Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
O.Z. 0050/43334
Simultaneous production of dicarboxylic acids and
diamines from polyamides
The present invention relates to a process for
the simultaneous production of dicarboxylic acids and
diamines from
a) polymers based on polyamides of dicarboxylic acids
or their derivatives with diamines, or
b) compositions containing essentially such polymers,
by splitting these polymers into their monomeric con-
stituents.
The splitting of polyamides such as nylon 66 (PA
66) into their monomeric constituents can be carried out
in a neutral or acid medium but in general it is prefer
ably carried out in a basic medium, inter alia because of
the shorter reaction time.
FR-A-926 873 describes the splitting of poly-
amides such as PA 66 and PA 610 with inorganic bases, for
example with a from 10 to 15% strength by weight alkali
metal hydroxide solution such as sodium hydroxide solu-
tion, at 200°C and about 15 bar. The resulting diamine is
then extracted or distilled out of the reaction mixture
and further purified by vacuum distillation. According to
this reference, the free dicarboxylic acid is obtained by
addition of a strong acid such as hydrochloric acid to
the diamine-free reaction mixture and subsequent preci-
pitation.
In IT-A-553 182 an excess of 20~ strength by
weight of sodium hydroxide solution at 220°C and 25 bar
reduces the reaction time compared with the process of
FR-A-926 873. The diamine is extracted from the aqueous
solution with n-butanol. One example concerns the removal
of insoluble titanium dioxide, previously present in the
polymer in the form of fibers, by filtration after the
splitting. The dicarboxylic acid is likewise freed by
addition of a strong mineral acid.
FR-A-1 070 841 describes the splitting of PA 66
- ~~~~ O.Z. 0050/43334
with alkali metal or alkaline earth metal hydroxide
solutions. According to this reference, the reaction
mixture is initially worked up by acidifying with sul-
furic acid and then the precipitated adipic acid is
separated off. Thereafter the filtrate is admixed with
potassium hydroxide solution, which brings down hexa-
methylenediamine as an oily layer which can be separated
off and purified. This reference also describes the
splitting and workup of polymers and copolymers that
contain polycaprolactam (PA 6).
DE-A-1 088 063 describes the splitting of PA 66
in a 10% strength by weight methanol NaOH solution. The
disodium adipate obtained is converted into the free acid
by acidification, while hexamethylenediamine (H1~) can be
obtained in pure form by distillation.
US-A-2 840 606 describes the splitting of PA 66
into disodium adipate and HIS in an isopropanol/water
mixture. According to this teaching, the HIS is isolated
from the alcohol phase by distillation. The adipic acid
is obtained by acidifying the aqueous phase with sulfuric
acid and may be purified by crystallization.
DE-A 39 26 642 describes a process and an
apparatus based on a four-compartment electrolysis cell
for obtaining an acid from its salt. However, no mention
is made of reaction parameters and examples in
DE-A 39 26 642.
A feature common to all these processes is the
isolation of adipic acid through acidification of the
respective alkali metal or alkaline earth metal salt
solutions. The inevitable inorganic salt coproduct,
usually sodium chloride or sodium sulfate, not only
interferes with the attempt to purify the dicarboxylic
acid by crystallization, since it inhibits the latter,
but also constitutes a considerable disposal problem.
A further disadvantage is that the processes
described cannot be suitably employed for working up
technical, for example fiber-reinforced, mineral-filled
AMENDED SHEET
2138154
3
and/or impact-modified, PA 66 molding compositions, since
the various additives would disrupt the smooth running of
the processes described.
It is an object of the present invention to
provide a process for the simultaneous production of
dicarboxylic acids and diamines that shall be free of the
abovementioned disadvantages.
We have found that this object is achieved by a
process for coproduction of dicarboxylic acids and diamines
from
(a) polymers based on polyamides of dicarboxylic acids or
their derivatives with diamines, or
(b) materials comprising essentially such polymers,
by cracking these polymers into their monomeric
constituents using a base in an aqueous medium and removing
the resulting diamines, characterized in that the base is
dissolved in a mixture of from 100 to 75% by weight of
water and from 0 to 25% by weight of a C1-C4-alkanol and
the resulting dicarboxylic acid salts of adipic acid or
sebacic acid are converted into the corresponding
dicarboxylic acids and bases by electrochemical means.
Suitable polymers based on polyamides of
dicarboxylic acids or their derivatives, for example the
corresponding acid halides, preferably the acid chlorides,
with diamines are from observations to date poly-
hexamethyleneadipamide, polyhexamethylenesebacamide and
polytetramethyleneadipamide, preferably polyhexamethylene-
adipamide.
Suitable compositions containing essentially, ie.
to at least 50% by weight, such polymers also include for
example copolyamides with PA 66 and also PA 66 or copoly
amides with PA 66 containing fibers and/or additives.
The bases used for splitting the polymers are in
general alkali metal hydroxides such as lithium hydroxide,
sodium hydroxide and potassium hydroxide, preferably sodium
hydroxide, or mixtures thereof, preferably a mixture of
sodium hydroxide and potassium hydroxide.
