Note: Descriptions are shown in the official language in which they were submitted.
~0504'76
This invention relates to an improvement in a single-
compartment electrolytic hydrodimerization process.
Production of paraffinic dinitriles, dicarboxamides or
dicarboxylates by electrolyic hydrodimerization of an alpha,
beta-olefinic nitrile, carboxamide or carboxylate is well
known, e.g. from u.S. Patents 3~193~475A79 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 thereon
have been continued with particular emphasis on lowering elec-
tric 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 divided by a
cation-permeable membrane into separate anolyte and catholyte
compartments. With the object o~ maintaining high electrolyte
conductivity while employing a relatively low proportion of
organic salts in the electrolysis medium, one approach to
improvement of the process has been to carry out the electroly-
sis 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
i is described in U.S. Patent 3,616,321 issued October 26, 1971,
to Albert Verheyden et alO As described in that patent,
adiponitrile is produced by electrolyzing an aqueous emulsion
of acrylonitrile, an acidic alkali metal salt of a polyacid such
as phosphoric acid and a surface-active substance such as a
quaternary ammonium salt. According to that patent, selectivi-
ties on the order of 75-~3% can be achieved when such a process
is carried out in a single-compartment (undivided) cell having
:,
.
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~050~76
a graphite cathode and an iron or magnetite anode. However,
corrosion of that type of anode proceeds at such a high rate that
even with the use of an anode corrosion inhibitor such as an
alkali metal pyrophosphate or metaphosphate, the process must
be carried out at such low temperatures (preferably about 20C.)
that expensive refrigeration of the electrolysis medium is
required and at low enough current densities (typically less
than 0.1 amp per square centimeter of anode surface area) that
the productive capacity of such a cell is quite low. The
suitability of other mate~rials (e.g. nickel, lead, lead dioxide,
stainless steel and alloy steel) for use as the anode in
similar processes has been suggested in U.S. Patent 3,511,765
issued May 12, 1970, to Fritz seck et al., U.S. Patent
3,630,861 issued December 28, 1971, to Jean Bizot et al. and
U~S. Patent 3,689,382 issued September 5, 1972, to Homer M.
Fox et al. However, the materials just mentioned are likewise
subject to relatively rapid corrosion when used as the anode
in processes of the kind just mentioned.
To avoid the costs of using a cell-dividing membrane
and for other reasons including those referred to hereinbefore,
a process by which an olefinic nitrile, carboxamide or car-
boxylate can be electrolytically hydrodimerized in an undivided
I cell with high selectivity and a low rate of anode corrosion
I is highly attractive for commercial use.
Thus, in accordance with this invention, there is
i described the use of carbon steel as an anode in an undivided
J cell wherein olefinic compounds are hydrodimerized to desired
I hydrodimers, and wherein corrosion of the carbon steel anode
J,~ iS inhibited by having present quaternary phosphonium
ions in the aqueous solution of the olefinic compound containing
-- 2 --
.~ .
~050476
at least about 0.1% by weight of an alkali metal phosphate,
borate or carbonate.
In the process of this invention, improved efficiencies
in obtaining high yields of the desired hydrodimers and cor-
rosion inhibition of the carbon steel anode can be obtained by
having present in the aqueous solution mono-quaternary
phosphonium ions, or divalent polymethylenebis(trialkylphos-
phonium) ions.
Olefinic compounds that can be hydrodimerized by the
process of this invention include those having the structural
formula R2C=CR-X wherein -X is -CN, -CONR2 or -COOR', R is
hydrogen or R' and R' is C1-C4 alkyl (i.e., methyl, ethyl,
n-propyl,isopropyl, n-butyl, isobutyl or tert-butyl). Com-
pounds 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-methylene-
valeronitrile, 2-methylenehexanenitrile, tiglonitrile or 2-
ethylidenehexanenitrile; olefinic carboxylates such as, for
example, methyl acrylate, ethyl acrylate or ethyl crotonate;
- and olefinic carboxamides such as, for example, acrylamide,
methacrylamide, N,N-diethylacrylamide or N,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 ~ the aforedes-
cribed structural formula. Also presently of greater utility
~ , .
