Insight into Friedel-Crafts acylation of 1,4-dimethoxybenzene to 2,5-dimethoxyacetophenone catalysed by solid acids—mechanism, kinetics and remedies for deactivation

Abstract

Friedel-Crafts acylation of aromatic ethers is challenging, which frequently encounters rapid catalyst deactivation by the ether. Although H-Y and H-β are known to perform better, there is still deactivation due to both the ether and the acylated ether. In the current work, the synthesis of 2,5-dimethoxyacetophenone, an intermediate used in the production of fine chemicals, was carried out via acylation of 1,4-dimethoxybenzene with acetic anhydride over various solid acid catalysts such as sulfated zirconia, UDCaT-1, UDCaT-5, 20% w/w H₃P₁₂W₄₀/K10, 20% w/w Cs_2.5H_0.5PW₁₂O₄₀/K-10, Amberlyst-15 and Indion-125. The cation exchange resins, Amberlyst-15 and Indion-125, were superior to other inorganic solid acids. A systematic study was undertaken to understand the reaction mechanism and catalyst functioning with Indion-125. The catalyst gets deactivated slowly over repeated use and this was studied independently. The adsorption of reactants and products was studied from pure component solutions and mixtures. The experimental data so generated were used to develop a model, incorporating deactivation. The model fits the experimental data very well. The current work gives an insight into choice of catalyst, kinetic modeling, studies in catalyst deactivation and methods to avoid deactivation.