Production of esters by biocatalysed transesterification in supercritical CO2 and Hexane

The development of sustainable processes that use renewable raw materials and minimize chemical and energetic waste has attracted considerable attention and represents a great challenge to both academic researchers and industrial experts. The use of enzymes to catalyse chemical transformations can constitute a more sustainable alternative to some traditional chemical processes and more than 100 biotransformations are already operated at an industrial scale [1]. Lipases usually operate in mild conditions and can catalyse both hydrolytic and synthetic reactions, depending on the surrounding medium. The hydrolysis of esters can be performed in water, but the reverse production reactions are not favoured in this medium, and are usually performed in organic solvents. Supercritical CO2 can constitute a more sustainable alternative to organic solvents as a reaction medium, provided that it does not have a direct adverse effect on the enzyme's active site or significantly reduce its activity. Decyl acetate was chosen as a model compound and its production by a transesterification reaction catalysed by Novozym 435 (immobilized Candida Antarctica Lipase B) was studied in both hexane and supercritical CO2. A comparative analysis between these two alternatives was performed, focusing on the differences on the enzyme's catalytic activity, solubilities of the substrates and mass transfer rates; which significantly affect the outcome of the reaction process and its productivity and provide information on when such solvents can be used. [1] Straathof, A.J.J., Panke, S., Schmid, A. The production of fine chemicals by biotransformations. Current Opinion in Biotechnology, 2002, 13(6), 548-556

Decyl acetate synthesis by enzyme catalysis in sc-CO2

The synthesis of decyl acetate, from the transesterification of vinyl acetate with decanol, was studied in a high-pressure experimental set-up, equipped with a variable volume batch reactor, operating isothermally at 35 ºC and 100 bar, using CO2 in supercritical conditions as solvent, and Candida antarctica lipase B (CALB), immobilized on the macroporous resin Lewatit B (Novozym 435®), as catalyst. The enzymatic content was determined for each particle size of the catalyst. It was shown that the smallest particles have a larger specific amount of enzyme, and the results indicate that the enzyme is located in an external shell of the particle, following an “egg-shell” model type, with a thickness of ca. 60 μm (assuming a homogeneous distribution), independent of the particle size. Both external and internal mass transfer resistances were evaluated. External resistances were easily eliminated by stirring and internal diffusional limitations can be considered as negligible. The effect of the feed concentration of substrates on the initial reaction rate was also studied. It was observed that, above a certain concentration of decanol in excess relatively to vinyl acetate, the reaction is inhibited by the alcohol. On the contrary, the reaction is favoured when the reactor is fed with excess of vinyl acetate, with a significant enhancement of its initial rate. These results are consistent with a Ping-pong bi-bi type mechanism with competitive inhibition by the alcohol, commonly used in the description of enzymatic reactions of esterification/ transesterification.Financial support for this work was in part provided by national research grant POCI/EQU/56732/2004 and by LSRE financing by FEDER/POCI/2010, for which the authors are thankful. A.S.Ribeiro and P.Vidinha acknowledge their Ph.D. scholarship by FCT (SFRH/BD/13084/2003 and SFRH/BD/13787/2003)

Enzymatic production of decyl acetate: kinetic study in n-hexane and comparison with supercritical CO2

The kinetics of the lipase-catalyzed synthesis of decyl acetate, by the transesterification reaction of vinyl acetate with decanol, was investigated at 30 °C using n-hexane as the solvent. Novozym 435 was found to be the most active catalyst among the immobilized lipases tested. Given the nonideality of the reaction mixture, only a thermodynamic activity-based kinetics was found to be suitable to represent the experimental data in the entire range of compositions tested (0.1-1.4 M). The reaction follows a ping-pong bi-bi mechanism, in which inhibition only by excess of alcohol was identified. Although intraparticle diffusional limitations were detected, intrinsic kinetic parameters were obtained by crushing the catalyst particles. The results were compared to those obtained with supercritical CO2 as the solvent. For the conditions tested, Candida antarctica lipase B showed higher activity in the organic medium.The authors thank Fundacção para a Ciência e Tecnologia (Portugal) for A.S.R.’s Ph.D. grant (SFRH/BD/13084/2003) and for the financial support of the project POCI/EQU/56732/2004, FEDER for financing LSRE (FEDER/POCI/2010), Professor Romualdo Salcedo from FEUP for the optimization code, and Novozymes and Amano Enzyme Inc. for kindly providing the catalysts