Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction Pathway Is Related to the Framework Aluminum Siting

As the conversion of methanol to olefins (MTO) over a zeolite catalyst is conducted on acid sites derived from framework aluminum (Al-F), it is possible to enhance the catalytic performance by altering the siting of Al-F if one knows the catalytic behavior of specified Al-F located at certain sites. 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 series of H-ZSM-5 zeolites exhibit similar acidity, morphology, and textual properties. However, they are quite different with respect to Al-F siting, as determined by UV-vis-DRS of Co(11) ions and Al-27 MAS NMR; Al-F of S-HZ-m is enriched in the sinusoidal and straight channels, whereas Al-F of T-HZ-m is concentrated in the channel intersections. When they are used as the catalyst in MTO, T-HZ-m gives higher selectivity to ethene and aromatics and a larger hydrogen transfer index (HTI) than S-HZ-m, whereas S-HZ-m exhibits higher selectivity to propene and higher olefins. Moreover, the C-13/C-12-methanol-switching experiments indicate that the incorporation of C-12 into pentamethylbenzene and hexarnethylbenzene is faster on T-HZ-m, whereas the scramble of C-12 for C-3-C-5 olefins is speedier on S-HZ-m. All of these illustrate that Al, in the channel intersections of H-ZSM-5 is probably more favorable to the propagation of the aromatic-based cycle, whereas Alp in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. These results help to clarify the catalytic behavior of given framework acid sites of H-ZSM-5 in MTO and then bring forward an effective approach to improving the catalytic performance by regulating the framework aluminum siting.