21381 5 4
3 a
A
It is preferable to use from 1.8 to 4.0,
preferably from 2.0 to 3.0, equivalents of alkali metal
- 4~~~~~~ ~ O. Z. 0050/43334
hydroxide per repeat unit of polymer, for example
- (- (CH2),,-CO-NH- (CHz) 6-NH-CO-] - in the case of PA 66. If
less than 1.8 equivalents of base are used, the result is
in general an undesirably high proportion of oligomer. If
more than 4.0 equivalents of base are used per repeat
unit, this leads in general, in particular in the case of
glass fiber-reinforced and/or mineral-filled polyamide
molding compositions, to a high degree of degradation of
the glass fibers or of the mineral filler.
In general, the alkali metal hydroxide is used in
the form of a from 5 to 25, preferably from 10 to 15,
strength by weight solution in water. If desired, instead
of water it is possible to use a water-C1-C4-alkanol
mixture which contains from 0 to 50, preferably from 0 to
25, % by weight of a C1-C4-alkanol or a mixture thereof.
The reaction is in general carried out at a
temperature within the range from 100 to 300°C, prefer
ably from 150 to 250°C. The pressure is in general within
the range from 0.1 to 10 MPa, although it is also
possible to employ a pressure outside this range. Prefer-
ence is given to working under the autogenous pressure.
Owing to the alkali metal hydroxide, the reaction
mixture pH is in general alkaline, preferably within the
range from 8 to 14.
The duration of the reaction depends essentially
on the concentrations of the starting materials, on the
temperature and on the pressure and will in general be
within the range from 0.5 to 15, preferably from 3 to 10,
h.
The splitting with a base can be carried out
continuously or batchwise.
It can be carried out in customary apparatus with
or without stirrer, preference being given to using a
pressure vessel equipped with a stirrer system that is
particularly suitable for solids dispersion, for example
a propeller stirrer or a cross-bar stirrer.
In a preferred embodiment, the starting polymer
21381 5 4
or polyamide-containing compositions are mechanically
comminuted to an average particle size of 0.1 to 50,
preferably from 1 to 10, mm before splitting. The
comminution can be carried out in a commercial mill, for
example in a cutting mill, or, preferably, in particular
when the compositions used contain hard materials such as
metal inserts, for example bolts, in a hammer mill.
Metal parts present in the material thus
comminuted can be removed in a drying separation process
using an air table, preferably with subsequent induction
separation, using for example a free-fall tube separator,
for complete removal of the metal parts, or in a wet
separation process, for example by means of a hydro-
cyclone.
Brief description of the drawings
Fig. 1 is a diagram of a three compartment electrolysis all
with three liquid cycles (KL1 to KL3).
Fig. 2 is a diagram of a four compartment electrolysis all
with four liquid cycles (KLl to KL4).
This invention will be better understood upon reading the
following non restrictive description and examples.
Detailed description of the invention
In a particularly preferred embodiment, the
polymer or composition feedstock is comminuted in a hammer
mill to a size of not more than 50 mm in length, any metal
parts present are separated off, and the millbase freed of
metal parts is then comminuted to a size within the range
from 5 to 12 mm in a cutting mill. If desired, the polymer
or polymer-containing composition thus pretreated can then
be additionally washed and dried before it is subjected to
splitting with a base.
2138154
5a
The reaction mixture obtained on splitting the
polyamides consists in general of a liquid phase, which
contains the diamine, the dicarboxylate salt, and insoluble
constituents.
According to the invention, the reaction mixture
obtained on splitting the polyamides is then subjected to
an electrochemical treatment to convert the dicarboxylate
salt into the corresponding dicarboxylic acid, for which it
is advantageous to remove troublesome impurities
beforehand.
For instance, in a further preferred embodiment,
constituents of the reaction mixture that are insoluble
after the polyamides have been split into their monomeric
constituents are removed.
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Examples of insoluble constituents are glass
fibers, carbon fibers, carbon black, minerals and rubber
and any metals not removed or not completely removed
beforehand, unless dissolved by the base.
Suitable processes for separating off the
insoluble constituents are known processes such as
filtration, sedimentation or centrifuging.
The removed insolubles may if desired be washed
in a further operation with water and/or an organic sol
vent, preferably with a C1-C4-alcohol or with the solvent
component used in the splitting with the base. This
operation can be carried out with the apparatus used for
the separation, for example a belt filter, and/or in a
further separating operation, in which case the
insolubles are in general first intimately mixed with the
water or solvent (mixture) used. The solids obtained at
this stage can if desired be further used as filler when
dry.
The filtrates from the washing operations can if
desired be combined with the filtrate from the reaction
mixture.
In a further preferred embodiment, the diamines
obtained in the base splitting are preferably separated
off after the insolubles have been separated off and
before the electrochemical treatment.
