Liquid phase hydrogenation of citral: Suppression of side reactions

Selective hydrogenation of citral was investigated at 70°C in four different alcohols over Ru, Rh and Ni supported catalysts. The main aim was to suppress the side reaction, like acetalization and cyclization of citronellal. Acetalization could be suppressed either by using chloride-free catalyst precursors and other supports than active carbon or by using longer chained alcohols, like 2-pentanol and 2-methyl-2-propanol as solvents. The highest maximum selectivity to citronellol (92%) was obtained in citral hydrogenation in ethanol over Ni/Al₂O₃ catalyst, which was pre-reduced ex situ at 350°C and in situ at 350°C for 2h in ethanol. The acidic alumina support was covered by spillover hydrogen, suppressing the acetalization reaction. Citronellal cyclization increased with the increasing solvent hydrophobicity and was also larger over active carbon supported Ru catalyst.