Mireille Wenkin

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.

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.

On the role of bismuth-based alloys in carbon-supported bimetallic Bi-Pd catalysts for the selective oxidation of glucose to gluconic acid (poster)

The formation of various Pd-Bi alloys on the surface of carbon-supported PdBi(10%wt)/C catalysts and their role in the catalytic oxidation of glucose to gluconic acid by oxygen were investigated. Supported catalysts characterized by different Bi/Pd ratios were prepared from Pd acetate and Bi oxoacetate, and activated upon thermal heating at 773 K under nitrogen. The pure Bi2Pd, BiPd and BiPd3 alloys were prepared from the same precursors upon thermal heating at 873 K, 973 K and 1123 K, respectively. The catalytic performances of the supported and unsupported catalysts for the above reaction were measured by keeping the palladium weight constant. The catalysts were characterized by XRD, XPS and BET. Bismuth losses from the catalysts in the reaction medium were analyzed by atomic absorption spectrometry after 4 h running. For the supported catalysts, the highest performances are observed for Bi/Pd equal to 1, and for Bi/Pd = 0.5 when the gluconic acid yields are normalized with respect to the initial or residual Bi amount, or to the catalyst mass. The most active alloy is Bi2Pd which is also the one that loses Bi at the largest extent.

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.

Bismuth derivatives of 2,3-dicarboxypyrazine and 3,5-dicarboxypyrazole as precursors for bismuth oxide based materials

Depending on the experimental conditions, different bismuth derivatives of 2,3-dicarboxypyrazine (2,3-H2pzdc) were obtained, corresponding to the compositions Bi(2,3-Hpzdc)3·2H2O (4) and Bi(2,3-Hpzdc)2·OH (5). In line with the analytical results and the infrared spectra, high resolution NMR studies (DMSO-) of anhydrous Bi(2,3Hpzdc)3 (4a) confirmed unequivocally the presence of one free carboxylic acid in the structure. Thermal degradation of 4a in air proceeds in three steps and results in the formation of α-Bi2O3 at moderate temperature (451°C). A bismuth derivative of 3,5-dicarboxypyrazole (3,5-H3Dcp) corresponding to the formula Bi2(3,5-Dcp)2·nH2O (3) was obtained from bismuth nitrate and the dicarboxylic acid in hot water. It decomposes into α-Bi2O3 when heated in air at 385°C. The starting compounds and their thermal degradation products were also examined with X-ray photoelectron spectroscopy.

The complex role of bismuth as promoting element to inhibit deactivation of Pd-Bi/C catalysts in the selective oxidation of glucose to gluconic acid

Bismuth is a well-established promoter of Pd/C catalysts used for the partial oxidation of alcohols, aldehydes and, in particular, sugars. It inhibits deactivation due to overoxidation but, simultaneously, gets leached in the reacting solution, a fact that would suggest disappearance of its promoting effect and subsequent deactivation. The extent of leaching changes in a complex way. The present paper deals with complementary experiments carried out to understand how the soluble fraction of Bi could be involved in the overall mechanistic scheme of glucose oxidation into gluconate. Monometallic Pd/C and bimetallic Pd−Bi/C catalysts of various compositions were used. Whatever the initial catalyst composition, the XPS surface intensity ratio measured in used catalysts lies in the range 0.4–0.6, suggesting that the dynamic state of the catalyst involves the association of 1 Bi and 2 to 3 Pd atoms. The performances of a monometallic Pd/C catalyst are significantly improved in the presence of adequate amounts of soluble Bi. Considering that Bi-glucose and highly stable Bi-gluconate complexes can form, it is suggested that one of them, most likely the second one, can get very strongly bound to the surface of Pd or Pd−Bi alloys and that this association constitutes the active site. A too large amount of soluble Bi complex inhibits the reaction, probably by preventing access of glucose to the catalytic site. Long term deactivation can be due to either the action of oxygen or progressive loss of the surface complex.