The removal of the diamines can be carried out by
known methods such as distillation, in particular recti-
fication, or extraction, an extraction with organic
solvents being preferable on energy grounds.
Prior to the extraction it can be advantageous to
remove volatile organic components, if present, for
example by rectification. The rectification can be
carried out in conventional apparatus (for example tray
columns or packed columns with arranged or dumped
packing).
To extract the diamines it is possible to employ
the customary known solvents (see DE-A-1,163,334) such as
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hal:ogenated hydrocarbons, for example chloroform or
methylene chloride, C,,-C8-alcohols such as n-butanol,
isobutanol, sec-butanol, 1-pentanol, 2-pentanol,
3-pentanol, neopentyl alcohol, 1-hexanol, 2-hexanol,
3-hexanol, 1-heptanol, 2-heptanol, 3-heptanol,
4-heptanol, 1-octanol, 2-octanol, 3-octanol, 4-octanol,
preferably n-butanol, isobutanol, C5-C8-cycloalkanes such
as cyclopentane, cyclohexane, cycloheptane, cyclooctane,
preferably cyclohexane, or aromatic hydrocarbons, prefer-
ably benzene and its C1-C4-alkyl derivatives such as
toluene and xylenes, or mixtures thereof. Particular
preference is given to using mixtures of benzene,
toluene, n-butanol and isobutanol. Very particular
preference is given to mixtures of from 25 to 60~ by
weight of benzene and/or toluene and from 40 to 75~ by
weight of a mixture of n-butanol and isobutanol with the
total isobutanol content of the mixture generally not
being higher than 40~ by weight.
The extraction can be carried out batchwise or
continuously, in general in conventional extraction
apparatus such as mixer-settlers or in pulsed or unpulsed
extraction columns (for example tray or packed columns).
The isolation of the diamines from the extract is
in general effected by rectification in a conventional
apparatus in which the diamines are advantageously
obtained in vapor form as a side takeoff from the
stripping portion of the rectification column. The
rectification is in general carried out at from 10 to
100 kPa, preferably at from 50 to 80 kPa.
Any diamine still present in the rectification
residue can if desired be separated therefrom in a
further distillation step, for example using a thin-film
evaporator, at from 0.5 to 50, preferably at from 2 to
30, kPa.
The extractant obtained at the top of the recti-
fication column is advantageously directly recirculated
into the extraction or freed in a further distillation
-~138154o,Z. 0050/43334
step of lower and/or higher boiling impurities.
The raffinate can if desired be subjected to a
further rectification in order to separate off dissolved
extractant and/or other volatile organic compounds.
Impurities that interfere with the
electrochemical treatment such as alkaline earth metal
cations, silica and polyphosphate anions or high mole-
cular weight organic amine compounds can advantageously
be removed from the aqueous solutions freed of insolubles
and diamines by treating these solutions for example with
adsorbents and/or suitable precipitants.
The adsorbents used are preferably activated
carbon or selective ion exchangers, if desired based on
chelates. Suitable precipitants include for example car-
bonates of alkali metals and/or ammonium carbonates.
From observations to date the manner of the
electrochemical treatment has in principle no bearing on
the success of the process of the invention.
The electrochemical treatment may for example
take one of the following forms (a) to (f):
(a) In this version the splitting of the dicarboxy-
late salt into the corresponding dicarboxylic acid
and the corresponding base can be carried out in a
two-part electrodialysis cell using bipolar
membranes. In general, the electrodialysis cell has
between the anode and the cathode from 1 to 200,
preferably from 20 to 70, electrodialysis units
separated from one another by bipolar membranes . The
bipolar membranes are separated from one another by
cation exchange membranes, so that an electrodialy-
sis unit has the following structure: bipolar
membrane (anode side) - anolyte compartment - cation
exchange membrane - catolyte compartment - bipolar
membrane (cathode side). The individual
electrodialysis units are preferably electrically
connected in series.
In this version it is advantageous to feed the
~13815~ ~.Z. 0050/43334
aqueous dicarboxylate salt solution into the anolyte
compartment. In the electric field of an applied
direct voltage the alkali metal cations generally
migrate through the cation exchange membrane into
the catolyte compartment. The hydroxyl anions
required for compensating the separated charges are
formed by the dissociation of the water in the
bipolar membranes on the cathode side. In this way
the corresponding alkali metal hydroxide solution
collects in the catolyte compartment. In the anolyte
compartment the dicarboxylate anion can combine with
the hydrogen ions from the bipolar membrane on the
anode side to form the free dicarboxylic acid.
It is advantageous to feed the dicarboxylate salt
solution into the anolyte compartments in parallel.
The product streams from the anolyte compartments,
containing the free acid and unconverted dicarboxyl
ate salt, and the product streams from the catolyte
compartments are advantageously combined with one
another. The free dicarboxylic acid is in general
obtained by crystallization from the combined
product streams from the anolyte compartment without
coprecipitation of the corresponding dicarboxylate
salt, which is preferably subjected again to the
electrodialysis process.
