Activation of Mo/HZSM-5 for methane aromatization

The effect of Mo/HZSM-5 pretreatment at 973 K in inert （He）, oxidizing （artificial air）, and carbu- rizing （CH4/He mixture） atmospheres on its performance in non-oxidative methane dehydroaroma- tization （MDA） was investigated. The effect of post-synthesis silylation on deactivation of external acid sites was also studied. Precarburization resulted in increased aromatic selectivity and im- proved catalyst stability. The benzene selectivity was the highest for the silylated Mo/HZSM-5 cata- lyst （benzene ＋ naphthalene selectivity after 1 h on stream was close to 100%）. The deactivation of precarbnrized zeolites was less pronounced than that of zeobtes heated in air or He. During heating in air or He, larger fractions of the molybdenum oxide species diffused into the micropores than during heating in methane. Carburization of the molybdenum oxide species in the micropores dur- ing MDA resulted in the formation of molybdenum carbide particles, and these contributed to pore blocking, making the Bronsted acid sites inaccessible. The formation of molybdenum carbides dur- ing heating in methane resulted in a less mobile Mo phase. It is argued that the presence of molyb- denum carbide particles in the micropores contributes to rapid catalyst deactivation, in addition to the formation of hard coke on the external surface.