KR820000459B1 - Process for preparation of terephthalic acid - Google Patents

Abstract

Terephthalic acid was prepd. by oxidn. of a mixt. of pxylene and p-toluic acid in H2O in the presence of at least one heavy metal salt as catalyst. Thus, a mixt. of p-xylene 100, p-toluic acid 180 and H2O 150 g contg. Co naphthenate 7.5 and Mn naphthenate 0.75 mmol was pressured with air to 20 atm and heated to <=185≰C, 250 min to give terephthalic acid 182, p-toluic acid 118, and 4-HO2CC6H4CHO 7 g. Omitting the Mn salt gave decreased O2 absorption and decreased amts. of terephthalic acid.

Description

Method of producing terephthalic acid

The accompanying drawings are schematic distribution diagrams of the continuous process of the present invention.

The present invention relates to a process for preparing terephthalic acid by oxidizing p-xylene in a liquid phase.

In the presence of liquid and heavy metal catalysts, the oxidation of p-xylene to terephthalic acid with oxygen molecules has been of considerable industrial importance and has been a constant effort in the past 30 years.

The main challenge encountered when carrying out the process is that it is much more difficult to oxidize p-toluic acid even though p-xylene is easily transformed into p-toluic acid, and when using the catalytic method, terephthalic acid This is hardly obtained. Numerous methods have been proposed to solve this problem, most of which consist of adding some active agent and accelerator. One of these methods consists of using bromine-containing compounds as active agents and lower fatty acids such as acetic acid as solvents. Although this method has been a complete success from a commercial standpoint, this method has serious drawbacks, since it causes severe corrosion problems when using bromine at high temperatures. This corrosion problem can be solved by using expensive and high corrosion resistance devices such as Hastelloy or titanium. Moreover, a lot of acetic acid solvent is consumed under the strong oxidation conditions used in the process, which is expensive.

In order to solve the corrosion problem, a method of using aldehyde and ketone compounds (eg, acetaldehyde and methyl ethyl ketone) instead of bromine as an active agent has been proposed. This method does not require harsh conditions and can use conventional stainless steel devices, even if acetic acid is used as the solvent. However, since the active agent is mainly oxidized and consumed with acetic acid during the reaction, acetic acid must be separated and purified in order for the reaction to be a coproduct and to perform the process economically. Another method has been proposed to solve the deficiencies that occur when using external compounds as active agents. For example, it has been found that p-xylene can be oxidized to terephthalic acid in good yield in acetic acid medium containing only a large amount of cobalt as a catalyst. Nevertheless, this method also leads to enormous consumption of acetic acid.

Recently, a method of oxidizing p-xylene to terephthalic acid without any solvent and active agent has been proposed. Temperatures close to or above the melting point of p-toluic acid (ie 179 ° C.) are used to satisfy the liquid phase conditions. Although this method looks simple in principle, it is difficult to do in practice. In other words, if the active agent is not used, the intermediate product, p-toluic acid, is difficult to oxidize, and if not used, technical problems arise in handling the solid and removing the heat of reaction. For example, it is difficult to separate terephthalic acid from other components of the reaction compound. It is generally separated by heating and washing at elevated temperatures of 230 ° -270 ° C (US Pat. No. 3,883,584 (or 290 ° -350 ° C (US Pat. No. 3,711,539)).

The treatment requires the use of expensive corrosion resistant pressure vessels, increased degradation and colored reaction products.

An object of the present invention is to provide a method for producing terephthalic acid with high purity and high yield by oxidizing p-xylene.

It is still another object of the present invention to provide a method that can be implemented in a conventional stainless steel device without requiring a device having high corrosion resistance.

Another object of the present invention is to provide a method which can be carried out in the absence of any additional solvent (especially lower fatty acids).

Another object of the present invention is to provide a method in which an accelerator such as a bromine compound is not used in addition to the oxidation catalyst.

