CA1067450A - Process for hydrodimerizing olefinic compounds - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In a process for hydrodimerizing an olefinic nitrile, amide or ester by electrolyzing an aqueous solution of the olefinic compound, alkali metal salt and quarternary phosphonium cations, the selectivity with which the hydrodimer is produced is surprisingly high when the solution contains less than about 5% by weight of the olefinic compound and alkali metal cations constituting more than half of the total weight of all cations in the solution and the solution is electrolyzed in contact with a cathode consisting essentially of cadmium.

Description

~L067450 This invention relates to a process for hydrodimeriz-ing olefinic co~pounds.
Production of paraffinic dinitxiles, dicarboxamides or dicarboxylates by electrolytic hydrodimerization of an alpha,beta-olefinic nitrile, carboxamide or carboxylate is well known, e.g~ from U.S. Patents 3,193,475-79 and 3,193,481-83 issued July 6, 1965, to M. M. Baizer. Although the process has been sufficiently attractive that it has been in com-mercial use for over nine years, efforts to develop improve-ments thereon have been continued with particular emphasis on lowering electric power costs and mitigating electrode~
corrosion and fouling tendencies because o which it has been heretofore commercially preferable to carry out the process with a cell-dividing membrane. With the object of maintain-ing high electrolyte conductivity while employing a relatively low proportion of organic salts in the electrolysis medium, one approach to lmprovement of the process has been to;carry ; out the electrolysis in an aqueous solution of a mixture of quaternary ammonium and alkali metal salts together with the olefinic compound to be hydrodimerized.
An example of a process utilizing such an approach is described in Netherlands Patent Application 66,10378 which was laid open for public inspection January 24, 1967.~ As described in that application, adiponitrile is produced by ele.ctrolyzing a neutral aqueous solution of acrylonitrile, an alkali metal salt of a polyvalent acid such as phosphoric, boric or sulfuric and a small quantity of an ammonium salt.
According to the examples of that application, goo~ selectivi-ties can be achieved when such a process is carried out in an ; 30 undivided tmembraneless) cell having a graphite cathode.
Further development of the process, also with use of a graphite cathode, is described in U.S. Patent 3,616,321 ' 1 ~

~067450 issued October 26, 1971, to A. Verheyden et al. As is known in the art, however, commercial-scale use of graphite cathodes in a process o~ the type discussed herein is not very attrac-tive, primarily because.graphite i5 quite brittle and, at the desirably-elevated hydrodimerization temperatures and generally-opt~mum electrolyte flo~ rates of at least about 0.3 meter per second, sufficiently subject to erosion and/or fouling that it soon becomes roughe~ed and the selectivity o~ the reaction ~hich takes place at t~e cathode) drops sharply.
Superficially, it might have seemed that various other materials having high hydrogen overvoltages could be satisfac-torily substituted for graphite as the cathode in electrolytic hydrodimerization (EHD) processes similar to that o~ Nether-lands Application 66,10378 to Tomilow published January 24, 1967, and U. S. 3,616,321 (corresponding to Canadian Patent 813,877) and, in fact, the suitability of a variety of such other materials for use in certa~n EHD processes has been suggested in the aforecited U. S. Patents 3,193,375-79 and '481-83, in U. S. Patent 3,511,765 issued May 12, 1970, to Fritz Beck et al, in U. S. Patent 3,595,764 issued July 27, 1971, to Maomi Seko et al and in U. S. Patent 3,689,382 issued September 5, 1972, to H. N. Fox et al. However, and not-withstanding those suggestions, it has been recognized in the art that at least in some instances when an olefinic compound EHD medium contains significant amounts of alkali metal salts, the selectivity with which the desired hydrodimer is produced is highly dependent on the specific cathode material employed.
To illustrate, in British Patent 1,014,428, issued December 22, 1965, to Ivan L. Rnoun~ants et al, the patentees demonstrated that the hydrodimer selectivity in

2 -~ 67450 electrolytic h~drodime~iz~tion of an olefinic compound such as acrylonitrile is quite high (70-80%~ when a graphite cathode is employed with a low-temperature (below 0C.) aqueous electrolyte containing alkali metal ions in sub-stantial concentration (0.7 to 1.2N) but that with the same electrolyte, an iron cathode yielded only about 20~ of the dinitrile (based on the converted monomer~ and a cadmium cathode provided pxactically no reaction product other than the saturated monomer (propionitrile). High ratios of propionitrile to adiponitrile obtained when various cathode materials other than graphite are employed with an electro-lyte containing alkali metal ions are also demonstrated and an explanation is provided by Baizer in Journal of the Electrochemical Society, Vol. 111, No. 2, at pages 215-22 (1964).
For reasons including those set forth hereinbefore, i a process by which an olefinic nitrile, carboxamide or carboxylate can be electrolytically hydrodimerized with high : :
selectivity while using as the electroLysis medium an aqueous solution containing an alkali metal salt in signi-flcant amount and in which the cathode is dimensionally stable and resistant to corrosion for long periods of time is highly attractive for commercial use. Accordingly, the provision of such a process is a primary object of the invention described herein. Another object of this inven-tlon is to provide such a process which can be satisfactorily carried out in an electrolytic cell in which the anode is in contact with the electrolysis medium and subject to corrosion under those conditions. Further objects of the invention will bejapparent from the following description and Examples in which all percentages are by weight except where otherwise noted.

