C5-C6 aldaric acids produced from purified pinewood hemicellulose hydrolysate, an upgraded waste stream from paper pulp production

A sustainable alternative source of raw materials for the production of specialty chemicals should involve the utilization of lignocellulosic biomass. It is a composite material mainly made up of carbohydrate polymers (cellulose and hemicelluloses) and an intricate aromatic polymer. In recent years, selective extraction and recovery of hemicelluloses monosaccharides by pretreatment of wood chips prior to pulping has gained increased importance in the biorefinery concept mainly to upgrade cellulose quality. Specifically, the protocols developped for aerobic oxidation of glucose to glucaric acid have been successfully transferred to solutions of the series of aldoses present in the purified hemicelluloses hydrolysate obtained from pinewood.

Recently, within the French R&D collaborative Polywood project aimed to develop biosourced polyamides from sugars contained in hemicelluloses, a team of CNRS-Université Lyon and SOLVAY Research and Innovation Center of Lyon (Elie Derrien, Catherine Pinel, Michèle Besson and Philippe Marion) performed catalytic aerobic oxidation of the C5-C6 sugars contained in the hemicelluloses hydrolysates to a mixture of the corresponding aldaric acids. Sugar hydrolysates were coming from autohydrolysis of softwood chips with hot water followed by a post-hydrolysis with diluted sulphuric acid,. Mohammed Ahmar, Emilie Martin-Sisteron, Guy Raffin, Yves Queneau (University Lyon and CNRS researchers) and Marlène Beyerle (Novasep) also contributed to the study for the purification and analytical aspects of the complex mixture. Their work is currently published in the journal Industrial & Engineering Chemistry Research.

Briefly, the research method employed involved the preparation of (Pt/C) and (AuPt/ZrO₂) catalysts via wet impregnation and liquid phase reduction. Next, these catalysts were engaged in air oxidation of non-purified and purified hydrolysates with air. Product analysis of monosaccharides and of aldarates was performed using Ion Chromatography. Non-commercial aldaric acids were synthesized for identification and calibration via multistep reaction from the aldoses via dimethylamide intermediates. Lastly, the researchers measured the total organic carbon in the liquid samples to get a clear understanding of the mass balance.

Unfortunately, the hydrolysate contains residual impurities and degraded compounds (acids, furfural, 5-hydroxymethylfurfural, lignin-derived aromatics, etc …). The catalytic air oxidation of the raw hydrolysates has confirmed the inhibitory ability of some of them, thereby necessitating pre-purification prior to oxidation. The authors noted that after the complete removal of inorganic salts and unsaturated compounds by a combined process consisting of filtration, demineralization, evaporation, and active carbon treatments to purify the hydrolysate, yields aldarates under alkaline conditions oxidation over Pt/C or aldaric acids in non-neutralized system over Au−Pt/ZrO₂ were close to those obtained in synthetic solutions of pure sugars.

In summary, CNRS and Solvay scientists presented successful catalytic oxidation with air of the aldoses contained in the sugar rich aqueous liquor, separated after pre-hydrolysis of softwood hemicelluloses with diluted sulphuric acid. Generally, they observed that combined purification process consisting of filtration, demineralization by ion-exchange resins, evaporation, and active carbon treatment of the raw aqueous-stream gave a purified hydrolysate of aldoses that was oxidized to hexaric and pentaric acids. Altogether, the yields of aldaric acids accounted for ca. 50% of hexaric acids and up to 70% of pentaric acids of the initial corresponding aldoses..