CATALYTIC ACTIVITIES OF METAL COMPLEXES IMMOBILIZED IN ZEOLITE-Y

Abstract

The term "catalyst" was first introduced by Brezelius in 1835, Catalysts help the reaction to achieve the chemical equilibrium by reducing the potential energy barrier in the reaction path. Catalyst activates the reactant molecules to bind with it at milder conditions where they react and finally products separate out leaving the catalyst for the next cycle. As large as 95 % processes used in the chemical industries today are catalyst based technologies. Directly or indirectly catalysts have contributed to more than 20 % GDP of developed nations. Important properties of the industrial catalysts are the substantial working life and the retention of their effectiveness while environmental issues stress on the development of such catalytic processes that are beneficial from industrial as well as environmental point of view. The efficient use of the solid supported catalysts can go a long way towards achieving these goals. Various methodologies have been evolved for the immobilization of homogeneous transition metal complexes on the solid support. Encapsulation of homogeneous catalysts in the super cages of zeolite matrix is one of the important methods for the immobilization. This method has provided opportunity to develop catalytic processes in the synthesis of fine chemicals and being used in various types of catalytic reactions like alkylation, hydrogenation, dehydrogenation, hydro-cracking, cyclization, amination, acylation, isometization, rearrangement and oxidation. The catalytic oxidations of organic substrates have been studied well due to its commercial and synthetic importance of the resulted functionalized molecules. An encouragement is thus derived to synthesize various zeolite encapsulated metal complexes and use them as catalysts for the oxidation of different organic substrates. The present thesis describes the synthesis of metal complexes with potential coordinating organic ligands encapsulated in the nano-cavity of zeolite-Y and their characterization by various physico-chemical techniques. Different types of catalytic oxidation reactions have been carried out and suitable reaction conditions have been obtained for the maximum oxidation of organic substrates. The reaction products have been analyzed by gas chromatograph (GC) and their identities confirmed by GC-MS. li For convenience the work presented in the thesis has been divided in the following chapters. First chapter is the introductory one and describes various types ofsolid inert support that have been used for the immobilization of homogeneous catalysts. Abrief introduction of zeolites and their importance, different methods for the immobilization of metal complexes in the nano-cavities of zeolites have been described. Literature on the catalytic applications of various encapsulated metal complexes has also been reviewed. Second chapter describes the interaction of oxovanadium(IV) exchanged zeolite-Y with the Schiff base derived from salicylaldehyde and 2- aminomethylbenzimidazole (Hsal-ambmz) in refluxing methanol followed by aerial oxidation leads to the formation of encapsulated dioxovanadium(V) complex, [V02(sal-ambmz)]-Y(2.1). Similar reaction with copper(II) exchanged zeolite-Y gave encapsulated copper(II) complex, [Cu(sal-ambmz)Cl]-Y(2.2). These encapsulated complexes have been characterized by spectroscopic studies, thermal analysis and scanning electron micrographs (SEM) as well as X-ray diffraction patterns. 3D model structure generated for neat complex [V02(sal-ambmz)] suggests that zeolite-Y can accommodate these complexes in its nano cavity without any stain. The encapsulated materials are found to be active catalysts for the oxidation of phenol, styrene and methyl phenyl sulfide using H202 as an oxidant. Under the optimised reaction conditions about 42 %conversion ofphenol was obtained with these catalysts where the selectivity of catechol varied in the order: 2.2 (73.9 %) > 2.1 (65.2 %). With the conversion of 97.0 %with 2.1 or 56.7 %with 2.2, the oxidation of styrene gave styrene oxide, benzaldehyde, benzoic acid, 1-phenylethane-1,2-diol and phenylacetaldehyde as major products. Amaximum of 96.1 %(with 2.1) and 91.0 % (with 2.2) conversion of methyl phenyl sulfide was observed in which the selectivity of major product methyl phenyl sulfoxide was found to be ca. 98 %. Reaction of N,N'-bis(salicylidene)cyclohexane-l,2-diamine (H2sal-dach) with oxovanadium(IV) and copper(II) exchanged zeolite-Y in refluxing methanol to yield the corresponding zeolite-Y encapsulated metal complexes, abbreviated herein as in [VO(sal-dach)]-Y (3.1) and [Cu(sal-dach)]-Y (3.2) have been described in Chapter three. Spectroscopic studies (IR, electronic and 'H NMR), thermal analysis, scanning electron micrographs (SEM) and X-ray diffraction patterns have been used to characterise these complexes. 