Improved process for preparing p-aminobenzoic acid-2-ethylhexyl ester

Abstract

The present invention relates to a method for preparing 2-ethylhexyl p-aminobenzoate, which comprises reacting 2-ethylhexyl p-nitrobenzoate with hydrogen in the presence of a catalyst, the reaction being carried out in water or a mixture of water and 2-ethylhexanol. The present invention also relates to the highly pure 2-ethylhexyl p-aminobenzoate obtained by this method.

Description

[Technical Field]

[0001] The present invention relates to a method for preparing 2-ethylhexyl p-aminobenzoate, which comprises reacting 2-ethylhexyl p-nitrobenzoate with hydrogen in the presence of a catalyst, the reaction being carried out in water or a mixture of water and 2-ethylhexanol. The present invention also relates to the highly pure 2-ethylhexyl p-aminobenzoate obtained by this method. [Background technology]

[0002] Ethylhexyl triazone (Uvinul T 150, CAS number: 88122-99-0) is a known, highly effective UVB filter. Due to its beneficial physical and chemical properties, it is widely used as an ingredient in various cosmetic products, such as anti-aging face care products and sunscreens. The exceptional photostability and absorption properties of ethylhexyl triazone mean that only small concentrations are required to achieve a high UV protection factor. Furthermore, the polarity of its structure provides it with good solubility in cosmetic oils and high affinity for skin keratin.

[0003] Therefore, an economically and environmentally optimal industrial process for the preparation of ethylhexyl triazone is highly desirable. Considering its contact with human skin in its use in cosmetics, ethylhexyl triazone must be highly pure. Typically, ethylhexyl triazone is prepared in a three-step process starting with the esterification of p-nitrobenzoic acid. The formed p-nitrobenzoic acid-2-ethylhexyl ester is then reduced to p-aminobenzoic acid-2-ethylhexyl ester, which is subsequently reacted with cyanuric halide to give the final product.

[0004] Regarding the second step, i.e., the reduction of p-nitrobenzoic acid 2-ethylhexyl ester to p-aminobenzoic acid 2-ethylhexyl ester, EP 3674293 A1 discloses the reduction of p-nitrobenzoic acid ester either by catalytic hydrogenation in a mixture of ethanol and methanol or by using a catalyst and a reducing agent in an aprotic organic solvent. Furthermore, protic solvents, especially water, are considered disadvantageous because they can cause undesirable side reactions in the subsequent reaction to produce ethylhexyl triazone. The resulting by-products must be removed, which necessitates an additional purification step.

[0005] Chinese Patent No. 112321522 discloses a method for preparing p-aminobenzoic acid-2-ethylhexyl ester, in which isooctyl p-aminobenzoate is reduced by catalytic hydrogenation in isooctyl alcohol. Summary of the Invention [Problem to be solved by the invention]

[0006] Considering the high requirements for the purity of ethylhexyl triazone, its starting material, p-aminobenzoic acid 2-ethylhexyl ester, also needs to have optimal product quality. Therefore, the object of the present invention was to provide an improved method for preparing p-aminobenzoic acid 2-ethylhexyl ester with optimal product quality.

[0007] In this regard, it was an object to provide a process which ensures high product quality, which is economically and environmentally advantageous, and which is technically and economically suitable for large-scale industrial applications. [Means for solving the problem]

[0008] Surprisingly, it has been found that at least some of the above objects can be achieved by the subject matter of the present invention, as described below and in the claims.

[0009] In one embodiment, the present invention relates to a process for preparing p-aminobenzoic acid-2-ethylhexyl ester, comprising reacting p-nitrobenzoic acid-2-ethylhexyl ester with hydrogen in the presence of a catalyst, the reaction comprising: (a) in water, or (b) in a mixture of water and 2-ethylhexanol It is executed in

[0010] Surprisingly, the present inventors have found that carrying out the reaction in water or a mixture of water and 2-ethylhexanol provides the desired product in high yield and purity, particularly without significant amounts of residual water. Carrying out the reaction in water or a mixture of water and 2-ethylhexanol is advantageous in terms of heat dissipation due to the beneficial specific heat capacity provided by water. This is particularly true because the preparation of 2-ethylhexyl p-aminobenzoate according to the present invention is exothermic. Furthermore, since water is formed as a by-product during the hydrogenation reaction, carrying out the reaction in water is also advantageous, thereby avoiding additional contamination of the product and simplifying purification and solvent recycling. When the reaction is carried out in water (a), there is no need to remove additional solvent at the end of the reaction. Furthermore, additional solvents that may need to be discarded or dehydrated for recycling are not contaminated with water, reducing complexity, waste, and costs. (b) It is also advantageous to carry out the reaction in a mixture of water and 2-ethylhexanol, since 2-ethylhexanol can be used as a solvent in the preceding process step to form the starting material, p-nitrobenzoic acid 2-ethylhexyl ester. In particular, p-nitrobenzoic acid 2-ethylhexyl ester can be dissolved in 2-ethylhexanol and directly applied, thereby reducing the complexity, waste, and cost of the overall process. However, compared to carrying out the reaction with 2-ethylhexanol alone as in the prior art, a mixture with water offers a favorable specific heat capacity and therefore favorable heat dissipation.