~06745~
In ~ccordance with this invention, there is pro~
vided a process for hydxodimerizing an olefinic compound having the formula R2C-CR-X, as defined hereinafter, which comprises electrolyzing an aqueous solution having dissolved -~ therein at least about 0.5% but less than about 5~ by weight of said olefinic compound, at least about 1~ by weight of sodium or potassium salt selected from the group consisting of phosphate, borate~ perchlorate, carbonate and sulfate sufficient to provide sodium or potassium ions con-stituting more than half of the total weight of all cations in the solution and from about 10 5 to about 0.5 gram mol per liter of cations selected from the group consisting of C8-C20 tetraalkylphosphonium ions containing at least three C~-C5 alkyl groups and C17-C36 polymethylenebis~trialkyl-; phosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups and the polymethylene ~ radical is C3-C8 in contact with a cathodic surface having :1 a cathode potential sufficient for hydrodimerization of said olefinic compound and consisting essentially of cadmium with :. 20 a current density between about 0.01 and about 1.0 amps per square centimeter of said cathodic surface and at a tempera-ture between about 5 and about 75C.
As noted above, the process of this invention is applicable to the electrohydrodimerization of olefinic compounds having the formula R2C=CR-X wherein -X is -CN, -CONR2 or -COOR', R is hydrogen or R', and R' is Cl-C4 alkyl, which compounds can be hydrodimerized to prepare a hydrodimer : having the formula X-CHR-CR2-CR2-CHR-X wherein X and R have the aforesaid significance with a high molar selectivity (generally at least about 75% and in many cases at least ~)67450 about 80%) based on the converted olefinic compound by elec-trolyzing an aqueous solution of the olefinic compound, multi-phosphonium cations or mono-quaternary phosphonium cations and an alkali metal salt in contact with a cathodic surface consisting essentially of cadmium.
In one embodiment o~ the invention, the aqueous solution has dissolved therein between about 0.5% and 5% of the olefinic compound, quaternary phosphonium cations in a concentration between about 10 5 and 0.5 gram mol per liter and at least about 1% of alkali metal salt sufficient to pro-vide alkali metal cations constituting more than half of the total weight of all cations in the solution. In another em-bodiment of the invention, the aqueous solution has dissolved therein at least about 1.0% of the alkali metal salt, quater-nary phosphonium cations in a concentration between about 10 5 and 0.5 gram mol per liter and at least about 0.1% but less than about 5% of the olefinic compound. In still another em-bodiment of the invention, the aqueous solution has dissolved therein between about 0.5% and 5% o~ the olefinic compound, quaternary phosphonium cations in a concentration of between about 10 5 and 0.5 gram mol`per liter and at least about 5%
of the alkali metal salt. As disclosed in greater detail hereinafter, the quaternary phosphonium cations employed in embodiments of the lnvention are mono-quaternary phos-phonium (e.g. tetraalkylphosphonium) cations while in other embodiments the quaternary phosphonium cations are multi-valent ions such as bis-quaternary phosphonium cations, e.g. polymethylenebis(trialkylphosphonium) cations, or a mixture of such monovalent and multivalent cations. Even when the process is carried out in an electrolytic cell in which the anode is in contact with the aqueous solution, ~' '`.

1~674S~

fouling o~ t~e cathode proceeds very slowl~ and the hydro-dimer selecti~ity remains high for an exceptionally long time, particularly when the cathodic surface has a center-line average not greater than about 90 microinches. Each of the embodiments of the invention is particularly useful in the preparation of adiponitrile, a nylon 66 intermediate, by the hydrodimerization of acrylonitrile.
Olefinic compounds that can be hydrodimerized by the process of this in~ention include those having the structural formula R2C=CR-X wherein -X is -CN, -CONR2 or -COOR', R iShydrogen or R' and R' is Cl-C4 alkyl (i.e., methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or tert-butyl). Compounds having that formula are known as having alpha, beta mono-unsaturation and in each such compound, at least one R may be R' while at least one other R is hydro-gen and at least one R', if present, may be an alkyl group containing a given number of carbon atoms while at least one other R', if present, is an alkyl group containing a different number of carbon atoms. Such compounds include olefinic nitriles such as, for example, acrylonitrile, methacrylonitrile, crotononitrile, 2-methylenebutyronitile, 2-pentenenitrile, 2-methylenevaleronitrile, 2-methylenehexanenitrile, tiglonitrile or 2-ethylidene-hexanenitrile; olefinic carboxylates such as, for example, methyl acrylate,~ethyl acrylate or ethyl croto-nate; and olefinic carboxamides such as, for example, acryl-amide, methacrylamide, N,N-diethylacrylamide or N,N-diethyl-crotonamide. Best results are generally obtained when the olefinic compound has at least one hydrogen atom directly attached to either of the two carbon atoms joined by the double bond in the aforedescribed structural formula.

