A green two-step process for adipic acid production from cyclohexene: a study on parameters affecting selectivity

In this paper, we report about an investigation of the effect of reaction parameters on catalytic performance in a two-step process aimed at the synthesis of adipic acid from cyclohexene. In the first step, cyclohexene reacts with an aqueous solution of hydrogen peroxide, under conditions leading to the formation of trans-1,2-cyclohexandiol as the prevailing product; the reaction is catalysed by tungstic acid, in the presence of phosphoric acid and of a PT agent. In the second step, 1,2-cyclohexandiol is then oxidized with air, in the presence of an heterogeneous catalyst made of alumina-supported Ru(OH)3. This process configuration is aimed at using the minimal amount of the costly hydrogen peroxide, since only one mole is theoretically needed. The first step afforded very high yield to the glycol, by means of a careful control of reaction parameters, and using only a slight excess of hydrogen peroxide. However, the second step turned out to be the more critical one, since the selectivity to adipic acid was very low because of the concomitant occurrence of side reactions. The latter were due to the reaction conditions used, which in turn were necessary for the activation of cyclohexandiol.