[0011] Thus, the present invention provides an environmentally and economically advantageous process for preparing p-nitrobenzoic acid-2-ethylhexyl ester in high purity and yield, suitable for large-scale industrial application.

[0012] In one embodiment of the present invention, the reaction is carried out in water.

[0013] In one embodiment of the present invention, the catalyst is a noble metal, preferably palladium, Raney nickel, rhodium, platinum, or mixtures thereof.

[0014] In one embodiment of the present invention, the reaction is carried out at a temperature between 70° C. and 150° C. under a hydrogen pressure of up to 100 bar for a period of up to 24 hours.

[0015] In one embodiment of the present invention, after completion of the reaction, the reaction mixture is held under hydrogen pressure for an ageing time of at least 60 minutes, preferably at least 90 minutes.

[0016] In one embodiment of the present invention, p-aminobenzoic acid-2-ethylhexyl ester is purified by distillation.

[0017] In one embodiment of the present invention, a stabilizer is added for the distillation, the stabilizer being selected from ascorbic acid, butylated hydroxytoluene (BHT), tocopherols such as vitamin E, carotenoids, or mixtures thereof.

[0018] In one embodiment of the present invention, the method comprises reacting a cyanuric halide with p-aminobenzoic acid-2-ethylhexyl ester in a non-polar solvent to produce a compound of the following formula: [ka] and preparing 2,4,6-trianilino-p-(carbo-2'-ethylhexyl-1'-oxy)-1,3,5-triazine having the formula:

[0019] In one embodiment of the present invention, the method further comprises preparing p-nitrobenzoic acid-2-ethylhexyl ester, comprising reacting p-nitrobenzoic acid with 2-ethylhexanol.

[0020] In one embodiment, the present invention relates to p-aminobenzoic acid-2-ethylhexyl ester having a purity of at least 94% by weight.

[0021] In one embodiment, the present invention relates to p-aminobenzoic acid-2-ethylhexyl ester, having the following chemical formula: [ka] The amount of bis(2-ethylhexyl)-4,4'-(diazene-1,2-diyl)(E)-dibenzoate having the formula: is less than 3000 ppm.

[0022] In one embodiment, the present invention relates to p-aminobenzoic acid-2-ethylhexyl ester obtainable by the process of the present invention.

[0023] Preferred embodiments of the invention can be found in the claims, the description and the examples. It is to be understood that the above-mentioned features of the subject matter of the invention and the features further indicated below are preferred not only in each given combination but also in other combinations without departing from the scope of the invention.

[0024] In connection with the above embodiments of the present invention, the following definitions are provided.

[0025] In the context of the present invention, p-aminobenzoic acid 2-ethylhexyl ester is obtained by a process comprising reacting p-nitrobenzoic acid 2-ethylhexyl ester with hydrogen in the presence of a catalyst, the reaction being carried out (a) in water or (b) in a mixture of water and 2-ethylhexanol.

[0026] As used herein, "p-aminobenzoic acid 2-ethylhexyl ester" refers to para-aminobenzoic acid 2-ethylhexyl ester, which represents 4-aminobenzoic acid 2-ethylhexyl ester.

[0027] As used herein, "p-nitrobenzoic acid 2-ethylhexyl ester" refers to para-nitrobenzoic acid 2-ethylhexyl ester, which represents 4-nitrobenzoic acid 2-ethylhexyl ester.

[0028] As used herein, a "catalyst" refers to a homogeneous or heterogeneous catalyst suitable for hydrogenation reactions. Exemplary catalysts are those containing palladium, nickel, rhodium, platinum, iridium, ruthenium, iron, or mixtures thereof. Preferably, the catalyst is a noble metal or a mixture thereof. More preferably, the catalyst is palladium, Raney nickel, rhodium, platinum, or a mixture thereof.

[0029] As used herein, "water" refers to deionized water, demineralized water, or distilled water, which refers to water purified to remove most of its mineral and salt ions. As used herein, when a reaction is carried out (a) in water or (b) in a mixture of water and 2-ethylhexanol, this refers to the active addition of water to the reaction mixture at the start of the reaction, not the presence of water only in the amount formed during the reaction. Thus, the method may include a step of adding water or a mixture of water and 2-ethylhexanol to the reactant, i.e., p-nitrobenzoic acid 2-ethylhexyl ester, or to a reaction mixture containing the reactant, at the start of the reaction. Thus, the method of the present invention may include a first step of combining p-nitrobenzoic acid 2-ethylhexyl ester, and optionally a catalyst, with a solvent, i.e., water or a water-2-ethylhexanol mixture, prior to the start of the reaction. The reaction may then be initiated by starting hydrogen supply to the already formed mixture.