It is still another object of the present invention to provide a process for the easy recovery of terephthalic acid from an oxidized reaction mixture at relatively mild temperatures.

It is still another object of the present invention to provide a process in which the catalyst and the oxidation intermediate can be recovered and reused in the oxidation reaction.

Another object of the present invention is to provide a method of easily controlling the temperature by effectively consuming heat generated from the oxidation reaction.

It is a further object of the present invention to provide a process which can be carried out not only batchwise but also continuously.

In order to achieve the above objects according to the present invention, there is provided a method for preparing teretylic acid by oxidizing p-xylene in a liquid phase as follows.

(B) a liquid mixture consisting of p-xylene, p-toluic acid and water as an oxygen molecule-containing gas in the presence of an oxidation catalyst composed of at least one heavy metal salt (cobalt and / or manganese salt is preferred) under sufficient oxidizing conditions to form terephthalic acid; Wherein the molar ratio of p-toluic acid to p-xylene is about 0.01-100 and the molar ratio of water to p-toluic acid is about 0.4-60),

(B) recovering the oxidized mixture containing terephthalic acid. The oxidation reaction is performed well at about 140 ° -220 ° C. with a pressure sufficient to keep at least some of the water in the liquid phase.

From the oxidized mixture, solid terephthalic acid can be separated at a temperature sufficient to keep the other components of the mixture in a liquid solution, and the remainder of the mixture containing the oxidizing intermediate and the catalyst can be reused for another oxidation reaction. have. This process can be carried out batchwise or continuously by introducing fresh p-xylene into the oxidizer at a rate sufficient to adequately maintain the molar ratio of p-toluic acid and p-xylene.

Still other objects and features of the present invention are apparent in the preferred embodiments with reference to the detailed description of the invention and the accompanying drawings.

When oxidizing organic compounds to oxygen molecules, the presence of water is generally undesirable and is known to be damaging.

In the most common view, it is believed that water interferes with radical initiation. For example, US Pat. No. 2,853,514, which refers to the oxidation of alkylbenzenes, especially xylenes, states that “the concentration of water should be kept below 5 moles to prevent exorbitant delays in induction periods”. In another expression, the water concentration should be kept below 5% by weight. Only when a strong radical generator such as bromine is present in the system can a significant amount of water be present. Thus, in the United States, in particular, US Pat. No. 3,139,459, which describes a method for oxidizing p-xylene to terephthalic acid in the presence of HBr and acetic acid as a solvent and cobalt catalyst, the amount of water exceeds 0.05 parts by weight If accumulated, the reaction is completely stopped. ” Thus, in view of the description of the prior art, not only does not use 50% by weight or more of water in the reaction under the conditions of the present invention, but also shows the flexibility of the reaction without causing any induction or inhibition problems, as shown below. It is surprising that you should use enough water to proceed.

The amount of water used in the process of the invention depends on the different factors, mainly the operating temperature and the composition of the reaction mixture. The amount of water is sufficient to allow a significant amount of p-toluic acid to be present in the solution at the operating temperature. Since the solubility of p-toluic acid in water increases rapidly with increasing temperature within the ranges considered in this chapter, the amount of water used will decrease with increasing temperature. However, usually the amount of water is at least 0.4 mole per mole of p-toluic acid shown in the reaction mixture.

For practical reasons, it is often advantageous to proceed the reaction with more water than necessary to keep the p-toluic acid in solution. For example, if a relatively large amount of terephthalic acid (generated from the oxidation reaction) is present in the system, a sufficient amount of water must be added to obtain a usable slurry. However, the amount of water that is higher than terephthalic acid (for example, moles of 160,000 10 ^-0.0715t per mole (T is the operating temperature in degrees Celsius) and other factors must be taken into account. When the amount is relatively high, especially when the mole ratio of terephthalic acid to p-toluic acid is more than ⅔, the above-mentioned upper limit is too high for the oxidation reaction to be carried out at a high rate. Addition adversely affects the reaction rate, especially if the amount of p-xylene in the system is relatively small, preferably the amount of water will be less than 60 moles per mole of p-toluic acid, and in most cases the p- There is no advantage to using more than 10 moles of water per mole of toluic acid.