The electrodialysis process can be carried out
not only continuously but also batchwise. A prefer-
red form of the continuous process involving a
plurality of electrodialysis cells comprises
dividing the total conversion between from 2 to 20,
preferably from 4 to 6, electrodialysis cells and
effecting only partial conversion in each electro-
dialysis cell.
It is particularly advantageous here to guide the
3 5 f lows in countercurrent . The outf low from an anolyte
compartment forms the inflow into the next anolyte
compartment, etc., so that the outflow from the last
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anolyte compartment is rich in dicarboxylic acid and
lean in dicarboxylate salt. The outflow from the
last catolyte compartment, containing a low
concentration of alkali metal hydroxide, forms the
inflow into the last but one catolyte compartment,
etc., so that the first unit has a high
concentration of dicarboxylate salt in the anolyte
compartment and a high concentration of alkali metal
hydroxide in the catolyte compartment. The result is
that the alkali metal hydroxide concentration
differences in the anolyte and catolyte compartments
are small within a unit. This ultimately leads in
general to an energy saving due to a higher current
yield and on average to lower cell voltages.
The current densities are in general within the
range from 0.1 to 2, preferably from 0.5 to 1.0,
kA/m~. The cell voltage is in general from 3 to 8,
preferably from 4 to 6, V per electrodialysis unit.
The pH is in general within the range from 2 to
10 in the anolyte compartments and within the range
greater than 13 in the catolyte compartments.
The compartment width is in general from 0.2 to
5, preferably from 0.5 to l, mm.
The electrodialysis temperature is in general
within the range from 40 to 110°C, preferably from
65 to 90°C.
The inflow and outflow velocities are in general
within the range from 0.05 to 0.2 m/sec.
The concentration of dicarboxylate salt used is
in general from 5 to 40~ by weight, preferably from
10 to 20~ by weight.
If desired, the conductivity in the anolyte
system can be increased by adding salts or acids
such as sodium sulfate or sulfuric acid. Substances
of this type are in general added within the range
from 0.1 to 10~ by weight, preferably from 1 to 60
by weight, based on the total weight of the solution
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present in the anolyte compartment.
To the catolyte compartment it is advantageous to
add the substances which are obtained in the course
of the operation, preferably the corresponding
alkali metal hydroxide such as sodium hydroxide or
potassium hydroxide, preferably sodium hydroxide.
The inflow into the catolyte compartment general-
ly comprises fully demineralized water, but at the
beginning it is preferable to employ the from 1 to
25, preferably from 5 to 10, o strength by weight
alkali metal hydroxide solution formed in the course
of the electrodialysis.
(b) A three-part electrodialysis cell with bipolar
membranes has the advantage over the procedure
described under (a) that the feed materials need not
be very pure. Furthermore, generally significantly
lower salt contents are obtained not only in the
dicarboxylic acid solution obtained but also in the
corresponding alkali metal hydroxide solution.
The three-compartment system contains not only a
cation exchange membrane but also an anion exchange
membrane, so that the structure of an electrodialy-
sis unit is as follows: bipolar membrane (anode
side) - anolyte compartment - anion exchange
membrane - center compartment - cation exchange
membrane - catolyte compartment - bipolar membrane
(cathode side) .
The dicarboxylate salt solution is advantageously
introduced into the center compartment. Under the
influence of a direct current electric field the
dicarboxylate anions generally migrate through the
anion exchange membrane into the anolyte compart-
ment, where they can combine with the hydrogen ions
present there to form the free acid. Apart from
selectivity losses of the anion exchange membrane
the free acid can be withdrawn from the anolyte
compartment devoid of salt. As in (a) the catolyte
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compartment yields the alkali metal hydroxide
solution. The outflow from the center compartment,
still containing residual quantities of
dicarboxylate salt, can be disposed of or
advantageously added again to the feed for the
electrodialysis process. Again as in (a) the flows
can be guided countercurrently in order to increase
the current yield.
To increase the conductivity the anolyte compart
went can have added to it for example an oxo acid
such as sulfuric acid, phosphoric acid or nitric
acid.
The catolyte compartment can advantageously have
added to it the substances which are obtained in the
course of the operation, preferably the correspond
ing alkali metal hydroxide such as sodium hydroxide
or potassium hydroxide, preferably sodium hydroxide.
As for the rest, the process of (b) can be
carried out under the same conditions as described
2 0 under ( a ) .
(c) In principle it is also possible to use electro-
dialysis cells having four compartments. The layout
generally resembles that of an electrodialysis cell
with three compartments except that, to protect the
bipolar membranes from possible fouling, a further
ion exchange membrane, preferably a cation exchange
membrane, is included. In general, an electrodialy-
sis unit will have the following structure: bipolar
membrane (anode side) - anolyte compartment - cation
exchange membrane - anode-near center compartment -
anion exchange membrane - cathode-near center
compartment - cation exchange membrane - catolyte
compartment - bipolar membrane (cathode side).
