Controllable synthesis of gallium-containing MFI zeolite for bifunctionally catalyzing methanol aromatization

Abstract

Ga-MFI zeolites with controllable crystal sizes were hydrothermally synthesized by using colloidal silicalite-1 as seed crystals and their catalytic performances for methanol-to-aromatics (MTA) were evaluated. It was found that the crystal size of as-made Ga-MFI zeolite became small with the adding amounts of seed crystals. Small-sized Ga-MFI zeolite with short channels shows much better coking-resistance and thus longer catalytic lifetime owning to its more favorable outward diffusion of aromatic products. In addition, the stability could be further improved by impregnating gallium species into Ga-MFI zeolite, but which slightly sacrificed its aromatic selectivity. The Ga species with different locations in zeolite play distinct roles in MTA. The weak intrinsic protonic acidity from tetrahedral framework Ga3+ could suppress hydrogen transfer reactions for light alkane formation while non-framework gallium species especially Ga2O3 nanoparticles promoted the dehydrocyclization process. The synergy between the strong Brønsted and weak Lewis acid sites contributes to the excellent aromatic yield.