Selectivity criteria for hydrogenation of citral in the liquid phase over unsupported bimetallic nickel-chromium and nickel-molybdenum catalysts

Abstract

The hydrogenation of citral has been investigated in cyclohexane with unsupported bimetallic Ni1-x Crx and Ni1-x Mo catalysts, prepared by co-reduction of mixtures of dry iodides of appropriate composition with sodium naphthalene as the reducing agent. The reaction was carried out in a static reactor and its progress was followed by measuring the concentrations of the components in the liquid phase. The measured product distributions versus citral conversion thus obtained allowed the calculation of the ratios kibM/kibN on which depend the theoretical equations for the concentration of the products deduced from a LANGMUIR-HINSELWOOD mechanism. The ratios kibM/kibN where ki, kj are the rate constants and bM, bN are the adsorption equilibrium constants for two competitive parallel reactions or two consecutive reactions, thus computed have been used as selectivity criteria. Whereas Ni0,88 Cr0,12 was the most active and the most selective catalyst in the hydrogenation of citral to citronellal, the molybdenum doped catalysts showed better selectivity in the hydrogenation of citral to citronellol. The initial hydrogenation rate increased with the molybdenum or the chromium content, but the selectivity was almost independent of the atomic composition.