The dicarboxylate salt solution is advantageously
introduced into the cathode-near center compartment
with the dicarboxylic acid solution being withdrawn
from the anode-near center compartment and the
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alkali metal hydroxide solution from the cathode
compartment.
In other respects, the process of (c) can be
carried out under the same conditions as described
under (b).
(d) The electrochemical cleavage of the dicarboxylate
salt into the dicarboxylic acid and the correspond-
ing base can be carried out in a further embodiment
in a two-part membrane electrolysis cell known per
se from chlor-alkali electrolysis. The membrane
electrolysis cell comprises in general from 1 to
100, preferably from 20 to 70, electrolysis units
grouped together in a block. In this block, the
individual electrolysis units can be electrically
connected in series by electrically connecting the
cathode of one unit to the anode of the next unit or
by using internally connected bipolar electrodes.
The products generally flow in and out via separate
collector lines for each compartment type. The two-
part membrane electrolysis unit generally has the
following structure going from the anode to the
cathode:
anode - anolyte compartment - cation exchange
membrane - catolyte compartment - cathode.
The aqueous dicarboxylate salt solution is advan-
tageously introduced into the anolyte compartment.
Under the electric field of the applied direct
voltage the alkali metal cations generally migrate
through the cation exchange membrane into the
catolyte compartment, where they are converted into
alkali. The hydroxyl anions required for compensat-
ing the separated charges are released in the
cathode reaction. The cathode reaction can be for
example the cathodic evolution of hydrogen or a
cathodic reduction of oxygen. The anolyte compart-
went generally retains the organic acid radical
which combines with the hydrogen ions or their
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hydrated forms released in the course of the anode
reaction to form the corresponding free acid. An
example of an anode reaction is the anodic evolution
of oxygen or the anodic oxidation of hydrogen. The
anode compartment will thus have in general become
leaner in the salt and richer in the free
dicarboxylic acid.
The membrane electrolysis process can be carried
out not only batchwise but also continuously. If it
is carried out over the continuous process, one
option is to divide the conversion between from 2 to
20, preferably from 4 to 6, cells and to guide the
flows countercurrently (see (a)).
The dicarboxylate salt solution used, which may
contain a plurality of such salts, has in general a
concentration of from 1% by weight up to the satura
tion limit of the salt(s), preferably from 5 to 35,
particularly preferably from 15 to 30, % by weight.
The current densities are in general within the
range from 0.5 to 10, preferably from 1 to 4, kA/m2.
The cell voltage is in general from 3 to 10 V,
preferably from 4 to 6 V, per membrane electrolysis
unit.
The pH is in general within the range from 2 to
10 in the anolyte compartment and within the range
greater than 13 in the catolyte compartment.
The compartment width is in general from 0.5 to
10, preferably from 1 to 5, mm.
The temperature selected for carrying out the
membrane electrolysis process is in general within
the range from 50 to 110°C, preferably from 65 to
90°C.
To ensure mass transport, the compartment con
tents are in general recirculated either by means of
3 5 pumps or through natural convection, ie . through the
mammoth pump effect due to gas evolution at elec-
trodes. The flow velocities in the compartments are
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in general within the range from 0.05 to 0.5,
preferably from 0.1 to 0.2, m/sec.
(e) A particularly preferred embodiment is the
electrochemical splitting of the dicarboxylate salts
into the corresponding dicarboxylic acids and bases
in a three-part membrane electrolysis cell.
The three-part membrane electrolysis unit has in
general the following structure:
anode - anolyte compartment - cation exchange mem
brane - center compartment - cation exchange mem
brane - catolyte compartment - cathode.
The aqueous dicarboxylate salt solution is in
general introduced into the center compartment. To
increase the electric conductivity in the center
compartment, a mineral acid or a salt can be added
to the center compartment electrolyte. Examples are
sulfuric acid, nitric acid, sodium sulfate and
sodium nitrate.
The center compartment generally retains the
organic acid radical, which can react with the
hydrogen ions liberated in the course of the anode
reaction and which have migrated into the center
compartment through the anode-side cation exchange
membrane to form the free acid. The acid is in
general removed from the center compartment system
together with unconverted salt. The anolyte used can
be an aqueous mineral acid such as sulfuric acid,
nitric acid or hydrochloric acid, preferably
sulfuric acid. The anolyte's essential function is,
together with the anode-side cation exchange mem-
brane, to protect the organic dicarboxylic acid from
anodic oxidation.
As for the rest, the process of (e) can be
carried out under the conditions described at (d).
(f) The electrochemical cleavage of the dicarboxylate
salts into the corresponding dicarboxylic acids and
bases can also be carried out in a four-part
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membrane electrolysis cell.
The four-part membrane electrolysis unit general-
ly has the following structure:
anode - anolyte compartment - cation exchange mem
brane - anode-near center compartment - anion
exchange membrane - cathode-near center compartment
- cation exchange membrane - catolyte compartment
cathode.
