Carboxylates which contain aldehydes, acetals and/or unsaturated compounds and are obtained by reacting an olefinically unsaturated compound with carbon monoxide and an alkanol are purified by a process wherein the said carboxylates are treated with hydrogen at elevated temperatures in the presence of an acidic ion exchanger or zeolite doped with one or more metals of group VIII of the periodic table, and the resulting low and/or high boilers are separated off by distillation.

The carbonylation of olefins, i.e. the reaction of, for example, ethylene, 
propylene or butylene with carbon monoxide and an alkanol in the presence 
of a carbonyl complex of a metal of group VIII of the periodic table, is 
used for the large-scale industrial production of carboxylates. If the 
starting material used is a diolefin, e.g. 1,3-butadiene, the reaction 
proceeds via the intermediate pentenoate to give dimethyl adipate, which 
is a useful starting material for the production of textile raw materials. 
Since carbon monoxide frequently contains a small amount of hydrogen or 
reacts with entrained water to from hydrogen, the carbonylation reaction 
is accompanied by a hydroformylation reaction. This gives aldehydes, which 
react with any alkanols present to give acetals. Moreover, depending on 
the synthesis, unsaturated ketones, e.g. tridecenones, and 
butenedicarboxylates are obtained as by-products in low concentrations. 
When the boiling points of the acetals, aldehydes, unsaturated ketones or 
butenedicarboxylates are very close to those of the resulting ester, a 
very technically complicated procedure is required in order to separate 
off these undesirable by-products by distillation. In the production of 
adipates, it is particularly important to separate off aldehydes, acetals 
and unsaturated compounds; if this is not done, the adipic acid prepared 
from these adipates is not very suitable for the production of 
textile-grade polymers. Moreover, when even small amounts of aldehydes, 
acetals and unsaturated compounds are present, the resulting products 
exhibit undesirable discolorations. 
It is an object of the present invention to provide a process which makes 
it possible to remove acetals, aldehydes and/or unsaturated compounds from 
carboxylates by a simple procedure. 
We have found that this object is achieved by a process for purifying 
carboxylates which contain aldehydes, acetals and/or unsaturated compounds 
and are obtained by reacting an olefinically unsaturated compound with 
carbon monoxide and an alkanol, wherein the said carboxylates are treated 
with hydrogen at elevated temperatures in the presence of an acidic ion 
exchanger or zeolite doped with one or more metals of group VIII of the 
periodic table, and the resulting low and/or high boilers are separated 
off by distillation. 
The novel process has the advantage that the esters can be freed from the 
contaminating aldehydes, acetals and/or unsaturated compounds by a simple 
procedure, which replaces the very expensive multi-stage purification 
process otherwise required. The carboxylates purified by the novel process 
are very pure since quantitative removal of the acetals, the aldehydes and 
the unsaturated compounds is effected, and the butenedicarboxylate present 
is even converted to a useful product. 
The general inventive concept is as follows: the acetals which have boiling 
points close to that of the carboxylate are converted catalytically to 
vinyl ethers, which are hydrogenated to low-boiling esters and separated 
off as such, or the aldehydes and acetals are converted to high boilers by 
an aldol reaction, and these are separated off. Butenedicarboxylates are 
converted to useful products, i.e. adipates, by hydrogenation. The 
unsaturated ketones are converted to saturated ketones, which can be 
separated off by distillation. 
