US3392093A - Electrolytic process for producing tetraalkyl lead compounds - Google Patents

Description

K. C. SMELTZ July 9, 1968 Filed June 23, 1964 CATHOLYTE CATHOLYTE E T u m b A 2 w 0 a a m Q. 5 m l m i ,& W. v Ill I I! l// f A N R 4 k m a: m a m l O N A INVENTOR KENNETH C. SMELTZ ATTORNEY United States Patent 3,392,093 ELECTROLYTIC PROCESS FOR PRODUCING TETRAALKYL LEAD COMPOUNDS Kenneth C. Smeltz, Wilmington, Del., assignor to E. I.

du Pont de Nemonrs and Company, Wilmington, Del.,

a corporation of Delaware Filed June 23, 1964, Ser. No. 377,312 17 Claims. (Cl. 20472) This invention relates to a process for producing tetraalkyl lead compounds and particularly to an improved process for preparing those compounds by the electrolytic reduction of an alkyl halide at a lead cathode.

Electrolytic syntheses of organometals are known. For example, Tafel, Ber., 44, 327 (1911), obtained tetraisopropyl lead by electrolyzing an aqueous acid solution of acetone at a lead cathode in the absence of air. Such method, however, is not general and appears impractical for organolead production because of low yields and side reactions. Later (in 1925) Calingaert and Mead disclosed tetraalkyl lead formation at a lead cathode by electrolyzing a catholyte consisting of an alkyl halide in either alcoholic caustic (Calingaert in US. Patent 1,539,- 297) or aqueous caustic containing casein (Mead in US. Patent 1,567,159). They hypothesized: Apparently the hydrogen formed at the cathode reduces the reaction mass, forming lead di-ethyl, which is unstable at the temperature of the catholyte, and breaks up thermally into lead and lead tetraethyl. The above disclosure suggests that a potential source of hydrogen is necessary, such as the hydroxylic solvent employed, and further that, if diethyl lead is an intermediate, the yield based on lead can be no more than 50% of the consumed lead. In fact, under the disclosed conditions the tetraethyl lead yields tend to be loW and the cathode deteriorates. Further, the ethylating agent tends readily to be destroyed in side reactions, by reaction for example with the caustic present, particularly in alcohol.

Besides the cathodic processes, anodic oxidations have also been employed to produce organometallics, as for example by Hein et al., Z. anorg. Allgem. Chem. 141, 161 (1924), who obtained tetraethyl lead by electrolyzing sodium zinc triethyl at a lead anode, and by Ziegler in British Patent 814,609 (1959) who discloses synthesis of Group IIV metal alkyls 'by electrolyzing a complex aluminum alkyl at an anode composed of the Group II-V metal. Numerous other references disclose similar processes. All such anodic processes are characterized by the fact that the source of the organic groups to begin with is invariably another organometal (often a complex of two Or more such compounds), the net result of the electrolytic oxidation being replacement of metal in the starting material by metal of the anode. That the anodic process requires organometal starting material is an important disadvantage, for such materials normally are difiicult or costly to make and require special storage and handling under inert atmospheres.

Ernest F. Silversmith and Walter J. Sloan, in their copending application, Ser. No. 156,128, filed Nov. 30, 1961, and now Patent No. 3,197,392, have disclosed a novel process for preparing organometal compounds, including tetraalkyl lead compounds, by electrolyzing, at a cathode of the selected metal, a solution of an alkylating agent in a normally liquid, non-hydroxylic catholyte. They disclose that it is not necessary to maintain rigorous anhydrous conditions in the catholyte and that traces of water therein can be tolerated. They also disclose that they can employ, as the anolyte, an aqueous solution of sodium carbonate. It is probable that, when such anolyte is used with the ion-permeable membrane employed by Silversmith and Sloan, water will gradually pass through the membrane into the catholyte so the catholyte, which 3,392,093 Patented July 9, 1968 initially is non-hydroxylic, will eventually contain considerable uncontrolled amounts of water.

The process of Silversmith and Sloan is very efiective and constitutes a significant advance in the art. However, improvements in that process are desirable; Current densities must be kept relatively low for eflicient current utilization. Attempts to achieve high cell production rates by operating at high current densities are stymied by the decreasing ability of the process to make tetraalkyl lead etficiently as current density is increased above certain limits. In other words, above such limiting current densities, the electrode reaction becomes less and less specific for the formation of tetraalkyl lead While gas-forming side reactions (at the expense of current and alkylating agent) become increasingly prominent. The process also tends to be erratic and difiicult to control over long periods of operation. The voltage required for maintaining current density at the selected operating level tends to increase with time while the highest current density that may be used for smooth operation (high tetraalkyl lead electrical yield with minimum side reactions) tends to decrease. The overall results are still higher electrical costs and lower production rates. Furthermore, the recovery of by-product bromine from the anolyte is more complicated, difiicult and costly than is desired.

It is an object of this invention to provide a new and improved electrolytic process for producing tetraalkyl lead compounds. Another object is to provide such a process which overcomes objections of the prior electrolytic processes and which produces higher yields of tetraalkyl lead compounds at higher production rates and in a more economical manner. A further object is to provide such a process which involves conditions that increase the specificity of the reaction to produce tetraalkyl lead compounds at the expense of side reactions and permits the use of higher current densities. A particular object is to provide such a process which can be operated in a continuous manner. A still further object is to provide such a continuous process which can be operated over long periods of time at high current densities and low voltages without loss of efficiency and decrease in yield and production rates. Other objects are to advance the art. Still other objects and advantages will be apparent from the detailed description presented hereinafter.

The foregoing and other objects may be accomplished in accord with this invention which comprises an electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a currentpermeable partition separating the catholyte from the anolyte, which process comprises:

(A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially (a) an alkyl halide in which the alkyl group has 1-10 carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration sufiicient to provide a catholyte having a conductivity of at least 0.001 ohm cm. and

(c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of l-4 carbon atoms; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, in a concentration sufiicient to provide an anolyte having a conductivity of ta least 0.001 ohrn cm.-

(2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said current-carrier;

(D) during the electrolysis, adjusting the amounts of the current-carrier in the catholyte and the anolyte as may be necessary to maintain their conductivities at at least 0.001 ohm cm. and adjusting the amount of the hydroxylic compound in the catholyte as may be necessary to maintain the concentration thereof within the range of from about 1 to about moles per mole of current-carrier; and

(E) recovering tetraalkyl lead from the catholyte.

While the process may be operated without a solvent in the catholyte, it usually will be preferred to employ an inert nonhydroxylic organic solvent for both the alkyl halide and the current-carrier, which solvent has a reduction potential higher than the alkyl halide and an oxidation potential higher than the current-carrier. By an inert" solvent is meant a solvent or mixture of solvents which is nonreactive to the other components of the catholyte and the anolyte and the reaction products produced therein, i.e. nonreactive to the tetraalkyl lead, the alkyl halide, the hydroxylic compound, the tetraalkyl ammonium monohalides and polyhalides, and halogens of atomic numbers of at least 17. Thus, there are excluded strong acids (reactive to tetraalkyl lead), strong bases (reactive to alkyl halide and halogen), and organometallic compounds such as diethyl zinc, triethyl aluminum and diethyl magnesium (reactive towards hydroxylic compound and tetraalkyl ammonium polyhalides), and the like.

The hydroxylic compound in the catholyte consists of water or an alkanol of 1-4 carbon atoms, i.e. methanol, ethanol, l-propanol, 2-propanol, l-butanol, 2-butanol, 2- methyl-l-propanol, 2-methy1-2-propanol, and mixtures of any two or more thereof. Water, methanol and ethanol, particularly water, are preferred. The hydroxylic compound will be employed in a proportion of from about 1 to 20 moles per mole of tetraalkyl ammonium monohalide, preferably from about 3 to about 10 moles per mole.

It has been found that, by the use of catholytes and anolytes of the foregoing defined compositions, the dilficulties encountered with the prior processes are largely overcome and many significant advantages are obtained. Particularly, the use of the controlled amounts of the hydroxylic compounds in the catholyte in the proportions and in the manner required by this invention materially improves the specificity of the tetraalkylation reaction, that is, it suppresses side reactions and causes more of the alkyl halide to react with the lead of the cathode to form tetraalkyl lead instead of other products, whereby the yields of tetraalkyl lead are increased, whether based on current or on cathodic lead consumed. Other things being equal, the production rate, i.e. the total amount of tetraalkyl lead produced at a given cathode in a given time, theoretically should increase with increase in the current density. However, as pointed out hereinbefore, increase in the current densities in the prior processes results in lower yields of tetraalkyl lead and large increases in side reactions and the production of gaseous by-products and loss of current efiiciency. The hydroxylic compounds in the catholyte in the amounts employed in this invention make it possible to use current densities considerably higher than can be used practically in the prior processes, by suppressing the side reactions that normally occur at such higher current densities, thereby making it possible to obtain production rates which cannot be obtained by the prior processes. Also, the use of the hydroxylic compounds in the catholyte according to this invention makes it possible, by judicious regulation of the concentration of the hydroxylic compound, to obtain a desired high current density and to maintain it throughout the electrolysis with the use of lower and more uniform cell voltages than could be used heretofore, particularly with cell partitions of substances which exhibit inherently high resistance in low hydroxylic or non-hydroxylic electrolyte systems, whereby reaction control is made easier and electrical costs lower. In other words, by regulation of the concentration of the hydroxylic compound in the catholyte, it is possible to maintain a highly efiicient production of tetraalkyl lead compound in high yields and at high produc tion rates throughout the electrolysis and particularly over long periods of continuous operation with minimum cell voltage requirements. These results are not obtainable by the use of the aqueous caustic, alcoholic caustic or the like aqueous solutions employed in the catholytes and/or anolytes of the prior processes and which are excluded by the present invention.

