Note: Descriptions are shown in the official language in which they were submitted.
1051819
This invention relates to an improvement in an
electrolytic hydrodimerization process.
Production of paraffinic dinitriles, 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 commercial
use for over nine years, efforts to develop improvements there-
on have been continued with particular emphasis on loweringelectric power costs and mitigating electrode corrosion and
fouling tendencies because of which it has been heretofore
commercially preferable to carry out the process with a cell-
dividing membrane.
With the object of maintaining high electrolyte con-
ductivity while employing an electrolysis medium containing
organic salts in a proportion small enough for attractive use
of a single-compartment (membraneless) cell, one approach to
improvement of the process has been to use as the electrolysis
medium an aqueous solution of a mixture of quaternary ammonium
and alkali metal salts together with the olefinic compound to
be hydrodimerized. An example of such an approach is described
in Netherlands Patent Application 66,10378 laid open for public
inspection January 24, 1967, and further development thereof
is described in U. S. Patent 3,616,321 issued October 26, 1971,
to A. Verheyden et al. and U.S. Patent 3,689,382 issued
September 5, 1972 to H. N. ~ox et al. However, all known
variations of the process are characterized by some degree of
inefficiency in use of the electrolyzing current, and this pro-
blem is typically even more significant in those process varia-
tions that utilize such an undivided cell.
-1-
105~819
For example, not all of the electroreduction that
occurs at the cell cathode takes the form of the desired hydro-
dimerization reaction or even the generally undesired simple
hydrogenation of the olefinic starting material. Instead, a
minor but significant proportion normally results in generation
of molecular hydrogen. This hydrogen ordinarily accumulates
in the electrolysis offgas together with oxygen produced at
the anode and, in fact, the proportion of hydrogen in the off-
gas is a fairly accurate indicator of the proportion of con-
sumed electrolysis current that was wasted on such hydrogenproduction. At relatively low concentrations of hydrogen in
the offgas, the percentage by volume of hydrogen in the offgas
is generally about twice the percentage of current consumed in
the electrolysis by undesired production of molecular hydrogen.
More specifically, the percentage of current consumed in the
electrolysis by undesired production of molecular hydrogen is
normally equal to fifty times the percentage by volume of
hydrogen in the offgas divided by one hundred less the percen-
tage by volume of hydrogen in the offgas, i.e., 50 x %H2/(100-
%H2). For example, a concentration of 10% by volume of hydrogenin an elect~ lysis offgas usually indicates that about 5.5%
of the current consumed in the electrolysis was wasted on
molecular hydrogen production and, accordingly, that the current
efficiency of the hydrodimerization process was not possibly
any greater than about 94.5%.
Clearly, the higher the proportion of the electroly-
zing current that produces molecular hydrogen rather than the
desired hydrodimer, the greater the cost of production of the
hydrodimer will be. Accordingly, a process improvement where-
by an olefinic compound from the aforementioned class can beelectrolytically hydrodimerized with a resultingly lowered
production of molecular hydrogen and a thereby increased current
--2--
~0518~9
efficiency is highly desirable, and it is an object of this
invention to provide such an improvement. Additional objects
of the invention will be apparent from the following descrip-
tion and Examples in which all percentages are by weight
except where otherwise noted.
More particularly, in accordance with the process of
the present invention, in a process for hydrodimerizing an
olefinic compound having the formula R2C=CR-X wherein -X is
-CN, -CONR2 or -COOR', R is hydrogen or R' and R' is Cl-C4
alkyl by electrolyzing an aqueous solution having dissolved
therein at least about 0.1% by weight of the olefinic com-
pound, and at least about 0.1% by weight of conductive salt in
contact with a cathodic surface having a cathode potential
sufficient for hydrodimerization of the olefinic compound, and
wherein the formation of hydrogen at the cathodic surface can
be substantially inhibited and the current efficiency of the
process significantly increased by including in the solution
at least one nitrilocarboxylic acid compound such as, for
example, a nitriloacetic or nitrilopropionic acid compound
having the formula Y2N ( Z YN )n R" - COOM wherein Y is a
monovalent radical such as hydrogen, - R"- COOM, ( CH2 )m+l
OH or Cl-C20 alkyl; -R"- is ( CH2 )m or --t- CHR"' )
R"'is hydroxy, -COOM, - ( CH2 )m COOM or Cl-Cg alkyl,
hydroxyalkyl or hydroxphenyl; Z is a divalent C2-C6 hydro-
carbon radical; M is a monovalent radical such as hydrogen,
alkali metal or ammonium; m is 1 or 2; n is 0-4; and at least
one Y is -R"-COOM or ( CH2 3m+lOH, there is provided the
improvement of including in the aqueous solution at least about
10 5 gram mol per liter of monoquaternary phosphonium cations
or multivalent, multiquaternary-phosphonium cations.
