Highly Active Nonpromoted Hydrotreating Catalysts through the Controlled Growth of a Supported Hexagonal WS 2 Phase Thibault Alphazan, † Audrey Bonduelle-Skrzypczak, † Christe ̀ le Legens, † Anne-Sophie Gay, † Zoubeyr Boudene, † Maria Girleanu, § Ovidiu Ersen, § Christophe Cope ́ ret,* ,‡ and Pascal Raybaud* ,† † IFP Energies nouvelles, Rond-point de l’e ́ changeur de Solaize, 69360 Solaize, France ‡ Department of chemistry and Applied Biosciences, ETH Zü rich, Vladimir Prelog Weg 2, CH-8093 Zü rich, Switzerland § IPCMS-UMR 7504, CNRS-Universite ́ de Strasbourg, 23 rue du Loess BP 43, 67034 Strasbourg cedex 2, France * S Supporting Information ABSTRACT: Highly active nonpromoted W-based hydrotreating catalysts are prepared through a molecular approach with a control of each step. This approach yields WS 2 crystallites exhibiting hexagonal 2D morphology, which have been characterized by combining XPS and HR HAADF-STEM techniques and ab initio molecular modeling. The first step is the impregnation of a well- defined precursor, [W(OEt) 5 ] 2 , grafted onto partially dehydroxy- lated amorphous silica-alumina (ASA) and characterized at the molecular level by spectroscopic techniques (NMR and IR). The use of increasing amounts of W precursor reveals the formation of (i) a layer of tungsten surface species grafted on the surface and (ii) layers of more mobile adsorbed species loosely bonded to the surface. Sulfidation of these materials provides WS 2 supported on ASA, which shows unprecedented lower sulfidation temperatures down to ambient temperature and improved activity by comparison with conventional references (polyoxometalate route). In addition, these improved activities are explained not only by a better level of sulfidation but also by the 2D hexagonal-like morphology of WS 2 crystallites (revealed by HR HAADF- STEM), in contrast to a truncated triangle-like morphology for conventional samples. This molecular approach thus opens new avenues to understand and improve the performances of hydrotreating catalysts. KEYWORDS: WS 2 , amorphous silica-alumina, hydrotreating catalyst, morphology, metallo-organic precursor, molecular approach, density functional theory ■ INTRODUCTION The evolution of oil world reserves and the stiffening of environmental regulations 1 call for a significant improvement of the performances of hydrotreating catalysts, which are used to remove O-, S-, and N-containing contaminants from petro- leum. 2 These catalysts are mainly based on supported transition-metal sulfides (TMS). 3 The TMS active phase is usually composed of tungsten (W) or molybdenum (Mo), promoted by nickel (Ni) or cobalt (Co). It forms a bimetallic sulfide phase, typically called the NiWS, NiMoS, or CoMoS phase, whose structure corresponds to a two-dimensional (2D) nanolayer of Mo(W)S 2 decorated by the promoter at its edges. 4,5 The hydrogenation properties of MoS 2 and WS 2 phases, profitably used in the hydrotreatment (HDT) process, were recently highlighted by H 2 /D 2 scrambling experiments. 6 While Ni-promoted WS 2 catalysts lead to activity improvement in aromatic hydrogenation, 7,8 optimizing the properties of WS 2 - based catalysts remains a challenge, hence limiting their applications in HDT. One of the major problems is the difficulty in improving the level of sulfidation of the active phase due to the formation of W(Mo)O x S y and amorphous W(Mo)O 3 inactive phases, probably as a result of the overly strong metal-support interactions. 9-11 The second challenge lies in the control of the morphology (bidimensional shape) and size of the WS 2 or MoS 2 nanocrystallites, observed either on model graphite supports, 2 or on industrial silica-alumina supports. 8 These challenges thus call for an improvement of synthetic methods with a full molecular control of the active phase. Industrial MoS 2 - and WS 2 -based catalysts are conventionally prepared by a succession of steps with little control over the metal-support interaction: (i) an incipient wetness impregna- tion of aqueous solutions of W salts such as ammonium metatungstate onto an oxide support, typically alumina or silica-alumina, (ii) a drying and possibly a calcination step, and (iii) an activation step using a sulfo-reductive agent (such as dimethyl disulfide (DMDS) or an H 2 S/H 2 gas mixture). The first stepimpregnationis critical because the deposition of Received: September 2, 2014 Revised: October 20, 2014 Published: October 22, 2014 Research Article pubs.acs.org/acscatalysis © 2014 American Chemical Society 4320 dx.doi.org/10.1021/cs501311m | ACS Catal. 2014, 4, 4320-4331