[0030] Thus, in various embodiments, the present invention relates to a method for preparing p-aminobenzoic acid 2-ethylhexyl ester by reacting p-nitrobenzoic acid 2-ethylhexyl ester with hydrogen in the presence of a catalyst, the method including providing a reaction mixture comprising p-nitrobenzoic acid 2-ethylhexyl ester and a catalyst in (a) water or (b) a mixture of water and 2-ethylhexanol, and reacting p-nitrobenzoic acid 2-ethylhexyl ester with hydrogen in the presence of a catalyst in the water or the mixture of water and 2-ethylhexanol.

[0031] Preferably, in (b), water is present in a ratio of at least 1:2 to 2-ethylhexanol, more preferably at least 1:1 to 2-ethylhexanol, and even more preferably at least 2:1 to 2-ethylhexanol. This ratio is preferably a weight ratio, but in some embodiments can be a volume ratio. Thus, both options are encompassed by the given ratios of at least 2:1, at least 1:1, and at least 1:2. In the preferred mixture of (b), the amount of 2-ethylhexanol is the amount of 2-ethylhexanol in which the starting material 2-ethylhexyl p-nitrobenzoate is dissolved, or the amount of 2-ethylhexanol in which the starting material 2-ethylhexyl p-nitrobenzoate is contaminated, and no additional 2-ethylhexanol is added.

[0032] In some embodiments of the present invention, after the reaction is completed, the reaction mixture is held under hydrogen pressure for an aging time. As used herein, "aging time" refers to the period after the reaction is completed during which the reaction mixture is held under hydrogen pressure. Thus, the reaction is carried out for the time necessary for completion of the reaction. When a period for the reaction is given, the period only refers to a range of time; the reaction is completed according to the given conditions, and even if the given period includes a longer reaction time, the reaction is not carried out for longer than the time necessary for completion of the reaction. For example, if the reaction is carried out for a maximum period of 24 hours, if the reaction is completed after 15 hours, the reaction is not carried out for 24 hours; in this case, the reaction is carried out for 15 hours. Thereafter, the aging time begins. As used herein, "held under hydrogen pressure" refers to not completely releasing hydrogen from the reaction vessel / reactor. This includes maintaining a similar, reduced, or increased hydrogen pressure compared to the reaction. Preferably, the hydrogen pressure during the aging time is similar to the hydrogen pressure during the reaction or refers to the hydrogen pressure generated in the vessel / reactor after the hydrogen valve is closed. The reaction time ends and the aging time begins when at least one of the reactants is completely converted and / or the amount of at least one of the reactants no longer changes. In particular, the aging time of the method of the present invention begins when 2-ethylhexyl p-nitrobenzoate is completely converted, or when the amount of 2-ethylhexyl p-nitrobenzoate reaches a minimum value and / or when the hydrogen consumption reaches a minimum value. As used herein, "reaching a minimum value" refers to the amount of 2-ethylhexyl p-nitrobenzoate or the hydrogen consumption reaching an absolute minimum value at which they no longer change. To determine the completion of the reaction and thus the start of the aging time, the reaction can be monitored by methods known to those skilled in the art, such as thin-layer chromatography, GC, HPLC, or NMR. When 2-ethylhexyl p-nitrobenzoate is no longer detected or a decrease in the amount of 2-ethylhexyl p-nitrobenzoate is no longer detected, the reaction is complete and the aging time begins.Furthermore, the completion of the reaction, and therefore the start of the aging period, can be determined by monitoring the hydrogen consumption, for example, by monitoring the hydrogen flow rate while maintaining a constant hydrogen pressure. When the hydrogen flow rate reaches a minimum while maintaining a constant hydrogen pressure, the reaction is complete and the aging period begins. During the aging period, the reaction mixture is maintained under hydrogen pressure. Therefore, the aging period ends with the release of residual hydrogen from the reaction vessel / reactor.

[0033] In some embodiments, the product is purified by distillation, in which a stabilizer is used. As used herein, "stabilizer" refers to a stabilizer or a mixture of stabilizers used to stabilize the product p-aminobenzoic acid 2-ethylhexyl ester during distillation, for example, by preventing polymerization and / or oxidation of the p-aminobenzoic acid 2-ethylhexyl ester. Thus, the stabilizer can be an acid and / or base, such as a carboxylic acid and / or a carboxylic acid salt, to neutralize the acid and / or base present in the mixture that induces polymerization of the p-aminobenzoic acid 2-ethylhexyl ester. Additionally, the stabilizer can be an antioxidant. Exemplary stabilizers are ascorbic acid, butylated hydroxytoluene (BHT), tocopherols such as vitamin E, carotenoids, or mixtures thereof. A preferred stabilizer is ascorbic acid.

