Synthesis and application of Nano-sized zeolite beta and its polymorphs

Abstract

The chemical industry has been acquiring the advantage of solid catalysts because they can be easily recovered from the reaction medium, possibly regenerated and reused. Zeolites are one type of widely used solid catalysts in various chemical processes because of many virtues, such as large surface area, molecular dimension of the pores for achieving shape selectivity, and excellent thermal and mechanical stability.

Mass transfer of reactants and products to and from the active sites of a zeolite catalyst is an important step in zeolite-catalyzed reactions. The Angstrom-level small pore size of the zeolite poses a strong mass transfer resistance, thus the intrinsic reaction rate is largely affected. Research on solving this mass transfer problem has been going on in two parallel areas, one is to synthesize nano-sized zeolites to shorten the diffusion length, and the other one is to synthesize zeolites with unique pore topologies to minimize pore tortuosity.

Zeolite beta having been commercially used as a catalyst is a large-pore microporous zeolite with three dimensionally interconnected channels/cages. It consists of highly faulted intergrowth of mixed polymorphs, namely polymorphs A, B and C. Each polymorph has a unique pore topology. Polymorph C possesses a three-dimensional pore topology, in which all pore channels are linear.

This thesis work attempted to synthesize 1) nano-sized zeolite beta, 2) a pure polymorph of zeolite beat, and 3) titanium¿containing pure polymorph. Potential applications of the materials were explored. Titanium has been successfully incorporated into the crystalline framework of pure polymorph C. The catalytic performance of the Ti-containing polymorph C catalyst was evaluated using cyclohexene epoxidation reaction. Key parameters influencing the crystallization of Ti-containing polymorph C were examined and elucidated. The thermal stability of the materials was studied. Approaches to improving the thermal stability and enhancing the catalytic performance were exploited. The kinetics of the epoxidation reaction over Ti-containing polymorph C was experimentally investigated and theoretically analyzed.