Carboxylic layer

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

Asahi s investigations showed that a Na+ concentration of 1.1 N was necessary in compartment II to maintain a current efficiency of 96% in the carboxylic membrane during operation with 3.5 N brine in compartment I and 32% caustic soda in compartment III. The Na+ concentration in compartment II was generally far lower than that in compartment I, and clearly indicates the tendency for depression of the Na+ concentration at the interface of the sulphonic and carboxylic layers in the normal... 

Fig. 17.7 Influence of Na+ concentration at interface of sulphonic acid and carboxylic layers on current efficiency 3-4 N NaCI 1.0-5.5 kA rrT2 33% NaOH.

In summary, the results indicate that the Na+ concentration at the interface between the sulphonic and carboxylic layers in the normal membrane is substantially depressed from that of the anolyte compartment, and that the concentration at this interface strongly affects the current density. 

The limiting current density may be defined as the current density at which the depression of the Na+ concentration at the interface of the membrane s sulphonic and carboxylic layers results in an abrupt rise in cell voltage and drop in current efficiency. 

The change from calcite to vaterite nucleation on stearate films at low [Ca] suggests that the extent of Ca binding is important for polymorph selection. The nucleation of calcite is favored by the formation of a well-defined Ca-carboxylate layer that mimics the first layer of the (110) face of the unit cell. By contrast, the structural requirements for vaterite formation are less precise. This is consistent with vaterite being the dominant phase on amine monolayers where no Ca binding is present, and suggests that kinetic factors of charge accumulation... 

Ionic additives to the electrolyte can influence the Kolbe electrolysis in a negative way. Anions other than the carboxylate should be excluded, because they hinder the formation of a carboxylate layer at the anode, that seems to be a prerequisite for the decarboxylation. In the electrolysis of phenyl acetate the coupling to dibenzyl is totally suppressed when sodium perchlorate is present in concentrations of 5 x 10" mol 1" benzyl methyl ether, the nonKolbe product, is formed instead. This shift from the radical... 

The membrane is exposed to chlorine and brine on one side and strong caustic solution on the other side at temperatures of around 90 °C. Only ion-exchange membranes made of perfluoropolymer can withstand such severe conditions. The ion-exchange groups are sulfonate (SOf) or carboxylate (CHCOO-). Modern membranes of this kind consist of three layers, a carboxylic layer on the cathode side, a sulfonate layer on the anode side and a reinforcement layer of fabric in between. In addition, both sides are provided with a hydrophilic coating [3, p. 81 15]. The thickness of the membrane is only one- to two-tenth of a millimeter. It must be mentioned that the hydrate water of the cations is taken along through the membrane. 

These performance goals have now largely been attained by continued improvements through several generations of materials. Currently, commercial perfluorinated ionomer materials for this application consist of membranes with carboxylate or mixed carboxylate-sulfonate functionality the latter membranes often have layered structures with the carboxylate layer exposed to the caustic catholyte solution. Fabric reinforcement is used in some instances to improve strength. 

Mechanisms of Inhibition. A number of diffusion experiments were run in which various concentrations of N,N -diphenyloxamide (< 0.1 wt % or 3 X 10 3 mol/kg) were loaded in the polyethylene films. The great majority of runs showed essentially no effect of the additive on the diffusion rate, nor were any unusual surface phases reproducibly noted (see Tables I and III for typical results). Previous work (2,5) has indicated that the inhibition effect of a deactivator may be caused by both surface and homogeneous scavenging effects. On the basis of the present results we conclude that the major effect of the deactivator involves surface-interface reactions rather than bulk scavenging mechanisms. The former may consist of poisoning of active surface sites on the Cu20/Cu film (13) and/or conversion of an interfacial copper carboxylate layer to a relatively inert phase of insoluble copper complex (5). Work is in progress to separate these mechanisms further. 

Bilayer carboxylate membranes can be produced by surface modification of Nafion-type membranes. In a process used by Asahi Chemical the sulfonate on a Nafion-type surface is reduced to sulfinic and sulfenic acids, then oxidized to a carboxylate layer of 2-10 pm thickness ... 

Perfluorinated membranes used in chlor-alkali cells normally have a thin layer of carboxylate on the cathode-facing surface of a sulfonate membrane. Nafion 901 was introduced as such a membrane [38]. It achieved 33% NaOH concentration with 95% current efficiency in cells operating at 3 kA/m and 3.3 to 3.9 V. The carboxylate layer can be prepared by lamination, but the layer can be... 

With the carboxylate layer on the membrane, the concentration of NaOH in the catholyte can be maintained at about 32%, which compares favorably with the 12% product from diaphragm cells. Consequently, considerably less thermal energy is needed for evaporation to the commercial 50% product. 

Scheme 6.5 Representation of formation of metal assisted ion pair at the film interface 4. a In the absence of M the supporting electrolyte can freely diffuses into the film, b Formation of strong M -carboxylate layer prevents the effective diffusion of counter ions into the film...

These composite membranes are prepared either by lamination of the perfluoro-carboxylate and perfluorosulfonate films [75-77] or by the chemical conversion [78-80] of one surface of the perfluorosulfonic acid to perfluorocarboxylic acid to realize a carboxylate layer thickness of 5-10 xm. 

The membrane conductivity data used in the present calculation are those for a carboxylic acid membrane [97-99], as the overall conductivity of a composite membrane is dictated by the carboxylic layer. Diffusion coefficient data for chloride and chlorate ions across the commercial composite membranes are not available. Hence, an average of the diffusion coefficient data for the chloride ion for carboxylic acid membranes [100,101] was used, assuming the same temperature dependence as that of membrane conductivity. The diffusion coefficient of the chlorate ion was assumed to be the same as that of the chloride ion. Variations in Dq- and k with anolyte concentration, under commercial operating conditions, were reported [102] to be weak, and hence, these dependencies were not considered here. 

FIGURE 4.8.33. Dependence of amount of NaCl in 50% caustic on the thickness of the carboxylic layer (98]. (Reproduced with permission of the Society of Chemical Industry.)... 

Impurities such as Ni and Mg, whose hydroxides have low solubilities, tend to precipitate near the membrane surface on the anode side, causing an increase in the ohmic drop across the membrane. On the other hand, impurities such as Ca, Sr, and Ba, with higher solubilities, are prone to deposit on the cathode side of the membrane in the carboxylate layer, leading to the formation of large voids and therefore, to decreased current efficiencies. The actual situation may be more complex because of mutual interactions of impurities to form complex species. 

Calcium in the brine is particularly harmful because of the moderate solubility product of its hydroxide ( 10 ). Calcium ions penetrate the membrane and precipitate as hydroxide near the membrane-catholyte interface or at the interface of the sulfonate and carboxylate layers. The voltage drop increases greatly when Ca(OH>2 accumulates to a concentration of about 1 mg cm [18]. 

There are three types of blisters, water, salt, and others. Water blisters are formed when there is high water transport across the layers. Salt blisters result finm localized heating of the membrane because of high local current density or nonuniform current distribution. Proper cell design can alleviate this problem (Chapter 5). The other type of blister arises when the acidity of the anolyte is high. When the anolyte pH is below 2, the carboxylate layer protonates to a nonconductive carboxylic acid. This will increase the voltage and the internal vapor pressure, and finally result in the formation of voids in the carboxylate layer. 

Precipitation of impurities in the sulfonate or the carboxylate layer disrupts the transport properties of water and sodium ions. Impurity precipitation usually roughens... 

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