Olefinic compounds that can be hydrodimerized by the
improved process of this invention include those having the
--3--
~05~8~9
structural formula R2C=CR-X wherein -X is -C~, -CO~R2 or
-COOR', R is hydrogen or R' and R' is Cl-C4alkyl (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 hydrogen 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-
methylenebutyronitrile, 2-pentenenitrile, 2-methylenevalero-
nitrile, 2-methylenehexanenitrile, tiglonitrile or 2-ethyli-
denehexanenitrile; olefinic carboxylates such as, for example,
methyl acrylate, ethyl acrylate or ethyl crotonate; and
olefinic carboxamides such as, for example, acrylamide, metha-
crylamide, N-N-diethylacrylamide or ~,N-diethylcrotonamide.
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 afore-
described structural formula. Also presently of greater util-
ity in the process of this invention are those olefinic com-
pounds wherein R' in that formula is methyl or ethyl, and par-
ticularly acrylonitrile, methyl acrylate and alpha-methyl
acrylonitrile.
Products of hydrodimerization of such compounds have
the structural formula X-CHR-CR2-CR2-CHR-X wherein X and R
have the aforesaid significance, i.e., paraffinic dinitriles
such as, for example, adiponitrile and 2,5-dimethyladiponitrile;
paraffinic dicarboxylates such as, for example, dimethyladi-
pate and diethyl-3,4-dimethyladipate; and paraffinic dicarbox-
amides such as, for example adipamide, dimethyladipamide and
1051819
~,N'-dimethyl-2,5-dimethyladipamide. Such hydrodimers can be
employed as monomers or as intermediates convertible 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
processes to prepare paraf~finic diamines especially useful in
the production of high molecular weight polyamides. Other
examples of various olefinic compounds that can be hydrodi-
merized 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 is herein also described in terms of
electrolyzing an aqueous solution having dissolved therein
certain proportions of the olefinic compound to be hydrodimer-
ized, the aforedescribed quaternary phosphonium cations and a
conductive salt. Such use of the term "aqueous solution" does
not imply, however, that the electrolysis medium may not also
contain an undissolved organic phase. To the contrary, the
process of this invention can be quite satisfactorily carried
out by electrolyzing the aqueous solution in an electrolysis
medium containing the recited aqueous solution and a dispersed
but undissolved organic phase in any proportions at which the
aqueous solution is the continuous phase of the electrolysis
medium. Hence in some embodiments of the invention, the
aqueous solution may be suitably electrolyzed in an electroly-
sis medium containing essentially no undissolved organic phase,
by which is meant either no measurable amount of undissolved
organic phase or a minute proportion of undissolved organic
phase having no significant effect on the hydrodimer selectivity
achieved when the aqueous solution is electrolyzed in accord-
ance with the process of this invention. Such a minute propor-
tion, if present, would be typically less than 5% of the
1051819
combined weight of the aqueous solution and the undissolved
organic phase in the electrolysis rnedium. In other embodi-
ments, the process of this invention can be carried out by
electrolyzing the aqueous solution in an electrolysis medium
consisting essentially of the recited aqueous solution and a
dispersed but undissolved organic phase in a larger proportion
(e.g. up to about 15%, 20% or even more of the combined weight
of the aqueous solution and the undissolved organic phase in
the electrolysis medium) which may or may not significantly
affect the hydrodimer selectivity depending on other conditions
of the process. In some continuous process embodiments in-
volving recycle of unconverted olefinic compound and whether
present in a minute or larger proportion, such an organic phase
is normally made up mainly (most commonly at least about 65%
and even more typically at least about 75%) of the olefinic
compound to be hydrodimerized and the hydrodimer product with
some small amounts of organic hydrodimerization by-products,
quaternary ammonium or phosphonium cations, etc. possibly also
present. Typically, such an organic phase contains at least
about 10%, preferably-between about 15% and about 50%, and
even more desirably between about 20% and about 40% of the
olefinic compound to be hydrodimerized. In any event, however,
the concentrations of the constituents dissolved in the aqueous
solution to be electrolyzed, as set forth herein, 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 electrolysis medium as the invention is
carried out. On the other hand, the weight percentages of
undissolved organic phase described herein are based on the
combined weight of the aqueous solution and the undissolved
organic phase in the electrolysis medium.
--6--
~OS~819
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 solution,
as described herein, results in a substantial amount of the
desired hydrodimer being produced. That proportion is gener-
ally at least about 0.1% of the aqueous solution, more typic-
ally at least about 0.5% and, in some embodiments, 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
proportions of the olefinic compound, e.g. at least about 5%
or even 10% or more, but in most embodiments, the aqueous
solution contains less than about 5% (e.g. not more than 4.5%)
of the olefinic compound and, in many of those embodiments,
preferably not more than about 1.8% of the olefinic compound.
