(1)

Cu(II)L,+OOH- P CU(II)L~(OOH)-,

(2)

CU(I)L~(OOH)~,

(3)

Cu(II)L2(OOH)-

-+

(4)

CU(I)L~(OOH)~+OOH+Cu(II)L2(OH)~- + 0 2

At [ H 2 0 2 ] o ~ K ;simultaneous , addition of two peroxoanions is possible at the second stage and not at stage (4). For the 2MSVP-AA-Cu(II) system, the role of the active center is fulfilled by the binuclear complex, Cu2(0H),(N),. In the presence of PA-Co(I1) complexes the reaction mechanism is apparently similar to that presented above. The catalytic properties of a Cu2+ complex with PVA (P = 1750) in an aqueous solution at 20 OC have been studied by means of spectrophotometry during the hydrolysis of 2,4-dinitrophenyl acetate (DNPA) to 2,4-dinitrophenol [88]. The mechanism proposed involved the activation of OH-group to form 2,4-dinitrophenolate ions, whereby the reaction is promoted by the formation of a ternary substrate-polymer-metal complex. Moreover, polymeric amine complexes with Cu2+ have been used as catalysts of the hydrolysis of organophosphorus esters [89]. The effect of structural and conformational ordering of macromolecular ligands on the properties of polymeric complexes of transition metals used as catalysts of various reactions has also been studied [90]. Furthermore, hydrolysis and oxidation reactions catalyzed by water-soluble copper complexes with poly-a-aminoacids have been analyzed. In addition, silver ions have been found to exhibit an accelerating effect altering the length of an alkyl group of a substrate [91]. This suggests the occurrence of hydrophobic interactions between the substrate and the PAA-Ag complex in the system. A marked increase in the hydrolysis rate at [Ag+]=0.02-0.03 mole/l corresponds to the change in the polymer chain conformation. The rate of p-nitrophenyl caproate hydrolysis by modified PEI, containing imidazole groups in the presence of metal ions, increased in the sequence Mn2+, Ni2+, Zn2+, Co2+, Cu2+ [92]. Among the selected metals, a copper complex was shown to be most effective, the maximum rate being achieved at a [imidazole]/[copper] ratio of 2.75. The catalysis of P4VP-Cu2+ complexes during the oxidation of such substrates as 3,4-dioxycinnamic, salicylic and ascorbic acids and substituted phenols has been discussed [93]. The rate of oxidation of the acids depends on the pH of the medium, its maximum being attained at pH2.5, 3.0 and 4.2, respectively. A maximum is reached at these pH values because at these values, the protonation of uncomplexed nitrogen atoms occurs. Moreover, the electrostatic interaction between the polyelectrolyte and the anionic substrate raises the nitrogen concentration near the active center. In the case of a positively charged substrate such as paraphenylene diamine, however, the oxidation rate sharply drops upon an increase in the solution acidity due to the electrostatic repulsion of the like-charged polyelectrolyte and substrate. +

1.2 Catalysis by Water-Soluble Polymer-Metal Complexes

19

An increase in the rate of oxidation of hydroquinone and pyrocatechol by the action of Cu(II), Co(II), Fe(II), and Mn(I1) metal complexes with PEI has been observed [94]. The rate of quinone accumulation depends on the type of metal ion entering the coordination sphere and increases in the following order: Cu(I1) > Co(I1) > Fe(I1) > Mn(I1). Furthermore, the oxidation of hydroquinone catalyzed by Cu(I1) complexes with PVI, PEI and vinyl-amine-vinyl acetate copolymer has been studied in an aqueous solution at 25 “C [95]. In addition, the oxidation of hydroquinone by PVI-Cu(I1) and vinylimidazole-vinyl sulfide VI-VS/Cu(II) complexes has exibited somewhat a typical behavior of copper complexes with copolymers [96]. For example, in respect of VI-VS/Cu(II) complexes, hydroquinone is oxidized quite rapidly only during the initial phase of the reaction. However, thereafter it changes only slightly. Deviation from linearity has been observed on increasing the content of vinyl sulfide units in the VI-VS copolymer. These results can be justified as follows. Initially, bivalent copper is reduced to the univalent state during hydroquinone oxidation. Then Cu(1) is stabilized by a sulfide group; i.e. an electron transfer from the substrate to Cu(I1) readily occurs, whereas a reverse process of oxidation of Cu(1) to Cu(I1) by molecular oxygen is impeded by the binding of Cu(1) by vinyl sulfide fragments of the copolymer. Other authors [97], who studied hydroquinone oxidation by polyampholyte-metal complexes, reached a similar conclusion. The catalytic activity of polymer-metal complexes has been determined from the value of the initial rate of substrate oxidation. For polyampholyte-copper (11) complexes the catalytic activity decreases in the sequence 1VI-AA > St-DMAPMAMA > MEAVEHC-MAA > 2MSVP-AA. The effect of copolymer composition on the rate of the hydroquinone oxidation reaction has been studied using IVI-AA copolymers. Curves of the change of the oxidized hydroquinone concentration as a function of reaction time in the presence of polyampholyte-copper(I1)complexes are presented in Fig. 10. An increase in the amount of basic groups in the copolymers leads to an increase in the rate of hydroquinone oxidation. Copolymers in which maleic and methacrylic acids are used as a second comonomer of 1V1, exhibit lower activity than 1VI-AA. The maximum catalytic activity for IVI-AA copolymer has been observed at a polymer: copper(I1) ratio of 17 : 1. Activation energies and effective rate constants, k,ff, have been determined from results of the measurement of the rate of quinone formation over a 288-313 K range of temperature. Thus, at pH 5.25 and [Cu”] = 1.2. lo-’ mole/l, these values are 1 1 1.4 kJ/mole and 1.32. min-’, and 79.5 kJ/mole and 8.55.10-* min-’ for low-molecular weight and polymeric catalysts respectively. Because the limiting step of the process is the oxidation from Cu(1) to Cu(II), the incorporation of polyampholytes which form more stable complexes with Cu(I1) than with Cu(1) is expected to increase the catalytic activity of the Cu(I1) ions. During the oxidative splitting of 4-methylcatechol, 4-tert-butylcatechol, and o-phenylenediamine catalyzed by styrene-4-vinylpyridine-copper(II) complexes, styrene units in the copolymer serve mainly as a stabilizer, whereas the highest process rate is achieved in benzene [98]. Respective muconates and muconitrile are products

Chapter 1: Catalytic Properties of Polymers in Solutions

20

6

-

4

-

1

2

3 4

0 0

20

10

t/min Fig. 10. Kinetic curves of hydroquinone oxidation in the presence of Cu(I1) ions (4) and 1VIAA/Cu(II) complexes (1-3) with the following proportion of 1VI units in the respective [O,]= 6.3. copolymer: 56 (l), 47 (2), 24 (3) mole Yo. [Hydroquinone] = 8. mol/l; pH 5.25; p = 0.1; T = 298 K [62] [Cu(II)] = 1.2. lo-' mol/l; [IVI-AA] = 1.2.

of the catechol and o-phenylenediamine splitting reaction. The yield of reaction products with polymeric catalysts is about three times higher than with pyridine-copper (11) catalyst complexes, while the maximum yield of muconate is observed at a [nitrogen]/[copper] ratio of 6 : 1. Poly-L-methylethyleneimine-ruthenium(II1) (P-LMEI-Ru(II1) complex is capable of catalyzing the asymmetric hydrogenation of methyl acetate in a homogeneous solution [99]. P-LMEI-Ru(II1) complex produces a higher yield of D(-) isomer than the L( +) isomer, whereas poly-DL-methylethyleneimine-ruthenium(II1) complex does not possess almost any selectivity. The main function of a multidentate ligand is to prevent the reduction of Ru(II1) to the metallic state. The application of complexes of water-soluble polymers (polyvinylpyrrolidone, polyethylene imine, polyethyleneiminodiacetic acid) with Rh2+, Pd2+, Pt4+, and Ni2+ have been cited [ 1001 during reactions involving the stereoselective hydrogenation of aromatic and heteroaromatic hydrocarbons (production of cyclohexanone from phenol, and of sugar alcohols from sugars), hydrogenation of double bonds, nitro groups, nitriles, partial hydrogenation of fats and fatty acids, and conversion of inner alkynes to cis- or trans-alkenes and of alkynes to cis-alkenes. Catalytic properties of complexes of multi-valenced metals with poly(ethy1ene glycol) (PEG) and polyurethane (PU) have been studied during liquid-phase oxidation processes such as the liquid-phase oxidation of hydrocarbons (phenanthrene, tetralin, cyclohexene), decomposition of hydroperoxides, hydrocarbons and decomposition of hydrogen peroxide [I01 - 1061. The kinetics of these reactions have been studied. The rate and selectivity of a particular reaction process depend not only on the properties

1.2 Catalysis by Water-Soluble Polymer-Metal Complexes

21

of the incorporated metal, but also on the structure and molecular weight (chain length) of the polymer. Thus the activity of complexes having the composition PEG: CoCl2 = 1 : 1 increases linearly with a rise in the molecular weight of PEG. Complexing of PEG with CoCl2 greatly reduces the intrinsic viscosity of PEG attributed to the formation of an intramolecular cycle. As illustrated below, during the catalysis of tetralin oxidation, Co2+ ions present in the intramolecular cycle (a) begin to interact with the molecular oxygen, and (b) break the ether-Co2+ bond. Then the activated O2 reacts with the substrate (tetralin molecule) to form hydroperoxide (c).

E+o-cl -c 0-

0-

I.D.0

+

0,

Cl--G

-CI

0-

I 0-

a

b

+

RH

-

0-

CI CI-CO-CI

+

ROOH

0-

C

The authors attribute these results to the contribution to the catalysis from the energy of non-equilibrium of polymer chain conformations manifested during the interaction of cobalt ions with reacting substrate molecules. When the rate of RHoxidation during reaction (b) is higher than that of the non-equilibrium conformation relaxation, its energy can be transferred to reacting substrate molecules thereby accelerating their conversion. This effect is greater the longer the polymer chain is. Therefore, energy parameters of the reaction are affected by the compression-extension energy of the polymer chain in the cyclization-decyclizationprocess depending on the molecular weight of PEG. The oxidation of tetralin in the presence of PEGCoCI, complex causes high selectivity of tetralin by hydroperoxide (up to 98%). Furthermore, the PU-CoC12 complex oxidizes tetralin mainly to tetralone-l (l-oxo1,2,3,4-tetrahydronaphthalene)and tetralone-2 (2-0x0- 1,2,3,4-tetrahydronaphthalene) the ratio of which is also determined by the chain length. The synthesis of manganese acetate tetra-p-aminophenylporphyrinate (MnAcTAPhP) chemically immobilized on different polymeric supports was carried out, and its catalytic activity during cholesterol oxidation by molecular oxygen was studied [ 1071. Soluble porphyrin-containing polymers were obtained by copolymerization of methacrylate or 4-vinylpyridine with the product of interaction of acrylic acid chloride with MnAcTAPhP. Cholesterol oxidation was performed in a 1 : 1 mixture of ethanol and chloroform. The effective rate constants, keff,were determined from the initial rate of product formation. The results showed the immobilization to cause an increase in the specific catalytic activity of MnAcTAPhP. This effect might be due to the additional binding of cholesterol molecules by hydrophobic fragments of the macromolecule chain resulting

Chapter I: Catalytic Properties of Polymers in Solutions

22

in an increase in the local concentration of the substrate near the active centers. The temperature dependence, k,ff,of cholesterol oxidation in the presence of copolymers of butyl- and methyl methacrylates is characterized by maxima over room temperature ranges. Such a monotonic behaviour of k,ff probably implies a phase change in the system under study. The kinetics and mechanism of 3,4-dioxyphenylalanine oxidation by hydrogen peroxide has been studied using two catalytic systems: hem-like ions of Fe(tetpy)(OH) (wherely “tetpy” denotes 2 3 :6‘,2”: 6”,2”’-tetrapyridil) immobilized on poly(L-glutamate) or pOly(D-glUtamate) [ 1081. The oxidation of 3,4-dioxyphenylalanine is a threestep process which includes the formation of an intermediate substrate-catalyst complex, an intramolecular electron transfer and the oxidation of both the lowest valency metal and the substrate radical by hydrogen peroxide. On the basis of the calculation of the energy of conformation changes, the model of a diastereomeric non-covalent complex with a mobile electron has been proposed. These data support the assumption that the catalyst stereospecificity depends on the mechanism of the attack on a central metal ion.

1.3 Oxidation of Disubstituted 2,6-Phenols The oxidative coupling of disubstituted 2,6-phenols catalyzed by polyamine-copper (11) complexes is somewhat similar to that of copper-containing proteins such as laccase and tyrosinase [109]. This similarity is manifested as follows: (1) binuclear com-

plexes of copper-containing proteins do not generate an EPR signal; (2) disubstituted 2,6-phenols are oxidized to diphenoquinones (similarly to laccase); (3) water is a product of oxygen reduction; the reaction kinetics are described by the Michaelis-Menten equation. At present, oxidative polymerization is the commercial means of obtaining polyesters [I 10, 1 111. Since it has been known that copper complexes of amines can catalyze the oxidative coupling of disubstituted 2,6-phenols [ 1121, many research projects have tackled the problem of elucidating the kinetics and mechanism of the given reaction [113- 1161. A significant contribution to the study of this process has been made by the pioneer research of ChaNa [117- 1221 and Tsuchida [66,67]. Depending on the reaction conditions, the oxidative coupling leads to formation of a C - 0 or C-C bond:

H

GCH3 CH3

This results in the formation of polyphenylene oxide (PPO) or diphenoquinone (4-(3,5-dimethyl-4-oxo-2,5-cyclohexadiene1-ilidene-2,6-dimethyl-2,5-cyclohexadieneI-one).

1.3 Oxidation of 2,6-Disubstituted Phenols

23

Besides disubstituted phenols, organic compounds containing mobile hydrogen atoms in terminal positions can undergo the oxidative coupling reaction [123- 1251:

-

H # J o N H z + no,

f

N

=

N

q

+ 2nHz0

The properties of active catalytic sites have been studied in detail in low-molecular weight systems such as N,N-dimethylbenzylamine/copper(II),N,N,N’,”-tetramethylethane- 1,2-diamine/copper(II) (TMED/Cu(II)) and other polymer ligands of compounds modeling monomer units. By means of IR, UV, EPR and X-ray spectroscopy methods, it has been unambiguously proved that the compounds are bonded with each other by CI- or OH- bridges [118]. Structures of some high- and low-molecular weight compounds used as ligands are presented below:

0 N

_______ p n 0 N-C -CH2-LN-CH,-CH,-NfCH2I , W I1 I CH3 ~

-C -N

cy3

L

-------

CH,-N-CHZ-CHZ-N-CH, I

I

CH3

CH3

4

r---------l

-CH2-CH2tN

-CHz-CHz-N

+CHzI

I

ILCH -3-

I

CH

- - - - - -3J

I

It appears that copper(I1) binuclear-type complexes bonded with each other by OHbridges play an important role in the oxidative coupling of substrates:

/

\

/

\

Chapter I: Catalytic Properties of Polymers in Solutions

24

-

100

1

60

0

2

1 NaOH / Cu

Fig. 11. Influence of NaOH/Cu(II) ratio on the rate of 2,6-dimethylphenol (DMP) oxidation (1) and diphenoquinone yield (DPQ) (2) in the presence of TMED-Cu(I1) complex; [CuCI2] = 0.0033 M, [DMP] = 0.06 M; T = 298 K; solvent: 1,2-dichlorobenzene/methanol(9: 1 vol/vol) [ 1201

The formation of the binuclear structure is confirmed by the following facts: IR spectroscopy detects absorption bands of OH- bridges at 3350cm-' and C u - 0 bands at 490cm-'; EPR spectroscopy shows the absence of the copper ion signal; the magnetic susceptibility value increases with temperature; and H20 is a product of O2 reduction, whereas for mononuclear copper complexes H202 is usually formed [120]. The structure of complexes considerably affects the character of oxidative coupling reactions [126, 1271. When chlorine ions act as a bridge-forming ligand, the reaction proceeds mainly towards formation of a C-C bond, whereas for OH-bridged complexes C - 0 coupling occurs. This in apparent from the dependence of the catalytic activity of TMED-Cu(I1) complex and the diphenoquinone yield on the NaOH/ Cu(I1) ratio (Fig. 11). In the copper(I1) complex with amino and chloro ligands the weak-acid substrate, 2,6-dimethylphenol (DMP), cannot replace bridge ions of chlorine, and the substrate coordinates with Cu(I1) in the free Z position. This, in its turn, leads to protonation of amino groups by DMP protons, formation of two phenol radicals and their subsequent dimerization to form diphenoquinone:

25

1.3 Oxidation of 2,6-Disubstituted Phenols

For copper(I1) hydrocomplexes, the substrate activation proceeds through the substitution of one bridge-forming 0 - ligand. Such coordination provides a twoelectron transfer from the substrate to a copper ion and formation of phenoxonium cations. Moreover, because of the coordination the original square-planar configuration, a, transforms to the tetrahedral form, b, thereby changing the bond angle:

b Thus the catalytic activity of amine complexes of copper (11) reaches a maximum after full replacement of Cl--bridged ligands by OH--bridged ones (Fig. 12). However, for copper complexes of polyamidoamines, the maximum of the rate of DMP oxidation has been achieved at a NaOH/Cu ratio of 0.5 instead of 1 as is the case for other polymer ligands [128]. The authors attribute this to the fact that mixed complexes of PAA * Cu(0H)Cl Cu * PAA-type behave as catalytically active species. e

\o

!s

i!i

Fig. 12. Dependence of starting rate (Ro)of DMP oxidation in the presence of the following complexes: styrene-N-vinylimidazole/copper (11) (1) and imidazole/copper (11) (2) on 4 NaOH/Cu(II) ratio; [CuCI2] = 3.33. mol/dm3, [DMP] = 0.06 mol/dm3, [N]/Cu(II) = 2: 1; I 2 2 NaOH/Cu 4 T = 298 L K; solvent: 1,2-dichIorobenzene/ methanol (13:2vol/vol) [118]

Chalia et ai. [ 1 15, 129, 1301 particularly examined the effect of chain length on the oxidation rate of disubstituted phenols. Consequently, several copolymers containing styrene and amine units were synthesized. By application of the following expression for the root-mean-square radius of gyration of a macromolecule,

Chapter I: Catalytic Properties of Polymers in Solutions

26

in which &In is the averaged molecular weight, [ q ]delineates the intrinsic viscosity, and po connotes the universal Flory constant, the average concentration (mole/dm3) of nitrogen atoms has been calculated with the equation below for styrene and NJV-dimethylbenzylamine copolymer containing different amounts of amino groups (a): “lcoi~=

103.7P;a

4 /-n(S33/2 .

NA

Substituting (S2)”2 into the equation (2) and using the numerical values and fin= mP,,, one finally obtains:

po = 2 . lo2‘, NA = 6.02.

wherein m is the molecular weight of a monomer unit. Concentrations of nitrogen atoms in the respective macromolecular coil of three copolymer are cited below:

a

Copolymer I I1 I11

h 1 9

0.10 0.18 0.10

1 .oo 0.17 0.17

dl/g

“lcoil

0.01 17 0.124 0.069

It can be seen that the local concentration of amino groups in the macromolecular coil is several orders of magnitude higher than the concentration of the respective lowmolecular weight analogue, dimethylbenzylamine (0.0033 mole/dm3), at the same initial concentration of high- and low-molecular weight catalysts in the solution. The catalytic activity of these polymer-metal complexes depends on the degree of functionalization, a; i.e. on the concentration of chelate-forming amino groups in the chain. The polymer chain effect has been studied in detail on molecular models depicted below (see scheme p. 27).

0

0.4

0,8 d

Fig. 13. Dependence of starting reaction rate (R,,) on the degree of functionalization ( a ) in a macromolecule ligand [I291

1.3 Oxidation of 2,ti-Disubstituted Phenols

27

1

C"

The reaction rate is maximum at a = 0.14 and 0.25 mole/dm3 when 6 and 3 inert units of styrene are present between two amino groups, respectively (Fig. 13). This means that at a given value of a, favorable coordination occurs between the amino groups of the copolymer and copper ions, on the one hand, and between copper ions and the substrate, on the other hand. As a result, the tetrahedral state proves itself more advantageous than the square-planar configuration; i.e. the sterically induced deformation of the complex favors electron transfer from the substrate to a copper ion. The authors contend [86] that during a catalytic reaction the configuration of the catalyst fluctuates, thereby accelerating the electron transfer. Furthermore upon addition of copper(I1) ions, the reduced viscosity of a partially quaternized poly4-vinylpyridine (QPVP) solution decreases from 6.23 to 0.84 dl/g, which implies the formation of intramolecular chelates. The reduction of QPVP-Cu(I1) complex to QPVP-Cu(1) causes an increase in the viscosity of the solution from 0.84 to 1.27 dl/g. This suggests a considerable increase in the macromolecular coil size during the reduction of copper ions. It should be noted that the composition of the product of oxidative coupling of 2,6-disubstituted phenols depends on the type of substrate [131]. For instance, the

Chapter I: Catalytic Properties of Polymers in Solutions

28

20

10

0,3

0,l

oc

Fig. 14. Dependence of the starting rate of oxygen absorption (R,)on the degree of functionalization a in the reaction of oxidation coupling of disubstituted 2,6-phenols: 1 - DMP; 2 - DBTP; 3 - DPP [131]

Table 3. The amount of diphenoquinone formed during the oxidation of DMP and DPP in the presence of the copper complex with partially dimethylaminomethylated polystyrene after 100% oxygen absorption Phenol

T, K

a , To

DMP

288 288 308 308

6.5 39.3 6.5 39.3

34.0 36.3 65.6 68.9

DPP

293 293 308 308

12.0 39.3 12.0 39.3

100 100 12.0 24.0

Diphenoquinone,

070

Table 4. Oxidative polymerization of DMP in the presence of copper(I1) polymer complexes Catalyst

Solvent

Cu2+, mol/l

Reaction time, rel. u.

PPO yield, wt. 070

PPO mol. wt. thou.

P4VP-Cu2+ Pyridine-Cu"

Benzene Benzene DMSO DMSO

0.005 0.005 0.01 0.01

1.5 1.5 5.5

99 88 64 96

7.1 5.8 1.6 1.5

QPVP-Cu2+

1 .o

conditions which favor C - 0 coupling for 2,6-dimethylphenol (DMP), yield C - C coupling for 2,6-diphenylphenol (DPP) and 2,6-di-tert-butylphenol (DTBP). From Fig. 14 it is evident that the reaction rate for different disubstituted phenols decreases in the sequence: DMP>DPP>J3TBP. This is attributed to steric effects. Moreover the reaction temperature and concentration of amino groups in the copolymer affects C - C and C - 0 coupling (see Table 3). The effect of the solvent on the yield and molecular weight of PPO formed is demonstrated well by the data given in Table 4.

1.3 Oxidation of 2,6-Disubstituted Phenols

29

Because polymer ligands consist of hydrophobic and hydrophilic sites, a particular domain can form around a metal ion depending on the solvent nature, thereby causing acceleration of electron transfer from substrate to the copper ion. Therefore, coils of functionalized macromolecules with surrounding solvent molecules fixed to chains at catalytic sites behave like isolated microreactors. The activity of sites inside the microreactor depends on the properties of loose low-molecular weight analogues [130, 132, 1331. The rate of DMP oxidation was also measured in the presence of mixed polymermetall-meta12 complexes [134]. Whenever an equivalent amount of the second metal is added to a P4VP-copper complex, the activity of the particular mixed polymermetal complexes changes. For instance, manganese ions promote the catalytic activity of this complex, whereas chromium and iron ions inhibit it. A mixed P4VP-Cu(II), Mn(II1) complex increases not only the reaction rate but also the yield and molecular weight of PPO. A rapid oxidation-reduction reaction as depicted below is expected in the presence of Mn ions: Cu(I)+Mn(III)

P4VP

Cu(II)+Mn(II)

As a result, DMP oxidation in the presence of the mixed catalyst involves the following cycles:

The substrate coordinates with the copper complex and thereby becomes activated. The reduced catalyst, P4VP-Cu(I) is oxidized to the original state by Mn(II1) ions. Subsequent oxidation of Mn(I1) ions by molecular oxygen, proceeds under mild conditions. Lately, researchers have been showing interest in the conversion of homogeneous catalysts to heterogeneous ones, fixing the former on organic and inorganic supports [135- 1381. This procedure makes it possible to separate the catalyst from reaction products thereby enabling multiple re-use of the catalyst. Two ways of immobilization of a homogeneous catalyst on a solid surface are possible: (1) the modification of

amorphous

.amorphous

Chapter I: Catalytic Properties of Polymers in Solutions

30

0

10

30

60 t/h

Fig. 15. Dependence of diphenoquinone yield on reaction time in the presence of [styrene-4-vinylpyridine]/[Cu(II)]/[OH] = 2 : 1 : 1; catalyst supported on Aerosil 200V; [DBTP] = 0.1 mol/dm3; T = 308 K; Po2= 101.3 kPa [130]

amorphous sites of crystalline isotactic polystyrene by chloromethylation, with subsequent amination not affecting these sites; and (2) the fixation of one end of the chain to non-porous silica gel. The supported polydentate coil effectively binds copper ions enabling such a catalyst to be used many times without an appreciable decrease in activity (Fig. 15). However, compared with the homogeneous system, the catalytic activity of the insoluble polymer-copper(I1) complex and molecular weight of the PPO formed both decrease. This is caused by steric hindrance and limited flexibility of ligands. Furthermore, the insoluble polymer matrix influences the electron-transfer step, and the activation energy of this process increases due to a decrease in the mobility of the ligand groups [139]. It has been ascertained [134- 1361 that immobilization does not alter the specific activity of catalysts. Whenever the surface coverage of silica gel by macromolecular ligands is increased, an increase is observed in the effective substrate complexing, analogous to that for a non-grafted polymer catalyst. However, the reaction kinetics and mechanism do not change upon fixation of a polyligand. This reaction is described by the Michaelis-Menten equation, and bridged dihydroxo complexes of copper(I1) are active. In addition, the oxidative polymerization of phenol derivatives is an important reaction which proceeds in vivo [134]. For example, let us consider the formation of melanin from tyrosine catalyzed by a tyrosinase-copper enzyme complex. The oxidation of tyrosinase has been described earlier [140]. It includes steps involving tyrosine oxidation to 4-(3,4-dihydroxyphenyl)-~-alanine(DOPA) and its further oxidation to DOPA-quinone, oxidative cyclization to DOPA-chromium with subsequent decarboxylation to 5,6-dihydroxyindole and, finally, oxidative coupling leading to the formation of melanin as a polymer product. Copper complexes with imidazole-containing ligands are found to possess the highest activity among metal complexes with synthetic polymers (Table 5).

31

1.4 Macromolecular Effects during Thiol Oxidation Table 5 . Formation of melanin catalyzed by polymer-copper (11) complexes Catalyst

Rate of melanin oxidation, I.mo1- . s pH7.0

Cu(I1)

pH4.0

0

0

23

0

Polyvinylimidazole-Cu(I1)

23

0.17

Vinylimidazole-vinylpyrrolidone-Cu(I1)

40

0.16

Imidazole-Cu(I1)

Polyvinylimidazole-Ni(I1)

0.04 900

DOPA (pH 4.0)

DOPA, DOPAChromium (pH 7.0)

0.04 0.04

Polyvinylimidazole-Fe(II1) Tyrosinase

Rate-determining step

DOPA DOPA, DOPAChromium

During the oxidation of DOPA by imidazole-copper(I1) complexes, a stable intermediate with A, = 480 nm attributed to DOPA-chromium is formed. Catalysis by vinylimidazole-vinylpyrrolidone-copper(I1) complexes is most related to that of t yrosinase. Catalytic properties of copper complexes with monomer, oligomer and polymer ligands containg f NCXNPh ( X = 0,S) fragments have been observed during DMP oxidation at room temperature in benzene-methanol (65 :35 vol/vol) and benzene-methanol-DMSO (64 : 35 : 1 vol/vol) solvent mixtures. It has been found that copper complexes with oligomer ligands possess the highest catalytic activity; this promotes the formation of the products of oxidative C - 0 coupling of PPO with 80% yield and more than 90% selectivity in 10-20 h. Similar results have been obtained for 2,4-xylenol oxidation. In the case of 2,6-dimethoxyphenol, 2,6-di-tert-butylphenol, and 2,4-di-tert-butylphenol oxidation the oxidative coupling of C - C with the formation of dimer products is catalyzed at similar rates and with high selectivity. The catalytic mechanism involving synthesized copper complexes and reasons for the maximum catalytic activity of copper complexes with oligomer ligands, have been discussed.

1.4 Macromolecular Effects during Thiol Oxidation Polymeric catalysts discussed in this section demonstrate high activity and selectivity during the oxidation of thiols to disulfides by molecular oxygen in the homogeneous phase. In practice, the catalysis of thiols is very important for removing mercaptans from oil and gases [142]. Another reason for studying this reaction on model systems is that disulfide bridges occur in biological systems. Systematic studies using a cobalt(I1) phthalocyanine complex with a soluble polymer, polyvinylamine (PVA),

Chapter I: Catalytic Properties of Polymers in Solutions

32

SO#Ja I

NaOS

CoPc INaSO,), SOflCl

during thioalcohol oxidation were performed by German et al. [I43 - 1471. Cobalt(I1) phthalocyaninetetrasodiumsulfonate,C O P C ( N ~ S Oadded ~ ) ~ , to PVA in the axial position exhibits high catalytic activity in the autooxidation of thioalcohols to disulfides [148, 1491. As shown above, a bivalent cobalt ion serves as the catalytic center [150]. The higher activity of this polymeric catalyst compared with such low-molecular weight amine is analogues such as CoPc(NaS03)JOH - or CoPc(NaS03)4/low-molecular attributed to the high local density of units in the macromolecular coil. This consequently causes an increase in the substrate concentration in the polymer coil bulk and simultaneously inhibits the formation of a catalytically inactive binuclear oxoproduct. In general, the oxidation of thioalcohols is presented by the following equilibrium state: CoPc(NaS03)4

4RSH+02.

2 RSSR+ H20

wherein the respective mechanism is described by the Michaelis-Menten equation. Because a polymeric catalyst maintains its solubility in water over a wide range of pH values, it is possible to follow the change in its catalytic activity as a function of pH. Depending on temperature, a maximum oxidation rate is observed between pH 8 and 9 (Fig. 16). This is attributed to the specific binding of the counter ions; i.e., in an acidic medium chlorine ions compete with thiol anions, and the concentration of the later decreases near the macromolecular coil. In alkali region, however, hydroxyl ions act as the competitor. Hence, the shape of the curve presented in Fig. 16 can be accounted for by the change in the local concentration of thiol anions near active groups of the polymer coil. Because of these considerable electrostatic effects during the 2-mercaptoethanol oxidation process, the addition of even a slight amount of a neutral salt (NaCI) sharply reduces the reaction rate (Fig. 17). This results from the decrease in the concentration of thiol anions, RS-, near the polymer chain caused by the specific binding of C1- ions by a polyion and subsequent twisting of the macromolecule. It was determined that the molecular weight of a polymer ligand greatly affects the rate of mercaptoethanol oxidation [ 15 1 - 1531. Fig. 18 presents the dependence of the

1.4 Macromolecular Effects during Thiol Oxidation

33

16

-

I6 -

c

.-‘cE

-,*

16L

-1

8

0

E

-EE -‘ F 0

-

8-

8- -

N

-

c

0

0-

0-

0

0,2

0,4 /(mol.dm-3)

Fig. 17. Influence of ionic strength (,u) of solution on the rate of RSH oxidation; [CoPc(NaSO,),] = 1.92.lO-’moI/l; [-NH2] = 1.5.10-3g.eq/1; pH 7.4; T = 298K [146]

Chapter I: Catalytic Properties of Polymers in Solutions

34

n

L

I

I

10’

102

I

103

n Fig. 18. Dependence of Michaelis-Menten constant, K,,, (1); reaction rate, k, (2); and activation enthalpy A H f (3); on the polymerization degree P of polymeric ligands at 298K; [-NH,] =3.8-10-3mol/dm3; [-NH2]/[Co] = 100; pH5.7 [I511

Michaelis-Menten constant, K,, the rate constant, k2, and the activation parameter, AH2, on the degree of polymerization ( P ) at 298K. A maximum can be seen at P = 20-40 at a low polymer concentration (<0.01 mole). The analysis of spectra in the visible region shows that PVA with low P causes the electron transfer from a thiol anion to Co(I1) and subsequent formation of Co(1). Moreover, both high- and lowmolecular weight PVA undergo various conformational changes and possess different base strength and various environmental influences due to CoPc(NaS03)4. For instance, potentiometric titration data show PVA amino groups with a low P to be more basic than the same groups with a high fj. This results in an increase of the local concentration of thiol anions near a more basic polymer. The actual independence of AH2 on P implies that the character of the reaction mechanism does not change with the chain length. In order to elucidate the effect that the charge density in the chain has on the catalytic activity, some copolymers based on vinylamine (VAm) and vinyl alcohol (VAl) were synthesized by acid hydrolysis of the copolymer N-vinyl-tert-butylcarbamate-vinyl acetate (NVTBC-VAc) [147]. It might be suggested that the sequence distribution of vinylamine units in the copolymer is related to the observed pattern of catalytic activity. This was investigated by calculating the number average block length, y, of ethyleneamine sequences in the copolymers VAm-VAl with various a

1.4 Macromolecular Effects during Thiol Oxidation

35

0,s

Fig. 19. Dependence of the molar fraction of charged monomer units (1); mean length of the block of ethyleneamine sequences (2); and rate of RSH oxidation reaction (3) on the molar fraction of ethyleneamine units in a polymeric ligand, a ; [-NH2] = 1.5.10-3 mol/l [CoPc(NaSO,),] = 1.9. lO-’mol/l; pH 7.8 11471

(where a is the molar fraction of ethyleneamine units in the copolymer) from the monomer feed and copolymer composition o n data and the reactivity ratios of NVTBC and VAc in benzene. The latter were determined from the values of ~ N V T B C= 1.12 and rvAc = 0.46 under the relevant conditions. Figure 19 (curve 3) shows the reaction rate as a function of a in the polymer chain. It is clearly seen that in the absence of amino groups ( a = 0) the reaction rate is vir-

Chapter I: Catalytic Properties of Polymers in Solutions

36

tually zero and the reaction rate does not increase linearly with a but levels off with increasing values of a . Figure 19 (curve 2) shows that the course of the catalytic activity versus a does not coincide with dependence of y on a . It can easily be seen that the degree of charge on the copolymer chain (i.e. mole fraction of charged monomeric units) at a certain pH value is represented by a ’ q (where 0 is the degree of protonation of amine groups). The dependencies of a - 0 on a (Fig. 19 curve 1) and V on a (Fig. 19, curve 3) are well correlated. This conformity is also confirmed by the fact that the reaction rate changes linearly with polymer charge. The linear increase of reaction rate with polymer charge suggests that the thiol anion is involved in the rate-determining step. The influence of charge density on the catalytic effect can be illustrated by comparing the catalytic activity of 2,4-, 2,6- and 2,lO-ionenes with M w =1 lo4. These data are compared with results obtained for the VAm-VAl copolymers. The dimensionless linear charge density parameter I of the polycations was calculated according to the rigid rod cell model for polyelectrolytes proposed by Kutchalsky [154]. Activation parameters of thiols in the presence of ionenes and VAm-VAI copolymers at pH 7.8 and T = 298 K are presented in Table 6.

Table 6. Activation parameters in the oxidation of 2-mercaptoethanol by polymeric catalysts Polymer

L

A G , kJ/mol

A H , kJ/mol

VAm-VAI copolymer a = 0.20 a = 0.39 a = 0.81

0.51 0.67 0.99

56.9

56.3 54.8

36k4 36 39

Ionenes 2,lO2,62,4-

0.82 1.15 1.45

61.1 56.3 55.4

25 38

11k3

A S , J/mol -7Ok 10 - 68 - 53

-168+8 - 105 - 58

The effect of the desolvation of reagents and an activated complex (commonly observed for all the systems) in which a number of charges is enclosed within a small space has been used to explain the effect of the chain charge density on the catalytic activity. The value of A S increases for both catalyst types due to an increase in the local concentration of the substrate and desolvation of the activated complex with a high electrostatic potential of macroions. For the VAm-VAl copolymers, however, the increase in activation entropy is very moderate in comparison with the ionenes. These differencies in entropy increment may be explained by the increasing of hydrophobicity of ionene polymers when the charge density decreases and the alkyl segments get longer. Thus, to accelerate oxidation reactions of thiols in the presence of c o P ~ ( N a S 0 3 ) ~ fixed to cationic polyelectrolytes, the following conditions are necessary: a high charge density along the polymer chain, a high basicity of amino groups in the

1.4 Macromolecular Effects during Thiol Oxidation

37 15

2

l,o

c

'

5

\ c

0,s

Fig. 20. Influence of DDS concentration on the starting rate of 2-mercaptoethanol oxidation ( I ) and the turbidity of the system (2) [147, 1561

polymer domain, and a low ionic strength of the solution. Depending on the characteristics of the polymer ligand, the effectiveness of thiol oxidation regarding the sequence of the applied polybases and polyamine basicity, changes as follows: poly(ethy1eneimine) > polyvinylamine > poly(L-lysine). The effect of surfactants on the autooxidation of different thiols such as 2-mercaptoethanol (ME), 3-mercapto-l,2-propanediol(MPD), mercaptoacetic acid (MAAc) and dodecylmercaptan (DDM) in the presence of a polymeric catalyst, has been studied [ 155, 1561. Fig. 20 illustrates the change in the initial rate of ME oxidation and the change in solution turbidity as a function of the concentration of the added anionic surfactant, sodium dodecylsulfate (DDS). The strong drop in the reaction rate and the increase in the turbidity of the system are apparently due to the formation of a complex between the polyion and the surfactant. However, the nonionic surfactant, oligoethylene oxide, does not influence ME oxidation. It is interesting to compare the activity of a PVA/CoPc(NaS03)4 catalyst crosslinked by a,a-dichloro-p-xylene with a water-soluble analogue [ 1571. The reaction rate for the immobilized system is 4-30 times lower than that of the water-soluble one. Both the basicity and activity of catalysts decrease with an increase in the degree of cross-linking. Two types of cross-linked catalyst have been studied; namely, catalysts obtained by fixing c o P ~ ( N a S 0 3 )to~ previously cross-linked PVA (Type A), and catalysts synthesized by binding c o P ~ ( N a S 0 3 )with ~ PVA prior to cross-linkage (Type B). The type A catalyst is 3 times more active than the type B catalyst.

