Optimal water coverage on alumina: a key to generate Lewis acid-base pairs that are reactive towards the C-H bond activation of methane.

The activation of methane is still a great challenge today, because it constitutes one of the largest hydrocarbon resources on earth. Many oxides catalyze processes involving the activation of C H bonds. Of these, g-alumina (g-Al2O3) is one of the most active: when treated beforehand at temperatures above 400 8C, it catalyses H/D exchange reactions of D2/CH4 and CH4/CD4 mixtures at room temperature with unexpectedly low activation energies (17–30 kJmol ). These reactions involve a very small number of active sites (defects) generated by the high-temperature pretreatment. Such a reactivity was attributed to the Lewis acidity of surface Al atoms, yielding Al CH3 and O H species, and more recently specifically to surface three-coordinate AlIII centers leading to the formation of four-coordinate AlIV CH3 moieties. However, it is not clear why AlIII, the expected most Lewis acidic site, would exist in realistic conditions, that is, on a hydroxylated alumina surface. The predominant termination of g-Al2O3 particles is the (110) facet (70–83%); [9] complete dehydration would require temperatures (> 900 8C) much higher than the window of stability of g-Al2O3. [11] Even after treatments at 400–500 8C, the OH density is 2–5 OH/nm 2 on g-Al2O3; [7,12] therefore, the strongest AlIII sites should be completely hydroxylated and thus their reactivity annihilated. Therefore, several questions emerge: what is the real nature of the active site for C H bond activation on alumina? Is it possible to have the two strong antagonists, highly Lewis acidic sites and a strong Lewis base (H2O), on the surface simultaneously without a direct annihilation between them? If these strong Lewis sites still exist, how would surface hydroxylation affect their reactivity towards CH4? Herein we provide answers to these questions and show the unexpected role of water by combining experiments and first-principle calculations. We underline that the reactive site is best described as an (Al,O) Lewis acid–base pair where oxygen basicity, apart from Al acidity, is also a key factor to reactivity. We first investigated the influence of thermal treatment of alumina on the density of sites involved in the formation of Al CH3 species through reaction with CH4 at 150 8C (Figure 1a). The density of sites gives a volcano-type curve as a function of the g-Al2O3 pretreatment temperature, reaching a maximum of 0.03 reactive Al sites per nm at about 700 8C. Below 400 8C, no site is generated, while at higher temperatures (> 800 8C) their density decreases. By comparison, both the hydroxy group coverage qOH and the specific surface area SBET decrease with increasing temperature (Figure 1b and Supporting Information, Figure S1): qOH decreases exponentially while SBET falls first slowly and then sharply above 800 8C. These phenomena are associated with a progressive change of the alumina bulk structure (g!d!q), as indicated by X-ray powder diffraction studies (Supporting Information, Figure S2). The nature of the active sites was studied by DFT calculations with a specific focus on low energy metastable terminations of partially hydrated alumina surfaces. The most abundant (110) surface was considered: its hypothetic fully dehydrated surface unit cell (s0) exposes three different aluminum Lewis acid centers: one three-coordinate (AlIII) and two types of four-coordinate (AlIVa and AlIVb) sites, with decreasing intrinsic Lewis acidity according to AlIII>AlIVb> AlIVa. It also exposes twoand threefold-coordinated O atoms with intrinsic Lewis basicity O2>O3 (Supporting Information, Figure S3a). This (110) termination has a rather high surface energy (1.5 J m ) and strongly interacts with water. 16] At a simulated qOH of about 3OH nm 2 (1 H2O per surface unit cell), close to the measured OH density at 500 8C, the OH group preferentially occupies the most Lewis acidic AlIII site (Supporting Information, Figure S3b). [15] However, from the various structures explored, a configuration with OH bridging two AlIVa centers, keeping AlIII free, is only 44 kJmol 1 less stable (s1, Figure 2a), making the [*] Dr. R. Wischert, Prof. Dr. C. Cop ret Universit de Lyon, CNRS Institut de Chimie de Lyon, C2P2, CPE Lyon 43, Bd. du 11 Novembre 1918, 69616 Villeurbanne Cedex (France)

[1]  C. Copéret.  C-H bond activation and organometallic intermediates on isolated metal centers on oxide surfaces. , 2010, Chemical reviews.

[2]  P. Nortier,et al.  Effects of crystallinity and morphology on the surface properties ofalumina , 1990 .

[3]  Y. Amenomiya,et al.  Exchange reaction of methane on some oxide catalysts , 1986 .

[4]  P. Sautet,et al.  Heterolytic Splitting of H2 and CH4 on γ-Alumina as a Structural Probe for Defect Sites , 2006 .

[5]  M. Scurrell,et al.  Exchange of alkanes with deuterium over $gamma$-alumina. A Broensted linear free energy relationship , 1975 .

[6]  R. Snyder,et al.  Structures and transformation mechanisms of the η, γ and θ transition aluminas , 1991 .

[7]  D. Stephan.  Frustrated Lewis pairs: a new strategy to small molecule activation and hydrogenation catalysis. , 2009, Dalton transactions.

[8]  D. Duprez,et al.  Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18O2 with 16O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by Noble Metals. Correlation with Oxide Basicity† , 1996 .

[9]  J. G. Larson,et al.  Studies of the Hydrogen Held by Solids. VII. The Exchange of the Hydroxyl Groups of Alumina and Silica-Alumina Catalysts with Deuterated Methane , 1965 .

[10]  G. Magnacca,et al.  A case study: surface chemistry and surface structure of catalytic aluminas, as studied by vibrational spectroscopy of adsorbed species , 1996 .

[11]  J. Beaufils,et al.  Détermination, par diffraction différentielle de neutrons, des faces cristallines exposées par des supports de catalyseurs en poudre , 1981 .

[12]  Philippe Sautet,et al.  Use of DFT to achieve a rational understanding of acid–basic properties of γ-alumina surfaces , 2004 .

[13]  J. Bercaw,et al.  Understanding and exploiting C–H bond activation , 2002, Nature.

[14]  P. Ratnasamy,et al.  Catalytic Aluminas: Surface Models and Characterization of Surface Sites , 1978 .

[15]  J. Boer,et al.  Study of phase transformations during calcination of aluminum hydroxides by selected area electron diffraction , 1964 .

[16]  S. Rossignol,et al.  Preparation of alumina through a sol–gel process. Synthesis, characterization, thermal evolution and model of intermediate boehmite , 2003 .

[17]  Jason D. Masuda,et al.  Reversible, Metal-Free Hydrogen Activation , 2006, Science.

[18]  G. B. Shul’pin,et al.  ACTIVATION OF C-H BONDS BY METAL COMPLEXES , 1997 .

[19]  Philippe Sautet,et al.  Hydroxyl Groups on γ-Alumina Surfaces: A DFT Study , 2002 .

[20]  J. Yates,et al.  IR spectroscopic detection of Lewis acid sites on alumina using adsorbed carbon monoxide. Correlation with aluminum-hydroxyl group removal , 1991 .

[21]  G. Hutchings,et al.  A study of the methane-deuterium exchange reaction over a range of metal oxides , 2002 .

[22]  J. Nørskov,et al.  A negative surface energy for alumina , 2004, Nature materials.

[23]  R. Crabtree.  Alkane C–H activation and functionalization with homogeneous transition metal catalysts: a century of progress—a new millennium in prospect , 2001 .

[24]  Robert H. Crabtree,et al.  Aspects of Methane Chemistry , 1995 .