Review
Vanadium complexes immobilized on solid supports and their use as catalysts for oxidation and functionalization of alkanes and alkenes

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Abstract

This review mainly discusses the immobilization strategies that have been used for vanadium complexes, typically mesoporous material, zeolites and polymers, the characterization procedures for the obtained materials, and their catalytic applications. The retention of the active metal compound within the catalyst may be based on (i) adsorption, (ii) the formation of covalent bonds between metal ligand and support, (iii) ion exchange, (iv) encapsulation, or (v) entrapment. The heterogenized complexes are used as catalysts for oxidations and functionalization of alkanes, alkenes and other substrates, and an account of the various applications reported is given.

Introduction

Catalysts have played a vital role in organic transformations and have major impact on the quality of human life as well as on economic progress. More than 90% of the processes in the petroleum, petrochemical, fertilizers and food industries are catalytically induced. While heterogeneous catalysis is preferred for commodity materials, most of the catalytic processes widely engaged in the manufacture of bulk as well as fine chemicals are homogenous in nature, producing large amount of side waste materials and imposing hazardous impact on the surrounding environment.

One of the major drawbacks of the homogeneous catalysts is the difficulty in separating the relatively expensive catalysts from the reaction mixture at the end of the process. The possible contamination of catalyst in the product also restricts their use in industry. Efficient anchoring of these catalysts on supports may overcome these drawbacks [1]. Since the catalytic action occurs at specific sites on the solid surface, often called as “active sites”, the uniform dispersion of metal catalysts is highly desirable for significant improvement of catalytic action. The inherent advantage of heterogeneous catalytic systems in the liquid phase over their homogeneous counterparts lies mainly on their easy separation and recycle ability. Various methodologies have been developed for the immobilization of homogeneous transition metal complexes. Organic polymers or inorganic solids like zeolites/molecular sieves, silica, alumina, other metal oxides, and carbon have been used as supports for the heterogenization of homogeneous catalysts.

The discovery of vanadate-dependent enzymes and their importance in various biological catalytic processes [2], [3] has stimulated research on the catalytic aspects of vanadium complexes. Many model vanadium complexes show catalytic activity towards various organic transformations [4], [5], [6], [7], [8], [9], [10]. In search of catalysts for organic transformations we have directed our research on the immobilization of complexes on various solid supports. As benefits of this process, in many cases these catalysts (or catalyst precursors) are more active and easily recyclable, and maintain their activity after several cycles of catalytic use. In addition these catalysts exhibit increased stability and improved selectivity, which is ascribed to contributions of site-isolation and confinement effects, as well as cooperation effects from the support.

Heterogenization of catalysts has been the subject of an enormous number of publications, several reviews have been written on this topic [11], [12], [13]. Reviews have addressed catalytic applications of polystyrene-supported ligands and metal complexes [14], [15], [16], zeolite and ordered mesoporous molecular sieves as catalysts supports [13], [17], [19] and immobilized asymmetric catalysts [12], [18], [20], [21]. In view of these, we present here a comprehensive overview of the vanadium complexes supported on cross-linked polystyrene, zeolites (mainly zeolite-NaY) and mesoporous materials (mainly MCM-41).

Four main distinct methodologies developed for the heterogenization of homogeneous catalysts, or the creation of heterogeneous catalysts, are: adsorption, encapsulation, covalent tethering and electrostatic interaction.

Catalysts immobilized by adsorption rely only on van der Waals interaction between the catalyst and the support. This is a weak interaction and the stability of the supported catalyst can be improved by modifying the catalyst and support to allow hydrogen bonding to occur.

Encapsulation is a process that does not require interactions between the catalyst and the support, and thus this method mimics the homogeneously catalyzed reactions. Other methods lead to changes in the catalyst. In fact, covalent tethering implies a modification of the ligand, which may influence its electronic character and/or its conformation, and physisorption and ion-exchange methods result in the catalyst being in close proximity to the support which may also affect electronic properties and ligand conformation. To satisfy the condition of encapsulation, the catalyst must be larger than the pores of the support material to prevent loss of the catalyst into solution during the course of the reaction, or recovery process. As the catalyst complex is larger than the pores of the support, techniques such as impregnation cannot be used to synthesize these catalysts. The supported catalyst can be prepared by either (i) assembling the catalyst within the pores of the support or (ii) assembling the support around the catalyst.