[0034] As used herein, pressure expressed in the pressure unit "bar" refers to relative or gauge pressure, which is zero-referenced to ambient air pressure and is therefore equal to absolute pressure minus atmospheric pressure. Gauge pressure is sometimes expressed in the pressure unit "barg." Only when the pressure unit "bar (abs)" is used does pressure refer to absolute pressure.

[0035] Preferred embodiments of the method of the present invention are described below, to which the following general considerations apply:

[0036] Generally, the reaction steps are carried out in a reaction vessel conventionally used for such reactions, such as a conventional stirred tank reactor. The reaction can be carried out continuously, semi-batch, or batchwise. Preferably, the process of the present invention is carried out batchwise, wherein all reactants are provided to the reaction vessel prior to the start of the reaction.

[0037] Details regarding reaction pressure and temperature are provided below. Unless otherwise indicated, process steps are preferably carried out under atmospheric pressure. Completion of the reaction can be monitored by methods known to those skilled in the art, such as thin layer chromatography, GC, HPLC, or NMR.

[0038] Unless otherwise indicated, the reactants can in principle be contacted with each other in any desired order.

[0039] It is further emphasized that the reaction can be carried out on a laboratory and industrial scale. DETAILED DESCRIPTION OF THE INVENTION

[0040] In the following, preferred embodiments of the present invention are provided. It should be understood that the preferred embodiments of the present invention are preferred either alone or in combination with each other.

[0041] As described above, the present invention relates to a method for preparing p-aminobenzoic acid 2-ethylhexyl ester, which comprises reacting p-nitrobenzoic acid 2-ethylhexyl ester with hydrogen in the presence of a catalyst, the reaction being carried out (a) in water or (b) in a mixture of water and 2-ethylhexanol. When the reaction is carried out in water or a mixture of water and 2-ethylhexanol, water provides a favorable specific heat capacity of the reaction mixture and therefore favorable heat dissipation, which is particularly relevant for the exothermic preparation of p-aminobenzoic acid 2-ethylhexyl ester. Furthermore, because water is also formed as a by-product during the reaction, adding water to the reaction mixture does not increase the number of contaminants in the mixture. When the reaction is carried out in (a) water, there is no need to remove solvent contaminants other than water. Because water is formed as a by-product during the reaction and must be removed in either case, the method of the present invention reduces the complexity of product purification and recycling, and therefore process waste and costs. Therefore, in one embodiment of the present invention, the reaction is carried out in water. It is also possible to apply the starting material p-nitrobenzoic acid 2-ethylhexyl ester dissolved in or contaminated with 2-ethylhexanol. This is particularly advantageous because the preceding step of preparing p-nitrobenzoic acid 2-ethylhexyl ester can be carried out in 2-ethylhexanol, thereby avoiding or reducing the solvent removal step prior to the reaction of the present invention, which has a positive impact on the complexity, waste, and cost of the overall process for preparing p-aminobenzoic acid 2-ethylhexyl ester. Also, in this case, the addition of water as a solvent provides a favorable specific heat capacity of the reaction mixture and therefore favorable heat dissipation. Therefore, in one embodiment of the present invention, the reaction is carried out in a mixture of water and 2-ethylhexanol. In one embodiment, the reaction is carried out in a mixture of water and 2-ethylhexanol in a weight ratio of water to 2-ethylhexanol of at least 1:2. In another embodiment, the reaction is carried out in a mixture of water and 2-ethylhexanol in a weight ratio of water to 2-ethylhexanol of at least 1:1.In another embodiment, the reaction is carried out in a mixture of water and 2-ethylhexanol in a weight ratio of water to 2-ethylhexanol of at least 2:1. Increasing the amount of water relative to the amount of 2-ethylhexanol increases the specific heat capacity of the reaction mixture, with a positive effect on heat dissipation. Thus, in a preferred embodiment, the amount of 2-ethylhexanol in the mixture of water and 2-ethylhexanol is the amount of 2-ethylhexanol in which the starting material 2-ethylhexyl p-nitrobenzoate is dissolved, or the amount of 2-ethylhexanol in which the starting material 2-ethylhexyl p-nitrobenzoate is contaminated, and no additional 2-ethylhexanol is added.

[0042] Furthermore, in one embodiment of the present invention, the pH value is adjusted to a value in the range of 4 to 10 before the reaction with hydrogen. In another embodiment, the pH value is adjusted to a neutral value. In one embodiment, the pH value is adjusted using sodium hydroxide. By adjusting the pH before the reaction with hydrogen, the purity of 2-ethylhexyl p-aminobenzoate can be increased.

