Roles of Hydroxyl Groups During Side‐Chain Alkylation of Toluene with Methanol over Zeolite Na‐Y: A Density Functional Theory Study

Abstract

AbstractThe side‐chain alkylation of toluene with methanol over alkali‐cation‐containing zeolite Y is an important reaction for industrial production of styrene, but the exact mechanism of this reaction is still unclear. The most accepted opinion is that the Lewis acid–base sites in zeolite Y activate the transformation from methanol to formaldehyde, the side‐chain alkylation of toluene with formaldehyde, and the formation of 2‐phenylethanol and styrene afterwards. In this study, we investigate the roles of various types of hydroxyl groups that could possibly exist in zeolite Na‐Y during this reaction, including the Brönsted acid sites and the terminal Al—OH and Si—OH groups, respectively. Through density functional theory (DFT) calculations, we find that the Brönsted acid sites in Na‐Y may catalyze the ring alkylation of toluene and be responsible for the formation of xylene, a side product discovered in experiments. More importantly, we find, for the first time, a new reaction pathway from 2‐phenylethanol to styrene over various types of hydroxyl groups in Na‐Y, which is kinetically more favorable than the conventional pathway. According to our calculation results, the most possible mechanism for this styrene production process may involve reactions over both the Lewis acid–base sites and the hydroxyl groups in Na‐Y.