The NH4Cl-assisted dehydration of xylose to furfural was studied using traditional and microwave heating. Significant differences in rate, pH profiles, and selectivity profiles were observed between both heating systems. A comparative kinetic analysis showed 7–13 times higher first-order rate constants for the xylose dehydration reaction under microwave heating. Dedicated experiments with varying irradiation power and liquid mixing intensity suggested that the differences are due to the development of overheated areas under microwave heating. This hypothesis was supported by modeling the rate enhancement observed under microwave heating using a simple kinetic model that assumes a minor overheated fraction amid a bulk liquid at target temperature.

Xylose is a promising feedstock for the fuel and chemical industry. We propose here an integrated process to recover and purify xylose from an acidic hydrolysate stream, e.g., coming from pretreated lignocellulose. This process consists of selectively extracting the xylose as the diboronate ester and back-extracting it into a clean aqueous solution. We report 85% xylose extraction efficiency using toluene/phenyl boronic acid in a single stage. We show that the extraction procedure is compatible with acidic and basic xylose feed (from pH 1 to 12), does not need a phase transfer agent (such as Aliquat), is strongly selective for xylose, and proceeds best with aromatic solvents. We demonstrate the structure of the diboronate ester by means of 2D-NMR and show evidence for the intermediate formation of a monoboronate ester. The release of xylose from the diboronate ester is explored, and an integrated process for xylose purification is proposed and its boundaries are discussed.

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 (PBA 2 X), under biphasic conditions. Experimental results obtained from reactions at 200 °C in a 1:1 v/v organic−aqueous biphasic system (composed of either 1methylnaphthalene or toluene and water at pH = 1 from H 2 SO 4 ) indicate that increasing the ionic strength (by adding Na 2 SO 4 ) 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 PBA 2 X 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.

This review focuses on the application of polar organic solvents, particularly aprotic ones, as reaction environments for the dehydration of C5 and C6 biomass-based sugars, with the aim to boost reactivity and selectivity towards furans.

The Cover Feature shows the local microwave heating of the reagent xylose in the water phase and the transfer of the product furfural into the cool organic phase. More information can be found in the Communication by L. Ricciardi et al. on page 3589 in Issue 14, 2020 (DOI: 10.1002/cssc.202000966).