Chiral supported ionic liquid phase (CSILP) catalysts for greener asymmetric hydrogenation processes

Chiral supported ionic liquid phase (CSILP) catalysts were prepared by physical adsorption (within highly porous carbons or mesoporous silica) of Ir, Ru and Rh complexes as IrCl(COD)-(S, S)-BDPP, [IrCl-(S)-BINAP](2), RuCl(p-cymene)[(S, S)-Ts-DPEN], RuOTf(p-cymene)[(S, S)-Ts-DPEN], [Rh(COD)(S, S)-DIPAMP][BF4], and [Rh(COD)(R, R)-Me-DuPHOS][BF4]. For the syntheses of CSILP catalysts [EMIM][NTf2], [BMIM][BF4] and [BMIM][PF6] ionic liquids were used. Comparative homogeneous and heterogeneous experiments were carried out using the asymmetric hydrogenation of double -C N- and -C C- bonds in trimethylindolenine, 2-methylquinoline and dimethylitaconate, respectively. The conversion and enantioselectivity was found to depend on the nature of the complex (metal and ligand), the immobilization method used, nature of the ionic liquid, nature of the support and the experimental conditions. (C) 2012 Elsevier B.V. All rights reserved.

Promotion of Ceria Catalysts by Precious Metals: Changes in Nature of the Interaction under Reducing and Oxidizing Conditions

By depositing ceria over supported precious metal (PM) catalysts and characterizing them with in situ diffuse reflectance UV (DR UV) and in situ Raman spectroscopy, we have been able to prove a direct correlation between a decrease in ceria band gap and the work function of the metal under reducing conditions. The PM ceria interaction results in changes on the ceria side of the metal ceria interface, such that the degree of oxygen vacancy formation on the ceria surface also correlates with the precious metal work function. Nevertheless, conclusive evidence for a purely electronic interaction could not be provided by X-ray photoelectron spectroscopy (XPS) analysis. On the contrary, the results highlight the complexity of the PM ceria interaction by supporting a spillover mechanism resulting from the electronic interaction under reducing conditions. Under oxidizing conditions, another effect has been observed; namely, a structural modification of ceria induced by the presence of PM cations. In particular, we have been able to demonstrate by in situ Raman spectroscopy that, depending on the PM ionic radius, it is possible to create PM ceria solid solutions. We observed that this structural modification prevails under an oxidizing atmosphere, whereas electronic and chemical interactions take place under reducing conditions.

A more direct way to make catalysts: one-pot ligand-assisted aerobic stripping and electrodeposition of copper on graphite

Electrodeposition of metals onto conductive supports such as graphite potentially provides a lower-waste method to form heterogeneous catalysts than the standard methods such as wet impregnation. Copper electrodeposition onto pressed graphite disc electrodes was investigated from aqueous CuSO4-ethylenediamine solutions by chronoamperometry with scanning electron microscopy used to ascertain the particle sizes obtained by this method. The particle size was studied as a function of pH, CuSO4-ethylenediamine concentration, and electrodeposition time. It was observed that decreasing the pH, copper-ethylenediamine concentration and time each decreased the size of the copper particles observed, with the smallest obtained being around 5-20 nm. Furthermore, electroless aerobic oxidation of copper metal in the presence of ethylenediamine was successfully coupled with the electrodeposition in the same vessel. In this way, deposition was achieved sequentially on up to twenty different graphite discs using the same ethylenediamine solution, demonstrating the recyclability of the ligand. The materials thus prepared were shown to be catalytically active for the mineralisation of phenol by hydrogen peroxide. Overall, the results provide a proof-of-principle that by making use of aerobic oxidation coupled with electrochemical deposition, elemental base metals can be used directly as starting materials to form heterogeneous catalysts without the need to use metal salts as catalyst precursors.

Redispersion of Gold Supported on Oxides

Although many gold heterogeneous catalysts have been shown to exhibit significant activity and high selectivity for a wide range of reactions in both the liquid and gas phases, they are prone to irreversible deactivation. This is often associated with sintering or loss of the interaction of the gold with the support. Herein, we report on the use of methyl iodide as a method of dispersing gold nanoparticles supported on silica, titania, and alumina supports. In the case of titania- and alumina-based catalysts, the gold was transformed from nanometer particles into small clusters and some atomically dispersed gold. In contrast, although there was a drop in the gold particle size on the silica support following CH3I treatment, the size remained in the submicrometer range. The structural changes were correlated with changes in the selectivity and activity for ethanol dehydration and benzyl alcohol oxidation. From these observations, it is clear that this treatment provides a method by which deactivated gold catalysts can be reactivated via redispersion of the gold.

