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

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.