In accordance with this invention, the minimum
required proportion of quaternary phosphonium 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 phos-
phonium cations are present in a concentration of at least
about 10 5 gram mol per liter of the aqueous solution. Even
more typically their concentration is at least about 10 4
gram mol per liter of the solution and, in some embodiments
employing monovalent mono-quaternary phosphonium cations, pre-
ferably at least about 5 x 10 4 gram mol per liter. Although
higher proportions may be present in some cases, as aforesaid,
the quaternary phosphonium cations are generally present in the
aqueous solution in a concentration not higher than about 0.5
gram mol per liter and even more usually not higher than about
105~8~
-1
gram mol per liter. In some preferred embodiments, the
concentration of quaternary phosphonium cations in the solu-
tion is between about 10 4 and about 10 2 gram mol per liter.
The quaternary phosphonium cations that are present
in such concentrations are thosepositively-charged ions in
which a phosphorous atom has a valence of five and is
directly linked to other atoms (e.g. carbon) satisfying
four fifths of tha~ va~ence. Such cations need contain only
oS,~7~ofc/~
one pentavalent ~}~e~ou_ atom as in, for example, various
mono-quaternary phosphonium (e.g. tetraalkylphosphonium)
cations, but they may contain more than one pentavalent atoms,
as in, for example, various multivalent multiquaternary phos-
phonium cations such as the bis-quaternary phosphonium cations,
e.g. polymethylenebis(trialkylphosphonium) cations. Mixtures
of such monovalent phosphonium cations and multivalent quater-
nary phosphonium cations can also be used. Suitable mono-
quaternary phosphonium cations may be cyclic, but they are
more generally of the type in which a pentavalent phosphorus
atom is directly ;inked to a total of four monovalent organic
groups preferably devoid of olefinic unsaturation and desir-
ably 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 typi-
cally 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 each 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 mono-
valent organic radicals. As such monovalent organic radicals,
suitable aryl groups contain typically from six to twelve
~5181C~
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
containing from one to twelve carbon atoms. Although quater-
nary phosphonium cations containing a combination of such
alkyl and aryl groups (e.g. benzyltriethylphosphonium ions)
can be used, many embodiments of the invention are preferably
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-, ethyltripropyl-,
ethyltributyl-, ethyltriamyl-, ethyltrihexyl-, octytriethyl-,
tetrapropyl-, methyltripropyl-, decyltripropyl-, methyltri-
butyl-, tetrabutyl-, amyltributyl-, tetraamyl-, tetrahexyl-,
ethyltrihexyl-, diethyldioctyl-phosphonium and many others
referred to in the aforecited U. S. Patent Nos. 3,193,~75-79
and '481-83. Generally most practical from the economic stand-
point are those C8-C20 tetraalkylphosphonium ions containing at
least three C2-C5 alkyl groups, e.g. methyltributyl-, tetra-
propyl-, ethyltriamyl-, octyltriethylphosphonium, etc. Parti-
cularly useful are the C8-C16 tetraalkylphosphonium ions con-
taining at least three C2-C4 alkyl groups.
Similarly good results 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 radi-
cals.
Presently most attractive from the economic stand-
point are the C18-C32 polymethylenebis(trialkylphosphonium) ions
~0518~9
in which each trialkylphosphonium radical contains at least
two C3-C5 alkyl groups and the polymethylene radical is C4-C6.
In many embodiments of the invention employing such polymethyl-
enebis(trialkylphosphonium) ions, the carbon atom content of
such ions is preferably from 20 to 34.
Presently of specific interest for potential com-
mercial 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 generally preferred are the hexamethylenebis
(trialkylphosphonium) ions containing from 20 to 30 carbon
atoms, e.g. those in which each trialkylphosphonium 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
preferably two n-butylgroups. Any of such cations can be in-
corporated into the aqueous solution to be electrolyzed in
any convenient manner, e.g. by dissolving the hydroxide or a
salt (e.g. a Cl-C2 alkylsulfate) of the desired quaternary
phosphonium cation 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 tetraalkylphosphon-
ium ions of the type described hereinbefore, they tend to dis-
tribute themselves in higher proportion toward the aqueous
phase of a mixture of an aqueous solution of the type electroly-
zed in accordance with the present invention and the undissol-
ved organic phase which, as aforesaid, may be present in the
--10--
105~8~9
aqueous solution during the electrolysis. Whether or not such
an organic phase is present in substantial proportion in the
aqueous solution during the electrolysis, product hydrodimer
is generally most conveniently removed from the electrolyzed
solution by adding to the solution (either before or after the
electrGlysis) 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 equili-
brated, and then separating (e.y. decanting) from the resulting
mixture a first portion thereof that is richer than said mix-
ture in the olefinic compound and therefore richer than said
mixture in the hydrodimer product which is normally substan-
tially more soluble in the olefinic compound than in the
electrolyzed aqueous solution. ~ormally, the hydrodimer pro-
duct is separated from said first portion of the mixture (e.g.
