Enzymatic reactions in supercritical carbon dioxide

Abstract

Lipases are enzymes that are widely used to catalyze a variety of industrially important reactions. However, lipase-catalyzed reactions are heterogeneous and are mass-transfer limited. Therefore, in most cases, the rates of reactions are very low in conventional aqueous and organic media, which limits the applicability of lipase reactions on an industrial scale. Due to their unique properties, supercritical fluids (SCFs) have proven to be efficient media for reactions with mass-transfer limitations. The present study aims at investigating hydrolysis and esterification reactions using a very cheap lipase (crude Hog pancreas lipase), in supercritical carbon dioxide (SCCO2).

The enzymatic hydrolysis of p-Nitrophenyl laurate (PNPL) to p-Nitrophenol (PNP) was studied in supercritical carbon dioxide using crude Hog pancreas lipase (HPL) and Penicillium roqueforti lipase (PRL). The two enzymes were compared for their optimal temperatures and kinetics. HPL had a higher optimal temperature (65°C) and better yield (22.5%) compared to PRL, whose optimal temperature was 50°C with a 15.3% yield. This is considerably higher than the optimum temperature of 37°C and yields of less than 1% observed in the aqueous medium.

The esterification of myristic acid with ethanol was studied with a commercial lipase (crude Hog pancreas lipase, HPL). The reaction was investigated in solvent-free conditions and in two solvents: supercritical carbon dioxide (SCCO2) and acetonitrile. Yields of up to 76% were obtained under solvent-free conditions. However, yields of only up to 37% and 4% were obtained in SCCO2 and acetonitrile, respectively. This was explained on the basis of very high enzyme and substrate concentrations in the case of solvent-free reactions. Conversions in SCCO2, though lower, seem better suited for commercialization. The influence of water and ethanol concentrations on the reaction was also investigated. Ethanol was found to be inhibitory for the reaction, and the addition of water led to a sharp reduction in conversions.

The synthesis of commercially important (banana) flavor esters of isoamyl alcohol, catalyzed by crude HPL, was studied under solvent-free and supercritical conditions. The esters synthesized were isoamyl acetate, isoamyl propionate, isoamyl butyrate, and isoamyl octanoate. Very low yields (3-4%) of isoamyl acetate were obtained, but high yields for the other three acids were obtained both in supercritical as well as solvent-free conditions. The yields of esters of even carbon acids, isoamyl acetate, butyrate, and octanoate, increased with increasing chain length, while the yields of isoamyl propionate were found to be higher than that of isoamyl butyrate. The optimum temperature of the reaction was found to be higher in supercritical conditions (45°C) than in solvent-free conditions (35-40°C). Reactions in SCCO2 seem commercially more viable than solvent-free reactions because of low enzyme loading and substrate concentrations

required