Method for producing α,α'-dichloroxylene

Abstract

To provide a high-purity α,α'-dichloroxylene by inhibiting the formation of impurities, such as nucleus-chlorinated compounds and chlorinated compounds formed by further chlorinating the side chains of the nucleus-chlorinated compounds, that have been difficult to separate out in purification by distillation.SOLUTION: A method for producing an α,α'-dichloroxylene according to the present invention includes a step in which, in a reaction vessel containing a boiling-state reaction fluid including starting-material xylene, the side-chain methyl groups of the xylene are chlorinated, the method being characterized by depressuring the inside of the reaction vessel with the degree of chlorination of the reaction fluid becoming higher, thereby keeping the reaction fluid boiling.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing α,α'-dichloroxylene. [Background technology]

[0002] α,α'-Dichloroxylene is a bifunctional compound with one chloro group in each of the side chain methyl groups of the aromatic ring. α,α'-Dichloroxylene exists as ortho, meta, and para isomers, and all of these isomers are useful as raw materials for high-performance polymers.

[0003] α,α'-Dichloroxylene can be produced by the side-chain chlorination reaction of xylene. Normally, the side-chain chlorination reaction is a radical reaction, and it is a sequential reaction, so it is difficult to stop the synthesis reaction when one chlorine atom is bonded to each of the two methyl groups, and it has been difficult to obtain the desired α,α'-dichloroxylene with high selectivity.

[0004] Patent Document 1 discloses a method for producing α,α'-dichloroxylene by a chlorine substitution reaction using concentrated hydrochloric acid from xylene glycol as a raw material. According to this method, by-products such as α-chloroxylene and α,α-dichloroxylene are not produced, but since the raw material xylene glycol is expensive and pressurized conditions are required, it is difficult to say that this method is industrially advantageous.

[0005] In addition, Non-Patent Document 1 discloses a method for producing α,α'-dichloroxylene by a chloromethylation reaction of benzyl chloride. Although the reaction proceeds under very mild conditions, the product is an isomer mixture of dichloroorthoxylene, dichlorometa-xylene, and dichloropara-xylene. In addition, large amounts of zinc chloride and perchloric acid are used as catalysts, and this method cannot be said to be industrially viable.

[0006] A method for producing α,α'-dichloroxylene is also known, which uses chlorine gas to chlorinate the side chain of xylene under light irradiation. Patent Document 2 discloses a method of photochlorination in an acetonitrile-benzene mixed solvent in order to increase the selectivity of chlorination. This method has a low efficiency and is expensive as a production method, because the conversion rate is reduced to increase the selectivity.

[0007] Patent Document 3 describes a method for producing high-purity α,α'-dichloroxylene. By keeping the reaction temperature low, α,α'-dichloroxylene, which has low solubility, is precipitated as a solid, and a product with a purity of 94-96% is obtained. In this production method, the reaction liquid becomes a slurry during the process, which is essentially disadvantageous as a photoreaction and does not result in a high yield. Furthermore, with the development of technology, the purity of monomers required for highly functional polymers is also high, and even this purity is not sufficient to meet market demands.

[0008] Considering production costs and efficiency, the side-chain chlorination method using chlorine gas is considered to be suitable for industrial use. It is known that impurities generated by this production method include α,α-dichloroxylene, in which two or more chlorines are bonded to one methyl group, perchlorinated products with even more advanced chlorination, and nuclear chlorinated products in which chlorine is directly bonded to the benzene ring. If it is possible to increase the conversion rate while suppressing the generation of these impurities, it is believed that it will be possible to efficiently produce α,α'-dichloroxylene.

[0009] Patent Document 4 describes a method for producing α,α'-dichloroxylene in which the concentration of α-monochloroxylene, an intermediate, in the reaction system is kept high in order to suppress the production of the above-mentioned impurities. Since this method requires a high concentration of α-monochloroxylene, the production efficiency of α,α'-dichloroxylene is low, and impurities tend to increase when the conversion rate is increased. Furthermore, while there is a description of side-chain perchlorinated products in the composition ratio of the resulting reaction solution, there is no description of nuclear chlorinated products.