by distillation) while a second portion of the mixture com-
prising an aqueous solution of the type subjected to electroly-
sis in accordance with the present invention is recycled and
the aqueous solution comprised by said second portion is sub-
jected to more of such electrolysis. In process embodimentsin which the hydrodimer product is separated from the electro-
lyzed 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 selecti-
vity 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 equili-
brated mixture of the type just described is highly attractive
from the standpoint of lessening the costs of recovering such
cations from the separated (e.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,
--11--
1051819
and despite their generally higher carbon content, various
bis-quaternary cations of the class definea hereinbefore have
been found to distribute themselves toward the aqueous solu-
tion in ratios significantly higher (e.g. up to at least 3-4
times higher) than those of the corresponding mono-quaternary
cations.
The type of conductive salt employed is not usually
critical to inhibition of hydrogen formation by use of a
nitrilocarboxylic acid compound as described herein. Hence
the conductive salt can be a quaternary phosphonium salt
such as, for example, a tetraalkylphosphonium phosphate,
sulfate, alkylsulfate, (e.g. ethylsulfate) or arylsulfonate
(e.g. toluene sulfonate). Although organic salts of that
general type can be employed as the conductive salt in a
divided or single-compartment (undivided) cell, it is generally
preferred to use an alkali metal conductive salt, i.e., a
salt of sodium, potassium, lithium, cesium or rubidium,
especially in undivided electrolytic hydrodimerization (EHD)
cells, and many attractive embodiments of the invention are
carried out with enough alkali metal salt dissolved in the
aqueous solution to provide alkali metal cations constituting
more than half of the total weight of all cations in the
solution. When such alkali metal salts are used, those of
lithium and especially sodium and potassium are generally
preferred for economic reasons.
Also preferred for such use are the salts of
inorganic and/or polyvalent acids, e.g. a tetraalkylphosphonium
or alkali metal orthophosphate, borate, perchlorate, carbonate
or sulfate and particularly an incompletely-substituted salt
of that type, e.g., a salt in which the anion has at least one
valence satisfied by hydrogen and at least one other valence
satisfied by an alkali metal. Examples of such salts include
-12-
1051819
disodium phosphate (Na2HP04), potassium acid phosphate
(KH2P04), sodium bicarbonate (NaHC03) and dipotassium borate
(K2HB03). Also useful are the alkali metal salts of condensed
acids such as pyrophosphoric, metaphosphoric, metaboric, pyro-
boric and the like (e.g. sodium pyrophosphate, potassium meta-
borate, borax, etc.) and/or products of hydrolysis of such
condensed acid salts. Depending on the acidity of the solution
to be electrolyzed, the stoichiometric proportions 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 of salts of dif-
ferent cations, e.g. different alkali metals, and/or different
acids, e.g. phosphoric and boric) are intended to be within the
scope of the expressions "conductive salt" and "alkali metal
phosphate, borate, perchlorate, carbonate and sulfate" 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 conductive salt in the solution
should be at least sufficlent to substantially increase the
electrical conductivity of the solution above its conductivity
without such a salt being present. In most cases, a concentra-
tion of at least about 0.1% is favored. More advantageous con-
ductivity levels are achieved when the solution has dissolved
therein at least about 1% of the conductive salt or, even more
preferably, at least about 2% of such a salt. In many cases,
optimum process conditions include the solution having dis-
solved therein more than 5% (typically at least 5.5%) of theconductive salt. The maximum amount of salt in the solution
is typically limited only by its solubility therein, which
-13-
105181g
varies with the particular salt employed. With salts such as
sodium or potassium phosphates and/or borates, it is generally
most desirable that the solution contain between about 8%
and about 15% of such a salt or mixture thereof.
As aforesaid, generation of molecular hydrogen at the
cathode of a process of the type discussed herein can be sub-
stantially inhibited by including in the aqueous electrolysis
medium at least one nitrilocarboxylic acid compound such as,
for example, a nitriloacetic or nitri~lopropionic acid compound
having the formula Y2N ( Z - YN ~~-n R" -COOM wherein Y is a
monovalent radical such as hydrogen, -R"-COOM, + CH2 ~ lOH
or Cl-C20alkyl (preferably Cl-Cl0 alkyl such as ethyl, n-propyl,
tert-butyl, n-hexyl, n-decyl, etc.); -R" is ~CH2 ~ or
~ CHR'''-~-; R''' is hydroxy, -COOM, ( CH2 ~ COOM or Cl-C8
alkyl, hydroxyalkyl (e.g. hydroxyethyl) or hydroxyphenyl (e.g.
