Electrochemical microreactors

Electrochemistry, to distinguish it from the topics discussed in previous sections, is concerned with low-energy charge transfer in solution. The electron transfer typically occurs on the surface of a charged (usually metal) electrode. Possible chemical reactions that may occur, and that may be of importance in chemical synthesis, are the generation or annihilation of gases (in an electrolysis or a fuel cell, respectively) and the generation or neutralization of ions, which may be accompanied with the dissolution or deposition of a solid material. 

The reasons to perform electrochemistry, in particular, electrosynthesis, in a microfluidic system are the following (Rode et al., 2009) (1) reduction of ohmic resistance in the electrochemical cell, by decreasing the distance between anode and cathode, (2) enhancement of mass transport by increase of electrode surface to cell volume ratio, also realized by small interelectrode gaps, (3) performing flow chemistry to establish single-pass conversion, and (4) coupling of cathode and anode processes, permitting simultaneous formation of products at both electrodes. The latter 

A fifth reason for using microfluidics in electrochemistry would be the possibility to combine flow chemistry with an ultrafast mixer, which allows the generation and subsequent use of short-lived reactive ions or radicals, for example, in a cation flow process (Suga et al., 2001 Yoshida, 2008). Finally, a sixth reason for performing electrochemistry in a microfluidic system may be the desire to efficiently remove reaction heat (or joule heat due to high currents in combination with a high ohmic resistance) in fast electrochemical reactions (Yoshida, 2008). 

The research on electrochemistry in microreactors has been reviewed in a number of recent publications (Hessel et al., 2004 Rode et al., 2009 Yoshida, 2008 Yoshida et al., 2008) therefore, we do not want to go into too much detail here. But since these reviews almost exclusively concern electroorganic synthesis, a number of other applications will be highlighted here. 

A similar distinction between a system with pre-electrolysis with only one electrode (in this case anodic) process, and a system with simultaneous anodic and cathodic processes (in which anode and cathode are on opposite walls of a microchannel so that each liquid is only in contact with the desired electrode potential, analogous to the fuel cell configurations discussed above) was made by Horii et al. (2008) in their work on the in situ generation of carbocations for nucleophilic reactions. The carbocation is formed at the anode, and the reaction with the nucleophile is either downstream (in the pre-electrolysis case) or after diffusion across the liquid-liquid interface (in the case with both electrodes present at opposite walls). The concept was used for the anodic substitution of cyclic carbamates with allyltrimethylsilane, with moderate to good conversion yields without the need for low-temperature conditions. The advantages of the approach as claimed by the authors are efficient nucleophilic reactions in a single-pass operation, selective oxidation of substrates without oxidation of nucleophile, stabilization of cationic intermediates at ambient temperatures, by the use of ionic liquids as reaction media, and effective trapping of unstable cationic intermediates with a nucleophile. 

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ZioGAS, A., Lowe, H., Kuppee, M., Eheeeld, W., Electrochemical microreactor a new approach in microreaction technology, in Eheeeld, W. (Ed.), Microreaction Technology 3rd International Conference on Microreaction Technology, Proc. of IMRET 3, pp. 136-156,... 

A ceramic electrochemical microreactor for the methoxylation of methyl-2-furoate with direct mass spectrometry coupling. 

Girault et at. developed a ceramic electrochemical microreactor (CEM) in which an array of platinum interdigitated band electrodes (gap between electrodes 500 pm) was screen printed on the ceramic surface (Scheme 4.39) [5 3], A methanolic solution... 

Kiipper et al. carried out a methoxylation reaction of 4-methoxytoluene in an electrochemical microreactor in which a glass carbon anode and a stainless steel cathode were separated by a microchannel foil 25 pm thick [54], The chemical resistance of the microchannel foils was very important because of the evolution of hydrogen and oxygen gases and the strong pH shifts during electrolysis. PEEK was found to be the most robust material. They also observed that selectivity of the oxidation of 4-methoxytoluene in acidified methanolic solution (pH 1, sulfuric acid) was influenced by the current density and flow rate. 

Yoshida et al. reported that generation and online detection of highly reactive carbocations from carbamates were accomplished by integrating an electrochemical microreactor with an FTIR spectrometer [57]. They also demonstrated that both the carbocations and nucleophiles could be generated using the paired electrochemical flow system to give the coupling products in reasonable yields (Scheme 4.42) [58]. 

Cheikhou K, and Tzedakis T, Electrochemical microreactor for chiral syntheses using the cofactor NADH, AIChE J. 2008 54(5) 1365—1376. 

The electrochemical microreactor is fairly effective for the oxidation of p-methox-ytoluene and 4-methoxybenzaldehyde is obtained after hydrolysis. The efficiency of the microreactor reaction (98%) is higher than that of the common industrial processes (85%) (Scheme 7.6) [29]. 

PyCN can be obtained as the sole product by using the electrochemical microreactor shown in Figure 7.4. 

V. Mengeaud, O. Bagel, R. Ferrigno, H. H. Girault, A. Haider, A ceramic electrochemical microreactor for the methoxylation of methyl-2-furoate with direct mass spectrometry coupling, Lab Chip 2002, 2, 39-44. 

S. K. Yoon, E. R. Choban, C. Kane, T. Tzedakis, P. J. A. Kenis, Laminar flow-based electrochemical microreactor for efficient regeneration of nicotinamide cofactors for biocatalysis, J. Am. Chem. Soc. 2005, 127, 10466-10467. 

Challenging applications, such as the synthesis of P-peptides [30] or examples of flash chemistry (like the reaction of electrochemically generated reactive cation pools [31]) have been successfully realized using microreactors. 

For microreactor synthesis using electrochemically generated reactive species, see Yoshida, J. (2005) Chem. Commun., 4509. 

EMM can also be effectively utihzed for fabrication of several of microfeatures for a wide range of microengineering applications such as fuel processing, aerospace, heat transfer, microfluidics, and biomedical applications. These microdevices have to often withstand high stresses at elevated temperatures during their service in different applications such as microcombustors, electrochemical reactions required at elevated temperatures in microreactors, and also in microthermal devices. For biomedical applications, microcomponents are to be made of biocompatible materials and... 

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