[0043] In one embodiment of the present invention, the catalyst is a noble metal. Preferably, the catalyst is selected from the group consisting of palladium, Raney nickel, rhodium, platinum, and mixtures thereof. In one embodiment, the catalyst is palladium. In another embodiment, the catalyst is palladium on carbon (Pd / C). In another embodiment, the catalyst is palladium on carbon (Pd / C), and the amount of palladium ranges from 1 wt% to 10 wt% palladium. In another embodiment, the catalyst is 2 wt% palladium on carbon (Pd / C). The amount of catalyst refers to the amount of noble metal relative to the amount of p-nitrobenzoic acid 2-ethylhexyl ester. In one embodiment, 0.002 to 0.005 wt% of noble metal is used relative to p-nitrobenzoic acid 2-ethylhexyl ester. In another embodiment, 0.003 to 0.004 wt% of noble metal is used relative to p-nitrobenzoic acid 2-ethylhexyl ester.

[0044] The reaction is carried out for a period of time until the reaction is complete. The aging period then begins. Therefore, the reaction time may vary depending on the applied hydrogen pressure and temperature. For example, the reaction time can be shortened by carrying out the reaction under a higher hydrogen pressure. To determine the completion of the reaction, the reaction can be monitored by methods known to those skilled in the art, such as thin-layer chromatography, GC, HPLC, or NMR. The reaction is complete when p-nitrobenzoic acid 2-ethylhexyl ester is no longer detected or a decrease in the amount of p-nitrobenzoic acid 2-ethylhexyl ester is no longer detected. Furthermore, the completion of the reaction can be determined by monitoring the hydrogen consumption, for example, by monitoring the hydrogen flow rate while maintaining a constant hydrogen pressure. The reaction is complete when the hydrogen flow rate reaches a minimum value while maintaining a constant hydrogen pressure. In one embodiment of the present invention, the reaction is carried out at a temperature of 70°C to 150°C under a hydrogen pressure of up to 100 bar for a period of up to 24 hours. In another embodiment, the reaction is carried out at a temperature of 70°C to 150°C under a hydrogen pressure of 1 to 50 bar for a period of up to 24 hours. In another embodiment, the reaction is carried out at a temperature between 70° C. and 150° C. under a hydrogen pressure of 2 to 20 bar for a period of up to 24 hours. In another embodiment, the reaction is carried out at a temperature between 70° C. and 150° C. under a hydrogen pressure of 7 to 10 bar for a period of up to 24 hours.

[0045] In one embodiment of the present invention, the reaction is carried out at a temperature between 80°C and 120°C for a period of up to 24 hours under a hydrogen pressure of up to 100 bar. In another embodiment, the reaction is carried out at a temperature between 80°C and 120°C for a period of up to 24 hours under a hydrogen pressure of 1 to 50 bar. In another embodiment, the reaction is carried out at a temperature between 80°C and 120°C for a period of up to 24 hours under a hydrogen pressure of 2 to 20 bar. In another embodiment, the reaction is carried out at a temperature between 80°C and 120°C for a period of up to 24 hours under a hydrogen pressure of 7 to 10 bar.

[0046] In one embodiment of the present invention, the reaction is carried out at a temperature of 90°C to 100°C for a period of up to 24 hours under a hydrogen pressure of up to 100 bar. In another embodiment, the reaction is carried out at a temperature of 90°C to 100°C for a period of up to 24 hours under a hydrogen pressure of 1 to 50 bar. In another embodiment, the reaction is carried out at a temperature of 90°C to 100°C for a period of up to 24 hours under a hydrogen pressure of 2 to 20 bar. In another embodiment, the reaction is carried out at a temperature of 90°C to 100°C for a period of up to 24 hours under a hydrogen pressure of 7 to 10 bar. In one particular embodiment of the present invention, the reaction is carried out at a temperature of 90°C to 100°C for a period of 10 to 20 hours under a hydrogen pressure of 2 to 20 bar. In another particular embodiment, the reaction is carried out at a temperature of 90°C to 100°C for a period of 13 to 17 hours under a hydrogen pressure of 7 to 10 bar. As noted above, the reaction is not carried out for longer than necessary for completion of the reaction. When a time period for a reaction is given, the time period refers only to a range of time, and the reaction will be completed under the given conditions. Even if the given time period includes a longer reaction time, the reaction will not be run for longer than necessary to complete the reaction. For example, if the reaction is run for a maximum of 24 hours, if the reaction is complete after 15 hours, the reaction will not be run for 24 hours; in this case, the reaction will be run for 15 hours. After that, the aging period begins.

