Recycling of the ionic liquid phase in process integrated biphasic whole-cell biocatalysis

Ionic liquids (IL) show a large set of interesting physicochemical properties, which make them an interesting alternative to commonly used organic solvents for applications in biphasic whole-cell biocatalysis. However, the currently still large cost of this class of solvents makes their use in industrial processes only competitive if the ionic liquid phase is recycled without loss of productivity during the process. Exemplarily for such an application, the asymmetric reduction of 2-octanone to (R)-2-octanol by a recombinant Escherichia coli overexpressing the Lactobacillus brevis alcohol dehydrogenase (ADH) and the Candida boidinii formate dehydrogenase (FDH) in biphasic ionic liquid/water systems was considered in this work. The repeated use of the ionic liquid phase was studied in a reaction system containing 20% (v/v) 1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([HMPL][NTF]) and a substrate concentration of 300 mM 2-octanone in the ionic liquid phase. 25 batch biotransformations were performed followed by phase separation, product isolation through distillation and reuse of the ionic liquid. No decrease in conversion was experienced during the 25 subsequent batch biotransformations. The average conversion was 98.5 (±0.7)%, and enantiomeric excesses were constant at values ≥99.5% (R). Over all cycles, a total of 999 (±6) g (R)-2-octanol L_IL^-1 was produced.