Liquid-liquid extraction and membrane separation are well-known separation method of extensive industrial application. Their incorporation into liquid membranes has the potential of several advantages, some of which are of particular interest for the recovery of carboxylic and amino acids: selectivities higher than those attainable by current separation methods, saving on energy costs for final concentration of separated products, high fluxes, compact installation, and low capital and operation costs. Stability of the liquid advantages, can be secured by utilizing extractant blocking polymeric membranes, Applicability, process consideration, and economic implications for recovery for carboxylic and amino acids by various extractant/membrane combinations are discussed.
The selective separation of citric and itaconic acids by amine extractants was studied. The stoichiometry of the carboxylate:amine complexes was established by various independent experimental techniques. Loading and distribution curves as well as FT±IR and¯uorescence spectra were recorded. Tri-n-octyl amine in 1-octanol was selective for the stronger carboxylate, citrate. Acid±base coupled extractants composed of di-(2-ethylhexyl) phosphoric acid and methyltriocyl ammonium cation in dichloromethane formed reverse micelles with uncharged surfaces. This extractant was selective for the more hydrophobic itaconate, which is located in the apolar envelope of the reverse micelle. Citrate was located in the aqueous inner core bound to the ammonium cation. The roles of amine salt, the diluent and of water-immiscible additives on the selectivity of the extraction are discussed.
Conversion of fumaric acid (FA) to L-malic acid (LMA) was carried out in a bioreactor divided by two supported liquid membranes (SLMs) into three compartments: Feed, Reaction, and Product. The Feed/Reaction SLM, made of tri-n-octylphosphine oxide (vol 10%) in ethyl acetate, was selective toward the substrate, fumaric acid (S(FA/LMA) = 10). The Reaction/Product SLM, made of di(2-ethylhexyl) phosphate (vol 10%) in dichloromethane, was selective toward the product, L-malic acid (S(LMA/FA) = 680). Immobilized yeast engineered to overproduce the enzyme fumarase [E.C. 4.2.1.2] was placed in the Reaction compartment and served as the catalyst. The yeast was immobilized in small glasslike beads of alginate-silicate sol-gel matrix. The construction of the bioreactor ensured unidirectional flow of the substrate from the Feed to the Reaction and of the product from the Reaction to the Product compartments, with the inorganic counterion traveling in the opposite direction. The conversion of almost 100%, above the equilibrium value of ca. 84% and higher than that for the industrial process, 70%, was achieved. In contrast to the existing industrial biocatalytic process resulting in L-malic acid salts, direct production of the free acid is described.
A novel liquid membrane reactor was designed to carry out the conversion of citric acid to itaconic (methylene succinic) acid by the fungus Aspergillus terreus. This bioreactor contained two supported liquid membranes (SLMs) that divided it into three compartments: (1) the feed chamber containing citric acid; (2) the reaction chamber containing the enzyme A terreus cells; and (3) the product collection chamber containing the counter-ion. The speci®city of SLMs allows the unidirectional¯ow, of the substrate from the feed to the reaction chamber, and of the product from the reaction to the product chamber in exchange for a counter-ion, thus maintaining low concentrations of both the product and the substrate, and limiting their inhibitory effects on the conversion process. The yield of itaconic acid in the membrane bioreactor was about two orders of magnitude higher than in the batch process.
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