The temperature at which the oxidation reaction takes place is not critical but usually ranges from 140 ° C. to 220 ° C. Below 140 ° C, the solubility of p-toluic acid in water is too small to enjoy the advantage of using water according to the present invention, while if it is operated above 220 ° C, it becomes excessively oxidized and undesirable side reactions occur. Serious corrosion problems are caused. The reaction temperature is most preferably 160 ° C-190 ° C.

The catalyst used in the process of the invention may be any heavy metal salt conventionally used to oxidize p-xylene to terephthalic acid. However, the catalyst may be dissolved in the reaction medium or may be dissolved by reacting with a component of the reaction mixture. Numerous salts of aliphatic carboxylic acids can be used, namely acetates, propionates, stearates, naprenates and the like. When the above salts are added in a system comprising p-toluic acid, anion exchange is readily accomplished, with the formation of the corresponding toluate as an effective catalyst. Particularly effective catalysts for the process of the invention are cobalt and manganese salts, which may be used alone or in combination. Other reaction variables are closely related to each other. In order to achieve the best activity, the catalyst has to be selected in consideration of different conditions. For example, cobalt salts exhibit particular efficacy in systems containing large amounts of unreacted p-xylene and relatively small amounts of water. Manganese, in contrast, is particularly useful in systems containing large amounts of water. In most cases, however, the best catalytic effect is obtained by combining manganese and cobalt together. The amount of catalyst can vary widely but usually the total concentration of the catalyst will comprise 0.001-0.1 M anhydrous base per liter of organic material present in the system.

When p-xylene, water and catalyst are in contact with each other and heated to a temperature within the above-mentioned range in the presence of oxygen, the oxidation reaction is not easily oxidized as long as p-toluic acid is present in the system. This is surprising considering the fact that p-toluic acid is not as easy to oxidize as oxygen molecules than p-xylene. In addition, when a small amount of p-toluic acid is present, the acid reaction proceeds actively for a short time but rarely after a while. This phenomenon is more pronounced when there is a large amount of water and a small amount of manganese catalyst in the system. Therefore, it is important in the present invention that p-toluic acid must always be sufficiently present in the reaction mixture. Conveniently, this amount is an amount in which the molar ratio of p-toluic acid to p-xylene is at least 0.01.

In practicing the present invention, an oxygen containing gas is passed through the mixture while heating p-xylene mixed with p-toluic acid and other intermediate oxidation products in the presence of water and a heavy metal catalyst. Stir effectively to bring in close contact with the different components. The reaction proceeds actively due to strong oxidative absorption and rapid temperature rise. An important advantage of the present invention is that temperature control can be made easily. That is, since there is a sufficient amount of water in the reaction mixture, the heat generated as a result of the exothermic oxidation reaction can be easily controlled by controlling the evaporation of water.

As the reaction proceeds, i.e., the conversion of the larger p-xylene, the oxygen uptake decreases and will stop unless p-xylene is added to the system. In fact, it is another important aspect of the present invention that p-xylene must be present in the system so that the molar ratio of p-toluic acid to p-xylene is less than 100 in order to activate the oxidation reaction and increase the yield of terephthalic acid. Thus, in the practice of the present invention, new p-xylene must be added to the coagulation mixture at a rate that maintains the satisfactory conditions. The addition can be carried out continuously or intermittently. For example, the reaction can be carried out batchwise until the oxygen uptake is actually stopped. Terephthalic acid can then be separated by simple filtration at a temperature at which the reaction temperature is maintained at least in p-toluic acid in solution. In fact, it is an important advantage of the present invention that there is a terephthalic acid formed in the reaction as a relatively large amount of crystals in which the terephthalic acid formed during the reaction is suspended in an aqueous medium in which most of the intermediate p-toluic acid is dissolved.

