CATALYTIC ACTIVITIES OF VANADIUM, MANGANESE AND COPPER COMPLEXES IMMOBILIZED IN ZEOLITE-Y

Abstract

Catalysts have played key role in the development of modern chemical technologies. They activate the chemical reaction at milder conditions through the bonding of reactant molecules with definite functional groups called 'active sites', where they react and finally detach from the catalyst for the next cycle. As large as 95 %processes used in the chemical industries there days are catalyst based technologies. Directly or indirectly catalysts have contributed to more than 20 % GDP of developed nations. Most of the catalytic processes, which are widely used in the manufacture of bulk as well as fine chemicals, are homogeneous in nature. Alarge amount of waste materials has been produced during these processes which imposed ahazardous impact on the environment. Homogeneous catalysts also face the problem of separation from the substrate and products. Therefore, there is anecessity to create new highly effective industrial processes, which are selective, ecologically safe and consume minimum energy. The efficient use of the solid supported catalysts can go along way towards achieving these goals. As a consequence of the inherent advantages ofthe heterogeneous catalytic system over their homogeneous counter part, efforts have been directed towards the development ofheterogeneous systems. Various methodologies have been evolved for the immobilization ofhomogeneous transition metal complexes. 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 oxidation of organic substrates has been studied well due to its commercial and synthetic importance of the resulted functionalized molecules. All these encouraged us to design zeolite-Y 11 encapsulated metal complexes and use them as catalysts for various oxidation reactions. The thesis entitled "Catalytic activities of vanadium, manganese and copper complexes immobilized in zeolite-Y", describes the synthesis of vanadium, manganese and copper complexes with potential coordinating organic ligands encapsulated in the nano-cavity ofzeolite-Y and their characterization by various physico-chemical techniques. Different types ofcatalytic 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. 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 of solid inert support that have been used for the immobilization ofhomogeneous catalysts. Literature on the catalytic applications of various encapsulated metal complexes has also been reviewed. In Second Chapter, complexes [Mnm(pydx-en)Cl(H20)] (1.1) [Mnm(pydxl, 3-pn)Cl(CH3OH)] (1.2) and [Mnni(pydx-l,2-pn)Cl(H20)] (1.3) have been prepared by the reaction of Mnn(CH3COO)2 with dibasic tetradentate ligands, AyV-ethylenebis (pyridoxylideneiminato) (H2pydx-en, I), NJVpropylenebis( pyridoxylideneiminato) (H2pydx-l,3-pn, II) and l-methyl-N,iVethylenebis( pyridoxylideneiminato) (H2pydx-l,2-pn, III), respectively followed by aerial oxidation in the presence of LiCl. Crystal and molecular structures of [Mn(pydx-en)Cl(H20)] (1.1) and [Mn(pydx-l,3-pn)Cl(CH3OH)] (1.2) confirm their octahedral geometry and the coordination of ligands through ONNO(2-) form. These complexes have also been encapsulated in the super cages of zeolite-Y. The encapsulated complexes have been used as catalysts for the oxidation, by H202, of methyl phenyl sulphide, styrene and benzoin efficiently. Oxidation of methyl phenyl sulphide under the optimized reaction conditions gave ca. 86 %conversion in with two major products methyl phenyl sulfoxide and methyl phenyl sulfone in the ca. 70 %and 30 %selectivity, respectively. Oxidation of styrene catalyzed by these complexes gave at least five products namely styrene oxide, benzaldehyde, benzoic acid, l-phenylethane-l,2-diol and phenylacetaldehyde with amaximum of 76.9 %conversion of styrene by [Mnni(pydx-en)Cl(H20)]-Y (2.4), 76.3 %by [Mnni(pydx-l,3-pn)Cl(H20)]-Y (2.5) and 76.0%by [Mnm(pydx-l,2-pn)Cl(H20)]- Y(2.6) under optimized conditions. Similarly, ca. 93% conversion ofbenzoin has been obtained by these catalysts, where the selectivity of the products followed the order: benzil > benzoic acid > benzaldehyde-dimethylacetal. Tests for the recyclability and heterogeneity of the reactions have also been carried. Neat complexes are equally active. However, the recycle ability of encapsulated complexes makes them better over