Showing papers in "Chemcatchem in 2013"

Synergistic Catalysis over Bimetallic Alloy Nanoparticles

Abstract: Bimetallic nanoparticles (NPs) have emerged as an important class of catalysts. In many cases, bimetallic alloy NPs have higher catalytic efficiencies than their monometallic counterparts, owing to strong synergy between the metals. In this Review, we survey the research progress on the synergistic effect of bimetallic alloy NPs for catalytic reactions related to fuel cells, such as the electrochemical oxidation of MeOH, EtOH, and formic acid, CO oxidation, the oxygen-reduction reaction, and the dehydrogenation of ammonia borane, formic acid, hydrous hydrazine, hydrazine borane, etc. In addition, the use of synergistic catalysis in some other important reactions has also been reviewed. Recent developments in synergistic catalysis over bimetallic alloy NPs will provide access to a variety of low-cost and high-performance catalysts for laboratory and industrial applications within the next few years.

Oxidative Dehydrogenation of Ethane: Common Principles and Mechanistic Aspects

Abstract: The increasing demand for light olefins and the changing nature of basic feedstock has stimulated substantial research activity into the development of new process routes Steam cracking remains the most industrially relevant pathway, but other routes for light-olefin production have emerged Fluid catalytic cracking only produces ethene in minor concentrations Challenged by marked catalyst deactivation, in contrast, catalytic dehydrogenation ethane up opens a more selective route to ethene The oxidative dehydrogenation (ODH) of ethane, which couples the endothermic dehydration of ethane with the strongly exothermic oxidation of hydrogen, would potentially be the most attractive alternative route because it avoids the need for excessive internal heat input, but also consumes valuable hydrogen In this Review, the current state of the ODH of ethane is compared with other routes for light-olefin production, with a focus on the catalyst and reactor system variants New catalyst systems and reactor designs have been developed to improve the industrial competitiveness of the ODH reaction of ethane The current state of our fundamental understanding of the ODH of light alkanes, in particular in terms of catalyst and reactor development, is critically reviewed The proposed mechanisms and the nature of the active site for the ODH reaction are described and discussed in detail for selected promising catalysts The reported catalytic performance and the possible limitations of these ODH catalysts will be examined and the performance of the emerging approaches is compared to the currently practiced methods

Insights into C ? C Coupling in CO2 Electroreduction on Copper Electrodes

Abstract: We present a first‐principles theoretical study of carbon–carbon coupling in CO2 electroreduction on the copper 2 1 1 surface. Using DFT, we have determined kinetic barriers to the formation of a CC bond between adsorbates derived from CO. The results of our nudged elastic band calculations demonstrate that kinetic barriers to CC coupling decrease significantly with the degree of hydrogenation of reacting adsorbates. We also show that this trend is not affected by the electrical fields present at the solid‐electrolyte interface during electrocatalysis. Our results explain how copper can catalyze the production of higher hydrocarbons and oxygenates in the electrochemical environment, despite producing only single carbon atom products in gas‐phase catalysis, and how CC bonds can be formed at room temperature in the electrochemical environment, whereas substantially higher temperatures are needed in the Fischer–Tropsch catalysis. The unique feature of the electrochemical environment is that the chemical potential of hydrogen (electrons and protons) can be varied through the applied potential. This allows a variation of the degree of hydrogenation of the reactants and thus the activation barrier for CC coupling.

Efficient BiVO4 Thin Film Photoanodes Modified with Cobalt Phosphate Catalyst and W‐doping

Abstract: Bismuth vanadate (BiVO4) thin film photoanodes for light-induced water oxidation are deposited by a low-cost and scalable spray pyrolysis method The resulting films are of high quality, as indicated by an internal quantum efficiency close to 100 % between 360 and 450 nm However, its performance under AM15 illumination is limited by slow water oxidation kinetics This can be addressed by using cobalt phosphate (Co-Pi) as a water oxidation co-catalyst Electrodeposition of 30 nm Co-Pi catalyst on the surface of BiVO4 increases the water oxidation efficiency from ≈30 % to more than 90 % at potentials higher than 12 V vs a reversible hydrogen electrode (RHE) Once the surface catalysis limitation is removed, the performance of the photoanode is limited by low charge separation efficiency; more than 60 % of the electron-hole pairs recombine before reaching the respective interfaces Slow electron transport is shown to be the main cause of this low efficiency We show that this can be remedied by introducing W as a donor type dopant in BiVO4, resulting in an AM15 photocurrent of ≈23 mA cm−2 at 123 V vs RHE for 1 % W-doped Co-Pi-catalyzed BiVO4

Selective Catalytic Oxidation of CH Bonds with Molecular Oxygen

Abstract: Although catalytic reductions, cross-couplings, metathesis, and oxidation of CC double bonds are well established, the corresponding catalytic hydroxylations of CH bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O2, the cheapest, “greenest”, and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial-scale, owing to a low space-time-yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O2 also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.

