Relation of Catalytic Performance to the Aluminum Siting of Acidic Zeolites in the Conversion of Methanol to Olefins, Viewed via a Comparison between ZSM-5 and ZSM-11

Abstract

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.