Capillary gap cells

In nonaqueous media, carbon (graphite) frequently is an optimal anode material, for example, for methoxylation reactions, even in an industrial scale (capillary gap cell, see Fig. 12). It shows an appreciable overvoltage for the oxidation of methanol compared to platinum. 

Fig. 12 Capillary gap cell (From Ref. [4]) 1 bipolar electrode plates,...

A concentric capillary gap cell (up to 800 cm active area) is described in [87]. 

Silyl enol ethers (Scheme 2, Y = OSi ( 113)3) can be dimerized to 1,4-dicarbonyl compounds in good yields. To suppress the methanolysis of the silyl enol ethers, MeCN-5% MeOH is used as solvent and the electrolysis is conducted within one hour by the use of a capillary gap cell, which allows high currents (Table 7, numbers 6 and 7) [59]. 

Some of these processes have been developed for technical conversions and have been summarized in Ref. [228, 229]. The anodic technical production of t-butylbenzaldehyde has been coupled with the cathodic reduction of phthahc anhydride to phthalide in a paired electrosynthesis in a capillary gap cell [230]. Indirect oxidations with Mn +/Mn + or as mediators... 

Ea and Ec are the applied potentials at the electrodes, while iR is the sum of the ohmic drops at the resistances of the electrolyte and, for example, of the diaphragm. The electrolyte resistance is linearly dependent on the electrode distance. Therefore, this distance should be small and a diaphragm should be avoided (undivided cell), if possible. These conditions are best fulfilled by capillary gap cells with gaps of 0.1-0.2 mm. In this case, the concentration of the supporting electrolyte can also be very small. 

If a large space-time yield is also required in the laboratory application, a capillary gap cell (disc-stack) is a good choice (Fig. 22.11). Only the upper-... 

For application in industry, the work-up of the product containing electrolyte must be easy and efficient. With capillary gap cells, the concentration of a supporting electrolyte may be so low that its recovery is not necessary from the economical standpoint. 

Efforts in this field of anodic oxidation are certainly to be expected. Difficulties that presently arise are due to the low conductivity in the usable solvents, e.g., ether, tetrahydrofurane, diglyme, glyme, and the reactivity of the anionic precursors, which could lead to serious side reactions on prolonged electrolysis. These problems may possibly be overcome by low temperature electrolysis in capillary gap cells 366CJ with small electrode distances to diminish the iR drop, and high electrode surface/ electrolyte volume ratios for fast electrolysis. 

The effectiveness of RE and PC electrochemical reactors for methoxylation of furan was examined by Thomas et al. (1988). The performance of various types of reactors is compared in Table XXIX. As shown, both RE and PC with cathode spinning gave better performance than a capillary gap cell. For the pump cell, the results differ depending on whether the cathode is spinning or... 

Capillary gap cell — The undivided capillary gap (or disc-stack) cell design is frequently used in industrial-scale electroorganic syntheses, but is also applicable for laboratory-scale experiments when a large space-time yield is required. Only the top and bottom electrodes of c.g.c. (see Figure) are electrically connected to - anode and cathode, respectively, whereas the other electrodes are polarized in the electrical field and act as -> bipolar electrodes. This makes c.g.c. s appropriate for dual electrosynthesis, i.e., pro duct-generating on both electrodes. 

Of course an appropriate ionic conductivity in the active layer is important as well. Organic solid polymer electrolytes (SPEs) provide only k = 0.1-1 mS/cm, and liquid organic electrolytes k — 1-lOmS/cm. But aqueous electrolytes have much better values in the order of lO-lOOmS/cm. One of the consequences of the low ionic conductivities of organic electrolytes is a minimization of transport length L (cf. Vetter s model [64]), or the capillary gap cell in organic electrosynthesis [4]. 

Undivided Flow Cells. A simple cell for large-scale laboratory electrolysis of organic compounds is shown in Fig. 5, and this concentric capillary gap cell has been used for many anodic reactions [53]. 

Figure 5. Cross section of concentric capillary gap cell. A, cathode and B2, anode support Cj, outlet C2, inlet D, anode E, silicon rubber seal. (From Ref. 53.)...

