Dehydroaromatization of methane under non-oxidative conditions over bifunctional Mo/ITQ-2 catalysts

Bifunctional Mo/ITQ-2 catalysts have been used for the methane dehydroaromatization (MDA) reaction. The Si/Al ratio, and thus the Brønsted acidity, of the zeolite affected the conversion, the formation rate of aromatics, and the selectivity to the different products (CO, C2, benzene, toluene, and naphthalene). Maximum activity and aromatics productivity were obtained for the zeolite with the lowest Si/Al ratio (Si/Al = 15) presenting the highest Brønsted acidity, while maximum benzene selectivity (∼70%) occurred for the zeolite with Si/Al = 25. The selectivity to naphthalene increased with decreasing the Si/Al ratio, that is, with increasing the density of surface Brønsted acid sites. At comparable Si/Al ratio, Mo/MCM-22 was more active and selective to benzene than Mo/ITQ-2. Reduction of surface acidity in ITQ-2 by treatment with oxalic acid significantly reduced the formation of naphthalene, resulting in enhanced benzene selectivity (∼75%). The acid-treated ITQ-2 was even more selective to benzene than the Mo/MCM-22 catalyst (∼65%). Molecular dynamics simulations were performed at the MDA reaction temperature (973 K) to study the diffusion of naphthalene in the two independent pore systems of MCM-22. The theoretical results allowed explaining the differences in selectivity between the two zeolite structures and the effect of surface dealumination in ITQ-2.