Selective carvone hydrogenation to dihydrocarvone over titania supported gold catalyst

Selective hydrogenation of natural carvone to industrially valuable dihydrocarvone was carried out at 100 °C under hydrogen pressure over a 1.9 wt.% Au/TiO₂ catalyst. The gold catalyst has demonstrated high activity as well as stereo- and chemoselectivity in conjugated CC double bond hydrogenation with predominant formation of trans-dihydrocarvone. The catalytic activity and trans- to cis-isomers ratio were shown to strongly depend on the solvent. In a range of C₁C₃ alcohol solvents both catalytic activity and trans- to cis-dihydrocarvone ratio increased following the order: 2-propanol < ethanol < methanol. The total selectivity to dihydrocarvone 62% was achieved at a nearly complete carvone conversion (90%) after 13 h in the case of methanol, with the trans- to cis-dihydrocarvone ratio being about 1.8. Based on the transition state theory a quantitative description of trans- to cis-dihydrocarvone ratio variations in different solvents was made.