Carbon Nanomaterials in Catalysis: Proton Affinity, Chemical and Electronic Properties, and their Catalytic Consequences

Abstract: The effects of the orientation of a graphene sheet, the edge structure of carbon nanofibers, and different surface functional groups on proton affinity, interactions with metal nanoparticles, and electronic modification of these structures, together with their catalytic consequences, have been reviewed. The ratio of prismatic to basal sites on the edge of carbon nanofibers has a remarkable influence on the properties of both the CNFs and the metal nanoparticles that are supported on it. The proton affinity, interactions with metal particles, and electronic density of metal particles can be further manipulated by fine-tuning the functional groups, such as oxygen groups, and by B/N-doping of the CNFs. The possibilities of using carbon nanofibers with different graphite-sheet orientations as a platform for rational catalyst design are discussed.

Carbon Nanofiber Supported Transition‐Metal Carbide Catalysts for the Hydrodeoxygenation of Guaiacol

Abstract: Hydrodeoxygenation (HDO) studies over carbon nanofiber-supported (CNF) W2C and Mo2C catalysts were performed on guaiacol, a prototypical substrate to evaluate the potential of a catalyst for valorization of depolymerized lignin streams. Typical reactions were executed at 55 bar hydrogen pressure over a temperature range of 300–375?°C for 4 h in dodecane, using a batch autoclave system. Combined selectivities of up to 87 and 69?% to phenol and methylated phenolics were obtained at 375?°C for W2C/CNF and Mo2C/CNF at >99?% conversion, respectively. The molybdenum carbide-based catalyst showed a higher activity than W2C/CNF and yielded more completely deoxygenated aromatic products, such as benzene and toluene. Catalyst recycling experiments, performed with and without regeneration of the carbide phase, showed that the Mo2C/CNF catalyst was stable during reusability experiments. The most promising results were obtained with the Mo2C/CNF catalyst, as it showed a much higher activity and higher selectivity to phenolics compared to W2C/CNF.

Morphology Effect of CeO2 Support in the Preparation, Metal–Support Interaction, and Catalytic Performance of Pt/CeO2 Catalysts

Abstract: Pt/CeO2 catalysts with various Pt loadings were prepared by a conventional incipient wetness impregnation method that employed CeO2 cubes (c-CeO2), rods (r-CeO2), and octahedra (o-CeO2) as the support and Pt(NH3)4(NO3)2 as the metal precursor. Their structures and catalytic activities in CO oxidation in excess O2 and the preferential oxidation of CO in a H2-rich gas (CO-PROX) were studied, and strong morphology effects were observed. The impregnated Pt precursor interacts more strongly with CeO2 rods and cubes than with CeO2 octahedra, and the reduction/decomposition of the Pt precursor impregnated on CeO2 octahedra is easier than that on CeO2 rods and cubes. With the same Pt loading, the Pt/o-CeO2 catalyst contains the largest fraction of metallic Pt, whereas the Pt/c-CeO2 catalyst contains the largest fraction of Pt2+ species. The reducibility of pure CeO2 and CeO2 in the Pt/CeO2 catalysts follows the order r-CeO2>c-CeO2>o-CeO2, and the reducibility of CeO2 depends on the Pt loading for the Pt/c-CeO2 catalysts but not much for the Pt/r-CeO2 and Pt/o-CeO2 catalysts. The catalytic performance of Pt/CeO2 catalysts in both CO oxidation and the CO-PROX reaction follows the order Pt/r-CeO2>Pt/c-CeO2> Pt/o-CeO2. The Pt0-CeO2 ensemble is more active than the Pt2+-CeO2 ensemble in the catalysis of CO oxidation in excess O2. H2-assisted CO oxidation catalyzed by the Pt/CeO2 catalysts was observed in the CO-PROX reaction, and the Pt2+ species and CeO2 with a large concentration of oxygen vacancies constitute the active structure of the Pt/CeO2 catalyst for the CO-PROX reaction. The effect of the morphology of the CeO2 support in the preparation, metal–support interaction, and catalytic performance of Pt/CeO2 catalysts can be correlated the exposed crystal planes and surface composition/structure of the CeO2 support with different morphologies. These results not only demonstrate that the structure and catalytic performance of oxide-supported catalysts can be tuned by controlling the morphology of the oxide support but also deepens the fundamental understanding of CO oxidation reactions catalyzed by Pt/CeO2 catalysts.