Immobilization of complexes using covalent tethering techniques is the most favored approach to design stable heterogeneous catalysts. Different strategies have been developed depending upon the reaction being catalyzed and several examples are described below. Additionally many porous solids, including zeolites and ordered mesoporous silicates can act as ion exchangers. This presents a mechanism for the immobilization of metal cations and complexes through electrostatic interaction.

Of these four strategies immobilizations by covalent tethering and electrostatic interaction form reasonably stable catalysts that are capable of reuse. Adsorption methodologies are a simple method of immobilization but tend to produce non-stable catalysts. Encapsulation of complexes during the preparation methods provides a very elegant methodology but often it is relatively complex compared with the more recently developed covalent grafting methods [20]. Immobilization via ionic interaction is conceptually simple and may be a useful method of immobilizing ionic catalysts.

Many early studies concerned with immobilization of asymmetric homogeneous catalysts showed that lower yields and selectivity and/or enantioselectivity were achieved with the corresponding immobilized catalysts. However, more recent studies have shown this need not be the case. Indeed, some homogeneous systems have been shown to be much more enantioselective when immobilized. Among other aspects, the reasons for this change are site isolation effects, achievable through the appropriate design of immobilized catalysts, and the containment (or confinement) effect i.e. the immobilized catalyst is constrained through interaction with the supporting matrix, and this can induce increased selectivity and/or enantioselectivity compared with the corresponding homogeneous catalyst [20].

Section snippets

Immobilization of vanadium complexes on polymer supports

Many polymers are non-reactive, but they can be made reactive by imparting a functionality. Thus, polymers bearing a reactive functional group are called functionalized polymers. Synthesis of polypeptide chains over chloromethylated polystyrene by Merrifield in 1963 [22] has inspired researchers to develop new polymer-supported catalyst. Various polymers such as polystyrene, polyvinylchloride, polyvinylpyridine, polyaniline, polyallyl, polyaminoacid, acrylic polymer, cellulose, silicate are

Analytical data

There are several methods for the characterization of heterogenized complexes and the choice of the set of methods to use depends on the particular case in hand, namely if the support is a polymer such as polystyrene, or a Si-based support such as zeolite-NaY or MCM-41, if the metal centre is paramagnetic i.e. a VIV-complex, or if it is diamagnetic i.e. a VV-complex, etc.

The complexation of a vanadium species with bound ligands is frequently accompanied by color changes of the solid support,

Catalytic activity studies

Vanadium(V) centres in complexes are usually strong Lewis acids due to their low radius/charge ratio [8] and act as catalysts in various organic transformations. Catalytic oxygen transfer reactions can be supported by a variety of oxygen donors, and the availability of active oxygen and nature of co-product(s) determine their practical utility, the most attractive oxidants, in terms of cost and environmental impact, being O2 and H2O2 [100].

Concluding remarks

The easy and reversible inter-conversion between the vanadium oxidation states +IV and +V, the easy formation of peroxido-vanadates, the capacity of vanadium complexes or oxides to act as Lewis acid and basic sites or undergo radical-mediated transformations during catalytic reactions, make vanadium one of the best suited elements for catalytic oxidations and oxygen-transfer reactions, and nature has indeed chosen vanadium for the active site in e.g. vanadium haloperoxidase enzymes.

It has been

Acknowledgments

MRM is thankful to Council of Scientific and Industrial Research, New Delhi, and Department of Science and Technology, Government of India, New Delhi for financial support. AK and JCP wish to thank Fundacão para a Ciência e a Tecnologia, FEDER and SFRH/BPD/34835/2007 for financial support.

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