(293a) Methods for Simultaneous Xylose Isomerization and Separation/ Concentration of Xylose and Xylulose by Liquid-Liquid Extration

Liquid-liquid extraction of sugar into organic phases has been used to recover sugars from aqueous solution. But native yeast is not able to ferment xylose (which is up 30% in biomass hydrolysate) to ethanol; this yeast can ferment xylulose, the ketose isomer of xylose, to ethanol. D-xylose isomerase is known to isomerize xylose to xylulose. However, the chemical equilibrium for this reaction is unfavorable for xylulose production, with an equilibrium xylulose:xylose ratio of ~2:8 at neutral pH and 50 °C.

Boronic acids have been known to form boronate esters with diols rapidly and reversibly in aqueous solution to shift the xylose:xylulose equilibrium. It’s shown that boronic acid can be confined to the organic phase by adding lipophilic quarternary ammonium salts to the organic phase. The sugar-loaded organic phase is mixed with a low pH aqueous phase to strip the sugar back.

In this mode of action, the boronic acid not only shift the equilibrium of xylose:xylulose, but also extract and concentrate sugar from the aqueous phase into the organic phase while the boronic acid itself is confined to the organic phase.