Simultaneous Isomerization and Reactive Extraction Followed By Back Extraction of Sugars from Biomass Hydrolysate for High Purity and Yield of Ketose Sugars

Biomass hydrolysates are often low in sugar concentration and contain contaminants associated with the pretreatment method and sugar and lignin degradation. Cost-effective and high-yield downstream processing requires sugar streams at higher concentration, free of contaminants generated during the pretreatment process. Instead of removing contaminants from the hydrolysate, we have taken the approach of removing sugars from the hydrolysate, leaving contaminants behind. In the process of removing sugars, we enzymatically isomerize them from their aldose (glucose, xylose) to their ketose (fructose, xylulose) forms. These ketose sugars are ideally suited for production of furans – HMF from fructose and furfural from xylulose.

Unfortunately, the isomerization reactions do not have favorable equilibria with respect to ketose formation. To overcome this limitation, we have previously developed a method of simultaneous-isomerization-and-reactive-extraction (SIRE) to produce the ketose isomer in high yield. We have demonstrated that with a xylose/xylulose mixture, SIRE followed by 2-stage back-extraction (BE) allows high yield recovery of xylulose in nearly pure form.

For mixtures of xylose and glucose found in biomass hydrolysate, separation and purification of the ketose sugars (xylulose and fructose) from the hydrolysate will be more complex. The extraction affinities of glucose and fructose are lower than xylulose, but higher than xylose. SIRE-BE has not been previously evaluated for biomass hydrolysates with high sugar concentrations (up to 90 g/L) for recovery of a high purity, mixed C5 and C6 ketose product stream. We have developed strategies for high purity recovery of both xylulose and fructose from mixed sugar streams using SIRE-BE. We will present these strategies as well as xylulose and fructose yields and purity obtained under different experimental configurations.