Synthesis and catalytic applications of inorganic and organic-inorganic hybrid porous frameworks

Abstract

Development of new class of porous materials is advantageous in numerous inter-disciplinary research areas. Porous materials especially zeolites play significant role in the heterogeneous catalysis. However, its application is limited in fine-chemical synthesis due to pore diffusion constraints. Microporosity and intricate channels are a boon for the petrochemical industry, but on the other hand they restrict the applications in fine-chemical synthesis involving large organic molecules. Thus in order to vanquish this problem encountered in micropors zeolites, two strategies have been adopted: (i) to reduce the crystal size of zeolite (termed as nanocrystalline zeolite), (ii) to develop novel synthesis strategies for the preparation of zeolites with inter-connected micropores-mesopores and additionally macropores in the framework structure. Applications of zeolites are limited in those reactions where redox active transition metal sites are required. There are only limited scopes for the introduction of transition metals in the zeolites such as ion-exchange process or framework substitution. Since transition metals are difficult to incorporate into the inorganic zeolite framework, attempts have been made to develop a class of porous materials, known as metal–organic frameworks (MOFs) in which various metals can be incorporated in the MOFs framework. However, the frameworks of MOFs are built with organic subunits; therefore, it is not possible to use them in catalytic reactions that are operated under harsh reaction conditions. Hence, it is important to choose MOFs that are stable under liquid phase conditions in different solvents and varied reaction conditions. Further to increase the catalytic applications of MOFs, efforts have been made to prepare functional MOFs via new ligands or by post-synthesis modification. New approaches are also being developed to prepare MOFs based nanocomposite materials. Keeping this in account, the first objective was the synthesis of mesoporous zeolites and metal organic-frameworks. Initially, organic templates were explored for the formation of different zeolite frameworks. The main advantage of the developed organic surfactant mediated strategy was that three different types of zeolite frameworks were prepared by using only one mesoporous structure directing agents. Furthermore, to develop a sustainable approach for the synthesis of mesoporous zeolite, natural templates such as pollen grains were explored for the formation of nanocrystalline zeolites. Microporous-mesoporous-macroporous (also known as tri-modal pores) containing ZSM-5 and Beta were prepared by a simple, economical, and eco-friendly strategy. Mesoporous zeolites and tri-modal porous zeolites exhibited significantly improved catalytic activity and recyclability in diverse acid catalyzed reactions involving bulky reactants/products, as compared to that of commercial zeolites. Next, the attempts were also made to prepare mesoporous metal organic frameworks by using di-cationic structure directing agents Copper and zirconium based metal organic frameworks were investigated in various catalytic reactions. In order to improve the catalytic efficiency, different functionality such as nanoparticles and organic bases were incorporated in the matrix of Cu/Zr based MOF by using the post-synthetic modification method. These functionalized MOFs were investigated in the coupling and condensation reactions. Next objective was to develop a highly sustainable zeolite and MOF based composite materials to catalyze those reactions, which have significant socio-economical value. Therefore, different synthesis strategies have been developed for the preparation of zeolite-MOF composite. In first synthesis strategy, less-useful extra-framework Al species present in the zeolite surrogates the growth of metal organic frameworks from the zeolite matrix. In the second strategy, the zeolite Beta was integrated with different Zr based metal–organic frameworks to prepare multi-functional composite catalysts. The composite catalysts contain the characteristic features of both the porous materials. The composite were employed as catalysts for acylation reaction, synthesis of pharmaceutical important compound such as coumarin, and biomass transformation to useful fine-chemicals and fuel additives such as γ-valerolactone, HMF and their derivatives. The obtained results reveal that the integration of both the frameworks in the composite catalyst is required to achieve the excellent catalytic activity. Overall, this thesis shall provide the details for the synthesis and catalytic applications of inorganic and organic-inorganic hybrid porous frameworks.