Selectivity enhancement by catalyst deactivation in three-phase hydrogenation of nerol

Selectivity enhancement by catalyst deactivation in the liquid phase has been investigated in this work with batchwise nerol hydrogenation over Rh/Al₂O₃ catalyst in 2-pentanol as a model system. The desired product is an unsaturated alcohol citronellol. Besides hydrogenation, isomerization of nerol to citronellal as well as hydrodeoxygenation reactions also took place. Other possible products were the undesired product of complete hydrogenation, 3,7-dimethyloctanol, and cis-3,7-dimethyl-2-octenol. The selectivity enhancement of citronellol as a function of nerol conversion was observed at higher reaction temperatures, with higher initial nerol concentrations and under lower hydrogen pressures. Under these conditions the final conversions even after prolonged reaction times were lower than 100% due to catalyst deactivation. Additionally, nearly 3-fold selectivity enhancement to citronellol compared to that of the fresh catalyst was achieved in the second reuse of the same catalyst. The main reason for the catalyst deactivation is most probably decarbonylation of nerol.