Local-field and excitonic effects in the optical response of alpha-alumina

This paper introduces key ingredients of the dielectric response of a-alumina that go beyond an independent-particle (IP) treatment of the valence-electron excitations. The optical-response functions were calculated from first-principles both at the Bethe-Salpeter and the random-phase approximation (RPA) levels. Excitonic effects obtained within the Bethe-Salpeter framework were found essential for reproducing the low-energy part of the experimental spectra (below 15 eV) and the bound exciton in particular. For higher energies, local-field effects introduced through the RPA modified considerably the IP results and provided a satisfactory account of the reflectivity spectra and of the position and shape of the dominant bulk plasmon resonance in the electron energy-loss spectra.

Palladium-catalyzed liquid-phase hydrogenation/hydrogenolysis of disulfides

For the first time, the hydrogenation/hydrogenolysis of a range of disulfides has been achieved over a supported palladium catalyst using hydrogen under relatively benign conditions. These unexpected results demonstrate that it is possible to avoid the poisoning of the catalyst by either the nitrogen-containing groups or the sulfur species, allowing both efficient reaction and recycling of the catalyst under the proper conditions (e.g., at low temperatures). A slight loss in activity was found on recycling; however, the catalyst activity can be recovered using hydrogen pretreatment. The reaction mechanism for the hydrogenolysis and hydrogenation of ortho-, meta-, and para-dinitrodiphenyldisulfide to the corresponding aminothiophenol has been elucidated. Density functional theory calculations were used to investigate the adsorption mode of the dinitrodiphenyldisulfides; a clear dependence on adsorption geometry was found regarding whether the molecule is cleaved at the S-S bond before the reduction of the nitro group or vice versa. This study demonstrates the versatility of these catalysts for the hydrogenation/hydrogenolysis of sulfur-containing molecules, which normally are considered poisons, and will extend their use to a new family of substrates. (C) 2007 Elsevier Inc. All rights reserved.

A fast transient kinetic study of the effect of H-2 on the selective catalytic reduction of NOx with octane using isotopically labelled (NO)-N-15

The H-2-assisted hydrocarbon selective catalytic reduction (HC-SCR) of NO, was investigated using fast transient kinetic analysis coupled with isotopically labelled (NO)-N-15. This allowed monitoring of the evolution of products and reactants during switches of H-2 in and out of the SCR reaction mix. The results obtained with a time resolution of less than 1 s showed that the effect on the reaction of the removal or addition of H-2 was essentially instantaneous. This is consistent with the view that H-2 has a direct chemical effect on the reaction mechanism rather than a secondary one through the formation of "active" Ag clusters. The effect of H-2 partial pressure was investigated at 245 degrees C, it was found that increasing partial pressure of H-2 resulted in increasing conversion of NO and octane. It was also found that the addition of H-2 at 245 degrees C had different effects on the product distribution depending on its partial pressure. The change of the nitrogen balance over time during switches in and out of hydrogen showed that significant quantities of N-containing species were stored when hydrogen was introduced to the system. The positive nitrogen balance on removal of H-2 from the gas phase showed that these stored species continued to react after removal of hydrogen to form N-2. (c) 2006 Elsevier Inc. All rights reserved.

Heterogeneous oxidation of pyrimidine and alkyl thioethers in ionic liquids over mesoporous Ti or Ti/Ge catalysts

Heterogeneous catalytic oxidation of a series of thioethers (2-thiomethylpyrimidine, 2-thiomethyl-4,6-dimethyl-pyrimidine, 2-thiobenzylpyrimidine, 2-thiobenzyl-4,6-dimethylpyrimidine, thioanisole, and n-heptyl methyl sulfide) was performed in ionic liquids by using MCM-41 and UVM-type mesoporous catalysts containing Ti, or Ti and Ge. A range of triflate, tetrafluoroborate, trifluoroacetate, lactate and bis(trifluoromethanesulfonyl)imide-based ionic liquids were used. The oxidations were carried out by using anhydrous hydrogen peroxide or the urea-hydrogen peroxide adduct and showed that ionic liquids are very effective solvents, achieving greater reactivity and selectivity than reactions performed in dioxane. The effects of halide and acid impurities on the reactions were also investigated. Recycling experiments on catalysts were carried out in order to evaluate Ti leaching and its effect on activity and selectivity.

