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As the conversion of methanol to olefins (MTO) over a zeolite catalyst is conducted on acid sites derived from framework aluminum (AlF), it is possible to enhance the catalytic performance by altering the siting of AlF if one knows the catalytic behavior of specified AlF 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 AlF siting, as determined by UV–vis–DRS of Co­(II) ions and 27Al MAS NMR; AlF of S-HZ-m is enriched in the sinusoidal and straight channels, whereas AlF 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 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. All of these illustrate that AlF in the channel intersections of H-ZSM-5 is probably more favorable to the propagation of the aromatic-based cycle, whereas AlF 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.

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Polymethylbenzene (polyMB) and alkene cycles are considered as two main routes forming light olefins in the process of methanol to olefins (MTO); however, the contribution that each cycle makes to MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the catalytic roles that the polyMB and the alkene cycles may play in forming ethene and propene from methanol in MTO over H-ZSM-5. The results demonstrated that ethene and propene can be produced in nearly the same probability via the polyMB cycle, as they have a very close free energy height as well as a similar free energy barrier for the rate-determining steps. Via the alkene cycle, however, propene is the dominant product, because the methylation and cracking steps to get propene have a much lower free energy barrier in comparison with those to form ethene. As a result, ethene is predominantly formed via the polyMB cycle, whereas propene is produced via both the polyMB and the alkene cycles. The contribution of the alkene cycle is probably larger than that of the polyMB cycle, resulting in a high fraction of propene in the MTO products. Meanwhile, both cycles are interdependent in MTO, as the aromatic species generated by aromatization via the alkene cycle can also serve as new active centers for the polyMB cycle, and vice versa. Moreover, the catalytic activity of H-ZSM-5 zeolite is directly related to its acid strength; weaker acid sites are unfavorable for the polyMB cycle and then enhance relatively the contribution of the alkene cycle to forming light olefins. These results can well interpret the recent experimental observations, and the theoretical insights shown in this work may improve our understanding of the MTO mechanism, which are conducive to developing better MTO catalysts and reaction processes.

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ZSM-5 and ZSM-11 zeolites are similar in their crystalline framework structure, acidity, morphology, and textual properties but considerably different in their catalytic performance for conversion of methanol to olefins (MTO). Such an unexpected but exciting finding was extensively explored by various techniques and density functional theory calculations. A detailed investigation shows that it is the different Al distribution in the ZSM-5 and ZSM-11 framework that causes the significant difference in MTO catalytic performance. In ZSM-5, Al atoms are enriched in the intersection, whereas in ZSM-11, the Al atoms are concentrated in the straight 10-membered ring channel. The acid sites located in the intersection enhance the arene-based cycle that generates more ethene, alkanes, and aromatics. Nevertheless, these hydrocarbon molecules can easily diffuse out of the zeolite channel, hence retarding the deposition of carbonaceous materials and increasing catalytic stability. However, the acid sites located in the straight channel promote the alkene-based cycle, thus preferentially generating higher olefins that could transform into aromatics and carbon precursors that have difficulty in diffusing out of ZSM-11. The fast accumulation of coke species leads to its short catalytic lifetime. Via a shift of the Al atoms of ZSM-11 from the straight channel to the intersection by incorporation of appropriate amounts of B or alteration of silica and alumina sources and addition of sodium cations, its MTO catalytic performance (activity, selectivity, and stability) becomes highly comparable to that of ZSM-5. The insights attained in this work not only help to clarify the relationship of Al siting in zeolite with its MTO catalytic performance but also provide a cue for improving the catalytic properties of zeolites by regulating the sitings of active sites in lattice sites.

Methods to synthesize zeolites with different crystal habits and assemble zeolite crystals into specific structures are reviewed for the rational design of zeolite particle morphologies.

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