Process for producing a carbonic acid ester

A process for producing a carbonic acid ester which comprises reacting an alcohol with carbon monoxide and oxygen in the presence of a catalyst comprising a compound of divalent copper is disclosed, wherein the catalyst deactivated as a result of the carbonic acid ester-producing reaction is regenerated by subjecting a reaction mixture or a concentrate thereof containing the deactivated catalyst to water-replacement treatment, followed by heat treatment and weak-acid treatment.

FIELD OF THE INVENTION 
The present invention relates to a process for producing a carbonic acid 
ester. More particularly, this invention relates to a carbonic acid 
ester-producing process which has enabled a catalyst to be industrially 
used repeatedly. The carbonic acid ester to be produced by the process of 
the present invention is an industrially important compound for use as an 
intermediate for polymers, medicines, and agricultural chemicals and as a 
solvent. 
BACKGROUND OF THE INVENTION 
In one conventional process for the industrial production of a carbonic 
acid ester, an alcohol has been reacted with phosgene. However, this 
process has problems, for example, that highly poisonous phosgene is used 
and that the reaction between an alcohol and phosgene yields highly 
corrosive hydrochloric acid as a by-product in a large quantity. 
Therefore, a large number of processes have been proposed for the 
production of a carbonic acid ester without using phosgene. Among these is 
a commonly employed process in which an alcohol to be esterified is 
reacted with carbon monoxide and oxygen in the presence of a catalyst. 
Representative examples of the catalyst used in this process include a 
catalyst comprising a copper compound (JP-B-45-11129) and a catalyst 
comprising a combination of a palladium compound, a copper compound, and a 
base (JP-B-61-8816). (The term "JP-B" as used herein means an "examined 
Japanese patent publication".) Further, known as an improvement of the 
latter catalyst is one comprising a combination of a palladium compound, a 
weak acid salt or/and halide of copper, and a weak acid salt or/and halide 
of an alkali metal or alkaline earth metal (JP-A-1-287062). (The term 
"JP-A" as used herein means an "unexamined published Japanese patent 
application.) 
The above-described catalysts, however, still have problems which should be 
overcome in order to utilize these catalysts in the industrial production 
of carbonic acid esters. Illustratively stated, in the case of using a 
copper compound as a catalyst, the copper compound, which has a low 
solubility, should be used in a large amount in order to obtain an 
effective reaction rate, since the catalytic activity of the copper 
compound is generally low. However, there are cases where the copper 
compound as a catalyst changes into copper hydroxychloride or other 
compounds according to the reaction conditions used and, as a result, the 
activity of the catalyst decreases. 
In the case of the catalyst comprising a combination of a palladium 
compound, a copper compound, and a base, since water and oxalic acid are 
formed as by-products of the reaction and accumulate with the progress of 
the reaction, part of the catalytic components react with these 
by-products and separate out as an insoluble hydroxide, oxalate, or metal, 
resulting in a decrease in catalytic activity. It is, therefore, difficult 
to industrially use the catalyst continuously or repeatedly. 
As apparent from the above, an industrially important theme for the 
conventional processes using either the catalyst comprising a copper 
compound or the catalyst based on the combination of a palladium compound 
and a copper compound is to establish a method of recovering or 
regenerating the catalyst. 
SUMMARY OF THE INVENTION 
The present inventors have made intensive studies in order to overcome the 
above-described problems and to enable carbonic acid ester production 
processes using the above-described catalysts to be utilized industrially. 
As a result, it has been found that catalyst regeneration can be attained 
by subjecting the reaction mixture containing insolubilized and 
deactivated catalytic components to water-replacement treatment, followed 
by heat treatment and treatment with a weak acid which, in the case where 
a salt of a weak acid has been used as a catalytic component, preferably 
is the same as the weak acid constituting part of the catalytic component. 
The present invention has been completed based on this finding. 
Accordingly, the present invention provides a process for producing a 
carbonic acid ester which comprises reacting an alcohol with carbon 
monoxide and oxygen in the presence of a catalyst comprising a compound of 
divalent copper, wherein the catalyst deactivated as a result of the 
carbonic acid ester-producing reaction is regenerated by subjecting the 
reaction mixture or a concentrate thereof containing the deactivated 
catalyst to water-replacement treatment, followed by heat treatment and 
weak-acid treatment. 
