Methane Dehydrogenation And Aromatization: A Further Study On The Bifunctionality Of Mo/HZSM-5 Catalysts

The direct conversion of methane into aromatics such as benzene and tolueneunder non-oxidative conditions opens a new approach to the catalytic transformationof methane into high value-added chemicals and the comprehensive utilization ofnatural gas. It is generally accepted that the molybdenum-modified zeolite catalysts,which show good catalytic performance in the reaction of methane dehydrogenationand aromatization (MDA), are bi-functional catalysts. The distribution of Br?nstedacid sites on the parent HZSM-5 zeolite was adjusted by two different methods,so-called acid pretreatment and silanation by chemical vapor deposition (CVD),respectively. The catalytic activity and stability in methane dehydro-aromatization(MDA) are remarkably improved on the Mo/HZSM-5 catalysts prepared usingmodified zeolites. Meanwhile, the formation of carbonaceous deposits is effectivelysuppressed on the modified catalysts. It is verified that the Br?nsted acid sitesprovided active sites not only for the polymerization and aromatization of the activeintermediate species, but also for the formation of carbonaceous deposits, which is acrucial factor leading to the deactivation of Mo/HZSM-5 catalyst. Only a smallfraction of Br?nsted acid sites, less than 0.6 per unit cell, is required to accomplishthe aromatization.Based on the results mentioned above, the bifunctionality of the Mo-based zeolitecatalysts was further investigated. A proper prolongation of the calcination time at773 K is in favor of the diffusion and migration of the Mo species into the channels,resulting in a significant decrease in the number of Br?nsted acid sites. In MDA, theMo species associated with the Br?nsted acid sites are more active and stable thanthose nonassociated with the Br?nsted acid sites. The MoOxCy species in face centered cubic (fcc) structure are more active andmore stable for methane activation and benzene formation than those in hexagonallyclose packed (hcp) structure. The pretreatment in hydrogen at a moderatetemperature (623K) can improve the catalytic activity, selectivity to one-ringaromatics and stability of Mo/HZSM-5 catalyst in MDA by enhancing the topotactictransformation of Mo specie from fcc structure to hcp structure.