SYNTHESIS, CHARACTERIZATION AND CATALYSIS BY PENTASILS TITANIUM AND VANADIUM SILICATES

Abstract

Zeolites are fascinating class of materials, which have attracted wide spread attention in basic science as well as in technology development. Zeolites are endowed with unique physical and chemical characteristics which offer opportunities to manipulate active site microenvironment. They are even compared with catalytic antibodies and metalloenzymes. Some of the prominent physical characteristics of zeolites are its ruggedness to temperature and pressure and its ability to recognise, discriminate and organise molecules with less than one angstrom level precision at the active sites. Moreover, non-corrosive, environment friendly and economically viable nature of the zeolite catalyzed reactions have provided the opportunity for the development of various industrial processes such as fluid catalytic cracking, hydrocracking, isomerization, and oxidation. These zeolite materials are amenable to modification by isomorphous substitution that make them attractive in tailor making of zeolite characteristics suitable for various applications. The possibility to incorporate certain transition metals e.g. Fe, Ti, Vetc. with redox characteristics, in zeolitic silicate network provides an opportunity to carry out selective oxidation reactions in a confined zeolite microenvironment around a catalytic active sites. Among these materials TS-1 (Titanium silicate-1) zeolites of pentasil family attracts special attention due to their use as shape selective oxidation catalysts for many organic substrates with hydrogen peroxide as oxidants. These materials are active in hydroxylation, epoxidation, ammoximation and oxyfunctionalization of aliphatics, amines and sulfur compounds. Sol-gel chemistry has generated a great deal of interest in various fields of science technology of late. Although the basic principle of sol-gel chemistry was known more than a hundred years ago, its technological potential has come to the light only in the last ten years. However, till recently, a majority of the materials prepared using the sol-gel method ceramics, refractories and glasses. The attempt to apply the sol-gel technique for the preparation of catalysts and particularly, metal substituted zeolites is relatively new. were Keeping in view of the above mentioned points, the present study was aimed to understand the effect of synthesis methodology of titanium silicate-1 (TS-1) synthesized by conventional as well as sol-gel methods. Synthesis of TS-1 by conventional method and by solgel method of wetness impregnation have been carried out in astainless steel reactor, following the procedure given in patent literature. Instrumental techniques such as X-ray diffraction (XRD), BET N2 sorption, energy dispersive x-ray (EDAX), Fourier Transform Infrared (FTIR), diffuse reflectance UV-visible (DR-UV-VIS), electron spin resonance (ESR), atomic absorption spectroscopy (AAS), differential thermal analysis (DTA), thermogravimetric analysis (TGA), microcalorimetry, scanning electron microscopy (SEM) and cyclic voltammetry (CV) were used for the physico-chemical characterization ofthe catalysts synthesized. The content of the thesis has been classified into six chapters. The introductory chapter (Chapter 1) is acritical survey of the literature on zeolites in general. The chapter also contains a review of the origin and development of zeolite materials with special emphasis on their potential applications in different organic transformations. Abrief account of about large number of new reactions are also described over a continuously increasing assemblage ofzeolitic structures that are described in literature. An attempt is made to review the present understanding of both the catalytic chemistry and structure-reactivity relationships over avariety ofzeolites. Detailed description ofthe experimental methods adopted for synthesis, i.e. conventional method (Alkaline hydrolysis) and sol-gel method (Acid hydrolysis) and their characterization has Ill been given in Chapter 2. Various methods employed for characterization and adetailed account of experimental procedures adopted are presented in this chapter in order to prepare the reader for the discussions in the forthcoming chapters. The Chapter 3deals with acomparative study of TS-1 synthesized by conventional as well as sol gel wetness impregnation. Acomparison of advantages of sol-gel method over the conventional method is also discussed. Astudy of kinetics of crystallization of the two systems revealed that there exists significant differences in nucleation and crystallization parameters between the two methods. The effect oftemperature, silica-titanium ratios, H20/Si02 ratios, are some of the variables that are studied. Adetailed characterization using XRD, IR, UV-DRS, cyclic voltammetery, and catalytic test reaction were employed in order to show that sol-gel wetness impregnation method results in fully crystalline pure phase with the same physicochemical characteristics as conventional method. In addition an XRD method for evaluation of framework Si/Ti ratios is also proposed. Acidity and acid site distribution is very important characteristic of zeolite. Adetailed study of acidity; acid strength distribution of TS-1 prepared by the conventional a well as by sol-gel methods using NH3 as probe molecules by microcalorimetric evaluation of heat of adsorption of the ammonia is also studied and forms the contents of Chapter 3. The type of zeolite, method of preparation, Si/Ti ratios are some of the variables as a function of which acidity and acid strength distribution was evaluated. These results indicated that titanium silicates contain predominantly acid sites of moderate strength. The acidity decreases with the increase of Si/Ti ratio, indicating that Ti ions in the structure are the seat of acidic activity. Catalytic oxygen transfer involving the reaction of an oxygen donor with an organic substrate in the presence of a catalyst holds considerable promise in commercial oxidation reactions. Phenol hydroxylation is a good example in this context. This reaction is also considered to be a test reaction for incorporation into the silicalite frame work. Chapter 4 describes adetailed account of four types of reactions catalyzed by TS-1 system. • Hydroxylation of phenol • Ammoximation of cyclohexanone to oxime. • Catalytic cleavage of Tosyl hydrozones and imines. • Baeyer-Villiger rearrangement by TS-1 /H202 systems The influence of different solvents on the hydroxylation of phenol over TS-l(WI) is interesting and complex. Chapter 4also deals with the solvent effect by using various solvents such as water, acetone, acetonitrile and methanol and also effect of different combinations of solvent in different proportions on the catalytic activity and selectivity. Chapter 5 deals with synthesis of TS-2 by sol-gel method as well as by conventional method. The TS-2 was synthesized for the first time by sol-gel wetness impregnation method. This method describes adetailed accounts of optimization of preparation conditions for synthesis and the intricacies involved in gel preparation and other synthesis variable like Si/Ti ratio, time of synthesis and kinetics of crystallization. In the same chapter acomparative study of conventional and sol-gel methods while highlighting the relative merits of sol-gel method is presented. The microcalorimetric study on synthesized TS-2 was also carriedout and presented in this chapter. A discussion of mechanism of crystallization of TS-2 by sol-gel wetness impregnation method is also presented. The formation of TS-2 and presence of TI ions in the structure was verified by IR, UV-DRS, Cyclic Voltammetry, XRD, N2-BET and t-plot method. Chapter 6 deals with the synthesis of Vanadium silicate-1 (VS-1) by sol-gel wetness impregnation method. The VS-1 was synthesized by sol-gel method for the first time. The synthetic methodology and its optimization is discussed. A detailed characterization of the product of hydrothermal synthesis of sol-gel wetness impregnated method was carried out to extract information about the presence of vanadium in structural positions. IR, UV studies indicated the framework presence ofvanadium. The valence state ofvanadium was confirmed by ESR and cyclic voltammetery. These studies indicated that both V5+ and V4+ are present in the structure to varying degrees depending on the preparation conditions and pretreatments. The catalytic activities were also characterized by phenol hydroxylation and toluene oxidation to benzaldehyde which indicated that performance of VS-1 synthesized by sol-gel method is comparable to the conventional preparation. Over all, a thorough studies ofthe synthesis characterization and catalysis ofTS-1 (MFI), TS-2 (MEL) and VS-1 (MFI) by wetness impregnation of amorphous precursors prepared by solgel method has been made. In conclusion the thesis deals with the interesting observations from academic point of view as well as industrial applications ofthese materials for various oxidation reactions.