Structure and Density of Active Zn Species in Zn/H-ZSM5 Propane Aromatization Catalysts

Abstract

Exchanged Zn cations increase propane conversion turnover rates, hydrogen formation rates, and selectivity to aromatics on H-ZSM5.In situZnK-edge X-ray absorption studies show that aqueous ion exchange with H-ZSM5 (Si/Al=14.5) leads to isolated Zn2+cations with tetrahedral symmetry. These Zn species reside at cation exchange sites as monomeric cations and form directly from nitrate solutions during ion exchange. X-ray absorption and temperature-programmed reduction studies show that Zn2+cations in Zn/H-ZSM5 do not reduce to zero-valent species during propane reactions at 773 K. Extrazeolitic ZnO crystals form on samples prepared by impregnation techniques. In contrast to exchanged Zn2+cations, these bulk ZnO crystals reduce to Zn metal and elute from the catalyst bed as Zn vapor under typical propane aromatization conditions. Condensation reactions of (Zn2+OH)+species with acidic OH groups appear to lead to the formation of the active Zn2+cations interacting with two Al sites (O−–Zn2+–O−). This active Zn species is supported by acid site density measurements from NH3titration and isotopic titration of remaining Brønsted acid sites with deuterium in Zn/H-ZSM5.