Inverse temperature dependence due to catalyst deactivation in liquid phase citral hydrogenation over Pt/Al₂O₃

Citral hydrogenation was studied over a Pt/Al₂O₃ catalysts in cyclohexane and in 2-pentanol. Several kinetic trends, which were correlated to catalyst deactivation, were achieved. Accumulation of trans isomers in both citral and in unsaturated alcohols was visible under deactivating conditions. The further hydrogenation of citronellal was nearly totally inhibited due to catalyst deactivation, whereas nerol, geraniol and citronellol formation were enhanced at higher temperatures and pressures. The main unusual kinetic phenomena in citral hydrogenation in 2-pentanol were, first, a maximum observed in the initial hydrogenation rates as a function of temperature caused by catalyst deactivation and, second, a minimum in citral conversion after prolonged reaction times. The reason for these unusual kinetic phenomena is decarbonylation reaction occurring during hydrogenation, i.e. formation of CO, which was confirmed by temperature programmed desorption of geraniol from a reduced Pt/Al₂O₃ catalyst.