PROCESS FOR PRODUCING CARBONIC ACID ESTER

Abstract: Disclosed herein is a process for producing a carbonic acid ester by reacting alcohol, carbon monoxide, and oxygen with one another in the presence of a copper compound alone or together with a platinum metal compound and a reaction accelerator, characterized in that (I) the reaction is carried out in a nitrile compound or amide compound as a solvent, (II) the reaction is carried out in the presence of a reaction accelerator which consists of a quaternary phosphonium halide and a quaternary phosphonium weak acid salt or quaternary phosphonium alkoxide, and/or (III) the reaction is carried out in the presence of a quinoid compound or a compound that changes into a quinoid compound under the reaction condition.

Description: The present invention relates to a process for producing a carbonic acid ester. A carbonic acid ester is an industrially important compound as an intermediate and solvent for the production of polymers and drugs and agricultural chemicals. A carbonic acid ester is industrially produced by the reaction of alcohol with phosgene. This process, however, has the disadvantage that phosgene is highly toxic and the reaction of alcohol with phosgene produces highly corrosive hydrochloric acid as a by-product. There is also a process for producing a carbonic acid ester without using phosgene. According to this method, alcohol, carbon monoxide, and oxygen are reacted the presence of a copper compound alone or together with a platinum metal compound and a basic compound. (See Japanese Patent Publication Nos. 11129/1970 and 8816/1986 and Japanese Patent Laid-open No. 185542/1983.) This process also has the disadvantage that the catalyst, especially the copper component, is poor in solubility and the formation of carbonic acid ester is rather slow. It has an additional disadvantage that a large amount of carbon dioxide as a by-product is formed as the result of combustion of carbon monoxide and the decomposition of carbonic acid ester. It is the object of the present invention to provide a process for producing a carbonic acid ester with a minimum of carbon dioxide formed as a by-product. This new process does not have the disadvantages of poor solubility an low reaction rate. This object is achieved by a process for producing a carbonic acid ester by reacting alcohol, carbon monoxide, and oxygen in the presence of a copper compound or together with a platinum metal compound and a further compound characterized in that the further compound is a) a quaternary phosphonium halide and a quaternary phosphonium weak acid salt or quaternary phosphonium alkoxide, and/or b) a quinoid compound or a compound that changes into a quinoid compound under the reaction condition. The materials used in the process of the present invention are described in detail in the following. The platinum metal compound includes halides, acetates, and nitrates of ruthenium, rhodium, and palladium. Of these platinum metal compounds, a palladium salt is most desirable. Usually the platinum metal compound is used in an amount of 0.1 to 1000 mmol, preferably 1 to 100 mmol, per 1l of alcohol. The copper compound includes cuprous and cupric halides, acetates, and nitrates. Usually these copper compounds are used in an amount of more than 1 mol, preferably more than 3 mol, per 1 mol of the platinum metal compound. The alcohol as a reactant in the process of the invention includes saturated aliphatic alcohols such as methanol and ethanol; unsaturated aliphatic alcohols such as allyl alcohol; aromatic alcohols such as phenol; and diols and polyols. They preferably have 1 to 20 carbon atoms. Particularly preferable is methanol. The carbon monoxide and oxygen may be used in the pure form or in diluted form. A diluent in the latter case may be argon or carbon dioxide which is inert in the reaction. Air may be used as the oxygen. The reaction is carried out under normal pressure or appropriate pressure. Carbon monoxide and oxygen are introduced into the reaction system such that their partial pressures are 10-3000 kPa (0.1-30atm) and 5-1000 kPa (0.05-10 atm), respectively. The reaction may also be concluded in the additional presence of a nitrile compound or amide compound. The compounds used in the present invention include trialkylamines, heterocyclic crosslinked amines, di-substituted-carbodiimides, alkali metal salts, and alkali metal alkoxides (which are basic substances described in Japanese Patent Publication