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