Preferred carboxylates are obtained by carbonylation of C.sub.2 -C.sub.12 
-monoolefins, C.sub.4 -C.sub.12 -diolefins, C.sub.5 -C.sub.12 
-cycloalkenes or C.sub.1 -C.sub.8 -alkyl C.sub.3 -C.sub.12 
-alkenemonocarboxylates. The carbonylation is carried out by a 
conventional method, by reaction with carbon monoxide and a C.sub.1 
-C.sub.8 -alkanol, in particular a C.sub.1 -C.sub.4 -alkanol, for example 
at from 100.degree. to 200.degree. C. and under from 50 to 1000 bar in the 
presence of a carbonyl complex of a metal of group VIII of the periodic 
table, in particular of a cobalt or rhodium carbonyl complex. This gives 
C.sub.3 -C.sub.13 -monocarboxylates of alkanols of 1 to 8 carbon atoms, 
C.sub.6 -C.sub.14 -dicarboxylates of alkanols of 1 to 8 carbon atoms or 
cycloalkanecarboxylates having 5 to 12 carbon atoms in the ring. Saturated 
mono- and dicarboxylates having the stated number of carbon atoms are 
particularly preferred. Esters prepared in this manner contain, as 
by-products, aldehydes and acetals, the aldehyde component having the same 
number of carbon atoms as the corresponding carboxylic acid. Moreover, the 
acetals contain radicals corresponding to the alkanols used. Other 
by-products are unsaturated ketones or unsaturated dicarboxylates, 
depending on the type of starting material used. The content of aldehydes 
and acetals is, for example, from 0.1 to 15% by weight. Suitable processes 
are described in, for example, U.S. Pat. No. 3,176,028 and German 
Laid-Open Application DOS No. 1,618,156. 
C.sub.1 -C.sub.4 -alkyl adipates have become particularly important 
industrially; they are prepared by carbonylation of butadiene or a C.sub.1 
-C.sub.4 -alkyl pentenoate with carbon monoxide and a C.sub.1 -C.sub.4 
-alkanol. A typical mixture contains, in addition to adipic acid, for 
example from 9 to 14% by weight of methylglutarates, from 2 to 5% by 
weight of ethylsuccinates, from 0.1 to 0.3% by weight of 
5-formylvalerates, from 0.2 to 0.5% by weight of 6,6-dimethoxycaproates, 
from 0.03 to 0.1% by weight of butenedicarboxylates and from 0.005 to 
0.02% by weight of tridecenones. Suitable processes are described in, for 
example, U.S. Pat. No. 2,801,263 and German Pat. No. 2,713,195. 
Suitable catalysts are acidic ion exchangers and zeolites which are doped 
with one or more metals of group VIII of the periodic table. Examples of 
suitable metals are cobalt, nickel, palladium and platinum, particularly 
preferred metals being palladium and platinum, in particular palladium. 
Advantageously, the acidic ion exchangers and zeolites contain from 0.5 to 
5% by weight of the stated metals. Examples of suitable zeolites are A, X 
and Y zeolites, as well as natural zeolites, such as fanjasites and 
nordenites. Strongly acidic ion exchangers (crosslinked polystyrene 
containing sulfonic acid groups) are particularly preferred. 
The catalysts according to the invention are prepared, for example, as 
follows: the acidic ion exchanger is suspended in water, a dilute 
palladium nitrate solution containing the calculated amount of palladium 
is added to the stirred suspension, and the product is filtered off, 
washed and isolated. When used in a batchwise procedure, the catalyst is 
washed water-free with methanol in a separate step; when used in a 
continuously operated reactor, the drying is advantageously carried out in 
the reactor itself. 
At the acidic centers of the catalyst, the most important impurity in terms 
of amount, i.e. the acetal (e.g. methyl 6,6-dimethoxycaproate), undergoes 
cleavage to give the corresponding vinyl ether and methanol, and the vinyl 
ether is hydrogenated to the saturated ether. These ethers have 
substantially lower boiling points than the acetals, and can therefore be 
separated off by distillation. The aldehyde (e.g. methyl 
5-formylvalerate), which is likewise an important impurity in terms of 
amount, undergoes an aldol reaction at the acidic centers to give the 
corresponding aldol, which has a high molecular weight, and hence a 
substantially higher boiling point than the aldehyde or the carboxylate. 
The other troublesome impurities, such as monounsaturated or 
polyunsaturated compounds, are converted either to useful products or to 
compounds which can be separated off by distillation. In the case of the 
butenedicarboxylate, the hydrogenation carried out during the purification 
process gives methyl adipate. The unsaturated ketones are converted to 
saturated ones which can be easily separated off by distillation. The 
process is carried out at elevated temperatures, preferably from 
50.degree. to 300.degree. C., in particular from 100.degree. to 
150.degree. C. Where acidic ion exchangers doped with the stated metals 
are used, it has been found to be particularly useful if the temperature 
is maintained at from 80.degree. to 140.degree. C. 