The aqueous dicarboxylate salt solution is advan
tageously introduced into the cathode-near center
compartment.
To increase the electric conductivity in the
center compartment, a mineral acid or a salt such as
sulfuric acid, nitric acid, sodium sulfate or sodium
nitrate can be added to the center compartment
electrolyte.
The acid anion generally passes from the cathode-
near center compartment into the anode-near center
compartment, where it reacts with hydrogen ions,
which are evolved in the course of the anode reac-
tion and pass into the anode-near center compartment
through the anode-side cation exchange membrane, to
form the free acid. The acid is in general withdrawn
from the center compartment system in high purity.
The remaining salt solution is in general withdrawn
from the cathode-near center compartment and recir-
culated into the adipate dissolution stage as a
part-stream or disposed of. The anolyte used is in
general an aqueous mineral acid, preferably sulfuric
acid. The anolyte's essential function, together
with the anode-side cation exchange membrane, is to
protect the organic acid from anodic oxidation.
As for the rest, the process of (f) can be
carried out under the conditions mentioned at (d).
In the above-described alternatives the cation
exchange membranes used are particularly preferably
polymers based on perfluorinated olefins or copolymers of
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styrene and divinylbenzene containing sulfonic acid and
if desired carboxyl groups as charge carriers. Very
particular preference is given to using membranes that
contain sulfonic acid groups only, since in general they
are more resistant to fouling by multivalent cations than
other membranes. Membranes of this type are known (for
example Nafion~ membranes of type 324). They consist of
a copolymer of tetrafluoroethylene with a perfluorinated
monomer that contains sulfone groups. In general they
have a high chemical and thermal stability. The ion
exchange membrane can be reinforced with a Teflon support
fabric. It is also possible to use copolymers based on
styrene and divinylbenzene.
Suitable anion exchange membranes are for example
the membranes described in detail in EP-A-449,071 so no
details will be given here.
The electrode materials used can be in general
perforated materials, constructed for example in the form
of nets, lamellae, oval profile webs or round profile
webs.
The oxygen overvoltage at the anodes is in
general set at less than 400 mV within the current
density range according to the invention in order that
the formation of ozone or per-compounds may be prevented.
Suitable anode materials of low oxygen overvol-
tage are for example titanium supports with electrically
conducting interlayers of borides and/or carbides and/or
silicides of subgroups IV to VI such has tantalum
borides, titanium borides or titanium suboxide, doped or
undoped tin oxides, or tantalum and/or niobium with or
without platinum metal doping, whose surface has in
general been doped with electrically conducting, non-
stoichiometric mixed oxides of subgroups IV to VI and
metals or metal oxides of the platinum group or platinum
metal compounds such as platinates. Atop these inter-
layers is in general the active electrode material, which
preferably consists of mixed oxides of tantalum with
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iridium, platinum or rhodium and platinates of the type
Lio,3Pt3O,,. To enlarge the surface area it is customary to
use superficially roughened or macroporous titanium
supports.
The cathodes are in general made of electrode
materials having a low hydrogen overvoltage in order to
avoid additional voltage losses in the membrane
electrolysis or electrodialysis cell. Suitable cathodes
are for example iron or nickel supports which have been
surface coated with finely divided cobalt, nickel,
molybdenum, tungsten, manganese, Raney metal compounds of
nickel or of cobalt, nickel- or cobalt-aluminum alloys,
or nickel-iron alloys or cobalt-iron alloys containing
from 65 to 90~ by weight of iron.
To improve selectivity and membrane life the
cathode side can be equipped with cation exchange mem
branes containing hydroxyl ion blockers. The selectivity
can be further improved by keeping the level of calcium,
magnesium and aluminum ions and also the silica content
in each case below 5 ppm.
The dicarboxylic acid obtained by the electro-
chemical treatment is in general present as an aqueous
solution having a concentration within the range from 1
to 30, preferably from 4 to 30, o by weight. This solu-
tion can contain the conductivity salt, if present, in a
concentration within the range from 0.05 to 15, prefer-
ably from 0.06 to 6, ~ by weight and the mineral acid, if
present, in a concentration within the range from 0.05 to
15, preferably from 0 to 6, ~ by weight.
The alkali obtained according to the invention
generally contains an alkali metal hydroxide in a con-
centration within the range from 5 to 35, preferably from
15 to 25, ~ by weight.
Particularly preferably, the alkali metal
hydroxide solution obtained according to the invention
can be recirculated or otherwise used, in which case if
desired it can be concentrated beforehand in a
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conventional manner, for example by evaporation.
To obtain the dicarboxylic acid in pure form, it
is in general crystallized out of the solution obtained
according to the invention, then separated off, for
example by filtration, and dried.
The dicarboxylic acid is preferably obtained from
the electrodialysis or membrane electrolysis solutions by
cooling or evaporation crystallization. Then the dicar-
boxylic acids are in general separated from the resulting
suspensions, for example by filtration, decanting or
centrifuging.