[0047] Surprisingly, the inventors have found that providing an aging period in which the reaction mixture is kept under hydrogen pressure after completion of the reaction increases the purity of the p-aminobenzoic acid-2-ethylhexyl ester. In particular, [ka] Contamination with the azo compound bis(2-ethylhexyl)-4,4'-(diazene-1,2-diyl)(E)-dibenzoate as a by-product having the formula

[0048] Thus, in one embodiment of the present invention, after completion of the reaction, the reaction mixture is maintained under hydrogen pressure for an aging time of at least 60 minutes, preferably at least 90 minutes. In another embodiment, after completion of the reaction, the reaction mixture is maintained under hydrogen pressure for an aging time of at least 2 hours. In another embodiment, after completion of the reaction, the reaction mixture is maintained under hydrogen pressure for an aging time of at least 3 hours. In another embodiment, after completion of the reaction, the reaction mixture is maintained under hydrogen pressure for an aging time of 2 to 4 hours. In another embodiment, after completion of the reaction, the reaction mixture is maintained under hydrogen pressure for an aging time of 2.5 to 3.5 hours. While extending the aging time does not adversely affect the reduction or avoidance of azo compound formation, it reduces the space-time yield, i.e., the yield obtained per unit space and time in the reactor / vessel, which increases costs. In one embodiment, the hydrogen pressure during the aging time is between 2 bar and 20 bar. In another embodiment, the hydrogen pressure during the aging time is between 7 bar and 10 bar. In one embodiment, the hydrogen pressure during the aging period is the same as the hydrogen pressure during the reaction or the hydrogen pressure that occurs in the reactor / vessel after the hydrogen valve is closed. In another embodiment, the hydrogen pressure during the aging period is the same as the hydrogen pressure during the reaction for 1.5 to 2 hours, followed by the hydrogen pressure that occurs in the reactor / vessel after the hydrogen valve is closed for 0.5 to 1.5 hours. In one embodiment, the temperature during the aging period is 70°C to 150°C. In another embodiment, the temperature during the aging period is 80°C to 120°C. In another embodiment, the temperature during the aging period is 90°C to 100°C. In one embodiment, the temperature during the aging period is the same as the reaction temperature. In another embodiment, the temperature during the aging period is the same as the reaction temperature, and the hydrogen pressure during the aging period is the same as the hydrogen pressure during the reaction for 1.5 to 2 hours, followed by the hydrogen pressure that occurs in the reactor / vessel after the hydrogen valve is closed for 0.5 to 1.5 hours. In another embodiment, the temperature during the aging period is between 70°C and 100°C, and the hydrogen pressure during the aging period is between 2 bar and 20 bar for 1.5 to 2 hours, followed by the hydrogen pressure that is generated in the reactor / vessel after closing the hydrogen valve for 0.5 to 1.5 hours.

[0049] To further purify the p-aminobenzoic acid 2-ethylhexyl ester, the solvent, and optionally the by-products of the reaction, can be removed by distillation. Therefore, in one embodiment of the present invention, the p-aminobenzoic acid 2-ethylhexyl ester is purified by distillation. The solvent removed by distillation can be recycled. In particular, the solvent can be reused in a subsequent process of reducing the p-nitrobenzoic acid 2-ethylhexyl ester to the p-aminobenzoic acid 2-ethylhexyl ester. Furthermore, in one embodiment of the present invention, a stabilizer is added for the distillation. In particular, the stabilizer can be one compound or two or more compounds. In one embodiment, the stabilizer is one compound. Adding a stabilizer increases the yield and purity of the p-aminobenzoic acid 2-ethylhexyl ester, in particular by preventing polymerization and / or oxidation of the p-aminobenzoic acid 2-ethylhexyl ester. Thus, in one embodiment, the stabilizer is a carboxylic acid and / or a salt of a carboxylic acid to neutralize the acid and / or base present in the mixture that induces polymerization of p-aminobenzoic acid-2-ethylhexyl ester. In another embodiment, the stabilizer is an antioxidant. In another embodiment, the stabilizer is an antioxidant and a carboxylic acid and / or a salt of a carboxylic acid. In one embodiment, the stabilizer is selected from the group consisting of ascorbic acid, butylated hydroxytoluene (BHT), tocopherol, or a mixture thereof. In one embodiment, the stabilizer is ascorbic acid or a mixture with ascorbic acid, preferably, the stabilizer is ascorbic acid.

[0050] In one embodiment of the present invention, after the reaction with hydrogen, the reaction mixture is filtered to remove the solid catalyst, and the filtrate is subjected to phase separation. In one embodiment, the catalyst is removed using a bag filter. In one embodiment, phase separation is performed by adding a carbonate salt. Performing phase separation by adding a carbonate salt further reduces the water content in the organic phase, thereby increasing the purity of the 2-ethylhexyl p-aminobenzoate in the organic phase. Furthermore, because cyanuric halide tends to hydrolyze to produce mono-, di-, and / or trihydroxytriazines, which may require additional process steps, removing water from the 2-ethylhexyl p-aminobenzoate product is particularly advantageous for the subsequent reaction of 2-ethylhexyl p-aminobenzoate with cyanuric halide to produce ethylhexyl triazone.