It appears that the hydroxylic compound, as employed in this invention, exerts its beneficial influences on the electrode reaction itself, by functioning within the electrical double layer around the cathode or at the cathode surface itself (in an unobvious manner in view of the prior art), to favor the conversion of alkyl halide and cathodic lead to tetraalkyl lead over their conversion to gaseous products and sludge-containing lead.

'Such beneficial eflects of the hydroxylic compound are not, as might have at first been expected, attributable to increased conductivity of the otherwise non-hydroxylic catholyte composition. Except at relatively high tetraalkyl ammonium monohalide (salt) concentrations (e.g. above about 0.5 mole/kg. of solution) where slight to moderate conductivity increases can be observed, addition of water, in the amounts here involved, has little or no effect on solution conductivity. Increase in solution conductivity, when observed at the higher salt concentrations, apparently derives from the etfect of water to decrease solution viscosity at those salt concentrationss. Decrease in the overall cell voltage requirement, when observed, apparently results from the unobvious effect of the hydroxylic compound to increase current flow across the cell partition, possibly by facilitating ion transport from one compartment to the other, irrespective of its elfect on the conductivity of the catholyte solution itself.

The concentration range of l to 20 moles of hydroxylic compound per mole of tetraalkyl ammonium monohalide appears to be critical. Below the specified lower limit of 1 mole, the beneficial elfects are not always attained to the desired degree and gassing occurs when the current density is increased to 0.1 amp/sq. cm., while above the range, gassing tends to be excessive and the tetraalkylation ineflicient. Within the broad range, the concentration of hydroxylic compound will vary depending primarily on the alkyl halide and the tetraalkyl ammonium monohalide, particularly on the nature of the halogens, and on the effect desired. Bromides and iodides respond better to the presence of the hydroxylic compound for the purposes of the invention, and, in general, the higher proportions of hydroxylic compound are used with these halides.

The hydroxylic compound, e.g. water, may be introduced into the cathode compartment directly, in an intermittent or a continuous manner, as such or in combination with any of the other catholyte ingredients, or it may be introduced in part from the anode compartment during the course of the electrolysis by employing an aqueous anolyte solution and a cell partition which permits water transport into the cathode compartment.

This invention is concerned primarily with the manufacture of tetraalkyl lead antiknock compounds such as tetramethyl lead, tetraethyl lead, and tetra(mixed methyl and ethyl) lead compounds, but the process is applicable to the preparation of other tetraalkyl lead compounds having up to carbon atoms in each alkyl group. Thus, the alkylating agent will be an alkyl halide in which the alkyl group has 1-10 carbon atoms and the halogen has an atomic number of at least 17, i.e. chlorine, bromine or iodine, or a mixture of any two or more of such alkyl halides. Such alkyl halides have reduction potentials of 1-2 volts (at a lead cathode relative to a saturated calomel electrode and determined in a solution of tetraethyl ammonium monobromide in acetonitrile at 20 C. to 80 C.). Preferably, the alkyl halides will have 1-2 carbon atoms, i.e. will be methyl halides, ethyl halides or a mixture of a methyl halide and an ethyl halide, and most preferably the bromide or bromides.

When a solvent (and diluent) for the alkyl halide is employed in the catholyte, it usually and preferably will be an alkanonitrile in which the alkyl group has 1-5 carbon atoms, it being understood that the term alkanonitrile means compounds of the formula RCN wherein R represents an alkyl radical. Representative alkanonitriles are acetonitrile, propionitrile, n-butyronitrile, isobutyronitrile, n-amylnitrile, isoamylnitrile, and pivalonitrile. Mixtures of any two or more of such alkanonitriles may be used. Acetonitrile is usually preferred. Other inert nonhydroxylic organic solvents, which have higher reduction potentials than the alkyl halides and higher oxidation potential than the tetraalkyl ammonium halides and which are solvents for both the alkyl halide and the tetraalkyl ammonium monohalide, can be employed in place of the alkanonitrile or in admixture therewith. Suitable solvents include carboxamides such as N,N-dimethylformamide and N,N-dimethylaceta.mide. The higher reduction potential is required to cause preferential reduction of the alkyl halide at the cathode. The higher oxidation potential is required to cause preferential oxidation of the tetraalkyl ammonium monohalide at the anode.

The current-carrier, which is employed in the catholyte and in the anolyte, must consist of at least one (one or more) current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than the alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen has an atomic number of at least 17. Such current-carrying tetraalkyl ammonium monohalides can be represented by the formula R NX wherein each R is an alkyl radical of from 1-18 carbon atoms and X is a halogen atom which has an atomic number of at least 17, i.e. chlorine, bromine and iodine. The alkyl radicals in a tetraalkyl ammonium monohalide may be the same or different. Mixtures of two or more tetraalkyl ammonium monohalides may be used. Also, the tetraalkyl ammonium monohalide must have a higher reduction potential than the alkyl halide employed, so as not to interfere with the alkyl halide-electrode reaction.

Representative tetraalkyl ammonium monohalides are:

Tetramethylammonium bromide Tetramethylammonium iodide Tetraethylammonium chloride Tetraethylammonium bromide Tetraethylammonium iodide Tetra(n-propyl)ammonium bromide Tetra(n-butyl)ammonium chloride Tetra(n-butyl)ammonium bromide Tetra(n-butyl)ammonium iodide Tetra(n-amyl)ammonium bromide Tetra (n-amyl ammonium iodide Trimethylethylammonium bromide Trimethylisoamylammonium bromide Dimethyldiethylammonium bromide Methyltriethylammonium bromide Dioctadecyldimethylammonium chloride Octadecyltrimethylammonium bromide The current-carrying tetraalkyl ammonium monohalides are employed in the catholyte and in the anolyte in concentrations suflicient to provide electrical conductivities in each of at least 0.001 ohm" cmr preferably at least about 0.01 ohmcmr and as high as is practicable. Thus, the tetraalkyl ammonium monohalide must be soluble in the other components of the catholyte and the anolyte, respectively, to the extent required to produce those concentrations. The maximum obtainable conductivity is determined by such factors as the particular tetraalkyl ammonium monohalide, the solvent and its dielectric constant and viscosity, and the other components of the electrolyte compositions and their character. The actual amount of tetraalkyl ammonium monohalide required to achieve a particular conductivity level (0.001 and higher) will vary with the various tetraalkyl ammonium monohalides, the various solvents, the various hydroxylic compounds, the relative concentrations of the different materials, and the temperature of operation. In view of these variables, it is impossible to specify absolute ranges for all possible catholytes and anolytes within the scope of this invention. However, it is well known to the art to impart electrical conductivity to normally nonconductive organic liquids, including nonhydroxylic organic liquds, by means of current-carryng solutes, and hence those skilled in the art have the knowledge to readily determine the proportions required for any particular electrolyte materials. This practice and the materials employed are disclosed by S. Swann, Jr., in the chapter beginning on page 385 of Technique of Organic Chemistry, A. Weissberger, editor, vol. II, 2nd ed. (1956), Interscience, N.Y., N.Y., and in the following literature:

(a) Harned and Owen, The Physical Chemistry of Electrolytic Solutions, 3rd ed., Reinhold, N.Y. (1950).

(b) Fuoss and Accascina, Electrolytic Conductance,

Interscience, N.Y. (1959).

(c) Tables Annuelles de Constantes et Donnies Numeriques 18, Conductivity of Electrolytes, Hermann, Paris (1937).

(d) Audrieth and Kleinberg, Non-Aqueous Solvents (Applications as Media for Chemical Reactions), John Wiley and Sons, N.Y. (1953).

The following illustrate the conductivities of some representative electrolyte compositions and variations thereof with variations in the materials, concentrations and temperatures. The conductivity of a 10% wt. solution of tetraethyl ammonium monobromide in acetonitrile is 0.027 ohm cm.- at 50 C. An electrolyte which consists of 10% wt. tetraethyl ammonium monobromide, 3% wt. water, and the rest acetonitrile, corresponding to 0.05 gm. rnole Et NBr/kg. of electrolyte, has a conductivity of 0.028 ohmcm. at 25 C. A 33% wt. tetrabutyl ammonium monobromide in 67% wt. of methyl bromide, corresponding to about 1.1 gm. mole Bu NBr/kg. total compositions, has a conductivity of 0.0011 ohmcm.- at 9 C.11 C. A 12% wt. Bu NBr/88% MeBr solution has a conductivity of 0.0015 ohm cm.- at 9 C.-11 C. The conductivities of compositions containing various amounts of tetraethyl ammonium monobromide (Et NBr) in a solution of 4% wt. water, 15% wt. MeBr and 5% wt. tetramethyl lead in acetonitrile to make at 25 C.

is shown in the following Table A:

TABLE A EtiNBr, Cone.

Conductivity, ohm- Percent wt. Gm.-Moleslkg. cm.-

These data shown that a conductivity of about 0.001 ohm cm.- can be achieved with as little as 0.01 gm. mole of tetraethyl ammonium monobromide per kilogram of catholyte; and that a conductivity of at least about 0.01 ohm" cm. can be obtained with at least about 2% by weight of tetraethyl ammonium monobromide, i.e. about 0.1 gm. mole Et NBr/kg. catholyte.

Generally, the concentration of the current-carrying tetraalkyl ammonium monohalide in the catholyte will be in the range of about 0.01 to about 3 gram moles per kilogram of catholyte, more usually from about 0.1 to about 2 gram moles per kilogram. When the catholyte does not include a solvent and diluent for the alkyl halide, the tetraalkyl ammonium monohalide usually will be in a concentration of from about 0.005 to about 0.5 mole, preferably from about 0.025 to about 0.25 mole, per mole of the alkyl halide, particularly when the alkyl groups in the alkyl halide have l2 carbon atoms and the alkyl groups in the tetraalkyl ammonium monohalides have l4 carbon atoms. Typical catholyte compositions which do not include a solvent are (I) 1 part by weight of methyl bromide, 0.5 part of tetrabutyl ammonium monobromide and 0.25 part of methanol (corresponding to a molar ratio of 5/1), and (II) 1 part by weight of methyl bromide, 0.27 part of tetrabutyl ammonium monobromide and 0.07 part of water.

