Liquid-Phase Hydrogenation of Citral over Pt/SiO2 Catalysts: II. Hydrogenation of Reaction Intermediate Compounds

Liquid-phase hydrogenation of the four principal reaction intermediates formed during citral hydrogenation, i.e., nerol, geraniol, citronellal, and citronellol, was studied at 298 and 373 K under 20 atm H2 at concentrations of 0.5 to 1.0 M in hexane. A decrease in the initial reaction rate as temperature increased from 298 to 373 K was exhibited during the hydrogenation of all four compounds, just as reported earlier for citral; however, the decrease in rate at 373 K was only one-half for citronellal whereas it was orders of magnitude greater for nerol and geraniol. Furthermore, simultaneous hydrogenation of citronellal and geraniol at 298 K resulted in a continuous decrease in the rate of citronellal disappearance in contrast to the nearly constant rate of disappearance observed during hydrogenation of citronellal alone. Competitive hydrogenation of citral with either geraniol or citronellal showed that geraniol hydrogenation to citronellol is kinetically insignificant during citral hydrogenation at 373 K. The initial activity for hydrogenation of the intermediates at 298 K follows the following trend: geraniol (1.9 s−1) > nerol (1.5 s−1) > citronellol (0.21 s−1) > E-citral (0.13 s−1), citronellal (0.13 s−1) > Z-citral (0.06 s−1). Based on the relative hydrogenation rates of the intermediate alone versus its hydrogenation in the presence of other reactants, the relative size of the adsorption equilibrium constants for the various organic compounds appears to be as follows: Kcitral > Kcitronellal > Kgeraniol, Knerol >Kcitronellol > K3,7-dimethyloctanol. This study indicates that activation of the CO bond should be performed at higher reaction temperatures to maximize selectivity to the unsaturated alcohols.