PROCESS FOR PRODUCING A HIGH PURITY QUATERNARY AMMONIUM HYDROXIDE

Abstract: A process for production of high purity quaternary ammonium hydroxides, comprising electrolyzing quaternary ammonium hydrogencarbonates represented by the general formula: (wherein the symbols are as defined in the appended claims) in an electrolytic cell comprising an anode compartment and a cathode compartment defined by a cation exchange membrane. In accordance with this process, high purity quaternary ammonium hydroxides can be produced with high electrolytic efficiency and further without causing corrosion of equip­ ment. Since the quaternary ammonium hydroxides produced by the present invention are of high purity, they can be effectively used as, for example, cleaners, etchants or developers for wafers in the production of IC and LSI in the field of electronics and semiconductors.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a process for producing a high purity quaternary ammonium hydroxide which comprises electrolyzing a quaternary ammonium hydrogencarbonate represented by the general formula (I): (wherein R¹, R², R³ and R⁴ may be the same or different and are each an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, or an aryl group or hydroxyaryl group), which has been prepared by reacting a tertiary amine represented by the general formula (II): (R¹R²R³)₃N (II) (wherein R¹, R² and R³ may be the same or different and are each an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group) with a dialkyl carbonate or diaryl carbonate represented by the general formula (III): (wherein R⁴ is an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group, and R⁵ is an alkyl group having 1 to 8 carbon atoms or an aryl group) in the presence of a solvent at a temperature of from 40 to 250°C, in an electrolytic cell comprising an anode compartment containing the quaternary ammonium hydrogen carbonate of formula (I) and a cathode compartment defined by a cation exchange membrane. 2. Description of the prior art Quaternary ammonium hydroxides of high purity are widely used in the electronics and semiconductor industry, specifically as cleaners, etchants and developers for wafers in the production of integrated circuits (IC) and large scale integrations (LSI). With a recent increase in the degree of integration in semiconductors, it has been increasingly demanded to increase the purity of chemicals for use in the production thereof. Thus, in order to increase the purity of quaternary ammonium hydroxides, the starting materials for use in production thereof and a process for the production thereof have been investigated. For electrolytic production of quaternary ammonium hydroxides, many methods have been proposed, including those as described in, for example, JP-B-28 564/1970 and 14 885/1971, JP-A-155 390/1982, 181 385/1982, 193 287/1984, 193 288/1984, 193 228/1984, 100 690/1985, 131 985/1985 and 131 986/1985. In the above methods, as quaternary ammonium salts to be subjected to hydrolysis, quaternary ammonium halides and quaternary ammonium sulfates are mainly used. However, when quaternary ammonium halides are used, part of halogen ions pass through the cation exchange membrane and enter the cathode compartment, thereby contaminating the final product of quaternary ammonium hydroxides and, therefore, high purity quaternary ammonium hydroxides are difficult to produce. Furthermore, halogen gas is generated during the electrolysis, thereby causing problems such as corrosion of the anode itself. Since the halogen gas generated is harmful, it is necessary to install equipment for removal or neutralization of the halogen gas. When quaternary ammonium sulfates are used as the starting material, problems arise in that they are difficult to handle, and sulfuric acid formed during the electrolysis corrodes the electrodes and equipment. Thus, high purity quaternary ammonium hydroxides are difficult to produce from quaternary ammonium sulfates. When quaternary ammonium organic carboxylic acid salts as described in JP-A-100 690/1985 are used as the starting material, organic carboxylic acids are formed during the electrolysis, which may undesirably corrode the anode itself. Furthermore, part of the organic carboxylic acids may pass through the cation exchange membrane and intermingle with the final product of quaternary ammonium hydroxides, thereby decreasing the purity thereof. Electrolysis of quaternary ammonium hydrogencarbonates using a diaphragm made of such materials as procelain, carborundum and arandum is disclosed in JP-B-28 564/1970 and 14 885/1981. By the use of such a diaphragm, however, high purity quaternary ammonium hydroxides cannot be obtained, and the method has disadvantages in that the current efficiency is low. From JP-A-61-170 588 a process for producing a quaternary ammonium hydroxide by electrolyzing quaternary ammonium hydrogen