Transformation of monosaccharides to furanic compounds and polymers in deep eutectic solvents

The conversion of lignocellulosic biomass saccharides into 5-hydroxymethylfurfural (5-HMF) and 2-furfural (2-F) has been proposed as a promising path to produce plastics, solvents and pharmaceuticals from renewable resources. 5-HMF and 2-F are generated from the dehydration of hexoses and pentoses respectively. Their cost effective production remains difficult to achieve because of a low reaction selectivity in conventional organic solvents or because of the high cost of more complex reaction media like ionic liquids. As an alternative solution, deep eutectic solvents (DES) prepared by mixing relatively cheap molecules like choline chloride, boric acid and an organic acid can be used to transform hexoses and pentoses to 5-HMF and 2-F at 90°C. In such reaction media, we demonstrate that a synergy between boric acid and α-hydroxylated acids (e.g. lactic acid) increases at least three-fold 5-HMF yield from glucose compared to mixture containing boric acid and an organic acid without an α-hydroxyl moiety. We proposed a mechanism involving the formation of borate esters between boric acid, glucose and the organic acid, bringing the reactants closer. DESs containing boric acid and an α-hydroxylated acid also catalyze the formation of insoluble substances (humins) from aldol additions/condensations reactions of monosaccharides and furanic compounds. Furan rings are incorporated in the insoluble substances but we show that humins formation starts from monosaccharides rather than from 5-HMF or 2-F. Adding another aldehyde in the reaction medium results in its incorporation in the formed humins and enhances selectivity of glucose transformation into 5-HMF. This finding suggests that simultaneous furanics synthesis and functionalization of the insoluble substances by aldol addition / condensation could increase humins value while improving 5-HMF and 2-F production.