Hydrodeoxygenation of Guaiacol over Carbon‐Supported Metal Catalysts

Abstract: Catalytic bio-oil upgrading to produce renewable fuels has attracted increasing attention in response to the decreasing oil reserves and the increased fuel demand worldwide. Herein, the catalytic hydrodeoxygenation (HDO) of guaiacol with carbon-supported non-sulfided metal catalysts was investigated. Catalytic tests were performed at 4.0 MPa and temperatures ranging from 623 to 673 K. Both Ru/C and Mo/C catalysts showed promising catalytic performance in HDO. The selectivity to benzene was 69.5 and 83.5 % at 653 K over Ru/C and 10Mo/C catalysts, respectively. Phenol, with a selectivity as high as 76.5 %, was observed mainly on 1Mo/C. However, the reaction pathway over both catalysts is different. Over the Ru/C catalyst, the OCH3 bond was cleaved to form the primary intermediate catechol, whereas only traces of catechol were detected over Mo/C catalysts. In addition, two types of active sites were detected over Mo samples after reduction in H2 at 973 K. Catalytic studies showed that the demethoxylation of guaiacol is performed over residual MoOx sites with high selectivity to phenol whereas the consecutive HDO of phenol is performed over molybdenum carbide species, which is widely available only on the 10Mo/C sample. Different deactivation patterns were also observed over Ru/C and Mo/C catalysts.

Catalyst Development for CO2 Hydrogenation to Fuels

Abstract: New active and selective catalyst compositions for the hydrogenation of CO2 to mainly fuel-type higher hydrocarbons were developed by application of an evolutionary strategy. It was shown that Fe and K supported on TiO2 and modified by Cu plus other modifiers resulted in highest selectivity for C5–C15 hydrocarbons at high degrees of CO2 conversion. Co containing catalysts were less suited since they produced methane and light hydrocarbons with high selectivities. A detailed study of reaction conditions showed that selected catalyst compositions were able to reach high CO2 conversion with still low selectivities to methane at higher reaction temperatures and a higher H2/CO2 ratio.

Catalytic Performance of Zeolite‐Supported Vanadia in the Aerobic Oxidation of 5‐hydroxymethylfurfural to 2,5‐diformylfuran

Abstract: The catalytic performance of zeolite-supported vanadia catalysts was examined for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in organic solvents such as N,N-dimethylformamide (DMF), methyl isobutyl ketone, toluene, trifluorotoluene and DMSO. Catalysts based on the four different zeolite supports H-beta, H-Y, H-mordenite, and H-ZSM-5 with 1–10 wt % vanadia loading were prepared and characterized by nitrogen physisorption, X-ray powder diffraction, scanning electron microscopy, ammonia temperature-programmed desorption, Raman spectroscopy and UV/Vis spectrophotometry. The H-beta zeolite catalysts were found to contain highly dispersed vanadium oxide species at all loadings, and provided the highest reaction selectivity towards DFF and the lowest metal leaching of the examined systems. In particular, 1w t % V 2O5/H-beta was found to be a stable, recyclable, and non-leaching catalyst for the production of DFF under mild conditions in DMF as solvent, although with low DFF yield. To increase the yield, oxidation of HMF at elevated pressures was also investigated with this catalyst. Under optimized conditions, a reaction selectivity towards DFF of > 99 % at 84 % HMF conversion was obtained, albeit with some contribution from lixiviated species to the total catalyst activity.

Highly Selective Synthesis of Phenol from Benzene over a Vanadium‐Doped Graphitic Carbon Nitride Catalyst

Abstract: Design and preparation of efficient and economical catalysts for direct hydroxylation of benzene to phenol is an important topic. In this work, a series of metal-doped graphitic carbon nitride catalyst (Cu-, Fe-, V-, Co-, and Ni-g-C3N4) were successfully synthesized by using urea as the precursor through a facile and efficient method. The catalysts were characterized systematically using N2 adsorption–desorption, FTIR, thermogravimetric analysis, powder X-ray diffraction, and X-ray photoelectron spectroscopy techniques. It was found that the vanadium-doped graphitic carbon nitride catalyst V-g-C3N4 was the most efficient catalyst for the direct synthesis of phenol from benzene with hydrogen peroxide as the oxidant and it could be recycled at least 4 times. The influence of reaction conditions such as the solvent, reaction temperature, reaction time, and the amounts of catalyst and hydrogen peroxide were investigated. Under optimized conditions, 18.2 % yield of phenol was obtained with the selectivity to phenol as high as 100 %.

Imaging Catalysts at Work: A Hierarchical Approach from the Macro‐ to the Meso‐ and Nano‐scale

Abstract: This review highlights the importance of developing multi-scale characterisation techniques for analysing operating catalysts in their working environment. We emphasise that a hierarchy of in situ techniques that provides macro-, meso- and nano-scale information is required to elucidate and optimise catalyst performance fully. This combined methodology should ideally embrace spatially resolved and spatio-temporal monitoring of a) the structure, catalytic activity, temperature and heat/mass transfer pattern in axial and radial directions in real reactors, b) the structure and temperature/heat/mass transport gradients in shaped catalysts and catalyst grains and c) meso- and nano-scale information about particles and clusters, whose physical and electronic properties are linked directly to the micro-kinetic behaviour of the catalysts. Techniques such as X-ray diffraction (XRD), infrared (IR), Raman, X-ray photoelectron spectroscopy (XPS), UV/Vis, and X-ray absorption spectroscopy (XAS), which have mainly provided global atomic scale information, are being developed to provide the same information on a more local scale, often with sub-second time resolution. X-ray microscopy, both in the soft and more recently in the hard X-ray regime, allows two- and three-dimensional information to be collected down to 10 nm spatial resolution in a gas atmosphere or liquid, although this improvement is at the expense of temporal resolution. Electron microscopy, which provides excellent local atomic scale structural and spectroscopic information, is being developed towards tomographic imaging and realistic conditions, allowing gaps to be bridged in pressure, reaction conditions and length scales, up to the meso- and macro-scale. In addition, new techniques such as single molecule fluorescence spectroscopy and non-linear spectroscopic techniques are emerging. In this review, we discuss prospects for the development and combined application of both existing and new techniques for in situ catalyst characterisation.

