A model compound (methyl oleate, oleic acid, triolein) study of triglycerides hydrodeoxygenation over alumina-supported NiMo sulfide

We studied hydrodeoxygenation of model compounds for vegetable oil into diesel-range hydrocarbons on a sulfided NiMo/γ-Al2O3 catalyst under trickle-flow conditions. Methyl oleate (methyl ester of oleic acid, a C18 fatty acid with one unsaturated bond in the chain) represented the C18 alkyl esters in natural fats, oils and greases. The effect of temperature and pressure on activity and product distribution (mainly C17 and C18 hydrocarbons) were studied. Hydrolysis of the methyl ester results in fatty acid intermediates, which are converted by direct hydrodeoxygenation to C18 hydrocarbons or decarbonated (by decarbonylation or decarboxylation) to C17 hydrocarbons. Reactant inhibition is more pronounced for the former route. The reaction is hardly inhibited by H2S, H2O, CO and tetralin solvent. H2S and to a lesser extent H2O increase the C17/C18 hydrocarbon ratio, because they inhibit direct hydrodeoxygenation more than decarbonation. The catalyst surface contains different sites for direct hydrodeoxygenation and decarbonation reactions. During methyl oleate HDO, the catalyst slowly deactivated, mainly due to blocking of Lewis acid sites of the alumina support that catalyze methyl oleate hydrolysis. The catalyst was much more active in the HDO of triolein (glyceryl trioleate, representative triglyceride model compound) than in methyl oleate HDO, to be attributed to very facile hydrolysis of triglycerides. Although the overall kinetics of methyl oleate and triolein HDO were similar, our results show that the catalyst and H2S play a much more important role in the hydrolysis of methyl oleate than in hydrolysis of triglycerides.