carbonates is known wherein quaternary ammonium hydrogen carbonates are used which have been prepared by reacting a tertiary amine with a dialkyl carbonate or diaryl carbonate in the presence of an alcohol solvent. SUMMARY OF THE INVENTION It is the object of the present invention to solve the above problems and to provide a method for producing a high purity quaternary ammonium hydroxide with high efficiency. It has been found that the demanded high purity quaternary ammonium hydroxides can be produced by electrolyzing quaternary ammonium hydrogen carbonates which have been prepared by reacting a compound of the above formula (II) with a compound of the above formula (III) in the presence of water as a solvent at a temperature of from 40 to 250°C in an electrolytic cell comprising an anode compartment and a cathode compartment defined by a cation exchange membrane. Subject-matter of the present invention is a process for producing a high purity quaternary ammonium hydroxide which comprises electrolyzing a quaternary ammonium hydrogencarbonate represented by the general formula (I): (wherein R¹, R², R³ and R⁴ may be the same or different and are each an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group), which has been prepared by reacting a tertiary amine represented by the general formula (II): (R¹R²R³)₃N (II) (wherein R¹, R² and R³ may be the same or different and are each an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group) with a dialkyl carbonate or diaryl carbonate represented by the general formula (III): (wherein R⁴ is an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group, and R⁵ is an alkyl group having 1 to 8 carbon atoms or an aryl group) in the presence of a solvent at a temperature of from 40 to 250°C, in an electrolytic cell comprising an anode compartment containing the quaternary ammonium hydrogen carbonate of formula (I) and a cathode compartment defined by a cation exchange membrane, which is characterized in that in the reaction of the compound of formula (II) with the compound of formula (III) water is used as a solvent. Preferred embodiments of the present invention are described in subclaims 2 to 21. DETAILED DESCRIPTION OF THE INVENTION The main reaction of the present invention is represented by the following reaction formula: (wherein R¹, R², R³ and R⁴ are the same as defined above). In accordance with the present invention, therefore, only carbon dioxide gas is formed along with the desired quaternary ammonium hydroxides. That is, neither corrosive substances nor impurities which may cause the contamination of the final product of quaternary ammonium hydroxides are formed during the electrolysis. The quaternary ammonium hydrogencarbonates which are used in the present invention are represented by the general formula (I): (wherein R¹, R², R³ and R⁴ are the same as defined above). Representative examples are tetramethylammonium hydrogencarbonate, tetraethylammonium hydrogencarbonate, tetrapropylammonium hydrogencarbonate, trimethylpropylammonium hydrogencarbonate, trimethylbutylammonium hydrogencarbonate, trimethylbenzylammonium hydrogencarbonate, trimethylhydroxyethylammonium hydrogencarbonate, trimethylmethoxyammonium hydrogencarbonate, dimethyldiethylammonium hydrogencarbonate, dimethyldihydroxyethylammonium hydrogencarbonate, methyltriethylammonium hydrogencarbonate and methyltrihydroxyethylammonium hydrogencarbonate. Since the object of the present invention is to produce high purity quaternary ammonium hydroxides, it is naturally necessary to use quaternary ammonium hydrogencarbonates which are of high purity as the starting material. From the above viewpoint, quaternary ammonium hydrogencarbonates prepared by reacting tertiary amines and dialkyl carbonates or diaryl carbonates in the presence of water are used in the present invention because of their high purity. This method which will hereinafter be explained in detail can be repesented by the following reaction formula: In the above formula R¹, R², R³ and R⁴ are the same as defined above, and R⁵ is an alkyl group having 1 to 8 carbon atoms or an aryl group. Representative examples of the tertiary amines represented by the above general formula: (R¹R²R³)₃N (II) are trimethylamine, triethylamine, tripropylamine, tributylamine, trioctylamine, dimethylethylamine, diethylmethylamine, N,N'-dimethylbenzylamine, N,N'-dimethylaniline, N,N'-dimethylcyclohexylamine, N,N'-diethylbenzylamine, N,N'-dimethylethanolamine, N,N'-diethylethanolamine, N-methyldiethanolamine, triethanolamine, N-methyldiethanolamine and N-ethyldiethanolamine. Representative examples of the dialkyl carbonates or diaryl carbonates represented by the above general formula: are dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, diphenyl carbonate, dibenzyl carbonate, dicyclohexyl carbonate, methylpropyl carbonate and ethylpropyl carbonate. Water is an essential