Hydrogenation of (–)-carvone in presence of gold catalysts: role of the support

The liquid phase hydrogenation of biomass derived (–)-carvone into industrially valuable dihydrocarvone was studied over monometallic Au catalysts supported on alumina, titania and zirconia, as well as on the mesoporous carbon support Sibunit in methanol as a solvent (100°C, hydrogen pressure 9 bar). It was shown that among the three types of functional groups present in carvone, which can be hydrogenated, namely C=O, conjugated and isolated C=C groups, hydrogenation of the latter was predominant. The catalytic activity was found to depend on the catalyst support type. Under comparative reaction conditions, the carvone conversion increased in the following sequence: Au/C ≪Au/ZrO₂ < Au/Al₂O₃ ≪Au/TiO₂. A higher activity of Au catalysts over metal oxides as compared to Au/C can be caused by the presence of acid sites as well as oxygen vacancies in their structure allowing strong adsorption of carvone through its carbonyl moiety. All catalysts supported on oxides showed similar selectivity towards trans- and cis-dihydrocarvone with the ratio between isomers (trans-/cis-isomer) being about 1.8, while this value for Au/C was close to 3.9, which can be related to a much lower carvone conversion in the latter case.