The cooling crystallization is customarily
carried out at from 0 to 50°C, preferably at from 10 to
40°C, advantageously at pressures within the range from
1 to 100 kPa, preferably from 4 to 20 kPa.
The dicarboxylic acids separated off can be
preferably obtained in a pure form by washing, for
example with water or Cl-C4-alkanols, and if desired by
recrystallization. If a plurality of dicarboxylic acids
are present at the same time, the individual dicarboxylic
acids can be isolated in pure form by utilizing the
solubility differences in a conventional manner such as
fractional crystallization.
The aqueous solutions obtained by crystallization
and washing can be concentrated in a conventional manner
and resubjected to a crystallization, for example by
adding them to as-electrodialyzed or as-electrolyzed
solutions that have still to be crystallized. They can
also be for example added to the reaction mixture
obtained from the base treatment of the polymers or
compounds used.
One advantage of the process of the invention
over known processes is that it obviates the formation
and disposal of salts which are customarily obtained When
the dicarboxylic acids are freed from their salts by
acidification. A further advantage is that even fiber-
reinforced, mineral-filled and/or impact-modified molding
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20 - O.Z. 0050/43334
compositions can be processed. Furthermore, the sub-
stances produced by the process of the invention, such as
dicarboxylic acids, diamines and bases and also, as the
case may be, glass fibers and mineral fillers, can be
used for making new products.
EXAMPLE 1
300 g of a nylon 66 having a viscosity number
(VN) - 149 (unit: 1 cm'/g (measured on a 0.5o strength by
weight solution of the nylon in 96~ strength by weight
sulfuric acid at 25°C in accordance with DIN 53727) and
comminuted to about 8 mm (average particle diameter) were
heated together with 780 g of 15% strength by weight
aqueous sodium hydroxide solution with stirring in a
pressure vessel at 220°C for 6 hours.
On cooling there was obtained a slightly yel-
lowish, homogeneous aqueous solution containing the
reaction products hexamethylenediamine and sodium adi-
pate.
To separate off the hexamethylenediamine, the
solution was repeatedly extracted with a total of 800 g
of a mixture of 50~ by volume of toluene and 50~ by
volume of n-butanol. The combined organic phases were
subjected to a fractional distillation. Toluene,
n-butanol and water were separated off first under
atmospheric pressure.
141 g of hexamethylenediamine were obtained in
the form of a colorless melt at 128 to 131°C/100 mbar.
The aqueous sodium adipate solution remaining
following the extraction step was evaporatively con
centrated under atmospheric pressure to a 27 o strength by
weight sodium adipate solution, extractant residues being
removed as well.
The concentrated sodium adipate solution was then
admixed with 0.5 g of pulverized activated carbon per
100 ml of solution and heated to 50°C. After 1 h the
activated carbon was filtered off and 80 mg of sodium
carbonate per 100 g of solution were added with stirring.
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After 1 h the stirrer was switched off and, after a
further 4 h, the solution was filtered. This prepurified
sodium adipate solution was then subjected to a treatment
with a selective ion exchange resin (Lewatit TP*208 (from
BAYER) ) .
EXAMPLE 2
This experiment was carried out using a pigmented
(with carbon black), (thermally stabilized) and glass
fiber-reinforced nylon 66 having a viscosity number (VN)
- 140 (measured in accordance with DIN 53727), see
Example 1) and a glass fiber content of 36 o by weight
(determination of the calcination loss of glass fiber-
reinforced plastics in accordance with DIN 53 395) which
had been comminuted to about 8 mm (average particle
diameter). In a pressure vessel 490 g of this composite
material were heated together with 1180 g of a 100
strength by weight sodium hydroxide solution with
stirring at 220°C for 8 hours. After the reaction mixture
had cooled down, the insoluble constituents such as glass
fibers were filtered off and repeatedly washed with
water.
The mother filtrate and the combined wash fil-
trates were repeatedly extracted with a total of 1100 g
of a mixture of 50% by volume of toluene and 50% by
volume of n-butanol to separate off the hexamethylenedi-
amine . The combined organic phases were subj ected to a
fractional distillation. First low boilers such as
toluene, n-butanol and water were separated off under
atmospheric pressure. 145 g of hexamethylenediamine were
obtained at 128 to 131°C/100 mbar. The aqueous sodium
adipate solution remaining following the extraction step
was evaporated to a concentration of 27.20 by weight of
sodium adipate, extractant residues being removed as
well. The rest of the workup was carried out similarly to
Example 1.
EXAMPLE 3
Batchwise electrolysis in a three-compartment
* trademark
A
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electrolysis cell as per variant e)
The three-compartment electrolysis cell used was
that diagrammatically depicted in Figure 1 with three
liquid cycles (RL1 to RL3). All product-contacting parts
with the exception of the electrodes consisted of
polypropylene, glass or quartz. The anode (El) (in
compartment (A)) was a titanium expanded-mesh anode
having an area of 100 cm2 and a coating suitable for
oxygen evolution. The cathode (E2) (in compartment (C))
likewise had an area of 100 cmz. It consisted of a
chromium-nickel stainless steel (1.4571) which had been
coated with a nickel network activated for hydrogen
evolution. The two membranes (M1 and M2) of the type
Nafion~ 324 were positioned directly on the electrodes
(El and E2) and were separated from each other by a 1 mm
wide center compartment (B) with a polypropylene spacer.
