Brønsted Acid-Site Density Controls the Mechanistic Cycle and Product Selectivity in the Methanol-to-Hydrocarbons Reaction in BEA Zeolite

Abstract

In this work, we show that the acid-site density controls the dominant cycle during the methanol-to-hydrocarbons reaction on beta zeolite. Our experimental evidence is based on the study of beta zeolites with very similar diffusional pathways and different aluminum content. High selectivity to propylene was observed for samples with low Brønsted acid-site density, which is a consequence of the promotion of the olefinic cycle. Our results also confirm that the production of ethylene via the olefinic cycle is negligible. In contrast, high ethylene and aromatics are found at a high Brønsted acid-site density, highlighting the predominancy of the aromatic cycle. Operando UV-vis data show that monoenylic carbocationic species predominate on the olefinic cycle, whereas the aromatic cycle is dominated by polyalkylated monoaromatics. Analysis of the spectroscopy data also shows a linear correlation of the formation of polyaromatic species with the Brønsted acid-site density.