Mesoporous TiO2: Preparation, Doping, and as a Composite for Photocatalysis

Abstract: Mesoporous materials are widely used in solar cells, lithium-ion batteries, sensors, and supercapacitors. Mesoporous TiO2 is also highly active in photocatalysis, owing to the benefits of the mesoporous network in promoting the diffusion of the reactants and products, as well as in facilitating access to the reactive sites on the surface of the photocatalysts. This Minireview focuses on the development of mesoporous TiO2, metal/nonmetal-doped mesoporous TiO2 networks, and mesoporous TiO2 composites. The design and synthesis of mesoporous TiO2, including the formation of various mesostructured frameworks, pore-size control, large-pore-size fabrication, and highly thermally stable mesoporous-network formation, are described in detail. From an application viewpoint, the subsequent fabrication of doped and multifunctional heterojunctions in mesoporous TiO2 is necessary for further improving its photocatalytic performance. The doping of mesoporous TiO2 with metals or nonmetals could extend the absorption edge into the visible-light spectrum and improve the separation efficiency of photogenerated electron–hole pairs. The fabrication of mesoporous TiO2 composites, especially heterojunctions, could make full use of the characteristics of each component for improving the overall photocatalytic performance. In view of this fact, mesoporous TiO2 still possesses significant room for development in photocatalysis and is worthy of greater attention in the future.

Efficient One‐Pot Preparation of Cu‐SSZ‐13 Materials using Cooperative OSDAs for their Catalytic Application in the SCR of NOx

Abstract: The cooperative use of the Cu-tetraethylenepentamine complex and N,N,N-trimethyl-1-adamantammonium as organic structure-directing agents (OSDAs) enabled the rationalized “one-pot” preparation of Cu-containing SSZ-13 zeolites. A detailed study of different synthetic variables permitted the control of the Si/Al and Cu/(Si+Al) ratios in the final solids. Cu-SSZ-13 molecular sieves synthesized in alkaline media demonstrate excellent catalytic activities and good hydrothermal stabilities for the selective catalytic reduction of NOx. Finally, the effect of synthesis media on the catalytic active sites is also demonstrated and a remarkable activity loss for samples synthesized in fluoride media is observed.

Zeolite Catalyzed Transformation of Carbohydrates to Alkyl Levulinates

Abstract: The carbon-based chemicals and fuels that are necessary to meet the energy demand for our society originate presently almost exclusively from inexpensive fossil resources. The forecast of diminishing and more expensive petroleum reserves has, however, engaged the chemical industry to find new feasible routes to convert biomass into potential platform chemicals, that can replace the fossil based chemicals in order to leave the chemical supply chain unaffected (value chain). In biomass, the major source of renewable carbon is confined in lignocellulose, but selective hydrolysis combined with efficient separation of the lignocellulosic fractions; lignin, cellulose, and hemicellulose, remain a technical challenging task. In particular, the direct conversion of lignocellulosic biomass to potential chemicals with high selectivity provides a bottleneck, owing to the recalcitrant feedstock. Conversion of the individual monomeric components in lignocellulose to chemicals is an alternative way to make platform molecules and improve the yield of the targeted product. 7] Consequently, we have focused on how levulinic acid esters can be directly derived from mono-, di-, and polysaccharides using heterogeneous catalysis. Levulinic acid has been listed as one of the twelve most important sugar-based building blocks obtainable from biomass, with a variety of potential applications as solvent, food flavoring agent, plasticizer, resin intermediate, and building blocks for, for example, tetrahydrofuran and succinic acid. To produce levulinic acid, carbohydrates are traditionally being treated with aqueous mineral acid (H2SO4 and HCl). [10] However, a major drawback in this homogeneous process is tedious work-up during the separation stages. Previously, few solid-acid catalysts have been investigated for the conversion of glucose/cellulose to levulinic acid in water. Yand Beta-type zeolites gave 10–40 % yield of levulinic acid from glucose/fructose/cellulose in water at temperatures between 110–240 8C. Despite the many advantages of zeolites, they are not hydrothermally stable in water during the reaction, which leads to a loss of structural integrity and can, therefore, not be recycled. Alternatively, Schraufnagel and Rase have reported that sucrose can be transformed into levulinic acid in water using acidic ion-exchange resins as catalysts at temperatures between 100 and 140 8C. However, owing to thermal instability of the resin at elevated temperatures, moderate yield of activity (25 % levulinic acid), and difficulty in removing humin by-products from the catalyst under mild conditions, they are unattractive for this transformation. A quantitative conversion of glucose was observed over Fe-pillared montmorillonite catalyst, but the selectivity to levulinic acid was only 20 % and a large amount of humins were also observed in this study. Recently, Peng et al. reported that glucose can be directly converted to ethyl levulinate (ELevu) over sulfated zirconia catalysts with a moderate yield up to 30 % at 200 8C. Furthermore, we found that sulfonic acid functionalized ionic liquids and solid Brønsted acids are more promising catalysts for the conversion of fructose to ELevu in ethanol than for the analogous conversion to levulinic acid in water. Here we introduce zeolites as catalysts for the conversion of carbohydrates to alkyl levulinates in alcohol medium to enhance the yield as well as to circumvent the drawbacks of hydrothermal and thermal instability of the catalysts. Aluminum-free Lewis acidic zeotype materials and mesoporous molecular sieves containing Sn, Ti, or Zr are capable of isomerizing hexoses, pentoses and trioses and converting them into useful chemicals. 16] Interestingly, we observed in the present study that glucose can efficiently isomerize to form fructose in alcohol media over zeolites without auxiliary Lewis acid metals (such as Sn, Ti, or Zr) other than aluminum. Under optimized reaction conditions, the transformation of glucose to methyl levulinate (MLevu) in methanol over various commercially available zeolites has been performed and the results are presented in Table 1. All of the experiments were performed with quantitative conversion of glucose and other monoand disaccharides causing the reported yields and se-

