Process for working up the liquid reaction products from the cu-catalysed preparation of dimethyl carbonate

A process is described for working up liquid reaction mixtures such as are formed during dimethyl carbonate preparation by oxidative carbonylation of methanol in the presence of a copper-containing catalyst, which process permits a simple separation of the water of reaction from the dimethyl carbonate. In this process, in a first distillation column, the water of reaction is taken from the bottom of the column; the top product from the first column is separated in a second column, under increased pressure, into dimethyl carbonate as bottom product and into a top product which is predominantly composed of methanol. The top product from the distillative working-up in the second column, which is predominantly composed of methanol, is recycled to the reaction process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for working up liquid reaction 
mixtures such as are formed during dimethyl carbonate preparation by 
oxidative carbonylation of methanol in the presence of a copper-containing 
catalyst under increased pressure at elevated temperature, which is 
characterized in that the reaction solution obtained after removal of the 
catalyst is freed from the water of reaction by simple distillation under 
normal pressure, and the anhydrous top product, comprising methanol and 
dimethyl carbonate, is separated by distillation under increased pressure 
into pure dimethyl carbonate as the bottom product and a dimethyl 
carbonate-depleted mixture of methanol and dimethyl carbonate as the top 
product, from which the top product is recycled to the normal-pressure 
column or the reaction. 
2. Description of the Related Art 
In the past, a number of processes have been developed for the preparation 
of dialkyl carbonates by catalytic reaction of the starting materials 
methanol, carbon monoxide and oxygen. 
In DE-A 2 110 194, metal complexes of groups IB, IIB and VIIIB of the 
Periodic Table, in particular the metals Cu, Ag, Au, Zn, Cd, Hg, Fe, Co 
and Ni, which can exist in two different oxidation levels in redox 
reactions, have been mentioned as suitable catalysts. This process gives 
good yields with complexed Cu.sub.2 Cl.sub.2, but has the disadvantage 
that the removal of the very expensive complex ligands and of the 
dissolved complexed catalyst from the reaction solution requires much 
effort. 
In DE-PC 2 743 690, simple monovalent salts of copper are used as catalysts 
instead of the copper complex compounds. Although this process variant 
gives good yields of dialkyl carbonates, working-up of the reaction 
solution also presents major problems here, since the partly dissolved 
catalyst must be removed from the reaction solution. According to the 
doctrine of this patent, this is effected by filtering off of the 
suspended content and rectification or crystallization of the dissolved 
catalyst. Since the catalyst-containing reaction solutions carry the 
catalyst to other parts of the plant, a high expenditure on apparatus is 
necessary for working up the reaction solution and catalyst. Because of 
the corrosive properties, all apparatuses (tanks, pipelines, distillation, 
crystallization and filtration apparatuses) which come into contact with 
the catalyst must be made of corrosion-resistant material. The process 
thereby loses its attractiveness. 
The same working-up problems mean that the use of synthesis gas instead of 
CO, as described in DE-C 3 045 767, must remain economically unattractive. 
The corrosion problems during working-up of the reaction solution caused 
by the copper-containing catalyst render processes which comprise further 
additions to the catalyst (for example EP-A 217 651, EP-B 090 977, U.S. 
Pat. No. 4,370,275) uneconomical. 
An alternative in process technology for removal of the catalyst is 
disclosed in DE-A 3 926 709. In this process the copper-containing 
catalyst remains in the reactor. The dialkyl carbonate formed during the 
reaction is stripped from the reaction mixture by the reaction gas 
together with the water of reaction and methanol. This effect is in 
general achieved by passing through the reaction mixture a gas stream of 
20 to 30 standard 1 of CO/O.sub.2 gas mixture per g of copper present as 
copper catalyst in the reactor. Disadvantages of this process are the very 
large amounts of gas which must be kept circulating, and the high energy 
costs thereby caused, as well as the problems in dispersion of the gas 
because of the large amounts of gas. In this procedure, the temperature 
and pressure of the reactor must additionally be regulated precisely in 
order to be able to maintain the level of liquid in the reactor, since 
even small variations in reactor temperature or pressure lead to the 
amounts discharged being changed significantly. The statements made in the 
case of DE-A 3 926 709 essentially apply to the processes described in EP 
0 460 732 A1and EP 0 460 735 A2. 
