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 degrees C, with 200 Nml min(-1) O2 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 C=C 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 degrees C.
Martin
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Leibniz-Institut für Katalyse e.V an der Universität Rostock, Berlin, Germany. andreas.martin@catalysis.de
Armbruster
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U
Decker
D
D
Gedig
T
T
Köckritz
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D000080462:Acyclic Monoterpenes; D000447:Aldehydes / Q000737:chemistry*; D002210:Caprylates / Q000737:chemistry*; D002384:Catalysis; D003247:Conservation of Natural Resources; D006046:Gold / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D053768:Metal Nanoparticles / Q000737:chemistry*; D039821:Monoterpenes / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D013997:Time Factors
D000080462:Acyclic Monoterpenes; D000447:Aldehydes; D002210:Caprylates; D039821:Monoterpenes; C530493:citronellic acid; D006046:Gold; C108217:citronellal; D010100:Oxygen