The chemical nature of carbonaceous deposits and their role in methane dehydro-aromatization on Mo/MCM-22 catalysts

Abstract

Various carbonaceous deposits are formed during the course of methane dehydro-aromatization (MDA) under non-oxidative conditions on Mo/MCM-22 catalysts. These carbon species were investigated by various temperature-programmed techniques such as TPH and TPCO 2, combining with TPO and TGA results in order to reveal their chemical nature and determine their amounts. The TPO profiles recorded from coked Mo/MCM-22 catalysts show two temperature peaks: one at about 756 K and the other at about 876 K. The coke amounts related to these two peaks were determined on the basis of the corresponding corrected and deconvoluted TPO profiles, combining with the TGA profiles concerned. The results of TPH and TPCO 2 suggested that both H 2 and CO 2 have the ability to eliminate the carbon species corresponding to both the high and low temperature peaks. The TGA results of the Mo/MCM-22 catalysts with different Mo loading revealed that the total amount of coke formed was the largest on a 6Mo/MCM-22 during TPSR with CH 4 for 3 h. Meanwhile, a kinetic analysis of the coke burning-off process revealed that there are two kinds of carbonaceous deposits: one is mainly located on the Brönsted acid sites and the other on the Mo species. The former is inert and causes deactivation of the catalyst for MDA, while the latter may be active and can be hydrogenated with H 2 to form ethylene and benzene. Therefore, the carbonaceous deposits formed in the course of MDA play a double role.