A multiphasic hollow fiber reactor for the whole-cell bioconversion of 2-methyl-1,3-propanediol to (R)-beta-hydroxyisobutyric acid

This paper describes the bioconversion of 2-methyl-1,3-propanediol to (R)-b eta -hydoxy-isobutyric acid (HIBA) by Acetobacter ALEI in a hollow fiber me mbrane bioreaction system arrangement that allows the integration of three liquid phases: the aqueous bioconversion phase, the organic phase consistin g of a solution of trioctyl phosphine oxide (TOPO) in isooctane, and the th ird phase consisting of a basic stripping solution that allows reextraction of HIBA from the organic phase. A comparison of HIBA mass transfer experim ents was carried out in the membrane reactor with two and three phases for different pH and TOPO concentrations. The use of the three-phase arrangemen t allows the extraction of high quantities of HIBA from the aqueous medium (higher than 85%) independently of the pH, whereas in the two-phase system the percentage of HIBA extracted from the aqueous medium was lower, 42% in the best case, and strongly influenced by the pH. The percentage of the ext ractive agent TOPO in the organic phase influenced on the mass transfer rat e in both bi- and triphasic arrangements. By simply integrating the re-extr action phase in the system it was possible to increase the extraction yield by 2-fold, reduce the amount of TOPO by 4-fold, and operate at the more fa vorable pH 4. A bioconversion experiment was done in these conditions (pH = 4, TOPO = 5%) to confirm the advantages of including the third stripping s olution. Fed-batch operation of the triphasic membrane reactor was maintain ed for more than 20 h, reaching an HIBA concentration in the stripping solu tion of 29 g L-1.