Asymmetric Reduction of Cyclic Imines Catalyzed by a Whole‐Cell Biocatalyst Containing an (S)‐Imine Reductase

Abstract: In view of the importance of chiral amines as building blocks for biologically active pharmaceutical drugs, considerable effort has been devoted to the development of asymmetric catalytic methods for their preparation. Notable advances have been made by using transition-metal catalysis, organocatalysis, and artificial transfer hydrogenases. However, biocatalytic approaches have emerged as being especially important, including those based upon transaminases, monoamine oxidases, and recently engineered NADH-dependent l-amino acid dehydrogenases (NADH is the reduced form of NAD = nicotinamide adenine dinucleotide). These methods are complementary in terms of the required substrates (amine, ketone) and also the amine that they generate (primary, secondary, or tertiary). In some cases, these biocatalytic processes have been successfully demonstrated at an industrial scale for the manufacture of recently launched drugs. However, there are no general biocatalytic strategies available that are based upon asymmetric reduction of imines as a route to enantiomerically pure amines. Imine reductases (IREDs) catalyze the reduction of imines to amines by utilizing NADH or NADPH as a cofactor (Scheme 1, NADPH is the reduced form of NADP = nicotinamide adenine dinucleotide phosphate).

Ag/AgBr‐Grafted Graphite‐like Carbon Nitride with Enhanced Plasmonic Photocatalytic Activity under Visible Light

Abstract: Ag/AgBr‐grafted graphite‐like carbon nitride (g‐C3N4) is fabricated by the in situ photoreduction of AgBr/g‐C3N4 hybrids prepared by a deposition–precipitation method. The Ag/AgBr/g‐C3N4 hybrids exhibit a strong absorbance in the visible and near‐IR region because of the surface plasmon resonance absorption of Ag nanocrystals. Compared with bare g‐C3N4 and Ag/AgBr nanoparticles, a 28‐fold and six‐fold enhancement in the degradation rate of rhodamine B is observed over Ag/AgBr/g‐C3N4 hybrids under visible‐light irradiation, respectively. The immense enhancement of the photocatalytic activity is attributed to the extended absorption in the visible‐light region, effective charge separation, and synergistic enhancement in the ternary Ag/AgBr/g‐C3N4 system. Moreover, the composite can be reclaimed easily by sedimentation without any decrease of its photocatalytic activity. This study provides new insight into the fabrication of highly efficient and stable g‐C3N4‐based plasmonic photocatalysts and facilitates their practical application to solve environmental issues.