DETAILED DESCRIPTION OF THE INVENTION 
Catalyst 
The catalyst used in the process of the present invention for producing a 
carbonic acid ester contains a compound of divalent copper. Examples of 
the divalent copper compound include cupric salts of such weak acids as 
acetic acid, pivalic acid, benzoic acid, and other carboxylic acids, 
cupric hydrobromate, cupric carbonate, cupric salts of such weak acids as 
phenol, cresol, p-chlorophenol, and other phenols, and cupric halides such 
as cupric chloride and cupric bromide. Such a divalent copper salt is used 
in an amount of generally from 1 to 3,000 mmol, preferably from 10 to 
1,000 mmol, per liter of the alcohol. 
In combination with the divalent copper salt, a compound of either an 
alkali metal or an alkaline earth metal may be used as a catalytic 
component. Examples of the alkali metal or alkaline earth metal compound 
include chlorides, bromides, iodides, acetates, and other compounds of 
lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, 
calcium, and barium. Such an alkali metal or alkaline earth metal compound 
may preferably be used in an amount of from 1/10 to 10 mol per mol of the 
divalent copper salt. A more preferred range of the amount thereof is such 
that the atomic ratio of halogen to copper in the catalyst is from 1/2 to 
4. 
A platinum group compound may further be used as a catalytic component. 
Examples thereof include halides, acetates, nitrates, and other compounds 
of ruthenium, rhodium, and palladium. Of these, palladium salts are 
particularly preferred. Although the amount of such a platinum group 
compound to be used is not particularly limited, it preferably is 1/10 mol 
or less per mol of the divalent copper salt from an economical standpoint. 
Raw Materials 
Examples of the alcohol to be used as a raw material for a carbonic acid 
ester in the present invention include saturated aliphatic alcohols such 
as methanol and ethanol, unsaturated aliphatic alcohols such as allyl 
alcohol, and aromatic alcohols such as phenols, and further include diols 
and polyols. Of these, alcohols having from 1 to 20 carbon atoms are 
preferred. The gaseous reactants, i.e., carbon monoxide and oxygen, each 
may be either a high-purity gas or a gas diluted with a gas inert in the 
reaction, such as nitrogen, argon, or carbon dioxide. Therefore, air may 
be used as an oxygen source. 
Reaction Conditions 
In the presence of the catalyst described above, the carbonic acid 
ester-producing reaction according to the present invention may be 
conducted by allowing the reactants to react at ordinary pressure or an 
increased pressure, preferably at from 1 to 100 atm. In the case where the 
gaseous reactants are used after being diluted with an inert gas, the 
partial pressure of carbon monoxide in the reaction system may be 
regulated in the range of from 0.1 to 30 atm and that of oxygen in the 
range of from 0.05 to 10 atm. The reaction temperature may be in the range 
of from 20.degree. to 250.degree. C. 
Regeneration of Catalyst 
The catalyst regeneration method in the present invention comprises the 
step of subjecting either the reaction mixture containing deactivated 
catalytic components that have been insolubilized and precipitated in 
various forms such as a metal oxalate, metal oxide, metal hydroxide, metal 
carbonate, elemental metal, etc. or a concentrate of the reaction mixture 
to water-replacement treatment, followed by the steps of heat treatment 
and weak-acid treatment. The weak acid used in the weak-acid treatment 
preferably is one which is the same as a constituent of one of the 
catalytic components if the catalytic component is a weak acid salt. Each 
of these steps is explained below. 
After completion of the carbonic acid ester-producing reaction, the 
resulting reaction mixture containing a precipitated deactivated catalyst 
is first subjected to water-replacement treatment prior to heat treatment. 
The reasons for this are as follows: (1) the reducing substances and 
combustible matter present in the reaction mixture should be removed 
before the heat treatment because it is usually preferred to conduct the 
heat treatment in the presence of oxygen or air in an oxidizing 
atmosphere; and (2) the raw-material alcohol remaining unreacted and the 
carbonic acid ester produced will be lost in the heat treatment because 
they are decomposed under high-temperature conditions. 