As pointed out heretofore, the catholyte may, and usually will contain, an inert nonhydroxylic organic solvent for both the alkyl halide and the current-carrying tetraalkyl ammonium monohalide, which solvent has a reduction potential higher than the alkyl halide and an oxidation potential higher than the tetraalkyl ammonium monohalide. Such solvent may be present in any desired amount to act as a diluent for the alkyl halide, but usually will be in an amount sufiicient to dissolve all of the alkyl halide, e.g. sufficient to constitute from about 39% to about 97% by weight of the catholyte composition. Preferably, when an alkanonitrile is employed as the solvent, and particularly when the alkyl halides have 1-2 carbon atoms and the alkyl groups of the tetra-alkyl ammonium monohalides have 1-4 carbon atoms, the catholyte initially will consist essentially of (a) from about 0.1 to about 3 gram moles, most preferably from about 1 to about 2 gram moles, of alkyl halide per kilogram of catholyte; (b) from about 0.1 to about 1 gram mole, most preferably from about 0.25 to about 0.5 gram mole, of current-carrying tetraalkyl ammonium monohalide per kilogram of catholyte; and (c) from about 1 to 20 moles, most preferably from about 3 to about moles, of hydroxylic compound per mole of current-carrying tetraalkyl ammonium monohalide; (d) the rest of the catholyte consisting essentially of the alkanonitrile.

The alkyl halide, the tetraalkyl ammonium monohalide, the hydroxylic compound and, when present, the solvent are readily coordinated in the catholyte to provide a suitably conductive reaction medium for the formation of tetraalkyl lead. When an alkyl chloride is employed as the alkylating agent, the tetraalkyl ammonium monohalide preferably will be a bromide or an iodide. However, an all bromide system is preferred, that is, one in which an alkyl bromide is employed in the catholyte and a tetraalkyl ammonium monobromide is employed in both the catholyte and the anolyte.

The catholyte may also contain substances which are thermal stabilizers for tetraalkyl lead compounds and which are otherwise inert in the electrolytic process. A wide variety of stabilizers are known (e.g. Calingaert in US. Patents 2,660,591-6, and others). iMore specifically, there may be used one or more hydrocarbons, in-

cluding saturated, unsaturated and aromatic hydrocarbons, boiling in the range of 70 C. to 250 C. at atmospheric pressure, e.g., isooctane, diisobutylene, terpenes, toluene, xylene, naphthalene and alpha-methylnaphthalene. In accordance with art-recognized principles, the stabilizer is normally chosen with due regard for the volatility of the tetraalkyl lead compound to be stabilized and usually amounts to from about 0.1% to about 30% by weight of such compound.

The anolyte, that is employed in the process of this invention, initially consists essentially of -a solution of (l) a current-carrier which consists of at least one ourrent-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than the alkyl halide employed and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, in a concentration sufficient to provide an anolyte having a conductivity of at least 0.001 ohrn cmf (2) in an inert solvent having a reduction potential at least as high as the alkyl halide and an oxidation potential higher than the current-carrying tetraalkyl ammonium monohalide. The suitable tetraalkyl ammonium monohalides and the amounts thereof required to produce a particular desired conductivity have been disclosed and discussed hereinbefore in connection with the catholyte. The solvents, which have been disclosed to be suitable for use in the catholyte, are also suitable for use in the anolyte. Also, the compositions, that have been disclosed hereinbefore to be suitable as catholytes, are suitable for use as the anolytes, and the composition of the anolyte initially may be the same as that of the catholyte. The solvent in the anolyte may be an alkyl halide of the class that is suitable for use as the alkylating agent, in which case, the solvent will have a reduction potential equal to that of the alkyl halide in the catholyte. When solvents, other than the alkyl halide, are used in the anolyte, they should have a reduction potential higher than the alkly halide in the catholyte. In addition, the solvent in the anolyte need not be a nonhydroxylic organic solvent (as required in the catholyte) provided that it has the other specified characteristics. Water and alkanols of 1-4 carbon atoms are particularly suitable as solvents in the anolyte. Usually, the solvent in the anolyte will consist of at least one solvent of the group consisting of water, an alkanol of 14 carbon atoms, and an alkanonitrile in which the alkyl group has l5 carbon atoms. The tetraalkyl ammonium monohalide, employed in the anolyte, may be the same as or dilferent from that employed in the catholyte, but usually will be the same. Preferably, it will be a bromide, particularly when the alkyl halide is a bromide. Preferably, the solvent in the anolyte consists of water, particularly when the halogen of the alkyl halide and of the tetraalkyl ammonium monohalide is bromine. Also, preferably, the alkyl groups in the tetraalkyl ammonium monohalide in the anolyte will have l2 carbon atoms, e.g. tetraethyl ammonium monobromide or tetramethyl ammonium monobromide.

The concentration of the current-carrying tetraalkyl ammonium monohalide in the anolyte will be sufiicient to provide the desired conductivity and will vary widely depending upon the particular tetraalkyl ammonium monohalide, the particular solvent, and the temperature employed, according to the principles hereinbefore discussed. In general, the current-carrying tetraalkyl ammonium monohalide may be in a concentration as low as 2% by weight up to about 50% by weight, particularly when a lower tetraalkyl ammonium monohalide is employed with a lower alkanol such as methanol and ethanol, as the solvent. Usually, the concentration of the current-carrying tetraalkyl ammonium monohalide will be in the range of from about 2% to about 30% by weight, preferably from about 5% to about 10%, and most usually about 5%, particularly when the alkyl groups of the tetraalkyl ammonium monohalide have l4 carbon atoms and the solvent is water, an alkanol of 1-2 carbon atoms, an alkyl halide of 12 carbon atoms, or acetonitrile.

In the electrolysis, halide ion is produced at the cathode; at the same time at the other side of the membrane halide ion is oxidized at the anode to halogen. Such halogen reacts with the current-carrying tetraalkyl ammonium monohalide of the anolyte to form a complex tetraalkyl ammonium polyhalide which, in the presence of excess tetraalkyl ammonium monohalide, normally is the trihalide, R NX and in some cases may be the tetrahalide, R NX or even the pentahalide, R NX Such tetraalkyl ammonium polyhalides normally are solids and are fairly insoluble in water, but are more soluble in alkanols and alkanonitriles. The tetraalkyl ammonium polyhalides can be recovered from the anolyte solutions, most readily from the solutions of current-carrying tetraalkyl ammonium monohalide in water, and then treated by known methods to readily release the excess halogen as elemental halogen and to regenerate the current-carrying tetraalkyl ammonium monohalide. When the halide compounds employed are bromides, the alkyl groups of the current-carrying tetraalkyl ammonium monobromides contain l2 carbon atoms and the anolyte SOlVent is water, the tetraalkyl ammonium polybromides formed in the anolyte include the tribromides, the tetrabromides and the pentabromides, all of which are normally solids which are insoluble in the anolyte solution and separate therefrom. If the anolyte is maintained at a temperature of about 45 C. or above, such mixture of tetraalkyl (methyl and ethyl) ammonium polybromides separates from the anolyte solution as a Water-immiscible, heavy liquid phase which can be most readily removed from the anode compartment.

The process of this invention is conducted in an elec trolytic cell which has a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition which separates the cell into a cathode compartment and an anode compartment, thereby separating the catholyte from the anolyte. Usually, the effective surfaces of the electrodes will be spaced apart by about 0.5 to about 2 cm. The catholyte and the anolyte are placed in their respective compartments and an electrolyzing direct electric current is passed through the cell, i.e. from the cathode through the catholyte, the current-permeable partitionand the anolyte to the anode. Electrolytic cells of the required type and the methods of operating them are conventional and well known in the art, and such electrolytic cells and methods may be used in the process of this invention. Some suitable apparatus and methods are described by S. Swann, Jr. in the chapter beginning on page 385 of Technique in Organic Chemistry, A. Weissberger, editor, Vol. II, 2nd ed. (1956), Interscience, N.Y., N.Y.; by Calingaert in US. Patent 1,539,297 by Mead in US. Patent 1,567,159; by Ziegler et al. in US. Patent 2,985,568; and by Foreman et al. in US. Patent 3,105,023.

The lead cathode may be in any form presenting a large surface for high tetraalkyl lead production, such as sheet, ribbon, rod or shot. By lead shot, it will be understood that small balls or pellets of lead are meant, which usually will have diameters initially in the range of about 0.05 to about 0.5 cm. The size of the pellets is not important so long as they are sufiiciently small for the purpose. With lead shot, good electrical contact is obtained by packing the particulate lead around a solid core of lead or other suitable electrode material which is attached to the potential source. The shot electrode is conveniently housed in a reticular plastic container through which catholyte solution can freely circulate. As reaction proceeds, the lead shot becomes smaller, eventually disappearing, and fresh make up shot is fed to the system to replace that consumed.

The anode, in general, may be made of any suitable electrode material, including lead, but usually will be of a different material. Since halogen (e.g. C1 Br I is produced in the anodic reaction, the anode should be resistant to corrosion by such halogen. Platinum is suitable, including Pt on Ti and Pt on Ta, and carbon may be used.