38

Chapter I: Catalytic Properties of Polymers in Solutions

The decomposition of hydrogen peroxide in an aqueous medium at p H 7 was investigated. This reaction employed the catalysts, Fe3+- and Co2+-2,9,16,23-tetracarboxyphthalocyanine, and their quaternized derivatives immobilized on 2-vinylpyridine and styrene copolymers by the Friedel-Kraft reaction [158]. The kinetics of H202 decomposition at pH 7 in the presence of immobilized catalysts, including the evaluation of the initial and maximum reaction rates, Vo and Vmax, were also studied. It was shown that for the fixed iron complex, V,, is 10 times higher than for the immobilized catalyst of a quaternized derivative, and 20 times higher than for non-bonded iron phthalocyanine. The mechanism of hydrogen peroxide splitting on the synthesized catalysts is similar to the reaction mechanism involving the catalase enzyme. The structure of the coordination sphere around the Fe3+-phthalocyanine ring has been studied by electron and EPR spectroscopy. Upon addition of potassium cyanide to the aqueous suspension of the catalyst, the concurrent inhibition of the reaction of hydrogen peroxide decomposition is observed. The synthesized catalysts can therefore be effectively used in the oxidation of organic compounds in the presence of 0 2 and H202. Nickel, vanadium, cobalt, iron and manganese complexes of phthalocyanine supported on polystyrene have been obtained by binding chlorosulfated phthalocyanines with styrene and divinylbenzene copolymer (8 and 20% DVB) [159] through direct sulfonylation and sulfamide bond formation. The resulting complexes have been characterized by EPR spectroscopy, electron microscopy and differential scanning calorimetry (DSC). Moreover, their catalytic activity during cyclohexene oxidation has also been studied; a slight increase, opposed to the activity of unsupported phthalocyanines, was observed. The dependence of the initial rate of oxidative decarboxylation of oxalic acid on the total acid concentration and the content of undissociated species of mono- and dianion in aqueous solutions in the presence of polymeric cobalt phthalocyanine has been examined [160]. Whereas the monoanion, [HC201], of oxalic acid is shown to enter the reaction, the undissociated species does not. Furthermore, [C20$-] dianion retards the process. This is attributed to the fact that in catalyst A, active particles of c o P ~ ( N a S 0 are ~ ) ~located mainly on the species' surface while in catalyst B they are distributed within the species and are hardly accessible.

1.5 Interfacial Catalysis One of the most promising directions of catalysis research concerns the ability of macromolecules to form complex compounds with low-molecular weight reagents. The solvating effect of polyesters, crown ethers and kryptands [161- 1681 on alkali metal salts has greatly influenced the development of interfacial catalysis [ 169- 1751. Reactions involving such polymers are similar to processes in which low-molecular weight catalysts migrate from one phase to another. Reactions of mainly SN2 type (biomolecular nucleophilic substitution), involving OH - , PO - , etc. halides, proceed by this route. The oxidation of trans-stilbene by potassium permanganate in benzene in the absence of polyester proceeds very slowly, and only 3% benzoic acid forms

1.5 Interfacial Catalysis

39

within 45 h [ 1761. However, the rate of substrate oxidation considerably increases in the presence of polyethylene oxide (PEO) due to the partial solubilization of KMn04 in benzene through the formation of a soluble complex of K + with PEO. It is assumed [I751 that the selective oxidation of primary alkyl halides by potassium chromate is realized through the transfer of a chromate ion to the organic phase by PEO. It has been found [117] that polyethylene glycol (PEG) produces a strong catalytic effect during the reaction of butyl bromide with KI. The yield of the butyl iodide product increases linearly with an increase in the length of the PEG chains. A similar acceleration of the reaction between solid potassium phenolate and 1 -bromobutane is observed in toluene in the presence of oligoethylene glycols [178]. The synthesis of 8 esters based on alcohols and phenols in two-phase system liquid-liquid catalyzed by PEG is discussed [178a]. Polylethylene glycols (PEG), HO(CH2CH20),H, in which n > 3, display a higher activity as dehydrohalogenation catalysts in two-phase systems of 2-bromobenzeneaqueous KOH, than the classical phase-transfer agents, benzyltriethylammonium chloride and 18-crown-6 [ 1791. From the data regarding the involvement of end OHgroups of PEG in this reaction and the sharp increase in the catalytic activity and concentration of PEG in the organic layer when "n" is increased from 2 to 4, the conclusion has been reached that new polymer alkoxides and (or) hydroxides are involved in this catalysis. Because maximum PEG activity is reached at n 2 5, an optimum structure for catalysis is similar to 18-crown-6 ether with this structure forming at the end of the polymer chain. Moreover, high catalytic activity is exhibited by polystyrene-graft PEG, which can be fully extracted by simple filtration and used repeatedly. Fluorescence was to study the reaction of 5-dimethyl-amino-1-naphthalenesulfonyl chloride (dansyl chloride) with butylamine in ethyl acetate and chloroform at 60" and 40"C, respectively. This study was conducted in the presence of polystyrene and polyoxyethylene (POE) as cosolvents as well as their respective low-molecular weight analogues, namely toluene and diethyloxyethane, respectively [ 1801. Acceleration of this reaction through the effect of POE in chloroform, as compared with its monomeric analogue, increased with an increase in the volume ratio of the cosolvent and the length of the polyoxyethylene chain. In the case of diethyloxyethylene, a maximum value of the second-order reaction rate constant was achieved at a 20 vol.070 concentration in the solvent mixture. Similarly, as compared with toluene, the addition of polystyrene to ethyl acetate or chloroform caused an increase in the reaction rate in accordance with the growth in the polystyrene chain length. The reaction rates (for polystyrene and toluene) decreased linearly with an increase in the solvent volume ratio. The reaction rate in ethyl acetate also decreased linearly with an increase in the diethyloxyethane concentration in the system. However, the presence of a maximum of 10 vol.070 POE in the mixture with ethyl acetate accelerated the reaction by several fold. Upon further increasing the POE concentration in the mixture to 30070, the acceleration effect stayed constant. The effect of polymeric cosolvents on chemical reactions between two low-molecular reagents has been interpreted in terms of thermodynamic properties of polymer solutions.

40

Chapter I: Catalytic Properties of Polymers in Solutions

The influence of oligomers on the chemical reaction between two low-molecular weight compounds in the solution was studied during the reaction of 5-dimethylamino-1-naphthalenesulfonyl chloride with butylamine in chloroform in the presence of a,odimethoxyoligooxyethylenes,CH30(C2H40),CH3 where n = 1-4 [181]. The thermodynamics of the acceleration effect of the oligomer solvent were examined, and the equation for its description was derived. It was determined that the reaction acceleration is influenced by three factors; i.e. oligomer chain length, the value of the parameter of pairwise interactions between system components, and the volume ratio of the oligomer solvent. To check this equation by means of fluorescence spectroscopy, second-order rate constants were measured. The degree of acceleration was shown to be highly dependent on the glym chain length and the oligomer volume ratio. It was concluded that the equation can be used for describing acceleration effects of the oligomer cosolvent. The interaction of mono- and dihalo benzenes with alkoxy ions is accelerated by the presence of polyethylene glycols which act as interfacial catalysts [182]. The percentage of conversion (0) of C12C6H4in the absence of catalysts but in the presence of 150, 1000, 6000, 8000 and of PEG with the weight-average molecular weight (Mw) 2.106 and at 140-150°C and 6 h reaction time, is 7.1, 8.2, 20.1, 33.0 and 28.1%, respectively. Bromobenzenes are more reactive than dichlorobenzenes. In the presence of PEG with MW= 6000, the conversion values are 71.5 and 33.0%, respectively. The effectiveness of alkoxylation depends on the alcohol type and decreases in the sequence: primary > secondary > tertiary. The 6 values for o-C12C6H4esterification of methyl, isopropyl and tert-butyl alcohols, in the presence of a catalyst with Mw= 8000 at 150°C and 6 h reaction time are 66.0, 13.5, and 5.1%, respectively. Furthermore, the hydrolysis of butyl acetate and methyl pivalate in benzene in the presence of KOH at 25 "C as well as the reaction of potassium phenolate with benzyl chloride in boiling acetonitrile are accelerated by addition of polyoxyethylene [ 1831. The catalytic effect of POE is augmented by an increase in the number of oxyethylene units, i.e. 1 < 6 < 12. PEO is also an interfacial catalyst of the reaction of phenol and 2,4,6-trimethylphenol with methyl iodide in water-chloroform and dichloromethane. The kinetic study of the reaction between benzyl chloride and potassium acetate in the presence of PEO of variable molecular weight in toluene and butanol has been performed with IR spectroscopy [184]. The dissolution of a reagent of poor solubility is apparently a rate-limiting step of the reaction in a solution of low polarity (toluene). The presence of PEO impurities in toluene has been detected. Moreover, effect of PEO and crown ethers as phase transfer catalysts has been compared. In a low-polarity solvent, oligoethylene oxides are more effective catalysts, while in a polar solvent (butanol) the effectiveness of PEO and crown ethers as phase transfer catalysts is similar. The effect of the concentration of interfacial transfer catalysts in aprotic solvents in contact with solid salts (sodium 2,4-dinitrophenolate) has been investigated with regard to their effectiveness for reaching the solid-liquid equilibrium in benzene, chlorobenzene, dichloromethane and acetonitrile at 25 "C [I 851. Polyethylene glycols with 300, 600 and 2000 mol. wt., trianthrylmethylammonium chloride, dodecyldimethylammonium chloride, tetrabutylammonium chloride and a crown ether, have

1.5 Interfacial Catalysis

41

been used as interfacial transfer catalysts acting as complexing agents. The study of the equilibrium in the presence of a catalyst shows that the amount of anion solubilized by one mole of the catalyst increases as the solvent polarity is increased. In addition, some catalysts obey the theoretical isotherm that the equilibrium state is observed over the whole concentration range, when the equilibrium constant is fixed. Furthermore, a limit of solubility is sometimes observed, whereby an increase in the catalyst concentration does not affect the reaction rate. The effect of polyester fragments on the kinetics of anionic cyanoethylation (CE) of polyoxyethylated (POE) and polyoxypropylated fatty alcohols (C,2 - c18) and nonylphenols in non-polar media has likewise been studied [186]. The degree of CE increases with an increase in the polymer chain length. This is especially pronounced for POE derivatives. The results obtained have been discussed in respect of the mechanism of interfacial transfer catalysis in which POE fragments similar to oligoglyms are more active due to effective cation solvation. Polyethylene glycol (PEG), when used as an interfacial catalyst, is tainted with lipophilic anions like quaternary ammonium salts [ 1871. Let us consider the following reaction involving the P-splitting of P-bromoethylbenzene catalyzed by PEG and BuNH4S04: PhCH2CH2Br(org)+ NaOH(aq.) catalysf_ PhCH = CH2(org.) + NaBr(aq.) + H20. The NaBr formed during the course of the reaction is bonded with the PEG more effectively than the reactant NaOH. Selectivity constants characterizing the ratio of extraction from the aqueous phase and PEG complexing have been determined for alkali metal salts. These constants can serve as a quantitative measure of the concentration of complexes and free anions in the aqueous phase and can enable the prediction of the degree of catalyst impurity. The proposed method of calculating selectivity constants can be common for all reactions involving alkali metal salts. Ultrasonic treatment has led to a considerable increase in the selectivity of alkylnitrobenzene (ANB) oxidation by molecular O2 to nitrobenzoic acids (NBA) in the presence of alkalis and polyethylene glycol as a phase transfer catalyst [188]. The rate of the CH3C6H4N02+KOH-+N02C6H4C02K' reaction involving PEG and 0 2 , obtained by mixing the reaction mixture by ultrasound, exceeds the rate of No2C6H4CH2CH2c6H4No2dimer formation, the only product of this reaction under mechanical mixing conditions. The para-positioning of substituents on the benzene ring is preferential in this reaction. It has furthermore been found that a 10-fold decrease in the initial ANB concentration (Cinit,= 0.01 M ANB) considerably increases the selectivity of the process; i.e. NBA becomes practically the only reaction product. Polyamides having polyoxyethylene blocks in the main chain of (NHCH2(CH20CH2),CH2NHCOOROO),,, in which x = 1 , 3, 5 and R = (CH2)8, (CH2)4, CH20CH2), as well as low-molecular weight model compounds (MC) of CH3(CH2)8CONHCH2X(CH20CH2)xCH2NHCO(CH2)8CH3 in which x = 1,3,5, have been studied in respect of their ability to complex with alkali metals [189]. Polyamides with the highest chain length of oligooxyethylene blocks (OOE) form complexes with large metal ions, Cs' and K', better than with Li' and Na'. This is inferred from the intramolecular cooperation of OOE blocks during complexing. Polyamides catalyze the interfacial nucleophilic substitution of hexyl bromide by

42

Chapter I: Catalytic Properties of Polymers in Solutions

potassium acetate in CH3CN. The catalytic activity of polyamide increases with an increase in the chain length of an oligooxyethylene unit. The activity of polyamide does not actually depend on the type of acid components. The polymer effect in the reaction, as compared with the model compounds, increases with a decrease in the number of oxyethylene units in the amine component. However, this involves a decrease in the absolute rates of the reactions catalyzed by the polyamide and the model compounds. At attempt has been made to use gels as phase transfer catalysts [190]. A considerable difference was observed between the effects of cross-linked and linear polymeric catalysts. Polymeric sulfones obtained by radical copolymerization of monomeric sulfones with styrene are active interfacial catalysts. Their activity was studied in the reaction of n-C8H17Brwith MI (M = Li, Na, K) in a toluene-water system [191]. Reaction of n-C8HI7Brwith NaI at 100°C for 48 hours produced only traces of n-C,8H171 when catalysts, including such low-molecular weight catalysts as DMSO, methyl phenyl sulfoxide and methyl benzyl sulfoxide, were absent. When this reaction was catalyzed by polymeric sulfone, the yield of n-Cl8HI7Iwas 43%. However, in the presence of polymeric catalyst an 82% yield was obtained after a reaction time of 160 h. Polymeric sulfones dissolve in toluene, but they are insoluble in H20. This allows the sulfonyl groups of the catalyst to interact with metal ions on the interface of organic and aqueous phases and to be transfered to the organic solvent. The catalytic activity of polymeric sulfones depends on their ability to bond cations and also depends on the lipophility of the active centre environment. The synthesized catalysts can be regenerated from a toluene solution by petroleum ether. Polymers, with [ q ]= 0.30-0.56 dl/g, containing methyl sulfonyl groups have been synthesized by copolymerization of methyl-4-vinylphenyl sulfoxide with styrene [92]. These copolymers are soluble in toluene but are insoluble in water. Their catalytic activity has been studied in SN2-type reactions of alkyl halides with different nucleophiles in water-toluene solutions at 100"C for 20 h. The copolymers effectively catalyze two-phase reactions of octyl bromide with KSCN, NaSCN and KI and of butyl bromide with NaOPh and NaSPh with a 11 -78% yield of respective reaction products. These reactions, however, are not catalyzed by DMSO, methylphenylsulfoxide and benzyl methyl sulfoxide. The copolymer catalytic activity is higher than that of the monomer analogues and increases with an increment in the number of styrene units in the copolymer. These copolymeric catalysts are easily extracted from the reaction medium. The influence of the hydrophobic environment of active centers on the catalytic activity of polymeric catalysts has already been discussed. The catalytic effect of polyethylene glycols (PEG) of different molecular weight and of 18-crown-6 on the interfacial transfer reaction during imidazole N-alkylation was assessed. It was found that PEG are more effective with an increase in the polymeric chain length [193]. The following activation parameters of these reactions in the presence of PEG-400, crown ethers and (C4H9)4NBr,were also determined, namely: E, = 51.9, 41, and 48.1 kJ/mole; A H = 48.9, 38.1, and 44.4 kJ/mole; A S = -34.7, -39.1, 34.7 e.u.; AG = 96.2, 92, and 92 kJ/mole, respectively. PEG complexing with imidazole sodium salt was studied by NMR, and the catalytic effect was shown to involve the formation of ionic pairs of PEG with the sodium cation.

1.5 Interfacial Catalysis

43

In addition, polymers and copolymers of N-methyl-N@-vinylbenzy1)formamide (MFA) and acetamide with styrene have been synthesized. They are used as interfacial transfer catalysts for the nucleophilic substitution of CH3(CH&Br by KSCN, NaSCN, KI, NaOPh, NaSPh in a toluene-water mixture [194, 1951. It is noteworthy that polystyrene, DMFA and MFA d o not show any individual catalytic activity. However, the activity of St-MFA copolymer increases with an increase in the styrene content. Linear polyvinyl esters and block copolymers accelerate the Williamson reaction which proceeds on a solid (potassium phenoxide)-liquid(n-butyl bromide) interface [ 1961. The phase transfer reactions between the liquid (1-bromoctane) and solid (potassium thiocyanate) interface catalyzed by polymer supported ureas have been studied. The catalytic activity of polyurea is higher than that of monomeric analogues [196a]. Other catalysts include polyvinyl alcohols with molecular weights of 115 000 (PVA-1) and 10000 (PVA-2), polyvinyl formaldehyde, polyvinylmethyl ester, polyvinyl methyl ketone, polyethylene oxide, polyvinylpyrrolidone, and polyacrylamide as well as block-copolymers such as poly(styrene-block ethylene oxide) and poly(ethy1ene oxide-block styrene-block ethylene oxide). Almost all of the homogeneous copolymers are involved in the solvation of alkali metal cations and in phase transfer induction. The low catalytic effect of PVA-I and PVA-2 evidently results from the formation of a hydrogen bond network in the particular polymer thereby preventing specific solvation. The high catalytic effect of block copolymers is accounted for by the presence of styrene blocks which cause an increase in the solubility of copolymers in toluene, the reaction solvent, which results in uncoiling of the macromolecules. This, in its turn, creates favorable conditions for the interaction of ethylene oxide blocks with solid potassium phenoxide and accelerates the transport of potassium ions to the liquid phase. The catalytic activity of polymer-bound crown ethers and cryptands as catalysts of interfacial transfer reactions was studied in anion-promoted, nucleophilic, aliphatic substitution reactions and was compared with the catalytic activity of polymer-supported phosphonium salts [197]. The activity of crown ethers was found to depend on the nucleophilic character and degree of substitution of the ring as well as on the distance between an active center and the main polymer chain. Potassium salts of mild nucleophiles (I -, SCN-) complexed well with 18-crown-6 groups, thereby increasing the catalytic activity of the polymeric catalyst by a factor of 2- 3 greater than structure-bound tributyl(alky1)phosphonium salts. The presence of a binding linear chain of 10 atoms between the crown ether and the main polymer chain caused an average 2-4-fold increase in the reaction rate depending on the degree of substitution in the ring. The catalytic activity of cryptands was higher than that of crown ethers and quatenary salts at the same degree of ring substitution. This activity, however, depended considerably less on anionic character and on the distance between the active center and the main polymer chain. Interfacial transfer reactions promoted by polymer crown ethers and cryptands proceed according to the same mechanism as for respective soluble catalysts. The reactions occur in the organic shell surrounded by the complexed ligand. Anions are exchanged on the surface of the water-organic solvent interface, whereby the exchange

Chapter 1: Catalytic Properties of Polymers in Solutions

44

does not require a concurrent transfer of countercations. Moreover, complexed fragments of crowns and cryptands surrounded by the aqueous phase are catalytically inactive. The catalytic effect of linear polyvinylbenzocrown ethers on the ionic transfer through membranes and the catalysis of reactions involving ionic pairs was compared with the catalytic effect of monomer analogues, i.e. benzocrown ethers [198]. The catalytic properties of polycrown ethers depended on the crown ether structure and the distance between crown ether ligands bonded by the main chain. This was especially illustrated whenever cations were simultaneously bound to two crown ether ligands (sandwich). This effect was studied in the decarboxylation of 6-nitrobenzyloxalol-3-carboxylic acid and its salts in the presence of copolymers of styrene with vinylbenzo-I 8-crown-6 and vinylbenzo-I 5-crown-5. This monomolecular reaction proceeded quantitatively and was rather sensitive to the ion pair structure, the solvent and the presence of cation-binding ligands. The interaction of the crown ether with a cation increased the interionic distance in the COO-K+ ion pair. In benzene solution this interaction sharply accelerated the decomposition. Polycrowns with closely spaced crown units, however, exerted a stronger effect. Decomposition of the acid in the ester solvent was also accelerated by the addition of crown ether ligands, whereby polycrown ethers were more effective than their low-molecular weight analogues. References concerning research studies of enyzme-like polymers conducted during the period of 1978- 1982, catalytic properties of water-soluble imidazole-containing polymers during ester hydrolysis, as well as the latest achievements in the manufacture and application of high-molecular weight catalysts containing thiocrown-ethers or crown-sulfides, can be found in previous reviews [ 199- 2011.

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[93] T.P. Dadze, M.L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim. 12, 2722 (1970) 1941 N.P. Butina, V.S. Pshezhetskii, D.I. Stom, Vysokomol. Soedin. Ser. B: 26, 812 (1984) [951 M. Sato, Y. Inaki, K. Kondo, K. Takemoto, J. Polym. Sci. Polym. Chem. Ed. 15, 2059 ( I 977) [961 M. Sato, H. Shindo, K. Kondo, K. Takemoto, J. Polym. Sci. Polym. Chem. Ed. 18, 101 (1 980) [97] V. B. Sigitov, S. E. Kudaibergenov, E. A. Bekturov, Abstr. 7th All-Union Conf. on Catalytic Reactions in Liquid Phase, Alma-Ata 1988, p. 101 [98] H. Nishide, T. Minakota, E. Tsuchida, J. Mol. Catal. 15, 327 (1982) [99] H. Hirai, T. Furuta, J. Polym. Sci. Polym. Chem. Ed. 9, 459 (1971) [IOO] E. Bayer, W. Schumann, J. Chem. SOC.Chem. Commun. 12, 949 (1986) [I011 V. A. Seleznev, Yu. P. Tyulenin, E. F. Vainstein, A. V. Artemov, Kinetika i Kataliz 24, 1085 (1 983) [I021 V.A. Artemov, V.A. Seleznev, E.F. Vainshtein, Izv. VUZ ov. Khim. Khim. Tekhnol. 26, 1298 (1 983) [I031 Yu. P. Tyulenin, V.A. Seleznev, E.F. Vainshtein, A. V. Artemov, Abstr. 5th Intern. Symp. on Heterogeneous Catalysis, Varna, 1983, Part 1 , p. 105 [I041 A. V. Artemov, E. F. Vainshtein, V.A. Seleznev, Yu. P. Tyulenin, Izv. VUZ ov. Khim. Khim. Tekhnol. 27, 64 (1984) [I051 A.V. Artemov, E.F. Vainshtein, Zh. Prikl. Khim. (Leningrad) 3, 2599 (1984) [lo61 A.V. Artemov, E.F. Vainshtein, V.N. Sapunov, Abstr. 5th Int. Symp. on Homogeneous and Heterogeneous Catalysis, Novosibirsk, 1986, p. 14 [I071 A. B. Solovieva, A. I. Samokhvalova, T. S. Lebedeva, V. S. Pshezhetzkii, L. V. Karmilova, N.S. Enikolopyan, Dokl. Akad. Nauk SSSR 290, 1383 (1986) [I081 B. Pispisa, A. Palleschi, Macromolecules 19, 904 (1986) [I091 A.J. Schouten, G. Challa, J. Reedijk, J. Mol. Catal. 9, 41 (1980) [110] A.S. Hay, H.S. Blanchard, C.F. Endres, J.W. Eustance, J. Am. Chem. SOC.81, 6335 (1 959) [ l 1 I] A. S. Hay, P. Shenian, A. C. Gowan, P. F. Erhardt, W. R. Haaf, J. E. Theberge in Encycl. Polym. Sci. Techno]., N.M. Bikales editor, Wiley, New York, 1969, Vol. 10, p. 92 [I 121 A.S. Hay, Polym. Eng. Sci. 16, 1 (1976) [113] E. Tsuchida, M. Kaneko, H. Nishide, Makromol. Chem. 151, 221 (1972) [114] E. Tsuchida, M. Kaneko, H. Nishide, Makromol. Chem. 164, 203 (1973) [115] A. J. Schouten, N. Prak, G. Challa, Makromol. Chem. 178, 401 (1977) (1161 A.J. Schouten, D. Wiedijk, J. Borkent, G. Challa, Makromol. Chem. 178, 1341 (1977) [I171 G. Challa, H.C. Meinders, J. Mol. Catal. 3, 185 (1977/78) [I 181 H.C. Meinders, F. Van Bolhuis, G. Challa, J. Mol. Catal. 5, 225 (1979) [119] A.J. Schouten, D. Noordegraaf, A.P. Jekel, G. Challa, J. Mol. Catal. 5, 331 (1979) [I201 G. Challa, A. J. Schouten, G. Ten Brinke, H. C. Meinders, Modification of Polymers. ACS Symp. Series. 121, 1 (1980) [I211 J.P. J. Verlaan, P. J.T. Alferink, G. Challa, J. Mol. Catal. 24, 235 (1984) [122] C.E. Koning, G. Challa, F.B. Hulsbergen, J. Reedijk, J. Mol. Catal. 34, 355 (1986) [I231 E. Tsuchida, M. Kaneko, Y. Kurimura, Makromol. Chem. 132, 209 (1970) [I241 E. Tsuchida, M. Kaneko, Y. Kurimura, Makromol. Chem. 155, 45 (1972) [125] H. J. Kevelam, K.P. de Jong, H.C. Meinders, G. Challa, Makromol. Chem. 176, 1369 ( 1 975) [126] G. Challa, Abstr. IUPAC Int. Symp. on Macromol. Florence, Italy 1980 [127] H.C. Meinders, G. Challa, J. Mol. Catal. 7, 321 (1980) [128] M. Flinterman, G. Challa, R. Barbucci, P. Ferruti, J. Mol. Catal. 18, 149 (1983) [I291 W. Breemhaar, H.C. Meinders, G. Challa, J. Mol. Catal. 10, 33 (1981) [130] G. Challa, J. Mol. Catal. 21, 1 (1983) [131] J.P. J. Verlaan, J.P.C. Bootsma, G. Challa, J. Mol. Catal. 14, 211 (1982) [132] G. Challa, J.P.J. Verlaan, Abstr. IUPAC Int. Symp. on Macromol., Amherst, 1982 [I331 C.E. Koning, J. J. W. Eshuis, F. J. Viersen, G. Challa, Reactive Polym. 5, (1986) [I341 E. Tsuchida, H. Nishide, Modification of Polymers, ACS Symp. Series 121, 147 (1980)

Chapter I: Catalytic Properties of Polymers in Solutions J.P. J. Verlaan, J. P.C. Bootsma, C. E. Koning, G. Challa, J. Mol. Catal. 18, 159 (1983) J.P. J. Verlaan, C. E. Koning, G. Challa, J. Mol. Catal. 20, 203 (1983) Tao Jailing, Lu Peizhi, Cao Mengjun, Polym. Commun. (P.R. China) 4, 263 (1985) C. E. Koning, B. L. Hiemstra, G. Challa, M. Van De Velde, E. J. Goethals, J. Mol. Catal. 32, 309 (1985) H. Nishide, Y. Suzuki, E. Tsuchida, Eur. Polym. J. 17, 573 (1981) H.S. Mason, J. Biol. Chem. 172, 83 (1948) H. Tsukube, K. Maruyama, T. Araki, J. Chem. SOC.Perkin Trans. Part 2. 10, 1485 (1983) P.C. Jocelyn, Biochemistry of SH Group, Acad. Press, London, 1972 P. Piet, J. H. Schutten, A. L. German, Prepr. IUPAC Int. Symp. on Macromol. Florence, Italy, 1980 W. M. Brouwer, P. Piet, A. L. German, Polym. Bull. 8, 245 (1982) W.M. Brouwer, P. Piet, A.L. German, Polym. Commun. 24, 216 (1983) W.M. Brouwer, P. Piet, A.L. German, J. Mol. Catal. 22, 297 (1984) W. M. Brouwer, Ph. D. Thesis, University of Technology, Eindhoven, The Netherlands, 1985 J.H. Schutten, J. Zwart, J. Mol. Catal. 5, 109 (1979) H.H. Schutten, T.P. Beelen, J. Mol. Catal. 10, 85 (1980) L.D. Rollman, J. Am. Chem. SOC.97, 2132 (1975) W.M. Brouwer, P. Piet, A.L. German, Makromol. Chem. 185, 363 (1984) J.H. Schutten, P. Piet, A.L. German, Makromol. Chem. 180, 2341 (1979) W.M. Brouwer, P. Piet, A.L. German, J. Mol. Catal. 22, 297 (1984) A. Katchalsky, Pure and Appl. Chem. 26, 327 (1971) W.M. Brouwer, P. Piet, A.L. German, ibid. 29, 347, 375 (1985) W.M. Brouwer, A.J. Robierst, A.L. German, Prepr. 5th Int. Symp. on Homogeneous and Heterogeneous Catalysis, Novosibirsk, 1986, Part 2, p. 58 W. M. Brouwer, A.M. Piet Traa, J. W. de Weerd Teun, P. Piet, Angew. Makromol. Chem. 128, 133 (1984) H. Shirai, S. Higaki, K. Hanabusa, N. Hojo, J. Polym. Sci., Polym. Chem. Ed. 22, 1309 ( 1984) M. Gebler, J. Inorg. and Nucl. Chem. 43, 2759 (1981) M. Chanda, K. Driscoll, G. Rempel, Abstr. IUPAC Int. Symp. on Macromol., Amherst, 1982, Sec. 1, p. 271 J. Smid, S.C. Shah, L. Wong, J. Hurley, J. Am. Chem. SOC.97, 5932 (1975) J. Smid, Pure and Appl. Chem. 48, 343 (1976) S. Shah, L. Wong, J. Smid, Ann. Chem. SOC.Polym. 18, 766 (1974) S. Shinkai, T. Kunitake, Kagaku, Chemistry 33, 588 (1978) J. Smith, S.C. Shah, R. Sinta, A. J. Varma, L. Wong, Pure and Appl. Chem. 51, 1 1 1 (1979) W. M. MacKenzie, D.C. Sherrington, Polymer 21, 791 (1980) L. J. Mathias, J. Macromol. Sci.-Chem. 15, 853 (1981) S. Akabori, S. Miyamoto, H. Tanabe, J. Polym. Sci., Polym. Lett. Ed. 16, 533 (1978) A. J. Varma, Eur. Polym. J. 22, 111 (1986) M. Hiraoka, Kraun Soedineniya, Mir, Moscow, 1986 W.M. MacKenzie, D.C. Sherrington, J. Chem. SOC.Perkin Trans. Part 2, 3, 514 (1981) G.T. Szabo, K. Gero, L. Toke, Acta Chim. Hung. 113, 17 (1983) Ch.U. Pittman, Polymer News 10, 75 (1984) Huang Xian, Huang Zhizhen, Chem. Reagents (P.R. China) 7, 20 (1985); 8, 18 (1986) Hou Wei, Jiang Xao, Hangzhou Univ. Natur. Sci. Ed. 22, 275 (1985) A. Hirao, S. Nakahama, M. Takahashi, N. Yamazaki, Makromol. Chem. 179, 915 (1978) F. Kh. Agaev, I. M. Abdullabekov, M. M. Movsumzade, A. L. Shabanov, Azerb. Khim. Zh. 1, 39 (1977) J.M. Harris, N. H. Hundly, T. G. Shannon, E. C. Struck, J. Org. Chem. 47, 4789 (1982) [178a] Y. Shanxin, H. Shiji, Chem. Reagents (P.R. China) 14, 246 (1992) [179] Y. Kimura, S.L. Regen, J. Org. Chem. 48, 195 (1983)

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A. Okamoto, A. Hayashi, J. Mita, K. Uchiyama, Eur. Polym. J. 19, 399 (1983) A. Okamoto, A. Hayashi, J. Mita, Polym. J. 15, 423 (1983) R. Neumann, Y. Sasson, Tetrahedron 39, 3437 (1983) S. Slaoui, R. Le Goaller, J.L. Pierre, J.L. Luche, Tetrahedron Lett. 23, 1681 (1982) 0.E. Filippova, I. N. Toptchieva, V. V. Lutzenko, V. P. Zubov, Vysokomol. Soedin. Ser. A: 26, 402 (1984) [185] G.T. Szabo, K. Gero, L. Toke, Acta Chim. Hung. 113, 11 (1983) [I861 I. Jianu, Abstr. IUPAC Int. Symp. on Macromol. Bucharest, 5-9 Sept. 1983, Sec. 1, p. 471 [187] R. Neumann, Y. Sasson, J. Mol. Catal. 31, 81 (1985) [188] R. Neumann, Y. Sasson, J. Chem. SOC.Chem. Commun. 9, 616 (1985) [189] S. Iwabuchi, T. Nakahira, A. Tsuchiya, K. Kojima, V. Bohmer, Makromol. Chem. 184, 535 (1983) [190] O.E. Filippova, I.N. Toptchieva, V.P. Zubov, Vestn. MGU, Khim. 24, 590 (1983) [191] S. Kondo, H. Yasui, K. Ohta, K. Tsuda, J. Chem. SOC.Chem. Commun. 7, 400 (1985) [192] S. Kondo, K. Ohto, K. Tsuda, Makromol. Chem., Rapid Commun. 4, 145 (1983) [193] Liu Han-Ming, Yuan Cheng-Xe, Acta Chim. Sci. (P.R. China) 20 (1983) [194] S. Kondo, Y. Inagaki, K. Tsuda, J. Polym. Sci, Polym. Lett. Ed. 22, 249 (1984) [195] S. Kondo, M. Minafuji, Y. Inagaki, K. Tsuda, Polym. Bull. 15, 77 (1986) [196] J. Kelly, W.M. MacKenzie, D.C. Sherrington, G. Reiss, Polymer 20, 1048 (1979) [196a] K. Shuje, 0. Takeshiro, T. Masakazu, K. Hideo, Y. Yasuo, Makromol. Chem. 193, 2265 (1992) [197] F. Montanari, S. Quici, P.L. Anelli, Brit. Polym. J. 16, 212 (1984) [198] J. Smid, K. Kimura, M. Shirai, R. Sinta, Polym. Prepr. Amer. Chem. SOC.22, 163 (1981) [199] Y. Imanishi, in “Bioactive Polym. Syst!’ London, New York, 1985 [200] J.A. Pavlisko, C.G. Overberger, Biomed and Dental Appl. Polym. ACS Symp. Houston Tex. New York, London, 1981 [201] T. Kadzuiti, Polym. Appl. 34, 545 (1985) [180] [181] [182] [183] [184]

Catalysis by Polymers

E. A. Bekturov and S . E. Kudaibergenov Copyright 0 2002 WILEY-VCH Verlag GmbH & Co. KGaA

Chapter 2 Catalysis by Ion-Exchange Resins

Catalysis by ion-exchange resins is one of the earliest applications of particles on polymeric supports and has therefore been studied most thoroughly [I]. The use of ion-exchange resins was spurred mainly during the early 1950s' by the development of methods to produce styrene and divinylbenzene (DVB) copolymers as beads. The most frequently occurring resins are products from chemical modification of gelatinous polystyrene (obtained as beads) with a nominal degree of cross-linking of 2 to 10%. The properties of the particles and the elasticity of the resin pores makes it possible to use them both in gas- and liquid-phase reactions. The maximal operation temperature of acidic resins is commonly 125 "C. At higher temperatures (about 200"C), partial or full deactivation occurs which somewhat limits their use in industry. Nevertheless, the local concentration of functional groups encapsulated in the resin, their swelling ability, thermal stability and some other advantages are important properties which classify ion-exchange resins as promising industrial catalysts [2]. Actually all acid-base type organic reactions can be performed in the presence of acidic or basic ion-exchange resins. Several kinds of organic syntheses involving hydrolysis, hydration, dehydration, cyclization, isomerization, polymerization, racemization, condensation, etc. can involve acid and base species of ion-exchange resins as catalysts. Furthermore, catalysts based on ion-exchange resins have particular technological advantages. For example, they are readily separated from the reaction mixture allowing the process to proceed continuously. Furthermore, because they do not require neutralization and concentration, high water and air expenditure and equipment corrosion is avoided. Both gas- and liquid-phase reactions catalyzed by ion-exchange resins considerably depend on substrate and resin particle sizes and the degree of cross-linking. Kinetic equations of these reactions have been described [4]. In the simplest case it is assumed that the polymer network plays an insignificant role, and the system behaves like a homogeneous one. A more complicated situation arises when one has to consider the diffusion of reagents inside a particle. Theoretical models of ion-exchange resins, have been presented in detail [I]. Presently occurring problems involving the manufacture and use of organic polymers and their applications have been discussed [5, 61. These cited reviews present mathematical models describing the catalytic effect of gelatinous and macroporous polymeric catalysts. They also discuss chemical and physical properties of polymer catalysts as well as possibilities of increasing their activity, selectivity and thermal stability by chemical modification of styrene and DVB copolymer through sulfurization, nitration, chlorination, fluoridation, and sulfoalkylation. The synthesis of polymer-supported Lewis acids is also examined. Because ion-exchange resin catalysis has been described earlier [7, 81, this chapter deals with only some examples of using resins having acid or base sites. Strong-acid ion-exchange resins have been obtained by sulfurization of polystyrene resins [9]:

2 Catalysis by Ion-Exchange Resins

51 SOjH’

Hfi04-

Acidic resins in water can be presented as a set of species in the following equilibrium:

e

S

0

j

H

’

e S O j H 3 0 i

Hzo

These acidic species have different catalytic activity, and each particular species can contribute to the reaction [lo, 1 I]. Moreover, it has been shown through spectroscopic methods [I21 that sulfoacid groups can form a network of hydrogen bonds. As an example of the catalytic effect of sulfogroups, let us consider alcohol dehydration. It is assumed [13] that alcohol dehydration proceeds to form carbonium ions according to the following scheme: 7

