Influence of Residues Contained in Softwood Hemicellulose Hydrolysates on the Catalytic Oxidation of Glucose to Glucarate in Alkaline Aqueous Solution
Résumé
The hydrolysate recovered after acid hydrolysis of the hemicellulose fraction of softwood represents a renewable and low-cost source of sugars which can be used for the production of chemicals such as glucaric acid. In this paper we first re-evaluated the metal-catalyzed aerobic oxidation of gluconate and one-step oxidation of glucose to glucarate in alkaline aqueous solution at 60 degrees C using air as oxidant. Then, we investigated the possible inhibitory effect of some inherent side (or by)-products liberated during the dilute mineral acid hydrolysis. High conversion efficiencies were obtained in glucose oxidation; however, the maximum glucarate yield was 54% over Pt/C since overoxidation products were formed under these conditions. As regards the residues in the hydrolysate, sodium salts (acetate and sulfate) or disaccharides (cellobiose and maltose) were without significant effect on the course of the reaction, whereas furan aldehydes (5-hydroxymethylfurfural and furfural) appeared to significantly slow down the reaction rates. Small phenolic residues from lignin showed a detrimental effect and should be eliminated prior to upgrading by catalytic oxidation of glucose in the hydrolysates.