Effect of silica source on zeolite MFI crystallization in fluoride media and its physicochemical and catalytic properties

Abstract

A series of MFI zeolites were synthesized in fluoride media from fumed silica hydrogel, fumed silica and TEOS to elucidate the influence of silica source on the physicochemical and catalytic properties of MFI zeolites. The materials obtained were characterized by nitrogen adsorption, XRD, SEM, TPD and 27Al NMR MAS methods and evaluated in MTO and hexane/3-methyl pentane cracking catalytic reactions. It has been demonstrated that crystallization proceeds via solution-mediated mechanism independently on silica source. However, the nature of silica source substantially affects the crystallization rate, the crystal size, the dynamics of Al incorporation into the MFI framework and Al distribution in the zeolite crystals. The application of fumed silica hydrogel and fumed silica with high polymerization degree yields crystals with an Al-enriched core and a siliceous shell. These materials demonstrate faster deactivation in the MTO reaction and higher selectivity to ethylene as compared to the MFI obtained from TEOS with a uniform Al distribution in the crystals. The effect observed is attributed to a longer diffusion path length of the reagent to active sites located mainly in the crystal core and the reaction products outside from the crystal core, resulting in a higher contribution of secondary reactions.