~ - 3 -
... . .
504'7~
in the process of this invention are those olefinic compounds
where R' is methyl or eth~l, and particularly acrylonitrile,
methyl acrylate and alpha-methyl acrylonitrile. Products of
hydrodimerization of such compounds include those having the
structural formula X-CHR-~R2-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, dimethyladipate
and diethyl-3,4-dimethyladipate; and paraffinic dicarboxamides
such as, for example, adipamide, dimethyladipamide and N,N'-
dimethyl-2,5-dimethyladipamide. Such hydrodimers can be em-
ployed 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 paraffinic diamines especially useful
in the production of high molecular weight polyamides.
Other examples of various olefinic compounds that can be hydro-
dimerized by the process of this invention and the hydrodimers
thereby produced are identified in the aforecited U.S. Patents
3,193,475-79 and '481-83.
The invention may also be described in terms of
electrol~zing an aqueous solution having dissolved therein
certain proportions of the olefinic compound to be hydro-
dimerized, quaternary phosphonium ions and an alkali metal
phosphate, borate or carbonate. 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 carried out
by electrolyzing the aqueous solution in an electrolysis
lOS047~i
medium containing the recited aqueous solution and a dis-
persed 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 in-
vention there may be suitably electrolyzed an aqueous solution
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 accordance with
the process of this invention. Such a minute proportion, if
present, would be typically less than 5% of the combined
weight of the aqueous solution and the undissolved organic
phase contained therein. In other embodiments, the 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 te.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 con-
tinuous process embodiments involving recycle of unconverted
olefinic compound and whether present in a minute or larger
proportion, such an organic phase would be 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 minor
amounts of organic hydrodimerization by-products, quaternary
phosphonium ions, etc. possibly also present. Typically, such
~()5(~7t~
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 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 electrolysis
medium as the process of this 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.
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% cf the aqueous solution, more
typically at least about 0.5% of the aqueous solution and,
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 proportions
of the olefinic compound, e.g. at least about 5~ or even 10%
or more, but in many embodiments of the invention, the aqueous
solution contains less than about 5% (e.g. not more than ~%)
of the olefinic compound and, in some of those embodiments,
preferably not more than about 1.8% of the olefinic compound.
-- 6 --
~()50~76
The minimum required proportion of quaternary phos-
phonium ions is very small. In general, there need be only
an amount sufficient to provide the desired hydrodimer selec-
tivity (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 concentration 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 gram mol per liter of the solution.
10 Although higher proportions may be present in some cases, as
aforesaid, the quaternary phosphonium cations are generally
present in the aqueous solutlon in a concentration not higher
than about 0.5 gram mol per liter and even more usually, in
a concentration not higher than about I0 l ~ra~ mol per liter.
In some preferred embodiments, the concentration of quaternary
phosphonium ions in the solution is between about 10 4 and
about 10 2 gram mol per liter.
The quaternary ions that are present in such concen-
trations are those positively charged ions in which a
phosphorus atom has a valence of five and is directly
linked to other atoms (e.g. carbon3 satisfying four fifths
of that valence. Such cations need contain only one penta-
valent phosphorus atom but may contain more than one of such
pentavalent atoms as in, e.g., various multivalent multi
quaternary ions such as the bis-quaternary phosphonium ions
referred to hereinafter. Suitable mono-quaternary ions may
be cyclic, but they are more generally of the type in which a
pentavalent phosphorus atom is directly linked to a total of
four monovalent organic groups preferably devoid of olefinic
30 unsaturation and desirably selected from the group consisting
of alkyl and aryl radicals and combinations thereof. Suitable
,
v ~, -- 7 --
~05~)47~
multi-quaternary phosphonium i~ns may likewise by cyclic,
and they are typically of a type in which the pentavalent phos-
phorus atoms are linked to one another by at least one divalent
organic (e.g. polymethylene) radical and each further substi-
tuted by monovalent orsanic groups of the kind just mentioned
sufficient in number tnormally 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 mono-
valent 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 containing from one to twelve carbon atoms.
