Operando UV–Vis Spectroscopy on the Role of Metals in the Methanol-to-Hydrocarbons Reaction Zeolite-Based Catalysts

Abstract

The methanol-to-hydrocarbons (MTH) process is a viable alternative to crude oil-based chemical processes for the production of essential chemical building blocks needed to produce our fuels, chemicals, and plastics. Zeolites are commonly used as MTH catalysts, and their performance is closely tied to their acid sites. Here, we explore the impact of the introduction of metals (Mg2+, Ca2+, Mn2+, and Zn2+) to zeolites ZSM-5, SSZ-13, and Beta. For the medium-pore-size zeolite ZSM-5, these metals reduce the zeolite's Brønsted acidity and generate different Lewis acid sites, altering catalyst lifetime and selectivity. Operando UV–vis diffuse reflectance spectroscopy (DRS) revealed that the introduction of Mg2+, Ca2+, and Mn2+ ions to zeolite ZSM-5 leads to an increased propylene selectivity due to a decreased concentration of neutral aromatic compounds, leading to a preference for the olefinic cycle. A more nuanced behavior was observed for the introduction of Zn2+, and depending on its loading, it can either favor the aromatic or olefinic pathway. Similar trends were observed in small-pore-size SSZ-13, however, less pronounced, and large-pore-size zeolite Beta. While Mg2⁺ also favors the olefinic cycle in zeolite Beta, this does not impact catalyst lifetime, suggesting that coking may not be the main deactivation mechanism.