Base-free selective oxidation of pectin derived galacturonic acid to galactaric acid using supported gold catalysts†
Agricultural residues like sugar beet pulp (SBP) are an interesting feedstock for the production of 2nd generation bio-based chemicals and materials. The pectin fraction of SBP is rich in galacturonic acid (GalA), a C6 sugar acid. The oxidation of this uronic acid at C1 yields galactaric acid (GA), which has several industrially interesting properties. It was previously shown that the Au catalysed oxidation of uronic acids under basic conditions is highly effective, yet leads to the co-production of salts. Hence, here we report for the first time on the oxidation of an acidic carbohydrate substrate, GalA, at its autogenic pH (2.2) in water, using carbon supported gold nanoparticles, under mild conditions in the presence of molecular oxygen. The comparison of the Au/C catalyst prepared by a colloidal deposition method with benchmark commercially available metal oxide supported gold catalysts shows that under acidic conditions, the Au/C catalyst is more active and more selective than Au/TiO2, and more stable than Au/Al2O3. The difference in selectivity is attributed to the H2O2 mediated chain scission reaction of the substrate (GalA) which is observed only in the case of metal oxide supported Au catalysts. The Au/C catalyst shows 100% GA selectivity at 76% GalA conversion (333 K, 21 h batch time) and a GA yield of up to 95% was obtained at 353 K. Detailed characterization of the fresh and spent Au/C catalysts by ICP-OES, TEM and XPS analyses showed no gold leaching, particle sintering or change in metal composition. The Au/C catalyst was fully regenerated by a mild alkaline wash, and used in five consecutive runs without any significant decrease in activity or selectivity. Labelling experiments with 18O2 and H218O2 revealed that under base-free conditions, the oxygen incorporated in the aldaric acid originates from the solvent water.