Undivided-cell process

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. 

Asahi also reports an undivided cell process employing a lead alloy cathode, a nickel—steel anode, and an electrolyte composed of an emulsion of 20 wt % of an oil phase and 80 wt % of an aqueous phase (125). The aqueous phase is 10 wt % K HPO, 3 wt % K B O, and 2 wt % (C2H (C4H )2N)2HP04. The oil phase is about 28 wt % acrylonitrile and 50 wt % adiponitrile. The balance of the oil phase consists of by-products and water. The cell operates at a current density of 20 A/dm at 50°C. Circulated across the cathode surface at a superficial velocity of 1.5 m/s is the electrolyte. A 91% selectivity to adiponitrile is claimed at a current efficiency of 90%. The respective anode and cathode corrosion rates are about mg/(Ah). Asahi s improved EHD process is reported to have been commercialized in 1987. 

Asahi also reports an undivided cell process employing a lead alloy cathode, a nickel—steel anode, and an electrolyte composed of an emulsion of 20 wt % of an oil phase and 80 wt % of an aqueous phase (125). The aqueous phase is 10 wt % KjHPO 3 wt % and 2 wt %... 

The undivided cell process is shown in Fig. 9 the electrodes are kept at a 2-mm distance by plastic spacers. The electrode assembly is installed in a pressure vessel. The life of the electrodes is only 2 or 3 months at best, so the cells have to be removed and opened regularly. For this purpose, there is a dedicated workshop for cell maintenance in the vicinity of the electrolysis facility. 

Clearly the undivided cell process is a very significant advance. 

Table 6.1 Comparison of the Monsanto 1965 divided cell process with the recent Monsanto undivided cell process.

It is also clear from the table that the absence of a membrane the decrease in the interelectrode gap and the increase tn the electrolyte conductivity combine to have a dramatic effect on the cell voltage. The change from —11.65 to — 3.84 V reduces the energy consumption by almost two-thirds. Moreover, the extraction of the product within the cell reduces the number of unit processes in the product-isolation procedure. This also improves the process economics substantially. Overall, the undivided cell process is a very significant advance. 

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

Electrocatalysis with nickel-bpy complexes has been shown useful for synthetic applications,202,211 especially when used in combination with the sacrificial anode process in an undivided cell (Equation (45)).207,211 Under these very simple experimental conditions, efficient nickel catalysts can be also generated in the presence of the cheap pyridine ligand.212... 

These are well known nonelectrodic reactions38. The electrochemical processes are meant to take care of the large amount of elemental lead set free in transmetallation, and has been devised to confine all the reactions to a single-compartment cell process. The cell in the present example is undivided, with Cd and Pb electrodes in DMF or DMSO solutions containing TBAP and Etl(10%) with Nal(5%). A sacrificial cadmium cathode is oxidized to diethylcadmium by ethyl iodide or, less readily, with ethyl bromide. 

The basic chemistry, the early Monsanto process brought on stream in 1965 by Baizer [102-104], and the new Monsanto process based on an undivided cell, combining simpler cell design, less complex product extraction and reduced energy consumption have all been summarized by Pletcher and Walsh [10] (see Scheme 4). 

In the USA Andco Chemical Corp. and Niagara Environmental Assoc., Inc. have widely used this procedure for the treatment of cooling tower blowdown waters and in treatment of metal finishing process streams. The undivided cell geometry makes use of cold-rolled steel plates as anodes. Disadvantages of this procedure Formation of second streams, e.g. large amounts of sludges which must be dumped or treated. 

The formation of S-oxides has also been observed when oxidizing a variety of 5-substituted 2-tert-butyl-l,3-dithianes in wet acetonitrile. In an undivided cell, 4-substituted 1,2-dithiolane-l-oxides were oblained (Scheme 25) [113]. A coupled cathodic process, in this case, was the reduction of protons formed in the anodic reaction. 

Because of the efficiency of this process, the anodic oxidation of amino acid precursors can serve as an excellent method for generating chiral building blocks for synthesis. For example (Scheme 17, Eq. 1) [39], the anodic oxidation of (45) was accomplished at a platinum anode using an undivided cell. An acetate nucleophile was used to trap... 

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... 

The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

The separator is often the weakest component in any electrochemical cell. There are also difficulties in employing ion-exchange diaphragms in aprotic media. Particularly with large industrial cells, it is advantageous to devise reaction conditions that allow the use of an undivided cell. One solution to these problems for an electrochemical reduction process employs a sacrificial anode of magnesium, alumin-... 

Cyclo-coupling between arylalkenes and an aliphatic ester function is achieved by electrolysis in tetrahydrofuran using cathode and anode both of magnesium in an undivided cell. The first electron addition is to the arylalkene. The bond forming steps involves nucleophilic attack by radical-anions or dianions derived from the alkene. Magnesium ions generated at the anode are essential to the process. The... 

The oxidation of propargyl alcohol to the acid and of but-2-yne-l,4-diol to acetylene dicarboxylic acid is carried out on a technical scale at a lead dioxide anode in sulphuric acid [4, 5]. Electrochemical oxidation of acetylenic secondary alcohols to the ketone at lead dioxide in aqueous sulphuric acid [4], gives better results than the cliromic acid based process of Jones [6], Oxidation of aminoalkan-1-ols to the amino acid at a lead dioxide anode in sulphuric acid is achieved in 31 -73 % 5delds [7]. This route is applied to the technical scale production of (l-alanine from 3-aminopropanol in an undivided cell [8]. 

In another procedure, oxidation is carried out in the presence of chloride ions and ruthenium dioxide [31]. Chlorine is generated at the anode and this oxidises ruthenium to the tetroxide level. The reaction medium is aqueous sodium chloride with an inert solvent for the alkanol. Ruthenium tetroxide dissolves in the organic layer and effects oxidation of the alkanol. An undivided cell is used so that the chlorine generated at the anode reacts with hydroxide generated at the cathode to form hypochlorite. Thus this electrochemical process is equivalent to the oxidation of alkanols by ruthenium dioxide and a stoichiometric amount of sodium hypochlorite. Secondary alcohols are oxidised to ketones in excellent yields. 1,4- and 1,5-Diols with at least one primary alcohol function, are oxidised to lactones while... 

The chemical dehydrodimcnzaiion of malonate esters can be carried out by the action of iodine on the sodium enolate [23], This route is adapted as an electrochemical process using alcoholic sodium iodide as electrolyte in an undivided cell with carbon electrodes [24, 25]. Base generated at the cathode and iodine generated at the anode serve in a catalytic cycle. 

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