Organic reactions catalysed by sheet silicates: ester production by the direct addition of carboxylic acids to alkenes

Abstract

The addition of aliphatic acids to a wide Variety of alkenes to form esters is heterogeneously catalysed by ion-exchanged sheet silicates. Cation exchange of the silicate is a prerequisite since natural montmorillonite is a poor catalyst for these reactions. Exchange with almost any multivalent ion induces some degree of catalytic activity but Al 3+, Cr 3+and H + ion-exchanged clays proved the most efficient catalysts in the present context. The reactions were carried out in pressure vessels, the reaction temperature range depending on the nature of the alkene employed. Esterification is in competition with alkene oligomerisation, and the synthesis of t-alkyi esters failed at temperatures much above room temperature due to the rapidity of the competing process. All other ester types, however, could be synthesised at temperatures up to 200 °C, and only above this temperature does the competitive process become significant. Initial reaction rates of esterification increase rapidly with temperature although, as would be expected, final equilibrium yields diminish. The number of ester products which may be obtained in a given reaction is determined by the number of stable carbocation intermediates which each alkene can produce. In those reactions where single carbocation intermediates are to be expected, excellent yields of a single ester are often obtained. For example ethyl ethanoate was prepared in 92% yield (based on ethanoic acid) with 96% selectivity, from ethanoic acid and excess ethene using Al 3+ ion-exchanged montmorillonite as the catalyst in a 40 h reaction at 200°C.