3

HO- C-CH3 \

0-H

CH3

I

0 C\

H3C HzO

+

HC, C,

=CHz

I

CH,

CH3

H3C

However, oxygen atoms of the SO, group are suitable bases for the separation of a proton from a tert-butylcarbonium ion at the final step of the process. At higher concentrations, the complex mechanism involving a network of hydrogen bonds seems more plausible:

I

\ O\ ...0-

o/

s-

Chapter 2 Catalysis by Ion-Exchange Resins

52

Sulfurized macroporous styrene divinylbenzene copolymers with different degree of cross-linking, exchange capacity, surface area, porosity and mean pore radius were synthesized, and their catalytic activity in 2-butanol dehydration was studied [ 14, 151. With an increase in the quantity of cross-link bonds from 20 to 40%, the specific surface of the resin increased from 31.5 to 114 mg/g. In addition, the porosity increased from 0.12 to 0.31 cm3/g, whereas the exchange capacity of the polymer decreased from 4.30 to 3.0 mg-eq. H /g. The dehydration reaction was conducted at temperatures between 110- 130°C. Maximal activity was exhibited by the catalyst with a 25% degree of cross-linking and a mean pore radius of 80.1 A. The rate of 2-propanol dehydration did not depend on catalyst particle sizes over the range from 18-25 to 52 - 60 mesh. Moreover, the ratio of cis/trans-Zbutenes forming during dehydration neither depended on the catalyst nature nor reaction temperature. However, it decreased by increasing the partial pressure of n-butanol up to a certain value after which it remained constant. Nonetheless, the total amount of olefins formed was actually constant. Because the substitution of Na' ions for H + reduced the catalytic activity, this infers that polymeric catalysts behave like aqueous acid solutions; i.e. protons are responsible for the reaction catalysis. The activity of macroporous ion-exchangers based on hydrophilic, glycidyl methacrylate copolymers containing sulfopropyl groups was studied with regard to their chemical structure, distribution of functional groups and morphology of the porous structure during the reaction of ethyl acetate transesterification by 1-propanol. This reaction was performed in the gas phase in a flow reactor at 120°C and in the liquid phase in a perfectly stirred reactor at 52°C [16-221. The obtained data was compared with the results of the study of the activity and properties of sulfurized macroporous styrene-divinylbenzene copolymers. The content and distribution of sulfo groups in catalyst layers and the surface sulfur concentration were evaluated. The properties of the ion-exchange resin were examined on the basis of the concentration of the cross-linking agent (varying over 30- 85%), the degree of sulfopropylation, the content of cyclohexanol used in copolymerization and the partial exchange of alkali metals. It was found that the distribution and concentration of acid centers depend on the preparation technique. The type of copolymer from which a strongacid ion-exchanger is prepared was not actually important. The methacrylate-based resin possessed higher concentration of sulfo groups than a catalyst based on styrenedivinylbenzene copolymer, this being exhibited only in gas reactions. The hydrolysis of dextrin over a 60-80°C temperature range in the presence of cross-linked copolymers of vinyl alcohol (VA) and styrene-sulfoacid (SSA) of different composition and of the industrial styrene-sulfoacid type resin, Amberlite 120B, was investigated [23]. The hydrolysis rate was proportional to substrate and resin concentrations. This differed from the results of homogeneous hydrolysis in the presence of soluble copolymers VA-SSA, by which the reaction rate is described by MichaelisMenten kinetics. The hydrolysis rate hereby increased with an increase in the VA concentration of the cross-linked copolymer, whereby the rate was always higher than for Amberlite 120B. The addition of cross-linked VA and styrene copolymers or PVA inhibited the hydrolysis in the presence of the copolymeric catalyst, VA-SSA. However, it slightly +

2 Catalysis by Ion-Exchange Resins

53

augmented the reaction rate in the presence of the ion-exchange resin. Because the ion-exchange resin had to be conditioned by the adsorption of such a large amount of water by the inhibiting resin, the substrate and product concentrations increased. The activation enthalpy and entropy of hydrolysis in the presence of three-dimensional resins was lower than for the ion-exchange resin and decreased slightly with an increase in the VA content in cross-linked copolymers. Apparently sequences of VA units in a catalytically active insoluble resin are significant during its interaction with the substrate. This is analogous to water-soluble copolymer catalysts despite the fact that, in this case, the interaction of the resin with the substrate is weaker. The alkylation of phenol by isobutylene catalyzed by a cation-exchange resin and sulfuric acid at 60- 100 "C in a batch reactor, showed that an increase in the concentration of hydrogen ions in the cation-exchange resin from 8 to 24 mmol/l augmented the conversion of phenol. However, the selectivity, determined as the ratio of obtained o-tert-butylphenol to reacted phenol, initially decreased but afterwards increased [24]. Upon reducing the cationite particle size from 0.63 to 0.004 mm, the conversion of phenol increased. Nonetheless, a further decrease in particle size did not affect the reaction rate. A model of the conversion of phenol to alkylsubstituted tertiary compounds with the first order for reagents, has been proposed. The apparent activation energy of otert-butylphenol, p-tert-butylphenol and 2,4-di-tert-butylphenol formation is 57.7, 51.5, and 46.4 kJ/mol, respectively. Sulfuric acid, when used as a catalyst, confers somewhat higher conversion yields but lower selectivity than the ion-exchange resin. During the experiment, the cationite does not lose its activity and can be used three times without regeneration. Reactions involving the iodation of acetone, methyl ethyl ketone, and methyl isobutyl ketone by elementary iodine on a strong-acid ion-exchange resin of polystyrene-type and on a weak-acid carboxyl resin were analyzed within a 25-45 "C temperature range [25]. The iodation reaction rate decreased with a drop in the solvent polarity in the sequence: methanol > ethanol > dioxane > n-hexane. Energy and activation entropy values as well as preexponential factor values were calculated on the basis of temperature dependencies on the rate of ketone iodation. The internal reaction of hydrogen ions in the ion-exchange resin matrix was a limiting step of the process. In addition, corresponding apdialdehydes and quinones formed during the oxidation of some cyclic 1,2-diols and 1,2- and 1,4-dihydroxybenzenesin the presence of strong-base ionites as periodates [26]. The highest product yield was obtained in polar aprotonic solvents. Further synthesis of isoquinoline and phenazine derivatives using anionites at the stage of o-quinoline formation was also feasible. Moreover, thermodynamic properties of the hydrolysis of carboxylic acid esters, dissociation of composite compounds and of other reactions catalyzed by functional polymers (sulfurized polystyrene, polyvinyl alcohol acetylated by o-benzosulfoacid) were also discussed [27]. Strong-base ion exchangers have been obained by treatment of chloromethylated resins with both aliphatic and aromatic tertiary amines. Similar weak-base ion-exchange resins have been synthesized using ammonia as well as primary and secondary amines instead of tertiary amines.

54

Chapter 2 Catalysis by Ion-Exchange Resins

OH

As opposed to acidic ion-exchange resins, base resins are less stable and can be used only up to 60°C without a loss in activity. An attempt has been made to study the working mechanism of ionites during the catalytic hydration of 4-cyanopyridine on AB-17-8 ionite in OH form [28-3la]. Two mechanisms have been proposed: the catalysis by counterions and the catalysis by fixed ionite ions. According to the former mechanism, the reaction proceeds by the homogeneous acid-base route:

OH- + R-C=NH bH

-:

R-C-NH,

According to the second mechanism, the reaction occurs according to the heterogeneous route, whereby quaternary nitrogen atoms are responsible for the process.

Thus, both the counterions and fixed ions can serve as catalytic sites where interaction with an organic molecule takes place. The influence of solvent type on the thermodynamics and kinetics of isobutylene and isoamylene hydration in the presence of the acidic catalyst, Amberlite-15, was studied [32]. Such solvents as dioxane, acetone, and nitromethane proved to be inert and provided 100% selectivity for tert-butyl alcohol formation at initial mole ratios of H20/isobutylene >4. Solvents containing an alcohol (butylcellulose, tetrahydrofurfuryl, isopropyl and cyclohexyl alcohols) or an acid (acetic acid) or acetate group

2 Catalysis by Zon-Exchange Resins

55

(methyl acetate) were not chemically inert, and their selectivity for tert-butyl alcohol formation varied from 37 - 97%. Solvents, which have boiling points that are very different from that of tert-butyl alcohol, were best suited for reaction product isolation. For example, dioxane was recognized as an optimal solvent. The kinetics of 1 -butene, cis-2-butene and trans-2-butene hydration to butanol was examined on the strong-acid ion-exchange resin, XE-307, with 0.6-0.7 mm particle sizes over a 75- 150 “C temperature range [33]. Besides hydration, isomerization of the butenes was also observed. The analysis of kinetic data showed the hydration and isomerization of butenes to proceed in equilibrium through formation of secondary butylcarbonium ions. During the butene hydration on catalysts with 0.65 mm particle size, the reactions were retarded by internal diffusion occurring at temperatures above or equal to 115 “C. Equilibrium constant values of butene hydration reactions beyond a 120- 150°C temperature range proved to be much lower than for propene hydration. Strong-base ion-exchange resins in OH - form, partially neutralized by 5-sulfosalicylaldehyde (SSA), exhibit catalytic activity in the racemization of optically active a-amino acids in the presence of copper(I1) ions over a 298 - 3 18 K temperature range [34]. The rate of S-Alanine racemization in the presence of heterogeneous catalysts increased 48-880 fold as compared with the rate of a homogeneous reaction. Apparently the mechanism of a-amino acid racemization by anion-exchangers modified by SSA involves the formation of a reactive copper(I1)-Schiff base complex. This assumption is supported by the following facts: (1) ionites themselves in OH- form do not racemize S-Alanine; (2) the racemization is observed only after SSA modification of resins; (3) the reaction rate increases upon addition of copper(I1) ions; (4) the composition of the reactant complex, Cu(I1) : SSA = 1 : 1, is not washed out; and ( 5 ) SSA is not washed out in the course of the experiments. Considering these facts, the following mechanism of racemization of a-amino acids has been suggested: Like homogeneous catalysis, the removal of a-hydrogen of the amino acid fragment by OH- ions, the local concentration of which is apparently high in the polymer phase, is probably the rate-determining step of heterogeneous racemization. Under similar conditions, the rate of a-amino acid racemization decreases in the sequence: Ala = Ser > Phe > Nva > Lys >Val, and correlates with the rate of substrate racemization in the presence of Schiff bases and transamination of amino acids by pyridoxal phosphate. Subsequently, the kinetics of S-Ala racemization has been investigated on heterogeneous catalysts obtained on the basis of eight modified anion-exchangers in the presence of Cu(I1) at 298 K [35]. For characterizing polymers the term “internal capacity value”, F, has been introduced, whereby F = mp/ (V, - Vp). The symbol mp denotes the number of quaternized amino groups in a polymer; V,, the total volume of a swollen polymer; and V,,, the dry polymer matrix volume. Good correlation has been established between the observed rate constant of the S-Ala racemization reaction and F values. All anionexchangers are divided into two groups according to the kind of nitrogen groups added to a quaternary nitrogen atom. The first group has at least one P-hydroxyalkyl substituent attached to the nitrogen atom, while the other consists of cationic groups

56

Chapter 2 Catalysis by Ion-Exchange Resins

0

II

R-Ce/c\o

\

I

HCr C r - O H

2 Catalysis by Ion-Exchange Resins

57

t

of the - N(CH&-type. Under identical conditions, catalysts of the first group possess much higher activity than those of the second group. An increase in the activity of heterogeneous catalysts, as opposer to homogeneous analogues, is attributed to the high concentration of OH- in the internal phase of ionites. The catalytic activity of macroporous anionite and Amberlite gel in the reaction of acrylic acid esterification by epichlorohydrin was studied at 50-90°C as a function of the catalyst concentration, molar ratio of reaction mixture components, counterion nature, reaction medium and catalyst particle size [36]. It was found that the reaction proceeds with an appreciable rate even in the presence of a very slight amount of catalyst. Furthermore, the product yield increased with an increment in the catalyst concentration and reaction temperature. The OH-form of ion-exchange resins was most active, and polar solvents promoted the process. The kinetics of octanoic acid esterification by 1 -butanol catalyzed by the macroreticular, sulfonylated ion-exchange resin, Amberlite-15, in a batch reactor was measured [37]. The effect of the catalyst amount, temperature and concentration of alcohol, water and butyl octanoate was investigated. Experimental data suggested that the solvated sulfo groups are bonded with the alcohol-water matrix. It was assumed that the examined reaction system included the heterogeneous reaction catalyzed by nonionized sulfo groups as well as the pseudohomogeneous reaction catalyzed by solvated protons. The majority of research on catalysis by anion-exchange resins is devoted to the use of resins in interfacial catalysis reactions [38 -431. In this connection, quaternary ammonium or phosphonium, onium salts have been commonly obtained [44]. Crownethers, cryptands and linear polyesters supported on polymers also catalyze similar reactions dealing with the interfacial transfer of reagents. In the simplest case, the ~ mechanism of interfacial catalysis represents a substitution reaction of S N type typical for the interaction of the nucleophile, Y - , present in the aqueous solution with the alkyl halogenide, R X , in the organic phase: Q+Y-+RX

-It

-

R Y + Q + X - organic

I t

Q + Y - + M + X - --+ M + Y - + Q + X - aqueous whereby M + Y - is the alkaline metal salt of the nucleophile; and Q ' X onium salt of the catalyst:

is the

X = C I . B r . I etc. yz1.2.3 ........

The onium salt is present in the aqueous phase in equilibrium with the ion pair Q ' X - . Because of the relative hydrophobic character of Q', it is effectively

58

Chapter 2 Catalysis by Ion-Exchange Resins

transferred to the organic phase. Catalysts on supports differ favorably from their nonbonded analogues [46,47]. The activity of polymeric phase-transfer catalysts depends on such experimental parameters as the active center structure, size of catalyst particles, structure and degree of cross-linking of a polymeric support, etc. [48]. Quaternary ammonium and phosphonium salts on a polystyrene support, which are effective interfacial transfer catalysts, can be separated from the reaction mixture by simple filtration and be used anew [49]. Some aspects of the use of catalysts on polymeric supports during organic synthesis have been analyzed [50-541. It has been found that phase-transfer catalysts facilitate the interaction of water-soluble reagents near the water-solvent interface. The reaction of I-bromoctane with the aqueous solution of sodium cyanide catalyzed by benzyltributylphosphonium ions bonded with the macroporous polystyrene matrix was studied [55 - 591. The main limiting factors were: the mass transfer of an organic substrate (1-bromoctane or benzyl bromide) from the organic phase bulk to the surface of catalyst particles; the diffusion of the substrate through the polymer matrix and the reactivity of active sites. The reaction rate is somewhat lower for a microporous polymeric catalyst than for a gelatinous one; i.e. the use of a macroporous polymer as a matrix does not facilitate the accelerated transport of ions within catalyst particles because of low swelling of the catalyst in aqueous media [55]. The reaction rate decreases with an increase in the degrees of cross-linking of the polymer from 2 to 10%. For instance, at 80°C and at 2,4, 6, and 10% degree of cross-linking, the respective reaction rate constants are 57.8. 24.10-5, 7.8.10-5, and s-'. This is attributed to the difficulty of the diffusion of reagents inside 1.92. a catalyst particle. Upon increasing the swelling of the polymer matrix in a solvent, the reaction rate increases in the order: decane < toluene < chlorobenzene. Concerning Ref. [51] the authors discussed the effect of different structural parameters on the rate of nucleophilic substitution reactions catalyzed by quaternary ammonium and phosphonium ions, as well as the effect of DVB cross-linking and the morphology (pore size). They also compared copolymers cross-linked by DVB and aliphatic dimethacrylate - rigid materials with a high surface area and gels with a low surface area. The intrinsic rotation mobility of chains in cross-linked gels of polystyrene was measured by the I3C NMR relaxation method. The relaxation time, T I , was insensitive to the cross-linking. However, during relaxation, T2 decreased when the time of cross-linking increased. 13C NMR spectroscopy was used to study the effect of the substitution degree in the ring on the mobility of the macro- and microenvironment and on the catalytic activity of methylenetributylphosphonium chloride groups and mesylate groups bonded with macroporous polystyrene crosslinked by 1070 DVB in three-phase systems [60]. The activity of tributylphosphonium groups, bonded directly with the polystyrene matrix through methylene bridges during the interfacial catalysis of nucleophilic substitution of aliphatic compounds in a liquid-liquid system, declined about ten fold by raising the degree of ring substitution from 10-60% [61]. Separation of the polymer matrix and phosphonium cation by a chain of 13 atoms at a 10-30% concentration of substituted rings raised the catalyst activity 1.3-3.1 fold. At a 60% degree of substitution, the catalyst activity increased 4.1 - 10 fold. The catalyst structure effect was explained by an increase in

2 Catalysis by Ion-Exchange Resins

59

the polarity of the catalytic center with an increase in the degree of substituted rings. Thus catalytic effectivity is augmented by increasing the bond flexibility between the catalyst and the main polymer chain [ I , 621. The chloromethylated copolymer of styrene with DVB (2%) was modified by 10-undecene-1-01or 1 1-bromo-1-undecene. A number of subsequent chemical conversions led to the formation of phosphonium salt fragments removed from the polymer skeleton by chains of methylene (spacer) groups [63, 641. Alternatively, such products were obtained by copolymerization of p-(6-bromoheptyloxy)styrene with styrene and DVB with subsequent interaction of the synthesized copolymer with tributylphosphorus in chlorobenzene. Interfacial catalysis reactions in C6HI7Br-NaCN(or KJ) systems in a toluene-water (2 :3 vol/vol) mixture at 90 "C were studied as a function of the radius of immobilized catalyst particles and the degree of substitution of benzene rings by phosphorus-containing groups. During these pseudo first-order reactions, the maximum activity was achieved at a 14-21 Vo degree of substitution for different catalyst types. It was shown that the insertion of spacer fragments between the reactive group and the main polymer chain leads to an increase in the activity of immobilized, interfacial transfer catalysts. The effectiveness of polymer-supported quaternary salts as interfacial transfer catalysts for alcohol 0-alkylation has been studied in the model reaction of benzyl alcohol with butyl bromide which yielded benzyl butyl ether [65]. Catalyst samples were prepared by reacting chloromethylated ion-exchange resins with quaternary amines at 70°C for 70 h. Ester synthesis was conducted at 40°C using 40% KOH. The effectiveness of the catalytic system under study was assessed, whereby the reaction rate depended on the concentration of benzyl alcohol and butyl bromide in the organic phase and the KOH concentration in the aqueous phase. With respect to kinetic data, it was shown that the limiting step is determined by the reagent concentration. During synthesis, K+ Ph-CH20- formed at the organic and aqueous phase boundary and (or) in the aqueous phase due to the reaction of benzyl alcohol with OH-. When the concentration of benzyl alcohol was very low, the latter reaction became the rate-limiting step. At a C6HSCH20Hconcentration greater than 0.3 M, the reaction rate sharply dropped with an increase in the benzyl alcohol content as a result of an increase in the water content in the organic phase. Thus, the concentration of benzyl alcohol greatly affected the water content in the organic phase, an important factor in the determination of optimal conditions of synthesis. The catalytic activity of cross-linked polymeric sulfoxides differing in the functionality of the polystyrene matrix (the general formula is @-(CH20CH2), -R, whereby @ is the polystyrene matrix cross-linked by divinylbenzene, R = H or CH3, and n = 1, 2 or 3), has been studied in nucleophilic substitution reactions between alkyl bromides (1 -bromobutane and 1-bromooctane) and phenoxides, iodides, thiocyanates or cyanides of alkaline metals under the conditions of catalytic three-phase reactions in the liquid-solid-liquid system [66]. The reaction is carried out in a toluene-water medium between 70- 100°C. In the systems the rate of anion transfer from the liquid phase to the organic phase decreases in the sequence: P h O - > J - > S C N - > CN-.

60

Chapter 2 Catalysis by Ion-Exchange Resins

By emulsion polymerization of styrene with chloromethylated styrene in the presence of divinylbenzene and hexadecyltrimethylammonium bromide as a surfactant, latex particles of the copolymer were synthesized with a diameter of 0.08-0.2 m [67,68]. The copolymer was transformed directly to phosphonium salts by boiling the emulsion with tributylphosphine for 24 h. The stirring rate over 510-670 rpm did not affect the rate constants of catalyzed, interfacial transfer reactions of the aqueous solution of sodium cyanide with I-bromooctane at 90°C and with benzyl bromide at 80 "C in toluene in the presence of the above catalysts. The catalyst activity increased with an increase in the latex particle size. Industrial anion-exchange resins are of little use for realization of three-phase catalysis because their interaction with olefin compounds does not yield the necessary stereochemical control of the reaction. It has been proposed to synthesize resins by reacting chloromethylated polystyrene (cross-linked by divinylbenzene) with amines of higher oleophility or by reacting amines with a styrene copolymer with a-olefins of appropriate structure [69]. Hereby interesting are linear and cross-linked copolymers which contain catalytically active functional groups in a homogeneous conformation environment; i.e. - CH,CH(CCH,N R3X-)CH2CH(R*) - and - CH2C(COOR*)(Y)CH,C(COOCH2N+ R3X-)(Y) -. Earlier [70], the synthesis of cross-linked polycations of controlled structure by spontaneous polymerization of 4-vinylpyridine with o-dibromoalkanes of Br(CH2),Br-type (whereby x changes from 2 to 12) was proven feasible. Cross-linked polycations catalyze the nucleophilic substitution reaction between n-octyl bromide and sodium cyanide in a three-phase system. The yield of n-octyl cyanide increases with an increment in x but is minimal at x = 3. This is attributed to the organophility of the catalyst and flexibility and pore sizes of the polymer network. Polymer-supported linear polyethers are more effective interfacial transfer catalysts [71]. Such polyethers readily activate charged nucleophiles through the specific interaction with their counterions, alkaline metal ions [72 - 761. Resin-supported oligoethylene glycol monoethers catalyze substitution reactions involving alkyl halogenides in aqueous solutions of potassium phenolate or sodium hydroxide [74,75]. Graft copolymers of polyethylene glycol (PEG) on DVB cross-linked polystyrene have been synthesized, and their catalytic activity in three-phase (liquidsolid-liquid) alkylation of 2-phenylacetonitrile with 1-bromobutane has been investigated [77]. The 2-phenylhexanenitrile yield is maximum for the copolymer obtained from the monoethyl ether of PEG with 12 ethylene oxide units and a 17% degree of ring substitution of chloromethylated polystyrene. The activity of this catalyst approximates that of benzyltriethylammonium chloride during the catalysis of the same reaction. The activity of these copolymeric catalysts decreases with reducing the length of PEG side chains and with an increase in the degree of ring substitution. Graft copolymers containing OH- and CH3- end groups as well as 18-crown-6 units show moderate activity. However, catalysts containing quaternary ammonium groups as substituents are the least active. In addition, PEG copolymers actively catalyze the alkylation of other nitriles, alcohols, and ketones. After quantitative recovery from the reaction medium, the polymeric catalyst remains active even after repeated use. +

2 Catalysis by Ion-Exchange Resins

61

Furthermore, graft copolymers of polyethylene glycol and cross-linked 18% divinyl-benzene of the general formula PS - CH2R, in which R = O(CH2CH20)6,4H (52% ring substitution and R = O(CH2CH20)~H(17% ring substitution), are active and stable three-phase catalysts for the saponification of esters (amyl acetate, methyl benzoate, ethyl decanoate, lactone of 16-hydroxyhexadecanoic acid, 2,4,6-trimethylbenzoate, tert-butyl benzoate) in an aqueous KOH solution in benzene, n-hexane or toluene at 25 "C and 70°C. The high activity of the catalysts is attributed to the end OH-groups. The grafted polystyrene copolymer in which R = O(CH2CH20)7,2CH3 with a 17% degree of ring substitution, does not actually catalyze the reaction. The activity of some polyethylene glycols bonded with macroporous copolymers of glycidylmethacrylate during the interfacial catalysis of the model reaction of sodium phenolate with n-BuBr, were investigated [79]. Rate constants of phenol alkylation in the aqueous system NaOPh-n-BuBr (in toluene)-PEG were measured at 60°C as a function of the molecular weight of PEG and its concentration. The obtained results were compared with the data on the kinetics of the reaction occurring in the presence of soluble PEG. Upon the insertion of immobilized and soluble PEG, alkylation rates increased 168 and 139 fold, respectively. The increase of the catalytic activity of immobilized PEG, which corresponded to a rise in the molecular weight of the polymer, was caused by an increase in the PEG ability to sorb alkali metal cations. PMMA cross-linked by ethylene dimethacrylate with immobilized side oligooxyethylene chains were used as interfacial transfer catalysts [80]. Also studied was the formation of benzyl acetate, benzyl bromide and alkali metal acetates in boiling chloroform and of n-octylphenyl ester from n-octyl iodide and K or Na phenoxide in toluene at 100 " C in the presence of a catalytic polymer amount. The product yield increased with an increase in the reaction time and the catalyst/acetate or catalyst/ phenoxide molar ratio. The conversion of benzyl bromide increased according to the following sequence of acetates: Na < K
62

Chapter 2 Catalysis by Ion-Exchange Resins

PhONa in dioxane [84]. It is assumed that each bipolar unit of insoluble CPVP is capable of solvating a cation, thereby increasing the nucleophilic activity. In the presence of CPVP, the conversion of BuBr increases more than 30 times compared with the reaction conducted in the presence of monomeric N-methylpyrrolidone. Moreover, the conversion is twice higher than in the presence of linear soluble PVP. The amount of CPVP, its particle sizes and reaction temperature do not markedly affect the degree of BuBr conversion. An increase in the density of cross-links impairs the effectiveness of its action. In reactions occurring in three-phase systems such as liquid-solid-liquid and solid-liquid-solid, e.g. butyl bromide with dioxane-insoluble potassium cyanide, CPVP behaves similarly to an interfacial transfer catalyst. Fifteen- and 18-memberedcrown ethers bonded to a polystyrene support, were used as interfacial transfer catalysts in halogen exchange reactions involving 1-chloro- and 1-bromooctane and aqueous solutions of NaJ or KJ in toluene, chlorobenzene and octane [85]. The reactions proceeded according to a pseudo first order, and their observed rate constants, Kobs, increased with an increase in the stirring rate up to 410 rpm. Thereafter Kobs remained constant. These values also decreased upon raising the degree of cross-linking (2-10molVo DVB). Absolute values of Kobs were higher for I-bromoctane, and depending on the solvent nature, they diminished in the sequence: chlorobenzene > toluene > octane. The rate constant increased with a rise in the degree of ring substitution in toluene and chlorobenzene. In octane, Kobs was maximum at 17% substitution. Furthermore, the catalyst activity depended on the length of methylene bridges bonding crown ethers with the polymer chain. The reaction rate was also affected by catalyst particle sizes, crown ether cavity sizes and by the nucleophilic structure of the salts. The mechanism of catalysis was discussed with regard to the mass transfer, inward particle diffusion and the catalyst structure effect. The synthesis of w-amino-substituted 18-crown-6 and [2.2.2]cryptand readily bonded with chloromethylated polystyrene cross-linked by different amounts of DVB, is described [86]. Such bonded polyesters are used as interfacial transfer catalysts promoted by anions. As in the case of analogous soluble systems, the catalytic activity of cryptands is higher than that of crown ethers and quaternary onium salts. Because of their high chemical stability, such catalysts can be regenerated without chemical decomposition. However, the impairment of mechanical properties caused by comminution of the polymer matrix remains to be solved. Polymeric cryptands with different porosity, polarity, network density and copolymer compositions have been obtained by polymerization and copolymerization 1,l0-diazabicyclo[8.8.8]hexacosane [87]. of 5,6-(vinylbenzo)-4,7,13,16,21,24-hexaoxaIt was found that copolymer composition and porosity do not affect the catalytic activity of polymeric cryptands in the nucleophilic substitution reaction between benzyl chloride and solid potassium acetate. Presumingly the reaction occurred on the polymer surface. These polymeric catalysts were more active during the nucleophilic substitution reaction between solid KCN and 1 -4-dichlorobutane. Polymeric catalysts containing fixed crown fragments were synthesized by condensation of hydroxymethyl- or w-hydroxynonyl-I8-crown-6 with a chloromethylated copolymer of styrene and DVB (1 '-70)[88]. The catalytic activity of polycrown ethers in the nucleophilic substitution of J -, Br -, CN- , SCN- in sodium salt of methanesulfonic

2 Catalysis by Ion-Exchange Resins

63

acid as well as in the exchange of J - , Br- in n-octyl bromide, were studied, and soluble crown ethers were used for comparison. Apparently the interfacial transfer reaction mechanism which involves the formation of the ion complex with a crown ether cavity, its dissociation and ligand substitution, did not change by crown ether fixation. To extend the use of high-molecular weight catalysts in organic synthesis, the synthesis and properties of some strong-base ionites [89,90], polymer-bonded crown ethers [91, 921 and open-chain analogues of crown ether compounds, have been investigated:

The catalytic effectiveness of polymeric reagents has -een demonstratei in nucleophilic substitution reactions [91], 0- and N-alkylation [92] and Knoevenagel condensation [91,93]. The preparation procedure of catalyst I during the reaction of halogen substitution in alkyl or aryl halogenides by an anion of a potassium salt is shown in Table 7: Table 7. Nucleophilic substitution reactions in the presence of catalyst I

R-X

KY

R-Y

Solvent

Yield R-Y or0

Poly(dibenz0-I 8-crown-6) obtained by polycondensation of dibenzo-18-crown-6 with formaldehyde in the presence of an acidic catalyst, is particularly interesting. Dibenzo-18-crown-6 is bonded with both macroporous and gel copolymers of styrene and DVB [91]. The former, which possess fixed-space structure, are bonded by approximately equal amounts of the crown ether, irrespective of the degree of cross-linking of the polymer matrix. In gel copolymers, one can easily observe a decrease in the amount of bonded groups of the crown ether with an increase in the DVB content of the matrix. The obtained polymers possess rigid structures. Furthermore, they swell relatively little and do not change their specific volume in chosen solvents. The above advantages determine their application, especially in column processes. Polymerbonded crown ethers have been tested in the Knoevenagel condensation reaction of aromatic aldehydes with compounds containing an activated methylene group.

64

Chapter 2 Catalysis by Ion-Exchange Resins

The influence of catalyst structure, solvent and the activity of methylene and aldehyde components on the yield of reaction products has been studied. Novel catalysts composed of three-dimensional copolymers of N-vinylpyrrolidone and the divinyl ester of diethylene glycol, have been proposed as interfacial catalysts [94]. Catalytic properties of these copolymers were analyzed during alcohol and phenolate alkylation by alkyl halogenides. The acquired data inferred that the copolymers effectively catalyze phase-transfer reactions. During alcohol alkylation, ether yields were somewhat higher than by the alkylation of phenolates. The authors attributed this to a better distribution of phases between the solid catalyst and the surrounding reaction mixture during alcohol alkylation than during phenolate alkylation. The glycoside yielding reactions of methyl and tert-butyl alcohols with tetra0-acetylbromoglucose in the presence of AgN03 (Koenigs-Knorr reaction) were also investigated. Additionally studied was the effect on the above reaction of additions of six macrocyclic compounds of monomeric and polymeric crown ethers as well as of those covalently bonded with polyacrylamide gel, which differed in the types of substituents, heteroatoms and cycle rigidity. The crown ether promoted the reaction and caused formation of sugar nitrate as a side product. The reaction selectively depended not only on macrocycle properties, alcohol molecular sizes and the dielectric constant of the liquid medium, but also on the type of the polymeric ligand with which the immobilized crown ether is bonded. In the reaction with the metal, only one product quantitatively formed regardless of the degree of macrocycle immobilization. During the reaction of tert-butanol in the absence of solvents, the role of the crown ether in promoting the substitution depended on the rigidity of the polymer matrix. The effect of the polymer on the selectivity was explained by steric hindrances occurring during the chemical reactions.

References [l] “Reaktzii na Polimernykh Podlozhkakh v Organitchesom Sinteze”, Mir, Moscow 1983; Engl. original “Polymer Supported Reactions in Organic Synthesis”, P. Hodge and D. Sherrington editors J. Wiley & Sons, New York, 1980 [2] J. Klein, Abstr IUPAC Int. Symp. on Macromol. Amherst, 1982 Section 1, p. 273 [3] W. Nier, Ind. Chem. 104, 632 (1981) [4] F. Helfferich, “Ion-Exchange”, McGraw-Hill, New York 1962 [5] K. Setinek, Izv. Khim. Akad. Nauk. Bulg. 13, 119 (1980) [6] H. Widdecke, Brit. Polym. J. 16, 188 (1984) [7] N.G. Polyanskii, Usp. Khimii 31, 1046 (1962); 39, 504 (1970) [8] Sugihara, Sci. and Ind. 37, 334 (1963) [9] J. R. Millar, D. G. Smith, W.E. Marr, T. R.E. Kressman, J. Chem. SOC.218 (1963) [lo] R. Thornton, B.C. Gates, J. Catal. 34, 275 (1974) [ l l ] B.C. Gates, W. Rodriguez, J. Catal. 31, 27 (1973) [12] E. Knozinger, H. Noller, Z. Phys. Chem. (BRD) 55, 59 (1967) [13] B.C. Gates, J. S. Wisnouskas, H. W. Heath, J. Catal. 24, 320 (1972) [14] S.P. Sivanand, B.V. Kamath, R.S. Singh, D.K. Chakrabarty, J. Catal. 69, 502 (1981) [15] S.P. Sivanand, R.S. Singh, D.K. Chakrabarty, Proc. Indian Acad. Sci. Chem. Sci. 92, 227 (1983)

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66

Chapter 2 Catalysis by Zon-Exchange Resins

[53] Ch.U. Pittman, Polym. News 10, 75 (1984) [54] W. Ford, J. Lee, M. Tomoi, Macromolecules 15, 1246 (1982) [55] P.F. Marconi, W.T. Ford, J. Catal. 83, 160 (1983) [56] W. Ford, J. Lee, M. Tomoi, Polym. Prepr. Am. Chem. SOC.23, (I), 183 (1982) [57] M. Tomoi, T. Ford, J. Am. Chem. SOC.103, 3821 (1981) [58] M. Tomoi, T. Ford, J. Am. Chem. SOC.102, 7140 (1980) [59] M. Tomoi, E. Ogawa, Y. Hosokawa, J. Polym. Sci., Polym. Chem. Ed. 20, 3421 (1982) [60] N. Ohtani, Ch.A. Wilke, A. Nigani, S.L. Regen, Macromolecules 14, 516 (1981) [61] P.L. Anelli, F. Montanari, S. Guici, J. Chem. SOC.Perkin Trans. Part 2, 12, 1827 (1983) [62] M. Tomoi, Y. Hosokawa, H. Kakiuchi, J. Polym. Sci., Polym. Chem. Ed. 22, 1243 (1984) [63] M. Tomoi, S. Shiiki, H. Kakiuchi, Makromol. Chem. 187, 357 (1986) [64] M. Tomoi, N. Kori, H. Kakiuchi, React. Polym. 3, 341 (1985) [65] H. Takeuchi, Y. Miwa, S. Morita, J. Okada, Chem. and Pharm. Bull. 33, 3101 (1985) [66] V. Janout, H. Hrudkova, P. Cefelin, Collect. Czech. Commun. 33, 3101 (1984) [67] M. Bernard, W.T. Ford, T. W. Taylor, Macromolecules, 17, 1812 (1984) [68] M. Bernard, W.T. Ford, T. W. Taylor, Polym. Prepr. Am. Chem. SOC.25, (I), 170 (1984) [69] E. Chiellini, IUPAC Macro Florence Int. Symp. on Macromol. 1980, Prepr. V. 1, Pisa, s.a. p. 102 [70] J. Noguchi, M. Sugawara, Y. Uchida, Polymer 21, 861 (1980) [71] T. Wakui, Wen Ying Xu, Chi-Sung Chen, J. Smid, Makromol. Chem. 187, 533 (1986) [72] S.L. Regen, J. J. Besse, J. McLich, J. Am. Chem. SOC.101, 116 (1979) [73] D.C. Sherrington, Polymer 21, 791 (1980) [74] D.C. Sherrington, Polymer 22, 431 (1981) [75] D.C. Sherrington, W.M. MacKenzie, IUPAC Macro Florence, 1980, Int. Symp. on Macromol. 1980, Prepr. v. 4, Pisa, s.a. p. 178 [76] J.C. Hefferman, D.C. Sherrington, Tetrahedron Lett. 24, 1661 (1983) [77] L. J. Mathias, J.B. Canterberry, Polym. Prepr. Am. Chem. SOC.22, (1) 38 (1981) [78] Y. Kimura, S.L. Regen, Synth. Commun. 13, 443 (1983) [79] J. Hradil, F. Svec, Polym. Bull. 11, 159 (1984) [SO] T. Wakui, Wen Ying Xu, J. Smid, Polym. Prepr. Am. Chem. SOC.25, (2), 126 (1984) [81] K. Sukata, J. Synth. Chem. Jpn. 39, 443 (1981) [82] S. D’Antone, M. Penco, R. Solaro, E. Chiellini, React. Polym. 3, 107 (1985) [83] E. Chiellini, R. Solaro, Chim. Ind. (Milan) 60, 1006 (1978) [84] M. Jelinkova, J. Kahovec, F. Svec, Polym. Prepr. Am. Chem. SOC.26, (l), 84 (1985) [85] M. Tomoi, N. Vanai, S. Shiiki, H. Kakiuchi, J. Polym. Sci. Polym. Chem. Ed. 22,911 (1984) [86] F. Montanari, J. Org. Chem. 46, 2125 (1981) [87] G. Manecke, P. Reuter, J. Mol. Catal. 13, 355 (1981) [88] P. L. Anelli, B. Czech, F. Montanari, S. Quici, J. Amer. Chem. SOC.106, 861 (1984) [89] Ya. Ya. Gibiets, A. A. Putze, A. Kh. Zitzmanis, Izv. Akad. Nauk Latv SSR, Ser. Khim. 600 (1986) [90] A. Kh. Zitzmanis, I. G. Zilnietze, A. 0. Berzinya, Izv. Akad. Nauk Latv SSR, Ser. Khim. 613 (1986) [91] A. S. Roska, M. K. Klyavinsh, S.A. Eiduka, A. Ya. Veveris, A. Kh. Zitzmanis, Izv. Akad. Nauk Latv SSR, Ser. Khim. 618 (1986) [92] M. K. Klyavinsh, A. S. Roska, A. Kh. Zitzmanis, Izv. Akad. Nauk Latv SSR, Ser. Khim. 21 5 (1985) [93] M.K. Klyavinsh, A. S. Roska, A. Kh. Zitzmanis, R. K. Shitze, Ya. R. Liepinsh, Izv. Akad. Nauk Latv SSR, Ser. Khim. 625 (1986) [94] A. Kh. Zitzmanis, M. K. Karulis, M. P. Kovaleva, Izv. Akad. Nauk Latv SSR, Ser. Khim. 353 (1986) [95] A. Ricard, J. Capillon, C. Quivoron, Polymer 25, 1136 (1984)

Catalysis by Polymers

E. A. Bekturov and S . E. Kudaibergenov Copyright 0 2002 WILEY-VCH Verlag GmbH & Co. KGaA

Chapter 3 Heterogenized Homogeneous Catalysts *

Recently much attention has been devoted to fixing homogeneous catalysts, namely transition metal compounds, onto polymeric supports [ 1 - 51. So-called heterogenized homogeneous catalysts combine the advantages of both homogeneous and heterogeneous systems. They retain such positive properties of homogeneous analogues as high selectivity, homogeneity of active centers, and ready modification. On the other hand, they possess some advantages of heterogeneous catalysts such as ease of separation, simple regeneration, long service life, thermostability and mechanical strength. Two basic mechanisms of fixation of a homogeneous catalyst onto a polymeric support have been cited. The first involves different means of catalyst component precipitation, impregnation, inclusion in gel, microencapsulation, erc. The second concerns fixation of catalytic sites by valence, coordination and ionic bonds, immobilization, ion exchange, etc.