component for the reaction and also acts as a solvent, and thus it can be used in a greater amount than the stoichiometrical amount. The amounts of the above dialkyl carbonates or diaryl carbonates and tertiary amines used vary with the kind of the dialkyl carbonates or diaryl carbonates, the kind of the tertiary amines and reaction conditions. In general, the molar ratio of the dialkyl carbonates or diaryl carbonates to the tertiary amines is 0,05:1 to 20:1 and preferably 0,1:1 to 10:1. It suffices basically that water is added in a stoichiometrically excessive amount in relation to the dialkyl carbonates or diaryl carbonates and tertiary amines. If, however, the amount of water used is too large, the separation and removal of the remaining water after the completion of the reaction needs a longer time, which is not advantageous from an economic standpoint. The reaction temperature is generally in the range of from 40 to 250°C and preferably 50 to 200°C. In practice, however, the reaction temperature should be determined taking into consideration the rate of reaction, the decomposition of the starting material of dialkyl carbonates or diaryl carbonates and of the reaction product of quaternary ammonium hydrogencarbonates. If necessary, the reaction can be carried out in an atmosphere of inert gas such as nitrogen, argon and helium, or hydrogen gas, which do not exert adverse influences on the reaction. The reaction can be carried out batchwise, semibatchwise or continously. In the present invention, an electrolytic cell comprising an anode compartment and a cathode compartment defined by a cation exchange membrane is usually used. In addition, an electrolytic cell comprising an anode compartment, a cathode compartment and at least one intermediate compartment defined by at least two cation exchange membranes can be used. As the cation exchange membrane which is used in the present invention, a membrane made of corrosion resistant fluorine-containing polymers having cation exchange groups such as sulfonic acid groups and carboxylic acid groups in suitable. In addition, those made of styrene-divinylbenzene copolymers having cation exchange groups as described above can be used. As the anode which is used in the present invention, electrodes commonly used in electrolysis of this type, such as a high purity carbon electrode and a platinum or platinum oxide-covered titanium electrode, are used. As the cathode which is used in the present invention, electrodes commonly used in electrolysis of this type, such as a stainless steel electrode and a nickel electrode, are used. These anode and cathode may be shaped in any desired form such as a plate, a bar, a net and a porous plate. The electrolytic cell and other equipment such as a reservoir, pipes and valves which are used in the present invention are preferably made of corrosion-resistant materials such as fluorine-containing polymers and polypropylene. In the present invention, electrolysis is carried out by applying a DC voltage. The current density is 1 to 100 A/dm² and preferably 3 to 50 A/dm². The electrolytic temperature is preferably in the range of 10 to 50°C. The electrolysis of the present invention can be carried out batchwise or continuously. The concentration of the starting material in an aqueous solution to be introduced in the anode compartment is adjusted to 1 to 60% by weight and preferably 3 to 40% by weight. In the cathode compartment is introduced preferably ultra pure water. If, however, only ultra pure water is introduced in the cathode compartment, the electric conductance is low at the start of the operation and electrolysis occurs only with difficulty. It is desirable, therefore, that the desired quaternary ammonium hydroxides be added in a small amount, e.g., in a proportion of 0.01 to 5% by weight. Preferably, prior to the electrolysis, the equipment is fully cleaned. It is also preferred that the electrolysis can be carried out in an atmosphere of clean inert gas such as nitrogen and argon. The present invention produces various advantages over the conventional methods. One of the major advantages is that high purity quaternary ammonium hydroxides can be easily produced with high electrolytic efficiency. Another advantage is that the problems encountered in the conventional methods, such as corrosion of equipment, can be overcome. EXAMPLE 1 In an electrolytic cell comprising an anode compartment and a cathode compartment defined by a cation exchange membrane Nafion 324 (trade name, for a fluorine-containing polymer-based cation exchange membrane produced by E.I. du Pont de Nemours & Co.), with a platinum-covered titanium electrode as anode and stainless steel (SUS 304) as cathode, a 30% by weight solution of tetramethylammonium hydrogencarbonate in ultra pure water was cycled in the anode compartment, and in the cathode compartment, a 0.5% by weight