This water-replacement treatment is conducted until the concentration of 
the organic substances, such as methanol, carbonic acid ester, etc., in 
the reaction mixture becomes 5% or less, preferably 2% or less. Although 
the reaction mixture may be subjected as it is to the water-replacement 
treatment, a concentrate of the reaction mixture is usually subjected to 
the treatment, because use of a concentrate is advantageous in the amount 
of water to be used and in apparatus. The concentrate of the reaction 
mixture can be obtained by concentrating the reaction mixture by 
distilling away about 95% of a carbonic acid ester, water and an unreacted 
alcohol in the reaction mixture. The carbonic acid ester can be taken out 
when the reaction mixture is concentrated to the concentrate and the 
water-replacement is carried out. 
After the water-replacement treatment, heat treatment is conducted. The 
heating temperature should be 100.degree. C. or higher, but it is selected 
in the range of generally from 150.degree. to 1,000.degree. C. from the 
standpoint of treating rate and apparatus. For example, the heat treatment 
can be carried out at from 170.degree. to 190.degree. C. for from 2 to 4 
hours. It should be noted that if the heat treatment is performed in an 
oxygen-free atmosphere, metallic ingredients in the catalyst, especially 
copper, are apt to be disadvantageously reduced into the monovalent state 
or elemental metal state, while if the heat treatment is performed in a 
reducing atmosphere, it is very difficult to decompose metal oxalates. 
Because of this, it is usually preferable to carry out the heat treatment 
in the presence of oxygen or air. 
The catalyst which has undergone the heat treatment is then treated with a 
weak acid, preferably with the same weak acid as that constituting part of 
a weak acid salt as one of the catalytic components. As the weak acid, use 
may be made of one whose copper salt dissolves in the raw-material alcohol 
or reaction mixture in a catalytic amount. For example, in the case where 
the alcohol is methanol or the like, a lower aliphatic carboxylic acid is 
advantageously used. 
The amount of the weak acid used for the weak-acid treatment is usually 1.5 
mol or more, preferably in the range of from 2 to 5 mol, per mol of the 
copper contained in the catalyst present in the mixture being treated. 
Although this treatment may be given to the heat-treated mixture as it is, 
the efficiency of the weak-acid treatment becomes higher as the 
concentration of the weak acid in the mixture being treated increases 
higher. For example, in the case of using acetic acid as a weak acid in an 
amount of 2 mol per mol of copper, the recovery of copper (the percentage 
of soluble copper after the treatment to the total amount of copper) was 
46% when the mixture being treated had an acetic acid concentration of 3%, 
whereas the recoveries of copper were 53%, 69%, and 95% at acetic acid 
concentrations of 32%, 45%, and 75%, respectively. 
Although the weak-acid treatment may be performed at room temperature, it 
is preferred to conduct the treatment at an elevated temperature in order 
to accelerate the treatment. For example, when acetic acid is used as a 
weak-acid, the weak-acid treatment is carried out preferably at a 
temperature of from 100.degree. to 115.degree. C. If a weak acid was added 
in an amount larger than the required amount, the excess weak acid may by 
removed after the treatment by evaporation. After completion of the 
weak-acid treatment, an alcohol to be a raw material in the subsequent 
carbonic acid ester synthesis or a solvent for the reaction is added to 
the weak acid-treated mixture, thereby to dissolve the soluble matter into 
the alcohol or solvent (this solution being referred to as solution A) and 
separate it from the insoluble matter. The insoluble matter can be reused 
after being treated by an adequate method. 
In addition, since the insoluble matter resulting from the regeneration 
treatments described above does not contain any expensive platinum group 
compound, e.g., palladium, the insoluble matter can be discarded without 
any fear of posing a cost problem. That is, platinum group compounds are 
soluble in the reaction mixture or a concentrate thereof in the present 
invention since the reaction mixture or a concentrate thereof contains 
much larger amount of cupric ions (Cu.sup.2+) than platinum group 
compounds. On the other hand, solution A containing the regenerated 
catalyst can be used after fresh catalytic components corresponding to the 
discarded insoluble matter are added thereto. 
According to the present invention, a catalyst containing a deactivated 
divalent copper compound can be regenerated efficiently and, hence, 
repeated industrial use of the catalyst has become possible. 
The present invention will be explained below in more detail with reference 
to the following Example and Comparative Example, but the invention is not 
construed as being limited thereto. Although the process of Example was 
conducted batchwise, it is a matter of course that either of the synthesis 
reaction and catalyst regeneration in the process can also be carried out 
continuously.