ortho-hydroxyphenyl); Z is a divalent C2-C6 hydrocarbon (e.g.
alkylene) radical such as, for example, n-hexylene, n-butylene,
iso-butylene or, generally more desirably, ethylene or n-propy-
I lene; M is a monovalent radical such as hydrogen, an alkali
metal (e.g. lithium or, usually more desirably, sodium or
potassium)or phosphonium; m is l or 2; n represents the number
of repeating --~Z - YN ) groups, if any, and may be 0, 1, 2,
3 or 4; and at least one Y in the formula is -R"-COOM or
-~ CH2-~--+1OH group in addition to the -R"-COOM group on the
right hand end of the formula as shown hereinbefore. At least
one such additional -R"-COOM or --~CH2-~m+lOH group is usually
desirably attached to the nitrogen atom at the left-hand end of
the formula but when n is l, such an additional group may be
attached (alternatively or otherwise) to the nitrogen atom in
the ( Z - YN ) unit, and when n is 2, 3 or 4, any one or
more of the nitrogen atoms in the repeating ( Z - YN ) units
may have such an additional -R"-COOM or ( CH2 } +lOH group
attached thereto.
-14-
1051~319
Preferably, but not necessarily, the nitrilocarboxy-
lic acid compound is an aminopolycarboxylic acid compound, i.e.
one in which there are at least two -R"-COOM groups. It is
also generally desirable for Y to be C2-C4 alkylene and for n
to be 0, 1, 2 or 3 (even more desirably 0, 1 or 2 and most
preferably 1 or 2). Representative of such compounds are
nitrilotriacetic acid, diethylenetriaminepentaacetic acid,
N,N-di(2-hydroxyethyl)-glycine, ethylenediaminetetrapropionic
acid, N-N'-ethylenebis[2-(o-hydroxyphenyl)]glycine and, typically
most favored, ethylenediaminetetraacetic acid and N-hydroxy-
ethylethylenediaminetriacetic acid (hereinafter sometimes
represented as EDTA and HEDTA, respectively). In the low
concentrations generally employed, they may be added to the
electrolysis medium as acids, or, usually more conveniently and
particularly at the alkaline pH' s favored for most embodiments
of the invention, as partially or fully neutralized salts there-
of (e.g. the water-soluble phosphonium or alkali metal salts
of such acids). In accordance with procedures known in the
art, alkali metal salts of such nitrilocarboxylic acid com-
pound can be prepared by reacting an appropriate amine (e.g.ethylenediamine) with an alkali metal salt of a chloracetic
acid in the presence of an alkali metal hydroxide, or with
hydrogen cyanide and formaldehyde and then an alkali metal
hydroxide, or with ethylene glycol to provide hydroxyethyl
substituents of nitrogen atom(s) of the amine and then
reacting the hydroxyethyl-substituted amine with an alkali
metal hydroxide in the presence of cadmium oxide to con-
vert the hydroxethyl substitutents to alkali metal acetate
substituents in the proportion desired, or with acrylonitrile
in the present of a base (e.g. sodium hydroxide) and then
hydrolyzing the cyanoethylated amine in the presence of an
alkali metal hydroxide. Conveniently utilized salts of EDTA,
HEDTA and other such nitrilocarboxylic acid compounds are also
~051~9
available commercially. See "Keys to Chelation", Dow Chemical
Company, Midland, Michigan (1969).
The minimum concentration of the nitrilocarboxylic
acid compound in the aqueous electrolysis medium is only that
sufficient to inhibit formation of molecular hydrogen at the
1~
cathodic surface 4~ the process. In general, at least about
0.025 millimol of the nitrilocarboxylic acid compound per liter
of the solution is desirable and at least about 0.1 millimol
per liter is preferred. In most cases having a greater attrac-
tion for commercial use, at least about 0.5 millimol per literis more desirable and at least about 2.5 millimols per liter
usually provide even better results. Generally, not more than
about 50 millimols per liter are required, although higher
concentrations may be employed if desired. Even more typically,
economic results are better when the concentration of the
nitrilocarboxylic acid compounds in the solution is not greater
than 25 millimols per liter. With reference to such concentra-
tions, it should be understood that the nitrilocarboxylic acid
compounds used herein may degrade under the conditions of the
process, e.g. to compounds that have lower molecular weight
and/or fewer -R"-COOM or ( CH2~--+-lOH groups but which never-
theless provide the advantages of this invention in substantial
measure, and accordingly such degradation products should be
considered as equivalent to the undegraded nitrilocarboxylic
acid compounds to the extent that they provide the advantages
thereof, when measuring or otherwise identifying a nitrilocar-
boxylic acid compound concentration with reference to the pro-
cess of this invention. Mixtures of two or more of the afore-
described nitrilocarboxylic acid compounds may also be used in
the process of this invention and accordingly, such mixtures
are meant to be within the scope of the expression "a nitrilo-
carboxylic acid compound" as used in this disclosure and the
appended claims.
