Abstract:

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.

Key words: Toluene, Side-chain alkylation, Radical mechanism, Zeolite X, Styrene, Isotope trace