Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Catalysts for Nonoxidative Methane Conversion

Abstract

Mo/H-ZSM5 (1.0−6.3 wt % Mo; Mo/Al = 0.11−0.68) catalysts for CH 4 aromatization were prepared from physical mixtures of MoO 3 and H-ZSM5 (Si/Al = 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoO x species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoO x species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H 2 O. The amount of H 2 O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D 2 showed that each Mo atom replaces one H + during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo 2 O 5 ) 2+ ditetrahedral structures interacting with two cation exchange sites. The proposed mechanism may provide a general framework to describe the exchange of multivalent cations onto Al sites in zeolites. As the Mo concentration exceeds that required to form a MoO x monolayer on the external zeolite surface (∼4 wt % Mo for the H-ZSM5 used), Mo species sublime as (MoO 3 ) n oligomers or extract Al from the zeolite framework to form inactive Al 2 (MoO 4 ) 3 domains detectable by 27 Al NMR. These (Mo 2 O 5 ) 2+ species reduce to form the active MoC x species during the initial stages of CH 4 conversion reactions. Optimum CH 4 aromatization rates were obtained on catalysts with intermediate Mo contents (∼0.4 Mo/Al), because both MoC x and acid sites are required to activate CH 4 and to convert the initial C 2 H 4 products into C 6+ aromatics favored by thermodynamics.