[0010] Patent Document 5 describes a method for producing a 1,4-bis(trichloromethyl)benzene compound. It is described that in this production method, the cause of the nuclear chlorination reaction to the benzene ring is HCl produced as a by-product, and that the HCl concentration in the reaction system can be reduced by keeping the reaction solution boiling during the reaction, thereby suppressing nuclear chlorine substitution. However, since the boiling point of the reaction solution usually increases as the side chain chlorine substitution progresses, it is not easy to maintain a boiling state. It is said that it is preferable to adopt a method of gradually increasing the reaction temperature while watching the progress of the reaction, but it is described that the reaction temperature should be limited to 160°C. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 02-292232 [Patent Document 2] U.S. Pat. No. 3,442,960 [Patent Document 3] Japanese Patent Application Publication No. 53-65830 [Patent Document 4] Japanese Patent Application Publication No. 63-72632 [Patent Document 5] JP 2019-156766 A [Non-patent literature]

[0012] [Non-Patent Document 1] Industrial Chemistry Journal 1968-71(6)-869 Summary of the Invention [Problem to be solved by the invention]

[0013] As described above, in the process for producing α,α'-dichloroxylene, the side chain chlorination reaction using chlorine gas is considered to be useful as an industrial process. However, all of the conventional processes are methods in which the reaction is stopped under conditions that keep the conversion rate low in order to suppress by-products such as overchlorinated products and chlorinated nuclei, and the raw xylene and low-chlorinated intermediates are recovered and recycled, resulting in poor production efficiency and high costs.

[0014] Usually, the nuclear chlorination reaction proceeds with the aid of a Lewis acid as a catalyst, but in the production of α,α'-dichloroxylene, it is believed that the reaction proceeds with the aid of HCl, a by-product of the reaction. Bronsted acids such as HCl can act as catalysts for nuclear chlorination reactions, just like Lewis acids. Therefore, if the HCl concentration in the reaction system can be reduced during the production of α,α'-dichloroxylene, it is possible to suppress the nuclear chlorination reaction.

[0015] Patent Document 5 describes the method of blowing chlorine gas while refluxing at the boiling point temperature, which describes that the reaction proceeds quickly and has the effect of removing the by-product HCl from the reaction system. However, since the boiling point of the reaction solution usually increases as the side chain chlorine substitution proceeds, it is not easy to maintain the boiling state. Therefore, it is considered to use a low-boiling point solvent that does not react with chlorine gas, but the type of solvent that meets this condition is limited. In addition, when a large amount of solvent is used, the production efficiency decreases and the cost increases.

[0016] In addition, since the boiling point of the reaction liquid rises as the chlorination proceeds, if the reaction is carried out without a solvent, the heat medium must also be heated to maintain the boiling state, which requires investment in equipment capable of high-temperature heating, resulting in increased fixed costs. Furthermore, even if high-temperature heating is required to maintain the boiling state, there is a limit to the temperature to which the temperature can be raised. The object of the present invention is to obtain high-purity α,α'-dichloroxylene by suppressing the generation of impurities such as nuclear chlorinated products and chlorinated products in which side chain chlorination has progressed further from the nuclear chlorinated products, which are difficult to separate during distillation purification. [Means for solving the problem]

[0017] In order to solve the above problems, the present inventors conducted extensive research and found that, even if the boiling point of the reaction liquid rises, it is possible to carry out chlorination in the boiling reaction liquid without using a solvent by reducing the pressure inside the reaction vessel and keeping the reaction liquid at a boil, and discovered the following method for producing α,α'-dichloroxylene. The boiling point of the reaction liquid rises as the chlorination proceeds, but by reducing the pressure inside the vessel, it is possible to keep the boiling point of the reaction liquid constant and to reduce the concentration of dissolved HCl in the reaction liquid, which causes the nuclear chlorination reaction.

[0018] That is, the present invention is a method for producing α,α'-dichloroxylene, comprising the steps of: and an additive selected from triphenylphosphine, triphenylphosphine oxide, pyridine, and hexamethylenetetramine and reducing the pressure inside the reaction vessel as the degree of chlorination of the reaction liquid increases, thereby maintaining the reaction liquid in a boiling state. Effect of the Invention