No. 8816/1956). The reaction is described in detail in the following. A nitrile compound or amide compound may be used as a solvent in the production of a carbonic acid ester by the reaction of alcohol, carbon monoxide, and oxygen which is carried out in the presence of a copper compound alone or together with a platinum metal compound and reaction accelerator. The nitrile compound includes benzonitrile and isobutyronitrile, and the amide compound includes N,N-dimethylformamide and N,N-dimethylacetamide. These solvents may be used individually or in combination with one another. In addition, the solvents should preferably have a boiling point higher than that of water and the carbonic acid ester to be produced. Such solvents make it possible to recycle the catalyst in the form of liquid or slurry to the reaction system in the step (such as flesh vaporization) of separating the catalyst from the resulting carbonic acid ester and water. The total amount of the solvent in the reaction liquid should be 5-80% by volume, preferably 10-50% by volume. With an amount in excess of 80% by volume, the concentration of the reactant is so low that the reaction rate is slow. With an amount less than 5% by volume, the reactants are not dissolved completely. The nitrile compound or amide compound used as a solvent increases the efficiency of the copper compound and hence increases the production rate of carbonic acid ester. According to the present invention, a carbonic acid ester may be produced by the reaction of alcohol, carbon monoxide, and oxygen in the presence of a copper compound alone or together with a platinum metal compound and (a) a quaternary phosphonium halide and a quaternary phosphonium weak acid salt or quaternary phosphonium alkoxide. The quaternary phosphonium halide used in the present invention is represented by the formula (I) below. where R¹, R², R³, and R⁴ represent alkyl groups (e.g., methyl, ethyl, and butyl), alkoxyl groups, aryl groups (e.g., phenyl), aryloxy groups, substituted alkyl groups, substituted alkoxy groups, substituted aryl groups, and substituted aryloxy groups. They may be the same or different. X represents a halogen such as chlorine, bromine, and iodine. Examples of the quaternary phosphonium halide include [(n-Bu)₄P]Cl, [(n-Bu)₃CH₃P]Cl, [(n-Pr)₄P]Cl, [(n-Bu)₄P]Br, and [(n-Pr)₄P]Br (Bu : butyl group, Pr : propyl group). The quaternary phosphonium halide is used in an amount of 0.05 wt% to saturation in the reaction liquid if it is a solid, and in an amount of 0.05 wt% to the amount of solvent if it is a liquid. The optimum amount depends on the rate of formation of the carbonic acid ester and carbon dioxide. The quaternary phosphonium weak acid salt used in the present invention is represented by the formula (II) below. where R¹, R², R³, and R⁴ are defined as in the formula (1); Y represents an anion of a carboxylic acid or carbonic acid; and n is a numeral equal to the valence of the anion. Examples of the quaternary phosphonium weak acid salt include those compounds which are represented by the formulas below. and [(n-Bu)₄P]₂CO₃. The quaternary phosphonium alkoxide is a compound represented by the formula (II) above, in which R¹, R², R³, and R⁴ are defined as in the quaternary phosphonium weak acid salt; n is 1; and Y represents an alkoxy group. Examples of the quaternary phosphonium alkoxide include compounds which are represented by [(n-Bu)₄P]OCH₃ and [(n-Bu)₄P]OC₂H₃. The quaternary phosphonium weak acid salt or the quaternary phosphonium alkoxide is used in an amount of 0.05 wt% to saturation in the reaction liquid if it is a solid, and in an amount of 0.05 wt% to the amount of solvent if it is a liquid. The optimum amount depends on the rate of formation of carbonic acid ester and carbon dioxide. The reaction of alcohol, carbon monoxide, and oxygen in the presence of the copper compound alone or together with the platinum metal compound, quaternary phosphonium halide, and quaternary phosphonium weak acid salt or quaternary phosphonium alkoxide is advantageous over the reaction in the presence of the catalyst system disclosed in Japanese Patent Publication No. 8816/1986 in that the formation of carbon dioxide as a by-product is greatly reduced. The catalyst system of the invention is a little slower in reaction rate than that disclosed in said Japanese Patent. However, the reaction rate can be readily increased by raising the reaction temperature and the oxygen partial pressure and also by