Françoise Delbecq

Tuning selectivity in catalysis by controlling particle shape.

A catalytic process for the selective formation of cis olefins would help minimize the production of unhealthy trans fats during the partial hydrogenation of edible oils. Here we report on the design of such a process on the basis of studies with model systems. Temperature programmed desorption data on single crystals showed that the isomerization of trans olefins to their cis counterparts is promoted by (111) facets of platinum, and that such selectivity is reversed on more open surfaces. Quantum mechanics calculations suggested that the extra stability of cis olefins seen on hydrogen-saturated Pt(111) surfaces may be due to a lesser degree of surface reconstruction, a factor found to be significant in the adsorption on close-packed platinum surfaces. Kinetic data using catalysts made out of dispersed tetrahedral Pt nanoparticles corroborated the selective promotion of the trans-to-cis isomerization on the (111) facets of the metal. Our work provides an example for how catalytic selectivity may be controlled by controlling the shape of the catalytic particles.

Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid–Base Pairs that are Reactive Towards the CH 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)

Nature and Structure of Aluminum Surface Sites Grafted on Silica from a Combination of High-Field Aluminum-27 Solid-State NMR Spectroscopy and First-Principles Calculations

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.

On the key role of hydroxyl groups in platinum-catalysed alcohol oxidation in aqueous medium

In the aerobic selective oxidation of alcohols in aqueous medium in a batch reactor, it was observed that the addition of water to dioxane solvent (10–50 vol%) substantially increased the activity of a Pt/C catalyst. Periodic density functional theory (DFT) calculations were carried out to compare the reactivity of alcohols on the bare Pt(111) surface and in the presence of adsorbed water or hydroxyl groups, to explain the effect of water. The calculations indicate that the presence of adsorbed hydroxyl groups promotes the catalytic activity by participating directly in the catalytic pathways and reducing the activation barrier. Good agreement was found between the experiments in aqueous phase and these calculations. Further, decarbonylation of the aldehyde may be involved in the deactivation during oxidation of a primary alcohol.

Asymmetric Synthesis of 2‐Methyl Cyclohexane Carboxylic Acids by Heterogeneous Catalysis: Mechanistic Aspects

The catalytic hydrogenation of (S)-alkyl-N-(2-methylbenzoyl)pyroglutamates was studied over supported rhodium and ruthenium catalysts at room temperature and a pressure of 5 MPa. The reaction was diastereoselective with the predominant formation of (1S,2R)-2-methylcyclohexane carboxylic acid with a diastereomeric excess (de) of up to 96%. The most stable conformation was determined by means of a combination of modelling calculations, NMR spectroscopy and X-ray structural determination. In this conformation, the carbonyl group of the pyroglutamate auxiliary shields one face of the aromatic ring. The observed selectivity may thus be explained by a preferential adsorption at the unshielded face which avoids steric repulsion by the C=O group to result in a cis hydrogenation. The addition of an amine, the nature of the support (alumina or active carbon) or of the metal (Rh or Ru) were shown to give additional stabilisation of the adsorption at the unshielded face to increase the diastereoisomeric excess.

Adsorption of CO and NO on (111) and (100) surfaces of Pd3Mn compared with Pd: a theoretical approach

Abstract By means of extended Huckel calculations, the electronic properties of the (111) and (100) surface Pd atoms of the Pd 3 Mn alloy are compared with those of pure Pd. An electron transfer occurs from Mn to Pd, resulting in an increased electron density on the surface Pd atom whatever Mn is present in the first or the second layer. The study of CO and NO adsorption shows for both molecules a decrease in the binding energy on the alloy surfaces compared with the pure metal surfaces. The NO overlap population is found to be much smaller on the alloy, which means that the NO bond is weaker and hence that NO dissociates more easily than on pure Pd. Moreover, the bent form of NO is found to be preferred in most cases on the alloy, which is even more favourable for NO dissociation. Therefore, these results explain why the alloying of Pd with Mn enhances the catalytic behaviour of Pd for the CO + NO reaction if NO dissociation is the determining step.

A TD-DFT investigation of two-photon absorption of fluorene derivatives

We report time-dependent density functional theory (TD-DFT) calculations on the two-photon absorption (TPA) properties of fluorene and derivatives. The influence of donor and acceptor groups and of dimerisation is investigated. Firstly the choice of a DFT functional and of the basis set is performed by comparison of experimental and calculated excitation energies and two-photon cross sections. Then, the calculations display an enhancement of the cross section with acceptor groups or with a combination of one donor and one acceptor groups (push-pull), at some positions on the cycles. Moreover, the largest cross section is obtained for bifluorene. The replacement of carbon atoms by nitrogen atoms, giving heterocycles, is not efficient. In chloroform as solvent, the excitation energy decreases and the two-photon cross section increases, mainly with a polar molecule. Finally, a rationalization of the results is given based on the three-level model by analysis of the transition moments and of the molecular orbitals.

NO chemisorption on a magnetic alloy surface: a density-functional periodic study of Pd3Mn(100) compared with Pd(100)

Self-consistent calculations based on density functional theory with gradient corrections are used to compare the adsorption of NO on Pd(100) and Pd3Mn(100). There are two types of Pd3Mn(100) surfaces, one with only Pd atoms (A) and one with Pd and Mn atoms alternately ordered (B). For adsorption on surface A, the adsorption sites are in the same stability order as for palladium, with the atop site less stable than the bridge and the hollow ones, but all the binding energies are slightly weaker. For adsorption on surface B, the stability order is totally different, the atop Mn site being the most stable one. Therefore, NO prefers to adsorb on a magnetic Mn atom rather than on a Pd atom. This result is interpreted in terms of orbital interactions by the existence of a strong interaction between the partially filled 2π NO orbitals and the empty d spin-orbital of Mn. NO keeps a large magnetic moment when adsorbed on surface B (0.6μB).

Revisiting the Structure of Methyltrioxorhenium Chemisorbed on Alumina

Obtaining a detailed understanding of the structures of surface species for heterogeneous catalysts is still a challenge today, despite the tremendous advances of spectroscopic techniques. For instance, for the promising alumina-supported alkene metathesis catalysts Re2O7/Al2O3, the structures of the surface species and the active sites are still a matter of debate after forty years of research. Recent advances on a model system based on chemisorbed CH3ReO3 on alumina [10–14] revealed that the active sites arise from surface aluminum atoms in threeor fourfold coordination (Al(3) and Al(4)) with strong Lewis acidity. In particular, CH3ReO3 reacts with the Lewis acidic sites of alumina to generate two types of surface species (Scheme 1), resulting from coordination of the oxo ligand (1, 85 %) or from heterolytic splitting of the C H bond on Lewis acidic Al sites to form a m-CH2 species and a surface OH group (2, 15 %). The minor species 2 is the precursor and the resting state of the active species in alkene metathesis, that is, the carbene (3).

Triisobutylaluminum: bulkier and yet more reactive towards silica surfaces than triethyl or trimethylaluminum

Triisobutylaluminum reacts with silica yielding three different Al sites according to high-field aluminum-27 NMR and first principle calculations: a quadruply grafted dimeric surface species and two incorporated Al(O)x species (x = 4 or 5). This result is in stark contrast to the bis-grafted species that forms during Et3Al silica grafting. Thus the isobutyl ligands, which render R3Al monomeric, lead to greater reactivity towards the silica surface.