Characterization of catalytic properties of nanomorphic zeolite using solid state NMR

Abstract

The work presented in this thesis addressed the characterization of catalytic properties of various kinds of nanomorphic zeolites using solid state NMR technique. Zeolites with crystalline microporous aluminosilicate frameworks are widely used as catalysts in petrochemical processes and fine-chemical syntheses due to their strong acid sites located on the frameworks. Conventional zeolites for catalytic applications are normally composed of crystallites of a micrometer scale (~ 1 μm). The external surface area of such conventional zeolites is quite small as compared to the total surface area of an enormous number of internal micropores. Accordingly, chemical reactions occurring on the external surfaces are disregarded in most zeolite catalysis. However, the situation is dramatically changing for zeolites with nanoscale morphologies (i.e., nanomorphic zeolites). Nanomorphic zeolites are available via post-synthetic treatments of conventional zeolites and direct synthesis routes. In particular, the surfactant-directed hydrothermal synthesis route is suitable for various kinds of nanomorphic zeolites with multi-level pore hierarchy. The surfactants are covalently bonded with multi-ammoniums which can function as zeolite structure-directing agents. The surfactants are dual structure-directing agents (SDAs). Microporous zeolite frameworks are generated by the multi-ammonium SDAs, while a large number of surfactant molecules are assembled into a micelle so that it can function as a mesoporous structure-directing agent. Typically, the nanomorphic zeolites are obtained in the form of nanosheets which are composed of a few layers of zeolitic micropores, or highly mesoporous nanosponges that are constructed with thin frameworks of microporous zeolites. These nanomorphic zeolites exhibited significantly larger external surfaces than conventional zeolites. It was confirmed that such an external surfaces have a important role in many catalytic applications to improve cataly...