Although quaternary phosphonium cations containing a combina-
tion of such alkyl and aryl groups (e.g. benzyltriethylphos-
phonium) 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-, ethyltripropyl-,
ethyltributyl-, ethyltriamyl-, ethyltrihexyl-, octyltriethyl-,
tetrapropyl-, methyltripropyl-, decyltripropyl-, methyltributyl-,
tetrabutyl-, amyltributyl-, tetraamyl-, tetrahexyl-, ethyltri-
hexyl-, diethyldioctyl -phosphonium 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 containing at least
` three C2-C5 alkyl groups, e.g. methyltributyl-, tetrapropyl-,
ethyltriamyl-, octyltriethylphosphonium,etc. Particularly
~- .
_ ~ _
-
()47~
useful are the C8-C16 tetraalkylphosphonium ions containing
at least three C2-C4 alkyl groups. Similarly good results
are obtained by use of the divalent polymethylenebis(trialkyl-
phosphonium) ions, particularly those containing a total of
from 17 to 36 carbon atoms and in which each trialkylphosphoniu~
radical contalns 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 standpoint are the C18-C32 polymethylenebis
(trialkylphosphonium) ions 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 polymethylenebis(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 hexamethylene-
bis(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-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
.
' : , ' ' . ~ - . ' '
~ 050476
hydroxide or a salt (e.g. a Cl-C2 alkylsulfa~e) 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 tetraalkyl-
phosphonium ions of the type described hereinbefore, they
tend to distribute themselves in higher proportion toward
the aqueous phase of a mixture of an aqueous solution of the
type electrolyzed 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 organic phase is present in substan-
tial proportion in the aqueous solution during the electroly-
sis, product hydrodimer is generally 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 equi]ibrated, 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 product
which is normally substantially more soluble in the olefinic
compound than in the electrol-yzed aqueous solution. Normally,
the hydrodimer product is separated from said first portion
of the mixture (e.g. by distillation) while a second portion
.
of the mixture comprisin~ an aqueous solution of the type
subjected to electrolysis in accordancewith the present inven-
tion is recycled and the aqueous solution comprised by said
-- 10 --
~05~76
second portion is subjected to ~ore of such electrolysis. In
process embodiments in which the hydrodimer product is separated
from the electrolyzed solution in t~ 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 a~ueous portion of a sub-
stantially equilibrated 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, and despite their generally higher carbon content,
various bis-quaternary cations of the class defined herein-
before have been found to distribute themselves toward the
aqueous solution in ratios significantly higher ~e.g. up to
at least 3-4 times high~r) than those of the corresponding
mono-quaternary cations.
The alkali metal salts which can be employed in the
invention are those of sodium, potassium, lithium, cesium
and rubidium. Generally preferred for economic reasons are
those of lithium and especially sodium and potassium. Also
preferred for such use are the alkali metal salts of inorganic
and/or polyvalent acids, e.g. an alkali metal orthophosphate,
borate or carbonate, and particularly an incompletely-
substituted salt of that type, i.e. a salt in which the anion
has at least one valence satisfied by hydrogen and at least
one other valenae satisfied by an alkali metal. Examples
of such salts include disodium phosphate (Na2HPO4), potassium
acid phosphate (KH2PO4), sodium bicarbonate (NaHCO3) and
dipotassium borate (K2HBO3). Also useful are the alkali metal
salts of condensed acids such as pyrophosphoric, metaphos-
. .
.
. . . .- ,~ ,, ~ . .
