Coke accumulation and deactivation behavior of microzeolite-based Mo/HZSM-5 in the non-oxidative methane aromatization under cyclic CH4-H2 feed switch mode

Abstract

The lifetime deactivation patterns of a microzeolite-based 6wt%Mo/HZSM-5 in the non-oxidative dehydroaromatization of methane were measured first at 1073K and a space velocity of 12,000ml/g-Cat/h in continuous CH4-feeding, cyclic 5min CH4–5min He feed switch and cyclic 5min CH4–5min H2 feed switch modes. Appearance of an obvious inflection point in the course of benzene selectivity under the cyclic CH4-H2 feed switch operation mode revealed the two stages deactivation feature of the catalyst: the deactivation was first slow and then accelerated. TG and TPO measurements of the spent samples were performed and the results exhibited strong similarity in the total coke content and the intensity and area of the low temperature TPO-COx peak. Then, a set of tests under the cyclic CH4-H2 feed switch mode over different time frames, followed by the TPO measurements of the spent samples, was performed to pursue the coke accumulation behavior of the catalyst over its lifetime. The low-temperature burning coke defined by the low-temperature TPO-COx peak was confirmed to be the dominant type of coke, reside on the external surface of the zeolite and in the inner mouth region of the zeolite channels, and accumulate over the catalyst lifetime at a constant rate. In parallel, a constant rate was also observed for the decrease of the benzene formation rate itself over the slow deactivation period. While these observations suggest the possible existence of a cause and effect relationship between the low-temperature burning coke and catalyst deactivation, a detailed discussion was made on the dominant effect of accumulation of graphite-like coke on the slow deactivation of catalyst.