Oxidation of Citronellal to Citronellic Acid by Molecular Oxygen Using Supported Gold Catalysts

The oxidation of citronellal to citronellic acid was studied using molecular oxygen as oxidant and gold‐containing supported catalysts under aqueous conditions. The reactions were carried out at 60–90 °C, with 200 Nml min −1 O 2 and at pH values from 9 to 12. The alumina‐ or titania‐supported catalysts were synthesized according to the deposition–precipitation procedure using urea or NaOH. Mechanistic studies have revealed that radical‐initiated reactions lead to undesired by‐products especially at pH<9, that is, the CC bond is attacked and a diol is primarily formed probably via an epoxide intermediate. This side reaction can be suppressed to a large extent by increasing the pH to 12 and by raising the catalyst/oxygen ratio. Furthermore, detailed studies on the influence of reaction time, pH value, reactant concentration and amount of catalyst show that citronellic acid can be obtained in over 90 % yield with total conversion of citronellal at pH 12 and a temperature of 80 °C.