~67~LS~
Also presently o~ greater utility in the process of this invention are those olefinic compounds wherein ~' in that formula is methyl or ethyl, and particularly acrylonitrile and alpha-methyl acrylonitrile. Products of hydrodimeriza-tion of such compounds have the structural formula X-CHR-CR2-CR2-CHR-X wherein X and R have the aforesaid signifi-cance, i.e., paraffinic dinitriles such as, for example, adiponitrile and 2,5-dimethyladiponitrile; paraffinic dicarboxylates such as, for example, dimethyladipate and diethyl-3,4-dimethyladipate; and paraffinic dicarboxamides such as, for exampl , adipamide, dimethyladipamide and N,N'-dimethyl-2,5-dimethyladipamide. Such hydrodimers can be employed as monomers or as intermediates convextible by known processes into monomers useful in the manufacture of high molecular weight polymers including polyamides and polyesters. The dinitriles, for example, can be hydrogenated by known pro-cesses to prepare paraffinic diamines especially useful in the production of high molecular weight polyamides. Other examples of various ole~inic compounds that can be hydro-dimerized by the process of this invention and the hydrodimers thereby produced are identified in the aforecited U.S. Patent Nos. 3,193,475-79 and '481-83.
The invention may also be described in terms of electrolyzing an aqueous solution having dissolved therein certain proportions of the olefinic co~pound to be hydro-dimerized, quaternary phosphonium cations and an alkali metal salt. Use o~ the term "aqueous solution" does not imply, however,that the solution may not also contain a dispersed but undissolved organic phase. To the contrary, the process of this invention can be quite satisfactorily carried out with 1~67~50 the recited ~queous solution containing anywhexe ;Erom a very small to a very substantial proportion of an undissolved organic phase during electrolysis of the solution. Hence in some embodiments of the invention there may be suitably elec-trolyzed an aqueous solution containing essentially no un-dissolved organic phase, by which is meant that the solution may contain either no measurable amount of undissolved organic phase or a minute proportion of undissolved organic phase such as mi~ht remain entrained in the aqueous solution despite the latter being permitted to stand without agitation after electrolysis and cooling and/or addition of more of the olefinic starting material to facilitate separation of a -~ product-containing organic phase, but the presence of which has no signiflcant effect on the olefinic compound conversion per pass or hydrodimer selectivity achieved when the separated aqueous phase is recycled for further electrolysis in accor-dance with the process of this lnvention. Such a minute proportion, if present, would be typically Iess than 5% of the combined weight of the aqueous solution and the undis-solved organic phase contained therein. In other embodiments, the invention can be carried out by electrolyzing an aqueous ; solution of the type described hereinbefore but having dis-persed therein an undissolved organic phase in a larger pro-portion (e.g. up to 15%, 20% or even more of the combined weight of the aqueous solution and the undissolved organic phase contained therein) which may or may not significantly affect the conversion per pass or hydrodimer selectivity depending on other conditions of the process. In some continuous process embodiments involving recycle of unconverted olefinic compound and whether present in a minute or larger ~)6~50 propoxtion, suc~ an organic phase i5 normally made up mainly (most commonly at least about 65% and even more typically at least about 75%) of the olefinic compound to be hydro-dimerized and the hydrodimer product with some minor amounts of organic hydrodimerization by-products, quaternary phos-phonium cations/ etc. possibly also present. In any event, however, the concentrations of the constituents dissolved in the aqueous solution to be electrolyzed, as set forth in this specification and the appended claims, are with reference to the recited aqueous solution alone and not the combined contents of said aqueous solution and an undissolved organic ` phase which, as aforesaid, may be present but need not be present in the aqueous solution as the invention is carried out. On the other hand, the weight percentages of undissolved organic phase in the aqueous solutions described herein are based on the combined weight of the aqueous solution and the undissolved organic phase contained therein.
:
Referring to the constituents of the aqueous phase, the olefinic compound to be hydrodimerized will be present in at least such a proportion that electrolysis of the solu-tion, as described herein, will result in a substantial amount`
of the desired hydrodimer being produced. That proportion is generally at least about 0.1~ of the aqueous solution, ~ more typically at least about 0.5% of the aqueous solution - andj in some embodiments of the invention, preferably at least about 1~ of the aqueous solution. Inclusion of one or more additional constituents which increase the solubility of the olefinic compound in the solution may permit carrying out the process with the solution containing relatively high pro-portions of the olefinic compound, e.g. at least about 5%

16~6745~

or even 10% or more, but in many embodiments of the invention, the aqueous solution contains less than abo~t 5~ ~e.g. not more than about 4%~ o~ the olefinic com]pound and, in some of those e~bodiments, preferably not more than about 1.8%
of the olefinic compound.
The minimum required proportion of quaternary phos-phonium cations is very small. In general, there need be only an amount sufficient to provide the desired hydrodimer selectivity (typically at least about 75%) although much higher proportions can be present if desired or convenient.
In most cases, the quaternary phosphonium cations are present in a concentra ~on of at least about 10 5 gram mol per liter of the aqueous solution. Even more typically their concentra-tion is at least about 10 4 gram mol per liter of the solu-tion. Although higher proportions may be present in some .
~ cases, as aforesaid, the quaternary phosphonium cations are ;l generally present in the aqueous solution in a concentration not higher than about 0.5 gram mol per liter and even more usually, in a concentration not higher than about 10 1 gram mol per liter. In some preferred embodiments, the concen-tration of quaternary phosphonium cations in the solution is between about 10 4 and about 10 2 gram mol per liter.
The quaternary phosphonium cations that are present in such concentrations are those positively charged ions in which a phosphorus atom has a valence of five and is directly linXed to other atoms (e.g. carbon) satisfying four fifths of that valence. Such cations neea contain only one pentavalent phosphorus atom but may contain more of such pentavalent atoms, e.g. as in the multivalent mul$i-quaternary phosphonium cations referred to hereinbefore.

