On the Induction Period of Methane Aromatization over Mo-Based Catalysts

The behavior of different species during the temperature-programmed surface reaction (TPSR) of methane over various catalysts is traced by an online mass spectrometer. It is demonstrated that the transformation of MoO3 to molybdenum carbide hinders the activation of methane as well as the succeeding aromatization in the TPSR. If this transformation process is done before the reaction, the temperature needed for methane activation and benzene formation will be greatly lowered (760 and 847 K, respectively). On the basis of comparison of the catalytic behavior of molybdenum supported on different zeolites, it is suggested that the initial activation of methane is the rate-determining step of this reaction. For the cobalt catalysts supported on HMCM-22 or Mo catalysts supported on TiO2, no benzene formation could be observed during the TPSR. However, the prohibition of benzene formation is different in nature over these two catalysts: the former lacks the special properties exhibited by molybdenum carbide, which can continuously activate methane even when multiple layers of carbonaceous species are formed on its surface, while the latter cannot accomplish the aromatization reaction since there are no Brønsted acid sites to which the activated intermediates can migrate, although the activation of methane can be achieved on it. Only for the catalysts that possess both of these properties, together with the special channel structure of zeolite, can efficient methane aromatization be accomplished.