US2883421A

US2883421A - Production of benzene dicarboxylic acids

Info

Publication number: US2883421A
Application number: US700979A
Authority: US
Inventors: Speer Walter
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1956-12-24
Filing date: 1957-12-06
Publication date: 1959-04-21
Anticipated expiration: 1976-04-21
Also published as: FR1192454A

Description

United States Patet PRODUCTION OF BENZENE DICARBOXYLIC ACIDS Walter Speer, Schwetzingen, Germany, assignor to Badische Anilin- & Soda-Fabrik Aktiengesellschaft, Ludwigshafen, Rhine, Germany No Drawing. Application December 6, 1957 Serial No. 700,979

Claims priority, application Germany December 24, 1956 9 Claims. (Cl. 260-524) The present invention relates to an improved process for the production of benzene dicarboxylic acids by oxidation of xylenes with oxygen or gases containing the same. In particular it relates to the production of benzene dicarboxylic acids, above all terephthalic acid, by oxidation of xylenes with oxygen, preferably in the presence of the usual oxidation catalysts, with the simultaneous co-employment of an additional substance promoting oxidation not hitherto used.

Among the aromatic dicarboxylic acids, terephthalic acid especially has gained importance in recent years as an intermediate product for the production of plastics and synthetic fibres. Various methods are known for its production. For example it may be obtained by rearrangement of potassium salts of other benzene dicarboxylic acids or by oxidation of dialkyl-substituted benzenes. Carrying out the oxidation by the use of oxygen or gases containing oxygen as oxidizing agent is however attended by difiiculty because the oxidation comes to a standstill at a certain incomplete conversion which depends on the dialkylbenzene used and then cannot be carried appreciably further even by intensifying the conditions of the reaction. Thus it is known that the oxidation of para-xylene itself with the use of catalysts practically proceeds only as far as the para-toluic acid stage because this can only be further oxidized with great diflfiiculty. Carrying out this process on a technical scale is therefore hardly possible, unless it be by esterifying the toluic acid formed and further oxidizing the ester.

It is an object of this invention to efiect, by co-employment of an additive, the activation of the oxidation of xylene with oxygen, air or other gaseous oxygen-containing oxidizing agent, if desired in the presence of the usual oxidation catalyst, so that both alkyl groups of the xylene to be oxidized are substantially converted in a single stage into the carboxylic acid.

Another object of the invention is to facilitate the oxidation of dialkyl benzenes by reaction-promoting substances so that it leads more readily over the stage of the monoalkylbenzene carboxylic acid and the intermediate oxidation products to an increased formation of benzene dicarboxylic acids.

A further object of the invention is therefore to obtain directly higher yields of benzene dicarboxylic acids by the oxidation of dialkylbenzenes with oxygen, air or other oxygen-containing gases in one oxidation stage.

I have found that this and other objects, such as may be seen in the following description, can be realised and good yields of benzene dicarboxylic acids, especially terephthalic acid obtained in a simple and economical way by carrying out the oxidation of a xylene with oxygen, air or another oxygen-containing gas in the liquid phase at temperatures of 120 to 200 C., if desired in the presence of one of the usual oxidation catalysts, in the presence of a lower saturated aliphatic alcohol. Of these, methanol is the most suitable alcohol for the process.

As the initial xylene there may be used the isomeric xylenes as such or also in admixture with each other. The xylene or mixture of xylenes may also contain small amounts of other hydrocarbons, for example benzene, toluene and ethylbenzene; for example it is possible to start from mixtures such as are present in crude xylene or of a technical grade xylene.

As alcohols in the presence of which the oxidation is carried out according to this invention, there come into question the lower members of the saturated aliphatic series, especially methanol, and also ethanol. The alcohols are added in small amounts to the oxidation mixture, for example amounts of 0.5 to 5% by weight per hour with reference to the weight of the xylene to be oxidized. When using methanol, good results are in general obtained when the methanol is uniformly dispersed in an amount of about 1 to 2% by weight per hour with reference to the amount of hydrocarbon being oxidized. Since the amount of catalyst employed as well as the amount of hydrocarbons other than xylene is small, the amount of methane added hourly can be approximately calculated with respect to the initial oxidation mixture of hydrocarbon and catalyst and will fall within the above-mentioned ranges. While the introduction of the alcohol may be begun with before the oxidation, it is the preferred practice of my invention to start the addition of the alcohol only when the absorption of oxygen is already in progress.