Therefore, p-toluic acid can be easily separated by filtration, centrifugation or other solid-liquid separator at a temperature that is usually in solution. The filtrate of this reaction contains a catalyst with the entire amount of the intermediate oxidation product and can be reused in another reaction with fresh p-xylene.

According to another embodiment of the invention, the reaction is carried out continuously as schematically shown in the accompanying drawings. The reaction is carried out in an oxidizer. The heat generated during the reaction is removed by evaporating some water from the reaction mixture. Some p-xylene is azeotropically evaporated with water and then the vapor is concentrated and then separated in decanter 2. The oxidation product produced in oxidizer 1 is sent to stripper 3 from which any unreacted p-xylene is removed by stripping with water.

The sediment in the effluent flowing out of stripper 3 is separated in separator 4 and then washed in washer 5 with a single hot line that can be provided at least in part from decanter 2. The filtrate and washings are recycled directly to the oxidizer 1. As is well known to those skilled in the art, it is particularly important that the water used as a washing solvent for terephthalic acid can be at least partially heated and evaporated by the heat generated during oxidation without the need for external energy supply. It is an advantage.

Example 1

The following material was charged to a 1 liter corrosion resistant autoclave equipped with a mechanical stirrer, heating jacket, condenser, gas inlet and outlet.

Cobalt naphthenate: 7.5 mmol (about 0.025 mol per liter of organic matter)

The molar ratios of p-toluic acid to p-xylene and water to p-toluic acid are 1.4 and 2.1, respectively.

After the reactor is compressed to air at 20 atm, the mixture is stirred and heated by introducing air at a flow rate of 300 liters per hour (measured at atmospheric pressure and 20 ° C.). Oxygen absorption starts when the temperature is about 100 ° C. The temperature then increases rapidly and is cooled down and maintained at 185 ° C.

Oxygen absorption rate increases dramatically during the first 20 minutes of the reaction but then slowly decreases. After 240 minutes of reaction, 101 liters of oxygen are absorbed. The reaction is stopped by cooling and the autoclave is opened. The contained precipitate is washed with water, filtered and dried at 80 ° C. and under vacuum. The filtrate is treated with a cation exchange resin to remove the metal catalyst and evaporated to dryness.

Each of the above components is analyzed using a combination of acid, polarography, and gas phase chromatography. Thus, it was found that the reaction mixture consisted of 137 g terephthalic acid, 177 g p-toluic acid and 7 g p-carboxybenz aldehyde. About 3 g of p-xylene is incorporated into the air stream during the reaction. Thus, under the conditions of the present invention, not only p-xylene is converted to p-toluic acid but also p-toluic acid is also converted to terephthalic acid.

Example 2

The following material is charged to the same autoclave as in the above embodiment.

The molar ratio of p-toluic acid to p-xylene is used in the same manner as in the above examples, and the molar ratio of molar to p-toluic acid is 6.3 instead of 2.1 and some manganese salt is added. When the temperature reaches 140 ° C., oxygen uptake clearly occurs. The temperature is maintained at 185 ° C. as in the above embodiment. After 295 minutes of reaction, 106 liters of oxygen are absorbed and the reaction is stopped by cooling. The reaction mixture is then processed and analyzed as in the above examples. 182 g of terephthalic acid and less than 118 g of p-toluic acid and 7 g of carboxybenzaldehyde earlier in the reaction.

Comparative experiments are conducted under the same conditions as above except that no manganese naphthenate is used. As above, oxygen absorption is apparent, but decreases rapidly after about 80 minutes of reaction. Total oxygen absorption is up to 58 liters and the amount of terephthalic acid in the reaction mixture is only 68 g. From these results, it can be seen that it is advantageous to add a certain amount of manganese salt with cobalt as a catalyst for oxidizing p-xylene to terephthalic acid when the reaction is carried out in the presence of a significant amount of water.

