Raman and X-Ray Absorption Studies of Mo Species in Mo/H-ZSM5 Catalysts for Non-Oxidative CH4 Reactions

Abstract

The structure of MoOx and MoCy species in Mo/H-ZSM5 after exchange and during CH4 reactions was probed by in situ Raman and X-ray absorption spectroscopy. Raman spectra of physical mixtures of H-ZSM5 and MoO3 powders initially showed strong Raman bands characteristic of bulk MoO3. The intensity of Raman bands for Mo-O-Mo decreased relative to those for Mo=O bonds during treatment in air at 773 K, suggesting that MoO3 spreads as (MoO3)n oligomers on external zeolite surfaces. The Raman bands for MoO3 crystallites became much weaker after treatment in air at 973 K; this occurred concurrently with H2O evolution, indicating that dispersed MoOx species exchanged with acidic OH groups in the zeolite. Weak bands appeared at 970 and 1045 cm−1; they were assigned to stretching modes in exchanged MoOx species. The X-ray near-edge spectrum of MoO3/H-ZSM5 mixtures gradually evolves to one characteristic of tetrahedral Mo6+ centers in bulk MgMo2O7 as H2O forms during treatment at 773–973 K, confirming that exchange occurs in this temperature range. Each Mo6+ replaces one H+ at all Mo contents (Mo/Al=0.11–0.37). The radial structure function derived from the absorption fine structure also evolves from that for bulk MoO3, to that expected of dispersed MoO3 without long-range periodicity, to one described using multiple scattering simulations as a (Mo2O7)2− dimer resembling MgMo2O7 with two of the O atoms located in the zeolite framework. This structure is consistent with the exchange stoichiometry from H2O desorption and with the removal of 2.5 O/Mo during reduction-carburization. The required number of Al pairs in the samples to accommodate the dimer structures is within the range predicted by Monte Carlo calculation of Al statistical distribution. The absorption edge and near-edge features in exchanged MoOx/H-ZSM5 evolve during CH4 reactions at 973 K to resemble bulk Mo2C without long-range periodicity, with the concurrent evolution of CO, CO2, and H2O and an increase in the rate of hydrocarbon formation. Multiple scattering analysis of a MoCx cluster bonded to an O atom leads to a radial structure function in excellent agreement with experiment.