The oxidation takes place in the usual way under the conditions known for working in the liquid phase. For example the procedure may be that the initial material, to which an oxidation catalyst has preferably been added, is heated to a temperature of about to 200 C., advantageously to C. and leading in oxygen, air or other oxygen-containing gas in such an amount that the oxygen or the oxygen component of the gas is absorbed wholly or partly, for example to the extent of about half. As soon as a good absorption of oxygen has been achieved and consequently the oxidation has appreciably commenced, the addition of alcohol is begun. The alcohol may be added either together with the oxygen or air or away from the oxygen or oxygen-containing gas. The introduction of the alcohol into the oxidation mixture should be distributed uniformly over the entire reaction time. In order to avoid an inordinately strong vaporization of the initial material, the oxidation may if desired be carried out under increased pressure, for example at 2 to 30, more preferably 10 to 20, atmospheres. Working under increased pressure is also especially of advantage when air or other oxygen-containing gas is used as the oxidizing agent. When Working under atmospheric pressure the use of oxygen is to be preferred to the use of air.

Although the oxidation already proceeds in a satisfactory way to the oxidation of the desired benzoic dicarboxylic acid even without an oxidation catalyst, and considerable amounts of dicarboxylic acid are formed, it is preferable to co-employ such reaction-accelerating substances.

As catalysts which may be dissolved or dispersed in the initial material there come into question oxidation catalysts which are known per se. For example there may be mentioned the salts of organic higher carboxylic acids, as for example of ethylhexane acid, first running fatty acids of the oxidation of parafiins or naphthenic acids, of which the cation is formed by a metal which is capable of occurring in more than one valency stage, for example cobalt, manganese, cerium and copper. I

prefer the cobalt salts. The amounts in which the cata-.

lysts may advantageously be co-employed lie in general between 0.01 and 1% by weight with reference to the hydrocarbon to be oxidized.

The gases leaving the oxidation zone are preferably cooled and the condensate which separates is returned to the oxidation vessel after separation of the water formed. The oxidation mixture contains as main product the benzene dicarboxylic acids, besides unoxidized hydrocarbon. There are also contained therein methylbenzene monocarboxylic acids and other partial oxidation products. These intermediate oxidation prodnets are however contained in considerably smaller amounts than in the prior oxidation methods which work without the presence of lower aliphatic alcohols in the oxidation of the dialkyl substituted benzenes. The oxidation mixture obtained when working according to this invention contains the benzene carboxylic acids partly in the form of the esters corresponding to the lower saturated aliphatic alcohol co-employed in the oxidation.

The benzene dicarboxylic acids can be separated from the oxidation mixture in a manner known per se, as for example by hot filtration and Washing of the filter residue with solvents, such as toluene, xylene or methanol or, after separation of unreacted hydrocarbon, by esterification of the whole oxidation product and separation of the ester.

The process may be carried out either discontinuously or continuously.

The following examples will further illustrate this invention but the invention is not restricted to these examples.

Example 1 450 grams of para-xylene and 0.9 gram of cobalt ethylhexanate are charged into an oxidation vessel which is provided with a stirrer, a gas inlet pipe, a dropping funnel and a condenser which is associated with a Water separator of which an overflow pipe leads back into the oxidation vessel, and the mixture is heated to 130 C. while leading in 15 litres of oxygen per hour. At the beginning the oxygen is absorbed in an amount of about litres per hour. After the temperature has risen to 140 C., about 12 ccs. of methanol per hour are allowed to drip uniformly into the oxidation mixture. As the conversion proceeds, the reaction temperature is raised to 170 C. and kept constant at this temperature. The gaseous and vaporous substances leaving the oxidation vessel are cooled in the condenser to a. temperature of about 15 C. so that the vaporous substances condense. The condensate passes into a water separator from which the aqueous phase containing methanol is run otf from time to time while the unreacted xylene flows back into the reaction vessel. The uncondensed gaseous constituents, which contain the unabsorbed oxygen, escape to atmosphere at the top of the water separator. After about 40 hours the absorption of oxygen is finished. The reaction mixture, which after distilling off the unoxidized xylene has an acid number of 330 and an ester number of 185, contains 359 grams of terephthalic acid and 140 grams of toluic acid which are partly present already in combined form as the methyl ester and after complete esterification with methanol under increased pressure and at elevated temperature can be separated by distillation.