In all these processes, although the catalyst is removed more or less 
effectively, the process problems during working-up of the catalyst-free 
reaction solution also have not been solved in these Applications, that is 
to say removal of the water from the catalyst-free reaction solution and 
the separation of methanol and dimethyl carbonate (DMC), since these form 
an azeotrope. 
According to DE-A 2450856, the removal of dimethyl carbonate from the water 
of reaction and methanol (MeOH) by means of a simple rectification is 
complicated, since various azeotropes form between DMC, MeOH and water. 
The doctrine of the patent application is separation by using extractive 
distillation with water as the solvent. However, the process is 
uneconomical, since considerable amounts of water are required for the 
separation (9.5 g of water per g of reaction solution). 
DE-A 2607003 describes a separation experiment for splitting the 
methanol/dimethyl carbonate azeotrope by applying pressure and elevated 
temperature. According to the doctrine of this application, although a 
bottom fraction of pure DMC is obtained, the top product is again a 
mixture of methanol and DMC which is depleted in DMC. How such a process 
fragment can be integrated appropriately into a working-up process is not 
disclosed. In addition, the separation is carried out without the water 
unavoidably obtained during oxidative carbonylation, which, as disclosed 
in DE-A 3926709, forms a ternary azeotrope with methanol and dimethyl 
carbonate. 
U.S. Pat. No. 4,162,200 also discloses a distillation by extraction where, 
for example, cyclohexane and chlorobenzene are employed as auxiliaries. 
Because of the use of reagents foreign to the system, additional 
distillation steps become necessary and these again impede an economic 
solution to the separation problem. In addition, these substances foreign 
to the system can lead to contamination of the dimethyl carbonate. A 
corrosive action is to be expected in particular if chlorobenzene is used, 
which leads to considerably higher costs. 
U.S. Pat. No. 3,803,201 describes a process in which the methanol/DMC 
azeotrope is worked up by a combination of low temperature 
crystallization, filtration and subsequent fractional distillation. This 
process is unusable for an industrial application merely because of the 
temperature of about -70.degree. C. required. The use of crystallization, 
filtration and distillations furthermore represents a considerable 
expenditure on apparatus, which makes the process look uneconomical. 
The object was to discover a simple and inexpensive process for the 
working-up of reaction solutions of dimethyl carbonate, methanol and water 
such as are formed during oxidative carbonylation of methanol using copper 
chloride as the catalyst. 
SUMMARY OF THE INVENTION 
Surprisingly, it has now been found that the water of reaction and dimethyl 
carbonate can be isolated from the reaction solutions, such as are formed 
during oxidative carbonylation of methanol using copper chloride as the 
catalyst, by the combination of normal-pressure distillation with 
distillation under increased pressure, instead of by cumbersome 
crystallization, filtration and distillation or by extractive 
distillation. This combination of normal-pressure distillation for removal 
of the water and subsequent pressure distillation of the anhydrous 
reaction solution for isolation of the dimethyl carbonate and recycling of 
the methanolic top product to the reactions is not only a simple process 
but, surprisingly, is also very inexpensive. This solution to the problem 
was all the more surprising since the effect of pressure distillation on 
the problematic component, DMC-methanol, was indeed known but was not 
classified as useful, and according to the prior art the assistance of an 
extraction agent or crystallization with subsequent filtration and 
distillation is necessary, since the DMC/methanol/water mixture obtained 
in the reaction could not be worked up by the pressure distillation 
described for the DMC/methanol mixture. 