Example 3

The experiment of Example 2 was repeated except that the oxidation reaction was stopped after heating at 180 ° C. for about 120 minutes. The reaction mixture is then recovered in a compressed filter heated to the same temperature. Here terephthalic acid is separated from other oxidation products. The resulting cake is washed twice with water at 185 ° C. and then dried. After cooling to room temperature, the filtrate and washings are filtered again and separated to wash the p-toluic acid precipitate with water. The final filtrate contains almost water.

The final filtrate containing the procatalyst and some p-toluic acid is evaporated to dryness. The resulting residue is charged to an autoclave with the same amount of p-xylene as well as water and precipitate.

The resulting mixture is then oxidized as mentioned above.

The same process is repeated nine times. As a result of the analysis, the average yield of terephthalic acid produced by the series of processes is 87% relative to the amount of p-xylene reacted.

Example 4

The experiment of Example 2 was repeated as it was except that 7.5 mmol of manganese naphthenate was used as the only catalyst. After 310 minutes of reaction, 95 liters of oxygen are absorbed. The reaction mixture is then cooled, treated and analyzed as mentioned in Example 1. The composition is 150 g of terephthalic acid, 146 g of p-toluic acid, and 7 g of p-carboxybenzaldehyde.

Example 5

The experiment of Example 2 is repeated except that 75 ml of water is placed in an autoclave and 7.5 millimoles of manganese toluate is used as the only catalyst. After 240 minutes of reaction, 81 liters of oxygen are absorbed. The analysis showed that the reaction mixture consisted of 111 g of terephthalic acid, approximately 179 g of p-toluic acid, 7 g of carboxybenzaldehyde and some unreacted p-xylene in an amount almost equal to the starting dose. Thus, as a general result of the reaction, p-xylene is converted to terephthalic acid in the presence of an almost constant concentration of p-toluic acid.

Example 6

The same amount of p-xylene, p-toluic acid and water as in Example 2 is charged into an autoclave with 3.75 mmol of cobalt and manganese naphthenate, respectively. The mixture was oxidized with air under the same conditions as in Example 2 except that the temperature was set at 170 ° C instead of 185 ° C. 74 liters of oxygen are absorbed after 230 minutes of reaction. The reaction mixture is cooled, treated and analyzed as described in Example 1. It consists of 98 g terephthalic acid, 179 g p-toluic acid, 7 g p-carboxybenzaldehyde and some unreacted p-xylene.

Example 7

Under the conditions of the above-described examples, a series of 13 continuous processes are carried out by recycling the catalyst and the reaction intermediate mentioned in Example 3. Terephthalic acid is white powder with an average yield of 87%. Analysis of the different air flows revealed that the current water consisted mainly of carbon dioxide, some non-recirculated p-tolualdehyde, and a light acid. The formation of heavy by-products, tars or other colored substances is not detected.

Example 8

The same amount of p-xylene, p-toluic acid and water as in Example 2 were charged at high pressure with 3.9 mmol of cobalt and manganese salt acetate, respectively. It is then oxidized with air under the same conditions as in Example 2 except that the temperature is set at 165 ° C.

After 300 minutes of reaction, 61 liters of oxygen was absorbed and analysis of the reaction compound revealed that 91 g of terephthalic acid was formed.

This example shows that by the process of the present invention p-xylene can be transformed into terephthalic acid at a temperature below the melting point of p-toluic acid without resorting to the use of lower fatty acid solvents.

Example 9

The following material is heated while injecting and stirring air at a pressure of 20 atm and a flow rate of 300 liters per hour.

The molar ratio of water to p-toluic acid is therefore 15.7, compared to 6.3 in most of the foregoing examples. Oxygen absorption begins when the temperature reaches about 125 ° C. The temperature is then maintained at 170 ° C.

