Uses of Microreactors

Some argue that miniaturized tools for both chemical synthesis and analysis need to be integrated onto a single chip to gain the true benefits of miniaturization [57], not least because of the problems associated with subsystem interconnectivity, dead volumes and chip-to-world interfaces. Demonstrations toward such a goal include, for example, a hyphenated mixing reaction channel coupled to a capillary electrophoresis column [58]. 

Microreactors offer a radical alternative platform for chemical synthesis, normally undertaken in macroscale flasks [68-70]. When reactions in microcapillary-scale reactors are compared with those in flask-scale batch reactors, they have been shown to offer yield, rate or selectivity advantages in a diversity of reactions schemes including carbonylative cross-coupling of arylhalides to secondary amides [32], 

Microreactors provide a safe means by which reactions, including multistage schemes, can be undertaken where, otherwise, products involving unstable intermediates may be formed. This is exemplified by Fortt who showed that for a serial diazonium salt formation and chlorination reaction performed in a microreactor under hydrodynamic pumping, significant yield enhancements (15-20%) could be observed and attributed them to enhanced heat and mass transfer [77]. This demonstrates the advantage of microreactor-based synthesis where diazonium salts are sensitive to electromagnetic radiation and static electricity, which in turn can lead to rapid decomposition. Microreactors facilitate the ability to achieve continuous-flow synthesis, which is often not possible with conventional macroscale reactors and batch production. 

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J. (2000) Experiences with the use of microreactors in organic synthesis, in Microreaction Technology - IMRET 3 Proceedings of the 3rd International Conference on Microreaction Technology (ed. W. Ehrfeld), Springer, Berlin, p. 181. 

Use of microreactors for nitration processes, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, pp. 194—200 (5-9 March 2000), AIChE Topical Conf Proc., Atlanta, USA. 

Garcia, E., Ferrari, F., Garcia, T, Martinez, M., Aracil, J., Use of microreactors in biotransformation processes study of the synthesis of diflycerd mono-laurate ester, in Proceedings of the 4th... 

As mentioned above, in order to extend the potentialities of the luminescence-based optical fibre biosensors to other analytes, auxiliary enzymes can be used. The classical approaches consist either of the coimmobilization of all the necessary enzymes on the same membrane or of the use of microreactors including immobilized auxiliary enzymes and... 

With the use of microreactors rapidly increasing, the laboratory manager, in dealing with what will certainly be multi-disciplinary teams working at the forefront of technology, will have to employ the interpersonal skills described elsewhere in this book. 

For some oxidations, toxic heavy metal oxidants are used [313], This can be circumvented by the use of microreactor technology due to safe handling of elemental fluorine that can be used to mediate oxidation reactions [313], This is possible in a direct way via fluorine introduction into the substrate and subsequent replacement by an oxygen moiety. In an indirect manner, intermediate oxygen transfer reagents such as HOF MeCN can be generated by the reaction of aqueous acetonitrile with elemental fluorine that then attacks the substrate. The only byproduct is hydrogen fluoride that could be recycled by electrolysis. 

With the use of microreactor processing, 70% conversion was achieved that gave only product and no side product [4]. Actually, the micro reactor conversion is lower than the conversion for batch, but the former is now the preferred route as the separation of the product from the reactant can be accomplished, whereas product, as mentioned, can be hardly purified from the side product. A throughput of 300 g/h was achieved. 

As in other fields of organic chemistry, the use of microreactors represents a new method to accelerate reactions significantly. Within a larger study on tin hydride and tris(trimethylsilyl)silane-mediated reactions, the continuous flow system has also been applied to conduct an intramolecular Meerwein reaction. The cyclization of bromide 6 to indane 7 was completed in less than 1 min (Scheme 3) [39]. 

Recent reviews have provided systematic coverage of the enzymatic microreactors used in chemical analysis [4]. Considering that the focus of this chapter is biocatalytic synthesis, it does not consider the analytical applications and the reader is referred to the cited literature ([4] and references given therein). The use of microreactors for high-throughput kinetic characterization of enzymes is another very interesting application of the technology [8], which, for reasons of limited space, is not discussed herein. 

A 2002 study reports the use of a microreactor system to increase rate of production of EO (72). The use of microreactors eliminates the formation of hot spots that are common in large reactor systems. Hot spots commonly affect selectivities by increasing the amount of combustion products formed. Microreactor systems also make harmful waste products easier to handle and limit the size of explosions if they occur. 

A quite different, but realistic, approach to temperature control and efficient mixing involves the use of microreactors [129,130,132,133] a simple design is shown in Figure 2.26 [129]. These techniques are under active development but microreactor designs are now available that could be used on an industrial scale for the efficient and safe use of fluorine. 

Historically, the use of microreactors dates back to the 1940s when they were developed to measure kinetics of catalytic reactions.One of the key early findings was Denbigh s 1965 observation that if a reactor were made small enough, temperature and concentration gradients with the reactor would be negligible, so that differential (i.e., gradientless) behavior would be observed. This allowed much more accurate kinetic... 

The size is critical to the use of microreactors in portable devices, such as many electronics goods. Microreactors could also be made small enough and be easy enough to mass produce so that they could be used in consumer products. Another advantage of the small size is that lesser amounts of chemicals must be used. This is highly desirable in dealing with chemicals that present significant environmental. 

Microreactors have been applied to many diverse applications and will doubtless be applied to many new reaction engineering problems in the future. Below are some specific examples of the uses of microreactors that illustrate some of the advantages of these systems. The applications described below clearly indicate the broad array of reaction engineering problems to which microreactors can be applied. 

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