Complete Catalytic Deoxygenation of CO2 into Formamidine Derivatives

Abstract: Because fossil resources are a limited feedstock and their extensive use results in the problematic accumulation of CO2 in the atmosphere, the organic-chemical industry will face important challenges over the coming few decades to circumvent the use of raw fossil materials. In particular, the fuel, petrochemical, and fine-chemicals industries have to find alternative feedstocks and carbon-free energy sources to embrace sustainability. In this regard, CO2 has been proposed as an “energy vector” for renewable energies, as a solution for hydrogen storage, and as a C1 building block for the synthesis of fine chemicals. 6] Yet, as a waste compound, CO2 is thermodynamically and kinetically difficult to transform and research efforts are still needed to promote shifts in technology in the chemical industry. Among the challenges that are associated with CO2 transformation, we must acknowledge that, despite recent progress, the scope of chemical functions that are available from CO2 is still very limited and mostly consists of molecules in which at least one C O bond from CO2 is retained. 6] In fact, the only catalytic reaction that results in the complete deoxygenation of CO2 is its reduction into methane by hydrogenation, hydrosilylation, or electrochemical methods. Interestingly, Wehmschulte and co-workers recently observed that toluene and diphenylmethane could be obtained as side-products in the silylium-catalyzed hydrosilylation of CO2 into methane in the presence of benzene. To utilize CO2 as a “true” C1 building block and to prepare a wide spectrum of chemicals, catalytic reactions that are able to promote the complete deoxygenation of CO2 with the complete reconstruction of the carbon valence sphere are required (Scheme 1). Herein, we report the first solution to tackling this problem by using the cascade reductive functionalization of CO2 into benzimidazoles, quinazolinones, formamidines, and their derivatives. We recently reported an organocatalytic formylation reaction of N H bonds by using CO2 and hydrosilanes to yield formamides. To substitute the C=O bond in the formamide derivative and achieve complete deoxygenation, we reasoned that the amide function could be reacted in a cascade reaction with a nucleophile (Scheme 1), such as an amine. The formylation step was efficiently catalyzed by N-heterocyclic carbenes (NHCs), which were found to be efficient at room temperature for the conversion of a large scope of amines, anilines, imines, and N heterocycles. This reaction was mild, robust, and selective; thus, it offered a good starting point for the development of new cascade reactions. Alternatively, nitrogen bases, such 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), were also active catalysts in this reaction, but at higher temperatures (100 8C). By using a primary amine as a nucleophile, the condensation step with a formamide is known to be thermally available without needing to resort to catalysts. However, hard drying agents, such as phosphorus oxychloride, trifluoroacetic anhydride, and thionyl chloride, are typically required to promote this reaction. To avoid the use of such additives, which increase waste formation, we investigated the reactivity of a diamine, o-phenylene diamine (1 a), in the presence of CO2 and hydrosilanes, so as to favor an intramolecular condensation reaction (Table 1). By using 5.0 mol % of IPr in the presence of CO2 (2 bar) and 1 equivalent of phenylsilane, compound 1 a was converted in high yield into its formyl and N,N’-bisformyl derivatives (compounds 2 a (31 %) and 3 a (38 %) respectively) after 24 h at 25 8C (Table 1, entry 5). To our delight, a significant amount (16 %) of the desired benzimidazole (4 a) was also observed in the reaction mixture. This reaction demonstrated that the complete deoxygenation product (4 a) was available under our reaction conditions. However, the observed selectivity indicates the high rate of the formylation reaction in the presence of PhSiH3. Because compound 3 a is unreactive towards condensation, a less-reactive silane, that is, poly(methylhydrosiloxane) (PMHS), 9] was employed to avoid the formylation of both amine functions. By using 3 equivalents of PMHS under similar reaction conditions (Table 1, entry 6), monoformyl derivative 2 a was formed as the major compound (42 % yield) and compounds 3 a and 4 a were formed as sideproducts (in 20 % and 5 % yield, respectively). Therefore, the condensation step appears to be rate determining in this cascade strategy. As a consequence, raising the operating temper[a] Dr. O. Jacquet, C. Das Neves Gomes, Dr. M. Ephritikhine, Dr. T. Cantat CEA, IRAMIS, SIS2M, CNRS UMR 3299 91191 Gif-sur-Yvette (France) Fax: (+ 33) 1-6908-6640 E-mail : thibault.cantat@cea.fr Homepage: http ://iramis.cea.fr/Pisp/thibault.cantat/index.htm Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200732. It includes a detailed description of the experimental and spectroscopic results. Scheme 1. Principles of our approach.

Synthesis of Cyclic Carbonates from Epoxides and CO2 under Mild Conditions Using a Simple, Highly Efficient Niobium-Based Catalyst

