Selectivity switch by phase switch – the key to a high-yield furfural process

Abstract

We show the effective conversion of xylose into furfural, with a selectivity >90 mol%, in an aqueous–organic three-solvent system, which is composed of an apolar aromatic solvent, a polar organic solvent and acidic water and helps in catalyzing the formation of furfural. We couple this with a pre-extraction step of xylose as a boronic diester into a promising integrated process for valorising a diluted xylose hydrolysate. This process promises facile recovery of furfural and good recycling of all solvent components. The use of the boronic diester was found to be irrelevant for obtaining high selectivity, as its hydrolysis under the reaction conditions is fast. Surprisingly, the >90 mol% selectivity requires the three-solvent system to transition from biphasic to monophasic under the reaction conditions. Phenylboronic acid (PBA) used to extract xylose was found to be instrumental to this transition; PBA-lean media (<20 mM PBA) remain biphasic under the reaction conditions and deliver only 70 mol% selectivity. Water partial pressure measurements across the phase transition temperature confirm the occurrence of the phase transition. The increase of the apolar nature of the reaction medium when transitioning from biphasic to monophasic operation, reached upon mixing of the aromatic solvent with the water–polar organic phase, is likely responsible for the improved selectivity. The presence of an aromatic solvent in the mixture is important, probably due to its interaction with PBA that is instrumental to achieving the phase transition. A 1 : 1 : 1 toluene–sulfolane–water (pH = 1) mixture and [PBA] > 20 mM resulted in the highest observed xylose-to-furfural selectivity (95 mol%).