The effect of dopants on the activity of MoO3/ZSM-5 catalysts for the dehydroaromatisation of methane

The effect of a number of dopants – Co2+, Fe3+, Al3+, Ga3+ and P(V) – on the catalytic activity of MoO3/ZSM-5 dehydroaromatisation catalysts has been investigated. Promotional effects were observed with Fe3+, Al3+ and Ga3+, whereas P(V) added in the form of phosphomolybdic acid decreased the benzene formation rate. Doping with Co2+ was not observed to produce any promotional effects, aside from an initial high conversion and burst of hydrogen which was short-lived. In the case of iron, although the benzene formation rate was close to that for the non-doped parent catalyst in the early stages, benzene formation was enhanced at longer times on stream and the addition of iron was observed to enhance methane conversion, principally through enhanced coke formation. The addition of gallium was found to produce an enhancement of benzene production in the early stages of reaction, but its principal effect was the reduction of hydrogen producing side reactions associated with the formation of coke. In comparing the effects of metal ion addition, it is notable that the impregnation solutions of Fe3+, Al3+ and Ga3+ are acidic. Possible effects of these dopants such as zeolite dealumination, modification of catalyst acidity and the formation of mixed molybdenum containing phases or re-dispersion of the MoO3 phase can be advanced as potential explanations for the observed effects.