Kinetics of Friedel–Crafts benzoylation of veratrole with benzoic anhydride using Cs_2.5H_0.5PW₁₂O₄₀/K-10 solid acid catalyst

Abstract

Friedel–Crafts alkylation and acylation reactions are ubiquitous in several industries and use highly polluting liquid acids as catalysts. Production of ketones and subsequently secondary alcohols is achieved by Friedel Crafts acylation using various acylating agents. Benzophenone and its family are used in intermediates, fine chemicals, dyes and pharmaceuticals industries. Use of homogeneous catalysts in benzoylation of veratrole (1,2-dimethoxybenzene) to produce 3,4-dimethoxybenzophenone (DMBP) is highly polluting. The current work deals with solvent free reaction of veratrole and benzoic anhydride to produce 3,4-DMBP by using different solid super acids such as 20% (w/w) Cs_2.5H_0.5PW₁₂O₄₀/K-10 clay (Cs-DTP/K-10), UDCaT-5, sulfated zirconia (S-ZrO₂), unsupported Cs-DTP and montmorillonite K-10 clay. Cs-DTP/K-10 was found to be best catalyst for this reaction. At a mole ratio of 1:5 of benzoic anhydride to veratrole, the solvent free reaction gave 89.3% conversion at 90 °C in 2 h at catalyst loading of 4% (w/v). The selectivity was 100%. The catalyst is robust and reusable. It was characterized before and after use to find its fidelity. The effect of different parameters on the rate of reaction and conversion of veratrole was studied systematically to deduce kinetics of the benzoylation reaction. The reaction follows Eley–Rideal mechanism with weak adsorption of benzoic anhydride. The apparent energy of activation is 20.8 kcal/mol.