Structure and density of active Zn species in Zn/H-ZSM5 propane aromatization catalysts

Exchanged Zn cations increase propane conversion turnover rates, hydrogen formation rates, and selectivity to aromatics on H-ZSM5. In situ Zn K-edge X-ray absorption studies show that aqueous ion exchange with H-ZSM5 (Si/Al = 14.5) leads to isolated Zn(2+) 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 Zn(2+) 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 Zn(2+) 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 (Zn(2+)OH)(+) species with acidic OH groups appear to lead to the formation of the active Zn(2+) cations interacting with two Al sites (O(-)-Zn(2+)-O(-)). This active Zn species is supported by acid site density measurements from NH(3) titration and isotopic titration of remaining Bronsted acid sites with deuterium in Zn/H-ZSM5. (C) 1998 Academic Press.