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-pa lladium catalysts for the selective oxidation of glucose to gluconic a cid, and to the understanding of the promoting role played by Bi in th ese catalysts. Catalysts were prepared according to various experiment al procedures from two kinds of precursors, containing either classica l inorganic ligands (chloride, nitrate) or organic ligands of the carb oxylate-type: the acetates and derivatives of the pyrazine-2,3-dicarbo xylic acid. Depending on the precursors used, the catalytic performanc es were found to be very different; catalysts prepared by deposition o f acetate-type precursors display the highest activity. The incorporat ion of bismuth in the Pd/C catalysts was confirmed to increase drastic ally the catalytic activity. The catalysts were characterized before a nd after their use in the catalytic operation by XRD, XPS, BET and IR. Depending on the preparation procedure used, the presence of BiOCl, B i2O3 and several Bi-Pd alloys in the bimetallic catalysts after the ac tivation step was deduced from XRD studies. Partial dissolution of bis muth during the catalytic tests was demonstrated by atomic absorption analysis of the reaction medium and elaborate investigations were unde rtaken to understand the individual effects of the various constituent s 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. Notwithst anding the increase in the conversion rate observed when two monometal lic Pd/C and Bi/C catalysts were used simultaneously, it was shown tha t the promoting role of bismuth was not merely dictated by the presenc e of bismuth in solution.