I osoY ~6
phoric, metaboric, py~oborîc and the like (e.g. sodium
pyrophosphate, potassium metaborate, etc.). Depending on the
acidity of the aqueous solution to be electrolyzed, the stoi-
chiometric 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 diso-
dium phosphate, and accordingly, such mixtures of salts (as
well as mixtures of salts of different alkali metals and/or -
different acids) are intended to be within the scope of the
expression "alkali metal phosphate, borate or carbonate" as
used herein. In fact, it has been found that the rates of
corrosion of the carbon steel anodes employed in the process
of this invention are significantly and surprisingly lower
when the electrolyzed solution has dissolved therein certain
mi~tures of such salts including mostnotably an alkali metal
phosphate and an alkali metal borate. 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
solution should be at least sufficient to substantially
increase the electrical conductivity of the solution above
its conductivity without such a salt being present. In
general, there is also enough alkali metal salt dissolved in
i the solution to provide alkali metal cations 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 alkali metal salt. More advantageous conductivity
levels are achieved when the solution has dissolved therein
at least about 1% of alkali metal salt or, even more preferably,
at least about 2% of such a salt. In many cases, optimum
- 12 -
. ~
~ . . . .. ~ . . ~
~()S047~
process conditions include the solution having dissolved
therein more than 5~ (typically at least 5.5~) of alkali metal
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 phosphates and/or borates, it is generally most
convenient when the solution contains between about 1% and
about 15% of such a salt or mixture thereof~ When the solution
contains an alkali metal phosphate and an alkali metal borate,
especially low rates of anode corrosion are normally achieved
when the solution has dissolved therein at least about 0.5%
and preferably at least about 2% of alkali metal phosphate
and at least about 0.25%, preferably at least about 0.5%
but in some cases desirably not more than about 4% of alkali
metal borate.
The acidity of the solution is preferably such that a
neutral or alkaline condition prevails at the cathode. Since
there is 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 is at least about
five, preferably at least about six and 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 eleven and, with the use of sodium or potassium phosphates
and/or borates as the main conductive salts, generally not
substantially higher than about ten.
The temperature of the solution may be at any level
compatible with e~istence as such of the solution itself, i.e.,
above its freezing point b~t 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
- 13 -
: . ~ . : , . . - , .
: . .
. , . .. . . ~:
- . . - . ~ . -. .
. .
~OSV~7~
temperature range will vary with the specific olefinic compound
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 65C. are especially desirable. In fact,
it is an important advantage of the present invention that it
can be carried out at such relatively high temperatures without
an economically intolerable rate of anode corrosion.
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 electrode surface of at least about
0.6 meter per second and even more preferably between about
0.9 and about 2.44 meters per second although a solution
velocity up to 6 meters per second or higher can be employed,
if desired. The gap between the anode and cathode can be
very narrow, e.g. about 1.0 millimeter or less, or as wide as
1.27 centimeters or even wider, but is usually most conven-
iently of a width between about 1.5 and about 6.35 millimeters.
ff As is well-known, electrolytic hydrodimerization ofan olefinic compound having a formula as set forth herein-
before must be carried out in contact with a cathodic surface
1 ~ ~
7 having a cathode potential sufficient for hydrodimerization
~' of that compound. In the process of this invention, the
., ~
3~ cathodic surface can be made of virtually any material at
¦ which such a cathode potential can be provided and which is
not dissolved, corroded or fouled by the electrolysis medium at
~ an intolerable rate. In general, the process is most
s ~ 30 desirably carried out with a cathode consisting essentially
;~ of cadmium, mercury, thallium, lead, zinc, tin (possibly not
suitable with some nitrile starting materials) or graphite,
.
- 14 -
:::
1a~5C~47~i
by which is meant that the cathodic surface contains a high
percenta~e ~yenerall~ at least about 95% and prefer~bly at
least about 98%1 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 other constituents that do not alter the
nature of the cathodic surface so as to prevent substantial
realization of the advantages of the present invention, parti-
cularly as describea herein. Such other constituents, if
present, are desirably other materials having relatively high
hydrogen overvoltages. Of particular preference are cathodes
consisting essentially of cadmium, mercury, thallium, lead
or an alloy of at least one of such metals, and especially
cathodes consisting essentially of ca~mium.
Cathodes employed in this invention can be prepared
by various techniques such as, for example, electroplating
of the desired cathode material on a suitably-s~aped substrate
of some other material, e.~. 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 con-
figuration consisting essentially of the desired cathode
material may be used without such a substrate, if convenient.