The anode (RL1) and cathode (RL2) cycles were
kept in natural circulation owing to the gas evolutions
at the electrodes. The cycle of the center compartment
(B), (RL3), was recirculated using a cycle pump (P). The
flow velocity in the center compartment (B) was
0.1 m/sec.
The anolyte used comprised 1131 g of 5o strength
by weight sulfuric acid introduced at location (1), the
catolyte comprised 1219 g of loo strength by weight
sodium hydroxide solution introduced at location (2), and
the center compartment electrolyte comprised 911 g of the
27% strength by weight sodium adipate solution of
Example 1 to which 19 g of 96o strength by weight of
sulfuric acid were added, so that 930 g of a solution
containing 21.7 by weight of sodium adipate, 2.9~ by
weight of adipic acid and 2.8~ by weight of sodium
sulfate were introduced at location (3).
A temperature of 80°C, atmospheric pressure, a
current density of 3.0 kA/m~, a cell voltage of 4.0 V (at
the beginning) and 5.1 V (at the end of the run) produced
in a current yield of 78o and after a reaction time of
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2 h 13 min the following electrolytes:
anolyte (removed at location (4)): 771 g of 6.6%
strength by weight sulfuric acid,
catolyte (removed at location (5)): 1348 g of
l4.lo strength by weight sodium hydroxide solution,
center compartment electrolyte (removed at
location (6)): 838 g of a solution containing 18.7%
strength by weight of adipic acid, 2.3o by weight of
sodium adipate and 3.2~ by weight of sodium sulfate.
EXAMPLE 4
Batchwise electrolysis in a three-compartment
electrolysis cell as per variant f)
The four-compartment electrolysis cell used is
diagrammatically depicted in Figure 2 with four liquid
cycles (RL1 to RL4). All product-contacting parts with
the exception of the electrodes consisted of polypropy-
lene, glass or quartz. Anode (E1) (in compartment (A))
was a titanium expanded-mesh anode having an area of
100 cm2 and a coating suitable for oxygen evolution.
Cathode (E2) (in compartment (D)) likewise had an area of
100 cm2. It consisted of a chromium-nickel stainless
steel (1.4571) which had been coated with a nickel
network activated for hydrogen evolution. The two elec-
trode-near cation exchange membranes (Ml and M3) of the
type Nafion~ 324 were positioned directly on the
electrodes (El and E2 respectively) and were separated by
two center compartments, (B) and (C), each 1 mm in width,
with a centrally disposed anion exchange membrane (M2) of
the type Tokuyama Soda° AMH. The center compartments, (B)
and (C), were provided with two polypropylene spacers
which served to keep the flow channel free and to prevent
direct contact between the membranes.
The anode (RL1) and cathode (RL4) cycles were
kept in natural circulation owing to the gas evolutions
at the electrodes. The cycles of the center compartments
(B) and (C), (RL2) and (RL3), were recirculated using the
cycle pumps (P1) and (P2). The flow velocities in the
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center compartments (B) and (C) were in each case
0.1 m/sec.
The anolyte used comprised 1099 g of 5.10
strength by weight sulfuric acid introduced at location
(1) , the catolyte comprised 1103 g of 4 .1 o strength by
weight sodium hydroxide solution introduced at location
(2) , the electrolyte of the anode-near center compartment
(B) comprised 1095 g of 2.Oo strength by weight sulfuric
acid introduced at location (3), and the electrolyte of
the cathode-near center compartment (C) comprised 1499 g
of the 27.2 strength by weight sodium adipate solution
of Example 2, introduced at location (4).
During the reaction a total of 915 g of water was
additionally introduced into the cathode-near center
compartment (C).
A temperature of 80°C, atmospheric pressure, a
current density of 3.0 kA/m~, a cell voltage of 7.0 V (at
the beginning) and 8.7 V (at the end of the run) produced
with a current yield of 72~, and after a reaction time of
5 h, during which the pH in the cathode-near center
compartment (C) was within the range from 10 to 12, the
following electrolytes.:
anolyte (removed at location (5)): 784 g of 7.1~
strength by weight sulfuric acid,
catolyte (removed at location (6)): 1579 g of
l3.lo strength by weight sodium hydroxide solution,
product of the anode-near center compartment (B)
(removed at location (7)): 2020 g of a solution
containing 14.6 strength by weight of adipic acid, 1.1%
by weight of sulfuric acid,
product of the cathode-near center compartment
(C) (removed at location (8)): 1086 g of a 2.3~ strength
by weight sodium adipate solution.