Cells with three-dimensional electrodes have bipolar electrodes such electrodes are characterized by the feature that one part of the electrode is anode and another part cathode. This can be realized in different ways, such as the pile capillary gap cell (Beck/Guthke cell. Chapter 32) [9,80], the Swiss roll cell [81], and packed- and fluidized-bell cells (Chapter 32) [82-84]. These cells are developed to meet economic demands, such as high space-time yield and simplicity in construction they are discussed in Chapter 31. 

Methoxylation lends itself well to operation on a relatively large scale [79-83]. For instance, several A-formyl derivatives have been anodically methoxylated on a 0.5-kg scale using a capillary gap cell with a graphite anode [80,82]. An example is given in Eq. (29). Polyalkoxylation of N, A-dimethylformamide (DMF) has also been achieved [81]. 

A classical example of this strategy is the capillary gap cell (see Sec. IV.B), which has been run by BASF for more than a quarter of a century [7]. Several dozen electrodes are piled onto each other to give a stocky pillar. The electrodes consist of graphite rings. The diameter of the central hole is a tenth of the outer diameter of the electrodes. The hydrodynamics and the current and potential distribution are far from optimal. 

Figure 5. Thin-film cells, (a) Capillary-gap cell, (b) Bipolar trickle tower.

The anodic methoxylation of ketones is a recent commercial example for an indirect process in an undivided cell [107]. The process (Fig. 18) was developed for BASF s undivided capillary gap cells with bipolar graphite electrodes [108] (Scheme 12). 

But new products, too, are ahead. Monsanto has developed a highly selective hydrodimerization of maleic acid esters [138] to prepare butanetetracarboxylic acid esters [139], a starting material for butanetetracarboxylic acid [140,141] (Scheme 14). BASF is continuously developing new specialities [142], especially for the capillary-gap cell. The meth-oxylation of ketones such as butanone and cyclohexanone has already been mentioned other developments may be found in the patent literature [143]. 

Cells with enlarged electrode area in a small cell volume are found in the multiple-cathode cell [64], the Swiss-role cell [65], the extended surface electrolysis (ESE) cell [66], and the capillary gap cell [13, 67]. 

A special cell concept combining ease of construction and scale-up is the capillary gap cell, which is mainly applied in electro-organic synthesis, where low conductivities of the electrolytes are a major problem, Figs 6 and 7, [13, 28, 67, 89], The cell consists of circular disk electrodes with a small interelectrode gap (1-2 mm) to minimize the ohmic voltage drop in the electrolyte. The electrolyte... 

In the case of well-known electrochemical reactions, as well as for electrolyses where larger scales are involved, two-electrode cells (connected to a galvanostat) can be used with continuous feed of the reactant to the working electrode. This type of electrolysis is suitable for industrial purposes where specific devices and cells are utilized. Since electrodes of large areas are necessary, the distance between the anode and the cathode is small and determines the cell geometry (e.g. capillary-gap cell or filter-press cell). The use of cells equipped of porous electrodes (materials like graphite or carbon moss, platinum, nickel) have also been developed to perform electrocatalytic reactions at very large surfaces. Some typical cells used in the laboratory and in industry are presented at the end of this review. 

The ideal cell in order to scale up an electrochemical reaction can depend on the reaction, the electroactivity of the substrate to convert, the concentration of the substrate, as well as the current density at the working electrode. The use of a separator is necessary when the electrode can affect the whole process negatively. With anodic oxidations, the reaction at the counter electrode is most frequently the cathodic formation of hydrogen. In these cases, a separator does not seem indispensable a tank cell (kind of Grignard type reactor equipped with cylindrical electrodes) or a capillary-gap cell (piling of bipolar electrodes in a cylinder-shaped vessel connected to an anodes and a cathode located at the top and the bottom of the cell) can be considered as suitable devices for anodic conversions. More generally, the so-called plate-and-frame cells (Fig. 4) are used in a battery. 

For larger scale eonversions, the circulation cell [2b], the capillary gap cell [14a] and the Swiss roll eell [14b] are available. All three eells work at low cell voltage... 

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