Hydrogen transfer versus olefins methylation: On the formation trend of propene in the methanol-to-hydrocarbons reaction over Beta zeolites

Abstract

Beta zeolites with similar textural properties but variable Brønsted acid site (BAS) densities in the range of 25.5–203.8 μmol g−1 were fabricated and investigated as catalysts in the methanol-to-hydrocarbons reaction. The intrinsic influence of BAS amount, BAS density and methanol conversion on the reaction mechanism was investigated. Interestingly, methanol conversion depends on the BAS amount regardless of the BAS density. The increase of methanol concentration could modulate the product distribution by promoting the methanol-induced hydrogen transfer reaction and enhancing the olefins methylation as well as inhibiting the cracking of higher olefins. As the BAS density decreases from 203.8 to 56.6 μmol g−1, hydrogen transfer reaction is gradually restrained while the olefin interconversion is enhanced, leading to gradual replacement of aromatics-based cycle by the olefins-based cycle and thus increasing the propene selectivity. However, as BAS density further decreases from 56.6 μmol g−1, the propene selectivity declines because the methylation route in the olefins-based cycle is much more favoured than the cracking route. After detailed investigation of the deactivation behaviours of involved Beta zeolites, it has been found that the accumulation of long-chain alkanes and large polycyclic aromatics is responsible for the catalyst deactivation. These results provide further insights into the effect of BAS density on the reaction mechanism and product selectivity.