Hierarchical structure and catalytic properties of a microspherical zeolite with intracrystalline mesopores

Abstract

An effective microspherical zeolite MFI for large molecules was prepared for the first time. The material is found in a microspherical form 5.0μm in size, and the microspheres are composed of 20–30-nm nanoparticles. The nanoparticles further contain uniform intracrystalline mesopores of ∼3.0nm. The mesopores in the material are generated in the course of crystallization, owing to bond-blocking from silanized silicon dioxide, and the mesoporous porosity can be effectively adjusted though different degrees of silanization. During the synthesis, an excess of grafted organic species on the silica surface inhibits the formation of zeolitic phases. The formation of the microsphere is attributed to the glomeration of halogen anions and/or the salts present during the synthesis. Fourier transform infrared spectrophotometry and 29Si and 27Al nuclear magnetic resonance results confirm that the existence of silanol groups is directly related to the rich crystalline defect sites. The adsorption of alkali molecules with different kinetic diameters (0.5–0.8nm) indicates that the mesopores generated in the zeolite catalyst markedly increase the accessibility of micropores and the availability of acid sites, which is confirmed by high performances observed in catalytic conversions, such as cumene cracking, benzylation of benzene, and methanol conversion to gasoline. The introduction of auxiliary mesoporosity to the zeolitic catalyst is believed to improve molecule transport and enhance the accessibility of active sites in the microporous structure.