JPH0136460B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for producing aniline by hydrogenation of nitrobenzene. U.S. patent for method for hydrogenating aromatic nitro compounds in liquid phase to produce corresponding aromatic amines
Proposed in No. 2292879. This method uses a nickel, cobalt, or copper catalyst supported on a finely divided carrier, suspended in a mixture of a nitro compound and its hydrogenation product, an amine, which is then charged with hydrogen. The reaction is carried out through The specification states that the activity of the catalyst is maintained at a high level by conducting the reaction under conditions such that the water produced in the reaction is continuously distilled off from the reaction system together with the amine, and that the aromatic amine is It is described that the activity of the catalyst is increased by using it as a solvent and reacting the amine at a relatively high concentration. In addition, Patent Publication No. 15779/1979 states that in a liquid phase reaction, the concentration of the produced aromatic amine used as a solvent is maintained at 95% by weight or more in the liquid phase, as in the above-mentioned US patent, and the reaction is carried out as much as possible. By carrying out the reaction at around normal pressure and at a temperature at or near the boiling point, the water produced by the reaction can be easily removed from the system, and the product distilled off from the reactor has an aromatic aroma with a low content of nitro compounds. It has been proposed that this is advantageous because it provides group amines. Keeping the amine concentration in the reaction solution high, that is, charging only the amount of nitro compounds commensurate with the consumption of the reaction, lowers the content of unreacted nitro compounds in the product that is distilled out of the system. It is also generally recommended to use a large amount of solvent to control the temperature in reactions that generate a large amount of heat, such as the present reaction, and to control the heat of reaction using the latent heat of vaporization. From this point of view, it seems advantageous to maintain the amine concentration in the reaction solution as close to 100% as possible and to reduce the concentration of the nitro compound.
However, if the reaction is carried out in an amine solvent with a low concentration of nitro compounds, nuclear hydrogenation of aromatic rings will easily occur as a side reaction, producing impurities such as cyclohexylamine, cyclohexanone, cyclohexylideneaniline, and cyclohexylaniline. It becomes easier. The rate of these by-products increases as the concentration of nitro compounds in the reaction system decreases. In order to suppress the formation of by-product impurities, which are these nuclear water additives, Japanese Patent Publication No. 47-27212 proposes the addition of organic bases such as alkanolamines. In this case, triethanolamine is added, but even in this case it is not satisfactory. In addition, it is said that it is desirable to contain nitrobenzene at around 10 ppm or less in commercially available high-purity aniline;
Even if the reaction is carried out at a low concentration of 0.5% by weight or less, more than 20 ppm of unreacted nitrobenzene will distill out together with aniline, and if an attempt is made to increase the amount of catalyst to suppress the distillation of unreacted nitrobenzene, nuclear water will increase. Additives increase proportionately. The present inventors have conducted intensive studies to solve these problems in a method for producing aniline from nitrobenzene by catalytic hydrogenation in the presence of a noble metal catalyst, and have found that by adding a specific compound under specific reaction conditions, They discovered a method for producing highly pure aniline containing almost no unreacted nitrobenzene and an extremely low content of nuclear hydrogenated products, and completed the method of the present invention. That is, the method of the present application is a method for continuously producing aniline by catalytic hydrogenation of nitrobenzene using a noble metal catalyst. This is a method for continuously producing high-purity aniline without going through a distillation process, which is characterized by carrying out the reaction in the presence of a compound containing nitrobenzene while maintaining the concentration of nitrobenzene in the aniline reaction solution at 0.5% by weight or less. The noble metal catalyst that can be used in the method of the present application is a catalyst that can usually be used in the catalytic hydrogenation reaction of nitro compounds, and is not specified. For example, catalysts containing palladium, platinum, rhodium, etc. can be used. Particularly preferred are palladium and or palladium-platinum catalysts, and when a palladium catalyst is used, it is preferably used by depositing it on lipophilic carbon having an oil absorption rate of at least 100 as a carrier. The catalyst concentration in the reaction system is usually 0.2
~2.0% by weight is suitable. Further, in the method of the present invention, compounds to be added include alkali metal hydroxides such as caustic soda and caustic potash, alkali metal carbonates or bicarbonates such as sodium carbonate, potassium carbonate, and sodium bicarbonate, or zinc acetate and nitric acid. subsalt, preferably 0.1 to 100 times the weight of the precious metal in the catalyst.
0.5 to 50 times the amount may be added directly to the catalyst or may be added to the reaction system as an aqueous solution. These compounds not only function as benzene ring hydrogenation reaction inhibitors, but also have a synergistic effect as co-catalysts for the noble metal catalyst in the present method, and can be continuously removed from the reaction system even with the usual amount of catalyst used. The distilled aniline contains almost no unreacted nitrobenzene. The reaction of the present method is carried out as follows.
