Biocatalytic Flow Reactors

Due to their cost, instability, and limited longevity, enzymes are not widely employed in production-scale syntheses however, through their immobilization and incorporation into flow reactors, biocatalysts have the potential to be employed in the synthesis of high-value products. Although the use of microfabricated reactors for the screening of biocatalysts for organic synthesis is a relatively new area of research, the field has been quick to employ those techniques developed for the use of solid-supported catalysts under continuous flow, a feature that is illustrated by the diverse array of immobilization techniques reported to date. 

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With numerous researchers investigating the advantages associated with thermally or biocatalytically controlled asymmetric syntheses, some of which have been performed in continuous flow reactors, few have considered the prospects of photochemical asymmetric synthesis, an idea... 

With numerous researchers investigating the advantages associated with the thermal or biocatalytic control of asymmetric reactions, Ichimura and co-workers [89] considered the potential of photochemical asymmetric syntheses performed in continuous flow reactors. To investigate the hypothesis, the authors employed the asymmetric photochemical addition of MeOH to (R)-( + )-(Z)-limonene (159) as a model reaction, comparing three quartz micro reactors, with a standard laboratory cell as a means of highlighting the synthetic potential of this approach. 

The process by BioCatalytics is comparable in terms of starting material and product but favors the use of isolated, silica-supported, immobilized L-aspar-tase. This process was claimed to have a higher productivity than a comparable whole cell process and takes place in a plug flow reactor, which is fed with 168 and ammonia solution. The immobilized enzyme is stable and keeps half of its initial activity for approximately half a year. The high activity can best be described by the fact that a single kilogram of enzyme produces 10,000 to more than 100,000 kg of 169, making it one of the most efficient biocatalytic processes known [144]. 

Biocatalytic membrane reactors for the removal of pollutants Wastewater flowing through lumen... 

Most recently, a modular microfluidic reactor and in-line filtration system for the rapid and small-scale evaluation of biocatalytic reactions have been demonstrated by O Sullivan and others [153]. The system combined a substrate with a biocatalyst in free solution. The PMMA enzymatic microreactor worked by co-flowing the enzyme and substrate through a T-channel and mixing was achieved by staggered herringbone micromixer (SHM). The filtration unit composed of gaskets made of PDMS... 

The reports mentioned above provide a systematic coverage of the nonimmobi-lized enzymatic reactors used in biocatalytic reactions under continuous flow operation. Results from microreactor experiments were comparatively higher than conventionally mixed batch reactors in terms of conversion rate and improvement of product yield as demonstrated for hydrolysis [140], dehalogenation [141], oxidation [142], esteriflcation [143], synthesis of isoamyl acetate [144,145], synthesis of cyanohydrins [147,148], synthesis of chiral metabolites [153], reduction [151], and bioluminescent reaction [149]. The small volumes involved and the favorable mass transfer inherent to these devices make them particularly useful for the screening of biocatalysts and rapid characterization of bioconversion systems. The remarkable results of such studies revealed that the product yield could be enhanced significantly in comparison with the conventional batch runs. 

Biocatalytic reactions performed using immobilized enzyme microreactors under continuous flow mode have been found effective for hydrolysis reactions [121,158-161], with the enzyme either trapped in the matrix [159], covalently linked to modified surface wall [160,121], enzymes entrapped in hydrogels [162], or enzymes immobilized on monolith [179]. The experimental setup consists of either chip-type microreactors with activated chaimel walls where enzymes bind, enzymes that bind to beads, enzymes entrapped in the matrix, enzymes adsorbed in nanoporous materials, and most recently, nanosprings as supports for immobilized enzymes in chip-based reactors, or enzyme immobilized monolith reactors, where support is packed inside a capillary tube (Table 10.4). 

Derivatization of channel walls has also been reported for the successful incorporation of biocatalytic surfaces in microreactors. Such a reactor has been used in the chiral resolution of a variety of substrates (Scheme 6.20). Such methodology may be used to screen substrates rapidly for enzymatic evaluation. In a manner comparable with chemical catalysts, the use of immobilized enzymes is a cost-effective method for their recycle and reuse. Other examples of continuous flow resolutions have been demonstrated... 

A fimctional, easily assembled, operated and cleaned microbioreactor packed with immobilized Candida antarctica lipase B (Novozyme 435) was recently developed by Pohar et al. (2010). So far, microbioreactor was used for studying continuous mode ester synthesis within bis(trifluoromethylsulfonyl)imide - based ionic liquid media. Ionic liquid containing substrates was pumped into the microbioreactor at various flow rates, and at the outlet of the reactor the product was collected and analyzed. With fuUy adjustable length, width and depth, the developed packed bed microbioreactor was proven to be a very successful and versatile tooling for biocatalytic reactions such as isoamyl acetate or butyl butyrate synthesis (Cvjetko et al, 2010 Pohar et al., 2010). 

The combination of a resin and covalently supported IL with SCCO2 was also used in the KR and dynamic kinetic resolution (DKR) of 1-phenylethanol with vinyl propionate catalyzed by Candida antarctica lipase B (CALB) [125]. The IL molecule covalently supported on Merrifield resin was realized through the reaction of 1-butyl imidazole with chloromethylated resin. Subsequently, NTf2 was introduced via ion exchange. Under improved conditions, the conversion of 1-phenylethanol was 50% with 99.9% ee to the product. In order to develop a more efficient process, the KR of 1-phenylethanol was tested on a flow system, and it remained stable for 6 days with 99% ee Moreover, by combing two fixed-bed reactors loaded with the supported enzyme (biocatalytic reactor, CALB-SILLP (SILLP, supported ionic liquid-like phase) 11, 150 mg) and an additional one with an acid zeolite (chemical racemization catalyst, 100 mg). Figure 2.40, the DKR of 1-phenylethanol... 

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