Free radical mechanism investigation of the side-chain alkylation of toluene with methanol on basic zeolites X

The side-chain alkylation of toluene represents a novel, environmentally friendly, and low cost route for the production of styrene. However, the yield of styrene produced in this way is currently low, and the mechanism responsible for the side-chain alkylation of toluene is poorly understood. Furthermore, the reason for the higher catalytic efficiency of CsX over NaX and KX remains unclear. In this work, the free radical mechanism of the side-chain alkylation of toluene over basic zeolite X has been elucidated using quantum chemical calculations, together with isotope tracing experiments and the reaction between p-nitrotoluene and methanol. The adsorption isotherm of methanol showed that Cs+ ions could block methanol from accessing the β-cage, which is where the side-chain alkylation reaction occurred. Furthermore, the H–D exchange results between toluene and deuterated toluene (C6D5CD3) showed that CsX was more efficient as a catalyst than KX for the conversion of toluene to the corresponding benzyl radical (C6H5CH2•). These two results therefore explain the higher catalytic activity of CsX towards side-chain alkylation than KX. Based on the free radical mechanism, the selectivity of styrene could be increased from 17.4% to 59.4% using CO2 as carrier gas instead of N2.

Graphical AbstractCs+ complexes with the aromatic ring and attracts the electron, simultaneously, the anion on the framework of zeolite donates the electron to methyl group of toluene. The synergistic effect makes the radical much more stable.Figure optionsDownload full-size imageDownload as PowerPoint slide