EXAMPLE 1 
Into a glass-lined reactor having a capacity of 5 liters was introduced 3 
liters of methanol containing, as a catalyst, 0.34 mmol/l of palladium 
chloride, 57 mmol/l of copper acetate, and 57 mmol/l of magnesium chloride 
dissolved therein. While CO and O.sub.2 were kept being fed to the reactor 
at rates of 200 Nl/h and 100 Nl/h, respectively, the methanol was reacted 
with the gaseous reactants at a total pressure of 21.1 atm and a 
temperature of 135.degree. C. After the reaction was conducted for 3 
hours, the reaction mixture was cooled and then analyzed by gas 
chromatography. As a result, it was found that dimethyl carbonate 
(hereinafter abbreviated as DMC) had been formed in a yield of 20.1% based 
on the methanol. Further, analysis of the reaction mixture by atomic 
absorption spectroscopy revealed that 99%, 24%, and 34% of the Pd, Cu, and 
Mg, respectively, introduced in the reactor had been insolubilized and 
precipitated. 
1,000 Grams of the reaction mixture (including the precipitate) was 
concentrated with a rotary evaporator and then with an evaporating vessel, 
thereby giving 50 g of a concentrate. Subsequently, 5 g of water was added 
thereto and the resulting mixture was concentrated to remove 5 g of 
volatile matter by evaporation. This procedure was repeated until the 
supernatant came to have a water content as determined by gas 
chromatography of 98% or more, thereby carrying out water-replacement 
treatment. The resulting liquid mixture was placed in a glass-lined 
autoclave having a capacity of 5 liters, and then heat-treated therein at 
190.degree. C. for 2 hours while air was kept being fed at a rate of 200 
Nl/h at a total pressure of 20 atm. After cooling and pressure release, 
the contents were analyzed by liquid chromatography. As a result, it was 
found that 98% of the metal oxalates had been decomposed. 
The heat-treated mixture was then concentrated with an evaporating vessel 
to give 25 g of a concentrate. Subsequently, 5.9 g of acetic acid was 
added to the concentrate and the resulting mixture was stirred at 
60.degree. C. for 1 hour. To the reaction mixture was added methanol in 
such an amount as to result in a total amount thereof of 1,000 g. This 
mixture was stirred at 20.degree. C. for 1 hour and the insoluble matter 
was then separated out by vacuum filtration. The thus-obtained filtrate 
and cake were analyzed by atomic absorption spectroscopy. As a result, it 
was found that the filtrate contained, dissolved therein, 100%, 96%, and 
97% of the Pd, Cu, and Mg, respectively, which had been introduced in the 
reactor for the DMC-producing reaction, while the cake contained 0%, 4%, 
and 3% of the introduced Pd, Cu, and Mg, respectively. 
To the filtrate obtained above were added 0.5 g of copper acetate and 0.3 g 
of magnesium acetate. (This liquid is referred to as "regenerated 
catalyst-containing raw-material liquid B".) The concentrations of 
palladium, copper, magnesium, chlorine ion, and acetic acid ion dissolved 
in raw-material liquid B were determined by atomic absorption 
spectroscopy, titrimetric analysis, and liquid chromatography. As a 
result, all of the determined values of these concentrations were 
substantially the same as those for the raw-material liquid containing a 
fresh catalyst before being used in the DMC-producing reaction described 
above, that is, the differences between the determined concentration 
values for raw-material liquid B and those for the raw-material liquid 
containing a fresh catalyst were within the determination errors. 
Using regenerated catalyst-containing raw-material liquid B, DMC synthesis 
was conducted in the same manner as in the above-described first synthesis 
employing a fresh catalyst. As a result, DMC was obtained in a yield of 
20.2%, demonstrating that the regenerated catalyst had an activity equal 
to that of the fresh catalyst. 
COMATIVE EXAMPLE 1 
(DMC Synthesis without Catalyst Regeneration) 
Using a fresh catalyst, DMC-producing reaction was conducted in the same 
manner as in Example 1, thereby obtaining DMC in a yield of 21.3%. 1,000 
Grams of the resulting reaction mixture was concentrated with a rotary 
evaporator to give 25 g of a concentrate. Methanol was added to the 
concentrate in such an amount as to result in a total amount thereof of 1 
liter. DMC synthesis was then conducted using the resulting mixture in the 
same manner as in the above-described first synthesis employing a fresh 
catalyst. As a result, the yield of DMC was 0.86%. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.