~L~67~5() Suitable mono-quaternary phosphonium cations may be cyclic, but they are more generally Qf the type in which a pentavalent phosphor~s atom is directly linked to a total of four monovalent organic groups preferably devoid of olefinic unsaturation and desirably selected from the group consisting of alkyl and aryl radicals and combinations thereof.
Suitable multi-quaternary phosphonium cations may likewise be cyclic, and they are typically of a type in which the pentavalent phosphorus atoms are linked to one another by at least one divalent organic (e.g. polymethylene) radical and further substituted by monovalent organic groups of the kind just mentioned sufficient in number (normally two or three) that four fifths of the valence of each such pentavalent atom is satisfied by such divalent and monovalent organic radicals. As such monovalent organic radicals, suitable aryl groups contain typically from six to twelve carbon atoms and preferably only one aromatic ring as in, for example, a ; phenyl or benzyl radical, and suitable alkyl groups can be straight-chain, branched or cyclic with each typically con-taining from one to twelve carbon atoms.
Although mono-quaternary phosphonium cations con-taining a combination of such alkyl and aryl groups (e.g.
benzyltriethyl phosphonium ions) can be used, many embodiments of the invention are carried out with quaternary cations having no olefinic or aromatic unsaturation. Good results are generally obtained with tetraalkylphosphonium ions containing at least three C2~C6 alkyl groups and a total of from 8 to 24 carbon atoms in the four alkyl groups, e.g. tetraethyl-, ethyltripropy~ ethyltributyl-, ethyltriamyl-, ethyltrihexyl-, 9679~S0 octyltriethyl-, tetrapropyl-, methyltripxopyl-, decyltripropyl-, methyltributyl-, tetrabutyl-, amyltributyl-, tetraamyl-, tetrahexyl-, ethyltrihexyl-, diethyldioctylphosphonium and many others referred to in the aforecited U.S. Patents 3,193,475-79 and '481-83. Generally most practical from the economic standpoint are those C8-C20 tetraalkylphosphonium ions con-taining at least three C2-C5 alkyl groups, e.g. methyltributyl-, tetrapropyl-, ethyltriamyl-, octyltriethylphosphonium, etc.
Particularly useful are the C8-C16 tetraalkylphosphonium ions containing at least three C2-C4 alkyl groups.
Similarly good resuIts are obtained by use of the divalent polymethylenebis(trialkylphosphonium) ions, particu-larly those containing a total of from 17 to 36 carbon atoms and in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups and the polymethylene radical is C3-C8, i.e., a straight chain of from three of eight methylene radicals. Presently most attractive from the economic stand-; point are the C18-C32 polymethylenebis (trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least ; 20 two C3-C5 alkyl groups and the polymethylene radical is C4-C6.
In many embodiments of the invention employing such polymethylene-bis(trialkylphosphonium) ions, the carbon atom content of such ions is preferably from 20 to 34. Presently of speciflc lnterest for potential commercial use in the process of this invention are the C20-C34 hexamethylenebis(trialkylphosphonium) ions, e.g. those in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups. Also generallypreferred ~:P67~50 are the hexamethylenebis(trialkylphosphonium) ions containing from 20 to 30 carbon atoms, e.g. those in which each trialkyl-phosphonium radical contains at least two C3-C5 alkyl groups, and especially the C24-C30 hexamethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least one and pre~erably two n-butyl groups. Any of such cations can be incorporated into the aqueous solution to be electrolyzed in any convenient manner, e.g. by dissolving the hydroxide or a salt (e.g. a C1-C2 alkylsulfate) of the desired quaternary phosphonium cation(s) in the solution in the amount required to provide the desired concentration of such cations.
One significant advantage of the polymethylenebis (trialkylphosphonium) ions for use in the present invention is that relative to most of the corresponding tetraalkylphos-phonium ions of the type described hereinbefore, they tend ~o distribute themselves in higher proportion toward the aqueous ~ phase of a mixture of an aqueous solution of the type electro-; lyzed in accordance with the present invention and the undissolved organic phase which, as aforesaid, may be present in the aqueous solution during the electrolysis. Whether or not such an or-ganic phase is present in substantial proportion in the aqueous solution during the electrolysis, product hydrodimer is gen-erally most conveniently removed from the electrolyzed solution by adding to the solution (either before or after the electrolysis) an amount of the olefinic starting material in excess of its solubility therein, mixing the solution and the excess olefinic compound until they are substantially equilibrated, and then separating (e.g. decanting) from the resulting mixture a first portion thereof that is richer than said mixture in the olefinic compound and therefore richer than said mixture in the hydrodimer ~)6745~

product ~ith is normally substantiall~ more soluble in the olefinic compound than in the electroly~ed aqueous solution.
Normally, the hydrodimer product is separated from said first portion of the mixutre (e.g. by distillation) while a second portion of the mixture comprising an aqueous solution of the type subjected to alectrolysis in accordance with the present invention is recycled and the aqueous solution comprised by said - second portion is subjected to more of such electrolysis. In process embodiments in which the hydrodimer product is separated from the electrolyzed solution in the manner just described and in view of the importance of having sufficient quaternary phosphonium cations in the aqueous solution to maintain a high hydrodimer selectivity on further electrolysis of the solution, the use of a quaternary cation that distributes itself in relatively high proportion in the aqueous portion of a substantially equilibrated mixture of the type just described is highly attractive from the standpoint of lessening the costs of recovering such cations from the separated te.g. decanted) organic portion of the mixture and/or loss of such cations due to incomplete recovery from said organic portion of the mixture.
Surprisingly, and despite their generally higher carbon con-tent, various bis-quaternary cations of the class defined hereinbefore have been found to distribute themselves toward the aqueous solution in ratios significantly higher (e.g. up to at least 3-4 times higher) than those of the corresponding mono-quaternary phosphonium cations.
The alkali metal salts which can be employed in the invention are those of sodium, potassium, lithium, cesium and rubidium. Generall~ preferred for economic reasons are those of lithium and especially sodium and potassium. They may be 1~67~5~