3.1 Catalysts Dispersed on Polymeric Support Surfaces Initial research [6- 191 on fixing platinum group metals onto polymeric supports was devoted mainly to studying the kinetics of hydrogenation of ally1 alcohol, dimethylethynylcarbinol (DMEC), o-nitrophenol, and methyl ethyl ketone by means of platinum and palladium catalysts on polymers of polyvinyl alcohol (PVA- CH2 CH(OH), polyhexamethylenadipamide - HN(CH&NHCO (CH2)4CO, polyacrylonitrile (PAN) - CH2 -CH(CN)-, and poly(ethy1ene terephthalate) (PETP)-(OC) C6H4(CO) OCH2CH2-. The catalysts were prepared by fixation of platinum and palladium simultaneously reduced by electrolytic hydrogen. Although kinetic parameters of the reaction and the influence of solvent and unsaturated compound properties were discussed in detail, the effect of the properties of the polymer support and its structure on catalyst activity were neglected. The influence of the structure of polymeric supports on the selectivity and activity of catalysts of platinum group metals supported as high-dispersed powders on polyamide, PAN, PVA, PETP and other polymers was studied during the hydrogenation of unsaturated organic compounds [20] and during hydrogen peroxide decomposition [21, 221. In some cases an optimum metal/polymer ratio was found [23 -261. The effect of the chemical properties of a support on the activity of dispersed zerovalent metals has been shown [27]. Chloromethylated polystyrene, cross-linked with 4-7% DVB with high porosity and surface area (0.1 -0.03 pm particles), was functionalized by the following groups: n-donor and n-acceptor groups capable of forming molecular associates with the substrate; hydrophobic and hydrophilic groups

*

S. S. Saltybaeva contributed to Chapter 3.

Chapter 3 Heterogenized Homogeneous Catalysts

68 I

@- CH20H

@-CH,NH,

I

7 I11 @-CH,NR2 IR=n-octyl)

@-CHNH2

+

F O N O ,

NO2 @-CH,OH

+!=&NO2

-0

@ C H 2 N H e N 0 2

P

CH20b

N

0

,

VI

VII

which elicit polar effects on the substrate; and groups that affect different physical properties of the polymers. Functionalized polymeric supports were formed by modifying chloromethylated polystyrene by means of various reagents. The polymeric supports shown above have the following functional groups: ndonor and n-acceptor groups (polymers V, VI, and VII); and hydrophobic or hydrophilic groups (polymers I - IV). Polymers with similar functional groups but different matrices constitute a separate group. All the experiments were conducted with the same amount of active metal (0.54 mg Pd) at 40 "C and at a H2-partial pressure of 500 mmHg. The molar ratio of Pdo to the substrate was 1 :2070. It was shown that catalysts, the functional groups of which decreased the retention time of the substrate in the polymer matrix or enhanced the substrate solubility in the polymer matrix, catalyzed the hydrogenation of styrene more effectively. Such catalyst types included n-acceptor or hydrophobic supports. During the hydrogenation of ally1 acrylate of the polar substrate model, the catalytic activity depended on both the n-acceptor and polar properties of the polymeric supports. Thus, a definite relationship was determined between properties of functional groups and the respective polymers. Polymer-supported platinum catalysts with different Pt concentrations were obtained by mixing powdered polymers with aqueous or alcohol solutions of H2PtC16, K2PtCI,, K2PtC16or (NH3),PtC12 at temperatures between 20- 100 "C [28]. Nylon-5, nylon-3, polyacrylamide, poly-p-phenyleneterephthalamide, polyacrylamidoxim, polyacrylonitrile, pyrolyzed polyacrylonitrile, Dowex ion-exchange resins and the molecular sieves NaY and A1203, were used as supports. The activity and selectivity of these catalysts were examined during benzene hydrogenation performed at 140- 19O"C, during a period of 0.2-2.2 h and at a H2: C6H6 ratio of 3 : 1. Before use, the catalysts were activated for 7 h in a H2 current at 160°C. According to their

3.1 Catalysts Dispersed on Polymeric Support Surfaces

69

selectivity for cyclohexene, all the applied catalysts were divided into 2 groups: platinum-polyamide catalysts selective for cyclohexene, and the remainder. Special selective properties of Pt-polyamide catalysts exhibited during benzene hydrogenation were attributed to the presence of bivalent platinum on the polyamide surface coordinatively bonded to two amide groups of the polymer chain. The hydrogenation of propylene by platinum [28] and rhodium [29] fixed on inorganic (A1203, MgO) and polymeric (nylon-66, nylon-610) supports, was conducted in flow and cyclic reactors. The reaction rate depended on the type of support used and was highest for platinum/nylon samples. Treatment of the catalyst at 130- 140 "C led to a full loss of the metal catalytic activity as a result of the destruction of the polymeric support. Similar results were also shown for tin. Apparently [29] the incorporation of tin caused formation of bimetallic clusters and influenced the electronic state of platinum. Polymeric catalysts were also synthesized by fixing metal compounds (Wilkinson catalyst, tertnuclear carbonyl cluster MoC15, Ni(acac)) onto the surfaces of polyethylene [30-321, polyacetylene [33], teflon [34], and polydibenzo-18-crown-6 [35]. In each case, the activity of dispersed catalysts was higher than that of the homogeneous catalysts. Highly effective catalysts have been prepared by using the protective properties of some hydrophilic polymers, such as polyvinylpyrrolidone (PVPD), polyvinylmethyl ester (PVME), dextrin and PVA. The protective role of these synthetic polymers is to prevent the aggregation of colloidal metal particles and to stabilize the homogeneous dispersion of small particles. These catalysts possess high activity and selectivity. They are, moreover, easily separated from reaction products and can be repeatedly reused [36, 371. The procedure for preparing colloidal dispersions is very simple. Boiling a PVA solution (polymerization degree of 500) and rhodium (111) chloride in a water-methano1 (1 : 1 vol.) mixture for 0.2 - 16 h at 79 "C yields a homogeneous solution, the colour of which changes from light- to dark-brown [36, 371. Colloidal dispersions of PVPD and PVME can also be obtained. Dispersed particles are also prepared by using ethanol instead of a water-methanol mixture or by adding a little NaOH to the methanol solution. By means of electron microscopy, the formation of colloidal particles with diameters of 8 A (small) and 40 A (large) was determined in the Rh-PVA-CH30H/H20 system. Fig. 21 presents the histogram of rhodium particle sizes after 4 h of boiling. It can be seen that large particle sizes occur mainly within the 30-70 A range and constitute 94% of the total number. The mean size of particles is 40 A.At the early stage of boiling of RhC13 and PVA solutions, particles of 8 A in diameter mainly form. The formation of small particles proceeds in two steps: the reduction of three-valent rhodium to the zero-valent state and aggregation of the latter to particles of 8 A diameter which consist of 20 atoms of rhodium. The crystal structure of metallic rhodium has a face-centered cubic lattice with the coordination number of 12 and corresponds to a RhI3 cluster. The transition of small particles (8 A) to more stable large aggregates (40 A) occurs by prolonged boiling of the RhCI3 and PVA mixture. Apparently, small particles serve as sites for the growth of

Chapter 3 Heterogenized Homogeneous Catalysts 40

20

0 100

50

Fig. 21. Distribution of colloidal particles of rhodium by size [36]

0

Diameter in A

large-particle crystals. In general, the process of formation of colloidal rhodium can be presented by the following scheme: RhC1, 5 complex PVA-Rh(II1) methanol_ small particles (8 A) boi’ingw large particles (40 A). The formation of a coordination complex at the early stage is shown by the appearance of an absorption band at 1 = 260 nm. However, the addition of the low-molecular weight complexing agent, ethylenediamine (EDA), completely inhibits the formation of colloidal dispersions. Consequently, in the presence of EDA the absorption band at 260 nm disappears. This implies that the first formation stage of colloidal dispersions includes the interaction of Rh(II1) and PVA. Colloidal dispersions of copper, palladium, silver, osmium, indium, platinum and gold can be prepared using both PVA and PVPD (Table 8). Table 9 lists some characteristics of the colloidal dispersion of rhodium. Figure 22 presents the change in the diameter of rhodium particles and their catalytic activity during cyclohexene hydrogenation. At the early boiling stage, the

Table 8. Formation of colloidal dispersion of various metals and hydrophilic polymers [36] Metal salts

PVA-CH,OH/H,O

PVPD-C2H,OH/H,O

PVPD-CH,OH/H,O

RuCI,.3HzO RhCI3.3H20 PdCl,.2H,O &NO,

-

-

-

+ +

+ + + + +

+ + + + + + +

oso,

Na21nC1, H,PtCI, HAuCl, (-):

-

+ + + -

no dispersion; (+): dispersion.

-

3.1 Catalysts Dispersed on Polymeric Support Surfaces

71

Table 9. Particle sizes and catalytic activity of colloidal rhodium dispersions ([Rh] = 0.01 mmol; [substrate] = 25 mmol; methanol is a solvent) Colloid

Meanparticle size, A

Rh-PVA-CH,OH/H,O Rh-PVME-CH,OH/H,O Rh-PVPD-CH,OH/H,O Rh-PVPD-C,H,OH Rh-PVPD-1 -propano1 Rh-PVPD-CH,OH/NaOH

0

40 43 34 22 24 9

5 10 15 Reflux time in h

Catalytic activity, mol H, (g-atom Rh)-' s 1-hexene

cyclohexene

15.2 22.5 15.8 14.5

3.1 9.6 5.5 10.3 9.3 19.2

-

16.9

'

Fig. 22. Dependence of the yield of colloidal particles of rhodium (1); formaldehyde (2); and catalytic activity of colloidal catalyst (3) on the time of boiling [36]

catalytic activity of rhodium is high; then it rapidly decreases reaching a minimum at 0.5 h. The maximum catalytic activity is attained after 3 h boiling. The catalytic activity of the colloidal rhodium dispersion during cyclohexene hydrogenation is much higher than that of conventional rhodium catalysts supported on charcoal because of the high surface area of the colloidal catalyst. The gradual decrease in the activity of colloidal rhodium on prolonged boiling ( - 8 h) results from the decrease in the effective catalytic surface of rhodium due to the formation of large aggregates. The elevated catalytic activity of rhodium particles within the initial boiling period ( - 1 min) can be attributed to the formation of secondary colloidal particles of rhodium caused by the reduction of Rh(II1)-PVA complex by hydrogen. During this reaction, colloidal dispersions of metals do not coagulate and show high stability. For instance, the Rh-PVA-CH30H/H20 system exposed to air for 20 days does not alter its dispersion state and by 85% of its catalytic activity is preserved.

Chapter 3 Heterogenized Homogeneous Catalysts

12

Colloidal dispersions of rhodium, palladium and platinum exhibit a strong catalytic effect during the hydrogenation of olefins and demonstrate high selectivity during the hydrogenation of cyclic dienes [38-401. The catalytic activity of colloidal dispersions, with respect to internal double bonds of cyclic olefins and especially conjugated double bonds, decreases in the sequence Rh- PVPD- CH30H/ NaOH > Rh - PVPD - C2H50H> Rh - PVPD - CH30H/H20. The Pd - PVPD CH30H/NaOH system possesses high catalytic activity and selectivity during the hydrogenation of cyclopentadiene, 1,5-cyclooctadiene, and methyl linoleate (Table 10) [421.

Table 10. Selective hydrogenation of dienes by colloidal palladium stabilized by PVPD or the polyionic complex, PAA-PEI Substrate

Catalyst

Cyclopentadiene

PVPD-CH,OH/NaOH PAA-PEI

8.8 0.09

97 98

1,3-cyclooctadiene

PVPD-CH,OH/NaOH PAA-PEI

35.0 0.13

100 99

1,5-cyclooctadiene

PVPD-CH,OH/NaOH PAA-PEI

1.8 0.009

99 95

Methyl ester of 9,12-octadecadienoic acid

PVPD-CH,OH/NaOH PVPD-CH,OH/H,O Pd/C

6.1 1.8 1.9

95 92

Catalytic activity, mol H, (g-atom Pd)-' s-'

Selectivity, Yo

71

Although the hydrogenation rate of 1,5-cyclooctadiene is lower in ethanol and 1-propanol than in methanol, the selectivity is the same [43]. The PVPD-Pd catalyst is insoluble in ethyl acetate and tetrahydrofuran and shows low activity. The rate of 1,3- and 1,5-cyclooctadiene hydrogenation is expressed by the following equation:

R = k *[H2] * [Pd] , where k = 4.86 and 1.06 mole-' .s-' for 1,3- and 1,5-cyclooctadiene, respectively. Large (65 A) colloidal particles of palladium show lower activity and selectivity than small particles (25 A). Colloidal palladium in the polyionic complex of polyacrylic acid-polyethylene imine (PAA-PEI) exhibits high selectivity but low catalytic activity 1431. Stable colloidal dispersions of copper have been obtained in water by reduction of copper (11) with NaBH4 in the presence of PVPD, PVA, dextrin and PVME at 25 "C [44,451. However, when high-molecular weight compounds such as polyethylene oxide, Pcyclodextrin and PAA are used protective colloids do not form. The mean colloidal diameter depends on the properties of the applied polymer and is 50 A for PVPD and 150 A for PVA. In contrast, the mean diameter of copper particles obtained

3.1 Catalysts Dispersed on Polymeric Support Surfaces

73

by NaBH4 reduction of Cu(I1) in the absence of polymers varies from 40 to 3500 A. Electron-diffraction experiments show that colloidal particles of copper are not merely aggregates of metal atoms but represent particles 20-250 A in diameter which possess face-centered cubic packing like a pure metal. Provided that protector polymers, have either N-substituted amido, alcohol, ester, or other groups, these polymeric functional groups apparently form polymer-metal complexes with Cu(I1) at the initial stage. Then, Cu(I1) ions are reduced to the zero-valent state by sodium borohydride or hydrazine. The colloidal dispersion stability is achieved by inhibition of the direct contact of metal particles with polymer molecules on the particle surface. Copper colloids protected by PVPD with a 3240 degree of polymerization, most effectively catalyze the selective (100%) hydration of acrylonitrile (AN) to acrylamide in water at 80°C at a molar Cu/AN ratio of 0.017. The acrylamide yield reaches 25.4 mole 'To within 2 h [46]. The reaction is first order with respect to the acrylonitrile concentration down to 47% conversion. The catalytic activity of all other protected colloidal dispersions is also much higher (2.5 -8.6 mole 070 in 2 h) than the activity of the copper precipitate which forms by the reduction of copper sulphate by NaBH, in the absence of a copolymer (0.3 mole Yo in 2 h). Hirai and Toshima [46] have reviewed the preparation and characterization of polymer-protected colloidal metal catalysts together with their characteristic properties and some of their applications. A means of synthesizing a supported palladium dichlorodimethyl sulfoxide catalyst (PdC12(DMS0)2), which is stabilized by PVPD (M = 20000) and fixed on solid support (A1203, Chromsorbs A and W) has been examined [47]. Such a supported complex catalyst (PdC12(DMS0)2- NaBH4 - PVPD on A1203 and Chromsorb A and W) exhibits selectivity with respect to 1,3-pentadiene and cyclopentadiene, whereas catalysts based on PVPD and phenylsulfonic complexes of Pd mainly catalyze the isomerization of alkylbenzenes [48]. The stabilizing effect of PVPD is attributed to the formation of coordination bonds between Pd and polymeric amide groups. Apparently, in these systems colloidal palladium stabilized by PVPD also forms because preservation of the ligand-metal coordination bond is likewise feasible. The possibility of preparing colloidal particles of Pt between 50-60 A in diameter by photoreduction of K2PtC14 inserted into polymer bubbles [49] was demonstrated. It was, moreover, found that the catalyst activity in during C2H4 hydrogenation increases with a decrease in Pt particle size. The dispersity of colloidal particles size depended on the concentration of the solvent used for K2PtC14. The possibilities and limitations of using the obtained catalysts in the hydrogenation of organic compounds is discussed. A process for synthesizing catalysts as colloids on ionite supports has been suggested. Rh, Pd, Pt and Ru hydrosols, prepared by reduction of aqueous solutions of the respective metal salts, were sorbed on different salt species of the strong-base anionite, Amberlite IRA-400, and of the sulfocationite, Amberlite IR-120. On the anionite, sorption proceeds irrespective of the salt species of the anionite, whereas on the cationite it proceeds only when the H-form is used. Nonetheless, the degree of aggregation of colloidal particles on the cationite is much higher than on the anionite.

74

Chapter 3 Heterogenized Homogeneous Catalysts

This results from differences in sorption mechanisms. The catalytic activity of colloidal catalysts was checked during hydrogenation reactions of styrene, cyclohexene, 1-hexene, methyl acrylate, a-methylstyrene and mesityl oxide. Actually, surfactants can be used as stabilizers of colloids. Results of the reduction of an aqueous solution of HZPtCl, by Hz and of photoreduction in the presence of sodium dodecylsulfate (NaDDS) and dodecyltrimethylammonium chloride (DDACI), are discussed [511. It was found that the photoreduction method produced platinum particles of 14 and 12 A mean sizes using NaDDS and DDACl. When chloroplatinic acid was reduced by hydrogen, mean particle sizes were 92 and 39 A in the presence of NaDDS and DDACl, respectively. It was noted that the photoreduction yields a more uniform distribution of particle sizes than from the reaction of H2PtC14 with hydrogen. Colloidal particles of platinum prepared by photoreduction exhibited high activity during vinyl acetate hydrogenation. Consequently, synthetic hydrophilic polymers such as polyvinyl alcohol, and polyvinylpyrrolidone, and polyvinylmethyl ester are interesting for two reasons: i.e. (1) they coordinate with metal ions thereby decreasing the rate of metal reduction, and (2) they prevent aggregation of colloidal particles. The simplicity of preparation of colloidal metal dispersions, the feasibility of controlling particle sizes, and some other advantages place these polymers as promising catalytic systems [52,53,53 a].

3.2 Catalytic Properties of Coordination Polymers Coordination polymer compounds have been successfully employed in various catalytic processes such as hydrogenation, oxidation, hydroformylation, polymerization, isomerization, asymmetric synthesis, etc. [54- 561. From literature data, it is evident that mainly carbo- and heterochain polymers containing carboxyl, hydroxyl, amino- and amido groups, sulfur- and phosphorus-containing fragments can be used to attach metals on the surface of polymers. The bond between a polymer ligand and a metal ion is effected either by their donor-acceptor interaction or by the metal's displacement of protons to form ion contacts. Polymer ligands currently applied as metal supports, can be divided into three basic groups, i.e. polyacids, polybases and non-ionic polymers. Typical representatives of each group are presented below (see scheme p. 74). PEI can be applied as a polymer matrix because the PEI molecules contain amino groups which are capable of bonding with group VIII metal salts. Catalytic properties of coordination compounds of PEI have been thoroughly studied [57-621 The structure or active sites of PEI complexes with nickel, cobalt, rhodium and palladium ions is presented as a five-membered chelate ring:

75

3.2 Catalytic Properties of Coordination Polymers Polymer ligands

Repeating monomer unit

Polyacids Poly(acry1ic acid) (PAA)

-CH,-CHI

COOH

Poly(methacry1ic acid) (PMAA) Maleic acid-styrene copolymer (MA-S)

COOH -kYH-

LOOH

- CH

-CHz

I

Poly(ethylenesu1fonic acid) (PESA)

S03H -CHz -CH-

Q

Poly(styrenesu1fonic acid) (PSSA) PoIybases Polyethyleneimine (PEI) Poly(2-vinylpyridine) (P2VP)

-CHz -CH

Poly(4-vinylpyridine) (P4VP)

-

Q

Poly(2-methyl-5-vinylpyridine) (P2M5VP)

CH3

-CH,-$H-

0 N

Nonionic polymers Poly(viny1 alcohol) (PVA)

-CHz-

Pol y(N-vinylpyrrolidone)

-CHz -CH

Poly(ethy1ene glycol) (PEG) Polyacrylamide (PAAm)

CHI OH

I

(PVPD)

6

CHUCH-CHz+

COOH

-

- CH, -0- CHZ -CHI -CHz

CONH,

76

Chapter 3 Heterogenized Homogeneous Catalysts

These complexes form active species upon treatment with molecular hydrogen or sodium borohydride. The analysis of IR spectra of starting and hydrogen-reduced palladium complexes of PEI implies the presence of coordination bonds. The catalytic activity of polymer-metal complexes is apparently attributed to the formation of hydride species of the coordinated palladium chloride when these complexes are treated with molecular hydrogen. Catalytic properties of these complexes have been studied during hydrogenation reactions of cyclic dienes [60], mixtures of diene isomers [59] and aromatic nitro groups [61]. In the presence of nickel, cobalt and palladium complexes the hydrogenation of l ,3-cyclohexadiene (I ,3-CHD) initiates the reaction of hydrogen disproportionation. This process shows selectivity for cyclohexene; at complete diene conversion the hydrogenation selectivity is 93 -98%. Depending on the metal properties, the hydrogenation of 1,3-CHD proceeds stepwise and involves hydrogen disproportionation whereby benzene and cyclohexene are formed. This reaction is of the first order for the substrate. In respect of the metals selected the activity decreases in the following order: Pd > Rh > Ru > Ni, Co. This sequence persists despite different methods of complex reduction. PEI-RhC13 and PEI-RuC13 complexes exhibit different catalytic activity with regard to a mixture of the cis- and frans isomers of pentadiene [61]. The former complex is more selective for pentene (0.94) than the latter. A quantitative aniline yield results from the reduction of nitrobenzene in the presence of PEI complexes with Ni(II), Co(II), Sn(II), Pd(I1) and Rh(II1) [61]. The reduction proceeds rapidly at 20-70°C and at 1-25 atm pressure of H2 both in a solvent and without. Besides aniline, cyclohexylamine is produced by further hydrogenation of the aromatic ring in the presence of the PEI-Rh(II1) complex. Polymer-metal catalysts do not lose their catalytic stability after repeated application. For example, when eight hydrogenation reactions were catalyzed with PEI-Pd(II), in each case a 100% product yield of aniline was attained. An attempt has been made to establish a relationship between mechanisms of formation of PEI complexes with group VIII metal salts and their catalytic activity [63,64]. The interaction of PEI and Rh proceeds intermolecularly. The change of the phase state and color of the complexes with time indicates extensive rearrangement of the structure of initial products. X-ray electron spectral data show that PEI-Rh complexes possess considerable structural inhomogeneity. Initially, the yellow A-type complex of PEI: Rh 6-7 forms. This complex maintains the ability to bond additional amounts of rhodium to form the B-type, N : Rh - 3.5. Presumably in A and B type structures, rhodium is fully coordination-saturated by chelate-forming amino groups and Rh-Rh bonds. Beyond the range 3.5 < N : Rh < 2, the chelation of subsequent rhodium ions is already impossible, and they coordinate with single amino groups to create C-type structures. Low-molecular weight ligands occupy the rest of the coordination sites. The material-balance data implies that a portion of Rh can be present in the complexes as clusters. Thus in complexes of N : Rh at a ratio of 2 : 1, out of 50 Rh(II1) atoms coordinated with 100 atoms of nitrogen, 15- 16 atoms of Rh are arranged in structures of A type, 13- 14 - in structures of B type, and 20-22 - in structures of C type. The preparation of palladium catalysts from polyethyleneimine-silica gel composites was reported [65]. PEI is first absorbed on silica beads or silica gel. Then the absorbed

3.2 Catalytic Properties of Coordination Polymers

polymer undergoes chemical cross-linking and leaching to remove the inorganic material. This creates high surface area of the polymer support and its chelation ability.

(3

@

1

PEI

'''1

Pd"

1. NaBH,

1NaBH, Pd/PEl/silicagel

The catalyst was synthesized as follows. Polymer amino groups were saturated (to 5 % ) with Pd(1I) ions which were then reduced by NaBH4 dissolved in water or in an organic solvent (e.g. formalin). The thus obtained Pd/PEI/silica beads and Pd/PEI/silica gel powders had pore areas of 50 and 500m2/g, respectively. These catalysts were employed for reduction of nitrobenzene in ethanol. Catalysts of both types were easily separated from reaction products by decanting and could be used repeatedly. The role of polymeric supports is determined in many respects by the chemical composition, configuration and conformation of chain molecules, a knowledge of which, in its turn, makes it possible to elucidate the effect of the polymer ligand structure on catalytic reactions [66, 671. Polyvinylpyridines are interesting polymer ligands which act as catalytic supports. The composition of P2VP, P4VP and P2M5VP includes nitrogen atoms in different positions in relation to the main chain. This affects their complexing ability and hence, should influence their catalytic activity. When Pd(I1) is supported on monofunctional anionites of varied structure, different coordination centers form. The number of pyridine nuclei in the coordination sphere of the metal and stability of the complex increase with the distance from the main chain [68]. This similarly, affects the activity and selectivity of hydrogenation of unsaturated compounds (Table 11). In particular, in the presence of the AN-40/Pd(II) complex, allylbenzene is mainly isomerized, whereas it is hydrogenated in the presence of AN-23/Pd(II).

78

Chapter 3 Heterogenized Homogeneous Catalysts

Table 1 1 . The effect of anionite nature on the structure of supported complexes of palladium (11) and their catalytic activity Support Ionite

N :Pd

The assumed compoistion of a coordination site

Ionogenic group

Initial rates mol/g. atom Pd min-' hydrogenation

AN-23

AN-25

isomerization 0.99

Q

N

CT

lo

0.56

1.45

2:1

CH3

3: 1

AN-40

6.19

6 N

L

J

Table 12. Hydrogenation of 4-vinylpyridine on catalysts obtained by PdCI, interaction with a copolymer of styrene and DVB modified by a-, P- or y-pyridyl N :Pd(I1) ratio 2: 1

Pyridyl position

k.102, min-'

a

2.7 0.4 1.3 2.9 0 1.6 2.5 0 1.7

P 4: 1

Y a

P Y 6: 1

a

P Y

Table 12 depicts rate constants of 4-vinylpyridine supported hydrogenation by catalysts obtained by interaction of PdCI2 with polymer ligands composed of copolymers of styrene with divinylbenzene modified by a-, p and y-pyridyl [69]. It can be seen that the most active catalysts are those obtained by WCI, heterogenation on

3.2 Catalytic Properties of Coordination Polymers

79

polymers modified by a- and y-pyridyl groups. These examples indicate that the activity of heterogenized complex catalysts depends on the arrangement of ligand groups. The catalytic effect of P2VP, P4VP and P2M5VP complexes with PdC12 in the reaction of allyl alcohol hydrogenation has been studied in order to clarify the effect of polymer ligand structure on the catalyst activity [70-751. In the presence of palladium catalysts, depending on reaction conditions, the substrate can either be isomerized to propionic aldehyde or be hydrogenated to propanol. Results of allyl alcohol hydrogenation on polyvinylpyridine complexes of palladium and Pd black in water at 293 K are compared in Table 13.

Table 13. Hydrogenation of allyl alcohol on complexes of P2VP-Pd(II) and P4VP-Pd(II) and Pd black in water Catalyst

Wspec,,ml/min 1 g of catalyst

Bsat.9

P2VP-Pd(II) P4VP-Pd(II) P2VP-Pd(O) P4VP-Pd(O) Pd(0)

150 490 710 800 17

400 410 630 640 470

mV

Po, mV

40 60 95 130 80

For all the catalysts studied, the hydrogenation proceeds in the external kinetic region. This is confirmed by the linear character of the change of hydrogenation rates with the catalyst amount. The reaction order for allyl alcohol is close to zero whereby the reaction rate does not depend on the substrate concentration. The decrease in the specific rate of allyl alcohol reduction, with the chosen catalysts proceeds in the following sequence: P4VP-Pd(O) > P2VP-Pd(O) > P4VP-Pd(II) > P2VP-Pd(II) > Pd(0). It can be seen that polymer complexes are much more active than Pd black. Even the reaction rate in the presence of NaBH4-reduced samples is one order of magnitude higher than for the palladium metal. Such low activity of Pd black is apparently attributed to the fact that two processes occur simultaneously on its surface; i.e. hydrogenation of allyl alcohol to propanol and isomerization to propionic aldehyde (Fig. 23 a). According to chromatographic analytical data, about 50- 52% of the catalysate consists of propionic aldehyde, in the presence of Pd black alone, whereas, the propanol yield reaches 84-89% (Fig. 24) in the presence of polymermetal catalysts which promote selective hydrogenation of allyl alcohol to propanol. The activity of polyvinylpyridine complexes of palladium also depends on their treatment. The higher activity of catalysts reduced by NaBH4 is apparently caused by the higher degree of destruction of complex-bound ions of palladium and the Pd(II)+Pd(O) reduction. The subsequent occurrence of free pyridine groups thereby increases the basicity of polymeric supports and thus the potential of catalyst saturation (Table 14, see p. 80).

80

Chapter 3 Heterogenized Homogeneous Catalysts 100

60

2 3

20 I

0,25

J

0,25

0,75

0,75 H2/mol

H2 /mot

Fig. 23. Composition of reaction mixture during hydrogenation of allyl alcohol in the presence of Pd black (a) and P4VP-Pd(O) (b) in water at 293 K. 1 - allyl alcohol; 2 - propanol; 3 propionic aldehyde [71]

t 1

' I\

\

13

I...

\

I. :;

\

/f* .;

. ..

I

.. ..

I

I6Oo

4 /cm-l

.. I

Fig.24. Part of IR spectra of' P4VP (l), P4VP-Pd(II) complex (2) and sodium borohydride-reduced complex (3) [711

With regard to IR spectra of the complexes, the ratio of complex-bound and free pyridine rings changes (Fig. 24) depending on the reductant concentration. These results agree well with data previously obtained [76]. When the P4VP-Pd(II) complex grafted on to polyethylene is treated with hydrogen or sodium borohydride, absorption bands of the coordinated pyridine rings and Pd-Cl bonds disappear in the IR

81

3.2 Catalytic Properties of Coordination Polymers

Table 14. Results of allyl alcohol hydrogenation on polyvinylpyridine complexes of palladium in water at 293 K depending o n the catalyst treatment Parameters

PZMSVP-Pd(I1)

P2VP-Pd(II)

P4VP-Pd(II)

H,

H,

NaBH,

H,

17.5 11.0

-

5.0

W , ml/min

E,,,., kJ/mole a mV Po9 mV

-

400 45

s, 070

-

NaBH, 6.5 39 400 45 73

4.5 -

400 40 -

540 100

94

NaBH, 25.0 19 630 120 91

13.0

410 50 -

spectrum. During activation of the complexes, Pd(I1) is supposedly reduced completely to Pd(0). By means of wide-angle X-ray scattering, Pd(0) has been found in P2VP-Pd(II) samples reduced by hydrogen or sodium borohydride. Furthermore, according to XPS data, palladium is present in the oxidized state in the initial P2VP-Pd(II) complex. In the complex reduced by hydrogen, palladium is characterized by a broad peak at 336.8 eV which suggests the superposition of lines of reduced and oxidized palladium. The spectrum of a treated catalyst has a peak with a maximum at 335.9eV and a shoulder at 337.1 eV. This confirms the presence of palladium in the reduced (Pd(0)) and oxidized (Pd(I1)) states. The structure of a polymeric support considerably affects catalyst activity. The decrease in the rate of allyl alcohol reduction with the chosen catalysts occurs in the sequence P4VP-Pd(O) > P2VP-Pd(O) > P2MSVP-Pd(O). The structure of coordination compounds of polyvinylpyridines with transition metal chlorides has been described in detail [76-791. The most likely structure of the complexes is ML2C12 or MLCl2, where by M denotes the metal ion and L the pyridine units. Unlike P4VP, which tends to form primarily intermolecular cross-links, coordination bonds between neighboring vinylpyridine units have mainly been obtained for P2VP and P2MSVP. As PdC12 is capable of coordinating with binuclear compounds [80],the following models of complexes are proposed:

a

b

C

Therefore in complex a, palladium is sterically more accessible for reduction and consequently for substrate activation and hydrogenation.

Chapter 3 Heterogenized Homogeneous Catalysts

82

Table 15. Influence of N : Pd(I1) molar ratio on the catalytic activity of P2VP-Pd(II) and P4VPPd(I1) complexes in allyl alcohol hydrogenation in water at 298 K, 0.03 g catalyst sample ~~

PVP: Pd

1:1 2: 1 5: 1 10: 1 25: 1 50: 1

P2VP-Pd(II)

P4VP-Pd(II)

W, ml/min

90,

21 12 10 10 9 10

630 640 600 600 600 550

mV

9hyd,,

530 440 430 450 450 300

mV

W, ml/min

90, mV

9hyd.p

26 15 10 10 10 10

640 610 650 650 640 580

520 410 480 500 500 350

mV

The catalyst activity can change with variation of the metal concentration supported on a particular polymer. Table 15 depicts the effect of the initial molar ratio of N : Pd(I1) on the rate of allyl alcohol hydrogenation. The maximum rate of allyl alcohol hydrogenation is attained with P2VP-Pd and 1O:l and25:1, P4VP-PdatN:Pdratiosof 1:1.AtmolarN:Pdratiosof2:1,5:1, the rate of allyl alcohol reduction does not particularly change. Somewhat underestimated values of saturation and hydrogenation potentials have been achieved for catalysts with a N :Pd ratio of 5 : 1. The change in the catalytic activity of PVP-Pd, obtained by altering the concentration of starting polymer and PdCl2 solutions, is apparently caused by a difference in the structure of synthesized complexes or by the formation of complexes with different composition. At a N : Pd ratio of 1, the structure of polymer-metal complexes can probably be presented as ML2Cl2, whereas at a N : Pd ratio of 2 : 1 they can be presented as MLC12. By examination of nickel complexes of P2M5VP during propylene dimerization, it was found [81] that in all the complexes, irrespective of the starting molar polymer-metal N : Ni ratio of 2 : 1, each nickel atom is bonded to two pyridine rings. The most important characteristic of heterogenized homogeneous catalysts is their stability. The stability of polyvinylpyridine complexes of palladium was studied during the hydrogenation of sequential portions of allyl alcohol. On all the examined samples, the hydrogenation of 10- 30 substrate portions occurs without a significant rate decrease (Fig.25). However with Pd black each subsequent sample is hydrogenated at a lower rate than a previous one.

10

2o Number of runs

Fig. 25. Hydrogenation of sequential portions of allyl alcohol in water with a P4VP-Pd(O) catalyst 1751

3.2 Catalytic Properties of Coordination Polymers

0,s

83

l,o

1,s

H2 / m o l

Fig. 26. Hydrogenation of DMEC on 0.01 g P4VP-Pd (NaBH,) in water at 298 K: (a) kinetic (1) and potentiometric (2) curves; (b) - state diagram of catalysate. (1) dimethylethynylcarbinol; (2) - dimethylvinylcarbinol; (3) - dimethylethylcarbinol [70]

The hydrogenation of dimethylethynylcarbinol (DMEC), an alcohol containing an acetylene bond was conducted with the most active catalyst, P4VP-Pd(NaBH4) [70]. The rate of reduction of the triple bond of DMEC is lower than that of the double bond. This is clearly seen from the kinetic curve in Fig. 26a; i.e. the reaction rate sharply increases after the absorption of a half of the calculated quantity of hydrogen. The chromatographic analysis of reaction products shows relatively selective hydrogenation of the acetylenic bond (Fig. 26b). The conducted experiments [70, 7 11 demonstrate that PVP-Pd complexes are active, selective and stable catalysts. The composition of such catalysts represents a composite system including Pd(I1) and Pd(0). The role of the polymer ligands evidently consists in the stabilization of the particular valent states of palladium which are optimum for the substrate hydrogenation. One can assume that in the given catalytic system, Pd(0) promotes the activation of hydrogen, whereas complex-bound Pd(I1) promotes the formation of a n-ally1 complex with unsaturated double bonds of the substrate and thus its activation. Furthermore, pyridine rings promote substrate orientation. This assumption enables polymer-metal heterogenized catalysts to be considered as models of catalytic enzyme systems. The influence of solvent type (hexane, water, methanol, ethanol, propanol, butanol and alcohol-water mixture) on the catalytic properties of the complex P2VP-Pd(II) has also been studied [75]. The catalytic activity of P2VP-Pd(II) catalyst increases in this order: hexane < water <ethanol < methanol. The swelling factor of P2VP also changes in the same order. How solvents influence the catalytic activity of polymer-metal catalysts was discussed by Klyuev [76]. Authors [76a] immobilized Pd(CH3C00)2onto polymers containing pyridyl and carboxylic groups and used them as catalysts in hydrogenation of dicycloheptadiene, 2-butyn-1,4-diene, diphenylacetylene. It has been proposed that the acetate groups are eliminated from the polymer catalysts during the hydrogenation.

