Michel Devillers

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.

Water-soluble niobium peroxo-complexes as precursors for the preparation of Nb-based oxide catalysts

In the frame of research aimed at developing new synthetic procedures of multimetallic Nb-based catalysts, peroxo complexes of niobium(V) of general formula A I 3 [Nb(O 2 ) 4 ] and A I 3 [Nb(O 2 ) x (H y L)]. n H 2 O (A I : NH 4 +, CN 3 H 6 + (gu); L: oxalate, tartrate, citrate) have been prepared and characterized on the basis of elemental and thermal analysis, FTIR and 13 C-NMR spectra. The crystal structure of (gu) 3 [Nb(O 2 ) 4 ] and (gu) 3 [Nb(O 2 ) 2 (C 2 O 4 ) 2 ].2H 2 O have been determined. The application of the obtained Nb complexes as precursors for the preparation of silica-supported Nb-Mo-O catalysts has been demonstrated. Combining Nb peroxo-carboxylato compounds with analogous Mo(VI) compounds in a silica-impregnation method carried out in aqueous medium leads to the formation of the supported Nb 2 Mo 3 O 14 phase.

Mixed-oxide catalysts involving V, Nb and Si obtained by a non-hydrolytic sol-gel route : preparation and catalytic behaviour in oxydative dehydrogenation of propane

Abstract One-step non-hydrolytic condensation reactions, starting from VO(O i Pr) 3 and NbCl 5 , have been used for the first time to prepare Nb-V oxide catalysts. Different materials containing the NbVO 5 phase at different purity levels have been obtained, after thermal treatments, depending on the experimental conditions. The synthetic procedure has also been applied to prepare ternary Nb-V-Si oxide systems. From preliminary catalytic reaction studies it appears that the catalysts so prepared are promising with respect to their interesting performances in the oxidative dehydrogenation (ODH) of propane. It has also been found that α-Sb 2 O 4 , physically blended with the Nb-V oxide materials, could act as an appropriate promoter to improve the catalytic performances of these systems.

Stability of bimetallic Bi-Pd and Pb-Pd carbon-supported catalysts during their use in glyoxal oxidation

Abstract The incorporation of Bi or Pb as promoting elements in Pd-based carbon-supported catalysts drastically increases the catalytic activity in the selective oxidation of glyoxal into glyoxylic acid. Because partial dissolution of the promoter was clearly demonstrated by atomic absorption analysis of the reaction medium, experiments are performed to examine the stability of these catalysts. Dissolution tests in the presence of the individual constituents of the reaction medium (glyoxal, glyoxylate, glycolate, oxalate) were carried out in air or nitrogen to identify the factors responsible for Pb or Bi leaching. Pb- or Bi-promoted Pd/C catalysts were prepared by thermal degradation of acetate-type precursors and characterized by X-ray diffraction and X-ray photoelectron spectroscopy before and after their use in glyoxal oxidation. Promoter leaching increases with the reaction time. Monometallic Bi/C and Pb/C catalysts were found to lose smaller amounts of promoting agent than the bimetallic M–Pd/C (M=Bi, Pb) catalysts. Losses are more pronounced from Pb–Pd/C catalysts than from their Bi-based partners. Both glyoxal and glyoxylate seem to be among the main factors responsible for the promoter losses in relation to their complexing properties.

Catalysis with gold complexes immobilised on carbon nanotubes by π-π Stacking interactions: Heterogeneous catalysis versus the boomerang effect

A new pyrene-tagged gold(I) complex has been synthesised and tested as a homogeneous catalyst. First, a simple 1,6-enyne was chosen as a model substrate for cyclisation by using different solvents to optimise the reaction conditions. The non-covalent immobilisation of our pyrene-tagged gold complex onto multi-walled carbon nanotubes through π-π stacking interactions was then explored to obtain a supported homogeneous catalyst. The heterogenised catalyst and its homogeneous counterpart exhibited similar activity in a range of enyne cyclisation reactions. Bearing in mind that π-π interactions are affected by temperature and solvent polarity, the reuse and robustness of the supported homogeneous catalyst was tested to explore the scope and limitations of the recyclability of this catalyst. Under the optimised conditions, recyclability was observed by using the concept of the boomerang effect.

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.

Bismuth-promoted palladium catalysts for the selective oxidation of glyoxal into glyoxalic acid

The incorporation of bismuth in carbon-supported Pd-based catalysts is shown to increase significantly the catalytic activity in the selective oxidation of glyoxal into glyoxalic acid. Main side products are glycolic acid resulting from Cannizzaro dismutation and oxalic acid, generated by further oxidation of glyoxalic acid. Catalysts characterized by different Bi/Pd ratios (with Pd + Bi = 10 wt.%) were prepared according to various experimental procedures from two kinds of precursors, containing either inorganic (chloride, nitrate) or organic (acetate) ligands. The fresh and used catalysts were characterized by X-ray diffractometry and X-ray photoelectron spectroscopy. When comparing the time dependence of the performances of catalysts having the same overall composition (Bi/Pd = 0.5), bimetallic catalysts prepared from inorganic ligands or from acetate precursors exhibit comparable activities, while the selectivity towards glyoxalic acid remains higher for the acetate-type catalysts. In addition, the catalytic performances of Pd-Bi/C catalysts were found to be dependent upon the catalyst composition, those characterized by molar ratios Bi/Pd between 0.5 and 1.0 representing the most adequate compromise between high activity and high selectivity. Complementary experiments were also conducted on the second step of the oxidation scheme, i.e. oxidation of glyoxalic acid to oxalic acid. The behavior of the bimetallic Pd-Bi catalysts is also compared to that of a commercial trimetallic PdPtBi/C catalyst