A variety of current-permeable membranes are known to the art and may be employed as cell partitions in this process. It will be understood that such materials may differ considerably in their resistance to the flow of ions as current-carriers and to the flow of non-ionics such as Water, nitrile solvent, and tetraalkyl lead. Suitable membrane materials are porous porcelain, asbestos, glass fiber paper, cellulosic substances such as porous cellophane and parchment, films of agar gel (supported on polyethylene screen or porous glass, for example), films of polyurethanes, polyvinylidene fluoride, porous polyethylene, polyvinyl butyral, and ion exchange resins. Laminates of these materials may be used.

In general, the current-permeable membranes will permit Water to flow through them from an aqueous anolyte to a catholyte which is composed wholly or predominantly of organic compounds and contains only small amounts of water. This transport of water through the membrane can occur in two ways: (a) by solution of the Water in and migration through the membrane, which is essentially a physicochemical phenomena highly dependent on membrane composition; (b) by streaming through pores or holes in the membrane wall, which is essentially a mechanical process and is largely independent of the membranes chemical make-up. Thus, the membranes can be characterized by water transport values. In general, the membranes have water transport values in the range 0.001 to 0.02 gram of H 0 per sq. cm. of membrane area per minute under process conditions. Membranes with low water transport values, below about 0.003 gram, are preferred in the process of this invention, as they permit easier control of the water level in the catholyte.

Parchment paper, a preferred material, is preferably pretreated before use with water or a solution of a tetraalkyl ammonium monohalide in Water for several hours at 20 C. to C. to increase its porosity and decrease its electrical resistance, the tetraalkyl ammonium monohalide preferably being selected from the class hereinbefore disclosed to be suitable for use as current-carriers in the catholyte and the anolyte. The parchment paper so treated is maintained in the wet porous state until ready for use. Parchment paper so treated has a water transport value of about 0.002 gram of Water per squ. cm. per minute. Cation exchange resins, wherein the fixed anionic sites are carboxylate or sulfonate groups which may be in the free acid, alkali metal or quaternary ammonium salt form, constitute another preferred class. When the cation exchange resin has alkali metal in its cationic sites, the metal may be replaced by tetraalkyl ammonium ions by ion-exchange during the process. While alkali metal is objectionable in the electrolyte compositions of this invention, the amount thereof so released from the resin normally is too small to produce significant adverse effects. The possibility of the introduction of objectionable amounts of alkali metal into the electrolyte from such a resin can be greatly decreased or eliminated by pretreating the resin with a solution of a tetraalkyl ammonium monohalide so as to replace the alkali metal with the tetraalkyl ammonium ion. Water-treated parchment and the cation exchange materials are preferred for their permselective characteristics. They retard passage of polyhalide ions from the anode compartment to the cathode compartment, said polyhalides being reactive towards tetraalkyl lead, and of tetraalkyl lead from the cathode compartment to the anode compartment Where it tends to be destroyed by reaction with polyhalide ions.

The voltage applied to the cell is not critical so long as it is sufficient to overcome the resistance of the cell (including that of the catholyte, the anolyte and the cell membrane) and to reduce the alkyl halide at the cathode, thereby establishing current flow. Once the alkyl halide begins to be reduced, the cell with its components constitutes an electrical conductor. Ohms law of electrical conductance states that the magnitude of the current flowing in a conductor is directly proportional to the difference of potential between the ends of the conductor (here between the cathode and the anode) and is inversely proportional to its resistance (here the total resistance, summing the resistances of all component parts through which the current must pass). In a given cell at a given instant, the resistance is essentially a constant so that the current is directly proportional to the applied potential, i.e. the potential difference between the electrodes. Thus, the greater the potential the more the current, and, with a given electrode geometry, the greater the current density at the electrode in question. In principle, a desired current density can be maintained by applying a constant voltage across the cell. In practice, however, the voltage requirement often varies, primarily because cell resistance tends to change during the course of the reaction as a result of changes (1) in the bulk chemical constitution of the catholyte and/ or the anolyte, or even of the cell membrane, and (2) at the electrode surfaces, sometimes referred to as polarization phenomena which is related to the speeds with which reactants are replenished at the reacting surfaces and at which the reaction products difluse away from these sites.

The reduction of the alkylating agent requires about 1-2 volts, and, normally, the overall voltage is adjusted so that the potential at the cathode itself is suflicient to effect this reduction but does not greatly exceed this minimum potential. The potential at the cathode itself is conveniently measured with a probe electrode versus a standard electrode, suitably a saturated calomel reference electrode, according to known techniques. Because of internal resistances, at least about 45 volts are required for cell operation. Higher voltages, up to 35 volts for example, can be used, but it is seldom necessary to exceed about 20 volts in the process of this invention.

Current densities of from about 0.05 to 0.5 amp./ sq. cm. of effective cathode area may be employed, but more usually will be from about 0.1 to about 0.4 amp./ sq. cm., most preferably about 0.2 amp./ sq. cm. Current densities of up to about 0.4 amp/sq. cm. have been employed. It is sometimes necessary to employ current densities above the optimum as far as electrical yield is concerned in order to obtain a higher production rate to meet a demand for the product. When a cathode is employed which is formed of lead shot the potential diiference between the anode and the cathode is greatest at the cathodes outermost surface, i.e. that closest to the anode. The outermost surface of such cathode can be regarded as being substantially smooth for calculating current density, that is the efiective area is approximately that of the outer cylindrical surface of the body of lead shot.

Operating temperatures are normally from about C. to about 80 C., consonant with a practical rate of production of the tetraalkyl lead compound and the thermal stability of the system, preferably from about 40 C. to about 50 C. The pressure should be at least suflicient to maintain the catholyte and the anolyte in the liquid state at the temperature of operation, but otherwise may be subatmospheric or superatmospheric, if desired. Usually, it will be preferred to employ about atmospheric pressure, when practicable. Reflux facilities may be used when necessary to retain volatile components in the system and to aid in controlling reaction temperatures. Also, if desired, an inert atmosphere, such as nitrogen, helium, argon or methane, may be employed.

The process may be operated batchwise. Usually, it will be necessary to add further amounts of the currentcarrying tetraalkyl ammonium monohalide and the hydroxylic compound to the catholyte and of the currentcarrying tetraalkyl ammonium monohalide to the anolyte during the electrolysis so as to adjust and maintain the concentrations thereof in the catholyte and in the anolyte within predetermined limits within the ranges hereinbefore disclosed, in order to maintain efficient operation until the electrolysis is completed. Also, if large batches are involved, it usually will be necessary to periodically or continuously replenish the lead cathode during the electrolysis so as to maintain the elfective area of cathode within reasonable limits. Means and methods for advancing a consumable electrode into electrical apparatus so as to maintain a desired effective electrode area therein are conventional, well-known and may be used. Replenishment of the lead cathode is conveniently accomplished with a cathode formed of lead shot by merely adding more lead shot thereto as may be necessary.

The process of this invention is particularly adapted for continuous operation and it preferably is so operated. Such a continuous operation will comprise:

(A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of '(a) an alkyl halide in which the alkyl group has 1- 10 carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1- 18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration sufficient to provide a catholyte having a conductivity of at least 0.001 ohmcmr and (c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of l-4 carbon atoms; and

(C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (l) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has l-18 carbon atoms and the halogen atom has an atomic number of at least 17, in a concentration suflicient to provide an anolyte having a conductivity of at least 0.001 ohrncm.-

(2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said current-carrier; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said alkyl halide, current-carrier and hydroxylic compound as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (c), inclusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the .anode compartment additional amounts of said current-carrier and said solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a desired conductivity of at least 0.001 ohm- CHI-.1; and

(F) recovering tetraalkyl lead from the withdrawn portion of the catholyte.

The le-ad cathode will be replenished as may be necessary to maintain a reasonably constant efiective cathode area. Also, in those operations in which there are formed tetraalkyl ammonium polyhalides which separate from the anolyte, such tetraalkyl amomnium polyhalides will be removed from the anode compartment continuously or periodically as may be necessary to avoid its accumulation therein to such an extent as to objectionably decrease the volume of the anode compartment or to otherwise interfere with the effective operation of the cell.

Apparatus suitable for the continuous operation of the process of this invention and the methods of operating such apparatus are well known to the art and are disclosed by S. Swann, Jr., Ziegler et a1. and Foreman et al., hereinbefore referred to.

One form of apparatus, which has been found to be particularly useful for the continuous operation of the process of this invention, is illustrated in vertical crosssection in the accompanying drawing as a cell having an outer cylindrical anode, a central axially disposed cathode, and an intermediate cylindrical current-permeable membrane providing an annular anode compartment and an inner cylindrical cathode compartment. More specifically, the cell has an outer cylindrical shell of glass which has supported on its inner surface a cylindrical liner or tube 11 of platinum that serves as the anode. A cylindrical membrane 14 is positioned about 0.5- cm. inwardly of the anode 11 and concentric therewith and extends from top to bottom of the cell to divide the cell into the anode compartment 12 and the cathode compartment 20. The shell 10 is provided with an inlet (or outlet) 13a and an outlet (or inlet) 13b connected to a source of circulating anolyte for the anode compartment 12. The membrane 14 is composed of parchment paper about 0.015 cm. thick.

The cathode is an assembly of (a) a lead (Pb) rod 15 having a diameter of about 0.32 cm. which is positioned axially of the cell and extends vertically from the bottom of the cell to the horizontal plane of the top of the anode 11; (b) cleaned lead shot 16, about 0.21 cm. in diameter packed around the rod 15, in good electrical contact with it, over a height vertically coextensive with the anode 11, and held in place by (c) a vertical cylindrical reticulated screen 17 of 80 mesh nylon extending from the bottom to the top of the cell, spaced about 0.5 cm. from membrane 14, and having similar nylon mesh grid 18 supporting the lead shot. The annular space between the screen 17 and the membrane 14 constitutes a cathode compartment which is connected to a source of circulating catholyte through the screen 17, below and above the body of lead shot 16, and catholyte inlet (or outlet) 19a and catholyte outlet (or inlet) 19b. The inside area of the nylon cylinder 17, which is coextensive with the body of lead shot 16, has an area of 103 sq. cm. which is taken as the effective area of the lead cathode.