using a proper solvent. Decreasing the side reaction and increasing the selectivity for carbonic acid ester are desirable because the unreacted reactants can be recycled. According to the present invention, a carbonic acid ester may also be produced by the reaction of alcohol, carbon monoxide, and oxygen in the presence of a quinoid compound or a compound which changes into a quinoid compound under the reaction condition. Examples of the quinoid compound include unsubstituted or substituted ortho- or para-benzoquinone (having a substituent cyano, carboxymethyl, and methyl group); polynuclear quinone such as anthraquinone; heterocyclic quinone; and their imino, N-alkyl, or N-arylimino derivatives. Examples of the compound which changes into a quinoid compound under the reaction condition include ketals of the corresponding quinoid compounds and hydrogenated compounds thereof, such as hydroquinone. The quinoid or the compound which changes into a quinoid under the reaction condition should be used in an amount of 0.1 to 5 wt% for the total amount of the reactants. The reaction system may contain a third component such as alkali metal salt disclosed in Japanese Patent Publication No. 8816/1986. The quinoid or the compound which changes into a quinoid under the reaction condition suppresses the formation of carbon dioxide as a by-product and affords a carbonic acid ester advantageously. The following examples illustrate the invention. Example 1 In a 500 ml jacketed flask equipped with a stirrer and condenser were placed 221 ml of methanol, 1.88 mmol of palladium chloride, 8.18 mmol of cuprous chloride, 3.76 mmol of tetra-n-butylphosphonium chloride, and 11.3 mmol of tetra-n-butylphosphonium acetate. Reaction was carried out at 60°C for 1 h while carbon monoxide and oxygen were being introduced into the flask under normal pressure at a flow rate of 8.0 l/h and 4.0 l/h, respectively. Dimethyl carbonate was formed at a rate of 93.7 mmol/l-h. Example 2 The same procedure as in Example 1 was repeated except that the amount of methanol was changed to 199 ml and 22 ml of benzonitrile was added. Dimethyl carbonate was formed at a rate of 128 mmol/l-h, which is 1.37 times greater than that in Example 1. This suggests that the nitrile compound also effectively increases the rate at which DMC is formed in the reaction that employs the catalyst system composed of copper, palladium, and phosphonium. Comparative Example 1 In a 500-ml jacketed flask equipped with a stirrer and condenser were placed 221 ml of methanol, 1.88 mmol of palladium chloride, 3.76 mmol of potassium chloride, 8.18 mmol of cuprous chloride, and 11.3 mmol of potassium acetate. Reaction was carried out at 60°C for 1 h while carbon monoxide and oxygen were being introduced into the flask under normal pressure at a flow rate of 8.0 l/h and 4.0 l/h, respectively. The liquid product and offgas were analyzed by gas chromatography. The results are shown in Table 1, in which r (DMC) and r (CO₂) represent the rate at which dimethyl carbonate and carbon dioxide were formed, respectively, and S(CO₂) represents the percentage of carbon dioxide formed as a by-product which is defined as follows: Example 3 The same procedure as in Comparative Example 1 was repeated except that the potassium chloride was replaced by 3.76 mmol of tetra-n-butylphosphonium chloride and the potassium acetate was replaced by 11.3 mmol of tetra-n-butylphosphonium chloride. The results of the analysis are shown in Table 1. Example r (DMC) [mmol/L-h] r (CO₂) [mmol/L-h] S(CO₂) (%) Comparative Example 1 160 45.6 22.2 Example 3 93.7 9.36 9.1 Comparative Example 2 In the same apparatus as used in Comparative Example were placed 188 ml of methanol, 33 ml of benzonitrile, 3.76 mmol of palladium bromide, 7.51 mmol of potassium bromide, 16.4 mmol of cuprous bromide, and 22.5 mmol of potassium acetate. The reaction was carried out at 60° for 1.5 h while carbon monoxide and oxygen were being introduced into the flask under normal pressure at a flow rate of 20.0 l/h and 10.0 l/h, respectively. The results or analyses are shown in Table 2. Example 4 The same procedure as in Comparative Example 2 was repeated except that the potassium bromide was replaced by 7.51 mmol of tetra-n-butylphosphonium bromide and the potassium acetate was replaced by 22.5 mmol of tetra-n-butylphosphonium acetate. The results of analysis are shown in Table 2. Example r (DMC) [mmol/L-h] r (CO₂) [mmol/L-h] S(CO₂) (%) Comparative Example 2 435 378 46.4 