After 300 minutes of reaction 43 liters of oxygen are absorbed and the reaction is stopped by cooling. The reaction mixture is processed and analyzed as in Example 1. The composition is 49 g terephthalic acid, 139 g p-toluic acid and 5 g p-carboxybenzaldehyde.

This example shows that p-xylene can be oxidized to terephthalic acid in the presence of large amounts of water according to the process of the invention.

Example 10

The experiments of the above-described examples are repeated in the same manner, except that manganese nattenate is used as the only catalyst without using p-xylene. When heating the mixture to 170 ° C. for 1 hour, very little oxygen is observed to be absorbed. Thereafter, even if the temperature is raised to 185 ° C, the change in absorption does not occur. After this one hour hold at 185 ° C. 50 g of p-xylene is introduced into the reactor. Sudden absorption of oxygen suddenly takes place and after 145 minutes 29 liters of oxygen are absorbed.

In this example, it is shown that under the conditions of the present invention, p-toluic acid cannot be oxidized to terephthalic acid, unless p-xylene is present in the system.

Example 11

Air is introduced at 20 atmospheres and a flow rate of 200 l / hr, and the next charge is heated while stirring at the same time.

Thus, the molar ratio of p-toluic acid to p-xylene and the molar ratio of water to p-toluic acid are 0.05 and 38, respectively. Oxygen absorption begins violently when the temperature reaches 180 ° C., and 92 liters of oxygen are absorbed after 245 minutes at 185 ° C., at which time the reaction is stopped by cooling. As a result, the reaction mixture consisted of 96 g of terephthalic acid, 108 g of p-toluic acid and 5 g of p-carboxybenzaldehyde.

Comparative experiments are conducted under the same conditions as above except that p-toluic acid is not used. The reaction of heating the mixture for 4 hours at 185 ° C. rarely occurs.

In another comparative experiment, 100 g of water is placed in an autoclave instead of 50 g. Therefore, the molar ratio of water to p-toluic acid is 78 rather than 38. Oxygen absorption begins at 75 ° C. However, after about 75 minutes at 185 [deg.] C., oxygen absorption suddenly drops to almost negligible. The total oxygen uptake is only 44 liters.

These comparative experiments show how important it is to have a sufficient amount of p-toluic acid relative to the amount of water in the system for oxidizing p-xylene to terephthalic acid according to the method of the present invention.

Example 12

The filling material is then heated under a pressure of 20 atmospheres while passing air at a flow rate of 300 liters per hour.

It can be seen that the filler is the same as in Example 6 except for water. Oxygen absorption begins when the temperature reaches about 160 ° C., but drops quickly after only 4.5 liters are absorbed. Heat at 185 ° C. for 2 hours without affecting the reaction rate. Thereafter, 30 ml of water is injected into the reactor, where oxygen absorption occurs violently. The reaction mixture is continued to heat at 185 ° C. for 240 minutes, then cooled and treated and analyzed as above. The composition of the reaction mixture consisted of 143 g terephthalic acid, 179 g p-toluic acid and 7 g p-carboxybenzaldehyde.

This example shows that under the conditions of the present invention, water not only harms the oxidation reaction but also has a favorable effect on the initial reaction.

Claims (1)

A method for producing terephthalic acid by oxidizing p-xylene and p-toluic acid with an oxygen molecule-containing gas in the presence of an oxidation catalyst having at least one heavy metal salt, wherein water is contained as a solvent and p-toluic acid to p-xylene The molar ratio of is about 0.01 to 100, the molar ratio of mole to p-toluic acid is about 0.4 to 60, the initial molar content of the mixture is 5% by weight, and the amount of water is sufficient to keep p-toluic acid in solution at the reaction temperature. A method for producing terephthalic acid, characterized in that a sufficient aqueous solution is mixed to oxidize.

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