solution of tetramethylammonium hydroxide in ultra pure water was cycled. Electrolysis was carried out by applying a DC current of 10 A/dm² between the anode and the cathode at a temperature of 40°C. At an electrolytic voltage of 7 to 11 V and an average current efficiency of 94%, a 4.13% by weight aqueous solution of tetramethylammonium hydroxide was obtained in the cathode compartment. The concentrations of impurities contained in the aqueous tetramethylammonium hydroxide solution as obtained above are shown below. Na, Fe, K, Ca: 0.001 ppm Al, Ag, Co, Cr, Mg, Mn, Ni, Zn: Less than 0.001 ppm Cl: Less than 0.01 ppm EXAMPLE 2 In the same electrolytic cell as used in Example 1 with the exception that H type Nafion 423 (trade name for a fluorine-containing polymer-based cation exchange membrane produced by E. I. du Pont de Nemours & Co.) was used as the cation exchange membrane, a 35% by weight solution of tetramethylammonium hydrogencarbonate in ultra pure water was cycled in the anode compartment, and in the cathode compartment, a 0.5% by weight solution of tetramethylammonium hydroxide in ultra pure water was cycled. Electrolysis was carried out by applying a DC current of 15 A/dm² between the anode and cathode at a temperature of 40°C. At an electrolytic voltage of 10 to 15 V and an average current efficiency of 93%, a 25.74% by weight aqueous solution of tetramethylammonium hydroxide in the cathode compartment was obtained. The concentrations of impurities contained in the aqueous tetramethylammonium hydroxide solution as obtained above are shown below: Na: 0.003 ppm Fe: 0.005 ppm K, Ca: 0.001 ppm Al, Ag, Co, Cr, Cu, Mg, Mn, Ni, Zn: Less than 0.001 ppm Cl: Less than 0.01 ppm PREPARATION EXAMPLE 1 The tetramethylammonium hydrogencarbonate used in Examples 1 and 2 was prepared as follows. 604 g of dimethyl carbonate, 394 g of trimethylamine and 250 g of water were introduced in a 3,000-milliliter Teflon®-lined reactor and heated with stirring. After the temperature in the reactor reached 100°C, the reaction was continued for 6 hours at 100°C. Tetramethylammonium hydrogencarbonate was obtained in a yield of 90.1 mol% (based on trimethylamine). EXAMPLE 3 604 g of dimethyl carbonate, 394 g of trimethylamine, 300 g of water and 500 g of methanol were introduced in the same reactor as used in Preparation Example 1 and heated with stirring. After the temperature in the reactor reached 100°C, the reaction was continued for 3 hours at 100°C. Tetramethylammonium hydrogencarbonate was obtained in a yield of 90.3 mol% (based on trimethylamine). The tetramethylammonium hydrogencarbonate thus obtained was electrolyzed in the same apparatus as used in Example 1 with the exception that a platinum-coated titanium electrode was used as anode and a nickel electrode, as cathode. A 20% by weight solution of tetramethylammonium hydrogencarbonate in ultra pure water was cycled in the anode compartment, and in the cathode compartment, a 1% by weight solution of tetramethylammonium hydroxide in ultra pure water was cycled. Electrolysis was carried out by applying a DC current of 13 A/dm² between the anode and the cathode at a temperature of 35°C. At an electrolytic voltage of 9 to 14 V and an average current efficiency of 90%, a 23.36% by weight aqueous solution of tetramethylammonium hydroxide was obtained in the cathode compartment. The concentrations of impurities contained in the aqueous tetramethylammonium hydroxide as obtained above are shown below: Fe: 0.003 ppm Na, K, Ca: 0.001 ppm Al, Ag, Co, Cr, Mg, Mn, Ni, Zn: Less than 0.001 ppm Cl: Less than 0.01 ppm EXAMPLE 4 In the same electrolytic apparatus as used in Example 3, a 30% by weight solution of tetraethylammonium hydrogencarbonate in ultra pure water was cycled in the anode compartment, and in the cathode compartment, a 1% by weight solution of tetraethylammonium hydroxide in ultra pure water was cycled. Electrolysis was carried out by applying a DC current of 10 A/dm² in the anode and the cathode at a temperature of 45°C. At an electrolytic voltage of 7 to 12 V and an average current efficiency of 89%, a 14.95% by weight aqueous solution of tetraethylammonium hydroxide was obtained. The concentrations of impurities contained in the aqueous tetraethylammonium hydroxide solution as obtained above are shown below: Fe: 0.005 ppm Na: 0.003 ppm K, Al, Ca: 0.001 ppm Ag, Co, Cr, Mg, Ni, Zn: Less than 0.001 ppm Cl: Less than 0.01 ppm PREPARATION EXAMPLE 2 The tetraethylammonium hydrogencarbonate used in Example 4 was prepared as follows. 63 g of diethyl carbonate, 63.4 g of triethylamine and 50.0 g of water were introduced in the same reactor as used in Preparation Example 1 and heated with stirring. After the temperature in the reactor reached 140°C, the reaction was continued for 5 hours at 140°C. Tetraethylammonium hydrogencarbonate was obtained in a yield of 87.9 mol% (based on triethylamine).