As aforesaid, the process of this invention is carried
out in an electrolytic cell having an anode consisting essen-
tially of carbon steel. By that is meant that the portion
of the positive pole of the cell that is in contact with the
solution undergoing electrolysis consists essentially of a
steel of a type conventionally recognized as a carbon steel
and not as iron or an alloy steel or stainless steel. A
standard definition of carbon steel, provided by the American -
Iron and Steel Institute (AISI) is as follows: "Carbon steel
is classed as such when no minimum content is specified or
~, - 15 -
105047~
guaranteed for aluminum, ch~omium, columbium, molybdenum,
nickel, titanium, tungsten, vanadium or zirconium; when the
minimum for copper does not exceed 0.40 percent; or when
the maximum content specifi~d or guaranteed for any of the
following elements does not exceed the percentages noted:
manganese 1.65, silicon 0.60, copper 0.60." Carbon steels
of various compositions are listed in the 1000, 1100 and 1200
series of AISI and SAE standard steel composition numbers,
many of which may be found on page 62 of Volume 1, Metals Hand-
book, 8th Edition (1961) published by the American Society
for Metals, Metals Park, Ohio. Carbon steels are readily
distinguishable from steels conventionally known as alloy
steels and listed in the 1300 and higher series of the
aforementioned standard steel composition numbers, from the
special alloy steels that are conventionally known as stainless
steels and normally contain substantial (usually more than
0.5~) other metals such as nickel and/or chromium, and from
commercially-pure iron which, by definition, contains not
more than about 0.01% carbon. In general, the carbon steels
that can be used as anode materials in the process of this
invention contain 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. Such proportions are expressed, of
course, without reference to any constituents of the electroly-
sis medium althou~h in operation of the process, certain of
those constituents may become associated with the surface of
the anode, either transiently or otherwise, so as to act as a
part of the anodic surface in the sense of serving as the
positive pole of the electrolytic cell. For example, in
- 16 -
~ '
~050~76
some embodiments of the process r it may be desirable to include
in the electrolysis medium a small amount (generally between
about 0.01~ and about 3%) o~ an inhibitor of corrosion of the
anode (e.g. an alkali metal salt of a condensed phosphoric
acid, such as tetrasodium pyrophosphate or the like) and/or a
similarly small amount of a heavy metal chelating agent ~e.g.
an alkali metal salt of a nitrilocarboxylic acid, such as
tetrasodium ethylenediaminetetraacetate or -t~trapropionate,
trisodium hydroxyethyleth~lenediaminetriacetate, trisodium
nitrilotriacetate or the like) and one or both of the same may
in some cases (generally in only very minor quantities)
become so associated with the carbon steel anode surface.
In operation of the undivided cell, in which an anode
and a cathode of the cell are simultaneously in direct physical
contact with the solution being electrolyzed, each anode may
be in the form of a plate, sheet, strip, rod or any other
suitable configuration. In a preferred embodiment, the anode
is a sheet of carbon steel closely spaced from and essentially
parallel to a sheet-like cathode in the same cell. As
typically used in the process of this invention, carbon steel
anodes are relatively inexpensive, highly conductive, have
good mechanical properties including excellent structural
strength and, as aforesaid, corrode at a rate that is lower,
to a surprisingly great degree, than the corrosion rate of
anode materials previously suggested for similar use. The
corrosion rate of the carbon steel anode is particularly and
importantly lower at relatively high current densities,
permitting much greater hydrodimer productivity in a cell
having a given anode surface area in contact with the
3~ electrolysis medium and thereby available for passage of the
electric current employed in the process of this inven~ion.
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1050476
In yeneral, there is no minimum current density with
which the present process can be carried out but economic
considerations usually requ~re the use of a current density of
at least about 0.01 and preferably at least about 0.05 amp per
square centimeter (amp/cm2~ o~ the anode surface in contact with
the solution being electrolyzed. Similar processes employing
more readily corroded anodes have had to be generally carried
out, as aforesaid, with current densities substantially lower
than 0.1 amp/cm2 of such anode surface, but the present process
can be very conveniently carried out with anode current densi-
A~ ties of at least about 0.1 amp/cm2 and, usually even more
desirably from an economic standpoint, with anode current
densities of at least about 0.15 amp/cm~ or even much higher.