The anode compartment 12 and the cathode compartment 20 are closed at the bottom by an end plug 21 and at the top by a like end plug 22 which are made of electrical insulating material and which serve to support and position the membrane 14 and screen 17 within the cell. An inlet 23 is provided at the top of the cell for charging the lead shot to the cathode assembly. The anode 11 and the cathode rod 15 are connected to a source of direct electric current through electrical leads 11a and 15a, respectively.

The cell is provided with a heat exchange jacket 24 which is connected with a source of circulating heat exchange fluid through inlet 24a and outlet 24b.

It will be understood that the specific materials of the cell are those used, and that other suitable materials may be employed in their place. For example, the shell 10 may be made of other suitable inert materials which are nonconductive of electric current, and the anode 11 may be of other suitable anode materials which are inert to the reactants. Other forms of lead cathode may be employed. Also, the screen 17 and grid 18 may be made of other reticulated material which is inert to the reactants employed and is non-conductive of electric current. The membrane 14 may be made of other suitable membrane materials which are permeable to electric current and ions, and which have been described in more detail hereinbefore.

The catholyte product may be treated by various methods to recover the tetraalkyl lead compound therefrom. For example, it may be diluted with a large excess of water, the tetraalkyl lead extracted with a hydrocarbon solvent such as pentane, and then separated from the solvent by fractional distillation under reduced pressure. Also, the catholyte product may be directly subjected to fractional distillation or steam distillation according to known methods. With acetonitrile as the solvent and tetra: methyl lead as the product, the catholyte product preferably will be subjected to fractional distillation whereby the tetramethyl lead is obtained as a low-boiling azeotropic mixture with acetonitrile. Such azeotropic mixture consists essentially of about 61% by weight of tetramethyl lead and about 39% by weight of acetonitrile and boils at about 73 C. at atmospheric pressure and, on cooling to 25 C. or below, separates into two phases, the upper phase being rich in acetonitrile and the lower phase being rich in tetramethyl lead. The tetramethyl lead, in either phase, can be obtained free of acetonitrile by washing the mixture with water which dissolves the acetonitrile, the tetramethyl lead being insoluble in Water. When a volatile thermal stabilizer, e.g. benzene, toluene, or xylene, is present in the catholyte composition, such material will co,- distill with the tetramethyl lead, alkanonitrile, and catholyte water, forming in effect a more complex azeotrope. The tetramethyl lead-hydrocarbon stabilizer composition can be recovered ready for blending and free of alkanonitrile by washing the distillate with water.

In order to more clearly illustrate this invention, presently preferred methods of operating it, and the advantages to be obtained thereby, the following examples are given in which the parts and proportions are by weight except where specifically indicated otherwise.

EXAMPLE 1 (A) Water-free run (control) (run 1) The electrolytic cell is of conventional structure and comprises essentially a lead cathode, a platinum anode, and separate cathode and anode compartments separated by a dry parchment paper membrane. The cathode and anode compartments are each charged with 25.2 parts (0.12 mole) of dry tetraethylammonium bromide (Et NBr) and 260 parts of dry acetonitrile. Then 46.7 parts (0.49 mole) of methyl bromide (MeBr) are added to the catholyte solution under a dry nitrogen atmosphere. The cell is closed, the anolyte and catholyte solutions are warmed to 45 C., and the direct current is turned on. Current is passed through the cell for 3.67 hours, during which time the input voltage is adjusted to provide a constant current density of about 0.06 amp./ sq. cm. and the temperature is maintained at 45 C. The resulting catholyte composition contains tetramethyl lead (TML) in yields of 73% based on the current passed and 81% based on the weight of lead lost by the cathode during the run. These and other pertinent data are included in the following summarizing Table I.

(B) Eifect of water (this invention) (runs 2-5) TABLE I.EFFECT OF H2O ON ELECTROYLYTIC TML F0 RMATION Catholyte composition:

MeBr m0les/kg 1.411.47 EtiNBr do 0. 34-0. 36 HzO As below acetonitrile solvent.

H O Content of Catholyte Molar Lowest Cell Percent 'IML Yield Run 2 Voltage Percent by Moles/kg. EtiNBl' Elect. Pb-Loss Wt. 0 0 26 73 81 0. 65 0. 36 ill 20 92 92 2. 5 1. 4 4/1 13 90 4. 9 2. 9 8/1 6. 4 87 91 9. 8 5. 8 17/1 4. 9 84 93 parts less acetonitrile in the catholyte.

The data show that the hydroxylic component affords a significantly more efiicient system in terms of cell voltage requirement and product yield.

EXAMPLE 2 The electrolytic cell was similar to that of Example 1 and was equipped with means for adding and removing catholyte and anolyte ingredients. The cell partition was a cation exchange resin consisting essentially of a polyethylene backbone having pendant carboxyl groups in the sodium form. The initial catholyte solution consisted of 6.66 parts (0.032 mole) of Et NBr, 69.1 parts of acetonitrile, and 13.7 parts (0.144 mole) of MeBr. A solution of 5.04 parts (0.024 mole) of Et NBr in 52.1 parts of acetonitrile served as the initial anolyte. With the solutions at 41 C., direct current flow was started. The voltage, initially at 6.5 volts corresponding to a current density of 0.025 amp/sq. cm, was increased gradually over a period of 7 minutes to 16.5 volts giving a current density of 0.1 amp./ sq. crn. At this point, vigorous gas evolution began in the cathode compartment. Methanol, 1.6

parts (0.05 mole), was added and the gassing ceased. After 4 more minutes, with the current density at 0.125 amp./ sq. cm., obtained by increasing the potential to 17.8 volts, slight gassing began which was suppressed with a second 0.05 mole portion of methanol. The electrolysis was continued with a current density at 0.15 amp/sq. cm., obtained by raising the applied voltage to about 18.7 volts, two more 0.05 mole additions of methanol being made after 48 and 55 minutes of elapsed operating time to suppress slight gassing. During the run, the cell voltage requirement was kept below about volts by periodically replacing anolyte solution with fresh anolyte of the original composition to replenish the supply of Et N+ and Br ions. The run was terminated after 1 hour. The tetramethyl lead yield was 83% based on the current passed, and' 89% based on the Weight of lead consumed.

In summary, the starting catholyte composition contained 1.5 moles MeBr and 0.33 mole Et NBr per kilogram of solution. Each 0.05 molar MeOH addition corresponded to 0.55 mole/kg. of solution and to a molar MeOH/Et NBr ratio of about 1.3/1, with the total amounting to about 2 moles/kg. of solution and a molar ratio of MeOH/Et NBr of about 6/ 1. It will be apparent that the tetramethyl lead could not have "been obtained as efliciently (83% electrical yield) in the absence of the methanol, because of the gas-forming side reactions that otherwise occur at the current density level utilized.

For comparison, the procedure of Example 1 was repeated except that methanol in one run and ethanol in another replaced the acetonitrile of both the cathlyte and the anolyte solutions. With methanol, the tetraJnethyl lead yields were about 2% based on current and 69% based on Pb consumed. With ethanol, the electrical yield was the Pb-loss yield 92%. In both systems, considerable gassing occurred throughout the runs in agreement with the low electrical yields. Furthermore, the addition to these systems of water corresponding to about 1.4 moles/kg. of catholyte and to a molar H O/Et -NBr ratio of about 4/1 had no beneficial effect on the tetramethyl lead yield or on the degree of gassing. In these runs, the solvent quantities of the alcohols corresponded to 24 moles MeOH and 17 moles EtOH per kilogram of solution and to molar alcohol/Et NBr ratios of about 67/1 for MeOH and 47/1 for EtOH.

Thus, it is apparent that use of alcohol in solvent quantities (Calingaert U.S. Patent 1,539,297) is not the equivalent of the use of the limited amounts of hydroxylic compound used in this invention. Since these results obtained with the alcohol solvent-based systems are much inferior to those obtained with the non-hydroxylic solvent system (control run 1 of Example 1), it is not obvious that use of an alcohol such as MeOH in limited amounts in the same non-hydroxylic solvent system would result in an overall superior system for the electrolytic production of tetraalkyl lead (Example 2).

EXAMPLE 3 Equipment The apparatus shown in the drawing and described in detail hereinbefore.

The Pb shot was cleaned by washing with aqueous (0.515%) HNO rinsed acid-free with water, rinsed with acetone and dried by evaporation, and stored under N before use.

The parchment paper membrane, in place in the cell, was conditioned by contacting it with boiling water for 0.5-2 hours. The water was drained and then the cathode compartment was filled with an acetonitrile solution of 7.5% Et NBr and 4% H O, the nylon cylinder was charged with clean Pb shot, and the anode compartment was filled with Water containing 5% Et NBr. The condi tioned membrane can be kept indefinitely under these conditions.

Start-up The cell was drained. A feed consisting of 15% CH Br (1.58 moles/kg), 7.5% Et NBr (0.33 mole/kg), 1% H O (0.56 mole/kg.) and 76.5% acetonitrile was circulated through the cathode chamber at a rate of 1500 mL/min. and at 42 C. At the same time, a fresh 5% Et NBr-H O solution at 45 C. was circulated through the anode compartment at about the same rate. While under circulation, catholyte solution was continually withdrawn from the circulating stream and fresh solution added at the rate of 15 ml./min. Circulation of the feed stocks was continued, without the application of electric current, until the H 0 content of the cathode chamber solution reached a steady state of 4.5% (2.5 moles/kg.) as a result of H 0 transport across the membrane from the anode side, in about 45 minutes.

