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

Abstract

The 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.

A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion

“…The reactions are studied at three distinct active sites as shown in Figure . We distinguish an isolated BAS, a BAS in the vicinity of a LAS, and an isolated LAS, since methylation is also known to occur over Lewis acid sites. , Furthermore the experimental IR spectra in Figure suggest that both BAS and LAS are encountered in every considered catalyst. On an isolated LAS, methanol is activated by adsorption on the LAS of the [M­(μ-OH) 2 M] 2+ moiety, rather than being adsorbed on the weak bridging Brønsted acidic OH site of the metal species.…”

Industrial catalysis: Strategies to enhance selectivity

“…It is widely accepted that an optimal synergy between acid and base sites is required for the efficient side-chain alkylation of toluene with methanol. Alkali-modified zeolite, especially cesium-modified zeolite X or Y, has been recognized as a suitable catalyst for this reaction [22,23]. Generally, the role of acid sites is to adsorb and stabilize toluene, while the activation of methyl in toluene and the dehydrogenation of methanol to formaldehyde occur on the base sites ( Fig.…”

Effects of the Pore Structure and Acid–Base Property of X Zeolites on Side-Chain Alkylation of Toluene with Methanol

“…The classical route for a large-scale styrene production is based on a gas-phase or liquid-phase alkylation reaction, typically starting from benzene alkylation with ethylene using zeolite catalysts (e.g., ZSM-5, Y, MCM-22, Beta, etc. ), followed by the dehydrogenation of ethylbenzene. − One of the most widely investigated alternative routes for styrene synthesis is the side-chain alkylation of toluene with methanol. − This process has the advantages of low-cost raw materials, low reaction temperature, and lower energy consumption compared to the traditional Friedel–Crafts alkylation of benzene with ethylene. − The side-chain alkylation of toluene with methanol is typically catalyzed by microporous X and Y zeolites, both belonging to the faujasite (FAU) family. The catalytic performance of K and Rb ion-exchanged X zeolites for this reaction was first reported by Sidorenko et al in as early as 1967.…”