Heterogeneously catalyzed hydrothermal processing of C5-C6 sugars

Biomass has been long exploited as an anthropogenic energy source; however, the 21st century challenges of energy security and climate change are driving resurgence in its utilization both as a renewable alternative to fossil fuels and as a sustainable carbon feedstock for chemicals production. Deconstruction of cellulose and hemicellulose carbohydrate polymers into their constituent C5 and C6 sugars, and subsequent heterogeneously catalyzed transformations, offer the promise of unlocking diverse oxygenates such as furfural, 5-hydroxymethylfurfural, xylitol, sorbitol, mannitol, and gluconic acid as biorefinery platform chemicals. Here, we review recent advances in the design and development of catalysts and processes for C5-C6 sugar reforming into chemical intermediates and products, and highlight the challenges of aqueous phase operation and catalyst evaluation, in addition to process considerations such as solvent and reactor selection

Catalysing sustainable fuel and chemical synthesis

Concerns over the economics of proven fossil fuel reserves, in concert with government and public acceptance of the anthropogenic origin of rising CO2 emissions and associated climate change from such combustible carbon, are driving academic and commercial research into new sustainable routes to fuel and chemicals. The quest for such sustainable resources to meet the demands of a rapidly rising global population represents one of this century’s grand challenges. Here, we discuss catalytic solutions to the clean synthesis of biodiesel, the most readily implemented and low cost, alternative source of transportation fuels, and oxygenated organic molecules for the manufacture of fine and speciality chemicals to meet future societal demands

Carboxylate-type palladium(II) complexes as soluble precursors for the preparation of carbon-supported Pd/C catalysts

Palladium-based carboxylate-type compounds were synthesized and used as precursors for the preparation of heterogeneous carbon-supported Pd/C catalysts. The selected compounds corresponded to the general formula Pd(O2CR)(2), Pd(O2CR)(2)L-2 or Pd(O2CR)(2)L', with R = CH3, C2H5, or C6H5, L = pyridine or diethylamine, and L' = 2,2'-bipyridine or 1,10-phenanthroline. The precursors were characterized by means of FTIR and XPS. Thermogravimetric analyses allowed us to study the way they decompose under nitrogen: X-ray diffraction investigations indicated they were converted mainly into Pd metal and PdO. Pd/C catalysts were prepared by impregnation from solutions of these complexes in water, benzene or acetone, followed by thermal activation under nitrogen. Catalysts were engaged in the partial oxidation of glucose to gluconic acid and have been characterized by XRD and XPS before and after their use in the catalytic reaction. Catalysts prepared in benzene showed better performances and higher Pd dispersion on the surface than the catalysts prepared in water or acetone. Activity was also found to be enhanced by monomeric, instead of trimeric, structure of the precursor. (C) 1998 Elsevier Science B.V. All rights reserved

The role of bismuth as promoter in Pd-Bi catalysts for the selective oxidation of glucose to gluconate

Bismuth is a well-established promoter of noble metal-based catalysts for the selective liquid phase oxidation of alcohols, aldehydes and carbohydrates with molecular oxygen. Experiments were carried out to improve the understanding of the promoting role of bismuth in bimetallic Pd-Bi catalysts used for the selective oxidation of glucose to gluconate. In relationship with the fact that these catalysts undergo substantial bismuth teaching under the reaction conditions, particular attention was paid to the possible role played by the soluble fraction of bismuth in. the oxidative process. Carbon-supported Pd-Bi/C catalysts characterized by various Bi-Pd compositions (0.33 less than or equal to Bi/Pd less than or equal to 3.0, 10wt.%Pd + Bi) were prepared from acetate-type precursors, tested under various experimental conditions and analyzed by X-ray diffractometry and X-ray photoelectron spectroscopy (XPS). Whatever the initial catalyst composition, the surface intensity ratio measured by XPS in used catalysts lies in the range 0.4-0.6, suggesting that the dynamic state of the catalyst involves the association of one Bi and two to three I'd atoms. The leaching process and the promoting effect itself are discussed in line with the formation of Bi-glucose and Bi-gluconate complexes present in solution but also as adsorbed species at the catalyst surface. The performances of a monometallic Pd/C catalyst are significantly improved in the presence of adequate amounts of soluble Bi. The involvement of the soluble fraction of bismuth in the overall mechanistic scheme of glucose oxidative dehydrogenation is suggested. The detrimental effect of large amounts of soluble bismuth is attributed to a too extensive adsorption of Bi-glucose complexes on the surface I'd atoms. (C) 2002 Elsevier Science B.V. All rights reserved

Diammine(pyrazine-2,3-dicarboxylato-N,O)palladium(II): Synthesis, crystal structure, spectroscopic and thermal properties

The synthesis, crystal structure and spectroscopic (IR, XPS) characterization of a new Pd complex with 2,3-pyrazinedicarboxylic acid (2,3-H(2)pzdc), Pd(2,3-pzdc)(NH3)(2) (1), are reported. This compound crystallizes in the monoclinic space group P2(1)/c with four molecules in the unit cell. Using 2697 independent reflections up to 2 theta=60 degrees, the structure was refined to R = 0.044. The lattice is formed of isolated zwitterionic moieties in which the square-planar coordination of palladium is ensured by three nitrogen atoms and one oxygen from a carboxylate group attached to the pyrazine ring. Both the structural and the infrared data confirm the absence of a polymeric network. Thermal degradation under nitrogen produces a mixture of palladium metal and PdO at 773 K. The XPS data of 1 are discussed together with those of a related compound with 3,5-pyrazoledicarboxylic acid (H(3)Dcp), (NBu4)(2)[Pd-2(Dcp)(2)] (2) and suggest the presence of a delocalized positive charge in the pyrazine ring

