A Study On Synthesis, Characterization And Catalysis Of Hierarchical MFI Zeolites

Mesoporosity is successfully induced into microporous MFI zeolites in order to create a hierarchical channel system for optimizing internal diffusion of substrates and products molecules. Two methods are employed to realize such intention. First, a Gemini-type surfactant structure-directing agent (SDA) was used to synthesize layered MFI zeolite with single-unit cell thickness along b-axis. The as-synthesized precursor was used as catalyst without further calcination. Second, the same SDA was introduced into a desilication system of TS-1 to induce both microporous and mesoporous channels into bulk crystals of parent zeolite.In the first part of the dissertation, layered MFI (LS-1) zeolite was synthesized according to literature in presence of C22-6-6 as SDA. Focusing on its relatively low stability in framework, C22-6-6 was replaced with C18-6-3 with similar surfactant-like structure but bulkier diammonium group. Characterization results indicated that LS-1 synthesized with C18-6-3 possessed a higher stability against leaching. Lewis base site was then generated through a simple acid wash of LS-1 precursor with halide acid solution. The activity of the catalyst was investigated in the cycloaddition of epoxides with carbon dioxide. The hybrid materials was demonstrated to be highly efficient catalysts for the reaction. The result testified the idea that the SDAs have potential to serve as catalytic active sites, providing a new solution to reduce the high cost of SDA in zeolite application.In the second part, the C18-6-3 SDA was introduced into a desilication system of large-crystal TS-1. The parent microsized TS-1 had a crystal size of 10 μm with neat surface and showed limited catalytic activity in epoxidation of alkenes. Through a hydrothermal treatment in presence of sodium hydroxide and C18-6-3, the mixture experienced a desilication-recrystallization process, resulting in a hierarchical TS-1 (TS-1). During this process, the framework silicon atoms were etched by NaOH, leaving disordered mesopores inside pristine crystals. Meanwhile, the dissolved Si species were further crystallized with the assistant of the structure-directing effect of C18-6-3 and formed layered MFI crystals on the external surface bulk TS-1 crystals. As a result, hierarchical TS-1 with multiple pore systems was successfully created, and it showed a significantly enhanced catalytic activity in the epoxidation of alkenes, even higher than nanosized TS-1 particles. The results testified that the use of high cost SDA could also be reduced by adding the SDA in post-synthesis treatments instead of in the original synthesis gel and was proven to be an effective method to improve the catalytic activity of catalysts.