salts of a monovalent acid, e.g. a perchlorate~ a nitrate, an acetate or a halide such as chloride or bromide. In some cases, e.g. where corrosion control is more of a factor, it may be desirable to use an alkali metal salt of a polyvalent acid, e.g. an orthophosphate, borate, carbonate or sulfate, and particularly an incompletely-substituted salt of that type, i.e. a salt in which the anion has at least one valence thexeof satisfied by hydrogen and at least one other valence thereof satisfied by an alkali metal. Examples of such salts include disodium phosphate (Na2HPO4), potassium acid phosphate (KH2PO4), sodium bicarbonate (NaHCO3~, dipotassium borate (K2HBO3), and sodium acid sulfate (NaHSO4). Also useful are the alkali metal salts of condensed acids such as pyrophosphoric, metaphosphoric, metaboric, pyroboric and the like (e.g. sodium pyrophosphate, potassium metaborate, etc.). Depending on the acidity of the aqueous solution to be electrolyzed, the stoichiometric pro-;~ portions of such anions and alkali metal cations in the solution may correspond to a mixture of two or more of such salts, e g.
a mixture of sodium acid phosphate and disodium phosphate, and accordingly, such mixtures of salts (as well as mixtures ofsalts of different alkali metals and/or different acids, e.g.
phosphoric and boric) are intended to be within the scope of the expressions l'alkali metal saltll and "sodium or potassium salt"
as used herein. Any of the alkali metal salts may be dissolved in the aqueous solution as such or otherwise, e.g. as the alkali metal hydroxide and the acid necessary to neutralize the hydroxide to the extent of the desired acidity of the aqueous solution.
The concentration of alkali metal salt in the solu-tion should be at least sufficient to substantially increase the electrical conductivity of the solution above its iC~67450 conductivity without such a salt bein~ present. In general, there is also enou~h alkali metal salt dissolved in the solution to provide alkali metal c~tions constituting more than half of the total weight of all cations in the solution. In most cases, the solution has dissolved therein at least about 0.1~
of the alkali metal salt. More advantageous conductivity levels are achieved when the solution has dissolved therein at least about 1% of alkali metal salt, or more preerably, at least about 2% of such a salt. In many cases, optimum process 10 ~ conditions include the solution having dlssolved therein more than 5% (typicalIy at least 5.5%) of alkali matal salt. The maximum amount of alkali metal salt in the solution is limited only by its solubility therein, which varies with the particular salt employed. With salts such as sodium or potassium phos-phates, it i5 generally most convenient when the solution contains between about 1~ and about 15% of such a salt or~mix-ture thereof.
The acidity of the solution is preferably such that an alkaline condition prevails at the cathode. Since there is .: ~ .
~20 normally an acidity gradient across the cell, pH at the anode can be lower than seven, if desired. In most cases, however, : .
pH of the overall solution should be at least about two, is preferably at least about five and when the solution is in contact with certain metals subjeot to corrosion, is most conveniently at least about seven. Also in most cases, the overall solution pH is not higher than about twelve, typically not higher than about eleYen and, with the use of sodium or potassium phosphates and~or borates, generally not higher than about ten~