-16-
1051819
In substantial measure when carrying out the present
process in a cell divided by a cation-permeable membrane and
particularly when carrying out the process in a single-
compartment cell, generation of hydrogen at the cathode is
even more significantly inhibited by including in the electroly-
sis medium a boric acid, a condensed phosphoric acid or an
alkali metal salt thereof. The boric acid or borate may be
added to the solution as orthoboric acid, metaboric acid or
pyroboric acid and then neutrallzed to the desired solution pH,
e.g. with an alkali metal (preferably the cation of the conduc-
tive salt) hydroxide or as a completely or incompletely sub-
stituted alkali metal salt of such an acid (e.g. disodium or
monosodium orthoborate, potassium metaborate, sodium tetra-
borate or the hydrated form thereof commonly called borax).
The condensed phosphoric acid~r pho-sphate may be added as a
polyphosphoric (e.g. pyrophosphoric or triphosphoric) acid and
then neutralized to the desired solution pH or as a completely
or incompletely substituted alkali metal salt thereof (e.g.
tetrasodium pyrophosphate or potassium hexametaphosphate or
triphosphate).
In general, the condensed phosphoric acids and their
alkali metal salts tend to hydrolyze in the electrolysis medium
at rates dependent on their concentration, the solution pH, etc.
It is believed, however, that the products of such hydrolysis
continue to inhibit the generation of hydrogen at the cathode
so long as they remain condensed to at least some degree, i.e.,
so long as they have not been hydrolyzed to the orthophosphate
form, and hence the preferred concentrations of such condensed
phosphoric acid compounds are herein expressed in terms of
weight percent of a condensed phosphoric acid (which may be
that originally added to the solution or hydrolysis products
thereof having a lower but conventionally recognizable degree
~os~9
of molecular condensation) or the molar equivalent of an alkali
metal salt thereof. When such a condensed phosphoric acid is
used in the process of this invention, and particularly in an
undivided cell having a metallic anode (e.g. an anode compris-
ing a ferrous metal such as carbon steel, alloy steel, iron or
magnetite), it is generally advantageous for the solution to
contain at least about 0.01%, preferably between about 0.02%
and about 3%, and often most desirably between about 0.02%
and about 2% of the condensed phosphoric acid or the molar
equivalent (molecularly equivalent amount) of an alkali metal
salt thereof.
The aforementioned boric acids and alkali metal salts
thereof, on the other hand, tend to relatively rapidly form
in the electrolysis medium a variety of boron-containing ions
having relative proportions normally dependent on their concen-
trations, the solution pH, etc., and generally including both
uncondensed (i.e., orthoborate) and condensed (e.g. metaborate,
tetraborate, polymeric ring-containing, etc.) ions, regardless
of whether the acids and/or salts originally added to the
electrolysis medium were in condensed or uncondensed form at
that time. In other words, condensed borates (e.g. tetrabor-
ates) normally convert in the electrolysis medium in part to
orthoborate ions and in part to other condensed borate ions,
while orthoborates added as such generally form various con-
densed borate ions, depending largely on the solution pH, etc.
In any event, it appears that the boron-containing ions are
effective for purposes of this invention whether they are
present in condensed or uncondensed forms or a mixture thereof
and accordingly, preferred concentrations of the boric acids
or salts are herein expressed (on the basis of one liter of
solution) in terms of gram atoms of boron which may be present
in the ionic form of condensed or uncondensed borates or other
-18-
1051819
boron-containing moieties provided by interaction between the
electrolysis medium and the boric acids and/or salts added
thereto. When such bori~ acids or salts are used in the pro-
cess of this invention, and particularly in an undivided cell
having a metallic anode (e.g. an anode comprising a ferrous
metal such as carbon steel, alloy steel, iron or magnetite),
it is generally desirable for the boron concentration in the
electrolysis medium to be at least about 0.01 and preferably
0.02 gram atom of boron per liter of solution. It is gener-
-ally not necessary that the boron concentration in the solution
be greater than about 0.9 gram atom per liter and in many
cases it need not be greater than about 0.5 gram atom per liter,
although higher concentrations are not necessarily detrimental
and may be advantageous, e.g. if it is intended that a boric
acid salt provide a substantial portion of the electrical
conducitivity of the electrolysis medium.
In most cases, the pH of the bulk of the electrolysis
medium is at least about two, preferably at least about five,
more preferably at least about six and most conveniently at
least about seven, especially when the process is carried out
in an undivided cell having a metallic anode. On the other
hand, the overall solution pH is generally not higher than
about twelve, typically not higher than about eleven and, with
the use of sodium or potassium phosphates and/or borates,
generally not substantially higher than about ten.
