The conversion of methanol to hydrocarbons (MTH) catalyzed by zeolites, including methanol to gasoline (MTG), methanol to olefins (MTO), and methanol to aromatics (MTA), has received wide attention in recent years because of its valuable industrial application. Among various mechanisms proposed for the MTH, the hydrocarbon pool (HCP) mechanism has been generally accepted. Derived from the HCP mechanism, the dual-cycle mechanism, which involves integrated aromatic and alkene cycles, could well interpret the formation and reactivity of the HCP species, and the products distribution in MTH. Based on the dual-cycle mechanism, it might be expected that the HCP species and products distribution can be tuned by altering the acidity of the zeolites, therefore, it is possible to enhance the catalytic performance by altering the siting of framework aluminum (AlF) if one knows the catalytic behavior of a specifically sited AlF. In this work, two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, were synthesized with silica sol and tetraethyl orthosilicate, respectively, as the silicon source. Both H-ZSM-5 zeolites exhibit similar acidity, morphology, and textural properties, but quite different in the AlF siting, as determined by UV-vis-DRS of Co(II) ions and 27Al MAS NMR. The AlF of S-HZ-m is enriched in the sinusoidal and straight channels, whereas the AlF of T-HZ-m is concentrated in the channel intersections. In term of catalytic performance in MTH, T-HZ-m shows higher selectivity to ethene and aromatics and larger hydrogen transfer index (HTI) than S-HZ-m, whereas S-HZ-m exhibits higher selectivity to propene and higher olefins. The 13C/12C-methanol-switching experiments indicate that the incorporation of 12C into pentamethylbenzene and hexamethylbenzene is faster on T-HZ-m, whereas the scramble of 12C for C3–C5 olefins is speedier on S-HZ-m. It might be concluded that the AlF in the channel intersections of H-ZSM-5 favors the propagation of aromatic-based cycle, whereas the AlF in the sinusoidal and straight channels promotes the alkene-based cycle. These results clarified the catalytic behavior of the given framework acid sites of H-ZSM-5 in MTH and bring forward an effective approach to improve the catalytic performance by regulating the framework aluminum siting.

Conversion of methanol to hydrocarbons over H-ZSM-5 zeolite-reaction pathway is related to the framework aluminum siting

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.