Abstract: Carbon dioxide is increasingly regarded as a ubiquitous and nontoxic C1 feedstock for the preparation of bulk commodity chemicals, and thus, it can be considered as a promising future alternative to depleting carbon-based fossil fuel sources. In contrast, the steadily increasing concentration of CO2 in the atmosphere has already reached unsustainably high levels as a result of human activities. Hence, the search for low-energy, carbon-neutral processes to convert CO2 into useful chemicals is of paramount importance. In this context, the exothermic reaction of epoxides and CO2 to form cyclic carbonates is of particular interest in catalysis research. Propylene carbonate (PC, 2 a) and ethylene carbonate (EC, 2 b) find wide application in industry. 5] Recently, sophisticated catalysts were reported for the homogeneous-phase synthesis of organic carbonates at room temperature and at atmospheric pressure, including twoor single-component bimetallic aluminum–salen systems, iron and bismuth complexes, and m-oxotetranuclear zinc and cobalt clusters. Alternative catalytic tools formed by combining Lewis acidic metal halides with nucleophilic co-catalysts (i.e. , MoCl5/PPh3, [10] ZnCl2/NBu4I, [11] InBr3/PPh3 ) have so far shown only low to moderate levels of activity for the synthesis of PC under ambient conditions. Nevertheless, inorganic metal complexes are inexpensive, readily available, and can serve as useful benchmarks to examine the potential of a metal towards the development of more elaborate organometallic complexes and clusters with enhanced activities and stabilities. With the exception of chromium, there are only few reports on the application of metals of group 4–6 for the synthesis of cyclic carbonates. 10] We explored group 4–6 transition-metal complexes (halides and oxychlorides) in combination with a standard nucleophilic co-catalyst : N,N-dimethylaminopyridine (DMAP). Preliminary screening was carried out under mild conditions [50 8C, 5 bar (1 bar = 100 kPa)] . All reactions led to the formation of PC as the only product. The halides and oxychlorides of 4d transition metals, including ZrCl4, NbCl5, MoCl5, and MoOCl4, were the most active (Table 1). In particular, NbCl5/ DMAP formed a very active catalyst (Table 1, entries 6 and 7).

Selective Production of Aromatics from Alkylfurans over Solid Acid Catalysts

Abstract: Solid acid catalysts were studied at temperatures near 523 K for the production of benzene, toluene, and p‐xylene by the reaction of ethylene with furan, 2‐methylfuran, and 2,5‐dimethylfuran, respectively, through the combination of cycloaddition and dehydrative aromatization reactions. Catalysts containing Brønsted acid and Lewis acid sites (i.e., WOx–ZrO2, niobic acid, zeolite Y, silica–alumina) were more active than catalysts containing predominantly Lewis acid sites (γ‐Al2O3, TiO2), which indicates the importance of Brønsted acidity in the production of aromatics. Microporosity is not required for this reaction, because amorphous solid acids and homogeneous Brønsted acids demonstrate significant activity for p‐xylene production. The production of p‐xylene from 2,5‐dimethylfuran proceeded at higher rates compared with the production of toluene and benzene from 2‐methylfuran and furan, respectively. Both WOx–ZrO2 and niobic acid demonstrate superior activity for aromatics production than does zeolite Y. WOx–ZrO2 demonstrates a turnover frequency for p‐xylene production that is 35 times higher than that demonstrated by zeolite Y. In addition, mesoporous materials such as WOx–ZrO2 offer higher resistance to deactivation by carbon deposition than do microporous materials. Results from Raman spectroscopy and the trend of turnover frequency with varying tungsten surface densities for a series of WOx–ZrO2 catalysts are consistent with previous investigations of other acid‐catalyzed reactions; this suggests that the high reactivity of WOx–ZrO2 is mainly associated with the presence of subnanometer WOx clusters mixed with zirconium, which reach a maximum surface concentration at intermediate tungsten coverage.

MOFs as Multifunctional Catalysts: Synthesis of Secondary Arylamines, Quinolines, Pyrroles, and Arylpyrrolidines over Bifunctional MIL-101

Abstract: Bifunctional MIL-101 MOFs containing Lewis acid Cr3+ sites and Pd or Pt hydrogenation/reduction centers, either as isolated metal complexes or in the form of encapsulated metal nanoparticles, have shown to be highly active catalysts for the one-pot nitroarene reduction and reductive amination of carbonyl compounds. This preparation procedure has been successfully applied to the synthesis of secondary arylamines, quinolines, pyrrols, and 3-arylpyrrolidines. In all the cases, the MOFs have shown superior performances with respect to commercially available Pd and Pt metal catalysts under the same conditions.

Three Carbons for Complexity! Recent Developments of Palladium‐Catalyzed Reactions of Allenes

Abstract: The three carbon atoms of allene moieties allow unique transformations and rapid generation of complexity. Not surprisingly, allenes became extremely versatile building blocks in organic synthesis. Transition-metal-catalyzed reactions of these cumulene π-systems have been particularly successful, and many applications in the synthesis of complex products have been reported. This review summarizes the palladium-catalyzed transformation of allenes published during the last decade. Many of the examples presented are impressive multicomponent processes or cascade reactions involving two or more steps leading to molecular complexity in simple one-pot operations. Consequently, several reactions have been developed with the goal of delivering new synthetic routes to natural products.

Biocatalytic Methods for C ? C Bond Formation

Abstract: Carbon-carbon bond formation is among the most challenging transformations in the organic synthetic chemistry. Enzymes capable to perform this reaction are of great interest. The enzymes for stereoselective CC bond formations have been investigated very intensively during the last two decades. New recombinant DNA technologies have paved the way for improved catalysts and broaden the application scope of the already known enzymes and reactions. On the other side new discoveries have brought more enzyme players in the arena of CC bond formation reactions. Novel enzymatic CC bond formation reactions have been applied, implying the most important benefit of biocatalysis, namely the high selectivity.

