Molybdenum Speciation and its Impact on Catalytic Activity during Methane Dehydroaromatization in Zeolite ZSM-5 as Revealed by Operando X-Ray Methods

Abstract

Combined high-resolution fluorescence detection X-ray absorption near-edge spectroscopy, X-ray diffraction, and X-ray emission spectroscopy have been employed under operando conditions to obtain detailed new insight into the nature of the Mo species on zeolite ZSM-5 during methane dehydroaromatization. The results show that isolated Mo-oxo species present after calcination are converted by CH4 into metastable MoCxOy species, which are primarily responsible for C2Hx/C3Hx formation. Further carburization leads to MoC3 clusters, whose presence coincides with benzene formation. Both sintering of MoC3 and accumulation of large hydrocarbons on the external surface, evidenced by fluorescence-lifetime imaging microscopy, are principally responsible for the decrease in catalytic performance. These results show the importance of controlling Mo speciation to achieve the desired product formation, which has important implications for realizing the impact of CH4 as a source for platform chemicals. "Watch" and learn: Combined high-resolution fluorescence detection X-ray absorption near-edge spectroscopy (HERFD-XANES), X-ray diffraction (XRD), and X-ray emission spectroscopy under real operando conditions provided detailed insight into the nature of the Mo species on zeolite ZSM-5 during methane dehydroaromatization, demonstrating the importance in controlling Mo speciation to influence selectivity towards light olefins and/or aromatic species.