The temperature of the solution may be at any level compatible with existence as such o~ the solution itself, i.e., ~0~7450 above its freezing point but below its boiling point under the pressure employed. Good results can be achieved between about 5 and about 75C. or even higher temperatures if pressures substantially above one atmosphere are employed. The optimum temperature range will ~ary with the specific olefinic compound and hydrodimer, among other factors, but in hydrodimerization of acrylonitrile to adiponitrile, an electrolysis temperature between about 25 and about 75C. is usually preferred.
Although nctnecessary, a liquid-impermeable cathode is 10 usually preferred. With the use of such a cathode, the aqueous solution to be electrolyzed is generally passed along the surface thereof at a linear velocity with reference to the adjacent cathodic surface of at least about 0.3 meter per second, preferably at least about 0.6 meter per .second and even more preferably between about 0.9 and about 2.4 meters per second although, if desired, a solution velocity up to 6 meters per second or higher can be employed. The gap between the anode and cathode can be very narrow, e.g. about 1 millimeter or less, or as wide as 12.5 millimeters or even wider, but is generally of a width between about 1.5 and about 6.2 millimeters.
As is well-known, electrolytic hydrodimerization of an olefinic compound having a formula as set forth hereinbefore must be carried out in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of that compound. In general, there is no minimum current density with which the process can be carried out at such a cathodic surface but in most cases, a current density of at least about 0.01 amp per square centimeter of the cathodic surface is used and a current density of at least about 0.05 amp per square centimeter of the cathodic surface is usually preferred. Although higher current densities may be practical in some instances, those generally employed in the present process are not higher than about 1.5 amps per square centimeter and even more typically ' ~al67~50 not hi~her than about 0.75 ~mp per square centimeter o~ theaforedescribed cathodic surface. Depending on o~her process variables, current densities not higher than about 0.5 amp per square centimeter may be preferred in some embodiments of the invention.
As aforesaid, the process of this invention is carried out with a cathodic surface consisting essentially of cadmium, meaning that the cathodic sur~ace contains a very high percentage of cadmium (generally at least about 90%, more typically at least about 95% and preferably at least about 98%) but that it may contain a small amount o~ one or more other constituents that do not alter the nature of the cadmium cathode so as to prevent it from providing the advantages of the present invention, particularly as described herein. Such other constituents, if present, are preferably other materials having relatively high hydrogen overvoltages.
When such other materials are present in a relatively high concentration such as, for example, from about 0.5% up to about 5% or higher, they are preferably lead and/or mercury.
However, best results are generally obtained when the cathodic surface has a cadmium content of at least about 99.5%, even -more typically at least about 99.8% and m~st desirably at least about 99.9% as in ~STM Designation B440-66T (issued 1966).
Cathodes employed in this invention can be prepared by any of various techniques such as, for example, electro-plating of cadmium on any suitably-shaped substrate of some other material, e.g. a metal having greater structural rigidity, or by chemically, thermally and/or mechanically bonding a layer of cadmium or an alloy thereof containing one or more of the aforementioned other optionally present cathode constituents to a similar substrate. Alternatively, a plate, sheet~ rod or 745~1 any other suitable confi~uration consisting essentially of cadmium may be used without such a substrate, if desired.
As aforesaid and contrary to expectations based on the disclosure o~ British Patent 1,014,42~, use of the process embodiments described herein including a cathodic surface consisting essentially of cadmium provides the desired hydrodimer with a high molar selectivity, based on the converted olefinic starting material, and for clearly long enough periods of time for attractive commercial practice of the process. The hydrodimer selectivity of the present in~ention, as contrasted with essentially zero in the British Patent 1,014,428 process embodiments using a cadmium cathode, is normally at least about 75%, i.e., at least about 75% of the moles of converted olefinic starting material are converted to the desired dinitrile, dicarboxylate or dicarboxamide. In many cases, the molar selectivity of the present process is at least about 80%~and, in some instances including certain embodiments employed in hydrodimerization of acrylonitrile to adiponitrile, as high as 85% or even higher.
The process o~ this invention can be satisfactorily carried out in a divided cell having a cation-permeable membrane, diaphragm or the li~e separating the anode and cathode compart-ments of the cell in such a way that the aqueous solution under-going electrolysis is not in contact with the anode of the cell and products of anode corrosion, if any, are substantially prevented from migrating to the cathode of the cell. The process can also be carried out in a cell not divided in that manner, i.e., in an electrolytic cell in which the aforedescribed aqueous solu-tion is simultaneously in direct physical contact with an anode and cathode of the cell, and in which the anode is composed of a material not corroded by the solution at a substantia:L rate (e.g. at least about 10 3 inch per year) such as, for example, 16~67~5~
one o~ the materials conventionall~ regarded as corrosion-proof (e.g. platinum, various alloys of platinum, other precious metals and alloys thereof, lead dioxide, etc.). In both of such embodiments, anode corrosion products normally do not reach the cathode of the cell in a ~uantity large enough to plate out on or foul the cathode to a degree sufficient to greatly lower the hydrodimer selectlvity of the process and it has been found that the surface smoothness of the cathode is generally not of critical importance to long-term maintenance of high selectivi-ties when that is the case.
In another embodiment, the process of ~his inventioncan be carried out in an undivided cell in which the anode is in contact with the aqueous solution, as aforesaid, and the anode is Gomposed of a material which, depending on process conditions such as the particular alkali metal salt employed~ the solution temperature, etc., may or may not be ~orroded by the solution at a substantial rate under the eIectrolysis conditions. Such less corrosion-resistant anode materials include the ferrous metals such as iron and steel, magnetite, nickel, and, in fact, any metal or alloy capable of being passivated, particularly if the solution undergoing electrolysis is alkaline or at least not strongly acidic (i.e., pH not substantially below seven).
When the process is carried out with an anode comprising such a less corrosion-resistant material in contact with the solution undergoing electrolysis and the anode is substantially corroded under the conditions of the process, e.g. such that products of corrosion of the anode become dispersed in the electrolysis medium and subsequently tend to plate out on and/or foul the cathodic surface to a degree which would otherwise substantially lower the hydrodimerization selectivity, it is generally most advantageous for long-term maintenance of high hydrodimer selec-tivities to inhibit such plating out and/or fouling by employing ~7450 a cathodic surface having a degree of smoothness corresponding to a centerline a~erage not greater than about 90 microinches (2.29 microns) as determined in accordance with the definition ; of centerline average set forth in American Standard ASA
B 46.0-1962 (Surface Texture~ published by The American Standard Society of Mechanical Engineers, 345 East 47th Street, New York, New York. In most cases, the centerline average o~ the cathodic surface employed in this embodiment of the present process is desirably less than about 70 microinches (1.78 microns), preferably less than about 50 microinches ~1.27 microns) and, for superior results in many cases, less than about 30 microinches (0.76 micron), aIl determined in accordance with the definition in the aforerecited ASA
publication. Centerline average, as the term is used herein, can be measured by various procedures and types of apparatus, exemplary o~ which are the Tank Taylor Hobson Talysur 4 and the procedures described in the Talysurf 4 Operator's Handbook distributed by Rank Precision Industries Ltd., Metrology Divlsion, P. O. Box 36, Leicester House, Lee Circle, Leicester LEl 9JB, England and in the U.S.A. by Engis Equipment Company, 8035 Austin Avenue, Morton Grove, Illinois 60053. In some embodiments in which anode corrosion may otherwise proceed at a relatively high rate, it may be desirable to also include in the electrolysis medium a small i amount (generally between about 0.02~ and about 3%) of an inhibitor of corrosion of the anode material employed (e.g. an alkali metal salt of a boric or condensed phosphoric acid when the anode material comprises a ferrous metal~ and/or a similarly small amount of a chelating agent for the anode metal (e.g. an alkali metal salt of a nitrilocarboxylic acid, such as tetrasodium ethylenediaminetetraacetate or -propionate, trisodium hydroxyethylethylenediaminetriacetate, trisodium ~L~67~5(3 nitrilotriacetate or the like).

