Stabilization of intermediate Mo oxidation states by Nb doping enhancing methane aromatization on Mo/HZSM-5 catalysts

Abstract

The dehydroaromatization of the naturally abundant methane is a promising process to produce aromatics and CO_x-free hydrogen. Low-temperature activity, regenerability and time-on-stream stability are fundamental challenges for the industrial use of the commonly studied benzenoid-selective Mo/HZSM-5 catalysts. We report a promotional effect of Nb doping on the activity and stability of Mo/HZSM-5 catalysts between 600 and 700 °C. Nb addition enhances benzene yields measurably at all investigated temperatures. An increased thermal stability of MoNb/HZSM-5 compared to Mo/HZSM-5 catalysts was found upon exposure to multiple consecutive reaction and oxidative treatment cycles, thus resulting in higher restorability of activity which extends the catalyst lifetime. While high-resolution electron microscopy showed homogeneous mixing of Mo and Nb on the catalyst, in situ time-resolved Mo K-edge X-ray absorption spectroscopy, supported by ex situ and time-resolved in situ near-ambient pressure X-ray photoelectron spectroscopy measurements, revealed a distinct interaction between the transition metals, involving a partial reduction of the otherwise stable Nb and concurrent oxidation of Mo which leads to increased stability of Mo⁵⁺/Mo⁴⁺ states. These effects were correlated to the enhanced activity and regenerability of the MoNb/HZSM-5 catalyst for methane dehydroaromatization.