[0051] As mentioned above, the present invention provides a method for preparing p-aminobenzoic acid 2-ethylhexyl ester, which is an important precursor for the preparation of the known highly effective UV absorber ethylhexyl triazone (Uvinul T 150, CAS number: 88122-99-0). The ethylhexyl triazone represents 2,4,6-trianilino-p-(carbo-2'-ethylhexyl-1'-oxy)-1,3,5-triazine. Thus, in another embodiment, the present method provides a compound of the following chemical formula by reacting cyanuric halide with p-aminobenzoic acid 2-ethylhexyl ester in a non-polar solvent: [ka] The method further comprises preparing 2,4,6-trianilino-p-(carbo-2'-ethylhexyl-1'-oxy)-1,3,5-triazine having the formula: Preferably, the cyanuric halide is reacted with 2-ethylhexyl p-aminobenzoate in a molar ratio of 1:3 to 1:5. Because 3 equivalents of 2-ethylhexyl p-aminobenzoate are required for 1 equivalent of cyanuric halide to form ethylhexyl triazone, reacting the cyanuric halide with 2-ethylhexyl p-aminobenzoate in a molar ratio of at least 1:3 ensures complete conversion of the cyanuric halide. Therefore, the improved method for preparing 2-ethylhexyl p-aminobenzoate is also advantageous for preparing ethylhexyl triazone. Improving the purity of p-aminobenzoic acid-2-ethylhexyl ester while simultaneously reducing the complexity, waste, and cost of its preparation also reduces the overall complexity, waste, and cost of the preparation of ethylhexyl triazone, which is particularly advantageous given the high demand for ethylhexyl triazone due to its various applications as a UV absorber.

[0052] Furthermore, p-aminobenzoic acid 2-ethylhexyl ester is prepared by reacting p-nitrobenzoic acid 2-ethylhexyl ester with hydrogen. Thus, in one embodiment of the present invention, the method further comprises preparing p-nitrobenzoic acid 2-ethylhexyl ester, which comprises reacting p-nitrobenzoic acid with 2-ethylhexanol. In one embodiment, the p-nitrobenzoic acid 2-ethylhexyl ester prepared by the method is directly used to prepare p-aminobenzoic acid 2-ethylhexyl ester in a subsequent reaction without purification or removal of the solvent. This is particularly true when the preparation of p-nitrobenzoic acid 2-ethylhexyl ester is carried out in water or 2-ethylhexanol or a mixture thereof, or without using any solvent at all.

[0053] As mentioned above, high purity can be obtained by the method of the present invention. Thus, in one embodiment, the present invention relates to p-nitrobenzoic acid-2-ethylhexyl ester having a purity of at least 94% by weight or at least 94% as determined by GC (AUC). The purity may be higher, for example, at least 95% or at least 96%, or even at least 97% or 98%. In particular, by maintaining the reaction mixture under hydrogen pressure for an aging time after completion of the reaction, contamination with the azo compound bis(2-ethylhexyl)-4,4'-(diazene-1,2-diyl)(E)-dibenzoate is significantly reduced or even avoided. Thus, in one embodiment, the present invention relates to p-aminobenzoic acid-2-ethylhexyl ester, which has the following chemical formula: [ka] is less than 3000 ppm, such as less than 2500 ppm or less than 2000 ppm.

[0054] Furthermore, in one embodiment, the present invention relates to p-nitrobenzoic acid-2-ethylhexyl ester obtained by the process of the present invention, which can be characterized by a small amount of the azo compound bis(2-ethylhexyl)-4,4'-(diazene-1,2-diyl)(E)-dibenzoate, as described above.

[0055] The present invention is further illustrated by the following examples. [Example]

[0056] In the examples, GC analysis is performed on an Agilent Technologies 6890N using the following conditions and parameters: Detector: FID Solvent: Acetonitrile Column: Optima 5 (30 m, ID 0.32 mm, film thickness 0.25 μm) Carrier gas: Nitrogen

[0057] Example 1: Preparation of p-aminobenzoic acid-2-ethylhexyl ester without aging 8 kg of p-nitrobenzoic acid-2-ethylhexyl ester is added under stirring to a reactor previously charged with 0.75 L of deionized water. The pH of the mixture is adjusted to a neutral value using sodium hydroxide (50% by weight in water).

[0058] 0.015 kg of catalyst (2 wt. % palladium on carbon) was added to the reactor under stirring. The temperature was adjusted to 95°C, and hydrogen was introduced at 9 barg while vigorously stirring. The hydrogen flow rate was carefully monitored throughout, maintaining a pressure of 9 barg. When the hydrogen flow rate reached a minimum, the reaction was stopped. Residual hydrogen was then released. Residual catalyst was removed from the reaction mixture using a bag filter. The agitator was stopped, and the organic phase containing the product was separated from the aqueous phase. The lower aqueous phase was then removed from the bottom of the reactor. The organic phase containing the product, p-aminobenzoic acid 2-ethylhexyl ester, remained in the reactor and was subsequently analyzed. The yield was 7.14 kg of organic phase. The final product was characterized using GC, and the water content was measured using Karl Fischer titration. The results are shown in Table 1.