Electrolysis At this point, in an attempt to compensate for the transport of anode water and maintain the Water content of the circulating catholyte solution approximately constant, the water level in the fresh catholyte feed solution was decreased to 0.2% (0.11 mole/kg). At the same time, the direct current was turned on. The current densitywas increased over a 0.25 hr. period to 0.18 amp/sq. cm. with a cell voltage of 11.3 volts. As a result, the temperature of the catholyte solution slowly rose to about 45 C.

The flow of a 30% Et NBr-H O solution was begun into the circulating anolyte composition at a rate of 5.8 g./min., while a portion of the total-was simultaneously removed at a rate of 3.6 g./min., in order to compensate for the depletion of the electrolyte as the result of (a) transport of Et N across the membrane to the cathode side and (b) formation on the anode side of tetraethylammonium polybromide which separated as a water-immiscible layer as the electrolysis progressed and which was removed from time to time.

After 1.5 hours, the cell voltage required for maintain- 1? ing the current density at 0.18 amp/sq. cm. had risen to 12.5 volts and the water content of the catholyte solution had dropped from 4.5% (2.5 moles/kg.) to 3.5% (1.95 moles/kg). Water was then added to the catholyte solution as needed to maintain a 5% level (2.7-8 moles/kg.) and a voltage of 11.4-11.5 volts. After 2.33 hours of operation, during which time the Et NBr content of the catholyte had increased to about 11% (0.52 mole/kg), the electrolysis was terminated.

The tetramethyl lead yield was 92% based on the total current passed through the cell, and 95% based on the weight of lead lost by the cathode during the electrolysis.

EXAMPLE 4 The electrolytic cell is similar to that of Example 1 and comprises a lead cathode, platinum anode, and cathode and anode compartments separated by a parchment paper membrane. The cathode and anode compartments are equipped with a reflux condenser for methyl bromide. The archment paper membrane had been soaked overnight in water; excess water was allowed to drain away just before the membrane was placed in the cell for immersion in the electrolyte solution. To each compartment was added a cold (about 8 C.) solution consisting of 75% MeBr, 20% tetra-n-butyl ammonium bromide and 5% water, all by weight, the Bu NBr concentration corresponding to 0.62 gm.-mole/kg. of catholyte and anolyte and the molar ratio of water to Bu NBr being 4.5 1. Under these conditions at 6 C.l C., a small amount of a white solid, apparently a MeBr hydrate, was present in the electrolyte composition.

The current was turned on. Little current flowed until the voltage was raised from an initial 10 volts to 20 volts, when the current density rose sharply from 0.001 amp/sq. cm. to 0.01 amp/sq. cm., then gradually to 0.015 amp/sq. cm. over a minute period. The voltage was raised to 25 volts, held there for about 8 minutes to give a steady current density of 0.02 amp./ sq. cm., and then increased to 30 volts for a current density of 0.03 amp./sq. cm. which was maintained for 40 minutes. MeBr, amounting to about A of the initial amount, was allowed to evaporate from the cell over a 5 minute period, whereupon the current density rose to 0.075 -amp./ sq. cm. at 30 volts. The reaction mixture was held for 10 minutes at 0.075 amp./ sq. cm. and 30 volts. At this point, the tetramethyl lead con-- tent of the catholyte corresponded to a yield of 79% based on the total current passed.

In comparison, when water was omitted from the anolyte and catholyte and dry parchment was used as the membrane, the maximum attainable current density was about 0.012 amp/sq. cm., which corresponds to a lower production rate, and the tetramethyl lead yield was 73% based on current.

EXAMPLE 5 The cell is of the sandwich type with a lead cathode and a platinum anode at a cell length of 1.0 cm., a cell partition area of 20 cm. and means for circulating catholyte and anolyte through the cathode'and anode compartments.

The membrane is that designated as AMF ion number C-103-DD by the American Machine and Foundry C0. and characterized as having an average resistance in KCl solution of 6.0-7.0 ohm/cmP, a wet thickness of 6 mils, and a 10% gel water. It has a high density polyethylene backbone which has been grafted with styrene and subsequently sulfonated to provide a cation exchange material of the sulfonic acid type. For use in this cell, the film is soaked for several hours in anolyte solution consisting of 7.5% by weight of Et NBr in water.

1000 grams of an anolyte solution, consisting of 7.5% by weight Et NBr in H O, is circulated and recycled continuously through the cell. 100 grams of a catholyte solution, consisting of by weight MeBr, 7.5% Et NBr, 0.7% H 0, the rest acetonitrile (providing a molar H O/Et NBr ratio of about 1/ 1), is circulated through the cathode compartment, with a portion being removed at the outlet end at the rate of 6 grams/min. while fresh catholyte is added at the same rate at the inlet end of the cell. Cell temperature is maintained at 45 C, Under these conditions, the water permeation rate through the membrane is 8.2 mg./cm. /min., and the acetonitrile permeation rate is 0.008 mg./cm. /min.

The current is turned on and the current density adjusted to O.lamp./cm. at a potential of 7 volts. After two hours, the water concentration in the catholyte has increased to about 4%. The tetramethyl lead yield is 92% based on current passed, and 95 based on the Pb consumed from the cathode.

It will be understood that the foregoing examples and the drawing have been given for illustrative purposes solely, and that this invention is not limited to the specific embodiments described and shown therein. On the other hand, it will be readily apparent to those skilled in the art that, subject to the limitations set forth in the general description, many variations can be made in the materials, proportions, conditions, techniques and apparatus employed without departing from the spirit or scope of this invention.

From the foregoing description, it will be apparent that this invention provides a new and improved process for producing tetraalkyl lead compounds. It constitutes a material improvement over the prior processes, overcoming problems involved in the prior processes, results in higher yields of tetraalkyl lead compounds, and makes it possible to produce the tetraalkyl lead compounds at much higher rates. It is simple and economical to operate and particularly it enables ready control of the conditions so that the production of tetraalkyl lead compounds can be maintained at maximum efiiciency throughout the electrolysis. Especially, it provides a continuous process which can be operated over long periods of time at high efiiciency with high yields of tetraalkyl lead compounds at high rates of production, maximum utilization of materials, minimum expenditures of energy, and low costs. It is believed to be the first commercially feasible electrolytic process for producing tetraalkyl lead antiknock compounds. Accordingly, it will be apparent that this invention constitutes an important and valuable advance in and contribution to the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially (a) an alkyl halide in which the alkyl group has 1-10 carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has l-18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration sufficient to provide a catholyte having a conductivity of at least 0.001 ohmcm. and

(c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of 14 carbon atoms; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl groups has 118 carbon atoms and the hlaogen atom has an atomic number of at least 17, in a concentration sufiicient to provide an anolyte having a conductivity of at least 0.001 ohm Gilli 2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said currentcarrier;

(D) during the electrolysis, adjusting the amounts of the current-carrier in the catholyte and the anolyte as may be necessary to maintain their conductivities at at least 0.001 ohm cm. and adjusting the amount of the hydroxylic compound in the catholyte as may be necessary to maintain the concentration thereof within the range of from about 1 to about moles per mole of said current-carrier; and

(E) recovering tetraalkyl lead from the catholyte.

2. An electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) an alkyl bromide in which the alkyl group has 1-10 carbon atoms,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction po tential than said alkyl bromide and in which each alkyl group has 1-18 carbon atoms, said current-carrier being in a concentration suflicient to provide a catholyte having a conductivity of at least 0.001 ohm cm.- and (c) from about 1 to 20 moles of water per mole of said current-carrier; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl group has 1-18 carbon atoms, in a concentration sutficient to provide an anolyte having a conductivity of at least 0.001 ohmcmr (2) in water;

(D) during the electrolysis, adjusting the amounts of the current-carrier in the catholyte and the anolyte as may be necessary to maintain their conductivities at at'least 0.001 ohmcm.- and adjusting the amount of the water in the catholyte as may be necessary to maintain the concentration thereof within the range of from about 1 to about 20 moles per mole of said current-carrier; and

(E) recovering tetraalkyl lead from the catholyte.

3. An electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) an alkyl bromide of l-2 carbon atoms,

'(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction po- 20 t ten-tial than said alkyl bromide and in which each alkyl group has 1-2 carbon atoms, said current-carrier being in a concentration sufiicient to provide a catholyte having a conductivity of at least 0.01 ohmcmf and (c) from about 3 to about 10 moles of water per mole of said current-carrier; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which hasa higher reduction potential than said alkyl bromide and in which each alkyl group has 12 carbon atoms, in a concentration suflicient to provide an anolyte having a conductivity of at least 0.01 ohm cmf (2) in water;

(D) during the electrolysis, adjusting the amounts of the current-carrier in the catholyte and the anolyte as may be necessary to maintain their conductivities at at least 0.01 ohm cmr and adjusting the amount of the water in the catholyte as may be necessary to maintain the concentration thereof within the range of from about 3 to about 10 moles per mole of said current-carrier; and

(E) recovering tetraalkyl lead from the catholyte.

4. An electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles per kilogram of catholyte of an alkyl halide in which the alkyl group has l-IO carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration sufiicient to provide a catholyte having a conductivity of at least 0.001 ohmcm.- and (c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of 1-4 carbon atoms,

(d) the rest of the catholyte consisting essentially of an inert nonhydroxylic organic solvent for both said alkyl halide and said current-carrier which solvent has a reduction potential higher than said alkyl halide and an oxidation potential higher than said current-carrier; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetra-alkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, in a concentration sufficient to provide an anolyte having a conductivity of at least 0.001 ohmcmr (2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said current-carner- (D) during the electrolysis, adjusting the amounts of the current-carrier in the catholyte and the anolyte as may be necessary to maintain their conductivities at at least 0.001 ohmcm.- and adjusting the amount of the hydroxylic compound in the catholyte as may be necessary to maintain the concentration thereof Within the range of from about 1 to about 20 moles per mole of said current-carrier; and

(E) recovering tetraalkyl lead from the catholyte.

