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

Abstract

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.

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

“…Although high Brønsted site densities result in faster deactivation due to faster coking rates, not only Brønsted acid site density but also distribution play an important role. Wang et al 7 showed that the same concentration of Brønsted acid sites can still lead to different lifetime due to an uneven Al distribution and formation of Al pairs. Analysis of Al distribution assessed by UV-Vis of Co-exchanged zeolites shows that this distribution within α-, β-and γ-sites is rather similar (Supplementary Figure 12), excluding the effect of Al location.…”

Progress in Developing a Structure‐Activity Relationship for the Direct Aromatization of Methane

“…The fact that these two structures outperform other structures with similar pore sizes and acid strength for a lot of reactions, like FCC, MTH and alkane/alkene conversion was also assigned to confinement effects. They influence in which configuration reactants interact with the active site and are believed to play a significant role in determining the activity of those two zeolite topologies . Kosinov et al .…”

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…The two materials showed similar intrinsic acidity, morphology and textural properties. Yet, the conversion of methanol by T‐HZ gives higher selectivity to ethylene and aromatics than by S‐HZ, which, in turn, provides higher selectivity to propene and higher olefins . Similarly, the crucial role of the pore size and shape for the methanol to olefin (MTO) conversion was revealed by DFT calculations .…”