Journal of Catalysis 251 (2007) 507–513 www.elsevier.com/locate/jcat Research Note Alkane metathesis by a tungsten carbyne complex grafted on gamma alumina: Is there a direct chemical role of the support? Jérôme Joubert, Françoise Delbecq, Philippe Sautet ∗ Laboratoire de Chimie, Institut de Chimie de Lyon, Université de Lyon, École Normale Supérieure de Lyon and CNRS, 46, Allée d’Italie, 69364 Lyon Cedex 07, France Received 6 April 2007; revised 12 July 2007; accepted 30 July 2007 Available online 14 September 2007 Abstract The activity in alkane metathesis of tungsten complexes grafted on gamma alumina raises many questions regarding the structure of the active site and the reaction pathways. We examine the case of a perhydrocarbyl tungsten complex on alumina from periodic density functional theory calculations. The quantum chemistry exploration of energy pathways suggests two possible mechanisms for alkane metathesis. The first mecha- nism occurs on the grafted complex only but is significantly activated. The second mechanism shows that hydration defects on the alumina surface can be active in alkane dehydrogenation toward alkenes, whereas the tungsten complex efficiently catalyzes the subsequent olefin metathesis step. The calculations thus underscore the role of the alumina support in the mechanism. © 2007 Elsevier Inc. All rights reserved. Keywords: Metathesis; Alkanes; Alumina; Surfaces; Tungsten; Mechanisms; Dehydrogenation; Density functional theory 1. Introduction The valorization of alkanes represents an important chal- lenge, especially in terms of the capability to selectively form longer or shorter alkanes chains to optimize their proper- ties as fuel. In this respect, alkane metathesis is a reaction with significant potential applicability. Such a reaction has been approached by homogeneous and heterogeneous cataly- sis [1–3]. At the midpoint between these two methods, surface organometallic chemistry offers an innovative approach and has proven its efficiency in alkane metathesis with Ta, Mo, or W complexes grafted onto silica or alumina [4–12]. This chem- istry is constructed with the tools of homogeneous catalysis brought to their limits, with the oxide surface considered as a ligand [13]. Besides the choice of the metal complex, the nature of the oxide support also may play a decisive role in the reactiv- ity thus obtained. This is especially so when a surface such as γ -Al 2 O 3 , with its partial hydroxylation and its rich acid– * Corresponding author. Fax: +33 472728860. E-mail address: philippe.sautet@ens-lyon.fr (P. Sautet). base chemical properties, is used. The development of tung- sten-based heterogeneous alkane metathesis catalysts from a surface organometallic chemistry approach has indeed uncov- ered some support effects that remain unexplained [10]. Graft- ing [W(Ct Bu)(CH 2 t Bu) 3 ] onto SiO 2 or γ -Al 2 O 3 after partial dehydroxylation at 500 ◦ C leads to well-characterized neutral mono-siloxy (Si s O)W(Ct Bu)(CH 2 t Bu) 2 and mono-aluminoxy (Al s O)W(Ct Bu)(CH 2 t Bu) 2 complexes, respectively, where Si s and Al s represent surface silicium and aluminum atoms. The complex grafted onto alumina is active in propane metathesis (turnover number [TON] = 28), whereas that grafted onto sil- ica is not (TON = 0) [10,13,14]. Recently, it was shown that a silica–alumina support, treated under similar conditions, yields a very similar surface complex as silica, with grafting onto the Si atom (Si s O)W(Ct Bu)(CH 2 t Bu) 2 but, surprisingly, gives a propane metathesis reactivity similar to that on the alumina sup- port [15]. The influence of the support thus seems especially confused. In addition, it has been proposed that, as for grafted tantalum complexes, in alkane metathesis olefin metathesis is the key homologation process, because olefins have been iden- tified as primary products, and the ratios of products of cross- metathesis between various alkanes are in agreement with those of the corresponding olefin metathesis [5–10]. All surface tung- 0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2007.07.035