5. An electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles per kilogram of catholyte of an alkyl halide in which the alkyl group has 1-2 carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-2 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration sutficient to provide a catholyte having a conductivity of at least 0.001 ohrncmf and (c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of Water and alkanols of 1-4 carbon atoms,

(d) the rest of the catholyte consisting essentially of at least one alkanonitrile in which the alkyl group has 1-5 carbon atoms; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17 in a concentration suflicient to provide an anolyte having a conductivity of at least 0.001 ohmcmr (2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said current-carrier;

(D) during the electrolysis, adjusting the amounts of the current-carrier in the catholyte and the anolyte as may be necessary to maintain their conductivities at at least 0.001 ohm and CH1.1 and adjusting the amount of the hydroxylic compound in the catholyte as may be necessary -to maintain the concentration thereof within the range of from about 1 to about 20 moles per mole of said current-carrier; and

(E) recovering tetraalkyl lead from the catholyte.

6. An electrolytic process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) from about 1 to about 2 gram moles per kilogram of catholyte of an alkyl bromide in which the alkyl group has 1-2 carbon atoms,

(b) from about 0.25 to about 0.5 gram mole per kilogram of catholyte of a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl group has 12 carbon atoms,

(c) from about 3 to about 10 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of acetonitrile; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl group has 1-2 carbon atoms, in a concentration of from about 2% to about 20% by weight (2) in water;

(D) during the electrolysis, adjusting the amounts of said current-carrier and water in the catholyte and the amount of said current-carrier in the anolyte as may be necessary to maintain the concentrations thereof in the catholyte and in the anolyte within the ranges specified in (b), (c) and (1), respectively; and

(E) recovering tetraalkyl lead from the catholyte.

7. An electrolytic process for producing tetramethyl lead at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) from about 1 to about 2 gram moles of methyl bromide per kilogram of catholyte,

(b) from about 0.25 to about 0.5 gram mole per kilogram of catholyte of a current-carrier which consists of tetraethyl ammonium monobromide,

(c) from about 3 to about 10 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of acetonitrile; and

(C) a liquid anolyte which initially consists essentially of a solution of (l) a current-carrier which consists of tetraethyl ammonium monobromide in a concentration of from about 5% to about 10% by weight (2) in acetonitrile;

(D) during the electrolysis, adjusting the amounts of said current-carrier and water in the catholyte and the amount of said current-carrier in the anolyte as may be necessary to maintain the concentrations thereof in the catholyte and in the anolyte within the ranges specified in (b), (c) and (1), respectively; and

(E) recovering tetramethyl lead from the catholyte.

8. An electrolytic process for producing tetramethyl lead at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the catholyte from the anolyte, which process comprises (A) passing an electrolyzing direct electric current through (B) a liquid catholyte which initially consists essentially of (a) from about 1 to about 2 gram moles of methyl bromide per kilogram of catholyte, (b) from about 0.25 to about 0.5 gram mole per kilogram of catholyte of a current-carrier which consists of tetraethyl ammonium monobromide,

(c) from about 3 to about 10 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of acetonritile; and

(C) a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of tetraethyl ammonium monobromide in a concentration of from about 5% to about by weight (2) in water;

(D) during the electrolysis, adjusting the amounts of said current-carrier and water in the catholyte and the amount of said current-carrier in the anolyte as may be necessary to maintain the concentrations thereof in the catholyte and in the anolyte within the ranges specified in (b), (c) and (1), respectively; and

(E) recovering tetramethyl lead from the catholyte.

9. A continuous process for the electrolytic production of tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of (a) an alkyl halide in which the alkyl group has 1-10 carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has l-18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration suflicient to provide a catholyte having a conductivity of at least 0.001 ohm cmf and (c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of 1-4 carbon atoms; and

(C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1l8 carbon atoms and the halogen atom has an atomic number of at least 17, in a concentration suflicient to provide an anolyte having a conductivity of at least 0.001 ohm cmf (2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said currentcarrier; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said alkyl halide, currentcarrier and hydroxylic compound as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (c), inelusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment'additional amounts of said current-carrier and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a conductivity of at least 0.001 ohm cmf and (F) recovering tetraalkyl lead from the withdrawn portion of the catholyte.

10. A continuous process for the electrolytic production of tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles per kilogram of catholyte of an alkyl halide in which the alkyl group has 1-10 carbon atoms and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration sufiicient to provide a catholyte having a conductivity of at least 0.001 ohm cmf and (c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of 1-4 carbon atoms,

(d) the rest of the catholyte consisting essentially of an inert nonhydroxylic organic solvent for both said alkyl halide and said current-carrier which solvent has a reduction potential higher than said alkyl halide and an oxidation potential higher than said current-carrier; and

(C) continuously recirculating through the anode com partment a stream of a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17 in a concentration sutficient to provide an anolyte having a conductivity of at least 0.001 ohm cmr (2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said current-carrier; and

(D) continuously withdrawing a portion of thecatholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said alkyl halide, currentcarrier, hydroxylic compound and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (d), inclusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said current-carrier and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a conductivity of at least 0.001 ohm cm. and

(F) recovering tetraalkyl lead from the withdrawn portion of the catholyte.

11. A continuous process for the electrolytic production of tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles per kilogram of catholyte of an alkyl bromide in which the alkyl group has 110 carbon atoms,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl group has 1-18 carbon atoms, said currentcarrier being in a concentration sufiicient to provide a catholyte having a conductivity of at least 0.01 ohm cmf and (c) from about 1 to 20 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of at least one alkanonitrile in which the alkyl group has 15 carbon atoms; and

' C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl Y group has 1-18 carbon atoms, in a concentration sut'ricient to provide an anolyte having a conductivity of at least 0.01 ohm cm. (2) in at least one solvent of the group consisting of water, an alkanol of 1-4 carbon atoms, and an alkanonitrile in which the alkyl group has 1-5 carbon atoms; and (D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said alkyl bromide, currentcarrier, water and alkanonitrile as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (d), inelusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said current-carrier and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a conductivity of at least 0.01 ohm GEL-'1; and

(F) recovering tetraalkyl lead from the withdrawn portion of the catholyte.

127 A continuous process for the electrolytic production of tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material Which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of liquid catholyte which initially consists essentially of (a) from about 1 to about 2 gram moles per kilogram of catholyte of an alkyl bromide in which the alkyl group has 1-2 carbon atoms,

(b) from about 0.25 to about 0.5 gram mole per kilogram of catholyte of a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl group has 1-2 carbon atoms,

(c) from about 3 to about 10 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of at least one alkanonitrile in which the alkyly group has 1-5 carbon atoms; and

(C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (1) a currentcarrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than said alkyl bromide and in which each alkyl group has 12 carbon atoms, in a concentration of from about 5% to about 10% by weight (2) in water; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said alkyl bromide, currentcarrier, water and alkanonitrile as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired relative proportions within the ranges specified in (a) to (d), inclusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it renters the anode compartment additional amounts of said current-carrier and water as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has those in- 27 gredients in desired relative proportions within the ranges specified in (l) and (2); and

(F) recovering tetraalkyl lead from the Withdrawn portion of the catholyte.

13. A continuous process for the electrolytic production of tetramethyl lead at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles of methyl bromide per kilogram of catholyte, (b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than methyl bromide and in which each alkyl group has 1-18 carbon atoms, said current-carrier being in a concentration sufficient to provide catholyte having a conductivity of at least 0.01 ohm cm. and

(c) from about 1 to 20 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of an inert nonhydroxylic organic solvent for 'both methyl bromide and said current-carrier which solvent has a reduction potential higher than methyl bromide and an oxidation potential higher than said current-carrier; and

(C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (l) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than methyl bromide and in which each alkyl group has 1-18 carbon atoms, in a con centration sutiicient to provide an anolyte having a conductivity of at least 0.01 ohmcmr (2) in an inert solvent having a reduction potential at least as high as methyl bromide and an oxidation potential higher than said current-carrier;

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said methyl bromide, currentcarrier, water and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (d), inelusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said current-carrier and solvent as may be necessary to replace those consumed in the electrolysis and those Withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a conductivity of at least 0.01 ohm cm.- and (F) recovering tetramethyl lead from the Withdrawn portion of the catholyte.

14. A continuous process for the electrolytic production of tetramethyl lead at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) while continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of (a) from about 1 to about 2 gram moles of methyl bromide per kilogram of catholyte,

(b) from about 0.25 to about 0.5 gram mole per kilogram of catholyte of a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than methyl bromide and in which each alkyl group has 1-2 carbon atoms,

(c) from about 3 to about 10 moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of at least one alkanonitrile in which the alkyl group has l-5 carbon atoms; and

(C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than methyl bromide and in which each alkyl group has 1-2 carbon atoms, in a concentration of from about 5% to about 10% by weight (2) in water; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said methyl bromide, current-carrier, water and alkanonitrile as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired relative proportions within the ranges specified in (a) to (d), inclusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said current-carrier and water as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has those ingredients in desired relative proportions within the ranges specified in (1) and (2); and

(F) recovering tetramethyl lead from the withdrawn portion of the catholyte.

