Journal of Catalysis 190, 92–103 (2000) doi:10.1006/jcat.1999.2741, available online at http://www.idealibrary.com on Microstructure and Characterization of a Highly Selective Catalyst for the Isomerization of Alkanes: A Molybdenum Oxycarbide Christophe Bouchy, Cuong Pham-Huu, Baudouin Heinrich, Christian Chaumont, and Marc J. Ledoux 1 L aboratoire de Chimie des Mat´ eriaux Catalytiques, GMI-IPCMS, ECPM/CNRS UMR 7504, Universit´ e L ouis Pasteur (UL P), 25 rue Becquerel, BP 08, 67087 Strasbourg Cedex 2, France Received June 10, 1999; revised October 22, 1999; accepted October 22, 1999 A molybdenum oxycarbide catalyst that is very active and se- lective for the isomerization of n-alkanes has been characterized by means of diffraction techniques and simulations. The proposed structure is confirmed by transmission electron microscopy. An el- emental analysis performed on the pure phase prepared from a bronze precursorprovides the stoichiometry of this new catalyst. Quantitative temperature-programmed oxidation measurements confirm the elemental composition. The stoichiometry of this new phase, MoO 2.42 C 0.23 H 0.78 , shows an excess of light elements O, C, and H only possible because of the presence of metal vacancies. A tentative explanation of its peculiar catalytic properties is deduced from these structural and chemical features. c  2000 Academic Press Key Words: molybdenum oxycarbide;structure;alkanes isomer- ization; catalyst. INTRODUCTION Analogieshave been found between transition metalcar- bides (Mo 2 C, WC, and/or W 2 C) and metals of the platinum family (Pt, Rh, Ru). Their respective electronic structures and catalytic properties have induced much research both from a synthetic point of view and for potential catalytic applications (1–10). Among these, the selective isomeriza- tion of n -alkanes into branched isomers without aromati- zation or cracking is one of the most studied systems (6, 11, 12). Different authors have reported that, in general, transition metal carbides catalyze hydrogenolysis or crack- ing reactions better than selective isomerization (13–16). Iglesia et al. (6, 12, 17) have improved isomerization selec- tivity on tungsten carbides by inducing a slight oxidation of the surface by O 2 or air treatment. They suggested that the surface has a bifunctional aspect, with patches of car- bide responsible for the dehydrogenation–hydrogenation of the alkane, and patches of acidic oxide responsible for the protonation and isomerization (bond shift), similar to what is observed on a conventional bifunctional catalyst such as Pt/acidic zeolite. At the same time, Ledoux et al. 1 To whom correspondence should be addressed. Fax: 33-388136880. E-mail: ledoux@cournot.u-strasbg.fr. (18, 19) have suggested that the large improvement in iso- merization selectivity observed when oxidizing the surface of Mo 2 C was due to the formation of a new phase contain- ing Mo, C, and O (and possibly H) atoms, and obtained by the reaction of the feed gas, a mixture of H 2 and hydro- carbon, partly re-carburizing the MoO 3 layer covering the top of the carbide’s surface. The presence of Mo 2 C in the bulk as support of this selective phase was probably not necessary. Indeed, starting from bulk MoO 3 , it has been possible to prepare the same phase by reacting a flow of H 2 /hydrocarbon directly with this bulk MoO 3 (20, 21). This oxide, crystallizing in an orthorhombic system, ex- hibits a strong anisotropy (22) due to its lamelar structure made of bilayers of MoO 6 octahedra, bonded by van der Waals forces. At 350 ◦ C, regardless of the total pressure (from 1to 40atm),in the presence ofH 2 /hydrocarbon(ratio from 2 to 150), this oxide is readily transformed into two phases,the sub-oxide MoO 2 and thisnewoxycarbide phase, MoO x C y . The catalytic selectivity of this solid mixture is identical to that observed on the partly oxidized carbide. This selectivity is unique because it is the only catalyst able to isomerize long-chain ( ≥C 7 ) linear alkanes into highly branched isomers at a very high conversion without crack- ing (19, 23, 24). This was attributed to the intervention of a reaction mechanism involving a metallacyclobutane inter- mediate (25) without any formation of carbocations, which thermodynamically favor the cracking of branched alka- nes. The existence and the catalytic activity of this new oxy- carbide are contested by different authors either attribut- ing the activity to MoO 2 (26), although it has been shown that this sub-oxide is not selective for the reaction (27), or not choosing between MoO 2 and the oxycarbide (28). The difficulty arises from three facts. First, in the condi- tions used for its preparation, this oxycarbide is poorly, or not at all, crystallized (8), contrary to the well-crystallized MoO 2 ,renderingconventionalX-raydiffraction (XRD) in- effective. Second, the degree of oxidation of Mo, +IV for MoO 2 , lies between +IV and +V for the oxycarbide (8), making ambiguous the X-ray photoelectron spectroscopy (XPS) analyses of the surface. Third, this oxycarbide is sta- ble in a reducing atmosphere containing hydrogen and/or 0021-9517/00 $35.00 Copyright c  2000 by Academic Press All rights of reproduction in any form reserved. 92