The following specific examples of the process of this invention are included ~or purposes of illustration only and do not imply any limitations on the scope o~ the invention.
Also in the ~ollowing examples, acrylonitrile and adi~onitrile are generally represented b~ AN and ADN, respectiveIy.

~XAMPLE`I
In a COntinUQUS process, a liquid electrolysis medium composed about 99% b~ (1) an aqueous solution having dissolved therein between 1.4% and 1.6% AN, about 1.2% ADN, 10% of a mixture of sodium orthophosphates, 0.6-1.4 x 10 3 mole per liter of methyltributylphosphonium ions and the sodium borates produced by neutralizing orthoboric acid in an amount cor-responding to about 2% of the solution to the solution pH of about 8.5 and about 1~ by (2) a dispersed but undissolved organic phase containing 27-29% AN, 54-58% ADN, 7-9% AN dimerization byproducts and 8% water was circulated at 55C. and 1.22 meters per second through a undivided electrolytic cell having an AISI 1020 carbon steel anode separated by a gap of 1.76 millimeters from a cathode composed of cadmium conforming to ASTM Designation B440-66T (at least 99.9% cadmium) and having a centerline average of 10-15 microinches (0.25-0.38 microns) measured according to American Standard ASAB46.1-1962 and electrolyzed as it passed through the cell with a current density of 0.185 amp per square centimeter of the surface of the cathode. Organic phase containing product ADN, byproducts and unreacted AN was separated from the electrolyzed medium and makeup AN was added a~ter which the medium was recirculated through the cell and electrolyzed again under the conditions just described ~067~50 For each F~raday of current passed thxough the medium, 0.4 millimole o~ Na4-~DTA ~as added to the'circulating medium and about 12 grams of the solution were purged from the system and replaced with water containing sufficient dissolved methyltri-butylphosphonium ions and sodium orthophosphates and borates to maintain the concentrations of those constituents of the solution at the aforedescribed levels and the total volume of the medium essentially constant. After 120 hours of electrolysis under those conditions,',it was ~ound that AN had been converted to ADN with average and final selectivities of 88% and the cathodic surface had corroded at an average rate less than 0.1 millimeter per year.
EXAMPLE II
; In processes essentially the same as ~escribed in Example I, except that the quaternary cations in the aqueous solution are any one of a mixture of those tested below the re-sults average and final selectivities of AN conversion to ADN
and cathodic surface corrosion'are substantially the same as those obtained in Example I.
Hexamethylenebis(tributylphosphonium) Hexamethylenebis(amyldipropylphosphonium) Hexamethylenebis(tripropylphosphonium) Hexamethylenebis(methyldibutylphosphonium) Hexamethylenebis(ethyldihexylphosphonium) Hexamethylenebis(decyldiethylphosphonium) Pentamethylenebis(,pxopyldibutylphosphonium) Pentamethylenebis(txiamylphosphonium) Tetramethylenebis(ethyldibutylphosphonium) Tetramethylenebis(octyldipropylphosphonium) Heptamethylenebis(ethyldibutylphosphonium) 10674~;~

EXAMPL~ III
In processes essentially as described abo~e in Example I, except that the quaternary cations in the aqueous solution are any one or a mixture of those identified below instead of methyltributylphosphonium ions~ the results obtained for average and final selectivities of AN conversion to ADN and cathodic surface corrosion are substantially the same as those obtained in Example I.
Amyltributylphosphonium lQ Tetrapropylphosphonium Diethyldihexylphosphonium Decyltriethylphosphonium Propyltributylphosphonium Tetraamylphosphonium Ethyltributylphosphonium Octyltributylphosphonium Diethyldibutylphosphonium

Claims (19)

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

1. A process for hydrodimerizing an olefinic compound having the formula R2C-CR-X, wherein -X is -CN, -CONR2 or -COOR1, R is hydrogen or R1, R1 is C1-C4 alkyl and at least one R directly attached to either of the two carbon atoms joined by the double bond in said formula is hydrogen which comprises electrolyzing an aqueous solution having a pH of 2-12 and having dissolved therein at least about 0.5% but less than about 5% by weight of said olefinic compound, at least about 1% by weight of sodium or potassium salt selected from the group consisting of phosphate, borate, perchlorate, carbon-ate and sulfate sufficient to provide sodium or potassium ions constituting more than half of the total weight of all cations in the solution and from about 10-5 to about 0.5 gram mol per liter of cations selected from the group consisting of Ce-C20 tetraalkylphosphonium ions containing at least three C2-C5 alkyl groups and C17-C36 polymethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups and the polymethylene radical is C3-C? in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of said olefinic compound and consisting essentially of cadmium with a current density between about 0.01 and about 1.0 amps per square centi-meter of said cathodic surface and at a temperature between about 5° and about 75°C.

2. The process of Claim 1, said solution having dissolved therein more than 5% by weight of the sodium or potassium salt.

