Cation-exchange resin-catalysed acylations and esterifications in fine chemical and perfumery industries

Abstract

Acylation and esterification reactions are typically carried out by using homogeneous acids as catalysts which can be profitably replaced with heterogeneous solid acid catalysts to develop green chemistry. Solid acids that give the desired level of activity but which can be easily removed from the reaction mixture with no residual inorganic contamination of the organic products offer obvious advantages over existing methods. This contribution is concerned with eco-friendly synthesis of some commercially valuable products such as p-methoxyacetophenone, dimethyl phthalate, diethyl phthalate, methyl anthranilate, methyl salicylate, and methyl p-hydroxybenzoate (methyl paraben). The specifications require the esters to be free of halide impurities on ppm level for perfumery use, and thus use of chlorine-containing agents or catalysts must be avoided. The following catalysts were evaluated:  Amberlyst-36, Bayer K2441, Amberlyst-15, Dowex 50Wx8, Indion-130, Deloxane ASP, Filtrol-24 clay, K-10 Montmorrilonite clay, and sulphated zirconia. Anisole was acylated with acetic anhydride to get 100% selectivity of p-methoxyacetophenone, and Amberlyst-36 was found to be the most active and reusable catalyst. A kinetic model is also presented. Amberlyst-36 was also the most active catalyst for the esterification reactions.