3D model structure generated for neat complex [VO(sal-dach)] (3.3) suggests that zeolite-Y can accommodate these complexes in its nano cavity without any stain. These encapsulated complexes catalyse the oxidation of styrene, cyclohexene and cyclohexane efficiently in good yield using H202. Under the optimized conditions, the oxidation of styrene catalysed by [VO(sal-dach)]-Y and [Cu(sal-dach)]-Y gave 94.6% and 21.7% conversion, respectively, where styreneoxide, benzaldehyde, benzoic acid, 1-phenylethane-1,2-diol and phenylacetaldehyde being the major products. Oxidation of cyclohexene catalysed by these complexes gave cyclohexeneoxide, 2-cyclohexene-l-ol, cyclohexane-1,2-diol and 2-cyclohexene-l-one as major products. Conversion of cyclohexene achieved was 86.6% with [VO(sal-dach)]-Y and 18.1 % with [Cu(sal-dach)]-Y. A maximum of 78.1 % conversion of cyclohexane catalysed by [Cu(sal-dach)]-Y and only 21.0 % conversion by [VO(sal-dach)]-Y with major reaction products of cyclohexanone, cyclohexanol and cyclohexane-1,2-diol have been obtained. Chapter four deals with the encapsulation of oxovanadium(IV), copper(II) and nickel(II) complexes of Schiff base derived from salicylaldehyde and oaminobenzyl alcohol (H2sal-oaba) in the nano pores of zeolite-Y by flexible ligand method and characterized by metal analysis, spectroscopic (IR and electronic) studies, scanning electron micrographs, thermal analysis and X-ray diffraction patterns. The encapsulated complexes abbreviated here as [VO(sal-oaba)(H20)]-Y (4.1), [Cu(saloaba)( H20)]-Y (4.2) and [Ni(sal-oaba)(H20)3]-Y (4.3) catalyze the oxidation of styrene, cyclohexane and methyl phenyl sulfide using H202 as oxidant in good yield. Styrene catalysed by [VO(sal-oaba)(H20)]-Y, [Cu(sal-oaba)(H20)]-Y under optimized reactions gave five reaction products, namely, styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid and phenylacetaldehyde, while [Ni(saloaba)( H20)3]-Y gave benzaldehyde selectively. In the presence of tertbutylhydroperoxide all catalysts gave styrene oxide in major yield, though overall IV conversion has been found low (10 - 30 %). The oxidation products of cyclohexane are cyclohexanone and cyclohexanol. Amaximum of 93.2 %conversion of methyl phenyl sulfide has been achieved with [VO(sal-oaba)(H20)]-Y using H202 as oxidant, where selectivity of sulfoxide was 96.9 %. Other catalysts were inactive towards the oxidation of methyl phenyl sulfide. Neat complex [VO(sal-oaba)(H20)] has been equally active. Encapsulation of oxovanadium(IV) and copper(II) complexes of monobasic bidentateftS donor ligand, 2-mercaptomethylbenzimidazole (Htbmz) in the cavity of zeolite-Y by flexible ligand method have been described in Chapter five. These complexes have been characterized by various physico-chemical methods. These encapsulated complexes, abbreviated as [VO(tbmz)2]-Y (5.1) and [Cu(tbmz)2]-Y (5.2), have been used as catalysts for the oxidation of styrene, methyl phenyl sulfide and diphenyl sulfide. Under the optimized reaction conditions, [VO(tbmz)2]-Y gave 96.4 % conversion of styrene with four oxidation products, styrene oxide, benzaldehyde, benzoic acid, 1-phenylethane-1,2-diol and phenylacetaldehyde. Conversion with [Cu(tbmz)2]-Y is considerably low (36.9 %) and gave only four products, styrene oxide, benzaldehyde, benzoic acid and phenylacetaldehyde. In both cases formation ofbenzaldehyde is relatively high. These catalysts are very active for the oxidation of methyl phenyl sulfide and diphenyl sulfide. The oxidation of diphenyl sulfide required at least H202 to diphenyl sulfide ratio of3:1 to give 91.7 % conversion in 7 h of reaction time. However, 94.3 %conversion of methyl phenyl sulfide has been achieved within 3 h ofcontact time at substrate to H202 ratio of 1:1. The catalytic activity of neat complexes using similar molar concentration as that used for encapsulated complexes under above reaction conditions have also been tested for comparison. It has been observed that the corresponding neat complexes have shown equally good catalytic activities. But high turn over frequency and recyclability make the zeolite encapsulated complexes better then their neat analogues.