A clue to exploration of the pathway of coke formation on Mo/HZSM-5 catalyst in the non-oxidative methane dehydroaromatization at 1073 K

Experimental investigation on the lifetime performance and coking behavior of a 6 wt% Mo/HZSM-5 catalyst in the non-oxidative methane dehydroaromatization was conducted in a three layers fixed-bed mode and at 1073 K and a space velocity of 5000 mL/g/h. Characterization of the coke in all spent samples recovered after different periods of the reaction was performed using TG, XPS and TPO techniques. The time-dependence of the catalytic performance revealed that the test catalyst undergoes three stages deactivation over its lifetime, the benzene selectivity remains high and constant in the second deactivation stage and then decreases very rapidly from 64 to 18% in the last stage, and the C2H4 formation rate keeps increasing in a wide region covering the whole second stage and the first one third of the last stage. On the other hand, quantification of the coke contents of the spent samples and their characterization revealed that the average coke formation rate in the last deactivation is obviously higher than that recorded over the second stage and the formation of the aromatic type of coke inside the zeolite channels is primarily accelerated in the last stage. Thus, both observations together lead to a conclusion that C2H4, which has a smaller molecular diameter than benzene and is allowed to form inside the narrowed zeolite channels, is the main source of coke formation in the last stage of catalyst deactivation. C2H4 has been well recognized to form throughout the whole course of the reaction, and therefore the present observations are expected to provide a clue to exploration of the pathway that dominates the coke formation in the period of the benzene selectivity remaining constant (the second stage of catalyst deactivation) as well. (C) 2014 Elsevier B.V. All rights reserved.