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.