The temperature of the solution may be at any level
compatible with extence of such of the solution itself, i.e.,
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 at even higher temperatures if pressures
substantially above one atmosphere are employed. The optimum
temperature range will vary with the specific olefinic compound
--19--
lOS1819
and hydrodimer, among other factors, but in hydrodimerization
of acrylonitrile to adiponitrile, electrolysis temperatures
of at least about 25 are usually preferred and those between
about 40 and about 65 C. are especially desirable.
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 present process must be carried out at such a
cathodic surface but in most cases, a current 2ensity of at
least about 0.01 amp per square centimeter (amp/cm2) of the
cathodic surface is used and a current density of at least
about O.OS amp/cm2 is usually preferred. Although hi'gher cur-
rent densities may be practical in some instances, those
generally employed in the present process are not higher than
about 1.5 amp/cm and even more typically not higher than
about 0.75 amp/cm of the aforedescribed cathodic surface.
Depending on other process variables, current densities not
higher than about 0.5 amp/cm2 may be preferred in some embodi-
ments of the process.
Although not necessary, a liquid-impermeable cathode
is usually preferred. With the use of such a cathode, the
aqueous solution to be electrolyzed is generally passed between
the anode and cathode 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 a solution velocity up to 6 meters per second or
higher can be employed if desired. The gap bet~.een 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 usually
-20-
1051819
most conveniently of a width between about 1.5 and about 6.2
millimeters. In the process of this invention, the cathodic
surface can be made of virtually any material at which the
requisite cathode potential can be provided and which is not
dissolved or corroded at an intolerable rate. In general, the
process can be carried out with a cathode consisting essenti-
ally of cadmium, mercury, thallium, lead, zinc, manganese, tin
(possibly not suitable with some nitrile reactants) or graphite,
by which is meant that the cathodic surface contains a high
percentage (generally at least about 95% and preferably at
least about 98%) of one or a combination (e.g. an alloy) of
two or more of such materials, but it may contain a small
amount of one or more constituents that do not alter the nature
of the cathodic surface so as to prevent substantial realiza- ,
tion of the advantages of the present invention, particularly
as described herein. Such other constituents, if present in
substantial concentration, are preferably other materials
having relatively high hydrogen overvoltages. Of particular
preferance are cathodes consisting essentially of cadmium,
lead, zinc, manganese, graphite or an alloy of one of such
metals, and especiaily cathodes consisting essentially of
cadmium. Best results are usually obtained with a cathodic
surface having a cadmium content of at least about 99.5%, even
more typically at least about 99,9% in ASTM Designation B440-
66T (issued 1966).
Cathodes employed in this invention can be prepared
by various techniques such as, for example, electroplating of
the desired cathode material on a 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 the cathode material to a similar substrate.
Alternatively, a plate, sheet, rod or any other suitable
~05~8~9
configuration consisting essentially of the desired cathode
material may be used without such a substrate, if convenient.
The process of this invention can be carried out in
a divided cell having a cation-permeable membrane, diaphragm,
or the like separating the anode and cathode compartments of
the cell in such a way that the aqueous solution containing
the olefinic compound undergoing hydrodimerization at the
cathode of the cell is not in simultaneous direct contact with
an anode of the cell. However, it is especially advantageously
carried out in cells not divided in that manner, i.e., in cells
which the solution being electrolyzed is in direct physical
contact with an anode and cathode of the cell. In fact, and
particularly without the presence of a boric or condensed
phosphoric acid or salt thereof in the preferred concentrations
described hereinbefore, it has been found that the aforemen-
tioned nitrilocarboxylic acid compounds, and especially in the
concentrations cited hereinbefore, generally substantially
inhibit the corrosion of metallic anodes when used in such
undivided cells. Anodes whose corrosion may be thereby inhi-
bited include those composed of the heavy metals (i.e., metalshaving a specific gravity greater than 4.0) such as, for
example, platinum, ruthenium, nickel, lead, lead dioxide and,
of particular advantage when the conductive salt is a phos-
phate, borate or carbonate, ferrous materials such as carbon
steels, alloy steels, iron and magnetite.
In fact, an especially preferred embodiment of the
invention is carried out in an undivided cell having an anode
comprising a ferrous metal and with the use of an alkali metal
phosphate, borate or carbonate conductive salt and an electroly-
sis medium having a pH not substantially below seven. Ofpotential interest from the economic standpoint are those
embodiments employing an anode consisting essentially of
~051819
carbon steel, exemplary compositions of which are listed in the
100, llU0 and 1200 series of American Iron and Steel Insitute
and Society of Automotive Engineers standard steel composition
numbers, many of which may be found on page 62 of Volume 1,
Metals Handbook, 8th Edition (1961) published by the American
Society for Metals, Metals Park, Ohio.