The present invention relates to a process for the preparation of dimethyl 
carbonate by reaction of methanol with oxygen and carbon monoxide in the 
presence of a copper-containing catalyst suspended or dissolved in the 
reaction medium under increased pressure at elevated temperature, removal 
of the catalyst and working-up of the reaction solution, which is 
characterized in that the catalyst-free solution is initially freed from 
the water of reaction as the bottom product in a first distillation column 
and the top product, comprising dimethyl carbonate and methanol, is 
separated in a subsequent pressure distillation at elevated temperature 
under increased pressure into a bottom product of pure dimethyl carbonate 
and a dimethyl carbonate-depleted, methanolic top product, and the top 
product is recycled to the process.

DETAILED DESCRIPTION OF THE INVENTION 
In the process according to the invention, which is preferably carried out 
continuously, the methanol to be reacted is brought into contact with the 
reaction gases oxygen and carbon monoxide, and if necessary additionally 
an inert gas, in the reactor in the presence of the catalyst. 
Copper compounds based on copper(I) and/or copper(II) salts are used as 
catalysts in the process according to the invention. Since the reaction is 
a redox reaction, both copper ion species are present during the reaction. 
Copper catalysts which are preferably employed are copper(I) halides, 
copper(I) acetylacetonate, copper(I) sulphate and/or copper(II) 
alkoxyhalides, copper(II) alkoxysulphate and copper(II) 
alkoxyacetylacetonate, and copper(II) methoxychloride is particular 
preferably employed. 
The liquid reaction medium essentially comprises the methanol to be 
reacted. In general, the molar ratio of methanol:dimethyl carbonate:copper 
(copper from the catalyst suspended and/or dissolved in the reaction 
mixture), standardized to 1 with respect to the methanol content, during 
continuous operation in the reactor is 1:(0.005-1):(0.001-5) , 
advantageously 1:(0.02-0.5):(0.005-1) and particularly preferably 
1:(0.04-0.3):(0.01-0.16). 
The reaction of the reaction gases with the methanol is carried out at a 
temperature from 60.degree. to 200.degree. C., preferably from 80.degree. 
to 140.degree. C., particularly preferably from 100.degree. to 130.degree. 
C. The reaction is carried out under pressures of 1 to 60 bar, preferably 
under 10 to 40 bar and particularly preferably under 15 to 35 bar. The 
pressure is expediently produced by forcing in the reaction gases. 
The gas stream fed to the reactor can be varied within wide limits, but a 
total gas stream comprising CO, oxygen and, if appropriate, an inert gas 
(such as, for example: N.sub.2, CO.sub.2 and the like), based on the 
copper of the catalyst present in the reaction solution, of 0.2-100 
standard 1/hour and g of Cu, preferably 0.6 to 80 standard 1/hour and g of 
Cu, particularly preferably 0.8 to 5 standard 1/hour and g of Cu, is 
expediently established. 
The composition of the reaction gases carbon monoxide and oxygen can be 
varied within wide concentration limits, but a CO:O.sub.2 molar ratio 
(standardized to CO) of 1:(0.005-1.0), and preferably of 1:(0.02-0.5), is 
expediently established. The oxygen partial pressure at these molar ratios 
is high enough to be able to achieve high space/time yields, and at the 
same time to allow no explosive carbon monoxide/oxygen gas mixtures to be 
formed. The reaction gases are not subject to particular purity 
requirements, and synthesis gas can thus serve as the CO source and air as 
the O.sub.2 carrier, but it should be ensured that no catalyst poisons, 
such as, for example, sulphur or compounds thereof, are introduced. 
The reaction of the catalyst-containing methanol with the reaction gases 
under the reaction conditions is expediently carried out at the lowest 
possible concentration of the water of reaction unavoidably obtained in 
the reaction mixture, in order to avoid secondary reactions, such as the 
formation of carbon dioxide and the simultaneous deactivation of the 
copper catalyst. The concentration of the water of reaction is in general 
not more than 8% by weight, advantageously not more than 6% by weight, 
based on the liquid phase. 
