Methane Dehydroaromatization Reaction On Mo-based Catalyst: Effect Of The Catalyst Acidity And Porosity

Methane dehydroaromatization (MDA) under nonoxidative condition is one ofthe important reactions in the field of direct conversion of methane. It is of significance for both the utilization of natural gas and the understanding of the nature of C-H bond activation in methane molecule. In this dissertation the effect of the zeolite acidity and porosity on the catalytic performance of MDA on Mo-based catalysts is investigated.The number of the Bronsted acid sites and the value of Mo/[H+] per unit cell on the Mo/HZSM-5 catalysts with different ratios of SiO2/M2O3 and the Mo/HZSM-5 catalysts with different Mo loadings are measured by the 1H MAS NMR experiments. From the results it is concluded that the migration ability of Mo species into the zeolite channels strongly depends on the number of the Bronsted acid sites per unit cell. The in situ 1H MAS NMR experiments also demonstrate that the Bronsted aicd sites, once replaced and occupied by Mo species, can not regenerate during the induction period of the reaction. Combining the 1H MAS NMR results with the reaction results we find that the Mo/HZSM-5 catalyst with the value of Mo/[H+] close to 1.0 shows the best catalytic performance of MDA reaction. The cooperated function of the Mo species with the Bronsted acid sites is the basis of the MDA reaction.The catalyst porosity can be modified by alkali treatment. Under an appropriate condition of the alkali treatment the micropore structure of the HZSM-5 and MCM-22 zeolites is not affected obviously, but some new mesopores can be developed. The Mo based catalyst derived from the appropriate alkali treated zeolite exhibits goodcatalytic performance due to the coexistence of micropores and mesopores, which is beneficial to the mass transfers of the aromatic product formed during the MDA reaction, thus increasing the reaction stability.The effect of synthesis condition of the HZSM-5 zeolite on the MDA reaction performance is also investigated. The catalytic performance of MDA on a Mo/HZSM-5 catalyst derived from the HZSM-5 zeolite prepared from soluble glass is better than that derived from the HZSM-5 zeolite prepared from silicon gel.