Supramolecular Organization in Organic–Inorganic Heterogeneous Hybrid Catalysts Formed from Polyoxometalate and Poly(ampholyte) Polymer

Hybridization of polyoxometalates (POMs) via the formation of an organic-inorganic association constitutes a new route to develop a heterogeneous POM catalyst with tunable functionality imparted through supramolecular assembly. Herein, we report on strategies to obtain tunable well-defined supramolecular architectures of an organic-inorganic heterogeneous hybrid catalyst formed by the association of a hydrophobically substituted polyampholyte copolymer (poly N, N-diallyl-N-hexylamine-alt-maleic acid) and phosphotungstic acid (H3PW12O40) POMs. The self-assembling property of the initial polyampholyte copolymer matrix is modulated by controlling the pH of the hybridization solution. When deposited on a mica surface, isolated, long and extended polymer chains are formed under basic conditions (pH 7.9), while globular or coiled structures are formed under acidic conditions (pH 2). The supramolecular assembly of the POM-polymer hybrid is found to be directed by the type and quantities of charges present on the polyampholyte copolymer, which themselves depend on the pH conditions. The hypothesis is that the Keggin type [PW12O40](3-) anions, which have a size of ~1 nm, electrostatically bind to the positive charge sites of the polymer backbone. The hybrid material stabilized at pH 5.3 consists of POM-decorated polymer chains. Statistical analysis of distances between pairs of POM entities show narrow density distributions, suggesting that POM entities are attached to the polymer chains with a high level of order. Conversely, under acidic conditions (pH 2), the hybrid shows the formation of a core-shell type of structure. The strategies reported here, to tune the supramolecular assembly of organic-inorganic hybrid materials, are highly valuable for the design and a more rational utilization of POM heterogeneous catalysts in several chemical transformations.

Metal getters for tritium storage

Whereas titanium is a getter material mainly suitable for the long-term storage of tritium, zirconium cobalt alloy can also be employed for the interim storage and transport of this gas. Activated zirconium cobalt alloy reacts within minutes with hydrogen at room temperature. At the composition of /ZrCoH/sub 0.8/ the dissociation pressure at room temperature is estimated to be 10/sup -3/ Pa. The zirconium cobalt/H/sub 2/ system is not pyrophoric at room temperature. Methane is partially cracked on Ti and on ZrCo at temperatures above 600 and 300/sup 0/C respectively. With titanium the corresponding carbide is formed without affecting the storage properties of the getter. After reaction of ZrCo with CH/sub 4/ or N/sub 2/ the hydrogen adsorption capacity is reduced. Titanium powder, sponge or sheet react with nitrogen at temperatures above 750/sup 0/C with a parabolic rate law. In the overlayer of the metal substrate the phases N dissolved in /alpha/-Ti, Ti/sub 2/N and TiN were identified. The same phases were observed when NH/sub 3/ reacts with this metal.

Enhancement of total oxidation of isobutene on bismuth-promoted tin oxide catalysts

The influence of the incorporation mode of Bi2O3 to SnO2 on the total oxidation of isobutene has been investigated. The catalysts have been characterized by XRD and XPS. Although the pure oxides do not exhibit any individual catalytic activity, certain bimetallic systems display substantial catalytic properties. The observed promoting effect of bismuth depends on the nature of the precursor but not on the incorporation medium. Bismuth oxide obtained from bismuth citrate, bismuth nitrate and the complexes with ethylenediaminetetraacetic acid or nitrilotriacetic acid enhances catalytic activity, whereas pure Bi2O3 and bismuth acetate do not increase catalytic properties. The formation of the ternary phase, Bi2Sn2O7, during the degradation step or during the catalytic test, cannot be excluded although it has never been detected when the calcination temperature did not exceed 803 K. This pure phase displays a very low activity but the catalysts containing at the same time Bi2Sn2O7 and SnO2 show appreciable catalytic performances. The possible intrinsic catalytic influence of highly dispersed Bi2O3 in these catalysts is discussed. In addition, several hypotheses are put forward to explain the synergy between SnO2 and Bi2O3: (i) a remote control mechanism based on the cooperation between separate Bi2O3 and SnO2 phases;(ii) the generation, during the reaction, of a highly dispersed ternary Bi2Sn2O7phase which cannot be detected, and could either display important catalytic activity itself, or act synergetically with SnO2.