Although greater anode current densities may be practical in
some instances, those employed in the present process are
generally not higher than about 0.75 amp/cm2 and even more
typically not higher than about 0.5 amp/cm of the anode
surface area in contact with the solution being electrolyzed. -
Depending on other process variables, anode current densities
of not more than about 0.35 amp/cm2 may be preferred in some
embodiments of the invention. However, the fact that any
of the aforementioned anode current densities of at least
., " .
about 0.1, and particularly at least about 0.15 amp/cm
can be advantageously employed in the process of this invention,
and especially at temperatures higher than about 25C.,
e.g. from about 40 up to about 65C. or even higher,
is very surprising in view of the much greater corrosion
:::
~ of other anode materials such as iron, magnetite, etc.,
, ~::
,~ - 18 -
,
105047~
in similar process use but under conditions generally con
sidered far less corrosive, e.g. at much lower temperatures
and/or anode current densities. As will be readily
apparent, the use of carbon steel anodes having advantages
of that magnitude in a process from which there are
available hydrodimerization selectivities of at least
about 75%, typically at least about 80% and commonly
as high as 85~ or even higher has provided that process
with a significant and unexpected commercial utility.
In addition, carbon steel does not contain substantial
proportions of other metals (such as nickel, etc.)
which are present in stainless and other alloy steels
and which, if released into the electrolysis medium, e.g.
by corrosion of an anode containing such metals, may tend
to plate out or otherwise become deposited on the cathode
and thereon alter the nature of the cathodic surface so
as to increase the generation of saturated starting
material (e.g. propionitrile) at the expense of hydrodimer
selectivity.
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 the following Examples, acrylonitrile
and adiponitrile are generally represented by AN and
A~N, respectively.
.
-- lg --
,
: . . - . . . ~
~050476
EXAMPLE I
In a continuous process, a liquid electrolysis medium
composed of about 99% by (1) an aqueous solution having dissolved
therein between 1.4% and 1.6% AN, about 1.2% ADN, 10% of a mix-
ture o~ sodium orthophosphates, 0.6-1.4 x 10 3 mol per liter
of methyltribut~lphosphonium ions, about 0.5~ of Na4EDTA and
the sodium borates produced by neutralizing orthoboric acid
- in an amount corresponding to about 2% of the solution to the
s.dution 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 byproducts and 8% 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 âadmium cathode composed of
cadmium conforming to ASTM 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/cm of the surface of the
anode (or cathode). Organic phase containing pxoduct ADN,
, AN E~D byproducts and unreacted AN was separated from the
i 20 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
o~ current passed through the medium, 0.4 millimol of
Na4EDTA was added to the circulating medium and about 12 grams
~ th solution were purged from the system and replaced with
water containing sufficient dissolved methyltributylphosphonium
ions and sodium orthophos~hates and borates to maintain the
concentrations of those constituents of the solution at the
aforedesc~ibed:levels and the total volume o~ the medium
,~ :30 essentlally constant. After 120 hours of electrolysis under
,
....
: i
,: . :, .. , . ,..... ,, ;
1050~7~
those conditions, it was ~ound that AN had been converted to
ADN with avera~e and ~inal selectivities of 88% and ~he steel
anode had corroded at an average rate substantially lower than
0.5 millimeter per ~Par.
Although the rate of corrosion of the carbon steel
anode is surprisingly low when the electrolysis medium contains
an alkali metal phosphate but no alkali metal borate (e.g.
0.97 and 0.86 millimeter per year~, the rate of such corrosion
is signi~icantly and even more surprisingly further lowered
when the electrolysis medium contains an alkali metal phosphate
and an alkali metal borate being no higher than 0.5 millimeter
per year as shown in Example I.
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
, 20 same as those obtained in Example I.
Hexamethylenebis(tributylphosphonium)
: Hexamethylenebis(amyldipropylphosphonium)
Hexamethylenebis(tripropylphosphonium)
Hexamethylenebis(methyldibutylphosphonium)
Hexamethylenebis(ethyldihexylphosphonium)
Hexamethylenebis(decyldiethylphosphonium)
Pentamethylenebis(propyldibutylphosphonium)
Pentamethylenebis(triamylphosphonium)
Tetramethylenebis(ethyldibutylphosphonium)
Tetramethylenebis(octyldipropylphosphonium)
Heptamethylenebis(ethyldibutylphosphonium)
- 21 -
. .
~05047~
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 ion$, 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
- 22 - :
