Understanding W/H-ZSM-5 catalysts for the dehydroaromatization of methane

Abstract

Tungsten is the most interesting and promising metal to replace molybdenum in methane dehydroaromatization (MDA) catalysis. Located in the same column of the periodic table, tungsten displays similar chemical features to molybdenum (i.e., formation and stability of oxidation states, acidity of trioxides, tendency toward formation of polynuclear species, atomic radii, ionic radii, etc.) but shows higher thermal stability. The latter could be an advantage during high-temperature reaction–regeneration cycles. However, the MDA activity of W–ZSM-5 catalysts is much lower than the activity obtained with their Mo counterpart. In order to gain a further understanding of such differences in catalytic activity, we present a thorough investigation of the effect of dispersion and distribution of W sites on the zeolite, their relation with catalytic activity, and the temporal evolution of dispersion with reaction–regeneration cycles. The structure of W sites is elucidated with advanced and detailed characterization techniques, including operando X-ray absorption spectroscopy (XAS). The information obtained can help the catalysis community to design better W catalysts for MDA and other reactions (i.e., metathesis, hydrocarbon cracking, hydrodesulfurization, isomerization, etc.) where this is the metal of choice.