Impact of Tungsten Loading on the Activation of Zeolite-Based Catalysts for Methane Dehydroaromatization.

To improve the performance of zeolite-based catalysts for the methane dehydroaromatization (MDA) reaction, it is of importance to understand the nature of the catalytically active phase. Although many studies have been devoted to unraveling the structure of the active site, there is still no consensus. Monomeric, dimeric, and/or clusters of molybdenum oxide or tungsten oxide are proposed precatalyst structures. This precatalyst is activated under reaction conditions, to form (oxy)carbidic species which are believed to be the active site. In this study, we investigated the effect of tungsten dispersion on the activation of W/ZSM-5 catalysts. We observed unexpected long activation times that could be shortened by inert or reductive pretreatment. Based on our investigations, we hypothesize that W/ZSM-5 catalysts with low weight loadings (i.e., 2 wt %) cannot be activated due to the presence of monomeric tungsten. For catalysts with medium weight loadings (i.e., 5 and 7 wt %), restructuring of the tungsten is required for the formation of the active site, which can be achieved through performing a thermal pretreatment. For higher weight loadings (i.e., 10 wt %), reduction plays a key role in the activation of the catalyst. We show that the activation of the catalyst is impacted by the precatalyst structure. These insights aid in the development of suitable activation treatments which could save time and energy if the reaction would be performed at an industrial scale.