84

Chapter 3 Heterogenized Homogeneous Catalysts

Complexes of PMEI/LiAIH4 in a 1 :4 ratio have been obtained by interaction of LiAIH, with linear poly-N-methylethyleneimine(PMEI) in THF solution. These complexes do not dissolve in ester and hydrocarbon solvents [82]. The reduction of different simple aldehydes, ketones and ethers at 298 K actually occurs inside a solid complex of PMEI-LiAlH, suspended in pentane. The pure alcohols formed can be isolated by treating only a separated solid phase. As a result of complexing with PMEI, lithium aluminium hydride in benzene and pentane becomes more active in the reduction of acetophenone. A complex of cross-linked P4VP with borane can serve as an efficient catalyst of the reduction of aldehydes and ketones [83].

An increase in solvent polarity leads to an increase in the reduction rate and the final product yield. The best results have been obtained with a buffer solution with pH 1 and with acetic acid; in these solvents the reduction of cyclohexanone is complete within 30 and 120 minutes, respectively. During this time, as shown by special test, hydrolysis of the borane complex does not occur. The increase in the reactivity of the borane-containing polymer in the acidic medium is attributed to the protonation of both the carbonyl compound and the polymer. The protonation of the carbonyl oxygen increases its reactivity, wheras the protonation of the polymer facilitates swelling of the macromolecule and causes conformation change which increase the accessibility of catalytically active sites. Poly-4-vinylpyridine complexes of copper(I1) catalyze the oxidation of thio salts such as sulfites (SO$-), thiosulfates (S20:-), dithionates ( S 2 0 f ) , disulfites (S20:-), trithionate (S30:-), tetrathionates (S40:-), by molecular oxygen to sulfate ions [84- 861.

RJO basic mechanisms of S20$- oxidation have been suggested. One involves the direct transfer of an electron from S20:- to Cu2+, whereas the other involves the transfer of an electron from S 2 0 f to O2 via the central atom of Cu(I1) in the Cu(P4VP)(S20$-)(0,) complex. Basic sulfur-containing species which form during the catalytic oxidation of S20$- in the presence of O2 by means of the P4VP-Cu(II) complex are SO:-, S402 and S 3 0 2 - . The quaternization and cross-linking of P4VP by dibromobutane considerably augment the catalytic activity of the copper complex compared with the linear complex, P4VP-Cu(II). The optimum stability and reaction rate are attained at a 31% degree of cross-linking. The kinetics of S20$- oxidation at pH 2.16, where the rate is maximum, are described well by the Langmuir-Hinshelwood model which assumes the coordination of both O2 and S20$- with Cu(I1).

3.2 Catalytic Properties of Coordination Polymers

b

n1L 1

5

9

85

Fig. 27. Dependence of initial rate of S& oxidation in the presence of complexes QP4VP-Cu(II)(l), P4VP-Cu (2) and pyridine-Cu(I1) (3) on pH of medium [85]

Figure 27 presents the dependence of the rate of thioanion oxidation on the pH of the medium. The sharp increase in the rate of S 2 0 f oxidation at pH 2.0 and pK 11.0 is caused by leaching of copper ions out of the complex and breakdown of copper particles. An increase in the rate of S 2 0 f oxidation caused by P4VP quaternization depends on the presence of quaternary nitrogen atoms responsible for the migration of the anionic substrate to the polymer matrix as a result of electrostatic attraction. In some cases, under the influence of ambient conditions (temperature, solvent, pressure) or during a catalytic process polymer substrates may undergo various conversions such as decomposition, oxidation, or hydrogenation. For instance, prolonged use of copper (11) complexes with AN-25 anionite during liquid-phase oxidation of cumene somewhat decreases catalytic activity [87, 881 as a result of a significant change in the coordination sphere of Cu(I1) ions. By EPR and IR spectroscopy, it has been found that during the course of catalytic cumene oxidation, the starting complex structure gradually changes to another complex type due to oxidation of a-methyl groups of P2MSVP.

M M

It has been moreover discovered that in the AN-25 ionite phase, copper (11) ions are present in two states, i.e. as isolated mononuclear tetracoordinated polymer com-

Chapter 3 Heterogenized Homogeneous Catalysts

86

plexes of C U ( N )and ~ as associates or regions with a very high, local Cu(I1) concentration. The liquid-phase oxidation of cumene by molecular oxygen causes formation of isolated complexes of C U ( N )composition ~ which sharply decreases the catalyst activity. Copper ions are partially washed out from ionite granules. Furthermore, it has been shown that the catalytic activity is exhibited by mononuclear complexes of Cu". Nickel(0) supported on linear polyvinylpyridine exhibits catalytic activity and selectivity during cyclotrimerization of butadiene [89]:

12 h

lc

lb

la

Table 16. Conversion of butadiene and yield of cycloenes Catalyst

Conversion,

Yield, 070

070

Ni-P4VP Ni-P4VP (repeated use) Ni-P2VP

84.9 74.7 74.5

la

lb

lc

76.4 72.2 69.3

7.9 6.0 9.4

10.6 7.8 10.7

Table 16 presents the butadiene conversion and the yield of various cycloenes. However, cross-linked (2% DVB) polyvinylpyridine complexes of nickel do not exhibit any reactivity. This is apparently a result of the lower swelling ability of the cross-linked polymer matrix and the limited diffusion of the substrate to nickel atoms. Heating of O S ~ ( C Owith ) ~ ~ P4VP in DMFA in a CO medium at 110°C for 3 h leads to formation of clusters attached to the polymer [90]. The structure of the obtained product, confirmed by IR (carbonyl bands at 1940 and 2040 cm-') and NMR spectroscopy data in CDC13 (6 = 10.5, 12.0 and 13.75 ppm, 0s-H), indicates that the basic metal species in the product is Os3H(C0)9(NCSH4)2.Both pyridine fragments originate from the P4VP chain. The synthesized product is active in the conversion of CO/methoxymethyl ether/H20 at 110 "C, whereby the osmium content decreases from 1.2 to 0.6% during the 120 h reaction. Several methods of obtaining polymeric complexes of vinyl azoles with iron, cobalt, nickel and copper salts have been developed and tested during hydrogenation of unsaturated organic compounds. However, the catalytic activity of such complexes during ester hydrogenation is not the same. The strongest catalytic effect is exhibited by nickel complexes of polyvinylbenzimidazole (PVBI) and its copolymers. The hydrogenation reaction proceeds stepwise in the presence of several double bonds in

3.2 Catalytic Properties of Coordination Polymers

87

the substrate. PVBI-Ni(I1) complexes are also active catalysts of the synthesis of hydrocarbons from CO and H2 [92]. Basic products of CO conversion are CH4 and C02. The type of nickel salt incorporated into the polymer matrix affects the catalyst activity. Thus, when salts of Ni(N03)2and NiC12 are employed, the conversion of CO is 51% and 40%, respectively. In the presence of Ni(OAc), and Ni12, however, it is 3% and 0.6%. Electrocatalytic properties of Cu2+ complexes with P4VP in oxygen reduction and their redox activity during adsorption on a carbon support have been reported [93]. Let us consider the catalytic effect of metals fixed on polyacids and copolymers of unsaturated carboxylic acids with different comonomers. Catalysts based on Pt, Pd, Rh, and Ni complexes with macromolecular ligands (styrene-maleic acid, maleic acidmethylmethacrylate, polyacrylic acid) have been synthesized [94- 971. The catalysts exhibit high activity and selectivity during the hydrogenation of furan and its derivatives, as well as benzofuran, benzodioxan, benzene, nitrobenzene, phenol, benzaldehyde, olefines, cyclic olefines and cyclic dienes. In the hydrogenation of furan, 2-methylfuran and benzofuran, 100% conversion and selectivity have been obtained. The highest activity during hydrogenation of unsaturated hydrocarbons is exhibited by rhodium-containing catalysts. Within one hour the turnover number, ii, i.e. the number of substrate molecules converted with one metal atom, reaches 289 during the hydrogenation of 1-hexene and 85 during benzene hydrogenation. Cyclic dienes as well as olefins are quantitatively hydrogenated to the respective saturated hydrocarbons. Complexes of PdC12, RhC13 and styrene copolymer with divinylbenzene-containing iminodiacetic acid groups, can be used for selective hydrogenation of olefins and dienes [98]. With polymer-metal catalysts, a-olefins are hydrogenated more readily than olefins with internal C = C bonds. Substituted alkenes, however, are hydrogenated at a much lower rate. These catalysts are less active in the hydrogenation of carbonyl and aromatic groups. Nonetheless, styrene and 5-hexen-2-one are seleetively hydrogenated to ethylbenzene and n-butyl methyl ketone with 100 and 99.5% yields. The oxidation of pinene and C,, -C14 a-olefins with a chain length of has been studied at 30- 50 "C in the presence of composite cobalt compounds as catalysts [99]. Oxygen-containing polymers (PVA, PAA, PMAA) as well as nitrogen-containing polymers (polyacrylamide) (PAAm), copolymers of acrylic acid and acrylamide (AAAAm), polyacrylonitrile (PAN)) and its oxime (PAN-0) were used as polymer ligands. The formation of a-pinene oxide, verbenol, verbenone and a-pinene hydroperoxide, which are basic products of pinene oxidation, indicates the occurrence both of double bond addition reactions and secondary hydrogen atom detachment reactions. Hydroperoxides formed during this process probably deactivate the catalyst. During pinene oxidation, catalytic activity decreases according to the following ligand sequence: PVA > PAN-0 >AA-AAm > PAA; PAN; PAAm. However, the activity of Co2+ complexes during oxidation of a-olefins decreases in the sequence: PVA > PMAA >AA-AAm > PAN-0 > PAAm. The reaction rate is here proportional to the catalyst concentration. The dependence of the oxidation rate on the concentration of peroxides initially incorporated in the reaction is significant. Apparently, the

88

Chapter 3 Heterogenized Homogeneous Catalysts

ligands in the studied complexes influence the pairing of d-electrons of an ionic complexing agent. The ligand type also affects the dynamics of oxidation-reduction conversions. The oxidation of CI1-C14 a-olefins by air in a flow apparatus (0.1 -2 Vmin rate of air feed) has been investigated at temperatures of 313-373 K in the presence of complex compounds of 111-period transition metals with polyvinyl-alcohol as a ligand [IOO]. The influence of reaction conditions like the concentration and character of a metal ion as a complexing agent, and the length of the macromolecular ligand during the oxidation process has been elucidated. A higher degree of oxidation and high selectivity regarding the formation of acids, peroxides and oxides depends on the ordinal number of the complexing metal. In a flow system, the stable formation of all the reaction products is achieved at much lower temperatures. The literature [ 101, 1021 cites the hydrogenation and isomerization of allylbenzene in the presence of palladium complexes on polymeric supports having oxygen-containing coordination groups like -OPh, -COOH, -OH. It was shown that the activity and selectivity of synthesized polymeric catalytic systems were affected by both the electron-donor properties of graft coordination groups and the solvent type. In the presence of the catalyst, the isomerization of allylbenzene to I-propenylbenzene mainly yields the trans isomer. Iron complexes attached to a polymeric support containing phosphinate and phosphonate groups have also been synthesized [ 1031. Their catalytic properties were studied during allylbenzene hydrogenation and isomerization reactions. An increase in the catalytic activity of these complexes is caused by diminished electron-acceptor properties of a substituent at a phosphorus atom. Although catalysts containing phosphinate groups catalyze hydrogenation with high selectivity, they are less stable than catalysts containing phosphonate groups. Methods of synthesizing polymers with carbonyl groups as supports of transition metal complexes have, moreover, been analyzed [ 1041. The activity of these catalysts in polymerization, oligomerization and hydrogenation reactions as a function of the polymer support structure has been discussed. Recently, much attention has been devoted to the problem of synthesizing catalysts containing non-ionic polymeric ligands such as polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinylpyrrolidone (PVPD), polyurethane (PU) and polyamides (PA). The interaction of group VIII metal salts with polyamides is attributed to the strong coordination bonds which form between polymeric peptide groups (CONH) and metal ions [105- 1071. According to Bernard et al. [108- I l l ] , the impregnation of PA-66 polyamide with a dilute solution of H2PtC14 at boiling point promotes deeper penetration of platinum into the polyamide bulk. Furthermore, about 75% Pt binds with PA-66 to form a donor-acceptor complex, and about 0.2% Pt is absorbed. The removal of chlorine during the reduction and drying of the catalyst leads to platinum reduction and to bond alteration in the complex. A considerable quantity of Pt crystallites is found both inside and on the surfaces of polyamide granules by means of electron microscopy. During examination of catalysts obtained by supporting platinum and palladium on polyamide 66, metal crystals are apparent mainly on the

3.2 Catalytic Properties of Coordination Polymers

89

surface of the polyamide support [I 121. Apparently a partial electron transfer from the polymer to the metal occurs. A similar conclusion has been made in the study of the structure and distribution of active support groups and the state of Pt attached to polyamides [113]. It is noted that functional polyamide groups located on the matrix surface and not in the bulk play an important role during catalysis. Most probably, platinum atoms in the form of (Pt)2 and (Pt)3 clusters are included in the space occupied by NHCO:

\ CH,ANHACHA ACO/ , CH,

High catalytic activity is exhibited when one unit cell of NHCO is occupied by one (Pt)2 or (Pt), cluster. Hydrogen activation is caused by decomposition of H2 molecules to the atomic state with subsequent formation of the hydride complex. Oxidation-reduction titration as well as electron-microscopy measurements show the incorporation of Pd crystallites inside nylon grains [I 141. Most have a mean diameter of about 30 A . However, under the assumption that all the metal is located on the polymeric support surface, the calculated accessible metal surface amounts to several hundred Angstroms. Therefore, during preparation of the Pd-nylon catalyst, part of the metal penetrates into the organic matrix. The mechanism of alkene hydrogenation is based on the sorption of the substrate onto the metal surface due to the transfer of an electron from the occupied n-bonding orbital to the vacant d-orbital of the metal. The alkene-metal bond is also stabilized by the return electron donation from the occupied d*-orbital of the metal to the vacant antibonding n * -orbital of the alkene. The catalytic activity and selectivity of platinum, palladium, rhodium and ruthenium salts supported on various polyamides was studied, and the correlation between their reduction capability to the metallic state and the formation of coordination complexes with polyamide was established [I 151. Concerning supported metals, the influence of the geometrical factor and the feasibility of electronic interaction of the supported metal with polyamide was discussed. It was found that the activity of Rh complexes decreases with an increase in the distance between amide groups of polyamide and correlates with the change in polymer crystallinity [107]. The kinetics of styrene hydrogenation to ethylbenzene have been investigated with a platinum/polyamide-66 catalyst and using a differential flow microreactor and a reactor with mixing [l 1 I]. The reaction mechanism changes at temperatures above 160°C. The activation energy of the reaction is 54 kJ/mol, at T < 160°C and 33 kJ/ mol at T = 160-225 "C. In the first case the reaction kinetics are described by the equation u = kO-k,,.Pst,whereas in the second case they are delineated by the equation u =

kO' kst.Pst'

kH,*PH,

. Thus,

1+ ~ H , * P H ,

at low temperatures the catalyst surface is

90

Chapter 3 Heterogenized Homogeneous Catalysts

saturated with hydrogen, whereas at T > 160°C the adsorption of hydrogen is described by the Langmuir model. The fact that the reaction kinetics on the tested catalyst vary from those of a conventional platinum catalyst for which the reaction order is approximately unity for hydrogen and zero for styrene, indicates that polyamide-66 affects the catalytic properties of platinum. Catalytic properties and activity depend on the type of impregnation solution applied, its concentration and pH, as well as on the specific surface value and conditions of preparation of a polymeric support. The most active catalyst has been obtained when a polymer with a high specific surface was used as a support. However, simultaneous supporting of AuCI3 on particles of H2PdC16/polyamide-66complex causes a 2 - 3 fold decrease in the specific surface of platinum and thereby decreases the catalyst activity significantly [116]. The incorporation of Au promotes formation of Pt-Au clusters on the polymer surface and decreases the accessibility of platinum atoms as the result of their screening by surface atoms of gold. Stable complexes of Rh, Pd, Pt, Ir are obtained by fixation on nylon [117]. These metals are probably each coordinatedly bonded with two amide groups of nylon. Such complexes exhibit high catalytic activity during benzene hydrogenation. With regard to the decrease in catalytic activity of their complexes, these metals are arranged in the order Ir > Pt > Rh, Pd. Whereas cyclohexene and cyclohexane were formed by hydrogenation of benzene with a Pt/nylon catalyst, only cyclohexane was detected with a Pt/Si02 catalyst [ 1191. Increased selectivity was observed for cyclohexane at elevated temperatures and after air pretreatment. Apparently oxidized platinum particles were responsible for cyclohexene formation. Polynaphthoylenebenzimidazoles (PBI) possess multiple properties which enables their use as supports in heterogeneous catalysis. Such polymers demonstrate high heat resistance and mechanical strength and low stability in organic solvents. They are moreover stable in the presence of acids and alkalis. A PBI-PdC12 catalyst containing 4&0.25% Pd has been prepared by PBI impregnation of an solution of Na2PdC14in aqueous dimethylformamide [ 1191. Palladium chloride supported on PBI, is reduced by hydrogen to metallic palladium which is an active catalyst for the hydrogenation of unsaturated compounds, isomerization of olefins and ally1 alcohol, as well as the disproportionation of cyclohexadiene to benzene and cyclohexene. After treating a PBI suspension with excess Cr03 at 20 "C, stirring for 5 hours and washing to remove the non-bonded reagent, PBI complexes containing 2.3 mmole of Cr03 per gram of PBI were obtained [120, 1211. Catalytic properties of the complexes were studied during the oxidation of alcohols to ketones. The polymer-metal complexes synthesized at different temperatures behaved differently because Cr6+ can be reduced to either Cr3+ or Cr2+. At 20°C, Cr03 splits benzimidazole rings to form COOH groups. The PBI complex with Pd(0) obtained by fixation of PdC12 and subsequent reduction of Pd(2+) to Pd(0) by methanolic NaOH in a H2 medium is effective at amine reduction. One important advantage of the PBI-Pd(0) catalyst is its ability to be fully regenerated by washing the reduced substrate from the polymer with methanol.

3.2 Catalytic Properties of Coordination Polymers

91

In the swelled state, polyamides based on 2,2-bipyridyl and 4,4'-dicarbonic acid are capable of bonding RhC13 and PdC12 [122, 1231. Polyamide metal complexes were reduced by hydrogen in alcoholic solutions of NaOH. These complex catalysts were highly effective during hydrogenation of saturated and unsaturated ketones to saturated alcohols (90- 100% yield), olefins (62- 100%) and nitro compounds (90- 100%). They were, moreover, reused thirty times without losing their activity. The catalytic activity of transition metal ions in PEG complexes of cobalt, manganese and copper chlorides is particularly affected by the polymer chain length and composition during tetralin oxidation [124- 1271. In complexes of 1 : 1 composition (one metal ion per polymer chain), the catalytic activity of the metal ion increases tenfold for Co2+ and Mn2+ and twice for Cu2+, when the polymer molecular weight is raised from 4000 to 40000. With an increase in the number of metal ions per polymer chain, the catalyst activity sharply decreases. This is caused by additional cross-linking of the polymer chain at metal/PEG ratios > 1. Different inorganic polymers, silica gel in particular, can be employed as matrices for fixing polymermetal complexes. Mechanisms of liquid-phase tetralin oxidation with PEG-CoCl2 complexes fixed on Aerosol-I75 surfaces, were investigated [ 1271. The mechanism, activity and selectivity of substrate oxidation with a heterogeneous catalyst are similar to those for a homogeneous one. Complexes of poly-y-mercaptopropynesiloxane-Pt [ 128, 1291, poly- y-cyanopropylenesiloxane-Pd,poly-y-aminopropylsiloxane-Pd[ 1301, polyvinylpyrrolidone-Pd supported on SiOz have been synthesized, and their catalytic properties during hydrogenation of aldehydes, ketones and aromatic nitro compounds have been studied. Silica gel-supported, cyclized polyacrylonitrile metal complexes as well as PMAA-Pd and PMAA-Pt complexes were prepared by copolymerization of MAA and a mixture of m- and p-DVB in the presence of silica gel [131]. The catalytic activity of the obtained complexes was examined in the reaction of cumene and ethylbenzene hydrogenation [131] and in the hydrogenation of aromatic and aliphatic nitrocompounds, alkenes and aliphatic aldehydes. Based on the decrease in the bond energy of cu2,3/2 from 935 to 932.4 eV and the increase in the bond energy of NIs from 399 to 399.6 eV, it was found, by XPS, that Cu+ is bound to a complex through a nitrogen atom of PAN. The activity of metal catalysts during cumene oxidation decreased in the following sequence: C u + >Cu2+ >Co2+ >Mn2+. Yields of 63% cumic alcohol and 28% acetophenone were obtained from cumene oxidation, whereas 87% acetophenone and 13% phenylethanol were obtained from ethylbenzene oxidation. The activity of such catalysts was influenced by cyclization conditions, the molar nitrogedmetal ratio of the complex, reaction temperature and different organic additions. It was found that nitrobenzene hydrogenation catalyzed by Si02-PMAA-Pd depends on the molar ratio of COOH/Pd. The maximum rate was observed at a COOH/Pd ratio of 10: 1. Both palladium and platinum catalysts were active and selective. In their presence, nitrobenzene was quantitatively hydrogenated to aniline within 6 min, whereas heptene was hydrogenated to heptane within 10 min. The catalytic activity of Si02-PMAA-Pt was higher than that of Si02-PMAA-Pd. Hydrogenation of aromatic nitrocompounds occurred faster than that of aliphatic nitrocompounds; the hydrogenation rates for p-, rn- and o-nitroanilines were 54.5, 50.0 and 28.0 mllmin, respectively.

92

Chapter 3 Heterogenized Homogeneous Catalysts

On the basis of the equation which accounts for the influence of swelling on the concentration of active centers and rate constant, experimental data from nitrobenzene hydrogenation with palladium containing polymers have been analyzed [ 1321. Synthesis of a coordination polymer of Rh(III), Pd(I1) with 2,3,6,7-octanetetraone-tetraoxime and selective hydrogenation of alkenes alkynes, carbonyl compounds and other substrates by this polymer complex have been described [133, 1341. By the study of the hydrogenation of various double and triple bonds (suspension in alcohol, T = 20°C, P H 2 =1 atm), it was found that this complex catalyzes the hydrogenation of the terminal double bond of alkenes under mild conditions. However, the hydrogenation of internal alkene double bonds proceeds very slowly. In this case, the polymer complex supposedly first catalyzes double bond migration to the end of an alkene molecule and only then accelerates the hydrogenation of the terminal double bond. Moreover, the complex readily catalyzes the hydrogenation of internal double bonds conjugated with aromatic rings. The complex, however, does not catalyze the hydrogenation of the - N = N bond in azobenzene. The hydrogenation of PhCECPh to PhCH2CH2Phoccurs very easily. The intermediate product is evidently cis-PhCH = CHPh because the trans isomer is hydrogenated more slowly. The selectivity of hydrogenation catalyzed by the complex apparently depends on steric factors.

3.3 Gel-Immobilized Catalytic Systems Catalysts display high specific activity provided that their contact with surfaces where reagents react at active sites are well developed. This can be achieved either by use of maximum dispersions or by synthesizing highly developed inner surfaces of catalyst granules. In the first instance, it is difficult to separate finely dispersed catalytic particles suspended in the reaction medium from final products and to reuse them. Concerning the latter alternative, an increase in the inner surface area of particles caused by high porosity is limited by a decrease in granular strength. Moreover, even heterogeneous catalyst samples which are mechanically strong in the initial state, break down and become pulverized in use. The following basic properties of catalysts are cited in the literature [135-1391: (1) A catalyst should consists of specifically formed particles of a separate phase which are insoluble but capable of swelling in the reaction medium, Le. permeable to both the substrate and solvent molecules. (2) Reaction products should diffuse readily in the catalyst phase and pass from this phase to the solution thereby vacating sites for new substrate molecules. (3) Active catalytic particles should be mechanically strong, elastic and morphologically stable so that they do not decompose or adhere to one another during a reaction. (4)Active catalytic sites should be uniformly distributed throughout the whole bulk of the catalytic system and should be attached strongly enough to polymer-support chains so that they are not washed out by reagent flows.

3.3 Gel-Immobilized Catalytic Systems

93

These requirements are partially met by catalytic systems consisting of a polymer (gel) swelled in the reaction medium and permeable to reagents and reaction products, and in which transition metal complexes are immobilized. In this case, the reaction proceeds not only on the surface but within the whole bulk of catalyst particles. Therefore the effectivness of such active sites can be as high as during homogeneous catalysis. At the same time, the catalyst is readily separated from reaction products. Furthermore, under conditions which regulate polymer gel swelling, it is possible to control the catalytic process [139]. Gel-immobilized catalytic systems (GCS) represent swelled polymer composites in which active sites of the particular metal complex are immobilized. Graft copolymers of ethylene-propylene rubber (EPRu) and ligands of 4-vinylpyridine, acrylic acid, vinylpyrrolidone, organophosphorus compounds etc. act as a polymeric supports (polymeric phases) [140]. The structure of metal complex sites immobilized in a polymer gel is presented by the following scheme:

Structural-morphological GCS studies show that graft components (e.g. poly-4vinylpyridine) are dispersed in the synthetic rubber matrix. The inclusion diameter is 1000-4000 A. The method of etching samples in high-frequency plasma of activated oxygen and tinge of heavy metal pairs allow delineation of the fine structure of the graft phase. The diameter of structural, graft component elements is 50-200 A. The complexing metal is sorbed in inclusions of the graft phase; this causes compaction of globular formations of the graft phase and coarsening of its internal elements. The polymer-base phase, however, remains intact [141, 1421. Structural peculiarities of coordination centers in the system EPRu-PVP * NiC12 were investigated [142]. This system is a microheterogeneous composite in which the EPRu phase is responsible for the transport of reagents, and the PVP phase serves as a so-called “microreactor”. Molecular mobility data of sondes in EPRu imply that in the active state (or swelling state), EPRu is characterized as a gel containing up to 300% more solvent than the non-swelled state as well as microheterogeneous PVP regions having low molecular mobility. A hierarchical structure of GCS consists of three phases [143]. The first phase represents an elastomer base in which inclusions of phase 2 of the graft copolymer are dispersed. Isolated inclusions of phase 2 contain separate domains of the polymer

94

Chapter 3 Heterogenized Homogeneous Catalysts

ligand [3] on which catalytically active sites with transition metals are immobilized. The typical width of such domains is 5 - 10 nm. The macrokinetics of catalytic reactions occurring in GCS particles have been studied in order to elucidate optimum characteristics of the hierarchical structure of GCS [ 1431. The catalytic reaction proceeds in phase 3, namely in domains. Applied analytical methods have made it possible to reveal the general macrokinetics of processes involving GCS for the first and second order reactions, and to show how the reaction rate depends on basic structural parameters. In addition, some characteristics of the optimum hierarchical structure can be estimated from the results of this analysis. One characteristic property of GCS is the preservation of constant catalytic activity over time. This allows catalytic reactions to be performed in both flow and batch reactors. It can be seen in Figure 28 that the rate of ethylene dimerization catalyzed by GCS in a flow system remains constant for several tens of hours, whereas their homogeneous analogues are quickly deactivated.

I 3

2

2

4

20

60

t/h

Fig. 28. Dimerization of ethylene on different nickel catalysts: 1 - EPR-P4VP-Ni(AcAc),AIR2C1; 2 - Ni(AcAc),-AIR,Cl; 3 - Ni(AcAc),-ethylpyridine-AlR2C1[ 1361

Gel-immobilized catalytic systems containing Ni are obtained in two separate stages: (1) immobilization of nickel(I1) salts in a polymeric support to form a nonactive gel complex, and (2) activation of the gel complex by alkylaluminum halides [144]. During the formation of active sites of nickel-GCS, the reaction of nickel(I1) compounds with organoaluminium compounds (OAC) causes alkylation and reduction of nickel. The process of active site formation in GCS is strongly influenced by OAC. According to their decreasing catalytic effect, OAC can be ordered in the following sequence: C2HSAlC12 > (C2H&AlCl> (C2H5)3Al,

C4H9AlC12> (C4H9)2AlCl> (C4H9)3A1

3.3 Gel-Immobilized Catalytic Systems

95

The formation of active sites in GCS is probably associated with the necessity for vacating the initial ligand environment of a central ion. One likely mechanism of GCS activation is shown as follows: EPRu-PVP-NiL2 * n H 2 0+n(AIRC12)2-+ EPRu-PVP-NiL2+ n(H20* AlC13)+ nAlR2Cl In the first stage of the activation of nickel complexes of (EPRu)-PVP-NiL2*nH20, water molecules act as donors, whereas AICI3 molecules function as acceptors. This stage is a limiting one. Subsequent, alkylation of nickel by OAC proceeds faster. With regard to the decreasing effect of the chosen solvent on GCS formation, hydrocarbons and chlorohydrocarbons can be arranged in the following sequence: 1,2-dichloroethane = chlorobenzene > paraffins > aromatic hydrocarbons > olefins (C5). Products of propylene dimerization contain 74 wt.070 methylpentene, 24% n-hexene and 2% 2,3-dimethylbutene. The composition of products depends neither on the type of nickel and aluminium compounds nor the solvent used. This implies that active sites of nickel GCS have similar properties which do not depend on the structure and composition of initial components of catalytic systems. Codimerization of ethylene by propylene in the presence of GCS containing different organophosphorus ligands, has been studied [145]. Nickel fixation leads to additional cross-linking of GCS. Catalysts with phosphate microligands are the most effective among phosphorus-containing GCS. Table 17 shows the effect of P ligand type on the composition of reaction products. In the sequence - P(C6HS)2; - P(OC6H5)2; - P(C6Hl1)2; - P(OC6H1')2; -P(iso-C3H7)2 a tendency is observed towards a reduction in the content of n-pentenes (from 70 to 60%) and an increase in the yield of isopentenes (from 3 1 to 39 wt.070). However, unlike homogeneous catalysts, the effect of the P ligand in GCS is much weaker. The oligomerization of furfuryl alcohol catalyzed with acidic catalysts (H2SO4, H3P04, erc.) or with AI2O3was examined earlier. In each case, the reaction occurred as intermolecular dehydration which formed linear products and preserved the aromatic furan structure. When GCS containing catalytically active compounds of Al, B and Ti were employed it was found that oligomerization of furfuryl alcohol (FAI), 2-furfurylidene acetate (FAc) and tetrachlorofuran (TCF) takes place without water formation. Moreover, the analysis of oligomerization products confirmed the preservation of their functional groups [146]. The UV spectrum of FAI, FAc and TCF oligomers indicated the disappearance of the adsorption band at 335 - 340 nm which is typical of a furan ring. Whereas IR spectrum of these oligomers showed a vivid decrease in the intensity of bands typical for a furan ring at 3120, 1500, 1020, 880, 800-740 cm-', the intensity of the band at 1220 cm-' (antisymmetric vibration of C - 0 - C) greatly increased. These changes as well as functional analytical data suggest that the yield of products in which cyclic fragments do not maintain the aromatic furan structure increases during oligomerization. The product yield of the conversion of a furan compound increases in the order: FA1 < FAc
6.2 8.0 3.2 2.8 2.6 2.2 2.4 2.1

2.5 2.3 1.5 2.3 1.5 1.7 1.3

-

Swelling in heptane, mg/g

Content of P, wt.%

69.0 66.9 69.4 68.4 65.2 62.3 60.7 61.8

32.8 32.7 33.7 33.9 32.7 32.9 31.8 30.9

18.6 21.8 20.5 21.2 21.6 21.8 22.7 23 .O 48.6 41 .O 45.8 44.9 45.7 45.3 45.5 46.1

31.0 33.1 30.6 31.6 34.8 37.7 39.3 38.2

iso-pentenes

n-pentenes

hexenes

butenes

pentenes

Composition of pentene fraction, wt.%

Composition of reaction products, wt.%

Note: Heptane is the solvent; P = 0.2 MPa; T = 40°C; AIRCl, is the cocatalyst; ethylene: propylene = 1 : 1 mol/mol.

Polymer ligands

330 410 140 151 132 145 128 134

Relative rate of codimerization, g of product

Table 17. Codimerization of ethylene and propylene with gel-immobilized nickel catalysts containing different macromolecular ligands

Q\

W

3.3 Gel-Immobilized Catalytic Systems

97

effects of substituents which, by withdrawing the electron density from a promoter heteroatom, enhance the character of the furan ring diene, two possible reaction mechanisms are feasible. The authors contend that, of these two mechanisms, the cycloaddition route is more likely. The difference in the molecular weight of the products is attributed to the fact that the polymer matrix restricts chain development, especially when oligomerization proceeds on active sites in a block of gel-immobilized catalytic systems. Basic characteristics and mechanisms of ethylene polymerization on gel-immobilized catalytic systems have been studied. Graft copolymers of polybutadiene, poly-4-vinylpyridine (P4VP), polyallyl alcohol and polymethacrylic acid (PMAA) on EPRu have been used as polymeric phases of GCS [147-1491. Titanomagnesium GCS were found to be most active. The effect of time and temperature on the ethylene polymerization rate as well as the effect of time on the polyethylene coverage of GCS were examined. How the degree of swelling affected the ethylene polymerization rate was also tested with different gel-immobilized catalytic systems. It was shown that immobilization of active sites in gel causes a decrease in the mean length of forming macromolecules. The dependence of the ethylene polymerization rate on the hydrogen concentration was significant, and the molecular weight of the polyethylene formed decreased upon increasing the hydrogen concentration in the system. By analyzing possible macrokinetic regimens of dimerization and polymerization reactions with different gel-immobilized catalytic systems, Neimark et al. discovered the existence of an optimum structure of such systems for each process and obtained optimum ratios of catalytic system components. These results agree well with experimental data and can be used for the target-oriented synthesis of GCS, in particular, for creating polyfunctional catalysts which catalyze specific chemical reactions. Methods of covalent and electron donor-acceptor fixation of metal complex catalysts on macromolecular supports have been intensively researched [151- 1541. The research cited covers achievements in the manufacture and study of immobilized polymerization catalysts based on transition metal compounds. Polymeric supports for such catalysts are obtained either by copolymerization of monomers with functional groups or by grafting such groups onto a polymeric base. Supports can contain electron-donor sites, i.e. complexing groups capable of bonding transition metal compounds. Polymeric supports based on polyolefins or other carbon chain polymers with a functional overlay resulting from grafting of monomers to functional groups, donor sites or chelate units have also been developed [ 151 - 1581. A number of carbon chain polymers (powdered polyethylene, polypropylene, polystyrene, EPRu, polyvinyl chloride, etc.) were subjected to gas-phase graft polymerization by monomers with polar groups (ally1 alcohol, acrylic acid, diallylamine, diallyl sulfide, vinyl acetate, 4-vinyl pyridine, etc.) initiated by ionizing irradiation. One general scheme for synthesis of such supports is depicted as follows:

98

Chapter 3 Heterogenized Homogeneous Catalysts

where R = H, CH3, C2H5, C3H6; R = R"' = H, CH3, C6H5; X = OH, NHR, SR, COCH,, CH2OH, CH2NHR, CH2SR, COOH, OCOCH3, COOCH3, C=N. However, the basic physicochemical properties of such polymeric supports do not change. Moreover, it is possible to control the length of graft chains and their density on the support surface. The density of graft fragments is 0.1 - 1.3 mmole/g, and the depth of a graft layer is 100-300 A. Transition metal compounds such as TiC14, T ~ ( O B U )(C5H5)2TiCI, ~, VC14, VO(Et)3 (0.018 -0.27 g-atom/g), can be attached to such supports by either covalent (a) or donor-acceptor (b) bonds. The catalytic activity of such systems greatly depends on the type of distribution and catalyst fixation on a polymeric support [l55]. In order to elucidate the effect of immobilization on catalytic properties, it is important to have information on the type of distribution of metal compounds on the support surface [156].

The spin label method is widely used to study the topochemistry of the polymeric support, synthesized by grafting diallylamine on low-pressure polyethylene, and immobilized catalytic systems based on this support [157]. The distance between fixed iminoxyl radicals and functional groups of the support is relatively large (- 25 -35 A), whereas the distance between these radicals and fixed components of a catalytic system is 1 3 5 A. Application of the spin label method [I581 and the theory of dipole broadening of EPR spectra [ 1591 has made it possible to distinguish two types of distribution of transition metal compounds on the support surface; namely uniform (isolated) and cluster distribution. With regard to VC14 distribution (PE-graft-P4VP), on the polymeric support of polyethylene-graft-poly-4-vinylpyridine the upper, local concentration limit of isolated complexes in the graft layer is 2.10-4 g-atom/cm3. The local concentration of V(+4) in clusters is estimated as (2-6). g-atom/cm3. Assuming that the concentration of isolated vanadium particles does not change upon increasing the vanadium content in the graft layer, one can evaluate the proportion of isolated and cluster ions as a function of the vanadium surface density

3.3 Gel-Immobilized Catalytic Systems

99

Fig. 29. Dependence of relative Cclust. (1) and Ciso,,(2) of vanadium atoms on surface density V on polyethylene-graft-poly-4-vinylpyridine[ 1581

(Fig. 29). It can be seen that by incrementing the surface density of vanadium, the amount of isolated particles sharply decreases, while the proportion of vanadium(1V) included in cluster formations increases. The obtained values of Cisol. and Cclust, enable evaluation of mean distances both between isolated vanadium complexes and between vanadium particles forming a cluster: Fisol,222A and FClust. = 6.8-9 A. Taking into consideration the six-coordinated environment of vanadium atoms (specially proven through the method of competing ligands), one can depict the intraand interchain interaction of VCI, with PE-graft-P4VP by the following scheme:

bbb@ N

N

N

848 t

VCI,-

c

y

t

- - -VCI,

c

Research studies [ 1601 conducted with static magnetic susceptibility and EPR spectroscopy methods have made it possible to define the distribution character of Cu(2 + ) ions fixed on polyethylene-graft-polyacrylicacid supports (PE-graft-PAA).

