Divided and Undivided Cells

Where possible, a single electrolyte compartment in an undivided cell geometry is favored as it considerably simplifies the constmction, electrolyte flow circuit and maintenance needs, while avoiding the potential drop and mass balance problems which can be associated with a microporous separator or ion-exchange membrane. 

Division of the cell can, however, offer a number of important benefits it can separate hazardous mixtures of products prevent unwanted chemical reactions prevent reactant or product loss at the other electrode permit the use of dissimilar electrolytes and protect an anode from corrosion by aggressive species. In addition, ion-permeable membranes can introduce selective ion migration through the membrane, and this can be used in designing the cell chemistry to achieve greater selectivity. When using a membrane it 

As mentioned in Section 17.1, the anodic and cathodic compartments of an electrochemical cell can be separated by an ion-exchange membrane or a porous diaphragm. The division of a cell is often practiced in industrial processes, despite the additional costs, the need for additional seals and possible maintenance problems. A separator may indeed allow a more independent choice of anode/anolyte or cathode/catholyte, enable current eftkiency to be maintained due to the exclusion of redox shuttles and help to isolate electrode products or prevent the formation of explosive or toxic mixtures, for example H2-O2. However, if possible, undivided cells are preferred, as they lead to lower ohmic drops and to much simpler technologies. 

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Inline flow-through cells dominate the market. Divided and undivided cells are known. Cells have been commercialized to a remarkable extent since the 1970s, especially in Eastern Eiuope and Germany. Selected schematic constructions and technologies are shown in Figs. 3 and 4. 

Alkynes can be reduced electrolytically. Internal alkynes gave 65-80% yields of cis alkenes when electrolysed in 10% sulfuric acid in ethanol at spongy nickel cathode [127], or predominantly trans alkenes if the electrolysis was carried out in a methylamine solution of lithium chloride. The yields of the alkenes and the ratios of trans to cis alkenes varied depending on whether the electrolysis was carried out in divided or undivided cells (yields 24-80%, composition of product 89-99% of trans alkene) [379]. 

In chloralkali electrolysis and conventional water electrolysis, the catholyte is strongly alkaline (cNaon, Ckoh = 30 wt%), whereas in orga-noelectrosynthesis it is neutral or acidic depending on whether a divided or undivided cell is used. 

The production of bipyridyls (1) and bipiperidyls (2) was observed on reduction of pyridine. Schering AG has a patent on a process for producing 4,4 -dipyridyls at the cathode of a divided or undivided cell using liquid ammonia as the solvent.26 The same bipyridyl was also formed during electrolysis of bromobenzene in pyridine solvent, using Mg electrodes.27 Bipiperidyls (2) were observed as products of pyridine reduction as early as... 

Table 19.1 Absolute second-order rate constant for the reaction of several organic pollutants with OH formed during the electro-Fenton process with Fe3+ as catalyst in divided and undivided Pt/carbon felt cells at room temperature (Oturan et al. 2000 Oturan et al. 2000, 2001 GOzmen et al. 2003 Edelahi et al. 2004 Hanna et al. 2005)...

Oxidants can be synthesized either at the anode or the cathode, and in some instances simultaneously at both the electrodes. Synthesis can also take place in either divided or undivided cells in some eases, the presence or lack of a separator dietates the type of oxidant formed. 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

Some filter-press type electrochemical reactors can be used both in a divided and undivided mode. This is illustrated in Ref. 51. The ICI filter-press cell system FM 21 is discussed in the context of several pilot-plant applications, most of them organic. 

Several configurations have been used in order to enhance the oxidation ability of the EF technology, with promising results for two- and three-electrode divided and undivided electrolytic cells. The efficiency of the process is always a function of temperature, pH, O2 feeding, stirring or liquid flow rate, electrolyte composition, applied potential or current, and catalysts and pollutant concentrations. 

E. A. Kaminski and R. F. SavineU, A Technique for Calculating Shunt Leakage and Cell Currents in Bipolar Stacks Having Divided or Undivided Cells, J. Electrochem. Soc. 130 1103 (1983). 

Two-Dimensional Electrode Flow Cells. The simplest and least expensive cell design is the undivided parallel plate cell with electrolyte flow by some form of manifold. Electrical power is monopolar to the cell pack (72). An exploded view of the Foreman and Veatch cell is shown in Figure 7. Note that electrolyte flow is in series and that it is not easily adapted for divided cell operation. 

It is clear from Table 7 that the undivided cell has considerable power usage savings over the divided cell operation. Also, there are no membrane costs, and cell fabrication is much cheaper. In addition, it was possible to simplify the product recovery in the undivided cell process. 

The significance of the supporting electrolyte cation depends crucially on whether a divided or an undivided cell is used. In a divided cell, the choice of cation is of minor importance but in an undivided cell the cathode process should not lead to formation of base and thereby to buffering of the solution. Metal cations such as Li+, Na+ or Mg + are often the choice since in aprotic solvents the metal cation may be the most easily reduced component. This has been observed as deposits of metal on the surface of the cathode arising from... 

An exceptionally interesting example of the electrohydrocyclization reaction involves the use of allenes which are tethered to a,/S-unsaturated esters (Table 3) [19]. The chemistry takes place in a manner wherein the new carbon-carbon bond forms between the central carbon of the allene and the /S-carbon of the unsaturated ester. Of particular value is the preservation of one of the double bonds of the original allene, thereby providing functionality for further elaboration. It is important to carry out these transformations in an undivided cell, as the use of a divided cell led to hydrogenation of the olefins instead of cyclization. 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

Technically interesting are the indirect electrochemical oxidations of benzylic alcohols (Table 11, No. 15-18) benzaldehyde dimethylacetals (Table 11, No. 19) and alkyl aromatic compounds (Table 11, No. 20, 21) It could be proven that benzylic alcohols are oxidizable using tris(2,4-dibromophenyl)amine as mediator not only in acetonitrile in a divided cell but also in methanol in an undivided cell... 

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