Example 4 340 169 33.2 Comparative Example 3 The same procedure as in Comparative Example 2 was repeated except that the potassium acetate was replaced by 22.5 mmol of potassium benzoate. The results of analyses are shown in Table 3. Example 5 The same procedure as in Example 4 was repeated except that the tetra-n-butylphosphonium acetate was replaced by 22.5 mmol of tetra-n-butylphosphonium benzoate. The results of analysis are shown in Table 3. Example r (DMC) [mmol/L-h] r (CO₂) [mmol/L-h] S(CO₂) (%) Comparative Example 3 443 348 44.0 Example 5 192 64.7 25.2 Comparative Example 4 The same procedure as in Comparative Example 2 was repeated except that the potassium acetate was replaced by 22.5 mmol of potassium pivalate. The results of analysis are shown in Table 4. Example 6 The same procedure as in Example 4 was repeated except that the tetra-n-butylphosphonium acetate was replaced by 22.5 mmol of tetra-n-butylphosphonium pivalate. The results of analysis are shown in Table 4. Example r (DMC) [mmol/L-h] r (CO₂) [mmol/L-h] S(CO₂) (%) Comparative Example 4 288 86.2 23.0 Example 6 136 21.1 13.4 Example 7 In a jacketed flask equipped with a stirrer and condenser were placed 221 ml of methanol, 3.7 mmol of palladium bromide, 16.4 mmol of cuprous bromide, 7.5 mmol of potassium bromide, 22.5 mmol of potassium acetate, and 15.5 mmol of duroquinone. The reaction was carried out at 60°C for 6h while carbon monoxide and oxygen were into the flask under normal pressure at a flow rate of 8.0 liters/hour and 4.0 l/h, respectively. The analysis of the liquid product by gas chromatography indicated the formation of 256.6 mmol of dimethyl carbonate. The reaction for 6 h formed 147.6 mmol of carbon dioxide, which is equal to 36.5% in the total amount of dimethyl carbonate and carbon dioxide formed. Comparative Example 5 The same procedure as in Example 7 was repeated except that the duroquinone was not added. The reaction formed 238.7 mmol of dimethyl carbonate and 250.8 mmol of carbon dioxide. The ratio of carbon dioxide as a by-product was 51.2%. Example 8 The same procedure as in Example 7 was repeated except that the duroquinone was replaced by 2.2 mmol of p-benzoquinone. The reaction formed 148.1 mmol of dimethyl carbonate and 57.0 mmol of carbon dioxide. The ratio of carbon dioxide as a by-product was 27.8%. Example 9 The same procedure as in Example 7 was repeated except that the duroquinone was replaced by 15.5 mmol of 2-tert-butylanthraquinone. The reaction formed 141.2 mmol of dimethyl carbonate and 49.1 mmol of carbon dioxide. The ratio of carbon dioxide as a by-product was 25.8%. Example 10. The same procedure as in Example 7 was repeated except that the duroquinone was replaced by 15.5 mmol of phenanthraquinone. The reaction formed 199.6 mmol of dimethyl carbonate and 108.3 mmol of carbon dioxide. The ratio of carbon dioxide as a by-product was 35.2%. Example 11 The same procedure as in Example 7 was repeated except that the duroquinone was replaced by 2.2 mmol of hydroquinone. The reaction formed 105.6 mmol of dimethyl carbonate and 49.9 mmol of carbon dioxide. The ratio of carbon dioxide as a by-product was 32.1%.

Claims: A process for producing a carbonic acid ester by reacting alcohol, carbon monoxide and oxygen in the presence of a copper compound or together with a platinum metal compound and a further compound, characterized in that the further compound is a) a quaternary phosphonium halide and a quaternary phosphonium weak acid salt or quaternary phosphonium alkoxide and/or b) a quinoid compound or a compound that changes into a quinoid compound under the reaction condition. The process of claim 1 in which furthermore a nitrile compound or amide compound is present. The process of claim 1, in which the quaternary phosphonium halide is (n-C₄H₉)₄PCl or (n-C₄H₉)₄PBr, and the quaternary phosphonium weak acid salt is The process of claim 1, in which the quinoid compound or a compound that changes into a quinoid compound under the reaction condition is added in an amount of 0.1 to 5 wt% based on the total amount of the reactants. The process of claim 1, in which the platinum metal compound is a palladium compound. The process of claim 1, in which the alcohol has 1 to 20 carbon atoms. The process of claim 1 in which the alcohol is methanol.

Inventors: KOJIMA, HIDETAKA, KOYAMA, HIROSHI, YOKOTA, SHIGERU
Applicants: DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1987
Main classification: C07C68
IPC codes: B01J31, C07C68