Chapter 3 Heterogenized Homogeneous Catalysts

100

The presence of three types of copper(I1) complexes was established: (1) isolated complexes separated by a mean distance of Fisol, 2 15 A;(2) complexes bonded by dipoleA; and (3) cluster complexes with strong exdipole interactions with Faggregate<7 change interactions. The content of the latter considerably increased with an increase in the concentration of fixed Cu(I1) complexes. The accumulation of complexing sites along a macromolecule caused formation of polynuclear complexes. Comparison of VC14 distribution in PE-graft-P4VP [I591 with Cu(I1) distribution in PE-graft-PAA shows that in the former case, the polymeric support is a rigid matrix which swells little under reaction conditions (CC14 solvent) and thus determines the spatial arrangement of metal ions. In contrast, PE-graft-PAA in an alcohol-water medium is a soft matrix, the spatial arrangement of paramagnetic sites in which is determined mainly by their properties. Therefore one can expect that the structure of copper(I1) complexes considerably affects the activity of copper-containing catalysts. Methods of synthesizing polymeric supports with an organometallic functional overlay have been determined, and titanomagnesium catalysts (PTMC) based on them have been prepared. The formation of isolated Ti(II1) ions has been discerned by EPR methods. Depending on synthetic conditions and polymeric support type, these isolated ions amount to 55% of fixed titanium. Experimental proof has been obtained for the formation of Ti(II1) through the step of Ti(1V) alkylation by magnesium-containing layers. From the dependence of the g-factor of Ti(II1) signals on formation conditions, one assumes that the polymeric support fragments enter the coordination sphere of Ti(II1) at low Ti(II1) concentrations in PTMC, and at high Ti(II1) concentration during reactions of Ti(II1). The presence of active phase crystallites of 40-75 A in diameter has been determined by X-ray diffraction. A correlation between the activity of PTMC during ethylene polymerization and the concentration of isolated Ti(II1) ions in these catalytic systems has been determined. Thus, complex catalysts on polymeric supports which catalyze olefin polymerization combine advantages of conventional heterogeneous and homogeneous catalysts such as high activity and selectivity, and their own inherent advantage of longevity [ 162- 1641. Consequently, supported catalysts contain stable active sites of homogeneous composition which enables the study of the structure and conversions of these sites by means of IR spectroscopy. IR spectroscopy [I651 was used to analyze complex catalysts based on TiC14 and AlR2Cl supported on different polymeric supports during olefin polymerization. In the course of these studies, the sequence of supporting catalytic components and mixing temperature were varied. It was found that synthesizing these immobilized catalytic systems requires organoaluminum components (AlEt2Cl) fixation and subsequent mixing with a transition metal compound at the necessary reaction temperature. It was moreover determined that the reactive sites of the active catalytic system of polyethylene-polyacrylic acid-A1Et2C1-TiC14 can be represented as a bimetallic complex with bridge bonds:

,c1..)Ti

Al,

'*

c1

3.3 Gel-Immobilized Catalytic Systems

101

TiCI4 immobilized on surface of PE modified by graft polymerization of vinylpyridine, acrylonitrile, methyl vinyl ketone, allyl alcohol, allylamine, allyl sulfide or acrylic acid, has been studied with IR spectroscopy and XPS [166]. Cluster aggregates of Ti(IV) fixed on the surface, were discovered. Their formation did not cause the reduction of Ti(1V). In binary catalytic systems of Ti(1V) on an organoaluminum polymeric support, Ti(IV) was reduced to Ti(III), and Ti(I1) appeared at 343 K. The high and stable activity of immobilized titanium compounds during ethylene polymerization was attributed to deceleration of Ti(1V) reduction processes. In most cases, specific catalytic activity depends on the surface concentration of the fixed transition metal [167] which influences the specific localization of fixed complexes on the support surface. For instance, in a PE-graft-PAA/Ni2+ catalytic system, depending on Ni(I1) content, either a complex structure with an anhydride ring can dominate: -CH2 -CH -CH2 - CH -

I

I

c=o

o=c

I

I

or the monosubstituted form can be favored: -CH2 - CHI

c=o I ONiX

Such complexes can be involved differently in ethylene dimerization reactions. During a catalytic process the chelate structure I exhibits higher activity than structure I1 [168]: CH -CH2 -CH -CH2

-CH2-

I

,Ca,

o,=," co

N

L o ' /

'0-c

I

I

4 \

H3C' -CH2

-

CH3

- CH -CH2 -CH I

I

- CH2 -

102

Chapter 3 Heterogenized Homogeneous Catalysts

In systems based on mononuclear cobalt chelates, virtually every metal atom contributes to the stereospecific polymerization of butadiene, whereas in the system based on binuclear chelate, only 24% of the cobalt atoms are involved in the formation of active sites.

3.4 Heterogeneous Catalysts Fixed on Ionites Ion-exchange resins (IER) are widely used as polymeric supports. Some advantages of using ionites as catalytic supports are their low solubility in water, limited swelling in organic solvents, high sorption capacity, thermal and chemical stability and microand macroporous structure [169- 1711. Such catalysts can be prepared relatively easily by impregnating IER with a solution of complex salts. It is also possible to widely vary the nature of ionic groups which changes the ligand environment of the metal. Consequently, this variation affects catalytic activity and selectivity. Ion-exchange resins (IER) in which functional groups are macrophase-separated from the hydrophobic polymer chain have been studied as ionic domains [172]. Transition metal ions find their way into such domains through ion exchange, thereby producing isolated, catalytically active sites 20- 100 A in diameter. One of the basic conditions necessary for preserving and increasing the effectiveness of complex catalysts on IER is that a complex should be fixed on a support without a change in the environment of the central metal atom [I73 - 1761. Heterogenized metal complexes have been obtained by ion exchange from aqueous solutions of complex salts with the general formula [ML,L&lq, ( M = Co, Ni, Ru, Rh, Pd, Os, Ir, Pt; L = Hal-, NOT, SCN-; L' = Me2S0, Et2S0, Et+, NH,), based on the strong-base anionite AB-17-8 and the strong-acid cationite KY-2-8. Many of these metal complexes exhibit catalytic activity during styrene hydrosilation by methyldichlorosilane and are stable enough to be used repeatedly. The internal coordination sphere of Ni(II), Co(II), Rh(III), Pd(II), Pt(I1, IV) and Ir(II1, IV) complexes is maintained during ion exchange. However, Ru, Os, and Pd(IV) complexes break down thereby causing heterogenized metal complexes (HMC) based on them to lose their catalytic activity. Catalysis in the presence of HMC proceeds directly on ionic group sites. The application of ionites as substrates makes it possible to obtain metal complexes with high stability under hydrosilation reaction conditions associated with hindrance of intermolecular exchange of ligands [177]. Fixation of complex ions on ionites increases the regioselectivity of catalysis (Table 18, see p. 103). Whereas the addition of methyldichlorosilane to styrene in the presence of H2PtC16 yields (2-pheny1ethyl)methyldichlorosilane (p isomer) and (I-phenylethy1)methyldichlorosilane (a isomer) in comparable quantities, heterogenized, hexachloroplatinate metal complexes catalyze the reaction with a 98.6% yield of the product of terminal addition. Depending on the ligand environment, the isomer yield increases in the element sequence: 3d <4d < 5d. The activity of metal complexes is also enhanced by increasing the number of the complexing metal. This increased activity is apparently attributed to a rise in the strength of metal-olefin bonds with increments

3.4 Heterogeneous Catalysts Fixed on Ionites

103

Table 18. Catalytic activity of some metal complexes heterogenized on ion-exchange resins Catalyst

Metal content

Degree of exchange

[701

[%I

Styrene hydrosilation by methyldichlorosilane Reaction time [h]

Total yield of reaction products

Yield from initial use of catalysts

p isomer

a isomer

[701

H2PtCl6

-

-

AB-17-8/PtC16

18.03

AB-17-8/PtC14 AB-17-8/OsC16 AB-17-8/Ir(III)C16

19.48

100 100

AB-1 7-8/Ir(IV)C16 KY-2-/Ir(NH3),CI KY-2-/Pt(NH3),C1

4.7 13.49 17.81 22.83 17.91

-

98.3 100

99.5 100

2 2 5

20 10 10 10 10

98 98.7 99.0 2.0 71.0 50.0 41 .O

90.6

73 98.3 83.4 91 .O

21 1.7 16.6 9.0

100.0

0 0

100.0 97.8 92.8

2.2 7.2

in the sequence 3d < 4d < 5d of group VIII elements [178]. Consequently, the lability of the double bond of coordinated styrene increases as well as the coordination ability of metals with respect to organosilicon hydrides in the same order [179]. Both amino groups and quaternary ammonium salts are coordination sites during H2PtC16 immobilization on weak-base anionites [180]. Fixation of H2PtCl6 is effected by ionic exchange and donor-acceptor interaction. However, the more the latter contributes to fixation, the faster is the reduction in catalytic activity. The reduced activity depends on the nature of active ionite groups [174]. The catalyst activity sharply decreases for AN-22-8P anionite containing ethylenediamine groups. Such a decrease, however, is less for AN-21- 16P (based on hexamethylenediamine) anionite. When the AN-1 8-1OP anionite (active groups NH+(CH3)20H- and N(CH3)2), which is structurally related to the strongly basic anionite AB-17-8, is employed, the reduced catalytic activity during repeated treatment with organosilane is insignificant. This is also the case despite the fact that the catalyst partially dissolves in the solution. Thus decomposition of the matrix bond and the transfer of the catalytic complex to solution occur when surface active groups form coordination bonds. The stability and activity of such catalysts is high because of their ionic character. Thus the surface groups of AN-18-1OP act as monodentate ligands. One suggested means of preserving the ligand environment of the central metal ion is to attach transition metal complexes containing sulfurized triphenylphosphine to different IER (Molselect, DEAE-25, Varion) [ 1811. Systematic studies of the liquid-phase oxidation of hydrogen sulfide and cysteine by molecular oxygen were performed using ionite complexes of iron (11) [182-1851. These reactions are interesting with regard to an economically feasible method of producing sulfur, an effective method of cleaning H2S from industrial and natural gases, and understanding the processes occurring in biological systems. Phosphorus-, nitrogen-, sulfur- and oxygen-containing ionites were used as supports. Table 19 lists

Chapter 3 Heterogenized Homogeneous Catalysts

104

Table 19. Catalytic activity of ionite complexes of iron in an aqueous medium (10ml) during H,S oxidation by oxygen (40"C;H,S/O, = 2/1; ionite: 0.05 g; NaOH: 0.05 g)

Ionite type

Ionite functional groups

Fe2 content, [mg-eq/l] +

Oxida-

Rate of

tion

(H2S + 0 2 )

mixture

product

absorption [ml/min]

ANKB-2

W

C

O

O

H

COOH

ANKB-50

-N: 1

so:

2.07

0.0082

S

1.47

0.250

so: -

0.80

COOH

K?

ANKF-I

0.0322

characteristics of ionites and their catalytic properties during the liquid-phase oxidation of H2S. High catalytic activity of ionite complexes of iron was observed in alkaline solutions. In the absence of base, all the catalysts studied proved themselves virtually inactive. The oxidation mechanism of both H2S and RSH in the presence of these catalysts is the same and involves electron transfer from RSH or S to oxygen via Fe"'aq(L) complexes:

e

P

I

4 1

H2S(RSH). . .Fe"'aq(L).

4

. .O2

The electron transfer from the iron complex to oxygen is a limiting step. The kinetics of cysteine oxidation are described by the equation W = k [RSH] [catalyst] [02] [OH-], the activation energy is 36.8 kJ/mole. The catalytic activity of the (RSH + 0 2 + FeZf/ionite+ H20) system can be controlled by changing the medium pH or by adding alkali to the system. The accelerating effect of alkali is attributed to the involvement of mixed dimer or polynuclear complexes of iron in the oxidation process, which facilitates the twoelectron transfer from cysteine to oxygen [ 1861. The enhancement effect of a catalyst has been discerned for iron-containing ionites. This is marked by an increment in catalytic activity, when repeated cycles of H2S oxidation are conducted under comparable conditions [ 182, 1831. The observed effect depends on the nature of the functional groups of the ionite.

-

-

3.4 Heterogeneous Catalysts Fixed on Ionites

105

In the presence of selected catalysts, H2S oxidation takes place by two routes, namely, the mild oxidation of hydrogen sulfide to elemental sulfur (route I) and further oxidation to SO:- (route 11): Fe"("')aq( L)

2H2S+02

S+2H20,

H20

Reaction selectivitydepends on the nature of ionite functional groups and is determined by the length of complexing groups (Table 20). In the presence of AN-5 1/8, H2S is oxidized to elemental sulfur, whereas in the presence of AN-221, it is oxidized to SO:-. Coordination complexes of copper(I1) ions with a carboxyl-containing ionite, KB-4, exhibited high catalytic activity during Na2S oxidation by molecular oxygen [187]. This complex consists of four ionite carbonyl groups lying in a square plane and two H 2 0 molecules lying in octahedral vertices. The increase in the catalyticactivity of copper ionite (EDE-1OP) complexes during sodium sulfide oxidation by molecular oxygen, depending on the copper ion concentration, is attributed to the formation of copper sulfide complexes of the following type [188]: L

/

L

S

S

\

cu

/

\

\

/

S

cu

L

/

\

\

/

S

cu

-L

/

\

\

/

L

/

cu

S

\

-L

L-

\ /

/

cu

S

\

L-

whereby L denotes functional groups of the polymer. The catalytic activity of Fe(I1)-phthalocyanine bound to PAAm gel (analogous to Cytochrom C) and ionites during the oxidation of cysteine as well as copper-containing cationite complexes during Na2S oxidation by molecular oxygen was also investigated [I 871. The applied matrix consisted of a fiber containing functional groups which are able to bind transition metal ions by coordination bonds. The selectivity of

Table 20. Catalytic activity and selectivity of ionite complexes of copper(I1) in an aqueous medium during H2S oxidation by oxygen (conditions are the same as in Table 19) Ionite type Functional groups of ionite

AN-221 AN-51/8 AN-251

- NH - CH2 - CH2 - NH2 - NH(CHz-CH2NH)Z - CH2 - CH, - NH2 - NHCH2 - CH2 - NH - CH2 - CH2 - NH2

Copper(I1) content [mg-eq/gl

Oxidation Rate of H2S + 0, product mixture absorption [ml/min]

1.82 1.83

so:S

2.1 1.6

1.76

S

1.3

106

Chapter 3 Heterogenized Homogeneous Catalysts

this oxidation reaction depends both on the nature of polymer matrix and the type of immobilized metal ions. By comparing data regarding ultra frequency conductivity and catalytic activity, it was found that in the polymer phase, stable sulfide structures form as electron transport channels. These sulfide complexes can be regarded as close analogues of natural electron transport agents, ferredoxins. The activity of phosphorus-containing ionites with complex-bound iron(II1) ions was studied during H2S oxidation by molecular oxygen in water at 20-40°C [189]. Iron sulfides on the ionite surface, which possess high catalytic activity and selectivity in oxidation of H2S to S, were formed during the reaction. These sulfides are readily oxidized in air to surface hydroxides (ionite-FeOOH) which are easily reduced back to sulfides by H2S. Thus, a mutual conversion of sulfides and iron hydroxides occurs during H2S oxidation. Iron ion complexes either facilitate electron transfer from H2S to O2 by alternately changing their valency from Fe3+ @ Fe2+, or they do not change their valence state. Knowing the reaction mechanism, one can specifically change the catalytic activity and selectivity of the system (H2S + 0 2 + Fe ionite+ solvent + OH -) by varying reaction conditions and the composition of the coordination centre of iron-containing ionites. On this way one can synthesize effective catalytic systems for selective H2S oxidation. The feasibility of using ionite complexes of Co2+, Mn2+ and Fe2+ as effective heterogeneous catalysts of liquid-phase oxidation of cumene was shown [ 190, 1911. The main products of cumene oxidation were its hydroperoxide and dimethylphenylcarbinol. The catalytic activity of ionite cobalt(I1) complexes depended on the type of IER ionic groups. The highest activity was demonstrated by Co2+ complexes with nitrogen-containing ligands, whereas the lowest activity was obtained from Co2+ complexes with carboxyl- and sulfur-containing groups. A medium level of activity was found for ionites with phosphate units. In a series of metals, ionite complexes of cobalt, manganese and copper were most active, whereas those of nickel and iron were least active. The kind of complexing ligand groups affected the selectivity of cumene oxidation. Amine complexes of transition metals oxidized cumene to dimethylphenylcarbinol. However, selective oxidation of cumene to its hydroperoxide was observed in the presence of phosphorus-containing ionite complexes. In practice, styrene divinylbenzene ion-exchange resins with different concentrations of divinylbenzene (DVB) as a cross-linking agent are often used as polymeric supports. It is assumed that the degree of cross-linking of a polymeric component affects the physical properties, texture and activity of these catalysts. Series of polymer-supported rhodium catalysts were prepared by reacting rhodium tetracarbonyldichloride with dimethylaminomethylated copolymers of styrene with divinylbenzene of different degrees of cross-linking (10, 15, 25, 40, 50, 60%) [192]. The change of texture of these catalysts during their preparation was studied with the method used to determine specific activity and porosity. The metal dispersion on the catalyst surface was ascertained by titration of the reduced catalyst by oxygen. It was found that the activity of these catalysts is enhanced by increasing the specific surface and pore radius. However, the selectivity of all the catalysts studied was approximately the same and was comparable with the selectivity of the dissolved rhodium complex.

3.4 Heterogeneous Catalysts Fixed on Ionites

107

Increasing catalytic activity by augmenting the specific surface area of catalysts with the same selectivity can be explained in two ways: either the reaction proceeds in catalyst pores which are large enough not to impose steric hindrances on reagents, or a polymer-supported complex acts as a primary, soluble, catalytically active site where the respective reaction occurs in solution. In the former case, a change in catalytic activity did not result from different diffusion rates of reactants to particular active sites. In the latter case, the activity depended on the readiness with which these sites separate themselves from a polymeric support. In preliminary experiments the authors found that a metal is partially dissolved. A comparison of the structure of catalysts synthesized by fixing anhydrous AIC13 and FeC13 on cation- and anion-exchange resins of porous and gelatinous types, shows that A1 and Fe are distributed uniformly in them [193]. High catalytic activity proportional to the content of A1 and Fe and the possibility of regenerating polymer and copolymer complexes without loss of activity, were ascertained. Copolymer complexes synthesized from porous ion-exchange resins, are more active than those synthesized from gel-type resins. The results of the study on the hydrogenation of different functional groups were summarized [194]. Here complexes of rhodium, palladium and nickel fixed on balls of densely cross-linked macroporous polystyrene of HAD-4 grade, on which anthranilic acid residues were bonded, were used as catalysts. Special attention was paid to the kinetic study of cyclohexene hydrogenation by rhodium( + ) derivatives and to elucidate the reaction mechanism using D2. The influence of diffusion restrictions on the reaction rate was discussed, in particular, that of the transport of hydrogen to pores and through the gas-liquid interface. Films of PS-S-Rh3+ were obtained by incorporating rhodium(II1) ions into ionic domains of sulfurized linear polystyrene. The chemical activity of rhodium-containing domains capable of acting as isolated reactors during interaction of rhodium-active sites with CO, H2 and H 2 0 under mild conditions was studied [195]. The reaction with CO at low temperatures without or before H2 reduction or full dehydration of PS-S-Rh3+ films caused formation of active sites of Rh+(C0)2. Partial reduction of H2 films preceding or following the exposure of the films to CO also led to formation of Rh(1 +)(CO), and, under different conditions, to formation of linear or bridge-bonded carbonyl groups as well as carbonyls bonded with small particles of rhodium(0). Catalytic properties of the obtained polymer films were discussed, and it was concluded that polymer-supported transition metals can be employed as effective catalysts. The catalytic activity of H and perfluorinated cation-exchange membrane salts, F-4SK and Nafion, was analyzed during olefin carboxymethylation [ 1961 and condensation of epichlorohydrin with cyclic ketones [ 1971. The possibility of using these membranes instead of Friedel-Krafts catalysts for synthesizing ethylbenzene and then styrene was cited. The activity of membrane salt species decreased in the sequence: Cu2+ >Sn2+ >A13+ >Co2+ >Zn2+. p-Toluic acid was obtained from toluene by the action of the rhodium salt. The structure and distribution of oxide particles of Ru and U in perfluorinated ionexchange membranes were investigated by means of X-ray fluorescence, X-ray diffrac+

108

Chapter 3 Heterogenized Homogeneous Catalysts

tion, scanning electron microprobe and XPS [198]. Samples were prepared by impregnating Nafion membranes with RuCl3 and UOz(NO3)2 dissolved in aqueous methanol and water solutions. Subsequent treatment with a water-methanol KOH solution sharply reduced the concentration of ruthenium and uranium in samples and thus increased their surface concentration, whereby Ru was present as Ru4+. It was demonstrated [199] that fibers with catalytic properties can be synthesized by modifying hydrate cellulose fibers by grafting 2-methyl-5-vinylpyridine. Methods of heterogenizing homogeneous Co(CO)4 or its analogue, C O ~ ( C O )are ~, described [200]. These procedures involve fast and quantitative exchange with either anion-exchange resins containing quaternary ammonium groups, or with weak-base anionites having no such groups. The reaction was studied by IR spectroscopy in an alcoholic medium. It was determined that the catalyst obtained is effective for carbonylation of organic halides (RX) forming either acids or esters depending on reaction conditions. By using benzyl chloride as a model, it was found that the product yield is influenced little by the support type, and that catalytic effectivness could be increased only by decreasing the RX/CO(CO)~ratio. IR spectra of [PdCl4I2- groups fixed on surfaces of Amberlite A-27, Dowex-1 and polystyrene-divinylbenzene resins containing - N +(CH3)3 surface groups and C1- , were examined by impregnating the polymers with K2PdC14 solutions [201]. The supported salts were active in decomposing cyclohexyl hydroperoxide to trans-2-hexanal. Nonetheless, the same results were obtained with soluble analogues of these catalysts when diffusion was unhindered in the reaction process. The kinetics of heterogeneous-catalytic epoxidation of cyclohexene (CH) by tertbutyl hydroperoxide (TBHP) was investigated [202]. Catalysts were prepared by impregnating the ion-exchange resin, Amberlite IRS-84 in H-form, with the acidic solution of ammonium molybdate. The concentration of Mo was 0.35 mmole/g resin. The reaction kinetics were studied in the absence of solvent and at high molecular ratios of CH and TBHP. The reaction proceeded by pseudo-zero-order in respect of CH. Reaction selectivity was 90-95%; i.e. the initial rate could be determined from the rate of TBHP consumption. It was found that the reaction order was 1.8 for TBHP and 1 for the catalyst. Analysis of EPR spectra of catalysts before and after the reaction showed partial oxidation of Mo3+ to Mo”. The authors supposed a stepwise reaction mechanism in which the interaction between CH and a complex of Mo and TBHP is considered to be the slow and irreversible step. In this complex, Mo was present in the oxidized state. A novel method has been proposed [203-2051 for obtaining polymeric sorbents with prearranged macromolecular sites for complexing with transition metal ions. Cobalt complexes with the prearranged sorbent were tested as catalysts by the liquidphase oxidation of styrene and ethylbenzene. These catalysts could be used repeatedly without a decrease in activity. The catalytic activity of polyacrylonitrile and polypropargyl methacrylate complexes with Co3+ was studied during ethylbenzene oxidation reactions [206, 2071. Anhydrous AlC13 and FeC13 were fixed on porous and gelatinous cation- and anionexchange resins such that the Fe concentration in the resin reached 0.80mg/g. By means of scanning electron microscopy and electron microprobes of a X-ray analyzer,

3.4 Heterogeneous Catalysts Fixed on Ionites

109

it was discovered that A1 and Fe are uniformly distributed in copolymer complexes [ 1931. The synthesized products were effective catalysts of esterification of dibasic acids by ethyl alcohol at 120 "C (5 h at a 1 : 1 molar ratio, and 10 h at a 1 :2 ratio). It was shown that fixed copolymer complexes of AICI3 are more active than FeCI3 and that separation of carbonyl groups by methylene groups reduces the yield of ether. The esterification of the trans isomer of maleic acid occurred faster than that of aromatic and alicyclic acids. A search for optimum conditions for obtaining vitamin K3 from 2-methylnaphthalene using metal ions fixed on sulfurized polystyrene rubbers was conducted [208]. Preliminary screening of different metal ions as catalysts of 2-methylnaphthalene oxidation by 30% hydrogen peroxide in acetic acid was carried out at 318 K. The catalytic activity of acetates of Pd, Fe, Cu, Co, Zn, Ag and Ni, H2PtC16*6H20, OsC13-3H20, HAuCI4.4H20, IrC13, NiC12.6H20 and CoC12*6H20was tested. The highest yield of vitamin K3 (57%) was attained in the presence of Pd(0Ac)z. It was found that the starting rate of 2-methylnaphthalene oxidation and vitamin K3 accumulation in the presence of Pd2+ salts decreases in the following anion sequence: SO:- > C l o y >NO, >AcO- >PO:- >C1- >Br- > I - . Oxidation of 2-methylnaphthalene by 60% H202 in the presence of Pd2+ ions supported on Dowex-SOW caused an increase in the conversion from 59.1 to 98.3% as well as in the yield of the desired product from 28.4 to 59.4%. Furthermore selectivity rose from 48.0 to 66.3% compared with the same reaction in the presence of palladium acetate. Catalytic properties of V 0 3 + and MOO;+ ions fixed on macro- and microporous polystyrene matrices containing functional groups of iminodiacetic acid, bis(phosphonomethy1)amino groups and bis(2-hydroxyethy1)amino groups were studied [209]. Polymeric catalysts of the complex containing 1 -4 mmole/g of V or Mo, were active catalysts of epoxidation of cyclohexene, E-geraniol, cyclohex-2-en-1-01and linalol by tertbutylhydroperoxide. During cyclohexene epoxidation, higher catalytic activity was exhibited by molybdenum compounds, whereas vanadium catalysts on a microporous support demonstrated higher activity during E-geraniol oxidation. It was noted that cross-linked polystyrene-telluric acid (a product of TeC14 polycondensation with a copolymer of divinylbenzene and styrene and subsequent hydrolysis in an alkaline medium) is an active catalyst of olefin epoxidation by hydrogen peroxide [210]. At 333K and in dioxane and tert-butanol solutions, the above catalyst quantitatively yielded epoxides from a wide range of unsaturated compounds such as aromatic and aliphatic hydrocarbons, alcohols and their derivatives. A plausible mechanism for the catalytic properties of a tellurium compound during polycondensation was discussed. A short review of some promising classes of catalysts based on IER containing metal cations has been presented [211]. These classes include partially substituted cation-exchange polymers, fully substituted cation-exchange polymers, as well as cationexchange polymers in which a metal cation is coordinated by another ligand. It is noted that the amount of acidic exchange sites can be controlled to change the bifunctional character of a catalyst. On the other hand, it is sometimes desirable to effect full exchange of all the acidic sites and to thereby obtain a polymeric, sulfonate salt

Chapter 3 Heterogenized Homogeneous Catalysts

110

catalyst containing metal cations. The third class includes catalytic metal cations which are bonded with a mobile ligand and a sulfonate site. Catalytic reactions involving the incorporation of metals or metal cations into IER, perfluorinated ion-exchange polymers and polystyrene-sulfoacid are described in detail.

3.5 Heterogeneous Metal Complex Catalysis Heterogeneous metal complex catalysis has recently developed into a separate branch of chemistry. This has been promoted by the immense development of two trends in chemistry: namely, (1) a broadening of synthetic routes and analysis of functional polymers, and (2) successful research in the field of homogeneous metal complex catalysis. By maintaining the specific properties of both homogeneous (high activity, selectivity, operation under mild conditions) and heterogeneous catalysts, (ease of separation of reaction products from reagents, multiple use), so-called heterogenized homogeneous catalysts combine advantages of both types and minimize the disadvantages of each. This can be done by adding homogeneous catalysts to organic polymers in such a way that the coordination sphere of the particular metal does not change significantly [3]. At present, the most frequently used heterogenizing agent is polystyrene crosslinked by divinylbenzene (DVB) to which various coordination ligands are chemically grafted. Several ways of synthesizing a polymeric support containing diphenylphosphonic groups are shown below [212]:

a

- CHZ-FH-

-CH,-CH-

BuLi

0

-CHz-F;HCIPPh, LiCl

An appropriate transition metal compound is added to ligands by complexation. The discovery of the Wilkinson (I), Vask (11) and Kramer (111) complexes depicted below, as well as other transition metal clusters, has made possible the synthesis of several different, highly effective heterogenized-type catalysts:

3.5 Heterogeneous Metal Complex Catalysis

RhCI(PPh3)3 ; I

Ir(CO)CI(PPh)& ; I1

111

Rh2CI2(C2H& 111

- CHZ-CHI

Q

The catalytic activity of polymeric RhCI(PPh3) during hydrogenation is higher than that of the free complex. This is explained by the fact that the rhodium complex preserves coordination unsaturation during the reaction as a result of spatial screening. A homogeneous catalyst fixed on a heterogeneous support apparently acts similarly as in solution. It nevertheless represents a separate non-stationary phase [3]. Some examples of heterogenized homogeneous catalysts are presented as follows [213]:

CH2-f2 RhCl (PPhJ, Cr ICOI,

-r-Pd-l['>O

~CH,PPh2Rh4~COlll Ph

co

In practice, polymeric supports of two types are often employed. These are strongly cross-linked beads of resin with a large surface area and weakly cross-linked gelatinous macroporous resins. Because the latter swell well in most organic solvents, almost their entire bulk is accessible to solvents and reagents [3, 213, 2141. This can be illustrated by hexene hydrogenation applying three catalyst types which differ from each other by the quantity of the cross-linking agent, divinylbenzene [215]. The change in the catalytic activity of gelatinous and macroporous catalysts with the same degree of chloromethylation, is compared in Figure 30. For densely cross-linked polymers (d, e, f) in which catalytically active sites are located mainly inside resin granules or in very small pores (b and c), the specific activity is lower than for sites located on the external surface (a). Hence it follows that the catalytic effectiveness of a supported complex is limited by the diffusion of reagents to catalytically active sites; i.e. dense cross-linking makes it difficult for the substrate to penetrate to the coordination center.

112

Chapter 3 Heterogenized Homogeneous Catalysts

1.5

L'

20

1.0

L c 3

I-

0.5

1

I

I

I

I

10

20

30

40

I

[Rh] / ( m o l . g - l )

Fig. 30. Influence of the degree of cross-linkingof gelatinous and macroporouspolystyrene on catalytic activity [215]

The support properties considerably affect the processes. A support can promote a change in the bond energy of reacting molecules with the catalyst surface thereby altering the direction of the catalytic reaction [216, 2171. Main requirements of supports for polymeric catalysts include: specific surface, size and distribution of pores, mechanical strength, particular particle size and distribution, minimum change in the volume with a change in the medium, and surface polarity with respect to the solvent. Furthermore, a polymeric support should be inert with regard to reagents and stable to temperature and pressure changes. To elucidate the effect of the degree of cross-linking of the polymeric component on the activity, texture and physical properties of catalysts, a number of polymer-supported catalysts were prepared by reacting rhodium tetracarbonyldichloride with dimethylaminomethylated copolymers of styrene and DVB with different degrees of cross-linking (10,- 15, 25, 40, 50, 60% DVB) [192]. Hydroxylation was chosen as a model reaction. The activity of catalysts was increased by increasing the specific surface and pore radius. Selectivity, however, did not change. To account for this fact, the authors suggest that either the reaction occurred in open catalyst pores which do not impose steric hindrances upon reactants (the change in activity resulting from the difference in the rate of diffusion of reactants to active sites), or a polymer-supported complex acts as a soluble catalyst. The interest of the scientific community in supported complex catalysts is mainly attributed to the possibility of obtaining highly dispersed metals with a high specific surface area. This is evidenced by BET data on the surface area of different supports after each treatment stage (Table 21) [220]. The presence of donor ligands on the support promotes stabilization of zero-valent metal centers due to strong coordination interactions. Moreover, a ligand should pre-

3.5 Heterogeneous Metal Complex Catalysis

113

Table 21. Surface areas of polymeric supports according to BET Compound

Surface area

m2 per g of catalyst

m2 per g of catalyst Treatment stage I1

Treatment stage I

12.2

56.1

109.8 144.8

vent migration of metal on the support surface. The existence of a definite correlation between rhodium particle sizes and the content of phosphine ligand groups on the support was reported [221]. Several novel rhodium-containing catalysts have been synthesized by the reaction of [Rh(COD)C1]2 (COD denotes cyclooctadiene) with styrene divinylbenzene copolymers functionalized by phosphine groups: polyPPh2 + [Rh(COD)C1]2+ (polyPPh2)Rh(COD)CI

.

Table 22. Analytical data of rhodium-containing catalysts on polymeric supports Catalyst

p

Rh-1 (polyPPh2) Rh-2 (polyPPh2) Rh-3 (polyPPh2) Rh-4 (polyPPh2) Rh-5 (polyPPh2) Rh-6 (polyPPh2) Rh-7 (polyPPh2) Rh-8 (polyPPh,) Rh-9 (polyPPh2) Rh-10 (PolyPPh,) Rh-11 (polyPPh,) Rh-12 (polyPPh,) Rh-13 (PolyPPh,) Rh-14 (PolyPPh,) Rh-15 (polyPPh2) Rh-16 (SO2)

1.6 6.2 1.7 1.7 1.2 1.2 1.2 1.2 1.7 0.5 0.5 0.5

10701

0.5

-

Rh [Yo]

Surface area by BET Im2/gl

1 .o

322 322 293 260 370 335 263 166 25 1 187 167 111 72 101 58 250

9.9 0.6 1.6 0.5 1 .o 2.0 5.0

3.0 0.5

1 .o

2.0 5.0 1.1 6.8 2.0

Chapter 3 Heterogenized Homogeneous Catalysts

114

60 o a

.-c ul

.W

U

t

40

0

n ) .

0

W

.-

N

v,

20

0

2

4 [R h] /%

Fig. 31. Correlation between the size of rhodium particles and rhodium concentration for three series of catalysts (070 means weight-%) [221]

The catalysts were reduced in a current of hydrogen by heating to 403 "C at 5 K intervals over a period of 18 h. Table 22 lists various analytical data pertaining to these. Structural and morphological properties of these rhodium catalysts after reaction were characterized by low- and wide-angle X-ray structure analysis. The correlation between P and Rh content and metal particle sizes is shown in Figure 31. At low P concentrations (0.5 or 1.2%) the particle diameter is increased by increasing the amount of Rh. At a P concentration of 1.770, however, the particle diameter was unaffected (18-20 A) at all metal concentrations. Ethylene hydrogenation was studied at 395 K and 1 atm in order to clarify the effect of phosphine groups on the overall activity of the catalyst [222]. The hydrogenation rate ( r ) , expressed in moles of ethane per hour per g-atom of metal, is presented in Table 23. The reproducibility and activity of these catalysts is illustrated by the fact that the values of r obtained for freshly prepared Rh-9, and the same material used two weeks and six months later were 0.31, 0.30 and 0.33, respectively. The rate of ethane formation is described by the conventional empirical equation, r = k . P2hylene P& (where x and y are exponential values characterizing reaction orders for ethylene and hydrogen). From Table 23 it follows that for almost all the catalysts x and y values are within the ranges of 0.0-0.05 and 0.9- 1.0, respectively. Figure 32 shows the dependence of the specific activity ( N ) of rhodium surface atoms on the total metal concentration for three series of catalysts with different phosphine group concentrations. These results correlate with the data in Fig. 31 and emphasize the key role of metal particle sizes in controlling catalytic activity. Accord-

3.5 Heterogeneous Metal Complex Catalysis

115

Table 23. Kinetic data of ethylene hydrogenation by rhodium-containing catalysts (r = hydrogenation rate). X and Yare exponents characterizing reaction order for ethylene and hydrogen, Y 2 respectively, derived from the formula r = kP&,,ene P H Catalyst

r

X

Y

E,,, [kcal/mole]

0.01 0.04 0.0 0.0 0.01 0.04 0.02 0.04 0.05 0.01 0.03 0.0 0.02 0.02 0.0 0.0

1.o 1.o 0.98 0.91 0.97 0.96 0.92 0.99 0.96 0.92 0.93 0.94 0.94 0.96

5.7 k 0.3 5.0k0.3 5.1 k0.3 5.1 k0.2 5.3 k 0.3 4.5 k 0.4 5.6k0.3 4.8 k 0.3 5.1 k0.3 4.7 k 0.2 5.4k0.3 5.2 k 0.3 6.1 k 0.3 12.3k0.6 12.1 k0.6 12.8 k0.6

[mol/(h.g-atom Rh)] at 395K 0.35 0.19 0.32 0.35 0.20 0.20 0.41 0.58 0.32 1.40 1.47 1.37 1.26 2.55

Rh-1 Rh-2 Rh-3 Rh-4 Rh-5 Rh-6 Rh-7 Rh-8 Rh-9 Rh-10 Rh-1 1 Rh- 12 Rh-13 Rh-14 Rh-15 Rh-16

1S O

79.4

1 .o

1.o

100

I

0

1

2 [Rh] in %

3

Fig. 32. Correlation between the turnover number, N , and rhodium content for three series of catalysts (070 means weight-%) [222]

Chapter 3 Heterogenized Homogeneous CataIysts

116

-t

.-c

400

E

2

n

E I

0

u

0 1

3 Catalyst in g

5

Fig. 33. Carbonylation of ally1 chloride in the presence of heterogeneous (1) and homogeneous heterogeneous cat(2) catalysts; homogeneous catalyst - [Pd(NH3)4]2' [@- CH,C,H,SO,],, alyst - [Pd(NH3)4]2+supported on polystyrenesulfoacid [212]

ing to activation energy values, the analyzed catalysts can be divided into two classes: Rh-14, Rh-15 and Rh-16 with Eact= 42-63 kJ/mole and Rh-1-Rh-13 with Eact= 17-25 kJ/mole. The difference between the two classes of catalysts results from the different chemical nature of the active sites. Despite the apparent simplicity of preparing heterogenized homogeneous complexes, in many cases the structure of complex-bound particles remains unknown [212]. Properties of homogeneous catalysts change little during the course of catalysis. For instance, in experiments conducted at 100°C and at a CO pressure of about 6.9 MPa, ally1 chloride carbonylation catalyzed by palladium proceeds selectively with a high yield of the desired product (Fig. 33): M

c

l

+ CO

@-C6H,SO;

[Pd(NH,)4]z+

t N C O C I

For a homogeneous catalyst, saturation is observed upon increasing the catalyst concentration, whereas a linear dependence is typical for a high-molecular weight analogue. This is attributed to the fact that, by raising its concentration, a homogeneous catalyst aggregates to form an inactive complex of indefinite composition [3]. However, the rigidity of the resin matrix prevents aggregation of the catalyst. It is, therefore, important to restrict mobility or maximally isolate catalytic sites in order to prevent the undesirable effect of aggregation. The formation of isolated zerovalent support metals which is responsible for hydrogenation and isomerization has been confirmed [223,224].

