Selective Extraction of Xylose from Acidic Hydrolysate-from Fundamentals to Process

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.