Toluene alkylation with propylene and ethylene on acidic mordenite zeolite

Abstract

Aromatic alkylation with ethylene and propylene are important reactions to produce cumene and ethylbenzene. Acidic zeolites have been widely used in the petrochemical industry since the 1980s for benzene alkylation due to their high reactivity, non-corrosivity, and environmentally friendly nature compared to traditional liquid acids. There are several challenges in the commercial benzene alkylation processes such as catalyst deactivation via alkene dimerization in addition to energy intensive and costly downstream separation processes primarily due to the high benzene to alkene ratio required to form the desired monoalkylated products. A variety of zeolites have been used for commercial benzene alkylation, however, there is still a lack of literature aimed at understanding how the microporous structures of zeolitic framework and the locations of acidic H+ sites impact their catalytic selectivity and stability. Furthermore, the reaction pathways of benzene alkylation on solid acids have remained uncertain. In this work, we combine kinetic, spectroscopic, and theoretical methods to identify the reaction pathways and to understand how the microporous structure impacts the reactivity and stability, taking the toluene-ethylene reaction as a model reaction. We reveal that exchanging H+ sites in the 8-membered ring (8-MR) significantly improves the stability of acidic mordenite zeolites by lowering the contribution of alkene dimerization reactions that lead to coke formation. By combining kinetic and spectroscopic techniques, we show that protons are saturated with π- bonded toluene, which sequentially react with ethylene to form ethyl toluene. These results can be utilized to provide design strategies for solid acid catalysts used in benzene alkylation with improved stability, selectivity, and reactivity.