Influence of metallic precursors on the properties of carbon-supported bismuth-promoted palladium catalysts for the selective oxidation of glucose to gluconic acid

This work is devoted to the preparation of carbon-supported bismuth-palladium catalysts for the selective oxidation of glucose to gluconic acid, and to the understanding of the promoting role played by Bi in these catalysts. Catalysts were prepared according to various experimental procedures from two kinds of precursors, containing either classical inorganic ligands (chloride, nitrate) or organic ligands of the carboxylate-type: the acetates and derivatives of the pyrazine-2,3-dicarboxylic acid. Depending on the precursors used, the catalytic performances were found to be very different; catalysts prepared by deposition of acetate-type precursors display the highest activity. The incorporation of bismuth in the Pd/C catalysts was confirmed to increase drastically the catalytic activity. The catalysts were characterized before and after their use in the catalytic operation by XRD, XPS, BET and IR. Depending on the preparation procedure used, the presence of BiOCl, Bi2O3 and several Bi-Pd alloys in the bimetallic catalysts after the activation step was deduced from XRD studies. Partial dissolution of bismuth during the catalytic tests was demonstrated by atomic absorption analysis of the reaction medium and elaborate investigations were undertaken to understand the individual effects of the various constituents of the reaction mixture on the dissolution process. Monometallic Bi/C catalysts were found to lose significantly larger amounts of bismuth than bimetallic Pd-Bi/C catalysts. Both glucose and gluconate appear as responsible for the dissolution of the promoting element. Notwithstanding the increase in the conversion rate observed when two monometallic Pd/C and Bi/C catalysts were used simultaneously, it was shown that the promoting role of bismuth was not merely dictated by the presence of bismuth in solution

Promoting effects of bismuth in carbon-supported bimetallic Pd-Bi catalysts for the selective oxidation of glucose to gluconic acid

Experiments are carried out to improve the understanding of the behaviour of Bi-promoted Pd/C catalysts during their use in the selective oxidation of glucose to gluconic acid by O-2. Supported Bi(5 wt.%)-Pd(5 wt.%)/C catalysts are prepared by deposition from a suspension of several carboxylate precursors in heptane, followed by thermal degradation under N-2 at 773 K. The catalysts are characterized by XRD, XPS and SEM-EDX. Because significant amounts of bismuth are leached from the catalysts under the reaction conditions, recycling experiments are performed to evaluate the influence of this process on the catalyst lifetime. Whereas the Bi losses are essentially restricted to the first few catalytic runs, the gluconic acid yield, normalized with respect to the catalyst mass, remains constant. Catalytic tests are also conducted in the presence of diethylenetriaminepentaacetate, which is a stronger chelating agent than the gluconate ions, to remove the major part of dissolved Bi from the solution. The behaviour of the bimetallic catalyst is also compared with that of a commercial trimetallic Pd(5 wt.%)-Pt(1 wt.%)-Bi(5 wt.%)/C catalyst

Comparison between wet deposition and plasma deposition of silane coatings on aluminium

Silane coatings are applied to metal surfaces for various purposes, e.g. to form a protective layer against corrosion or to act as a primer for subsequent coating. In this work bis-1,2-(triethoxysilyl) ethane (BTSE) was used as a precursor to deposit coatings on Al 99.99% substrates with three different techniques: dipcoating (water based solution), vacuum plasma and atmospheric plasma. Infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and field emission gun-scanning electron microscope (FE-SEM) were used to characterise the structure, composition and surface morphology of the silane coatings. The aim of this investigation is to compare the surface and bulk characteristics of the films prepared with the three different methods, in order to get information on how the BTSE molecule is modified by the deposition technique. The results show that films could be deposited by both vacuum and atmospheric plasma, besides the more traditional wet dipcoating. The layers deposited by vacuum plasma can be considered as hybrid organic-inorganic, comparable to the silane layers obtained by dipcoating. Atmospheric plasma treatment, however, leads to the formation of more inorganic films richer in Si-O bonds. XPS and FTIR measurements show the presence of Si-O-Si bonds, while Si-O-Si, Si-O-C, Si-O and Si-CH3 absorption bands are revealed by IRRAS measurements. © 2010 Elsevier B.V.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Characterization of thin water-based silane pre-treatments on aluminium with the incorporation of nano-dispersed CeO2 particles

Thin silane films with the incorporation of nano-dispersed CeO2 particles have been deposited by dip-coating over aluminium substrates. The silane molecule under investigation is bis-1,2-(triethoxysilyl)ethane (BTSE), which has been used as a water-based solution in order to fulfill the industry need for reducing the impact on human health and environment. The influence of the nano-oxides on the silane was first analyzed in the solution by means of 29Si NMR, which showed that the ageing and stability of BTSE are not affected by the presence of CeO2 nanoparticles in the silane matrix. The chemical interactions between CeO2 nanoparticles and the BTSE in the cured coating were investigated by XPS, and the results pointed to the possible formation of a bond between Ce and Si. Field emission scanning electron microscopy (FE-SEM), field emission Auger electron spectroscopy (FE-AES) and focused ion beam coupled with transmission electron microscope (FIB-TEM) were used to investigate the film structure and surface morphology. A major challenge in the formation of thin layers with the incorporation of nano-oxides is to avoid the formation of particles agglomeration, which affects the film uniformity and might create preferential paths for corrosion attack. This issue has been overcome and the resulting films appear uniform and homogeneous, with the nanoparticles well embedded in the silane matrix and always covered by a thin silane layer. Electrochemical impedance spectroscopy (EIS) measurements have been performed on the samples and the results show how the barrier properties of the thin silane films are improved by the addition of CeO2 nano-dispersed particles. © 2010 Elsevier B.V.SCOPUS: ar.jinfo:eu-repo/semantics/publishe