INFLUENCE OF RU PRECURSOR, SUPPORT AND SOLVENT IN THE HYDROGENATION OF CITRAL OVER RUTHENIUM CATALYSTS

A series of ruthenium supported catalysts were prepared and investigated in the liquid-phase hydrogenation of citral. The mechanism of the reaction was found to be dependent on the Ru precursor, support and solvent used. On the sample prepared from RuCl3 the acetals of citronellal were formed with the subsequent hydrogenation of the isolated C=C double bond. On the samples obtained from precursors which do not contain chloride ions, citronellal was the main reaction product. Hydrogenation of the isolated C=C double bond was negligible. Using cyclohexane as solvent, a large amount of isopulegol was also obtained. The results of the hydrogenation of citral have been correlated to the presence of acid sites and incompletely reduced ruthenium which favour cyclization and acetals formation from citronellal. On the basis of the reported results, a reaction scheme for hydrogenation of citral over ruthenium catalysts is proposed.