Enzyme-emulsion membrane reactor

Figure 9.6 Scheme of the S- and R-naproxen isomer production in the enzyme-emulsion membrane reactor. From Ref. [8] with permission. 

In addition to this, that an interesting novel emulsion membrane reactor concept overcomes the difficulties of the large solvent volume otherwise required for the reduction of poorly soluble ketones [30]. 2-Octanone was reduced by a carbonyl reductase from Candida parapsilosis to (S)-2-octanol with > 99.5 % ee and total turnover number of 124 - the 9-fold value of that obtained in a classical enzyme reactor. 

An important problem in emulsified organic-aqueous systems is that of scale-up, which is concerned with the realization of stable emulsions and the separation of phases after the reaction. The use of biphasic membrane systems that contain the enzyme and keep the two phases separated is likely to solve this problem. In the case of 5-naproxen an ee of 92% has been demonstrated without any decay in activity over a period of two weeks of continuous operation. A number of examples of biocatalytic membrane reactors have been provided by Giorno and Drioli (2000) and include the conversion of fumaric acid to L-aspartic acid, L-aspartic acid to L-alanine, and cortexolone to hydrocortisone and prednisolone. 

The reaction in a homogeneous solution with a polar organic solvent in which the enzymes and substrates are both soluble, occurs often at the expense of the enzyme stability [4, 5]. Besides immobilised enzymes in organic solvents [6], emulsion reactors, especially enzyme-membrane-reactors coupled with a product separation by membrane based extractive processes [7-9] and two-phase membrane reactors [10-12], are already established on a production scale. 

Figure 8.4 Reactor types used in organic-aqueous biphasic systems (a) Emulsion reactor, (b) Lewis cell, (c) passive membrane reactor, (d) active membrane reactor. E represents enzyme molecules.

Lipases (E.C. 3.1.1.3.) catalyze the hydrolysis of lipids at an oil/water interface. In a membrane reactor, the enzymes were immobilized both on the side of the water phase of a hydrophobic membrane as well as on the side of the organic phase of a hydrophilic membrane. In both cases, no other means for stabilization of the emulsion at the membrane were required. The synthesis reaction to n-butyl oleate was achieved with lipase from Mucor miehei, which had been immobilized at the wall of a hollow fiber module. The degree of conversion reached 88%, but the substrate butanol decomposed the membrane before the enzyme was deactivated. 

A further improvement of the multiphase reactor concept using lipase for enantioselective transformation has been recently reported, that is, an emulsion enzyme membrane reactor. Here, the organic/water interface within the pores at the enzyme level is achieved by stable oil-in-water emulsion, prepared by membrane emulsification. In this way, each pore forms a microreactor containing immobilized... 

Recent studies in the pharmaceutical field using MBR technology are related to optical resolution of racemic mixtures or esters synthesis. The kinetic resolution of (R,S)-naproxen methyl esters to produce (S)-naproxen in emulsion enzyme membrane reactors (E-EMRs) where emulsion is produced by crossflow membrane emulsification [38, 39], and of racemic ibuprofen ester [40] were developed. The esters synthesis, like for example butyl laurate, by a covalent attachment of Candida antarctica lipase B (CALB) onto a ceramic support previously coated by polymers was recently described [41]. An enzymatic membrane reactor based on the immobilization of lipase on a ceramic support was used to perform interesterification between castor oil triglycerides and methyl oleate, reducing the viscosity of the substrate by injecting supercritical CO2 [42],... 

Membrane reactors using biological catalysts can be used in enantioselective processes. Methodologies for the preparation of emulsions (sub-micron) of oil in water have been developed and such emulsions have been used for kinetic resolutions in heterogeneous reactions catalyzed by enantioselective enzyme (Figure 43.4). A catalytic reactor containing membrane immobilized lipase has been realized. In this reactor, the substrate has been fed as emulsion [18]. The distribution of the water organic interface at the level of the immobUized enzyme has remarkably improved the property of transport, kinetic, and selectivity of the immobilized biocatalyst. 

Pal, P., Dutta, S. and Bhattacharya, P. (2002). Multi-enzyme immobUization in eco-friendly emulsion liquid membrane reactor—A new approach to membrane formulation. Sep. Purif. Technol., 27, 145-54. 

Li N, and Sakaki K, Performance of an emulsion enzyme membrane reactor combined with premix membrane emulsification for lipase-catalyzed resolution of enantiomers, J. Mem. Sci. 2008 314 183-192. 

Giomo L, D Amore E, Mazzei R, Piacentini E, Zhang J, Drioli E, Cassano R, Picci N (2007), An innovative approach to improve the performance of a two separate phase enzyme membrane reactor by immobihzing lipase in presence of emulsion , / Memb. Sci, 295,95-101. 

Heterogeneous photooxidation of phenol by catal)4ic membranes. Chin. J. Process Eng. 6, 54-59. Giorno, L., D Amoie, E., Mazzei, R., Piacentini, E., Zhang, J., Drioli, E., Cassano, R., and Picci, N. (2007). An innovative approach to improve the performance of a two separate phase enzyme membrane reactor by immobilizing lipase in presence of emulsion. J. Membr. Sci. 295, 95-101. Giorno, L., and Drioli, E. (2000). Biocatalytic membrane reactors Applications and perspectives. Trends Biotechnol. 18, 339-348. 

The water phase can also be recharged with fresh substrate in an emulsion reactor, where a hydrophilic membrane is used to cleave the emulsion (Fig. lOd) [49]. In this type of reactor as well as in the bimembrane reactor the enzyme is well separated from the organic phase. This is important as interphases, e.g., between two immiscible solvents or between a liquid and a gas differing widely in the dielectric constants, may lead to protein denaturation. A more detailed description of these reactors can be found in the references given or for membrane reactors in general [35,106-108]. 

The first step in downstream processing is the separation of the product-rich phase from the second phase and the biocatalyst. This may be simplified if the enzyme is immobilized or if a membrane module is included in the experimental set-up. In the case of emulsion reactors, centrifugation for liquid phase separation is a likely separation process [58], although the small size of droplets, the possibility of stable emulsion formation during the reaction, particularly if surface-active... 

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