The hydrogenation with hydrogen can be carried out under atmospheric 
pressure, but, because of the increased hydrogen solubility, it has been 
found to be advantageous to maintain superatmospheric pressure, for 
example not more than 10 bar. In the subsequent distillation, the 
carboxylates have only to be separated off from components which have 
substantially higher or lower boiling points. It should be particularly 
emphasized that this distillation does not make any especially high 
demands with respect to the number of separation stages. 
The process is advantageous firstly because carboxylates can be freed from 
several types of impurities at the same time by a simple procedure, and 
can be obtained in a very pure form, so that very expensive separation 
processes which are otherwise required can be dispensed with. These would 
otherwise be absolutely necessary in order to ensure that the dicarboxylic 
acids prepared from these products were of textile-grade quality. Thus, 
the technical complexity of the preparation of pure carboxylates is 
greatly reduced by the novel purification process. 
Secondly, the use, according to the invention, of a catalyst possessing 
acidic centers, for example a zeolite or strongly acidic ion exchanger 
which is doped with one or more metals of group VIII of the periodic 
table, ensures that only undesirable compounds, and not the ester 
functions of the desired product, are attacked. In contrast, if a 
conventional industrial hydrogenation were carried out using a 
conventional catalyst without the above double function, even under mild 
conditions the ester function would be hydrogenated to produce 
ester-alcohols (e.g. methyl 6-hydroxycaproate), which, even under mild 
thermal conditions, undergo cyclization to lactones, e.g. 
.epsilon.-caprolactone, with elimination of methanol. Frequently, such 
lactones cannot be separated off from the desired product by distillation. 
This applies in particular to the preparation of dimethyl adipate, which 
in such a case would be unsuitable for the textile sector. Moreover, 
partial hydrogenation of ester functions must be regarded as a real loss 
of desired product. In contrast, the novel purification process is an 
elegant method for the purification of carboxylates which is easy to carry 
out technically and even gives additional small amounts of the desired 
product. 
The esters purified according to the invention are useful as solvents. 
Adipates are hydrolyzed to adipic acid, which is a starting material for 
the preparation of polycondensates with nylon 6,6.

The Examples which follow illustrate the process according to the 
invention. 
EXAMPLE 1 
0.2 liter of a strongly acidic ion exchanger (crosslinked polystyrene 
possessing sulfonic acid groups) which is doped with 1.0% by weight of 
palladium, and 0.8 kg of dimethyl adipate which contains 0.3% by weight of 
methyl 6,6-dimethoxycaproate, 0.25% by weight of methyl 5-formylvalerate, 
0.07% by weight of dimethyl butenedicarboxylate, 0.015% by weight of 
tridecenones and 0.2% by weight of dimethyl 2-methylglutarate and has an 
extinction coefficient of 20,260, are introduced into a 2 liter shaken 
autoclave. Hydrogen is forced in under a pressure of 5 bar, and the 
mixture is then heated for 2.5 hours at 125.degree. C., the autoclave 
being shaken at the same time. The product obtained after cooling has a UV 
number of 2,800. Rectification gives 99.75% strength by weight dimethyl 
adipate which has a UV number of 1,900 and still contains 0.1% by weight 
of dimethyl 2-methylglutarate. 
EXAMPLE 2 
18 liters/hour of 99.2% strength by weight dimethyl adipate and 80 liters 
(S.T.P.)/hour of hydrogen are introduced into a 9 m tube reactor which 
contains 40 liters of a strongly acidic ion exchanger doped with 1.5 g of 
palladium per liter of dry ion exchanger. The reactor is operated in a 
flooded state, and kept at 110.degree. C. and under a pressure of 8 bar. 
The ester employed contains on average 0.5% by weight of methyl 
6,6-dimethoxycaproate, 0.13% by weight of methyl 5-formylvalerate, 0.02% 
by weight of dimethyl butenedicarboxylate, 0.02% by weight of tridecenones 
and tetradecenones and 0.05% by weight of dimethyl 2-methylglutarate and 
has an average UV number of 14,000. The product from the reactor is fed to 
a distillation column having 30 theoretical plates, from which column the 
desired product is taken off as a sidestream. The dimethyl adipate 
obtained is 99.85% pure and has an average UV number of 1,200.