Effect of addition of a second metal in Mo/ZSM-5 catalyst for methane aromatization reaction under elevated pressures

Non-oxidative methane dehydro-aromatization (MDA) over transitional metal-doped zeolites (i.e. ZSM-5) has a potential to be an alternative for methane transformation processes to higher hydrocarbons. Despite of systematic studies of the effects of the catalyst composition, preparation procedure, pretreatment, and the reaction conditions systematically followed since the pioneering works of the Chinese group in 1993, the suppression of fast deactivation of catalyst by coke deposition was still not solved successfully. In the present work the long-term MDA experiments at 973 K and elevated pressures up to 600 kPa were carried out with metal-doped Mo/ZSM-5 catalysts. Several Mo-based catalysts doped with various amount of Co, Mn, and Ce were prepared and tested in the MDA reaction. Experimental results are presented in terms of profiles of methane conversion as well as ethylene, benzene, and toluene concentrations as a function of time on stream, accompanied by the TGA analysis of the coke residue accumulated on the spent catalysts. The amount and nature of coke deposits which were influenced by introduction of different metals were also investigated. The Co content of 0.8 wt% in Co-Mo/ZSM-5 catalyst was evaluated as optimal for total benzene production. Prolonged experiments showed the evident positive effect of higher reaction pressure on catalyst stability for all tested catalysts. Moreover, the Co-Mo/ZSM-5 catalyst showed better resistance to the deactivation by coke formation in comparison to Mo/ZSM-5 and M-Mo/ZSM-5 (where M = Mn, Ce) catalysts. In addition, the Ce-modified catalyst showed comparable higher production of toluene at elevated pressures. (C) 2015 Elsevier B.V. All rights reserved.