15. A continuous process for the electrolytic production of tetramethyl lead at a lead cathode in an electrolytic cell having a lead cathode, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell (B) While continuously recirculating through the cathode compartment a stream of a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles of methyl bromide per kilogram of catholyte,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than methyl bromide and in which each alkyl group has 1-18 carbon atoms, said current-carrier being in a concentration sufiicient to provide a catholyte having a conductivity of at least 0.01 ohm* cm.- and (c) from about 1 to moles of water per mole of said current-carrier,

(d) the rest of the catholyte consisting essentially of at least one alkanonitrile in which the alkyl group has 1-5 carbon atoms; and

(C) continuously recirculating through the anode compartment a stream of a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monobromide which has a higher reduction potential than methyl bromide and in which each alkyl group has 1-18 carbon atoms, in a concentration sufficient to provide an anolyte having a conductivity of at least 0.01 ohm cmf (2) in an alkanonitrile in which the alkyl group has 1-5 carbon atoms; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said methyl bromide, currentcarrier, Water and alkanonitrile as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a re circulating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (d), inelusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said current-carrier and alkanonitrile as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a conductivity of at least 0.01 ohm" cmf and (F) recovering tetramethyl lead from the withdrawn portion of the catholyte.

16. A continuous process for the electrolytic production of tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a cathode of lead shot, an anode of a material which is resistant to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition made of member of the group consisting of parchment paper and an ion exchange resin separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell at a voltage sufficient to provide and maintain a current density of from about 0.05 to about 0.5 amp./ sq. cm. of efiective cathode area (B) while continuously recirculating through the cathode compartment at a temperature of from about 20 C. to about 80 C. a stream of a liquid catholyte which initially consists essentially of (a) from about 0.1 to about 3 gram moles per kilogram of catholyte of an alkyl halide in which the alkyl group has 1-10 carbon atoms 30 and the halogen atom has an atomic number of at least 17,

(b) a current-carrier which consists of at least one current-carrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17, said current-carrier being in a concentration suflicient to provide a catholyte having a conductivity of at least 0.001 ohmcmrand (c) from about 1 to 20 moles per mole of said current-carrier of at least one hydroxylic compound of the class consisting of water and alkanols of 1-4 carbon atoms,

((1) the rest of the catholyte consisting essentially of an inert nonhydroxylic organic solvent for both said alkyl halide and said current-carrier which solvent has a reduction potential higher than said alkyl halide and an oxidation potential higher than said current-carrier; and

(C) continuously recirculating through the anode compartment at a temperature of from about 20 C. to about C. a stream of a liquid anolyte which initially consists essentially of a solution of (1) a current-carrier which consists of at least one currentcarrying tetraalkyl ammonium monohalide which has a higher reduction potential than said alkyl halide and in which each alkyl group has 1-18 carbon atoms and the halogen atom has an atomic number of at least 17 in a concentration sutficient to provide an anolyte having a conductivity of at least 0.001 ohmcm.

(2) in an inert solvent having a reduction potential at least as high as said alkyl halide and an oxidation potential higher than said currentcarrier; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said alkyl halide, currentcarrier, hydroxylic compound and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte stream entering the cathode compartment which has those ingredients in desired concentrations and relative proportions within the ranges specified in (a) to (d), inclusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said current-carrier and solvent as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has a conductivity of at least 0.001 ohm-V cm.

(F) adding lead shot to the cathode as may be necessary to replace that consumed by the electrolysis and to maintain the effective area of the cathode substantially constant; and

(G) recovering tetraalkyl lead from the withdrawn portion of the catholyte.

17. A continuous process for the electrolytic production of tetramethyl lead at a lead cathode in an electrolytic cell having a cathode of lead shot, an anode of a material 7 difierent from that of the cathode and which is resistant 31 to attack by halogens of atomic numbers 17 to 53, and a current-permeable partition of parchment paper separating the cell into a cathode compartment and an anode compartment, which process comprises (A) passing an electrolyzing direct electric current through the cell at a voltage sufiicient to provide and maintain a current density of from about 0.05 to about 0.5 amp/sq. cm. of effective cathode area (B) while continuously recirculating through the cathode compartment at a temperature of from about 40 C. to about 50 C. a stream of a liquid catholyte which initially consists essentially of (a) from about 1 to about 2 gram moles of methyl bromide per kilogram of catholyte,

(b) from about 0.25 to about 0.5 gram mole of tetraethyl ammonium monobromide per kilogram of catholyte,

(c) from about 3 to about 10 moles of water per mole of said tetraethyl ammonium monobromide,

(d) the rest of the catholyte consisting essentially of acetonitrile; and

(C) continuously recirculating through the anode compartment at a temperature of from about 40 C. to about 50 C. a stream of a liquid anolyte which initially consists essentially of a solution of (1) tetraethyl ammonium monobromide in a concentration of from about 5% to about by weight (2) in water; and

(D) continuously withdrawing a portion of the catholyte from the recirculating catholyte stream that has passed through the cathode compartment and (D adding to the recirculating catholyte stream before it reenters the cathode compartment additional amounts of said methyl bromide, tetra ethyl ammonium monobromide, water and acetonitrile as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating catholyte 32 stream entering the cathode compartment which has those ingredients in desired relative proportions within the ranges specified in (a) to (d), inclusive;

(E) simultaneously withdrawing a portion of the anolyte from the recirculating anolyte stream that has passed through the anode compartment and (E adding to the recirculating anolyte stream before it reenters the anode compartment additional amounts of said tetraethyl ammonium monobromide and water as may be necessary to replace those consumed in the electrolysis and those withdrawn and to provide a recirculating anolyte stream entering the anode compartment which has those ingredients in desired relative proportions within the ranges specified in (1) and (2), and

(E withdrawing from the anode compartment water-immiscible tetraethyl ammonium polybromide as may be necessary to avoid its accumulation in the anode compartment to an undesirable extent;

(F) adding lead shot to the cathode as may be necessary to replace that consumed by the electrolysis and to maintain the effective area of the cathode substantially constant; and

(G) recovering tetramethyl lead from the withdrawn portion of the catholyte.

References Cited UNITED STATES PATENTS 1 ,539,297 5/ 1925 Calingaert 20472 1,567,159 12/1925 Mead 20472 3,234,112 2/1966 Braithwaite 20459 3,254,009 5/1966 Ziegler et a1. 20459 JOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner.

Claims (1)

1. AN ELECTROLYTIC PROCESS FOR PRODUCING TETRALKYL LEAD COMPOUNDS AT A LEAD CATHODE IN AN ELECTROLYTIC CELL HAVING A LEAD CATHODE, AN ANODE OF A MATERIAL WHICH IS RESISTANT TO ATTACK BY HALOGENS OF ATOMIC NUMBERS 17 TO 53, AND A CURRENT-PERMEABLE PARTITION SEPARATING THE CATHOLYTE FROM THE ANOLYTE, WHICH PROCESS COMPRISES (A) PASSING AN ELECTROLYZING DIRECT ELECTRIC CURRENT THROUGH (B) A LIQUID CATHOLYTE WHICH INITIALLY CONSISTS ESSENTIALLY OF (A) AN ALKYL HALIDE IN WHICH THE ALKYL GROUP HAS 1-10 CARBON ATOMS AND THE HALOGEN ATOM HAS AN ATOMIC NUMBER OF AT LEAST 17, (B) A CURRENT-CARRIER WHICH CONSISTS OF AT LEAST ONE CURRENT-CARRYING TETRALKYL AMMONIUM MONOHALIDE WHICH HAS A HIGHER REDUCTION POTENTIAL THAN SAID ALKYL HALIDE AND IN WHICH EACH ALKYL GROUP HAS 1-18 CARBON ATOMS AND THE HALOGEN ATOM HAS AN ATOMIC NUMBER OF AT LEAST 17, SAID CURRENT-CARRIER BEING IN A CONCENTRATION SUFFICIENT TO PROVIDE A CATHOLYTE HAVING A CONDUCTIVITY OF AT LEAST 0.001 OHM-1 CM.-1, AND (C) FROM ABOUT 1 TO 20 MOLES PER MOLE OF SAID CURRENT-CARRIER OF AT LEAST ONE HYDROXYLIC COMPOUND OF THE CLASS CONSISTING OF WATER AND ALKANOLS OF 1-4 CARBON ATOMS; AND (C) A LIQUID ANOLYTE WHICH INITIALLY CONSISTS ESSENTIALLY OF A SOLUTION OF (1) A CURRENT-CARRIER WHICH CONSISTS OF AT LEAST ONE CURRENT-CARRYING TETRAALKYL AMMONIUM MONOHALIDE WHICH HAS A HIGHER REDUCTION POTENTIAL THAN SAID ALKYL HALIDE AND IN WHICH EACH ALKYL GROUPS HAS 1-18 CARBON ATOMS AND THE HLAOGEN ATOM HAS AN ATOMIC NUMBER OF AT LEAST 17, IN A CONCENTRATION SUFFICIENT TO PROVIDE AN ANOLYTE HAVING A CONDUCTIVITY OF AT LEAST 0.001 OHM-1CM.-1 0.001 OHM-1CM.-U, (2) IN AN INERT SOLVENT HAVING A REDUCTION POTENTIAL AT LEAST AS HIGH AS SAID ALKYL HALIDE AND AN OXIDATION POTENTIAL HIGHER THAN SAID CURRENTCARRIER; (D) DURING THE ELECTROLYSIS, ADJUSTING THE AMOUNTS OF THE CURRENT-CARRIER IN THE CATHOLYTE AND THE ANOLYTE AS MAY BE NECESSARY TO MAINTAIN THEIR CONDUCTIVITIES AT AT LEAST 0.001 OHM-1CM9-1 AND ADJUSTING THE AMOUNT OF THE HYDROXYLIC COMPOUND IN THE CATHOLYTE AS MAY BE NECESSARY TO MAINTAIN THE CONCENTRATION THEREOF WITHIN THE RANGE OF FROM ABOUT 1 TO ABOUT 20 MOLES PER MOLE OF SAID CURRENT-CARRIER; AND (E) RECOVERING TETRAALKYL LEAD FROM THE CATHOLYTE.

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