The location, structure, and role of MoOx and MoCy species in Mo/H-ZSM5 catalysts for methane aromatization

Abstract

The location, structure, and role of MoO x and MoC y species in solid-state exchanged Mo/H-ZSM5 catalysts were probed by infrared, UV-visible, and X-ray absorption (XAS) spectroscopy, and by time-resolved mass spectrometric measurements of the initial products of methane reactions. UV-vis spectra of MoO 3 /H-ZSM5 mixtures showed a shift in edge to higher energies during treatment in air at 773 K, showing the dispersion of MoO 3 crystallites as smaller domains. The intensity of the OH infrared band at 3610 cm −1 decreased only above 773 K, indicating that Mo +6 exchange occurs only after MoO 3 dispersion on external zeolite surfaces. The amount of H 2 O evolved during exchange and the intensity decrease for the OH band give similar values for the residual OH density and these values indicate that each Mo +6 replaces one H + . Analysis of XAS showed that Mo species exist as (Mo 2 O 5 ) +2 dimers interacting with two exchange sites in all samples with Mo/Al F ratios In situ XAS detected the reduction and carburization of these dimers during CH 4 reactions at 930–973 K, concurrently with an increase in methane aromatization rates. Infrared OH bands became weaker during this process, indicating that reduction-carburization processes do not remove Mo species from exchange sites to re-form OH groups. The rate of hydrocarbon formation increased as O-atoms (O/Mo=2.5) are removed from (Mo 2 O 5 ) +2 to form MoC y species detected by XAS, which remain isolated and retain bonding to framework oxygens.