Electronic, structural, and reactive properties of ultrathin aluminum oxide films on Pt(111)

Low-energy electron diffraction, X-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction spectrometry results are reported for the structural and reactive behavior of alumina films grown on Pt(111) as a function of thickness and oxidation temperature. Submonolayer Al films undergo compete oxidation at 300 K, annealing at 1100 K resulting in formation of somewhat distorted crystalline gamma-alumina, Thicker deposits require 800 K oxidation to produce Al2O3, and these too undergo crystallization at 800 K, yielding islands of apparently undistorted gamma-alumina on the Pt(111) surface. Oxidation of a p(2 x 2) Pt3Al surface alloy occurs only at>800 K, resulting in Al extraction, These alumina films on Pt(lll) markedly increase the coverage of adsorbed SO4 resulting from SO2 chemisorption onto oxygen-precovered surfaces. This results in enhanced propane uptake and subsequent reactivity relative to SO4/Pt(111). A bifunctional mechanism is proposed to account for our observations, and the relevance of these to an understanding of the corresponding dispersed systems is discussed.

Structure-reactivity Correlations in the catalytic coupling of ethyne over novel bimetallic Pd/Sn catalysts

The structure, thermal stability, and catalytic behavior of a novel highly dispersed silica-supported Pd/Sn catalyst prepared by an organometallic route have been examined by X-ray photoelectron, X-ray diffraction, and X-ray absorption, fine structure spectroscopies, the latter two measurements being carried outwith an in situ reaction cell. Additional reactor measurements were performed on a more Sn-rich catalyst and on a pure Pd catalyst. Varying the temperature of reduction induced large variations in catalytic performance toward ethyne-coupling reactions. These changes are understandable in terms of the destruction of SnO2-like structures surrounding the Pd core, yielding a skin of metallic Sn which subsequently undergoes intermixing with Pd. The overall thermal and catalytic behavior of these highly dispersed materials accords well with the analogous single-crystal model system.

A dramatic shift from multiple to simple exchange in the cyclopentane/D-2 probe reaction on palladium catalysts

Electron deficient active sites in Pd catalysts, either as films or on supports, are deliberately generated by calcining in O-2 at high temperature followed by the mildest possible reduction (with the reaction mixture itself), and are manifested by a marked shift from multiple to simple exchange in the cyclopentane/D-2 probe reaction.

Ethylene oxidation over platinum: In situ electrochemically controlled promotion using Na-beta'' alumina and studies with a Pt(111)/Na model catalyst

Electrochemically modified ethylene oxidation over a PI film supported on the Na+ ion conductor beta '' alumina has been studied over a range of conditions encompassing both promotion and poisoning, The system exhibits reversible behavior, and the data are interpreted in terms of (i) Na-enhanced oxygen chemisorption and (ii) poisoning of the surface by accumulation of Na compounds. At low Na coverages the first effect results in increased competitive adsorption of oxygen at the expense of ethylene, resulting in an increased rate, At very negative catalyst potentials (high Na coverage) both effects operate to poison the system: the increased strength of the Pt-O bond and coverage of the catalytic surface by compounds of Na strongly suppress the rate, Kinetic and spectroscopic results for ethylene oxidation over a Pt(111)-Na model catalyst shed light on important aspects of the electrochemically controlled system, Low levels of Na promote the reaction and high levels poison it, mirroring the behavior observed under electrochemical control and strongly suggesting that sodium pumped from the solid electrolyte is the key species, XP and Auger spectra show that under reaction conditions, the sodium exists as a surface carbonate. Post-reaction TPD spectra and the use of (CO)-C-13 demonstrate that CO is formed as a stable reaction intermediate, The observed activation energy (56 +/- 3 kJ/mol) is similar to that measured for CO oxidation under comparable conditions, suggesting that the rate limiting step is CO oxidation. (C) 1996 Academic Press, Inc.