Effect Of Steam-treatment Of Mo/HZSM-5 Catalysts On Their Catalytic Performance Of Methane Dehydroaromatization Reaction And The Nature Of Carbonaceous Deposits Formed During The Reaction

Among those tested catalysts for methane dehydroaromatization (MDA) reaction, Mo/HZSM-5 shows obvious superiority in catalytic performance, however, it's also prone to deactivation due to the formation of heavy carbonaceous deposits during the reaction. Therefore, the work done in this thesis is mainly foucused on investigating the nature of coke deposits and enhancing the catalytic stability of Mo/HZSM-5 catalyst.A facile and effective method, post-steam treating, is used to modify Mo/HZSM-5 catalyst before running MDA reaction, and the result of which demonstrates that catalytic stability and reaction activity of Mo/HZSM-5 are improved obviously after post-steam-treatment. Further studies by using XRF, XRD, XPS, TG, NMR indicates that more molybdenum species will migrate into HZSM-5 channels accompanying a decrease in Bronsted acid sites over Mo/HZSM-5 catalyst during the post-steam treatment process and the amount of coke deposits is suppressed greatly as well.The morphology of coke deposits formed over Mo/HZSM-5 and Mo/HMCM-22 are monitored by using electron microscopy. It's reasonable to conclude that there are different catalytic behaviors for Mo species over HZSM-5 and HMCM-22 zeolites due to the fact that carbon nanotubes can be formed on the external surface of Mo/HZSM-5 catalyst while not on Mo/HMCM-22 catalyst during MDA reaction.Hydrogenation activities of various carbonaceous species formed over Mo/HZSM-5 catalyst are compared under different hydrogenation conditions. The results show that hydrogenation reaction will easily occur for hydrogen-poor carbonaceous species, while the carbon nanotubes at the external surface and the carbonaceous species over Bronsted acid sites will react with H2 under more rigorouscondition, i.e. at higher temperature.