[0019] In comparison with conventional methods, the production method of the present invention suppresses the generation of impurities such as nuclear chlorinated compounds and chlorinated compounds in which side chain chlorination has progressed further from the nuclear chlorinated compounds, which have been difficult to separate during distillation purification, and can obtain high-purity α,α'-dichloroxylene by normal distillation purification. Furthermore, in this production method, since chlorination can be performed while adjusting the boiling point by reducing the pressure inside the vessel, it is not necessary to use a solvent as in conventional methods, and it is possible to maximize the vessel efficiency, which is also very advantageous in terms of cost. By using this production method, it is possible to suppress the production of the above-mentioned by-products, which are difficult to separate during distillation purification, and to produce high-purity α,α'-dichloroxylene. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention relates to a method for producing α,α'-dichloroxylene, comprising the steps of: and an additive selected from triphenylphosphine, triphenylphosphine oxide, pyridine, and hexamethylenetetramineand reducing the pressure inside the reaction vessel as the degree of chlorination of the reaction liquid increases, thereby maintaining the reaction liquid in a boiling state.

[0021] As used herein, the term xylene means ortho-xylene, meta-xylene, or para-xylene, and α,α'-dichloroxylene means α,α'-dichloroortho-xylene, α,α'-dichlorometa-xylene, or α,α'-dichloropara-xylene.

[0022] The chlorination reaction of xylene is usually carried out by blowing chlorine gas without solvent under light irradiation. Even when a solvent is used, if the chlorination reaction can be carried out while maintaining the boiling point of the reaction liquid, it is possible to suppress the generation of nuclear chlorinated compounds. However, the solvent that can withstand this chlorination condition must have a property that does not cause chlorination reactions, and the types are limited. For example, benzotrifluoride, 4-chlorobenzotrifluoride, chlorobenzene, dichlorobenzene, etc. can be mentioned. However, in order to suppress the generation of nuclear chlorinated compounds, a large amount of solvent is required to maintain the boiling state, which results in a deterioration of the efficiency of the vessel and a decrease in the yield, so it is industrially preferable not to use a solvent.

[0023] In the production method of the present invention, the chlorination reaction of xylene is carried out by reducing and adjusting the pressure inside the reaction vessel in accordance with the rise in the boiling point of the reaction liquid so as to maintain the reaction liquid in a boiling state. The pressure reduction can be adjusted by the pressure regulating valve of the vacuum pump so that the reaction liquid having the composition calculated from the chlorination degree boils at the heat transfer medium temperature at that time. Here, the chlorination degree is a numerical value representing the number of moles of chlorine bonded to 1 mole of xylene by reaction. In the production method of the present invention, for example, the chlorination degree of the reaction liquid is monitored, and the pressure is reduced by the pressure regulating valve according to the chlorination degree. For example, in the case of meta-xylene, when the chlorination degree is 1 and the vessel pressure is 53 kPa, the boiling point of the reaction liquid is about 150°C. When the chlorination degree is 2, the boiling point of the reaction liquid is about 156°C at the vessel pressure of 13 kPa.

[0024] The operation of reducing the pressure inside the vessel can be performed by using a conventional pressure adjustment means. In the production method of the present invention, for example, the specific gravity at each degree of chlorination of meta-xylene is measured in advance, and the degree of reduction in pressure can be automatically adjusted in conjunction with a specific gravity meter in the apparatus according to the degree of chlorination of meta-xylene.

[0025] In the production method of the present invention, the reaction liquid can be kept in a boiling state by reducing the pressure inside the vessel in accordance with the rise in the boiling point of the reaction liquid, and by adjusting the temperature of the reaction liquid. By simultaneously adjusting the pressure inside the vessel and the temperature, the reaction liquid can be kept in a boiling state without excessively increasing the temperature of the reaction liquid, and even if the degree of pressure reduction is fixed, the reaction liquid temperature can be adjusted to operate within a range that is usually easy to operate. Even in this case, it is possible to increase the degree of chlorination, and since the reaction liquid is kept in a boiling state, the dissolved HCl concentration in the reaction liquid is kept low, thereby suppressing the production of nuclear chlorinated products.

[0026] The reaction temperature of the chlorination reaction in the production method of the present invention is the temperature at which the boiling state of the reaction liquid is maintained by reducing the pressure. As long as the boiling state is maintained by reducing the pressure, the temperature of the reaction liquid can be controlled to a constant temperature without increasing the temperature with a heat medium. Since the boiling point of the reaction liquid increases with increasing chlorination degree, the pressure inside the vessel can be adjusted from normal pressure to absolute pressure in increments of 0.1 kPa to control the temperature of the reaction liquid to a constant temperature. When the reaction liquid reaches a degree of chlorination of 2 at normal pressure, the boiling point of the reaction liquid is 235°C. Therefore, as long as the pressure inside the vessel can be controlled so that the reaction liquid temperature is kept below 235°C and the boiling state is maintained, there is no need to adjust the liquid temperature.