The conversion is carried out up to a desired and adjustable value, based 
on the methanol employed, of less than 35% and particularly preferably 
less than 25%. 
The catalyst can be removed in various ways in the process according to the 
invention. 
In a preferred embodiment, the catalyst is removed continuously from the 
reactor together with the reaction solution and separated off by 
sedimentation, as described in DE-A 4203796, and the catalyst-free 
reaction solution is fed to the working-up. 
In another preferred embodiment of the process according to the invention, 
the reaction solution is stripped continuously from the reactor with the 
aid of the excess reaction gas, as described in EP 0460732 A1, EP 0460735 
A2 and DE-A 3926709 A1 and is thus obtained in catalyst-free form after 
subsequent condensation. 
The reaction solutions which can be worked up according to the invention 
comprise methanol to the extent of 50 to 90% by weight, preferably to the 
extent of 55 to 80% by weight and particularly preferably to the extent of 
60 to 75% by weight; and dimethyl carbonate to the extent of 8 to 45% by 
weight, preferably 15 to 40% by weight and particularly preferably to the 
extent of 20 to 37% by weight. The water content of these reaction 
solutions is between 0.5 and 15% by weight, preferably between 1 and 10% 
by weight, particularly preferably between 1.5 and 8% by weight. All the 
data are based on the total weight of the reaction solution. 
According to the invention, the catalyst-free reaction solutions thus 
obtained are initially freed from the water of reaction by simple 
rectification. The rectification is initially carried out under 0.1 to 8 
bar, preferably 0.5 to 4 bar and particularly preferably under 0.8 to 3 
bar. The bottom of the column here is heated up to 65.degree. to 
200.degree. C., preferably 80.degree. to 160.degree. C. and particularly 
preferably to 90.degree.-150.degree. C. 
DMC and methanol are obtained as the top product and water is obtained as 
the bottom product. In contrast to the known literature, no 
DMC/MeOH/H.sub.2 O or DMC/H.sub.2 O azeotrope was obtained, but separation 
of the DMC and methanol from the water of reaction was achieved. 
On the basis of the reaction solution composition sought in the reaction, 
DMC/MeOH is initially distilled off selectively as an azeotrope in the 
distillation, followed by MeOH, which forms no azeotrope with water. 
In the preferred embodiment of the separation, a mixture of DMC and MeOH is 
obtained as the top product and water is obtained as the bottom product. 
The top product comprising DMC and MeOH still has a water content of less 
than 1% by weight, and preferably less than 0.1% by weight of water. 
To separate the DMC and MeOH, the DMC/MeOH mixture is then separated by 
distillation in a pressure column, without addition of an auxiliary, into 
pure DMC as the bottom product and a DMC-depleted methanolic top product. 
The DMC thus obtained already has such a high purity that it can be 
employed for most applications without further purification. 
The rectification is carried out under 1 to 30 bar, preferably 4 to 20 bar 
and particularly preferably under 8 to 15 bar. The bottom of the column 
here is heated up to 65.degree. to 250.degree. C., preferably 100.degree. 
to 220.degree. C. and particularly preferably to 130.degree. to 
200.degree. C. 
The top product of the pressure column is recycled to the process. For 
this, in the preferred embodiment of the process according to the 
invention, this top product can be recycled to the reactor, if appropriate 
together with fresh methanol. 
In another embodiment, this top product of the pressure column is recycled 
to a suitable point in the first column operating under normal pressure. 
In yet another embodiment, the top product of the pressure distillation can 
be fed to the reactor of another working-up column before the recycling. 
In this other working-up column, the azeotrope of dimethyl carbonate and 
methanol is obtained as the top product and is recycled to a corresponding 
point in the pressure column. Methanol comprising more than 90% by weight, 
preferably more than 95% by weight, particularly preferably more than 98% 
by weight of methanol is obtained as the bottom product and is recycled to 
the reactor. 
