Effect of Co additive on formation of coke deposits in methane dehydro-aromatization over Mo/HZSM-5 catalyst

Methane dehydro-aromatization (MDA) in the absence of gas-phase oxygen is one of the promising routes for direct conversion of methane into fuels and chemicals, particularly into aromatics such as benzene and toluene. MDA under non-oxidative conditions was carried out over Co-Mo/HZSM-5 catalyst, and the result suggested that the addition of Co dramatically improved the stability of Mo/HZSM-5 catalyst. Furthermore, the chemical nature and physical properties of the coke deposits formed on Co-Mo/HZSM-5 catalyst were studied by various temperature-programmed surface reactions (TPSR), such as TPH, TPCO2 and TPO, combining with TGA and TEM. The results were compared with those obtained over Mo/HZSM-5 catalyst. The TPO profiles recorded during the procedure of coke burning-off showed two peaks. The TPH experiments had effect on the coke burning-off only at high temperature. However, TPCO2 mainly resulted in the diminishing of the coke burning-off at low temperature. The TEM pictures gave evidence that a lot of filamentous carbon formed on the surface of coked catalysts during the course of TPSR, and this kind of coke deposits was not a contributor to the deactivation of catalyst. Hydrogen had no ability to react with carbon filaments, but TPCO2 experiment could eliminate them completely.