[0027] In addition, the temperature of the heat medium can be raised at the same time as controlling the pressure inside the vessel to maintain the boiling state. Even when controlling the pressure inside the vessel, the liquid temperature can be adjusted to operate within a range that is normally easy to operate. In addition, the range of vacuum pump capacity options is expanded. The reaction liquid can be adjusted so that the boiling point of the reaction liquid is less than 235°C, preferably 230°C or less, more preferably 200°C or less.

[0028] The production method of the present invention can be carried out by irradiating ultraviolet-containing light or by activation by heat, or by a combination of these. In this case, a radical initiator may be added. As the light, as long as it is ultraviolet-containing light, a light source that is usually used in photoreactions, such as natural light, a mercury lamp, or a hydrogen discharge tube, can be used for external or internal irradiation. As the radical initiator, a peroxide-based radical initiator such as benzoyl peroxide or an azo-based radical initiator such as azobisisobutyronitrile can be used. The amount of the radical initiator used is preferably 0.001 to 1 mol %, more preferably 0.001 to 0.1 mol %, based on the xylene used as the raw material.

[0029] In general, in the radical chlorination reaction of the benzene ring side chain, the presence of trace amounts of metals from the raw material or the equipment can be a problem. Even a very small amount of these metal components acts as a Lewis acid, and the undesired chlorination to the benzene nucleus may proceed. Among the xylene isomers, meta-xylene in particular has a strong tendency to cause this nuclear chlorination reaction. In order to suppress the action of this trace amount of metal components, additives can be added. As additives, phosphorus compounds such as triphenylphosphine and triphenylphosphine oxide, or Lewis bases such as nitrogen compounds such as pyridine and hexamethylenetetramine can be used. The amount of additive used is preferably 0.1 to 10,000 wtppm, more preferably 1 to 1,000 wtppm, based on the xylene used as the raw material.

[0030] In the production method of the present invention, chlorine gas is used for chlorinating xylene. The chlorine gas used in the chlorination reaction may be supplied alone or may be mixed with an inert gas such as nitrogen. The amount of chlorine gas used is preferably 1 to 5 moles, more preferably 1.5 to 2.5 moles, relative to the xylene used as a raw material.

[0031] The reaction solution containing α,α'-dichloroxylene obtained by the chlorination reaction of the present invention contains very little nuclear chlorinated compounds and side chain chlorinated compounds of the nuclear chlorinated compounds, which are by-products that are difficult to separate by distillation. Therefore, high-purity α,α'-dichloroxylene can be obtained by purifying the solution under normal reduced pressure distillation conditions.

[0032] The low-boiling components that are distilled off during the distillation, such as unreacted xylene and α-chloroxylene, can be recovered and recycled.

[0033] The α,α'-dichloroxylene obtained in the present invention has a high purity and can therefore be used in many applications, such as as a raw material for high-performance polymers and electronic information materials. EXAMPLES

[0034] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0035] The method for producing α,α'-dichloroxylene of the present invention is characterized in that the reaction liquid is kept in a boiling state by reducing the pressure inside the reaction vessel as the degree of chlorination of the reaction liquid increases. As described above, when the degree of chlorination of the reaction liquid becomes 2 at normal pressure, the boiling point of the reaction liquid becomes 235°C. Therefore, it is theoretically clear that, as long as the reaction liquid temperature is in a range below 235°C, there is no need to adjust the liquid temperature, and only the pressure inside the vessel is controlled to maintain the boiling state. In the following examples, as an example of actual operation, an example is described in which the reaction liquid is kept in a boiling state by reducing the pressure inside the reaction vessel and adjusting the temperature of the reaction liquid.

