Synthesis and catalytic applications of nanocrystalline zeolites and their nanocomposites

Abstract

Zeolites are crystalline microporous aluminosilicates. Zeolites are prepared in highly basic medium using either additives or soft templates. Catalytic applications of microporous zeolites are limited to small molecules because of mass transfer limitation. Furthermore they do not contain any redox metal in their framework structure to carry out multi-functional activities. Catalytic activity of zeolites can be tailored by two ways; either surface functionalization or by incorporating transition metal through ion-exchange or framework substitution into the zeolite. Nanocrystalline zeolite is better suited because it contains large number of surface silanol groups, and large external surface area that helps to anchor the active sites to catalyze appointed organic reactions. Furthermore, small particle size and intercrystalline mesoporosity present in the nanocrystalline zeolite decreases the diffusion path length and provides readily accessible active sites that make it an interesting candidate for catalysis. Keeping this in account, the first objective was the synthesis of nanocrystalline zeolites such as Nano-Beta and Nano-ZSM-5. The intercrystalline porosity and the inherent acidity present in these materials were explored in the synthesis of fine chemicals, especially alkyl phenyl ether and 5, 6, & 7-membered heterocycles of pharmaceutical importance. Next attempt was made to explore another important feature present in nanocrystalline zeolites e.g. the functional nature of surface silanol group. This unique feature was exploited to prepare multi-functional catalysts by the surface functionalization and metal incorporation. Therefore, the surface of nanocrystalline ZSM-5 was functionalized with a wide range of amines, ionic liquids and metal containing ionic liquids. These materials were further investigated in the activation and utilization of CO2 in the heterocycle synthesis. Halometallate ionic liquid functionalized materials were investigated in the reduction of carbon-carbon multiple bonds. Multifunctional activity in parent nanocrystalline ZSM-5 was introduced by decorating transition metal nanoparticles on the external surface of zeolite. Transition metal nanoparticles (e.g. Pd, Cu, Ag, and Ru) supported nanocrystalline zeolite based multifunctional catalysts were prepared. Pd-based bi-functional catalysts were investigated in the tandem reaction to prepare fine chemicals especially heterocyclic compounds. Ru-based bi-functional catalyst was investigated in the biomass transformation to fuel additives or fine chemicals. Ag-based bi-functional catalyst was investigated in the hydration and epoxidation reaction. Cu-based bi-functional catalyst was explored in the synthesis of bulkier heterocycles and fine chemicals. Finally, an attempt was made to disperse metal oxide, especially octahedral manganese oxide molecular sieve (OMS), on the external surface of nanocrystalline zeolites to afford multifunctional activity in zeolite. OMS was chosen as a metal oxide because it shows similar properties & structural similarity to zeolites. In this study, OMS was prepared by three different routes and investigated in the hydration reaction and tandem reaction where oxidation, condensation, and cyclization are the key steps. OMS supported Nano-ZSM-5 based bi-functional catalysts were also prepared and investigated in the condensation and protection of different amines for the synthesis of industrially important fine chemicals. OMS and H-Beta both catalysts were investigated in the two-step selective synthesis of 2,5-difrormylfuran (DFF) from carbohydrates. In most of the reactions, structure activity relationship was established and reaction mechanisms were proposed based on the time dependent NMR study, control experiments and other physicochemical characterizations. Overall, this thesis reports the synthesis of a wide range of multi-functional catalysts based on zeolites. Acidic zeolites and multi-functional zeolites reported in this thesis were used as catalysts in the synthesis of various industrially important synthetic intermediates, especially heterocyclic compounds. The work presented in this thesis contributes to the area of Green and Sustainable Chemistry.