When carrying out hydrogenation reactions using noble metal catalysts,
Normally, if water exists in the reaction system, the catalytic activity tends to decrease and by-products increase at the same time, so the water produced during the reaction must be constantly removed from the system to make it virtually water-free. Preferably, the reaction is carried out in the presence of A small amount of nitrobenzene is introduced into a previously charged aniline solvent to carry out a reaction, and the nitrobenzene introduced into the reactor is instantaneously converted into aniline and water, and the reaction products are removed from the system as vapor. Although the pressure during the reaction can be carried out at atmospheric pressure, it is preferably 1.5 to 10 atm, and it is necessary to carry out the reaction at a reaction temperature of 150 to 250°C. The nitrobenzene introduced into the reactor is supplied into the aniline solvent in an amount approximately equivalent to the conversion of nitrobenzene to aniline so that the concentration of nitrobenzene in the reaction solution is 0.5% by weight or less, preferably at a concentration of 0.1% by weight or less. Maintain and react. If the nitrobenzene concentration exceeds 0.5% by weight, unreacted nitrobenzene will increase, and a distillation step will be necessary to achieve the specs of high-purity aniline, which is not preferable. Therefore, it would be very industrially advantageous if the reaction could be carried out at a nitrobenzene concentration of 0.5% by weight or less in the reaction solution, but the production rate of nuclear hydrogenation by-products represented by N-cyclohexylaniline decreases around this nitrobenzene concentration. It tends to increase rapidly, and this tendency becomes even more pronounced as the concentration of nitrobenzene becomes lower. The aniline that is continuously taken out as vapor from the reactor and condensed in this way contains almost no unreacted nitrobenzene, and contains no nuclear hydrogenation of cyclohexanol, cyclohexanone, cyclohexylideneaniline, cyclohexylaniline, etc. High purity aniline containing almost no substances can be obtained continuously. After water is separated from the condensed and distilled aniline, if necessary, a portion of the aniline is returned to the reaction system to adjust the aniline solvent in the reactor. As described above, by carrying out the method of the present invention, it is possible to obtain highly pure aniline with an extremely low content of unreacted nitrobenzene and extremely suppressed nuclear hydrogenated products, and therefore almost no purification steps are required. The effect can be said to be extremely large. Examples are shown below. Example 1 Equipped with an inlet for continuously supplying nitrobenzene, a catalyst, and hydrogen gas, an outlet for discharging unreacted hydrogen gas and products from the reaction system, a condenser, a stirrer, and a thermometer. Aniline in a stainless steel autoclave with an internal volume of 1
500g Palladium in carbon powder with oil absorption rate of 260
0.3 g of hydrogenation catalyst obtained by depositing 0.8% by weight, 0.1% by weight of platinum, and 0.8% by weight of iron, and 0.1g of sodium bicarbonate, the internal temperature was set to 190-200°C, and the total pressure was set to 3Kg/cm ² G.
Nitrobenzene, catalyst, and hydrogen gas are supplied at a rate of 100 g, 0.025 g, and 60 to 80 g/hour, respectively, and the produced water and aniline are continuously discharged from the system together with unreacted hydrogen gas. Water and aniline were condensed by introducing the mixture into a condenser connected to an autoclave and cooling it. During this time, the flow rate of hydrogen gas was adjusted so that an amount of aniline corresponding to the amount produced from the supplied nitrobenzene was distilled out, that is, so that the weight of the liquid phase containing aniline as the main component in the autoclave was always maintained at around 500 g. Adjusted. On the other hand, the reactor liquid was withdrawn from the reactor at regular intervals in an amount corresponding to the amount of catalyst supplied (approximately 10% by weight of the nitrobenzene supplied) to keep the catalyst concentration in the system constant. At that time, by filtering and analyzing the extracted bottom liquid, it was possible to know the concentration of nitrobenzene in the bottom liquid and the amount of nuclear hydrogenated products such as N-cyclohexylaniline and other by-products produced. The resulting condensate was separated into two layers, yielding a colorless to pale yellow transparent aniline layer. This aniline layer contains about 4.5% water, but as a result of analysis by gas chromatography, impurities include cyclohexanol, cyclohexanone, cyclohexylideneaniline, and nitrobenzene each of less than 5 ppm.
It only contains 40ppm or less, 20ppm or less, and 10ppm or less, and the purity of aniline is 99.99.
% or more. The aniline solution in the liquid phase in the reactor was analyzed one by one, and it was confirmed that the nitrobenzene concentration was maintained at 0.02% or less. During that time, the nitrobenzene concentration in the liquid phase became too high, and there was no need to make changes such as reducing the amount of nitrobenzene supplied or increasing the amount of catalyst supplied. Although N-cyclohexylaniline was observed in this liquid phase, its content was only 0.05% 30 hours after the start of the reaction. Examples 2 to 7 and Comparative Examples 1 to 2 Example 1 except that the types of catalyst and additives were changed
It was carried out in the same manner. The results are shown in the table. Comparative Example 3 When the reaction was carried out in the same manner as in Example 1 except that the supply rates of nitrobenzene, catalyst, and hydrogen gas were set to 130 g (average), 0.033 g (average), and 60 to 90 g/hour, respectively, impurities in the distilled aniline layer is cyclohexanol 30ppm or less, cyclohexanone 50ppm,
A slightly yellow to yellow anline containing 10 ppm or less of cyclohexylideneaniline and 400 ppm or less of nitrobenzene was obtained, and the purity was 99.96% or more. In addition, the aniline solution in the liquid phase in the reactor during the reaction was sequentially analyzed, and the nitrobenzene concentration was maintained at 0.52 to 0.58%. During this period, in order to maintain this nitrobenzene concentration, the amount of nitrobenzene supplied is 120 to 140 g per hour.
(Average 130g), catalyst supply amount 0.025-0.038g
(average 0.033g). In addition, N-cyclohexylaniline was observed in this liquid phase, but 0.01% was detected 30 hours after the start of the reaction.
It was hot. 【table】

Claims (1)

[Claims] 1. Presence of a compound selected from alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, zinc acetate, zinc nitrate in a continuous process for producing aniline by catalytic hydrogenation of nitrobenzene using a noble metal catalyst. Below, a method for producing high-purity aniline, which is characterized in that the reaction is carried out while maintaining the concentration of nitrobenzene in the aniline reaction solution at 0.5% by weight or less.