If the oxidation is carried out under the same conditions in the same way but without the addition of methanol during the oxidation, there are obtained, after the absorption of oxygen is finished and working up has been carried out in the manner described above, 78 grams of terephthalic acid and 299 grams of toluic acid in the form of the methyl ester.

Example 2 Into 500 grams of para-xylene in which 0.9 gram of cobalt ethyl-hexanate have been dissolved there are led 15 litres of oxygen per hour at 125 C. in the Way described in Example 1 and at the same time 8 ccs. of methanol are dripped into the reaction vessel per hour uniformly distributed over the time. After 35 hours, the reaction is discontinued and the contents of the reaction vessel, after distilling off the unreacted paraxylene, are esterified with 4 times the weight of methanol for 6 hours at 240 C. After Working up the esterification mixture there are obtained 146 grams of dimethyl terephthalate and 352 grams of methyl paratoluate.

By Working in the same way and using the same oxidation mixture but with the difference that the oxidation is carried out at the oxidation temperatures set out in the following table, the following amounts of oxidation products are obtained after esterification of the oxidation mixture in way described in the preceding paragraph:

423 grams of crude xylene obtainable in commerce, after the addition of 1 gram of cobalt ethyl-hexanate and after heating the mixture to 140 C. are oxidized in the way described in Example 1 by leading in 25 litres of oxygen per hour; after the absorption of oxygen has evidently started, 10 ccs. of methanol per hour are introduced in uniform distribution over the whole of the 1 hour period. The temperature is raised to 160 C. as the reaction proceeds and while leading in oxygen and adding methanol in the hourly amounts stated, and kept at the said temperature. After 35 hours the oxidation is discontinued. The oxidation mixture, after distilling off the unreacted crude xylene, is worked up by esterification of the oxidation product remaining as a residue, and separating the ester. 224 grams of a mixture of dimethyl phthalates and 204 grams of a mixture of the methyl esters of the isomeric toluic acids are obtained.

Example 4 500 grams of meta-xylene are oxidized in the way described in Example 1 with oxygen with the simultaneous addition of 20 ccs. of methanol per hour. The reaction temperature is keptfirst at 140 C. for 12 hours and then at 160 C. for 38 hours. By working up the oxidation mixture, after distilling off the unreacted xylene and esterifying the residue, there are obtained 402 grams of dimethyl isophthalate and 151 grams of methyl metatoluate.

Example 5 500 grams of para-xylene and 1 gram of cobalt ethyl hexanate are charged into the apparatus described in Example 1 and heated up while leading in 20 litres of oxygen per hour. As soon as the reaction has commenced, which is the case between and C., 6 ccs. of methanol per hour are dripped uniformly into the reaction vessel. The temperature is raised to 155 C. corresponding to the progress of the reaction and kept at this temperature. After 20 hours, the constituents which have separated in solid form in the reaction mass are filtered oil in a suction filter heated to C. by super heated steam. The filter residue is washed with 200 grams of para-xylene heated to 130 C. The filtrate containing xylene, the xylene used for washing and also the xylene which is recovered by evaporation in vacuo from the filter cake are recovered and further oxidized" under the said conditions. After 1,200 grams of para xylene have been used up in this way, there are obtained, after esterification of the combined filter residues with methanol, a total of 943 grams of dimethyl terephthalate and 240 grams of para-toluic acid ester. The methyl paratoluate can be returned to the reaction vessel.

