STRUCTURE AND DENSITY OF ACTIVE ZN SPECIES IN ZN H-ZSM5 PROPANE AROMATIZATION CATALYSTS/

Exchanged Zn cations increase propane conversion turnover rates, hydro gen formation rates, and selectivity to aromatics on H-ZSM5. In situ Z n K-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 monomeri c cations and form directly from nitrate solutions during ion exchange . X-ray absorption and temperature-programmed reduction studies show t hat Zn2+ cations in Zn/H-ZSM5 do not reduce to zero-valent species dur ing propane reactions at 773 K. Extrazeolitic ZnO crystals form on sam ples prepared by impregnation techniques. In contrast to exchanged Zn2 + cations, these bulk ZnO crystals reduce to Zn metal and elute from t he catalyst bed as Zn vapor under typical propane aromatization condit ions. Condensation reactions of (Zn2+OH)(+) species with acidic OH gro ups appear to lead to the formation of the active Zn2+ cations interac ting with two Al sites (O--Zn2+-O-). This active Zn species is support ed by acid site density measurements from NH3 titration and isotopic t itration of remaining Bronsted acid sites with deuterium in Zn/H-ZSM5. (C) 1998 Academic Press.