Hydrodeoxygenation pathways catalyzed by MoS_2 and NiMoS active phases: A DFT study

摘要

Due to the increasing need for purifying renewable feeds (such as biomass effluents) by means of catalytic hydrotreatment processes, the atomic-scale understanding of the catalytic properties of transition metal sulfide active phases in the presence of oxygenated molecules becomes crucial. Using density functional theory (DFT) calculations, we evaluate the adsorption properties and the hydrodeoxygenation pathways of relevant model O-containing molecules on the M-edge sites of the MoS_2 and NiMoS active phase. We show first that the adsorption energies of methyl propanoate, propanoic acid, propanal, propanol and water are stronger on MoS_2 than on NiMoS. The interaction with the accessible Mo site is directed by the oxygen atom of either the C=O group for ester and acid or the OH group for alcohol and water molecules. For propanal, the adsorption mode depends on the nature of the active site: it is found to be bidentate on NiMoS, where the C and O atoms of the carbonyl group simultaneously interact with the dual Ni-Mo sites of the M-edge. The investigation into hydrodeoxygenation pathways reveals how the C=O hydrogenation and the C-O bond cleavage occur on transition metal sulfides. The specific adsorption mode provides a lower activation energy for the hydrogenation of propanal into propanol on NiMoS than on MoS_2. The propanol is further deoxygenated by a nucleophilic substitution mechanism involving a sulfhydryl group and leading to a thiol intermediate before propane formation. The rate-limiting step of the aldehyde HDO process is determined by the C-O bond cleavage step for which the activation energy is found smaller for NiMoS than for MoS_2.