[0059] [Table 1]

[0060] Example 2: Preparation of p-aminobenzoic acid-2-ethylhexyl ester with aging 8 kg of p-nitrobenzoic acid-2-ethylhexyl ester is added to a reactor pre-charged with 0.75 L of deionized water under stirring. The pH of the mixture is adjusted to a neutral value using sodium hydroxide (50 wt % in water). 0.015 kg of catalyst (2 wt % palladium on carbon) is added to the reactor under stirring.

[0061] The temperature is adjusted to 95°C and hydrogen is introduced at 9 barg with vigorous stirring. The hydrogen flow rate is carefully monitored throughout, maintaining a pressure of 9 barg. Once the hydrogen flow rate reaches a minimum, the hydrogen pressure is maintained for an additional 120 minutes. The hydrogen valve is then closed and aging is continued for an additional 60 minutes. The remaining hydrogen is then released.

[0062] Residual catalyst is removed from the reaction mixture using a bag filter.

[0063] The agitator was stopped, and the organic phase containing the product was allowed to separate from the aqueous phase. The lower aqueous phase was then removed from the bottom of the reactor. The organic phase containing the product, p-aminobenzoic acid 2-ethylhexyl ester, remained in the reactor and was subsequently analyzed. The yield was 7.14 kg of organic phase. The final product was characterized using GC, and the water content was determined using Karl Fischer titration. The results are shown in Table 2.

[0064] [Table 2]

Claims

1. 1. A process for preparing p-aminobenzoic acid 2-ethylhexyl ester, comprising reacting p-nitrobenzoic acid 2-ethylhexyl ester with hydrogen in the presence of a catalyst, said reaction comprising: (a) in water, or (b) in a mixture of water and 2-ethylhexanol It is executed with adding said water (a) or a mixture of water and 2-ethylhexanol to said reaction mixture at the start of said reaction.

2. 10. The method of claim 1, wherein the reaction is carried out in water, and the method comprises adding the water to the reaction mixture at the start of the reaction.

3. 2. The method of claim 1, wherein the reaction is carried out in a mixture of water and 2-ethylhexanol, the ratio of water to 2-ethylhexanol being at least 1:2, preferably at least 1:1, more preferably at least 2:1, and the method comprising adding the mixture of water and 2-ethylhexanol to the reaction mixture at the start of the reaction.

4. The method according to any one of claims 1 to 3, wherein the catalyst is a noble metal, preferably palladium, Raney nickel, rhodium, platinum, or a mixture thereof.

5. 5. A process according to any one of claims 1 to 4, wherein the reaction is carried out at a temperature between 70°C and 150°C under a hydrogen pressure of up to 100 bar for a period of up to 24 hours.

6. 6. The process according to any one of claims 1 to 5, wherein after completion of the reaction, the reaction mixture is kept under hydrogen pressure for an ageing time of at least 60 minutes, preferably at least 90 minutes.

7. The method according to any one of claims 1 to 6, wherein the p-aminobenzoic acid-2-ethylhexyl ester is purified by distillation.

8. 8. The method of claim 7, wherein a stabilizer is added for the distillation, the stabilizer being selected from ascorbic acid, butylated hydroxytoluene (BHT), tocopherols such as vitamin E, carotenoids, or mixtures thereof.

9. The method comprises reacting a cyanuric halide with p-aminobenzoic acid-2-ethylhexyl ester in a non-polar solvent to produce a compound of the following formula: 【Chemical 1】 The method of any one of claims 1 to 8, further comprising preparing 2,4,6-trianilino-p-(carbo-2'-ethylhexyl-1'-oxy)-1,3,5-triazine having the formula:

10. 10. The method of any one of claims 1 to 9, wherein the method further comprises preparing p-nitrobenzoic acid-2-ethylhexyl ester, comprising reacting p-nitrobenzoic acid with 2-ethylhexanol.

11. p-aminobenzoic acid 2-ethylhexyl ester having a purity of at least 94% by weight or 94% as determined by GC, and having the following chemical formula: 【Chemistry 2】 p-aminobenzoic acid-2-ethylhexyl ester, having an amount of bis(2-ethylhexyl)-4,4'-(diazene-1,2-diyl)(E)-dibenzoate of the formula:

12. 12. The p-aminobenzoic acid 2-ethylhexyl ester according to claim 11, obtained by the process according to any one of claims 1 to 7, wherein after completion of the reaction, the reaction mixture is kept under hydrogen pressure for an aging time of at least 60 minutes, preferably at least 90 minutes.

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