Activity and stability of sulfur catalytic phases for hydrotreating of vegetable oils
By combining well defined experiments and density functional theory (DFT) calculations, we propose a rational understanding of the deoxygenation mechanisms of relevant oxygenate molecules (ethyl heptanoate and heptanal) so as to provide guides to define optimal catalytic systems for the hydrotreating of renewable feedstocks (vegetable oils, animals fats). The hydrotreatment of vegetable oils, with the deoxygenation reaction, is an alternative route to transesterification and can be used to obtain high quality diesel. The transformation of oxygenated model compounds was studied under a total pressure of 1.5 MPa, at 523 K, in a fixed bed reactor over various unsupported transition metal sulfide catalytic phases (TMS). Results have shown the influence of the sulfide phase on the selectivity for deoxygenation reaction (hydrodeoxygenation (HDO) and/or decarbonylation/ decarboxylation (DCO)) and the specific reactivity of the rhodium sulfide for the transformation of ethyl heptanoate. The study of the transformation of heptanal shows the reaction of deoxygenation is preferentially following the HDO pathway over all the catalysts. In our reaction conditions, heptanal was identified as a reaction intermediate of this deoxygenation pathway. The promoting effect of cobalt and nickel on the activity of monometallic molybdenum sulphide was observed for the transformation of heptanal. The relation between the deoxygenation and HDO rates and the ab initio calculated sulphur-metal bond energy E(MS) in the bulk TMS is following a volcano curve. Bimetallic sulfide NiMoS (0.43), with an intermediate E(MS) (127 kJ.mol-1), is found as the most active TMS for both reactions. The maximum of the HDO/DCO selectivity is obtained for the mixed catalyst CoMoS (0.1). Thanks to catalytic results and ab initio calculations, two reaction pathways are proposed for HDO and DCO reactions