Effect of Si/Al₂ ratios in Mo/H-MCM-22 on methane dehydroaromatization

A series of Mo(5)/H-MCM-22 catalysts with various Si/Al₂ ratios were applied to methane dehydroaromatization reaction. According to the NH₃ TPD, XPS, TPO, UV Raman and STEM-EDS analysis, it is clearly verified that Brønsted acid site, determined by the Si/Al₂ ratios, stabilizes monomeric Mo oxide species, by facilitating to migrate Mo oxides into the microchannel of zeolite, so that it can influence the distribution of catalytically active Mo oxide species. Combined TPO and UV Raman analysis of post-reaction catalysts obviously confirmed that HT (high temperature) type coke is primarily composed of polyaromatics such as naphthalene and anthracene, which brings about the deactivation of the catalyst during MDA reaction. The formation of HT type coke is mainly affected by the amount of Mo oxides present on the external surface of zeolite, which indicates that the major role of zeolite microchannel is to provide a shape-selective environment during the conversion of methane to benzene. To sum up, an increment in Brønsted acid site enhances the dispersion of Mo oxides, thus leading to the improved methane conversion rate and benzene formation rate while suppressing the formation of HT type coke concomitantly.