Simultaneous isomerization and reactive extraction of biomass sugars for high yield production of ketose sugars

Abstract

A desire to achieve one-pot syntheses of value-added products from biomass sugars has led to intense research efforts to convert these sugars with high yields into their ketose forms. Aldose-to-ketose isomerization has an unfavorable equilibrium, and to date, high yield conversion to ketoses in a manner that allows for further conversion economically has yet to be demonstrated. Here, a novel strategy for simultaneous-isomerization and reactive-extraction (SIRE), where the isomerization of aldose sugars and the reactive-extraction of ketoses occur concurrently in the same vessel, is presented. In SIRE, the unfavorable aldose–ketose equilibrium is overcome by selective extraction of ketose from the reaction medium maintained at the optimal pH of the xylose–glucose isomerase (XI) enzyme; the ketose sugar is back-extracted into a low pH strip solution as a pure, concentrated sugar stream. Starting with pure xylose and using Gensweet^® for isomerization, best results for xylulose concentration and purification were achieved with an extraction phase of 1-octanol containing naphthalene-2-boronic acid (N2B) and Aliquat^® 336. SIRE, followed by a two-stage strip, resulted in a nearly pure xylulose stream at high concentration and yield. Sugar can be concentrated in both the extraction and stripping steps by reducing the volume ratio of the sugar-receiving phase relative to the sugar-donating phase for each step. This result is significant as the separation of isomers is regarded as one of the most challenging and high cost purification processes. When implemented with biomass hydrolyzates, this technology has the potential to provide contaminant-free, concentrated ketose sugar streams without the need for energy-intensive separation and concentration schemes.