Isolation of Ketose Sugars from Mixed Sugar Solutions Using Simultaneous-Isomerization-and-Reactive-Extraction (SIRE), Followed by Back-Extraction (BE)

Converting aldoses to ketoses involves isomerization, typically conducted enzymatically. However, the isomerization does not have a favorable equilibrium with respect to ketose formation. We have previously developed a method of simultaneous-isomerization-and-reactive-extraction (SIRE) to produce the ketose isomer of xylose (xylulose) in high yield and purity. SIRE followed by 2-stage back-extraction (BE) allows recovery of xylulose in nearly pure form when starting from xylose. Results for SIRE-BE have not been reported for high concentrations of sugars likely to be found in biomass hydrolysates.

For C5 sugars, the extraction affinity is significantly higher for the ketose than the aldose. For C6 sugars, the aldose/ketose extraction affinities are much closer, suggesting that glucose/fructose separation will be more difficult. By changing the temperature and sugar to boronic acid extractant ratio during mixed glucose/xylose SIRE, conditions were identified that allow production of different composition product streams. The output from SIRE-BE can thus be tailored to meet the needs of the downstream process.

We present here results for SIRE-BE with concentrations of sugar representative of those found in biomass hydrolysates. Results for pure C5, pure C6 and mixed C5/C6 sugars subjected to SIRE-BE will be presented, including conditions that maximize either mixed ketose or xylulose separation from the sugar mixtures.