Structure and density of Mo and acid sites in Mo-exchanged H-ZSM5 catalysts for nonoxidative methane conversion

Mo/H-ZSM5 (1.0-6.3 wt % Mo; Mo/Al = 0.11-0.68) catalysts for CH4 aromatizat ion were prepared from physical mixtures of MoO3 and H-ZSM5 (Si/Al = 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in a ir indicate that MoOx species migrate onto the external ZSM5 surface at abo ut 623 K. Between 773 and 973 K, MoOx species migrate inside zeolite channe ls via surface and gas phase transport, exchange at acid sites, and react t o form H2O. The amount of H2O evolved during exchange and the amount of res idual OH groups detected by isotopic equilibration with D-2 showed that eac h Mo atom replaces one H+ during exchange. This stoichiometry and the requi rement for charge compensation suggest that exchanged species consist of (M o2O5)(2+) ditetrahedral structures interacting with two cation exchange sit es. The proposed mechanism may provide a general framework to describe the exchange of multivalent cadons onto Al sites in zeolites. As the Mo concent ration exceeds that required to form a MoOx monolayer on the external zeoli te surface (similar to 4 wt % Mo for the H-ZSM5 used), Mo species sublime a s (MoO3), oligomers or extract Al from the zeolite framework to form inacti ve Al-2(MoO4)(3) domains detectable by Al-27 NMR. These (Mo2O5)(2+) species reduce to form the active MoCx species during the initial stages of CH4 co nversion reactions. Optimum CH4 aromatization rates were obtained on cataly sts with intermediate Mo contents (similar to 0.4 Mo/Al), because both MoCx and acid sites are required to activate CH4 and to convert the initial C2H 4 products into C6+ aromatics favored by thermodynamics.