Liquid-phase hydrogenation of citral for production of citronellol: Catalyst selection

Selective hydrogenation of citral to citronellol has been investigated over Ni supported catalysts. The aim was to compare different support materials and catalyst preparation methods. Selectivity to citronellol was dependent on conversion. The highest maximum selectivity towards citronellol for a Ni/Al₂O₃ (16.7wt.% Ni) catalyst at 2.3bar hydrogen increased from 70% in ethanol to 85% 2-pentanol, where acetalisation could be totally eliminated. In ethanol the highest maximum selectivities to citronellol were about 83% over a 8.8wt.% Ni/Al₂O₃ catalyst prepared by atomic layer epitaxy (ALE) method. The lower hydrogenation pressure (2.3bar) favoured the formation of citronellol compared to results obtained at 5.3bar hydrogen for all other catalysts, but not for Ni/Al₂O₃ (16.7wt.%). Both a bimetallic Ni-Cu/SiO₂ (10wt.% Ni, 3wt.% Cu) catalyst and a Ni/SiO₂ (17.6wt.% Ni) catalyst prepared by ALE method showed high selectivities to citronellol, 79 and 76%, respectively at 5.3bar hydrogen. An optimum hydrogenation rate and an optimum specific metal surface area in the catalyst gave the highest selectivities to the desired intermediate, citronellol. Ni supported Y zeolite catalysts were not selective towards formation of citronellol. Instead these catalysts favoured other reactions, like cyclisation, dehydrogenation and cracking.