In general, the carbon steels that are advantageously
used as anode materials in the process of this invention con-
tain between about 0.02% carbon (more typically at least about
0.05% carbon) and about 2% carbon. Normally, carbon steels
such as those of the AISI and SAE 1000 series of standard
steel composition numbers are preferred and those containing
between about 0.1% and about 1.5% carbon are typically most
desirable. Regardless of the material from which it is made,
each anode in the cell may be in the form of a plate, sheet,
strip, rod or any other configuration suitable for the use
intended. In a preferred embodiment, however, the anode is in
the form of a sheet (e.g. of cold-rolled carbon steel) essen-
tially parallel to and closely spaced from a cathodic surface
of approximately the same dimensions.
Although the invention described and claimed herein
is not to be regarded as limited to any particular mechanism
proposed therefor, it is presently believed that the nitrilo-
carboxylic acid compounds (and probably to a lesser extent, if
present, the boric and/or condensed phosphoric acid compounds)
at least partially sequester heavy metals which tend to accumu-
late in the electrolysis medium (e.g. as a result of corrosion
of the cathode and/or, with use of an undivided cell, corrosion
of the anode) and that such sequestration inhibits the deposi-
tion of those metals on the cathode of the cell. It is furtherbelieved that unsequestered heavy metals (or oxides and/or
hydroxides thereof) tend to form colloidal particles in the
105181~
electrolysis medium and after such deposition, alter the
nature of the cathodic surface so as to increase the generation
of molecular hydrogen at the expense of process current effi-
ciency. Those beliefs are mainly based on observations that
increases in hydrogen production normally accompany increased
deposition on the cathode of a relatively dense precipitate
which has been identified as essentially completely composed of
such heavy metals (principally iron in an undivided cell having
a steel anode) and their oxides and hydroxides, and that
deposition of the precipitate is substantially inhibited by use
of the process improvement described and claimed herein.
The following specific examples of the process of
this invention are included for purposes of illustration only
and do not imply any limitations on the scope of the invention.
Also in these examples, acrylonitrile and adiponitrile are
generally represented by AN and ADN, respectively.
EXAMPLE I
In a continuous process, a liquid electrolysis med-
ium composed about 99% by (l) an aqueous solution having dis-
solved 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, about 0.5% (14.2
millimoles per liter) of Na4EDTA and the sodium borates pro-
duced by neutralizing orthoboric acid in an amount corresponding
to about 2% of the solution to the solution of 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 EHD by-products and
3% water was circulated at 55C. and 1.22 meters per second
through an undivided electrolytic cell having an AISI 1020
carbon steel anode separated by a gap of 1.76 millimeters
from a cadmium cathode composed of cadmium conforming to ASTM
1051819
Designation B 440-66T (at least 99.9% Cd), and electrolyzed
as it passed through the cell with a current density of 0.185
amp/cm2 of the surface of the cathode. Organic phase
containing product ADN, AN, EHD by-products and unreacted AN
was separated from the electrolyzed medium and make-up AN was
added after which the medium was recirculated through the cell
and electrolyzed again under the conditions just described.
For each Faraday of current passed through the medium, 0.4
millimole of Na4-EDTA was 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 electroly-
sis under those conditions, it was found that AN had been con-
verted to ADN with average and final selectivities of 88%, the
steel anode had corroded at an average rate less than 0.5
millimeter per year and the volume percent of hydrogen in the
offgas had averaged below 1% with a final value of 0.8%.
EXAMPLE II
In processes essentially as described in Example I
except that the quaternary cations in the aqueous solution are
any one or a mixture of those identified below, instead of
hexamethylenebis(ethyldibutylphosphonium) ions, the results
average and final selectivities of AN conversion to ADN and
anodic surface corrosion are substantially the same as those
obtained in Example I:
-25-
1051819
Hexamethylenebis(tributylphosphonium)
Hexamethylenebis(amyldipropylphosphonium)
Hexamethylenebis(tripropylphosphonium)
Hexamethylenebis(methyldibutylphosphonium)
Hexamethylenebis(ethyldihexylphosphonium)
Hexamethylenebis(decyldiethylphosphonium)
Pentamethylenebis(propyldibutylphosphonium)
Pentamethylenebis(triamylphosphonium)
Tetramethylenebis(ethyldibutylphosphonium)
Tetramethylenebis(octyldipropylphosphonium)
Heptamethylenebis(ethyldibutylphosphonium)
EXAMPLE III
In processes essentially as described 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 average and final
selectivities of AN conversion to ADN and anodic surface
corrosion are substantially the same as those obtained in
Example I:
Amyltributylphosphonium
Tetrapropylphosphonium
Diethyldihexylphosphonium
Decyltriethylphosphonium
Propyltributylphosphonium
Tetraamylphosphonium
Ethyltributylphosphonium
Octyltributylphosphonium
Diethyldibutylphosphonium