Claims: A process for producing a high purity quaternary ammonium hydroxide which comprises electrolyzing a quaternary ammonium hydrogencarbonate represented by the general formula (I): (wherein R¹, R², R³ and R⁴ may be the same or different and are each an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group), which has been prepared by reacting a tertiary amine represented by the general formula (II): (R¹R²R³)₃N (II) (wherein R¹, R² and R³ may be the same or different and are each an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group) with a dialkyl carbonate or diaryl carbonate represented by the general formula (III): (wherein R⁴ is an alkyl group or hydroxyalkyl group having 1 to 8 carbon atoms, an alkoxyalkyl group having 2 to 9 carbon atoms, or an aryl group or hydroxyaryl group, and R⁵ is an alkyl group having 1 to 8 carbon atoms or an aryl group) in the presence of a solvent at a temperature of from 40 to 250°C, in an electrolytic cell comprising an anode compartment containing the quaternary ammonium hydrogen carbonate of formula (I) and a cathode compartment defined by a cation exchange membrane, characterized in that in the reaction of the compound of formula (II) with the compound of formula (III) water is used as a solvent. The process as claimed in claim 1 wherein the cation exchange membrane is made of a fluorine-containing polymer or a styrene-divinylbenzene copolymer, having cation exchange groups. The process as claimed in claim 2 wherein the cation exchange membrane is made of a fluorine-containing polymer having cation exchange groups. The process as claimed in claim 3 wherein the anode is a carbon electrode or a platinum or platinum oxide-coated titanium electrode. The process as claimed in claim 3 wherein the cathode is a stainless steel electrode or a nickel electrode. The process as claimed in any of claims 1 to 5 wherein the electrolytic cell is made of a corrosion-resistant material. The process as claimed in claim 6 wherein the corrosion-resistant material is a fluorine-containing polymer or polypropylene. The process as claimed in any of claims 1 to 7 wherein the electrolysis is carried out at a current density of 1 to 100 A/dm². The process as claimed in claim 8 wherein the current density is 3 to 50 A/dm². The process as claimed in any of claims 1 to 9 wherein the electrolysis is carried out at a temperature of 10 to 50°C. The process as claimed in any of claims 1 to 10 wherein the quaternary ammonium hydrogencarbonate is used as a 1 to 60 % by weight aqueous solution. The process as claimed in claim 11 wherein the concentration of the quaternary ammonium hydrogencarbonate is 3 to 40 % by weight. The process as claimed in any of claims 1 to 12 wherein water is introduced into the cathode compartment. The process as claimed in claim 13 wherein the water contains 0,01 to 5 % by weight of the quaternary ammonium hydroxide. The process as claimed in claim 13 or 14 wherein the water is ultra pure water. The process as claimed in any of claims 1 to 15 wherein the quaternary ammonium hydrogencarbonate is selected from the group consisting of tetramethylammonium hydrogencarbonate, tetraethylammonium hydrogencarbonate, tetrapropylammonium hydrogencarbonate, trimethylpropylammonium hydrogencarbonate, trimethylbutylammonium hydrogencarbonate, trimethylbenzylam-monium hydrogencarbonate, trimethylhydroxyethylammonium hydro-gencarbonate, trimethylmethoxyammonium hydrogencarbonate, dimethyldiethylammonium hydrogencarbonate, dimethyldihydroxyethylammonium hydrogencarbonate, methyltriethylammonium hydrogencarbonate, and methyltrihydroxyethylammonium hydrogencarbonate. The process as claimed in any of claims 1 to 16 wherein the molar ratio of the dialkyl carbonate or diaryl carbonate to the tertiary amine is 0,05:1 to 20:1. The process as claimed in claim 17 wherein the molar ratio of the dialkyl carbonate or diaryl carbonate to the tertiary amine is 0,1:1 to 10:1. The process as claimed in any of claims 1 to 18 wherein the water is used in a stoichiometrically excessive amount in relation to the dialkyl carbonate or diaryl carbonate, or the tertiary amine. The process as claimed in any of claims 1 to 19 wherein the reaction temperature is 50 to 200°C. The process as claimed in any of claims 1 to 20 wherein R¹, R², R³ and R⁴ are each an alkyl group having 1 to 4 carbon atoms.

Inventors: AOYAMA, TETSUO, ISHIKAWA, JIRO, SAKURAI, NAOTO, SHIMA, EIJI
Applicants: MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1987
Main classification: C25B3
IPC codes: C25B3