POLYMER-ANCHORED METAL COMPLEXES AS CATALYST FOR SOME OXIDATION REACTIONS

Abstract

The acceleration of chemical reactions by means of a substance is called catalalysis. A catalyst provides an alternative route to products, the catalytic route being subject to lower activation energy thereby increasing the reaction rate than the uncatalyzed reaction. Catalysts generally change in the course of a reaction but are regenerated. Modern catalytic science faces big challenges as most of the catalytic processes, which are widely engaged in the manufacture of bulk as well as fine chemicals, are homogeneous in nature. Production of a large amount of side waste materials by these processes has imposed a hazardous impact on the environment. Homogeneous catalysts also face the problem of separation from the substrate and products, and very often decompose or polymerize during catalytic action. There is, therefore, necessity to create new highly effective industrial processes, which are selective, ecologically safe and consume minimum energy. In this direction, polymer-anchored catalyst fulfils the industrial demand of easy separation, thermal stability, recycle ability and good recovery of products. Various solid supports like alumina, silica, encapsulated in the super cages of zeolites and mesoporous molecular sieves such as MCM-41 and SBA-15 have also been used for the immobilization of homogeneous catalysts. The discovery of Merrifield resin i.e. chloromethylated polystyrene crosslinked with divinylbenzene, provide a new path to researchers for the synthesis of heterogeneous catalyst. Immobilisation of homogeneous catalysts through covalent bonding with chloromethylated polystyrene cross-linked with divinylbenzene and develops them as environmentally safe heterogeneous catalysts for oxidation reaction have attracted attention in recent years. This method has now become a very specialized method because polymer-anchored catalysts inherit the advantages of both homogenous as well as heterogeneous catalysts as they are thermally more stable, selective and recyclable, and allow their easy recovery from the reaction products at the end of the reaction. Many polymer-anchored complexes have been reported to show potential catalytic activity towards the oxidation and sulfoxidations of organic substrates along with other u reactions. All these encouraged us to design polymer-anchored catalysts and use them for various oxidation reactions. The thesis entitled "Polymer-anchored metal complexes as catalyst for some oxidation reactions" mainly deals with the syntheses of vanadium, copper, molybdenum and tungsten complexes anchored onto chloromethylated polystyrene (cross-linked with 5%divinylbenzene) and their characterization by various physicochemical techniques. Catalytic activities towards oxidation of various substrates have also been carried out under optimized reaction conditions to achieve maximum oxidation products. For convenience the work presented in the thesis has been divided in the following five chapters: First chapter is the introductory one and describes various types of solid inert support that have been used for anchoring of homogeneous catalysts. A brief introduction on functionalized polymers and different modes for anchoring of catalysts has been described. Literature on polymer-anchored metal complexes and their catalytic applications for various reactions have also been reviewed. Second chapter describes the covalent anchoring of 2-(2-pyridyl)benzimidazole (2- pybmz) and 2-(3-pyridyl)benzimidazole (3-pybmz) on to chloromethylated polystyrene crossed-linked with 5%divinylbenzene. These chelating resins readily react with non-isolable metal peroxo species, prepared in situ by stirring V2Os (in presence ofaqueous KOH), Mo03 or W03.H20 with an excess of30 %H202, to give the corresponding oxodiperoxo complexes, PS-K[VO(02)2(L)] (L = 2-pybmz , L = 3- pybmz, PS-[MoO(02)2(L)] )] (L = 2-pybmz, L = 3-pybmz and PS-[WO(02)2(L)] )] (L = 2-pybmz, L = 3-pybmz. PS-2-pybmz and PS-3-pybmz represent polymeranchored ligands. Structures of these complexes have been established on the basis of spectroscopic (IR and electronic), scanning electron microscopy, thermogravimetric studies and elemental analyses. Oxidation of phenol with PS-K[VO(02)2(L)] while oxidation of styrene with all catalysts synthesized have been tested using H202 as an oxidant. Oxidation of phenol with ca. 35%conversion gave a mixture of catechol and jp-hydroquinone where selectivity towards catechol is ca. 62 %. Oxidation of styrene gave five products, styrene oxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-phenyl-1,2-ethanediol. Various parameters such as amount of oxidant and catalyst, and temperature of