3.5 Heterogeneous Metal Complex Catalysis

117

One assumes that a catalyst is more active when equilibrium in the polymeric matrix is shifted to the region of formation of a complex with a lower coordination number [3]. A free metal coordination site is necessary for the activation of both hydrogen and the substrate. From the study of olefin hydrogenation using TiC12, TiLC13, ZnLC13, HfLCl, and TiL2C12(L is cyclopentadiene) fixed to a copolymer of styrene and DVB, it was found that in the initial stage, a hydrogen atom is bound by a a-bond with the

P

metal and approaches the olefin which is at first n-bonded. In the next stage, hydrogen migrates to one carbon atom of olefin which then forms a a-bond with the particular metal. The approach of the second hydrogen atom causes formation of a saturated hydrocarbon and the rupture of the metal-carbon bond. However, for hydrogenation of dienes catalyzed by palladium(0) (polystyrenepyridine), another mechanism is possible [226]. In the authors' opinion, it includes the stage of a diene complexing on palladium and the transfer of hydrogen to one of the olefin carbon atoms to form a trihaptoenyl complex:

The enyl-palladium bond formed can further be transformed by different means: i.e. hydrogen transfer to the enyl system creates an alkene system and free alkene; or hydrogen transfer from the enyl complex yields the starting or isomerized diene.

118

Chapter 3 Heterogenized Homogeneous Catalysts

Whether hydrogenation or isomerization occurs depends on the relative height of potential barriers leading to absorption of additional H2 or the reversible process [227]. This can be seen by examining the mechanism of olefin hydrogenation on a heterogenized catalyst depicted as follows:

paraffin

The addition of neighboring hydrogen results in formation of paraffin, whereas the reversible process leads to formation of an olefin-hydride complex and subsequently to an olefin isomer. CH3 I +CH,-f*CHz-

CH3 I

RhCl (PPhJ, C&OH

t.0

70OC

I

OCH,

+CH2-C

I

+CH,

-

c.0 I

OCH,

The fact that isomerization does not occur in a homogeneous system suggests that the reversible reaction rate is lower than that of further H2 absorption. In a heterogeneous, rather than homogeneous, system, neighboring vacant sites are not blocked by solvent molecules. This, in its turn, causes reversible absorption of

3.5 Heterogeneous Metal Complex Catalysis

119

hydrogen from the intermediate Rh-alkyl complex. An interesting feature of the catalytic system obtained by interaction of RhCI(PPh& with a copolymer of methyl methacrylate and 1 -isopropenyl-(3)-l,2-dicarboundecarbonate salts, is the fact that already during synthesis, a mobile hydrogen atom from dicarboundecarbonate anion is transferred to Rh to form a monohydride complex [228], L,RhH+CH2 =CHCH2R: CH~CH~CHZR

+

L,RhH

L,RhH

+ CH2 = CHCH2R P CH2 7 CHCHZR

L,- 1 RhH

I

RhLx- 1

CH3CH=CHR (cis, trans)

The authors assume that in the first stage of hydrogenation and isomerization, the olefin is attached by a Rh-H bond to form a a-alkyl complex. During the course of the reaction, the Rh-C bond is severed resulting in formation of alkane and regeneration of the hydride complex. The intermediate formation of Rh-2-alkyl correlates with isomerization catalyzed by the complex. Under mild conditions, the rate of coordination displacement of a-olefin by p-olefin somewhat exceeds the rate of hydrogenolysis. An increase in the hydrogen pressure causes an increase in the hydrogenolysis rate, the conditions under which alkane forms. It is assumed [229] that two conditions are necessary for catalytic activity by a flexible polymeric support: i.e. (1) the absence of free phosphine groups in the polymer backbone, and (2) the presence of a binuclear complex. It is known that Rh2C12(PPh3) does not catalyze hydrogenation reactions in a homogeneous phase. However, a hydride species forms when it is supported on a polymer. This catalytic activity is apparently attributed to formation of a dimer by means of a halogen bridge:

When a metal complex is in equilibrium with phosphinated polystyrene, the metal binds with the polymer to form the structures [230]:

Chapter 3 Heterogenized Homogeneous Catalysts

120

P3RhCI ,

P2LRhCI ,

and

[P2RhC1]2

where P denotes polymeric phosphine, and L signifies triphenylphosphine (PPh3). Each type of complex exhibits different rates of substrate reduction. The activity of both homogeneous and polymer-supported catalysts is increased by reducing the ligand/metal ratio [3]. In contrast with a heterogeneous analogue, increasing the concentration of a homogeneous catalyst accelerates the reaction rate until a particular limit is reached at which the rate stays constant (Fig. 34). The reaction rate for the supported catalyst greatly deviates from that of the homogeneous catalyst at similar concentrations of palladium and phosphine. Therefore, this example illustrates the effect of catalytic site isolation and of increasing the coordination unsaturation of the polymeric matrix.

I

I

I

I

I

I

Fig. 34. Change in the rate of dimerization and simultaneous methoxylation of butadiene at 100 "Cby altering the catalyst concentration.Comparison of homogeneous and polymer-bound palladium-containing catalysts (a - degree of substitution (To) of phenyl rings of polystyrene by diphenylphosphine groups (PPh2)). Resins with a = 47, P : Pd = 2,5 (1); a = 15, P : Pd = 4,3 (2); a = 47, P :Pd = 3,7 (3); a = 15, P:Pd = 5,7 (4); Homogeneous catalysts with P :Pd = 4 (5); P :Pd = 6 (6); P : Pd = 8 (7) [3]

Although reaction rate and selectivities of heterogeneous catalysis do depend on ligand dimensions ligand/metal complex ratios, these values are in most cases unknown. In order to clarify this, the hydrogenation of olefins on rhodium/phosphine The compounds p-n-butyl- and p-n-dodecylphenylcatalysts has been studied [23I]. diphenylphosphine as well as triphenylphosphine have been used as models of triphenylphosphine polymeric ligands

121

3.5 Heterogeneous Metal Complex Catalysis

where n = 0,1,2,3. To establish the dependence of reaction selectivity on phosphine and olefin structure, four olefins and four phosphine ligands at a phosphine/rhodium ratio of 2 or 3 were selected. Figure 35 depicts the cyclohexene hydrogenation rate as a function of P/Rh ratio in the range 1.3-3.5. It can be seen that the maximum rate is reached at a P/Rh ratio in the range 2-2.3, and that reaction selectivity for the different olefins depends little on their structure.

I

Y

.-C 6N L

-E d

\

>

1

2

Fig. 35. Dependence of cyclohexene hydrogenation rate on phosphinehhodium ratio in benzene. (1) - triphenylphosphine; (2) - p-n-butylphenyldiphenylphosphine; (3) - p-n-octylphenyldiphenylphosphine; (4) - p-n-dodecylphenyldiphenylphosphine 12311

3

[P hos phin e] / [R h]

The activity and selectivity of heterogenized homogeneous catalysts often depend on the preparation technique [229, 2321. For instance, a catalyst obtained by chlorophosphonylation of a macro-cross-linked copolymer of styrene and DVB is stable and remains activated for 500 h. However, catalysts prepared by chloromethylation of this copolymer are deactivated faster: .@-CH2CI @1@-PC12 @-cH2kPRi

3@-CH2PR2 3@-PRi RhHCO(PPh3)3 t

@=HAD-2

R = P h ; OPh R’ = Ph ; OPh ; OMe

@CHZkPRiRhHCO(PPh3)

or HAD-4

Chapter 3 Heterogenized Homogeneous Catalysts

122

The rapid decrease in the activity of the latter is caused by residual chloromethyl groups which react with phosphine groups to form the following type of salt:

Let us consider how phenyl phosphonium units separated from the main polymer chain by methylene groups can affect catalytic structure and activity [235]. Polychlorostyrene interacting with RhC1(C6Hl2)in CO yields a complex of structure I shown below. When polystyrene resin has anchor sites of -CH2PPh2 type, a structure of type I1 forms. This is attributed to the fact that the incorporation of additional methylene groups imparts the polymer chain some flexibility and promotes bis-coordination of the complex.

I

I1

High selectivity observed during hydroformylation results from rhodium bonding with one or two phosphine ligands. Under mild conditions (313 K, PH = 1 atm in ethylbenzene) the heterogeneous polystyrene, promotes the selective catalyst, N i ( a ~ a c-A12(C2H5)3C13-phosphinated )~ hydrogenation of 1,4-cyclohexadiene to cyclohexane [234]. The phosphinated polystyrene has been obtained by two methods: copolymerization of styrene with p-diphenylphosphinostyrene and phosphination of industrial polystyrene. High selectivity is achieved at a low degree of cross-linking and at moderate quantities of phosphine ligands in the polymer. Applying a polymer matrix, one can control the catalyst selectivity with respect to substrate molecules of different sizes [3]. The hydrogenation of various olefins in the presence of cross-linked polystyrene (2% DVB) and using beads containing phosphine groups (size of beads is 40-80 mesh for catalyst I and 100-200mesh for catalyst 2) to which RhCl(PPh3)3 is added, has shown that in both cases the reaction rate decreases with an increase in the length of the olefin chain [235,236]. The dependence of the relative rate of reduction on the size of the olefin molecule for catalyst I is characterized by a smooth curve with respect to saturation of a large-sized substrate. The greatest difference in selectivity of hydrogenation is observed between I-hexene, cyclohexene and cyclooctene. However, for catalyst 2 in benzene solution, the difference between hexene and cyclooctene disappears, and the largest deviation is observed between cyclohexene and cyclooctene. However, upon addition of a polar solvent (ethanol) to benzene, which causes a decrease in the pore size of the polymeric

3.5 Heterogeneous Metal CompIex Catalysis

123

Table 24. Selectivity in respect of particle sizes in hydrogenation of olefins Substrate

1 -Hexene Cyclohexene Cyclooctene A’-Cholestene

Relative rate (compared with cyclohexene)

8(Ch,PPh,),RhCl

RhCI(PPh,),

2.5

1.4

1

1

0.22 0.031

0.67 0.71

support (the ratio of benzene: ethanol = 1 : 1), the rates of reduction of cyclohexene and cyclooctene are almost equal. The catalyst selectivity in respect of particle sizes in olefin hydrogenation is compared in Table 24. It can be seen that when a polymeric catalyst is used, cyclohexene is reduced about 32 times faster than A 2-cholestene, whereas for RhCI(PPh3)3complexes in solution the rates differ by a factor of 1.4. The mean channel size in a polymeric catalyst is 7.4 A. R u C I H ( P P ~attached ~)~ to phosphinated polystyrene (2% DVB) effectively catalyzes the hydrogenation of olefins with terminal double bonds under mild conditions [237]; short-chained olefins are hydrogenated more quickly. For instance, hydrogenation rates for 1 -hexene, 1 -octene, 1-decene, 1 -dodecene, 1-hexadecene and 1 -0ctadecene are 1.6, 1.49, 1.26, 1.21, 1.03 and 0.99 ml/min, respectively. Apparently for larger molecules more stringent diffusion limitations rule within the matrix, especially within active site domains in which rhodium atoms act as bonding sites. In the authors’ opinion [238], however, the effect of such limitations may be neglected when support particle sizes fall within the 200-400mesh range. It is noted that the change in reduction rate from cyclohexene to cyclooctene cannot be caused by mass transfer effects. Compared with the activity of the homogeneous complex, a decrease in the activity of polymer-bound RhCI(PPh3)3 is attributed to the change in the rhodium state due to formation of bonds on the polymeric support surface. The selection of a solvent is important with regard to controlling the activity and selectivity of polymer-supported complex catalysts. The literature presents no clearcut correlation between catalytic activity and solvent type. Figure 36 demonstrates the influence of the swelling factor of polymeric supports in various solvents on the rate og hydrogenation of I-octene in the presence of a Pd/polystyrene catalyst [239]. The swelling factor is evaluated as the ratio of the volume of beads after 4 hours of swelling in various solvents to the initial volume of catalyst beads before solvent addition. It can be seen that the reaction rate is higher in solvents of medium polarity, such as tetrahydrofuran, acetone and ethyl acetate. Hexane and ethyl acetate fall in the category of poor solvents. The effect of a solvent is confined mainly to the change of the degree of swelling of a polymeric support which, in its turn, changes the pore size and restricts diffusion of reactants to catalytically active sites [240]. Hydrogenation of styrene in the presence of Pd and Pt complexes with phosphinated cellulose has shown that a polymeric catalyst is more active in ethanol, T H F and acetone [241]:

Chapter 3 Heterogenized Homogeneous CataIysts

124

5E

601

THF e

Ethylacetate

F!O >O

-

Methanol Butanol

Toluene

I

2 Swelling

I

Fig. 36. Influence of the swelling factor of the polymeric support on the rate of 1-octene hydrogenation in different solvents [239]

4 factor

Solvent: ethanol acetone THF Rate 8.14 4.65 4.52 ml/min

ethyl acetate 3.29

acetic acid 2.70

Solvent: chloroform dichloromethane nitromethane benzene DMSO cyclohexane Rate 1.12 0.96 0.91 0.73 0.31 0.2 ml/min

Extensive research has recently been performed on polynuclear metal carbonyls fixed on polymeric supports. Metal clusters supported on polymers can be used as industrial catalysts [242-2441 For instance, the rhodium carbonyl cluster [Rh,2(C0)30I2-, in solution is capable of catalyzing conversion of CO+H2 to glycols, methanol and other products. The formation of glycols indicates a considerable difference between a mononuclear complex and a metal cluster, because a cluster with metal-metal bonds can favor binding of a carbon-containing group with neighboring metal atoms thereby resulting in formation of carbon-carbon bonds. The literature data [245] implies that both the cluster arrangement of metal atoms and the kind of polymeric ligand groups are very significant for catalysis. Binding a cluster with a solid surface considerably limits the interaction between clusters and formation of catalytically inactive aggregates. Statistical methods of fixing intact clusters on solid surfaces have been summarized [246, 2471. A cross-linked copolymer of styrene with DVB functionalized by phosphine groups was employed as a support. Carbonyl clusters of tetrahydrotetraruthenium, dihydridoosmium and the bimetallic carbonyl clusters, Fe2Pt, RuPt2, AuOs3, were fixed by means of ligand exchange. Ethylene hydrogenation was studied under atmospheric pressure at 50-90 "C using tetraruthenium clusters grafted to a polymer matrix [248]. Polymer grafted H4Ru(C0)12-x(PPh)x analogues (where x = 1, 3 or 4) were synthesized by exchanging H ~ R U ~ ( Cligands O ) ~ ~with polystyreneDVB membranes containing phosphine ligands. It is suggested that catalytic sites

3.5 Heterogeneous Metal Complex Catalysis

125

form from Ru4 clusters by the reversible rupture of Ru - Ru, bonds which yields unsaturated, metal coordination sites. A similar conclusion was made [249] from the study of a polystyrene-DVB copolymer modified by phosphine groups to which tetrairidiumcarbonyl clusters were attached. The structures of anionic, triple-nuclear osmium and iron cluster catalysts supported on copolymers of styrene and divinylbenzene were analyzed by means of IR spectroscopy. Their catalytic activity during 1 -hexene hydroformylation [250] and C6HsNO2 carbonylation [25 11 were investigated. It was found that isomerization proceeds simultaneously in the presence conventional catalysts. In the absence of moisture, a triple-nuclear osmium complex could be removed from a polymeric support after reaction. This suggests catalytic activity for this complex, particularly in the fixed state. Furthermore, a definite correlation was found to exist between polynuclearity and selectivity of heptanol formation. For iron, however, the cluster structure altered during the course of the reaction. The distribution of palladium and phosphorus along the diameter of spherical granules of two catalysts representing triple-nuclear clusters of palladium grafted to phosphinated polystyrene surfaces was examined by using scanning microscopy [252]. In the first catalyst, palladium was uniformly distributed along the granular axis, whereas in the second it was distributed mainly in the subsurface layer, 0.2 mm from the center point of the 1 mm diameter granule. Phosphorus, however, was uniformly as distributed along the granular diameter in both catalysts. With regard to 2-butyne hydrogenation, the first catalyst could be applied for 35 time more reaction cycles than the second catalyst. However, the selectivity of the second catalyst for butenes and cis-2-butene hydrogenation was higher than that of the former by a factor of 1.4-3.7. On the basis of the obtained values of the P/Pd ratio in the granular bulk, models of the structure of active sites have been proposed for catalysts differing in the degree of coordination unsaturation of palladium atoms which determines the selectivity in 2-butyne hydrogenation reactions. Triple-osmium clusters supported on polystyrene-divinylbenzene, silica gel and A1203 catalyze alkene isomerization [253]. IR spectra are indicative of the formation of a stable structure of @-(CH2)2 -O-(CO)loOsH-type. The authors suggest that these structures serve as a pool of catalytically active particles from which unsaturated coordination clusters form. A catalyst has been synthesized by reacting polystyrene-divinylbenzene n-styrenediphenylphosphine with H2RhOs3(acac)(CO),o [254]. The results of IR spectral, XPS and EM analyses and of the study of the catalytic activity of polymeric catalysts during 1-butene isomerization and ethylene hydrogenation indicate that catalytic activity during polymerization reactions is related to the formation of stable, grafted, tripleosmium carbonyl clusters. This is also the case for hydrogenation reaction with regard to rhodium. A catalyst of diisobutylene hydroformylation was synthesized by interaction of Rh2(C0)&I2 with nitrogen-containing polymer ligands obtained by treating chloromethylated copolymers of styrene and DVB with primary, secondary and tertiary amines [255]. The effect of diamine or substituted pyridine-type additions o n the activity of carbonyl clusters of Rh was studied on R h ~ ( C 0 ) ~during 6 a

126

Chapter 3 Heterogenized Homogeneous Catalysts

homogeneous, water-vapor shift catalytic reaction vapor [256]. It was shown that the activity considerably increases in the presence of additions. It was, moreover, found that catalysts prepared under mild conditions (lOO°C, below 1 atm pressure), by fixing Rh6(C0)16and Rh2(C0)4C12 onto the surface of polystyrene containing grafted polyamines, possess high activity during the watervapor shift reaction. The polyamines studied in this reaction were 2-aminopropylamine (I), 2-aminoethylamine (11), polyethylamine (111), diethylenetriamine (IV) and linear polyethylenediamine (V). With respect to Rh6(C0),6 at 100°C, the maximum effect occurred by first incorporating polyamine IV and then V. The effectiveness of the remaining ligands decreased in the sequence: I > I1 > 111. Unlike the homogeneous analogue, the activity of fixed Rh clusters remained constant for a long time. It was shown that the quantity of H20 added strongly affects catalytic activity. According to IR studies, the active catalyst species were the cluster anions, [Rh4(co)26]4- and [Rh,4(C0)2513-. Complexes of H4R~4(C0)12 and H4Ru4(CO)lo(PPh)2 were also prepared and bound to polymer supports BPhCH2XPPh2, SiO2 -PhCH2PPh2 and PhCH2N(C2H5)P(N(C2H5)2)2[257]. The synthesized products were used as catalysts of 1-hexene hydrogenation and isomerization. How the solvent nature, reaction time, density of support cross-linking, ligand type as well as the ligand/metal ratio affected the catalytic properties of clusters was moreover investigated. Polymeric-ruthenium catalysts have been prepared by the reaction of (q 6-cycloocta-I ,3,5-triene) (q4-cycloocta-1,5-diene) ruthenium(O), (q 6-cycloocta-l,3,5-triene) (COT); (~4-cycloocta-l ,5-diene) (COD) with polystyrene in hydrogen at room temperature [258]. Elemental analysis, IR and mass-spectrometry data show that in these polymer-metal complexes, two cycloolefin ligands, present in the starting Ru(COT)(COD) complex, are substituted by two phenyl rings of polystyrene:

8-

Under the same conditions, Ru(COT)(COD) reacts with 1,3-diphenylpropane, an analogue of a polystyrene unit, and yields the insoluble product, diphenylpropaneRu2 besides Ru(q6-diphenylpropane) (COD). This implies that the catalytic structure is similar to a heterogeneous polystyrene complex. The authors proposed a structural catalyst model based on the assumption of polymeric bonding of ruthenium metal clusters through the formation of rutheniumaromatic ring bonds [259]. The validity of this contention was confirmed by synthesis of ruthenium complexes based on poly-I -vinylnaphthalene instead of polystyrene. It was demonstrated that both catalytic ruthenium-containing systems are active in the hydrogenation of unsaturated bonds in olefins, mononuclear aromatic hydrocarbons,

3.5 Heterogeneous Metal Complex Catalysis

127

ketones, nitriles, nitroaromatics and oximes. Factors determining the catalytic activity, selectivity and stereospecificity of supported ruthenium-polymer catalysts were discussed in relation to the proposed structure. By using synchronous irradiation as a source of excitation, catalytic fine structure was analyzed by means of X-ray spectra of K absorption for a conventional ruthenium metal catalyst and a polymeric catalyst (Ru-polystyrene) prepared by interaction of the complex Ru(q6-cycloocta-l,3,5-triene)(q4-cycloocta-1,5-diene)with polystyrene at 20°C in tetrahydrofuran at P H =~ 1 atm [260]. The results of elementary analysis, IR and mass-spectrometry showed that cycloolefin ligands present in Ru complexes, are not included in the polymeric catalyst but are presumably substituted by polystyrene phenyl rings. Measurements were made on samples of an immobilized ruthenium catalyst with a phenyl/Ru molar ratio of two. The analytical data demonstrated that in the ruthenium complex, the Ru-Ru bond length is 2.68 A. In a polymeric catalyst, Ru-Ru and Ru-C bond lengths were 2.88k0.1 A and 2.05k0.05 A , respectively. The number of carbon atoms bonded with ruthenium amounted to 6 k 1. The results obtained correlated with the assumption that a bond forms between a ruthenium atom and a phenyl ring of polystyrene. The Ru-C distance determined for a polymeric catalyst agreed well with that observed for homogeneous monomeric complexes of ruthenium. The immobilized catalyst apparently consisted of small clusters of ruthenium added to polystyrene by complexing of a ruthenium atom and a phenyl ring. The presence of clusters in unsaturated coordination sites enabled an explanation of the change in catalytic properties of polymerfixated ruthenium complexes compared with those of analagous, homogeneous ruthenium complexes. A nickel complex fixed to polystyrene by means of Ni-C o-bonds was obtained through oxidative addition of halogenated polystyrene to Ni(0)-tetra(tripheny1phosphine) [261]. The catalyst was synthesized to an approximate degree of polymerization of 1100 in toluene solution at room temperature in the presence of nitrogen. It was found that the obtained complex was highly active during C2H4-dimerization reactions conducted in the presence of catalyst-toluene suspensions as well as in the gas-solid phase. It was shown that the solvent affects catalyst selectivity. Selectivity was lower in the gas-solid system, and considerable quantities of hexenes and octenes were formed. Methods of synthesizing organometallic compounds attached to polymers and their catalytic applications have been cited [262]. Polymeric fixation of a metalloenes yields hydrogenation catalysts. Their reduction products are applied as catalysts of olefin isomerization and ethyl propionate oligomerization to create linear and cyclic trimers. Cyclopentadienyl (Cp)-containing CpzTiCl2 and TiCpC1, compounds fixed on polymers are used for epoxidation of cyclohexene and cyclooctene. The compounds Cp2TiC12and CpZrC12 fixed on supports in which the metal atoms are closely arranged to each other, enable the synthesis of complexes containing two titanium and two nitrogen atoms. Data on catalytic properties of transition metals fixed with alkyl, porphyrins and phthalocyanine groups have been analyzed. Polymeric and low-molecular weight complexes based on organostannyltricarbonyl nickel complexes were employed as catalysts of oligomerization of propargyl alcohol,

128

Chapter 3 Heterogenized Homogeneous Catalysts

phenylacetylene and propargylic acid esters [263]. This reaction produced cyclic or linear oligomers in 90% yield. In all cases, the formation of cyclooctatetraene derivatives was observed. The type of substitutent attached to a phosphorus atom in the monomeric and polymeric complexes determined the composition of oligomers being formed. With low-bulk phenyl substituents, for example, mainly cyclotrimers were obtained, whereas for bulky substituents such as tert-butyl or polymeric ones, cyclotetraenes were primarily formed. Low-molecular weight complexes decomposed during reaction, while high-molecular weight complexes could be isolated by filtration and reused. Although their activity decreased with time, polymer complexes resisted washing and could be reused many times. However, air and moisture destroyed both polymeric and monomeric complexes. The literature reports the synthesis of two types of catalyst based on tungsten and molybdenum chemically bonded with the polymer, and their use in the metathesis of olefins with internal C = C bonds [264]. The catalysts are synthesized by bromination of polystyrene containing 2% divinylbenzene with subsequent treatment of the brominated polymer by n-BuLi in tetrahydrofuran. Lithium-polystyrene derivatives are thus formed. After reaction with a , a'-dipyridyl or Ph2PCl, they are converted, respectively, to dipyridyl (A) or phosphine (B) derivatives of polystyrene that form active complexes after being heated with W(CO)6 or Mo(CO)~. During the metathesis of 2-pentene in the presence of ethylaluminium dichloride, it was shown that tungsten catalysts of types A and B are one order of magnitude more active than respective non-polymeric analogues. The most effective system, a catalyst of A-type which contains tungsten and ethylaluminium dichloride, can be used many times, although its activity gradually decrease because of splitting off transition metal atoms. To achieve maximum catalytic activity during metathesis, it is advisable to first add ethylaluminum dichloride to tungsten or molybdenum catalysts and only thereafter, olefin. A dipyridyVtungsten molar ratio approaching unity is a significant factor determining high catalytic activity. The incorporation of low amounts of oxygen into the reaction system causes a sharp increase in the metathesis rate. These catalysts have been proven inactive in the metathesis of a-olefins. However, the presence of the latter does not affect the conversion of olefins with internal bonds. bromide was synthesized by reaction Polymeric 2,4,6-trimethylphenoxymagnesium of EtMgBr with the phenolic OH-group in trimethylphenol bonded with polystyrene partially cross-linked by divinylbenzene. The activity of this polymeric catalyst was studied in several reactions catalyzed according to the acid-base mechanism [265]. In the presence of a polymeric catalyst, self-condensation of linear aliphatic aldehydes in toluene at 100°C proceeds more slowly. However, the total yield (90-95%) of a,punsaturated products is comparable with the yield after catalysis by the unsupported homogeneous analogue, AgOMgBr. Unlike homogeneous catalyses, epoxide compounds are isomerized at 80°C in toluene to simple carbonyl compounds with ketones as the predominant product. In the former case, the ketone and aldehyde yield decreases as a result of the formation of products from the self-condensation of aldehydes and the condensation of aldehydes with ketones. This reduction is probably decelerated in reactions with a polymeric catalyst.

3.5 Heterogeneous Metal Complex Catalysis

129

Bis-44socyanatophenylmethaneand diarylcarbodiimide condensation was studied at 125-200°C using a triphenylarsenic oxide catalyst grafted to the surface of polystyrene [266]. The catalyst was prepared by bromination of polystyrene-Br2 in nitromethane in the presence of BrF3(C2H5)0, TI(OAC)~,FeC13, AlC13, SnCl, and TiC14 with subsequent treatment by diphenylarsenic chloride with addition of Li in tetrahydrofuran and oxidation with H202. It was found that the catalyst possesses maximum activity when the polystyrene bromination stage is conducted in the presence of Tl(OAc)3. Furthermore, traces of HCl in bis(4-isocyanatopheny1)methane triggered catalyst deactivation which can be prevented by pretreating bis-4-azo-cyanophenylmethane with propylene or cyclohexene oxides. After epoxide treatment of bis(4-isocyanatophenyl)methane, condensation turnover at 160"C reached 5.6 x lo4. It was shown that the catalyst may be also used for synthesizing di-o-tolylcarbodiimide and di-p-tolylcarbodiimide from phenyl isocyanate with a 94% yield. Organic polymers composing a divinylbenzylphosphine functional group attached to PtCI2 and PdC12 yield heterogeneous hydrogenation catalysts which are analogues of the homogeneous catalysts, PtC12(PPh3)2and PdC12(PPh& [267]. Such catalytic systems were tested regarding their activity and selectivity during soybean methyl ester hydrogenation. It was determined that the activity of palladium catalysts at 25 "C is proportional to their metal content. An increase in the reaction temperature caused their decomposition. Although such catalysts are inactive at temperatures below 15O"C, upon addition of SnCI2.2H20 (Sn: Pt = 10: l), they became active as well as selective, reducing only trienes to dienes without forming saturated compounds or isomerizing dienes with conjugated bonds. Both catalysts could be easily regenerated by filtration, and reused several times. Nonetheless, the activity and selectivity of the platinum catalyst necessitated addition of SnC12*2 H20. Polystyrene-bonded RhCl[P(C6H5)3]3, RuC12[P(C6H5)3]3 and IrCl(C0) [P(C6H5)3]2exhibit catalytic activity and selectivity during the selective hydrogenolysis of trichloromethyl substrates in the presence of propan-2-01 at 140- 160°C [268]. In argon and in the presence of polymeric RhCl[P(C6H5)3]3, 2,2,2-trichloro1-phenylethanol is transformed to 2,2-dichloro-l -phenylethanol with a 62% yield. The rhodium catalyst is especially active in solvents in which the polymer base swells well. Under similar conditions, 2,2,2-trichloroacetophenoneand 2-propyl trichloroacetate change to 2,2-dichloroacetophenone and 2-propyl dichloroacetate, respectively. A mixture of 1,3,3- and 3,3,3-trichloropropylbenzeneare formed in a 1 : 1 ratio by hydrogenolysis of 1,3,3,3-tetrachloropropylbenzene.Under these reaction conditions, the complex does not separated from the polymer base. Because saturation of active sites of the HC1 catalyst reduces its activity during the course of the reaction, catalytic activity can be restored by means of air or oxygen through removal of chlorine which is coordination-bonded to the catalyst metal. Two routes of activating homogeneous catalysts by polymers are cited; namely, immobilization of active sites and selective absorption of nonvolatile phosphine ligands [269]. The first route is illustrated by the preparation of Ti4+ fixed on a polymer reduced by organomagnesium ester compounds which yields catalysts active in olefin isomerization and hydrogenation. Considerable differences between organomagnesium and organolithium compounds as reductants and between their respective

130

Chapter 3 Heterogenized Homogeneous Catalysts

homogeneous systems have been discussed. The second route is illustrated by the activation by Ag-containing polymers of homogeneous hydrogenation catalysts composed of tris-(tripheny1phosphine)rhodium chloride. These polymers are mild acids that readily absorb triphenylphosphine ligands from solution, thereby restoring the activity of the catalytic system. Polystyrene-bonded IrCl(CO)(PPh3)2 promotes hydrogen transfer from formic acid to different olefins [270]. After several initial cycles during which the substrateiridium complex forms, the activity of this catalyst increases. The reaction kinetics have been thoroughly studied using benzalacetophenone as a hydrogen acceptor. The catalytic mechanism involves splitting the Ir - P bond in the immobilized complex, P-(PPh3)21rCI(CO) (where P is a polymer matrix), coordination of an acceptor by a vacant site and oxidative addition of HCOOH. After subsequent olefin incorporation whereby iridium propenylformate forms, hydrogen transfer occurs with loss of CO and reduction. Values of kinetic isotope effects from the application of HCOOD, DCOOH and DCOOD indicate the limiting role of the hydrogen transfer step. The polymeric catalyst activity decreases with its increasing resistance against swelling. Completely activated catalysts are stable in air and do not lose their effectiveness after repeated catalytic cycles. Cyclopentadienyllithium attached to a polymer (P-1) has been synthesized [27 11 by reacting alkyl lithium with a cross-linked copolymer of styrene and divinylbenzene (20%). Furthermore Rh3+ attached to the polymer (2% Rh content; P-2) has been obtained by reaction of P-I with RhC13*3H20in methanol at room temperature. Catalytic properties of P-2 were studied in the hydrogenation of benzene (100% formation of cyclohexanes), o-xylene (formation of a mixture of cis-1,2 and trans-1,2dimethylcyclohexanones), acetophenone (formation of a mixture of ethyl-benzene, 1-phenylethanol and methyl cyclohexenyl ketone) and I-hexene (formation of n-hexane and traces of 2-hexene). The effect of N(C2H5)3was linked with the removal of HCl (as (C6H5)3N. HCl) from a saturated, intermediate coordination complex of Rh3+. The catalytic isomerization of allylbenzene by P-2, resulting in the synthesis of cis- and trans-propenylbenzene (25 and 75 070 respectively) was also investigated. The incorporation of N(C2H5)3 completely inhibited isomerization because it prevented the oxidative addition of C - H bonds of ally1 ligands to Rh3+. Low activity of P- 2 in 1,Ccyclohexadiene disproportionation was moreover observed. Phenylsiloxane polymers containing complex-bound Cr(C0)3 groups have been obtained by interacting Cr(C0)3 with ladder polyphenylsesquioxane and linear polyphenylsiloxane [272]. The synthesized polymers were characterized by elemental analysis, IR spectroscopy, gel-permeation chromatography, thermogravimetry and viscometry. According to thermogravimetric data, they are more stable at 180- 200 "C in nitrogen than analogues complexes based on a cross-linked polystyrene. The polymers are effective as catalysts of stereoselective hydrogenation of methyl sorbate to cis-3-hexenoate at 160 "C in cyclohexane or tetrahydrofuran. Soluble complexes have been isolated after hydrogenation. Then, the catalytic activity decreases; this is probably caused by the loss of Cr(C0)3 groups. The results agree with the mechanism involving the dissociation of CI-(CO)~groups, a considerable amount of which cannot be reassociated with polymeric phenyl groups.

3.5 Heterogeneous Metal Complex Catalysis

131

Telomerization of butadiene with different nucleophiles such as alcohols, amines, carboxylic acids, phenol, water and silane was analyzed using palladium(0) catalysts bonded with phosphinated polystyrene [273]. Adducts of butadiene and amines with a 2: 1 composition selectively form in reactions with secondary amines. Reactivity decreases with increasing steric hindrance caused by the considerable bulk of substituents attached to a nitrogen atom: morpholine > piperidine > diethylamine > diisopropylamine. Butadiene selectively reacts with primary amines to form adducts of 4: 1 composition depending on reaction conditions. Triethylamine is a prerequisite in reactions with carboxylic acid in order to achieve high yields of adducts of 2: 1 composition. In this reaction, the isomerization of linear products to branched ones occurs. At a high degree of butadiene conversion, the isomer ratio reaches thermodynamic equilibrium. The reaction of butadiene with formic acid yields 1,7-octadiene only. Benzene should be used as a solvent for the interaction of butadiene with alcohols if high yields are required. In this case, the reactions proceed at high rates. Polymeric catalysts can be easily separated from the reaction mixture, and be regenerated with a slight decrease in subsequent activity. According to the literature [274], polystyrene-immobilized complexes of (PPh,),MCI(X)-type (where M = Rh or Ir; X = CO; n = 2 or 3) are effective catalysts of selective hydrogen transfer from formic acid to a,punsaturated ketones. The activity of such catalysts is sensitive to the solvent type. For example, the replacement of toluene by ethylene glycol, decane or decalin increases the conversion of unsaturated ketone from 62 to 89.98 and 99%, respectively. The basic disadvantage of metal complex catalysts fixed on polymeric supports is the copolymeric character of the latter [275]. Not only do they contain the desired functional groups, but they also possess residual chlorine and bromine atoms and alkali metals, which in their turn are capable of interacting with transition metal compounds. A further disadvantage of polymeric metal complexes is that the surface distribution of ligand groups is often random, and distances between these groups necessary for complexing are not certain. Moreover, thermal processes can either break polymeric chains or cause cross-linking and consequent formation of free metal particles. Free ligand groups present on the surface are thus capable of changing the neighboring microenvironment of active catalytic sites which inevitably affects catalytic activity and selectivity. All these disadvantages involved in complexing with macromolecular ligands should be considered before the synthesis and application of transition metal complexes fixed on support surfaces. The method of identifying functional groups of a DVB (2%)-styrene copolymer consisting of - Br, -P(C6H5), -CH2CI and -CH2C5H5 has been suggested [276]. The decisive factor which determines the prolonged use of heterogenized homogeneous catalysts is the preservation of the metal complex during the course of the catalytic reaction. Although some types of catalyst operate for a long time without losing their activity and selectivity, most polymer-supported catalysts eventually lose their original activity. Such catalysts cannot be used repeatedly. The loss of catalytic activity is mainly associated with the reversible dissociation of the metal complex from the ligand [277]. Ways of reducing metal loss have been cited [278]. They involve increasing the polymeric ligand concentration and employing swelling

Chapter 3 Heterogenized Homogeneous Catalysts

132

microporous resins in which functional groups are uniformly distributed within the matrix, instead of using macroreticular beads with surface functional groups. Lastly, reduction in catalytic activity can be prevented by avoiding the use of complexing solvents which can compete for metal ions.