neat ones. Third Chapter describes the synthesis of zeolite-Y encapsulated oxidovanadium(IV) complexes, abbreviated herein as [VIV0(pydx-en)]-Y (3.4), [VIV0(pydx-l,3-pn)]-Y (3.5) and [V,vO(pydx-l,2-pn)]-Y (3.6), with H2pydx-en, H2pydx-l,3-pn and H2pydx-1,2-pn, respectively. Neat complexes [VIV0(pydx-en)] (3.1), [VIV0(pydx-l,3-pn)] (3.2) and [V,vO(pydx-l,2-pn)] (3.3) have also been prepared. Spectroscopic studies (IR, UV/Vis and EPR), elemental analyses, thermal studies, field emission scanning electron micrographs (FE-SEM) and Xray diffraction patterns are used to characterize these complexes. Oxidations of styrene, cyclohexene and methyl phenyl sulfide have been investigated using these complexes as catalyst precursors in the presence of H202 as oxidant. Under the optimized reaction conditions, a maximum of 85.5 %conversion of styrene has been obtained with 3.4, 84.6 %conversion with 3.5 and 82.9 %conversion with 3.6 in 6hofreaction time. The selectivity ofthe various products is similar for the catalyst precursors (i.e. complexes 3.4 to 3.6) and follows the order: benzaldehyde >l-phenylethane-l,2-diol > benzoic acid > phenyl acetaldehyde. With cyclohexene, a maximum conversion of 95.9 %has been achieved with 3.4, 94.5 %with 3.5 and 94.2 %conversion with 3.6, also in 6h ofreaction time. The IV selectivity ofthe various products is similar for the three catalysts: 2-cyclohexene- 1-one > 2-cyclohexene-l-ol > cyclohexane-l,2-diol. The oxidation of methyl phenyl sulfide is achieved with 3.4, 3.5 and 3.6 in 2.5 h of reaction time with 85.5 %, 82.1% and 80% conversion, with higher selectivity towards sulfoxide. UV-Vis and 51V NMR experiments with 3.1 confirm the plausible formation of VvO(02)L as intermediates in the catalytic oxidations. Reaction of Mnu(CH3COO)2 with tribasic pentadentate ligand, H3sal-dahp obtained by the condensation of salicylaldehyde and l,3-diamino-2- hydroxypropane followed by aerial oxidation gives [Mnm(sal-dahp)(H20)] (4.1). Its encapsulation in zeolite-Y, abbreviated herein as [Mnm(sal-dahp)(H20)]-Y (4.2), has been achieved by the reaction of Mn(II)-exchanged zeolite-Y with H3sal-dahp in refluxing methanol, followed by aerial oxidation. Studies on these complexes are described in Fourth Chapter. Both the complexes are characterized by various physico-chemical studies. Oxidation of benzoin with H202 has been investigated using [Mnm(sal-dahp)(H20)]-Y as catalyst as oxidant. Under optimized reaction conditions a maximum of 86.1% conversion of benzoin is achieved in 6 h of reaction time. The selectivity of the various products follows the order: benzoic acid (64.3 %)> benzil (22.3 %) > benzaldehyde-dimethylacetal (13.4 %). Neat complex is equally active and the oxidation products obtained also follow the same order of selectivity. Two possible mechanisms, one via Mn =0 containing intermediate compound formation and second via direct interaction of benzoin with manganese have been proposed through which various substrates form. In Fifth Chapter, synthesis of [Cu"(acpy-oap)Cl] by the reaction of monobasic tridentate ligand, Hacpy-oap (Hacpy-oap = Schiff base derived from 2- acetylpyridine and o-aminophenol) with CunCl2 in refluxing methanol is presented. Elemental analysis and spectral (IR and electronic) studies confirm its distorted square planar structure. Complex [Cun(acpy-oap)Cl] has also been encapsulated in the nano cavity of zeolite-Y and its encapsulation ensured by various physicochemical techniques. Neat as well as encapsulated both complexes are active catalysts for the oxidation of styrene and cyclohexene using tertbutylhydroperoxide (TBHP). Reaction conditions for the maximum oxidation of these substrates have been optimized by considering the concentration of oxidant, amount of catalyst, volume of solvent and temperature of the reaction mixture. Under the optimised conditions styrene gave a maximum of60.2% conversion in 7 h with mainly two reaction products namely styrene oxide and benzaldehyde. Oxidation of cyclohexene required 6 h to give 61.3% conversion where cyclohexene epoxide, 2-cyclohexene-l-one, 2-cyclohexene-l-ol and cyclohexane- 1,2-diol are obtained as major oxidation products. Catalyst [Cu"(acpy-oap)Cl] does not leach metal ion during catalytic activity and is recyclable. Finally, summary and over all conclusions based on the achievements are presented.