One-pot synthesis of [Mn,H]ZSM-5 and the role of Mn in methanol-to-propylene reaction

A Mn-modified ZSM-5 zeolite (Mn-HZ) was synthesized using a quasi-solid-phase method, and its physicochemical properties were characterized. The presence of tetracoordinated framework Mn(III) was confirmed using ultraviolet–visible and Raman spectroscopy measurements, X-ray photoelectron spectroscopy, and H2-temperature programmed reduction, and the results suggest that Mn entered the framework structure of ZSM-5. Crucially, the introduction of Mn increased the weak Brønsted acid site content and decreased the strong acid site content and resulted in an increase in the amount of framework Al in the straight zeolite channels. During application for the methanol-to-propylene reaction (MTP), Mn-HZ produced fewer aromatics and had a lower carbon deposition rate than the unmodified zeolite, resulting in a longer catalyst lifetime and higher selectivity for propylene and butene. In addition, a mechanism for the MTP reaction is proposed based on the detection of ethylene in high concentrations in the initial stages of the reaction, suggesting that it could be the initial hydrocarbon pool species. Overall, our results indicate that Mn-HZ favored the olefin-based cyclic mechanism to the aromatic-based mechanism.