Lignin-Based Green Catalyst for the Chemical Fixation of Carbon Dioxide with Epoxides To Form Cyclic Carbonates under Solvent-Free Conditions

Abstract: lignin-based green catalyst for the chemical fixation of carbon dioxide with epoxides to form cyclic carbonates under solvent-free conditions

Organometallic Ruthenium Nanoparticles: A Comparative Study of the Influence of the Stabilizer on their Characteristics and Reactivity

Abstract: The use of metal nanoparticles as catalysts is a topic of growing interest at the frontier between homogeneous and heterogeneous catalysis. Metal nanoparticles are highly interesting systems owing to their high number of surface atoms, which give rise to numerous active sites. Furthermore, the surface properties of metal nanoparticles can be tuned by the addition of a stabilizer, for example, a polymer, a surfactant, or a ligand, or by combining a metal with a support to take profit of their synergy to orientate a catalytic reaction. Significant efforts are being made towards the synthesis of metal nanoparticles in general and, more precisely, towards the preparation of ligand-stabilized nanoparticles in which the size, shape, and surface state are controlled. Since ligands can modulate both the electronic and steric environment at the surface of the particles, numerous studies are presently devoted to analyze the influence of ligands on the stabilization of nanoparticles and on their surface properties. Such studies are of key importance to develop more active and selective nanocatalysts. In that context, ruthenium nanoparticles are candidates of choice as they can be characterized inter alia by nuclear magnetic resonance, as ruthenium displays little or no Knight shift and since they are active catalysts for hydrogenation reactions of, for example, arenes, olefins, and alkynes. In this Review, we present an overview of our group's efforts in the synthesis of ligand-stabilized ruthenium nanoparticles of controlled size and surface state using different types of ligands. We report the influence of nitrogen-, sulfur-, silicon-, phosphorus- and carbon- containing ligands as coordinating atoms to the metal surface, on their stabilization, as well as on their surface reactivity, in comparison with sterically-stabilized Ru nanoparticles prepared following the same organometallic approach, but using polymers or "nanoreactors" made of alcohols or ionic liquids that allow for control of the growth of the particles by a confinement effect. Nanoparticles of other metals are also described when appropriate.

Rh1/γ-Al2O3 Single-Atom Catalysis of O2 Activation and CO Oxidation: Mechanism, Effects of Hydration, Oxidation State, and Cluster Size

Abstract: The single‐atom catalysis of O2 activation and CO oxidation with Rh1 supported on γ‐Al2O3 is investigated here through ab initio molecular dynamics techniques. We scrutinize the molecular details of the mechanism for the full catalytic cycle that involves the oxidation of two CO molecules in succession. The effect of the surface hydration and oxidation state of Rh on the kinetics of O2 activation and CO oxidation is presented. We also report here the catalytic activity of experimentally intercepted RhI(CO)2 on γ‐Al2O3. Furthermore, we delineate the importance of single‐atom catalysis by comparing the performance of the Rh6/Al2O3 catalyst. A molecular level understanding of the differential reactivity on hydration, on oxidation, and of a larger Rh cluster size is reported.

Influence of Extraframework Aluminum on the Brønsted Acidity and Catalytic Reactivity of Faujasite Zeolite

Abstract: A series of faujasite zeolites was modified by extraframework Al (AlEF) with the goal to investigate the influence of such species on the intrinsic Bronsted acidity and catalytic activity towards paraffin cracking. The chemical state of AlEF and zeolite acidity were investigated by 27Al MAS NMR and COads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect-free Y zeolites were obtained by substitution of framework Al by Si with (NH4)2SiF6. In accordance with the next-nearest-neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in COads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by AlEF only resulted in a small fraction of cationic AlEF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less AlEF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Bronsted acid sites perturbed by cationic AlEF species. The results of MQMAS 27Al NMR spectroscopy confirmed the presence of sites perturbed by AlEF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although COads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from AlEF, they were less suitable to predict the reactivity if the Bronsted acid sites were affected by cationic AlEF species.

Mechanistic Study of Ethanol Dehydrogenation over Silica‐Supported Silver

Abstract: A silica-supported Ag catalyst has been shown to be an efficient heterogeneous catalyst for the oxidant-free dehydrogenation of ethanol into acetaldehyde. The reaction mechanism has been investigated by in situ FTIR spectroscopy. The kinetic isotope effects for proton and hydride abstraction have been studied by using CH3CD2OH and CH3CH2OD as labeled reactants. The results indicate that OH bond activation and the formation of a hydrogen-bonded complex take place on the silica support and that the Ag particles are necessary for the activation of the CH bond. The kinetic isotope effect (kH/kD) is 1.9 for CH3CD2OH and 1.8 for CH3CH2OD. The concerted mechanism of CH cleavage on the Ag sites and proton abstraction on the silica sites is proposed to account for the results of the spectroscopic and kinetic experiments.