Methanol aromatization over Ga-doped desilicated HZSM-5

Abstract

A series of desilicated, Ga-impregnated, and a combination thereof HZSM-5 catalysts were synthesized and tested in methanol aromatization at high temperatures (400–500 °C). Small extents of ZSM-5 framework deterioration, mesopore expansion, and improvements of Lewis and Brønsted acidities were caused by desilication. For Ga-doped catalysts, enlarged mesopores reduced the mass transfer limitation and allowed more hydrated gallium ions, i.e., precursors of gallyl species, to migrate into zeolite cavities, compared to those for un-treated ZSM-5. This increased the number of (GaO)⁺-Brønsted sites, bifunctional centers that promote aromatics through dehydrogenation and cyclization. These sites also facilitate the secondary dehydrogenation of alkanes (especially isobutane) to propylene, and the subsequent conversion of propylene to aromatics. Aromatization activity can be enhanced by sacrificing the catalyst lifetime, which is shortened by severe coking.