Hydrodeoxygenation pathways catalyzed by MoS 2 and NiMoS active phases: A DFT study C. Dupont, R. Lemeur, A. Daudin, P. Raybaud ⇑ IFP Energies Nouvelles, Direction Catalyse et Séparation, Rond-point de l’échangeur de Solaize, BP 3 – 69360 Solaize, France article info Article history: Received 3 December 2010 Revised 19 January 2011 Accepted 22 January 2011 Available online 4 March 2011 Keywords: Hydrotreatment catalysts MoS 2 NiMoS Hydrodeoxygenation Density functional theory Nucleophilic substitution Reaction pathway abstract Due to the increasing need for purifying renewable feeds (such as biomass effluents) by means of cata- lytic 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 func- tional 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, pro- panol and water are stronger on MoS 2 than on NiMoS. The interaction with the accessible Mo site is direc- ted 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 NiAMo sites of the M-edge. The investigation into hydrodeoxygenation pathways reveals how the C@O hydrogenation and the CAO bond cleavage occur on transition metal sulfides. The specific adsorp- tion 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 CAO bond cleavage step for which the activation energy is found smaller for NiMoS than for MoS 2 . Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction The decrease in available fossil fuel resources induces an increasing interest for alternative and renewable solutions, like the use of biomass. Hence from 2010, European Union imposes the introduction of 5.75% of renewable fuels extracted from bio- mass in fuels [1]. This kind of renewable sources of energy allows reduction in greenhouse gas emission compared to fossil fuels. Nevertheless, due to their high content in oxygenated compounds, the biomass effluents also present some disadvantages, like high viscosity, immiscibility to standard fuels or poor heating value. Hence, reducing the oxygen content is required to make bio-oils valuable for fuels. One possible solution is provided by the hydro- deoxygenation (HDO) route, which leads to the removal of oxygen atoms in the presence of hydrogen. The process may be rather sim- ilar to conventional hydrotreating, and existing refining processes can be used for the co-treatment of biomass effluents and petro- leum fractions [2–5]. From the catalytic point of view, MoS 2 -based catalysts promoted by either Co or Ni well known to be active in hydrodesulfurization (HDS) are also active in HDO (as described below). Moreover, the hydrotreatment of biomass effluents such as vegetable oils leads to paraffinic diesel, which exhibits proper- ties close to conventional diesels. However, the HDO mechanism (including C@O hydrogenation and CAO bond cleavage) on MoS 2 - based active phase remains unknown. At the same time, the hydro- gen consumption induced by HDO processes makes crucial to bet- ter understand the HDO mechanisms taking place on such catalysts, subject of the present report. According to previous experimental studies [6–8], complete hydrodeoxygenation occurs through three proposed routes: hydro- deoxygenation (HDO), decarbonylation and/or decarboxylation, producing H 2 O, CO and CO 2 , respectively, together with the tar- geted hydrocarbon compounds. The selectivity between the HDO route and the decarbonylation/decarboxylation one is governed by the relative easiness for breaking CAO bond versus CAC bond. These different pathways have been proposed for different kinds of oxygenated compounds, both aromatic derived from lignocellu- losic biomass [4] and aliphatic derived from esters, acids or alco- hols [7,8]. Concerning aliphatic esters, which are the major component of vegetable oils, Krause and coworkers have recently published experimental works [7–9] to investigate their transfor- mation mechanisms on alumina-supported Co- and Ni-promoted MoS 2 catalysts. According to their studies, one key intermediate 0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2011.01.025 ⇑ Corresponding author. Fax: +33 4 37 70 20 66. E-mail address: pascal.raybaud@ifpen.fr (P. Raybaud). Journal of Catalysis 279 (2011) 276–286 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat