Synergic Effects of Boronate Diester Formation and High-Ionic Strength Biphasic Operation on Xylose-to-Furfural Selectivity

Developing strategies to boost the selectivity for furfural from biomass-based xylose is important for the development of green fuels and chemicals. This study explores the effects of ionic strength on the dehydration of xylose, starting from its phenylboronate diester (PBA2X), under biphasic conditions. Experimental results obtained from reactions at 200 °C in a 1:1 v/v organic–aqueous biphasic system (composed of either 1-methylnaphthalene or toluene and water at pH = 1 from H2SO4) indicate that increasing the ionic strength (by adding Na2SO4) from 0.1 to 6.1 M results in an increased xylose-to-furfural selectivity (from ∼70 to ∼90 mol %). This is partly due to the effect of salt on the partitioning of furfural, which is pushed into the organic phase, while the rate of furfural formation is enhanced, as reported in the literature. Remarkably, however, starting from PBA2X increases the xylose-to-furfural selectivity (88 mol %) beyond the level observed when starting from free xylose (75 mol %). Combined, these results indicate a synergic effect of the use of the PBA diester of xylose as the starting material, biphasic operation, and high ionic strength on the overall xylose-to-furfural selectivity. Based on these results, a process concept is proposed, which connects an extraction step, to retrieve xylose as the boronate diester from a xylose-rich biomass hydrolysate, to the selective furfural production at high-ionic strength under biphasic conditions. Such a process avoids addition of salt to the original biomass feed and thus combines the benefits and selectivity enhancements of ester formation, biphasic operation, and high ionic strength while allowing the recovery of the product and the closing of the process cycles. The validity of the process concept is supported by additional data on partitioning, losses, and product isolation, as well as an analysis of sustainability metrics.