Columns having fixed baffles, fillings and packings are suitable 
distillation columns for these separation processes. The fillings or 
ordered packings to be used are those which are customary per se for 
distillations, such as are described, for example, in Ullmanns 
Encyclopadie der Techn. Chemie (Ullmann's Encyclopaedia of Industrial 
Chemistry), 4th Edition, Volume 2, page 528 et seq. Examples which may be 
mentioned are: Raschig or Pall rings, Berl, Intalex or torus saddles or 
interpacking. These fillings can be made of various materials, such as 
glass, stoneware, porcelain, stainless steel, plastic and metal, which can 
be processed in woven or mesh form, especially if metal is used. Preferred 
fillings or packings are, for example, Pall and Novolax rings, Berl 
saddles, BX packing, Montz-Pak, Metal-pak, Melladur, Kerepak and CY 
packing. 
Suitable tray columns are, for example, perforated trays, bubble trays, 
valve trays, tunnel trays and centrifugal trays, which in turn can be of 
various designs. 
Columns having fillings or packings are particularly suitable for the 
column for removal of the water. The number of theoretical plates is 1 to 
200, preferably 5 to 100, particularly preferably 10 to 60. 
Both tray and filled or packed columns are suitable for the pressure 
distillation column. The number of theoretical plates is 1 to 200, 
preferably 5 to 100, particularly preferably 10 to 60. 
FIG. 1 shows the process according to the invention by way of example: 
The catalyst-free reaction solution comprising methanol, dimethyl carbonate 
and water is introduced via line 1 into the middle third of column A. The 
water of reaction which has been removed leaves the column as the bottom 
product via line 2. The top product, comprising dimethyl carbonate and 
methanol, is metered via line 3, pump P and line 4 into the middle third 
of pressure column B. The methanolic top product depleted in dimethyl 
carbonate leaves the pressure column via line 5 and heats the bottom of 
column A via heat exchanger C. After leaving heat exchanger C, the top 
product is further cooled, if appropriate, in heat exchanger D and 
recycled to the reaction via line 7. The useful product dimethyl carbonate 
leaves column B via line 8 and is of adequate purity for most cases of 
use. 
The invention is illustrated in more detail by the following example, but 
without being limited thereto. 
EXAMPLE 1 
An apparatus as shown in FIG. 2 is used. FIG. 2 differs from FIG. 1 in that 
line 5 is not connected to heat exchanger C. The catalyst-free reaction 
solution was metered into the middle of the first distillation column A. 
This distillation column of glass (d=4.5 cm, 1=60 cm) for removal of the 
water was filled with 800 ml of 4.times.4 mm V.sub.4 A wire mesh coils and 
had an oil-heated bottom evaporator with a discharge and an attached 
condenser with reflux divider. The top product from A was metered via a 
buffer vessel and a pump into the middle of pressure column B. Pressure 
column B was identical to the glass column A in structure, but with the 
difference that it was made of steel and had a pressure retention system 
which allowed precise regulation of the pressure. 
The apparatus was charged with 560 g/hour of a reaction mixture comprising 
66% by weight of methanol, 31% by weight of dimethyl carbonate and 3% by 
weight of water. Column A operated under normal pressure and had a bottom 
temperature of 100.degree. C. Column B was operated under a pressure of 10 
bar and had a bottom temperature of 185.degree. C. After 6 hours, the 
apparatus was in equilibrium and gave 16.6 g per hour of water of reaction 
as the bottom discharge of A. Distillation column B produced 75.5 g/hour 
of DMC having a purity of 99.89%, and the top product of B was 469.4 
g/hour and comprised methanol to the extent of 79.1% by weight and DMC to 
the extent of 20.9% by weight. The water content of this return stream was 
about 250 ppm.