Microreactors electrochemical flow

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]. 

Yunus K, Marks CB, Fisher AC, Allsopp DWE, Ryan TJ, Dryfe RAW, Hill SS, Roberts EPL, Brennan CM (2002) Hydrodynamic voltammetry in microreactors multiphase flow. Electrochem Commun 4 579-583... 

Figure 9.1 Cyanation of pyrene in an electrochemical flow microreactor with a serial electrode configuration.

Figure 9.2 A stack-type electrochemical flow microreactor with an interdigitated electrode configuration.

A stack-type electrochemical flow microreactor having interdigitated band electrodes on a ceramic body was developed (Figure 9.2). The anodic oxidation of furans in methanol using H2SO4 as the supporting electrolyte was carried out, and the effect of the residence time was investigated by mass spectrometry analysis [13]. 

Figure 9.3 An electrochemical flow microreactor having a parallel electrode configuration.

One of the most interesting features of electrochemical flow microreactor systems seems to be electrolysis without using an intentionally added supporting electrolyte by virtue of short distance between the electrodes. This field has attracted significant research interest, although various electrolyte-free electrochemical systems have been developed [17,18]. High electrode surface area to reactor volume ratios are also advantageous for conductivity and reaction efficiency. 

Marken and coworkers accomplished electrolysis without using intentionally added electrolyte using an electrochemical flow microreactor having a parallel plate-to-plate electrode configuration [19]. A substrate solution was introduced into the chamber, in which two electrodes were placed facing each other at a distance of micrometer order. In this system, the substrate solution flow and the current flow were perpendicular. 

Figure 9.4 Electrochemical flow microreactor system without using intentionally added supporting electrolyte. Electrochemical oxidation of furan.

There is another type of electrochemical flow microreactors that can be used for electrolyte-free electrolysis (Figure 9.5) [24]. In this system, two carbon fiber electrodes are separated by a spacer (porous PTFE membrane, pore size 3 pm, thickness 75 pm) at a distance of micrometer order. A substrate solution is introduced into the anodic chamber. The anodic solution flows through the spacer membrane into the cathodic chamber. The product solution leaves the system from the cathodic chamber. In this system, the electric current flow and the liquid flow are parallel. Using this electrochemical flow microreactor having a serial electrode configuration, the anodic methoxylation of p-methoxytoluene was accomplished effectively without intentionally added electrolyte. Protons generated by the anodic oxidation acted as carriers of the electricity. This process will be discussed in detail in the practical part of this chapter. The device could also be used for the anodic methoxylation of N-methoxycarbonyl pyrrolidine and acenaphthylene. 

Atobe and coworkers also reported the generation of unstable o-benzoquinones using flow electrolysis (Figure 9.10). The anodic oxidation of catechols in an electrochemical flow microreactor gave o-benzoquinones, which were reacted with benzenethiols to obtain the coupled products [29]. 

Figure 9.10 Generation and reactions of o-benzoquinones using an electrochemical flow microreactor.

Reaction System Anodic oxidation of p-methoxytoluene is carried out in the absence of intentionally added supporting electrolyte using an electrochemical flow microreactor with a serial electrode configuration to obtain p-methoxyben-zaldehyde dimethyl acetal. 

Experimental Setup and Equipment An electrochemical flow microreactor shown in Figure 9.11, which is composed of diflon and stainless steel bodies made by a mechanical manufacturing technique, is used. The two-compartment cell is... 

Figure 9.11 Electrochemical flow microreactor, (a) Exterior look of the reactor, (b) Interior look of the reactor, (c) Diagram of the system.

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. 

Another characteristic feature of microreactors derived from their much greater surface-to-volume ratios is that they make phase-boundary reactions such as gas-liquid, liquid-liquid, or solid-liquid reactions more efficient. This feature of flow microreactors is also advantageous for photochemical [66-75] and electrochemical [76-86] reactions, which have received significant attention from the viewpoint of environmentally benign syntheses. 

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. 

Figure 4.S Electrochemical cofactor regeneration is simply accomplished in microreactor devices making use ofthe flow/ characteristics on a microscale. By applying different flow rates for and Q2, the anode is isolated making use of almost strictly laminar flow regimes.

A solution to this problem is provided by making use of laminar flow in a microreactor environment, where FADH2 is regenerated electrochemically with a turnover of 21 mmol lactate/(mol NAD s), which is 2.7 times faster than the classical approach (Figure 4.5) [18]. 

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