Mechanistic and kinetic studies on glyoxal oxidation with Bi- and Pb-promoted Pd/C catalysts

Bimetallic Pd–M/C catalysts (M = Bi, Pb) for the selective oxidation of glyoxal into glyoxalic acid were prepared either from inorganic salts or from acetate-type precursors. The bimetallic catalysts were found to be very active, confirming the promoting ability of both bismuth and lead in this reaction. The magnitude of their effect in terms of activity and selectivity was similar, and dependent on the preparation method and on the composition (optimum for ratios Pd:M = 1). The heavy elements could play their promoter role when introduced in solution in combination with a monometallic Pd/C catalyst. A kinetic treatment of the complex reaction scheme was carried out, and the rate constants for each step were determined. This showed that the reaction proceeds through a real heterogeneous mechanism involving the catalyst for the oxidation of glyoxal into glyoxalic acid and then, in a second step, into oxalic acid. The direct formation of oxalic acid from glyoxal could be neglected, and the formation of glycolic acid was confirmed to depend only on the pH. The absence of deactivation with our catalysts was also demonstrated. Correlations between the rate constants and some surface characteristics of the promoted catalysts were evidenced.