3.6 Decomposition of Water by Polymer-Supported Complex Catalysts The conversion of solar energy to chemical energy by photocatalytic reduction of water is one of the most important tasks aimed at solving energy problems. Consequently, the search for effective catalysts for decomposing water to H2 and 0 2 is necessary. At present, attempts have been made to apply heterogenized homogeneous catalysts for water decomposition. Basic components of the system for conducting reductive decomposition of water include a photosensitizer, electron donor, electron mediator and a multielectron redox-catalyst [279 -2811. Tris(2,2-bipyridyl)ruthenium (11), Ru(bpy)t+, is often employed as a photosensibilizer. The disodium salt of ethylenediamine tetraacetic acid (EDTA) serves as an electron donor. Methylviologen (MV), a mediator, transports electrons to a multielectron redox-catalyst and binds two protons and electrons, thereby producing molecular hydrogen. Noble metals stabilized by water-soluble polymers such as PVPD, PVC, etc., are used as a multielectron redox catalyst. Hydrogen evolution rates from use of polymeric viologen (PV) and PVPD as a multielectron redox-catalyst, are compared below:

Electron donor

Photosensitizer

Mediator

Stabilizer of colloidal platinum

Rate of H formation (mmo1.h- 21)

EDTA EDTA

Ru(bpy):+ Ru(bpy):+

MV2+ PV(100)

PVPD PV(100)

10 3.5

The model of electron transport along pendant viologen vanadium(I1) groups to colloidal platinum is presented below. The dotted line illustrates the direction of the flow of electrons (see scheme p. 132). The electron transport to a multi-electron redox-catalyst in the presence of PVPD is higher than in the presence of polymeric viologen because of the reduced probability of collision of a photosensitizer with an electron mediator. Based on the fact that heteropolyacids (HPA), e.g. H4[SiWi2040] and H3[PMoI2O40], and their reduced species are effective photocatalysts of selective dehydrogenation of alcohols and H2 evolution from water in the presence of V i l and Cr:gf, production of a new type of heterogeneous catalyst by immobilizing HPA on cationic polymers was proposed [282]. Polymeric quaternary ammonium salts,

3.6 Decomposition of Water by Polymer-Supported Complex Catalysts

133

J-,

poly-2-methyl-5-vinylpyridine50% alkylated with benzyl chloride (M = 40000), and polydiallyldimethylammonium chloride ( M = 60000) were used as polymeric supports.

m=n

=

50mol%

Spectrophotometric titration data imply the formation of stable HPA-polyelectrolyte complexes. On average, 3 - 3.5 monomer polycation units correspond to one heteropolyanion. Immobilized HPA maintains specific properties of homogeneous HPA solutions. For instance, they are capable of both chemical and photochemical reduction. By addition of a C r i i solution to the polymeric complex suspensions, or by ultraviolet irradiation of this suspension, the complexes acquire a reddish tint which indicates the formation of a reduced species of HPA. The ability of highlyreduced HPA species to evolve H, is attributed to the presence of multi-valenced cations in HPA (e.g. W, Mo, etc.) that promote the double-electron reduction of water to H2 without evolution of free radicals. The study of the problem of H2 evolution from water has shown that it is promising to use rhodium complexes with polyamines (PEI, AN-221) as active and stable catalysts of H2 evolution from water in the presence of strong single-electron reductants such as V i i and Cri: [283]. Polymeric rhodium complexes are highly stable. For example, in a special test, a catalyst based on Rh-AN-221 maintained its original activity during more than 6500 catalytic cycles per 1 g of rhodium, while the activated species preserved its catalytic activity for a year. Catalytic properties of different immobilized catalysts are compared in Table 25. The specific catalytic activity, a , expressed as the maximum rate of H2 evolution per gram-atom of incorporated rhodium, has been introduced for convenience in comparison.

Chapter 3 Heterogenized Homogeneous Catalysts

134

Table 25. Specific catalytic activity, a , of rhodium complexes activated in situ in the evolution of H, from 1.5-10-2 M aqueous solution of VSO, at 298K Starting catalyst species

pH

Starting ratio

a [mol H, (g-atom Rh)-'.h-']

Rh(EDA),CI,

0 0 2 0 2 2

4 30 30 30 30 120 120

16 0.82 400 300 8.7 640

Rh-PEI (Rh: N

=

1 :9)

Rh-PEI (Rh: N = 1 :2.5) Rh-AN-221 (Rh: N = 1 : 3.4)

0.002

The fact that catalysts with a Rh :N ratio of 1 : 2 are more active than those having a 1 :4 ratio indicates the effectiveness of unsaturated coordination complexes. Detailed kinetic analysis was conducted on a granulated complex of Rh-AN-221 with 7% Rh(Rh: N = 1 :2). It was found that the rate of H2 evolution increased linearly with catalyst concentration; i.e. under these conditions, H2 evolution was not only determined by mass transfer but also by catalytic processes. Hydrogen evolution increased for highly dispersed samples, whereby the a value was increased from 8.7 to 640mol H2 (g-atom Rh)-'.h-' by reducing the diameter of catalyst particles from 1-2 to 0.1 nm. For Rh-AN-221, a increased with the V2+ concentration starting at a low concentration ( 5 . M) and did not depend on the V2+ concentration after reaching a high concentration of (0.5 -2). M. The effect of medium pH on this process was confined mainly to the effect of pH on rhodium particle sizes. Electron microscopy measurements showed polymer-metal catalyst particles at pH 2 to have complex fractional composition. The presence of small (10-30 A) particles and clusters (300-400 A) was observed. These particles then aggregated to larger sized particles. The high catalytic activity was evidently the result of both structural types. The photocatalytic decomposition of hydrogen sulfide immobilized on polymers containing cadmium, zinc and tin sulfides was studied [248]. A photocatalyst was activated by finely dispersed particles of Pd and Pt. Electron microphotographic data depicted cadmium sulfide as being uniformly distributed in the polymer globule and, according to microdiffraction data, as composed of hexagonal crystal planes 3000 - 5000 A in diameter. Photocatalytic activity of cadmium sulfide immobilized on granulated ion-exchange resin AN-221 was also detected. By using a film of a cation-exchange polymer based on sulfurized fluoroplastic 0.3 mm thick, the stability of the photocatalytic film exceeded that of the respective sulfide in the colloidal state. Irradiation (1> 290 nm) of a water suspension of polyparaphenylene (PPP) in the presence of triethylamine or diethylamine caused photocatalytic evolution of H2 from water [285]. The surface area of PPP determined by BET was 39.1 m2/g. Although the photocatalytic activity of PPP was lower than that of a commercial monocrystal, such systems could eventually become promising photocatalysts for obtaining low-cost feedstock from water (Fig. 37).

3.6 Decomposition of Water by Polymer-Supported Complex Catalysts

135

-

EE

2

N

z

0

20 Irradiation t i m e

40 in h

Fig. 37. Isolation of H, by irradiation (A = 313 nm) of a ZnS suspension (15 mg) containing diethylamine (3 cm3) and water (1 cm3) (1); and a polyparaphenylene suspension (10 mg), containing diethylamine (3 cm3) and water (1 cm3) ( 2 ) [285]

The literature also describes the application, to water decomposition, of “Nafion” (perfluorosu1furized)-type polymeric films [286], water-soluble polymers, PEI, PAA, PVME, PVA and PAAm [287] and water-insoluble polymers [288] as polymer matrices for supporting semiconductor crystals of cadmium sulfide and small particles of metallic platinum in a water suspension of TiO,/Pt and Ru(bpy):+. Tsuchida et al. [289] illustrated one possibility of using different titanium-containing polymers for the reduction of molecular nitrogen to ammonia and hydrazine. The authors employed polystyrene or polycarboxylic acids of linear o r cross-linked structure as a polymer matrix. Cross-linking of the polymer chain caused an increase in the ammonium yield compared with a linear analogue. Titanium cyclopentadienyl chloride bonded with a carboxylic group proved itself as an active catalyst due to a n increase in the d-electron energy levels of the metal atoms. An increase in the ammonia yield as a result of augmenting the degree of cross-linking was attributed to the fact that mononuclear complex particles predominate in cross-linked samples. Polymer-metal complexes were found effective at low titanium concentrations in the polymer. The ratio of the yield of reaction products, ammonia and hydrazine, changed with reaction temperature. By increasing the temperature the yield of ammonia increased, while that of hydrazine decreased. A mechanism of molecular nitrogen reduction has been proposed on the basis of the data obtained. By the photoreduction of H2PtC16 in the presence of unsaturated surfactants (7 0 types), Pt clusters covered by polymeric micelles were obtained and further polymerized by UV radiation or by y-irradiation [290]. The formed clusters protected by polymerized micelles exhibited greater catalytic activity during the photochemical synthesis of H2 in the presence of ELlTA/Ru(bpy):+/MV2+/Pt in aqueous solution than those clusters protected by linear polymers. The use of non-ionic surfactants resulted in the best catalytic activity.

136

Chapter 3 Heterogenized Homogeneous Catalysts

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Catalysis by Polymers

E. A. Bekturov and S . E. Kudaibergenov Copyright 0 2002 WILEY-VCH Verlag GmbH & Co. KGaA

Conclusion

The analysis of recent literature data shows that catalysis by polymers has become an independent and thriving branch of chemistry. Extensive development of this field is attributed to the success achieved in synthesis and investigation of so-called functional polymers as well as to success attained in homogeneous, metal complex catalysis. The fruitful cooperation of these two directions, namely the fixation of homogeneous catalysts o r transition metal compounds o n organic polymers, has led to the novel idea of heterogenization of homogeneous metal complex catalysts. Such catalysts obtained by the heterogenization of various polymeric supports by homogeneous complexes of transition metals, retain the advantages of both homogeneous (high selectivity) and heterogeneous (convenient manufacture) catalysts [ 1 - 61. While the former are helpful for elucidating the kinetics and mechanisms of catalytic reactions, the latter are more promising for the production of stable catalytic systems. Although combining the advantages and simultaneously avoiding the drawbacks of each catalyst type would be ideal for heterogenized homogeneous catalysts, their catalytic sites, too, eventually become deactivated. Two aspects of catalysis involving polymers should be discussed: (1) the catalytic effect of functional groups of polymers and (2) the use of polymers as supports for homogeneous metal complexes. Such an approach is useful because it enables one to establish a relationship between enzyme-like, homogeneous and heterogeneous catalysis. Enzymes and synthetic polymers are very similar in many respects. However, the main limitation of applying polymers for enzyme catalysis is their insufficient variety of functional groups; polymers cannot, for example, yet perform complex enzymatic functions. Nevertheless, the following functions of both polymers and enzymes are analogous: considerable reaction acceleration realized under normal conditions in neutral media and aqueous solutions; high operational effectivity and selectivity for both reagents and reaction products; as well as reaction control. Polymeric catalysts, like enzymes, operate at stereospecific sites. Reactive selectivity is provided by hydrophobic “traps”, coordination and hydrogen bonds, and electrostatic interactions. Thus, by applying some general concepts of enzyme catalysis, effective polymeric catalysts can be synthesized. The reactivity of a chain molecule is determined in many respects by (the respective macromolecular) chemical composition, configuration and conformation. Knowledge of this enables clarification of the kinetics and catalytic mechanisms of the different macromolecular functional groups involved in active sites. The high catalytic activity of polymeric catalysts is caused by the effect of several, active macromolecular functional groups on substrate molecules. The influence of a multifunctional catalyst is restricted to cooperative, electrostatic and hydrophobic effects which cause formation of a transition state complex with decreased activation energy followed by decomposition to reaction products, and catalyst regeneration. The catalytic reaction rate depends on the properties and structure of active functional groups on

146

Conclusion

the polymers, macromolecular conformation, solvent type, medium pH, ionic strength of solution as well as other factors. The combination of various nucleophilic units in one macromolecule makes it possible to obtain highly effective and specific polymeric catalysts. Hydrophobic interactions play an important role in the formation of an intermediate catalyst-substrate complex. Generally speaking, the combination of groups with enzymatic capabilities is apparently a prerequisite for producing effective catalysts. By controlling the number and length of side ally1 groups, one can specifically alter the primary structure of polymeric catalysts, their conformation and adsorption properties, and create high local density of nucleophilic groups in a macromolecule coil. The bifunctional effect of active groups is strongest when they are closest to each other, i.e. when the polymeric chain assumes the conformation of a twisted coil in solution. Therefore, the effectiveness of polymeric catalysts is maximum in hydrophobic domains which act as individual microreactors [7, 81. Hydrophobic and polar sites are also present in enzymes, and determine an enzyme’s ability to accelerate particular reactions. In this respect, the catalytic effect of “polysoaps’’ is similar to that of some enzymes. Synthetic polyesters of linear (polyoxyethylene) and cyclic (polycrown ethers, kryptands) structure are presumably like enzymes because they incorporate cavities suitable for substrate activation. Homogeneous catalysts of this type make it possible to effect new reactions of reagents distributed in two or more phases such as gas-liquid, liquid-liquid or liquid-solid (interfacial catalysis). Halogen exchange and nucleophilic substitution reactions in organic halides are successfully conducted by such catalytic systems. They possess some selectivity with respect to substrates because of different solubilities of alkyl halides in the organic solvent and polymer phases [9]. Coordination compounds of metal ions with macromolecular ligands are interesting with respect to bioinorganic chemistry [lo- 121. It is known that iron, copper, zinc, cobalt, etc. ions are directly involved in catalytic processes realized by enzymes. Polymer-metal complexes are often more effective as catalysts than their components. Such high catalytic activity of polymer-metal complexes is caused by formation of an intermediate, polymer-metal ion-substrate complex [ 13, 141. The effect of metalpolymer complexes is evidently based on the arrangement of substrates in a definite order around an active metal site, whereas macromolecular functional groups fix substrates and carry out the nucleophilic attack. If it were possible, for instance, to synthesize catalysts which combine organizational functions of organometallic catalysts and the spatial hydrophobic-hydrophilic topology of enzymes, then such catalysts would be equal to natural ones in their activity and selectivity. The problems of complexing and catalysis are closely linked with each other [15, 161. The generally acknowledged concept of coordination unsaturation of a central metal-ion is a necessary condition for exhibition of catalytic properties by polymermetal complexes [17, 181. It is noted [14] that the catalytic activity of coordination compounds of polymers depends on the ligand-metal bond strength and correlates well with the Irving-Williams series. The role of macromolecular conformational changes in carrying out a catalytic act should be emphasized. In some cases the creation of a favorable chain conformation considerably increases the reaction rate, as in the oxidation of 2,6-disubstituted phenols by polymer-copper complexes [8]. A con-

Conclusion

147

tribution of the energy of non-equilibrium of polymer chains to catalysis has been detected [ 191. The conformation of macromolecules is affected by solvent properties, change in medium pH, ionic strength of the solution and temperature. Hence, by controlling the polymer chain conformation, one can greatly change its complexing ability and thereby the catalytic effect. Unlike linear polyelectrolytes, catalysis by ion-exchange resins has several technological advantages (easy separation of reaction products, the possibility of conducting the process in flow reactors, etc.) [20]. The effectiveness of catalysts based on ion-exchange resins is determined, among other factors, by the active site structure, degree of cross-linking of the resin and mean pore size. The characteristics of the particles and the elasticity of the pores of ion-exchange resins make it possible to use them in both gas- and liquid-phase reactions. Micro- and macroporous resins are the most suitable for operation in flow reactors because they alter their volume relatively little when the solvent is changed. In contrast, gelatinous resin pore sizes change over a wide range depending on solvent polarity. One of the basic advantages of acidic and basic ion-exchange resins, as opposed to linear analogues, is the effective encapsulation of active groups inside a resin protecting equipment from corrosion [21]. Using ion-exchange resins, one can transform organic compounds which catalyze homogeneous acids and bases. Unlike homogeneous analogues, ion-exchange resins provide prolonged contact of a catalyst with a reagent enabling sequential reactions to be carried out [22]. Provided that the functional groups of ion-exchange resins are located mainly in internal regions of cross-linked polymers and are isolated from each other, it is sometimes possible to also perform sequential catalytic reactions [23]. A sharp increase in the solubility of alkali metal salts in organic media in the presence of linear polyesters, polymeric crown ethers and kryptands as well as quaternary ammonium and phosphonium salts has led to synthesis of a new type of catalyst, namely, interfacial catalysts. By means of interfacial catalysis a number of interesting organic syntheses have been conducted [22]. Unlike micellar and polyelectrolytic catalysis, in interfacial catalysis, the catalytic process involves solution transfer or a change in the distribution of ions due to formation of ionic pairs. The activity of phase-transfer catalysts depends on experimental parameters such as the active site structure, size of catalyst particles, structure and degree of cross-linking of a polymeric support. Basic limiting factors are the mass transfer, diffusion and reactivity of active sites. It can be expected that the influence of interfacial catalysis will grow and that researchers’ efforts will be directed towards expanding its use in organic synthesis. Catalysis by polymer-supported complexes of transition metals is currently one of the most promising fields of catalysis. This is attributed to the fact that such catalysts combine the specificity and selectivity of homogeneous catalysts and the manufacturing convenience of conventional heterogeneous catalysts. So-called “heterogenized homogeneous catalysts” are increasingly being applied in the production of bulk organic substances [22, 241. Whereas the number of research papers on this subject, reached 370 by 1977 [25], by the end of 1980 the number exceeded 800 [24]. This increase reflects the intense development of this field. There are several ways of attaching homogeneous metal complexes to a polymeric support [24, 261; namely, adsorption, covalent and coordination bonding and ion ex-

148

Conclusion

change. A detailed analysis of the methods of heterogenization of homogeneous complexes of transition metals has been presented [24]. The role of a polymer matrix is restricted mainly by the fact that a support contributes to coordination unsaturation of a central metal ion and at the same time, prevents aggregation of active sites. Some scientists [27] contend that the following requirements should be satisfied when choosing a support. A support must be chemically and thermally stable. It should have a high specific surface area, prevent metal migration on its surface, be permeable to substrate and solvent molecules and exhibit mechanical strength, elasticity and structural stability. The bond between a support and a transition metal complex should be strong enough not to be washed out by the flow of reagents. In general, the effect of a support on the catalytic properties of heterogenized metal complexes is complicated and often unpredictable. Apparently the role of a support is limited not only to the matrix isolation of catalytically active sites and prevention of their aggregation, but also to stabilization of the homogeneous dispersion state. Most likely a support can form molecular associations at the expense of hydrophilic and hydrophobic groups and greatly influence the substrate. The role of polymeric supports is determined in many respects by the chemical composition, configuration and conformation of chain molecules. Knowledge of these properties makes it possible to elucidate the effect of polymeric ligand structure on catalytic reactions [28]. Interest of the scientific community in heterogeneous metal-complex catalysts has been aroused by the possibility of obtaining highly dispersed metals with a high specific surface area. Therefore, dispersion methods consist mainly in different ways of impregnating metal compounds on a polymeric support such as nylon, polyacrylonitrile, poly(viny1 alcohol), polyvinylpyrrolidone, efc. [29]. However, such an approach has not solved and cannot solve the problem of increasing catalytic effectiveness; during a catalytic reaction metal particles are often washed out, not only from the surface but also from the bulk of a support. A reliable means of heterogenizing transition metal complexes is coordination binding of a metal with a respective functional group of a polymeric support [30]. In the literature, carbon- and heterochained polymers containing such groups as -OH, - COOH, -S03H, ?N, >NH, -NH2, P, 0, and S are often cited as polymeric ligands. To summarize, the structure of active coordination sites attached to organic polymers is determined by the properties and structure of polymeric ligands, the composition, structure and stability of coordination compounds and by heterogenization conditions. The catalytic reaction rate is affected by solvent composition, substrate type and structure, temperature, efc. The choice of solvent is important with regard to controlling the activity and selectivity of polymer-supported composite catalysts. The literature, however, mentions no precise correlation between catalytic behavior and solvent type. One may maintain that the solvent effect is limited to altering the degree of swelling of a polymeric support which, in its turn, leads to a change in the degree of accessibility of catalytically active sites [31]. One basic limitation of polymeric supports is their inhomogeneity with respect to chemical composition. After its modification by organic and inorganic reagents, undesirable atoms or groups of atoms such as chlorine, bromine, or alkali metals remain in the supports. In many cases, they are capable of interacting with transition metal compounds and can reduce

References

149

the catalytic activity [32]. Catalytic activity suffers from random distribution of ligand groups on the matrix surface, as well as destruction, oxidation and hydrogenation of the polymeric support. All these characteristics should be considered when transition metal complexes are attached to organic polymers. A decisive factor concerning the prolonged use of heterogenized homogeneous catalysts is preservation of the respective metal complex during catalysis. Although some of these catalysts function for a long time without significant loss of activity, multiple use of catalysts causes washing out of an active component into the solution. Catalyst deactivation is attributed to the following mechanisms: reversible dissociation of a metal complex with a ligand, and decomplexing of a metal by a strong ligand (solvent o r reaction products). It is assumed [24] that the problem of regeneration of a homogeneous metal complex cannot be totally solved. Two ways of reducing metal loss [22] involve increasing the polymeric ligand concentration through employment of swelling macroporous resins with equal distribution of groups both on surface and inside a matrix, as well as complexing solvents and chelate-forming ligands. Cluster complexes of transition metals fixed o n polymers are interesting from both theoretical and practical points of view. For instance, the catalysis of the C O + H 2 reaction, effected by many heterogeneous cluster catalysts at high temperatures, can proceed under milder conditions through polymer supports. The idea of supporting two or more transition metal complexes on the same polymer or using several different polymeric supports during the same reaction is also attractive because several reactions could be conducted sequentially using such catalysts. To ensure the high surface area of a polymeric support and to increase its chelation ability, it is sometimes advisable to support polymer-metal complexes on the surface of inorganic materials (silica gel, alumina, etc.). Consequently, catalysis by polymers is one of the most rapidly developing branches of macromolecular chemistry, the successful development of which can lead to substantial changes in chemical production methods. This field will continue to thrive as a result of improvements in methods of synthesizing functional polymers and heterogenized homogeneous catalysts, as well as the development procedures for isolating and increasing the longevity of catalytically active sites.

References [I] A. D. Pomogailo, “Polimernye Immobilizovannye Metallokompleksnye Katalizatory”. Nauka, Moscow 1988 (Polymer Immobilized Metal Complex Catalysts) (in Russian) [2] P. Hodge, Ann. Repts. Progr. Chem. 83, 283 (1986) [3] W.T. Ford (ed) Polymeric Reagents and Catalysts, Washington D.C. ACS, 1986 [4]A. B. Stiles, Catalysis Supports and Supported Catalysts. Butterworth, London 1987 [ 5 ] Ch. U. Pittman, E. Ch. Carraher, J. R. Reynolds, in Encycl. Polym. Sci. and Eng. 10, 541 ( 1987) [6] F. Svec, Polymerny Katalyzatory. Praha 1987 [7] S. Shinkai, Sh. Hirakawa, M. Shimomura, T. Kunitake, J. Org. Chem. 46, 868 (1981) [8] G. Challa, J. Mol. Catal. 21, 1 (1983)

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Conclusion

[9] Yu. Sh. Goldberg, M. V. Shimanskaya, Zhurnal Vsesoyuznogo Khimitcheskogo and Obshchestva imeni D.I. Mendeleeva 31, 149 (1986) [ 101 K. M. Yatzimirskii, “Biologitcheskie Aspekty Koordinatzionnykh Soedinenii”, Naukova Dumka, Kiev 1979 (Biological Aspects of Coordination Compounds) (in Russian) [l 11 M. Hughes, “Neorganitcheskaya Khimiya Biologotcheskikh Protzessov”. Mir, MOSCOW 1983; Engl. original “Inorganic Chemistry of Biological Processes”. J. Wiley and Sons, Chichester, New York, Brisbane, Toronto, 1981 [12] S. L. Davydova, in “Iony Metallov v Biologitcheskikh Sistemakh”. Mir, Moscow 1982; Engl. original “Metal Ions in Biological Systems” [13] N.A. Vengerova, Yu.E. Kirsh, V.A. Kabanov, Vysokomol. Soedin., Ser. A: 13, 2509 (1971) [14] N.A. Vengerova, N.N. Lukashina, Yu. E. Kirsh, V. A. Kabanov, Vysokomol. Soedin., Ser. A 15, 773 (1973) [15] E. A. Bekturov, L. A. Bimendina, S. Kudaibergenov, “Polimernye Kompleksy i Katalizatory”. Nauka KazSSR, Alma-Ata 1982; “Polymeric Complexes and Catalysts” (in Russian) [16] E. A. Bekturov, S. E. Kudaibergenov, R. E. Khamzamulina, “Kationnye Polimery”. Nauka KazSSR, Alma-Ata 1986; (Cationic Polymers) (in Russian) [17] E. Tsuchida, H. Nishide, Adv. Polym. Sci. 24, 1 (1977) [18] M. Kaneko, E. Tsuchida, J. Polym. Sci. Macromol. Rev. 16, 397 (1981) [19] V.A. Seleznyev, Yu.P. Pulenin, E.F. Vainstein, A. V. Artemov, Kinetika i Kataliz, 24, 1085 (1 983) [20] V. D. Kopylova, A. N. Astanina, “Ionitnye Kompleksy v Katalize” (Ionite Complexes in Catalysis). Khimiya, Moscow 1987 [21] W. Nier, Ind. Chem. 104, 632 (1981) [22] P. Hodge and P. Sherrington (ed) “Reaktzii na Polimernykh Podlozhkakh v Organitcheskom Sinteze”. Mir, Moscow 1983; Engl. original “Polymer Supported Reactions in Organic Synthesis”. J. Wiley and Sons, New York 1980 [23] M. J. Astle, J.A. Zaslawsky, Ind. Eng. Chem. 44, 2871 (1952) [24] G. V. Lisichkin, A. Ya. Yuffa, “Geterogennye Metallokompleksnye Katalizatory” (Heterogeneous Metal Complex Catalysts). Nauka, Moscow 1981 (in Russian) [25] Y. Chauvin, D. Commereuc, F. Dawans, Prog. Polym. Sci. 5, 95 (1977) [26] A.Ya. Yuffa, G.V. Lisichkin, Uspekhi Khimii 47, 1414 (1978); 55, 1452 (1986) [27] K. Zhang, D.C. Neckers, J. Polym. Sci., Polym. Chem. Ed. 21, 3115 (1983) [28] D. V. Sokolskii, A. K. Zharmagambetova, S. G. Mukhamedzhanova, E. A. Bekturov, S. E. Kudaibergenov, S.S. Saltybaeva, Dokl. Akad. Nauk SSSR 283, 678 (1985) [29] D.V. Sokolskii, S.F. Lankin, O.A. Tyurenkova, Zh. Fiz. Khim. 41, 3126 (1967) [30] S. L. Davydova, in “Katalizatory, Soderzhashchie Nanesennye Kompleksy”, Novosibirsk 1978 [31] D.E. Bergbreiter, Bushi Chen, T.J. Lynch, J. Org. Chem. 48, 4179 (1983) [32] N.A. De Munck, M.V. Verbruggen, J.J.F. Scholten, J. Mol. Catal. 10, 313 (1981)

Catalysis by Polymers

E. A. Bekturov and S . E. Kudaibergenov Copyright 0 2002 WILEY-VCH Verlag GmbH & Co. KGaA

Subject Index

4-acetoxy-3-nitrobenzoic acid (ANBA) 5,7ff. 4-acetoxy-3-nitrobenzosulfonate(ANBS) 7 3-acetoxy-N-trimethylaniliniumiodide (ATMAI) 8f. activated complex 36 activation 1, 5 activation energy 13 acylation 4 aggregation 3 alkylation 4 alkylnitrobenzene (ANB) 41 amino acid 55 association 3 autooxidation 37 binuclear structure 24 biocatalysts 10 branched polyethylene imine (BPEI)

5

calorimetry 38 catalysate 79 catalyst-metal ion-substrate complex 10 catalysts 1, 92, 96, 106, 108, 114, 116, 120, 123 catalyticactivity 51,71,89,94,106, 127f., 133 catalytic cycles 133 charge transfer 4 chelate ring 74 clusters 69, 98, 124 coil 2 colloidal dispersions 69 colloidal metal particles 69 compact-coil conformation 6 condensation 50 configuration 77 conformation 77 cooperative interaction 1 coordination bond 11 coordination compounds 10 coordination sphere 85, 102 copolymers (CPL) 3f., 50 core 2 Cosover-Mike 5 cross-linked copolymer 124 cross-linked polyvinylpyrrolidone (CPVP) 62 cross-linking 50 crown ethers 43, 44, 63 cyclization 50

deactivation 50 decarboxylation 3 decomposition 11, 14ff., 85, 89 degree of polymerization 34 dehydration 50 denaturation 4 dextrin 69, 72 differential scanning 38 diffusion 50

4-(3,4-dihydroxyphenyl)-Lalanine (DOPA) 30f. 2,6-dimethylphenol (DMP) 24f., 28 2,4-dinitrophenyl acetate (DNPA) 18 2,6-diphenylphenol (DPP) 28 dispersed metals 112 dispersion 69 2,6-di-tert-butylphenol (DTBP) 28, 30 divinylbenzene (DVB) 58ff., 62f., 78, 106, 110, 112, 124 dodecylmercaptan (DDM) 37 dodecyltrimethylammonium chloride (DDACI) 74 donor-acceptor interaction 74 electron-acceptor 88 electron-transfer 30 electron transport 132 enthalpy 5 entropy of activation 5 EPR spectroscopy 23, 24, 38, 85, 98f., 108 ESR 17 esters 2 ethylenediamine (EDA) 70 ethylene-propylene rubber (EPRu) 93, 97 exchange capacity 52 fixation 67 Flory constant 26 GCS 94f., 97 gel 64 gel-immobilized catalytic systems 92f. globules 2 heterogeneous catalysts 57 heterogenized homogeneous catalysts 67 heterogeneous systems 67 homogeneous systems 67 hydration 50

152 hydroformylation 122 hydrogenation 67, 79, 81ff., 85, 92, 111, 114f., 118, 120ff. hydrogen bonds 2 hydrolysis 1, 67 hydrolytic enzymes 1 hydrophobic domains 3, 7 hydrophobic-hydrophilic balance 4 hydrophobic interaction 2 hydrosols 73 imidazole 1 immobilization 67 impregnation 67 inhibition 38, 73 interfacial catalysis 1, 38 intramolecular chelates 27 intrinsic viscosity 4 ion-exchange resins 50 ionites 63, 102ff., 106 IR spectroscopy 1OOf. isomerization 50 Langmuir 90 Langmuir-Hinshelwood 84 limiting step 19 linear polyethylene imine (LPEI) 5 local concentration 32 macro-microporous 109 macroporous antionite 57 maleic acid-styrene (MA-S) copolymers 75 mean pore radius 52 mercaptoacetic acid (MAAc) 37 2-mercaptoethanol (ME) 36f. 3-mercapto-l,2-propanediol(MPD) 37 metal ions 1

N-methyl-N(p-vinylbenzy1)formamide (MFA) 43 methylviologen (MV) 132 micelles 3 Michaelis-Menten 5, 9, 15, 22, 30, 32, 34 microdomain 6 microencapsulation 67 microenvironments 5 microreactors 2, 29 mononuclear complexes 86 network 50 p-nitrophenyl acetate (p-NPA) 2, 4f., 8f. NMR spectroscopy 58 noble metals 132 oligooxyethylene (OOE) blocks 41 organometallic compounds 127

Subject Index organophosphorus esters 18 oxidation 1 , 21f., 29, 31, 36f., 85, 104 phenylimidazole-acrylic acid (PIAA) copolymers 8 phenylimidazole-methacrylic acid (PI-MAA) copolymers 8 phenylimidazole-vinylpyrrolidone(PI-VP) copolymers 8 photoreduction 74 photosensitizer 132 phthalocyanine (PC) 31 ff., 37 polyacrylamide (PAAm) 75, 87, 105, 135 polyacrylic acid (PAA) 7f., 11, 13, 18, 25, 12, 75, 87, 135 polyacrylic acid-polyethyleneimine (PAA-PEI) copolymers 72 polyacrylonitrile (PAN) 67, 87 polyacrylonitrile-oxime (PAN-0) copolymers 87 polyacryloyllupinine (PAL) 13 polyallyl alcohol 97 polyamide (PA) 16f., 88f. polyampholytes 9 polychlorostyrene 122 polyelectrolyte 3 polyethylenalanine (PEA) 9 f. poly(ethy1ene glycol) (PEG) 20f., 39, 41 f., 60f., 75, 88, 91 polyethylene imine (PEI) 4f., 19,74ff., 134f. polyethylene oxide (PEO) 39f., 72 poly(ethylenesu1fonic acid) (PESA) 75 poly(ethy1ene terephthalate) (PETP) 67 poly(D-glutamate) 22 poly(Lg1utamate) 22 polymerization 50 polymer matrix 58, 68 polymer-metal complexes 10 poly(methacry1ic acid) (PMAA) 6f., 1 1 , 75, 87, 91, 97 poly-lmethylethyleneimine (P-LMEI) 20 poly-N-methylethyleneimine(PMEI) 84 polynaphthoylenebenzimidazoles (PBI) 90 polyoxyethylene (POE) 39 ff. polyparaphenylene (PPP) 134 polyphenylene oxide (PPO) 22, 28ff. polypropyleneglycine (PPG) 9 f. polysoaps 2, 3 polystyrene 126

polystyrene-N-(2-~arboxybutylamine) (PBA) 9 poly(styrenesu1fonic acid) (PSSA) 75 polyurethane (PV) 20, 88 poly(viny1 alcohol) (PVA) 5, 13, 31, 33, 37, 43, 52, 67, 69ff., 75, 87f., 135

Subject Index

153

polyvinylamine (PVA) 5 polyvinylbenzimidazole (PVBI) 86f. polyvinylchloride (PVC) 132 polyvinylimidazole (PVI) 1 f., 8, 19 polyvinylmethylester (PVME) 69 poly-1-vinylnaphthalene 126 poly-4-vinyl-N-benzylpyridiniumchloride (PVBPCI) 3 poly-4-vinyl-N-propylpyridinium chloride (PVPPCI) 3f. polyvinylpyridjne (PVP) 7, 62, 93 poly(2-vinylpyridine) (P2VP) 75 poly-4-vinylpyridine (P4VP) 3, 11, 18, 27 ff., 79f., 93, 97 polyvinylpyrrolidone (PVPD) 61, 69ff., 75, 88, 132 porosity 52 preservation 73 pseudo-first order rate constants 6 quaternized poly-4-vinylpyridine (QPVP) 27f. racemization 50, 55 random-coil transition reagents 1 reductant 80 regeneration 67

6

saturation 79 second-order rate 8 sedimentation coefficients 4 selectivity 105 separation 67 sodium dodecylsulfate (NaDDS) 74 solvating effect 38 specific binding 32

stereoregularity 6 steric hindrance 30 structure-reactivity relationship 5 substrate I 5-sulfosalicylaldehyde (SSA) 55 supports 1 surface area 52, 71, 112 surfactants 8 swelling ability 50 swelling factor 124 ternary complexes 10 tert-butylhydroperoxide (TBHP)

108

N-tetramethyl-ethane-1,2-diamine (TMED) 23f. titanomagnesium catalysts (PTMC) 100 transition metal IOf., 13, 15, 19, 20, 67 transition state 1 turnover number 87, 115 vinyl alcohol (VA) 52 vinylimidazole-acrylic acid (VIAA) copolymers Sf., 15f., 19 vinylimidazole-maleic acid (VI-MA) copolymers 9 vinylimidazole-methacrylic acid (VI-MAA) copolymers 8 f. vinylimidazole-vinylalcohol(VI-VA) copolymers 7 vinylimidazole-vinylphenol (VI-VP) copolymers 7 vinylimidazole-vinylsulfide(VI-VS) copolymers 19 Wilkinson catalyst 69 yield 57

Catalysis by Polymers

E. A. Bekturov and S . E. Kudaibergenov Copyright 0 2002 WILEY-VCH Verlag GmbH & Co. KGaA

Fakirov (Editor)

Oriented Polymer Materials Edited by Stoyko Fakirov. 1996. XXII, 512 pages with 337 figures and 42 tables. Hardcover. DM 188,- 6s 1.373,- sFr 169,- US $129,25. Huthig & Wepf Verlag, Huthig GmbH, Zug Heidelberg Oxford CT/USA. ISBN 3-85739-123-5

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The orientation phenomenon is a basic inherent peculiarity of polymers, arising from the chain character of macromolecules and their ability to adopt different conformations - from a coil to an extended chain. The realization of these two extreme conformations or some intermediate ones determines to the greatest extent the mechanical properties of polymers as materials. In addition to the synthesis of new polymers, orientation is a basic approach to the obtaining of polymeric materials with superior properties. The book starts with an overview of the orientation phenomenon, followed by chapters dealing with basic techniques for the study and characterization of orientation and oriented polymers. Another goal was to include as much as possible different representatives of oriented polymer materials, as well as approaches to the improvement of their mechanical properties. The diversity of styles and ways of presentation of the chapters - like in many books of this type - is compensated Ic R by the high professionalismof the co-authors and by the B cu updated results they have offered. 7

Malkin/Kulichikhin

Rheokinetics Rheological Transformations in Synthesis and Reactions of Oligomers and Polymers. By A. Ya. Malkin and S. G. Kulichikhin. 1996.326 pages with 202 figures and 8 tables. Hardcover. DM 148,- 6s 1.081,- sFr 133,- US $101,75. Huthig & Wepf Verlag, Huthig GmbH, Zug Heidelberg Oxford CTAJSA ISBN 3-85739-120-0

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Numerous operations in the technology of polymer synthesis and processing are connected with the flow of material caused by the rotation of the mixer in the reactor, the screw in the extruder or by melt flow filling the form. Evaluations of such operations inevitably require the determination of the properties of a flowing liquid. Experts in lacquers, pigments and polymer solutions came across this problem more than 50 years ago. The determination of the fundamental principle - dependence of material properties on the regime of deformation - gave rise to a new scientific field - rheology investigating anomalous media. "Rheokinetics" offers a comprehensive overview of both fundamental and applied aspects of the science.

From the contents: Rheokineticsof Linear Polymer Formation; Ionic Polymerization; Free Radical Polymerization; Polycondensation; Rheokinetics of Oligomer Curing; Determination of the Point of Gel Formation; Viscosity Increase to the Point of Gel Formation; Microphase Separation in Reactive Systems; Curing after the Point of Gel Formation; Rheokinetic Equations of Curing; Non-isothermal Curing; Structuring Peculiarities of Different Forms of Oligomers and Polymers; Transformation in Polymeric Systems; Polymer-analogous Transformations; Gelation (Curing in Solution); Rheokinetics of Phase Transitions.

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