Example 6 5 kilograms of para-xylene and 10 grams of cobalt ethyl hexanate are charged into a pressure-tight vessel of litres capacity which is provided with a rapidly rotating stirrer, a condenser and a water separator, and which has a separator which is connected to the reaction vessel through an overflow. The reaction vessel and also the overflow arranged at a level of 10 litres and the separator are capable of being heated by a steam jacket. Reaction vessel, overflow and separator are heated to 160 C. and air is led through the system under a pressure of 15 atmospheres. 1,500 litres of residual gas are released from pressure per hour behind the condenser. When the reaction has commenced, which can be ascertained by a diminution of the oxygen content in the residual gas, a mixture of 220 grams of para-xylene, in which 0.1% of cobalt ethyl-hexanate is dissolved, and 100 grams of methanol is pumped in hourly by means of a cam shaft pump. The residence time of the xylene present in the pressure vessel amounts to about 35 hours under these conditions. From the separator, the hot reaction product which overflows is released from pressure in a vessel which is provided with a condenser by which para-xylene distilled off is condensed and pumped back into the reaction vessel by the cam shaft pump. The residue which becomes solid when cold is worked up by esterification with methanol as described in the foregoing examples. There are thus obtained in continuous operation from each 1,000 grams of para-xylene by crystallizing out an average of 900 grams of dimethyl terephthalate and by distillation 310 grams of methyl para-toluate. The methyl para-toluate is mixed with para-xylene and pumped back into the reaction vessel for further oxidation.

What I claim is:

1. In the process for the production of a benzene dicarboxylic acid by the oxidation of a xylene in the liquid phase in an oxidation zone with an oxidizing gas containing oxygen at a temperature of between 120 and 200 C. and in the presence of an oxidizing catalyst, the improvement which comprises carrying out the oxidation in the presence of a lower saturated aliphatic alcohol introduced uniformly into the oxidation zone at a rate of 0.5 to 5% by weight an hour, percentage with reference to the amount of xylene to be oxidized.

2. The process as claimed in claim 1 wherein the lower saturated alcohol is methanol.

3. In the process for the production of a benzene dicarboxylic acid by the oxidation of a xylene in the liquid phase in an oxidation zone with an oxidizing gas selected from the group consisting of oxygen and air at a temperature of between and 200 C. and in the presence of an oxidizing catalyst, the improvement which comprises introducing into the oxidation zone a lower saturated aliphatic alcohol uniformly at a rate of 0.5 to 5% by weight an hour, percentage with reference to the amount of the xylene to be oxidized, the lower saturated aliphatic alcohol being introduced after the absorption of the oxygen has commenced.

4. The process as claimed in claim 3 wherein the lower saturated alcohol is methanol.

5 The process as claimed in claim 3 wherein the xylene is a mixture of technical-grade xylene isomers.

6. In the process for the production of terephthalic acid by the oxidation of para-xylene in the liquid phase in an oxidation zone with oxygen at a temperature of between 120 and 200 C. and in the presence of an oxidizing catalyst the improvement which comprises introducing methanol uniformly into the oxidation zone at a rate of 0.5 to 5% by weight an hour, percentage with reference to the amount of the para-xylene to be oxidized, the methanol being introduced after the absorption of oxygen has commenced.

7. The process as claimed in claim 3 wherein the oxidation reaction is carried out under a pressure of from 2 to 30 atmospheres.

8. The process as claimed in claim 4 wherein the oxidation reaction is carried out under a pressure of from 10 to 20 atmospheres.

9. The process as claimed in claim 4 wherein the oxidation reaction is carried out at a temperature of from C. to C.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. IN THE PROCESS FOR THE PRODUCTION OF A BENZENE DICARBOXYLIC ACID BY THE OXIDIATION OF A XYLENE IN THE LIQUID PHASE IN AN OXIDATION ZONE WITH AN OXIDIZING GAS CONTAINING OXYGEN AT A TEMPERATURE OF BETWEEN 120* AND 200* C. AND IN THE PRESENCE OF AN OXIDIZING CATALYST, THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE OXIDATION IN THE PRESENCE LOWER SATURATED ALIPHATIC ALCOHOL INTRODUCED UNIFORMLY INTO THE OXIDATION ZONE AT A RATE OF 0.5 TO 5% BY WEIGHT AN HOUR, PERCENTAGE WITH REFERENCE TO THE AMOUNT OF XYLENE TO BE OXIDIZED.