3. The process of Claim 1, wherein the olefinic compound is acrylonitrile, said solution having dissolved therein at least about 10-4 gram mol per liter of C8-C16 tetraalkylphosphonium ions containing at least three C2-C4 alkyl groups.

4. The process of Claim 1, wherein the olefinic com-pound is acrylonitrile, said solution having dissolved therein at least about 10-4 gram mol per liter of C1?-C32 polymethyl-enebis(trialkylphosphonium) ions in which each trialkylphos-phonium radical contains at least two C3-C5 alkyl groups and the polymethylene radical is C4-C6.

5. The process of Claim 4 which further comprises mixing said solution with acrylonitrile in excess of its solu-bility in said solution until said solution and said excess acrylonitrile are substantially equilibrated, separating from the mixture a first portion richer than said mixture in acrylo-nitrile and a second portion comprising an aqueous solution having dissolved therein at least about 0.5% but less than 5%
by weight of acrylonitrile, at least about 1% by weight of said sodium or potassium salt and between about 10-4 and about 0.5 gram mol per liter of said C16-C32 polymethylenebis(tri-alkylphosphonium) ions, and subjecting the aqueous solution comprised by said second portion to more of said electrolyzing.

6. The process of Claim 1, said solution having dissolved therein between about 10-4 and about 10-2 gram mol per liter of C20-C34 hexamethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups.

7. A process for hydrodimerizing acrylonitrile which comprises electrolyzing an aqueous solution electrolyte having dissolved therein from about 0.5 to about 4% by weight of acrylonitrile, from about 1% to about 15% by weight of sodium or potassium salt selected from the group consisting of phos-phate, borate and carbonate sufficient to provide sodium or potassium ions constituting more than half of the total weight of all cations in the solution and from about 10-? to about 10-l gram mol per liter of cations selected from the group consisting of C?-C20 tetraalkylphosphonium ions containing at least three C2-C? alkyl groups and C17-C36 polymethylenebis-(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups and the poly-methylene radical is C3-C? in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of acrylonitrile and consisting essentially of cadmium with a current density between about 0.05 and about 0.75 amp per square centimeter of said cathodic surface while passing the solution along said cathodic surface at a velocity of at least about 0.3 meter per second, said solution having a pH between about 5 and about 11 and a temperature between about 20° and about 75°C.

8. The process of Claim 7, said solution having dis-solved therein more than 5% by weight of the sodium or potas-sium salt.

9. The process of Claim 8, said solution having dis-solved therein not more than about 1.8% by weight of acrylo-

10. A process for hydrodimerizing acrylonitrile which comprises electrolyzing a mixture comprising as a first component an aqueous solution electrolyte having dissolved therein from about 0.5 to about 4% by weight of acrylonitrile, from about 1% to about 15% by weight of sodium or potassium salt selected from the group consisting of phosphate, borate and carbonate sufficient to provide sodium or potassium ions consitituting more than half of the total weight of all cations in the solution and from about 10-4 to about 10-1 gram mol per liter of cations selected from the group consisting of C8-C20 tetraalkylphosphonium ions containing at least three C2-C5 alkyl groups and C17-C36 polymethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C6 alkyl groups and the polymethylene radical is C3-C8, and as a second component an undissolved organic phase in a propor-tion of about 20% of the combined weight of the solution and the organic phase in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of acrylonitrile and consisting essentially of cadmium with a current density between about 0.05 and about 0.75 amp per square centimeter of said cathodic surface while passing the mixture along said cathodic surface at a velocity of at least about 0.3 meter per second, said mixture having a pH between about 5 and about 11 and a temperature between about 20° and about 75°C.

11. The process of Claim 7, said solution having dis-solved therein between about 10-4 and about 10-2 gram mol per liter of C?-Cl6 tetraalkylphosphonium ions containing at least three C2-C4 alkyl groups.

12. The process of Claim 7, said solution having dis-solved therein between about 10-4 and 10-2 gram mol per liter of C20-C34 polymethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C5 alkyl groups and the polymethylene radical is C4-C6.

13. The process of Claim 10, which further comprises mixing said electrolyte with acrylonitrile in excess of its solubility in said electrolyte until said electrolyte and said excess acrylonitrile are substantially eguilibrated, separating from the mixture a first portion richer than said mixture in acrylonitrile and a second portion comprising an aqueous solu-tion having dissolved therein from at least about 0.5% but less than 5% by weight of acrylonitrile, at least about 1% by weight said sodium potassium salt and between about 10-4 and about 0.5 gram mol per liter of said C16-C32 polymethylenebis(trialkyl-phosphonium) ions, and subjecting the aqueous electrolyte com-prised by said second portion to further electrolysis.

14. The process of Claim 7, said solution having dissolved therein between about 10-4 and about 10-2 gram mol per liter of C20-C30 hexamethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least two C3-C5 alkyl groups.

15. The process of Claim 7, said solution having dis-solved therein between about 10-4 and 10-2 gram mol per liter of C24-C30 hexamethylenebis(trialkylphosphonium) ions in which each trialkylphosphonium radical contains at least one butyl group.

16. The process of Claim 1, in which the tetraalkylphos-phonium ions are methyltributylphosphonium.

17. The process of Claim 1, in which the polymethylene-bis(trialkylphosphonium) ions are tetramethylenebis(tributyl-phosphonium) ions.

18. The process of Claim 1, in which the polymethylene-bis(trialkylphosphonium) ions are hexamethylenebis(ethyldibu-tylphosphonium) ions.

19. The process of Claim 1, in which the olefinic compound is acrylonitrile.