Liquid-phase hydrogenation of citral over Pt/SiO2 catalysts II. Hydrogenation of reaction intermediate compounds

Liquid-phase hydrogenation of the four principal reaction intermediates for med during citral hydrogenation, i.e., nerol, geraniol, citronellal, and ci tronellol was studied at 298 and 373 K under 20 atm H-2 at concentrations o f 0.5 to 1.0 Ril in hexane. A decrease in the initial reaction rate as temp erature increased from 298 to 373 K was exhibited during the hydrogenation of all four compounds, just as reported earlier for citral; however, the de crease in rate at 373 K was only one-half for citronellal whereas it was or ders of magnitude greater for nerol and geraniol. Furthermore, simultaneous hydrogenation of citronellal and geraniol at 298 K resulted in a continuou s decrease in the rate of citronellal disappearance in contrast to the near ly constant rate of disappearance observed during hydrogenation of citronel lal alone. Competitive hydrogenation of citral with either geraniol or citr onellal showed that geraniol hydrogenation to citronellol is kinetically in significant during citral hydrogenation at 373 K. The initial activity for hydrogenation of the intermediates at 298 K follows the following trend: ge raniol (1.9 s(-1))> nerol (1.5 s(-1))> citronellol (0.21 s(-1))r 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 hydrogena tion in the presence of other reactants, the relative size of the adsorptio n equilibrium constants for the various organic compounds appears to be as follows: K-citral > K-citronellal > K-geraniol, K-nerol >K-citronellol > 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. (C) 2000 academic Press.