the reaction mixture have been taken into consideration in for the maximum conversion of substrates. These catalysts are recyclable without considerable loss in their catalytic activities. The covalent bonding of monobasic bidentate ligand 2-(2'- hydroxyphenyl)bezimidazole (Hhpbmz) to chloromethylated polystyrene crosslinkined with 5 %divinylbenzene has been described in third chapter. Treatment of the above chelating resin, abbreviated herein as PS-Hhpbmz, with [Mo02(acac)2] (Hacac = acetylacetone) and Cu(CH3COO)2-H20, gave polymer-anchored complexes PS-[Mo02(hpbmz)2] and PS-[Cu(hpbmz)2], respectively. Structures of these complexes have been established on the basis of various studies. These complexes have been tested as catalyst for the oxidation of styrene, ethylbenzene and methyl phenyl sulfide. Reaction conditions for the maximum oxidation of these substrates have been optimised by considering the concentration of oxidant, amount of catalyst, volume of solvent and temperature of the reaction mixture. Under the optimized conditions styrene gave a maximum of 58.9 %conversion with five reaction products namely styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid, and phenyl acetaldehyde. Oxidation of ethylbenzene gave 47.7 % conversion where benzaldehyde, phenyl acetic acid, styrene, 1-phenylethane1,2-diol and benzoic acid have been obtained as major oxidation products. Amaximum of 75.1 %conversion of methyl phenyl sulfide catalyzed by PS-[Mo02(hpbmz)2] and 44.8 %conversion with PS-[Cu(hpbmz)2] have been achieved. These catalysts do not leach metal ions during catalytic activityand are recyclable up to two cycles. fou^i cWyfe* presents the reaction of 3-Formylsalicylic acid covalently bonded to chloromethylated polystyrene (PS) cross-linked with 5% divinylbenzene reacts with DL-alanine and L-isoleucine. The obtained tridentate Schiff-base ligands are PS-H2fsal-DL-ala and PS-H2fsal-L-isol, respectively. These anchored ligands on reaction with [VO(acac)2] and Cu(CH3COO)2.H20 form complexes PS-[VO(fsal-DLala)( H20)], PS-[Cu(fsal-DL-ala)(H20)], PS-[VO(fsal-L-isol)(H20)] and PS-[Cu(fsal- L-isol)(H20)]. Structures of these immobilized complexes have been established on the basis of various studies. Oxidation of p-chlorotoluene and cyclohexene has been investigated using these complexes as catalyst in the presence of H202 as oxidant. Reaction conditions have been optimized by considering the concentration of oxidant, amount of catalyst and temperature of the reaction mixture. Under the optimized IV condition/>-chlorotoluene gave a maximum of 13.8 % conversion catalysed by PS- [VO(fsal-DL-ala)(H20)] with four products having selectivity order: pchlorobenzaldehyde > /?-chlorobenzylalcohol > p-chlorobenzoic Acid > 2-methyl-5- chlorophenol > 3-methyl-6-chlorophenol. Oxidation of cyclohexene with PS- [VO(fsal-DL-ala)(H20)] is 78.8 % which is followed by PS-[VO(fsal-L-isol)(H20)] with 76.6 % conversion and oxidation of cyclohexene by copper based catalysts are considerably low (29.4 to 32.5 %). The selectivity of the products follows the order: 2-cyclohexene-l-ol > cyclohexene oxide > cyclohexane-1,2-diol > 2-cyclohexene-1- one. Recycle study indicates that these catalysts can be reused at least three times without any significant loss in their catalytic potential. The corresponding neat complexes [VO(fsal-DL-ala)(H20)], [Cu(fsal-DL-ala)(H20)], [VO(fsal-L-isol)(H20)] and [Cu(fsal-L-isol)(H20)] exhibit lower conversion along with lower turn over frequency as compared to their polymer-anchored analogues. Dioxovanadium(V) complexes, K[V02(sal-inh)(H20)] and K[V02(salbhz)( H20)]] (where H2sal-inh = Schiff base derived from salicylaldehyde and isonicotinoylhydrazide, and H2sal-bhz = Schiff base derived from salicylaldehyde and benzoylhydrazide) have been reacted with polymer bound imidazole (PS-im) to give polymer-supported complexes, abbreviated as PS-K[V02(sal-inh)(im)] and PS-K[V02(sal-bhz)(im)]. Characterization of these complexes and their catalytic activity for the oxidative bromination of salicylaldehyde, and oxidation of methyl phenyl sulfide and benzene oxidation are reported in Fifth Chapter. Oxidative bromination of salicylaldehyde using H202/KBr gave 5-bromosalicylaldehyde selectively with a maximum of 90.4 % conversion of salicylaldehyde. Under the best suited reaction conditions, a maximum of 97.8 % conversion of methyl phenyl sulfide with 96.7 % selectivity towards sulfoxide and 3.3 % towards sulfone using PS-K[V02(sal-inh)(im)] while 98.2 % conversion with 97.5 % selectivity towards sulfoxide and 2.5 % towards sulfone using PS-K[V02(salinh)( im)] as catalyst have been achieved. Oxidation of benzene gave phenol with ca. 15 % conversion. These catalysts are reusable.