Enantioselective reduction of sparingly water-soluble ketones: continuous process and recycle of the aqueous buffer system

Abstract

Low solubility of starting materials and products in water and low enzyme utilisation are the two main obstacles for the production of enantiopure long-chain alcohols with alcohol dehydrogenases. A combination of techniques was used to overcome these limitations. A previously identified ionic liquid was used as a detergent to increase the low solubility for the starting materials as well as for the products. The low enzyme utilisation was increased by using ultrafiltration in an enzyme membrane reactor. Based on kinetic characterisation and stability data of the alcohol dehydrogenase from Lactobacillus brevis and glucose dehydrogenase from Bacillus sp. used for ketone reduction and cofactor regeneration, respectively, a continuous process was realised. In a configuration of a cascade of two enzyme membrane reactors the process could be demonstrated for more than 1000 hours with turnover numbers of more than 10⁶ and space time yields of up to 34 g L⁻¹ d⁻¹ with 99.9% enantioselectivity. Furthermore, downstream processing via adsorption of the alcohols was included, allowing 90% recycle of the aqueous buffer. Thus, the E-factor (amount of waste stream per product) was reduced to 13.