[0036] Example 1 A 200 mL flask was charged with 90 g (0.848 mol) of meta-xylene and 0.01 g (100 wtppm) of triphenylphosphine oxide. The mixture was heated to 140°C, and chlorine gas was bubbled for 10 hours at a flow rate of 0.21 g / min under irradiation of a 200 W high-pressure mercury lamp. During this time, the chlorination degree of the reaction solution was monitored, and the pressure inside the vessel was gradually reduced from 101 kPa (normal pressure) by manually operating the pressure control valve of the vacuum pump, and the reaction was terminated at a final pressure inside the vessel of about 7 kPa. The chlorination degree at this time was 2.1, and the α,α'-dichlorometa-xylene in the reaction solution was 47.8 GC%, and the total of the nuclear chlorinated bodies was 0.3 GC%. The reaction mixture was transferred to a distillation flask, the pressure was reduced to 2.7 kPa, and the heat medium was heated to 120°C. After total reflux for 2 hours, the mixture was distilled at a reflux ratio of 10 to obtain α,α'-dichlorometa-xylene in a yield of 36.8% and purity of 99.9 GC%. The total amount of chlorinated compounds contained in the distillate was 0.04 GC%.

[0037] Comparative Example 1 Example of using a solvent under normal pressure A 1000mL flask was charged with 360g (3.39mol) of meta-xylene, 360g (7.24mol) of 4-chlorobenzotrifluoride, and 0.04g (100wtppm) of triphenylphosphine oxide. Under normal pressure, the mixture was heated to 140-160℃ to maintain a boiling state, and chlorine gas was bubbled at a flow rate of 1.68g / min for 6 hours under irradiation of a 200W high-pressure mercury lamp. The chlorination degree at this time was 2.0, and the α,α'-dichlorometa-xylene in the reaction solution was 47.8GC%, and the total of the nuclear chlorinated substances was 0.4GC%. The reaction mixture was transferred to a distillation flask, the pressure was reduced to 2.7 kPa, and the heat medium was heated to 120°C. After total reflux for 2 hours, the mixture was distilled at a reflux ratio of 10 to obtain α,α'-dichlorometa-xylene with a yield of 22.7% and a purity of 99.9 GC%. The total amount of chlorinated compounds contained in the distillate was 0.05 GC%.

[0038] Comparative Example 1 is an example in which a solvent was used, the reaction solution was heated and kept boiling under normal pressure, and α,α'-dichlorometa-xylene was obtained in the reaction solution with a chlorination degree of 2.0. Although a high-purity target product was obtained even when a solvent was used, the efficiency of the reactor deteriorated. In this comparative example, the same amount of solvent as the raw material xylene was used, so that the yield in one operation was half the amount compared to the case without a solvent, by simple calculation. In addition, an operation for recovering the low-boiling point solvent was required, making the overall operation complicated.

[0039] Comparative Example 2 An example where the reaction was carried out under normal pressure and no solvent was used. A 500 mL flask was charged with 500 g (4.71 mol) of meta-xylene and 0.05 g (100 wtppm) of triphenylphosphine oxide. The mixture was heated to 120°C under normal pressure, and chlorine gas was bubbled in at a flow rate of 0.68 g / min for 16 hours under irradiation of a 200 W high-pressure mercury lamp. The chlorination degree at this time was 2.1, and the α,α'-dichlorometa-xylene in the reaction solution was 44.6 GC%, and the total of the nuclear chlorinated compounds was 7.7 GC%. The reaction mixture was transferred to a distillation flask, the pressure was reduced to 2.7 kPa, and the heat medium was heated to 120°C. After total reflux for 2 hours, the mixture was distilled at a reflux ratio of 10 to obtain α,α'-dichlorometa-xylene in a yield of 25.6% and purity of 97.8 GC%. The total amount of chlorinated compounds contained in the distillate was 2.2 GC%.

[0040] In Comparative Example 2, no solvent was used and the reaction solution was not heated. As a result, when the degree of chlorination increased, the boiling state could not be maintained and a high purity target product could not be obtained.

Claims

1. A method for producing α,α'-dichloroxylene, comprising a step of chlorinating a side chain methyl group of xylene in a reaction vessel having a boiling reaction liquid containing raw material xylene and an additive selected from triphenylphosphine, triphenylphosphine oxide, pyridine and hexamethylenetetramine, and reducing the pressure inside the reaction vessel as the degree of chlorination of the reaction liquid increases, thereby maintaining the reaction liquid in a boiling state.

2. 2. The method for producing α,α'-dichloroxylene according to claim 1, wherein the reaction liquid is maintained in a boiling state by reducing the pressure inside the reaction vessel and adjusting the temperature of the reaction liquid.

3. 3. The method for producing α,α'-dichloroxylene according to claim 1, wherein the pressure inside the reaction vessel is reduced so that the temperature of the reaction liquid becomes less than 235°C.