CH642343A5 - METHOD FOR PRODUCING TEREPHTHALIC ACID. - Google Patents

Description

The present invention relates to an oxidation process for the production of terephthalic acid by oxidation of p-toluic acid or mixtures thereof with p-xylene and / or with partially oxidized derivatives thereof, such as p-tolualdehyde.

Terephthalic acid is of great importance because it is being used to an increasing extent for the production of high-molecular resins, such as fiber- and film-forming polyester.

The prior art knows many processes for the oxidation of alkyl-substituted aromatic compounds in the liquid phase to aromatic carboxylic acids. One of the first publications in this field is US Pat. No. 2,245,528, which describes a one-step process for the oxidation of aromatic alkyl compounds by molecular oxygen using a metal catalyst, a solvent such as acetic acid, and optionally an oxidation initiator. Even under harsh conditions that is

Yield of dicarboxylic acids, however, low. After oxidation of a mixture of xylenes in air in acetic acid 55 containing cobalt and manganese acetates as catalysts at 185 to 200 ° C. under a pressure of 50 at and in the presence of diethyl ketone as initiator, the yield of phthalic acids is only 2%, and the main reaction products were toluic acid along with other oxidation intermediates.

60 Numerous other patents describe processes for the oxidation of p-xylene in one step with improved yields of terephthalic acid; they essentially relate to the use of specific activators, such as bromine-containing compounds (US Pat. No. 2,833,816), ketones (US Pat. No. 2,583,514) or aldehydes (US Pat. No. 3,036,122). Although some of these methods are used on an industrial scale, they have serious disadvantages. So z. B. severe corrosion problems when using bromhal

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activators. If ketones or aldehydes are used, some of them are inevitably lost, and the remainder is mainly converted into acetic acid, which has to be recovered, purified and economically exploited so that the process is affordable. Despite these disadvantages, the use of an activator is nevertheless considered to be an essential requirement for an effective production of phthalic acids from xylenes.

In most cases, the use of a solvent is also claimed to be necessary. Low molecular weight fatty acids, especially acetic acid, are often used for this purpose. The amount of solvent added must be sufficient to keep the reactants and reaction products in solution or at least in suspension without difficulty. Thus, the reaction mixture is gently stirred, oxygen dispersion is improved, the formation of by-products is kept to a minimum, and the heat of reaction is easily removed by solvent evaporation. However, under the commonly used reaction conditions, a substantial amount of solvent is lost through cooxidation. Furthermore, the solvent must be separated from the other components of the reaction mixture and then cleaned and recycled. The consumption of part of the solvent and the work steps to recover the remaining part naturally lead to additional process costs.

In order to avoid the above-mentioned essential problems, it has been proposed to carry out the oxidation of p-xylene in the absence of a solvent. In this case, of course, at a temperature at least in the range of the melting point of p-toluic acid, i.e. about 180 ° C, work to obtain a liquid reaction mixture. The choice of temperature is therefore limited. In the absence of a solvent, the handling of the reaction mixture and the separation and purification of the terephthalic acid are still difficult. The reaction is usually carried out to a point where the terephthalic acid content in the mixture is not more than 60%, preferably not more than 45% by weight. Beyond this point, "it becomes difficult to handle the oxidation reaction mixture as a slurry, which is why the reaction is adversely affected" (see US Pat. No. 3,883,584).

In the absence of a solvent, the removal of the heat of reaction in industrial production poses another difficulty, since the reactor is very heavily contaminated even if terephthalic acid is not present in large quantities. For example, in US Pat. No. 2,696,499, which relates to the oxidation of xylene in toluic acid in the presence of a solvent, it is stated that “the essential design problem inherent in cooling the xylene oxidation mixture is the avoidance of separation of solids is ».

US Pat. No. 3,406,196 describes a two-step process in which water from suspending agent for terephthalic acid is used. In the first stage, an aromatic alkyl compound, in particular p-xylene, is oxidized to partially oxidized compounds by means of air in the absence of an additional solvent, which are further oxidized in the second stage at a higher temperature in the absence of substantial amounts of water as the suspension medium. Bromine or a compound containing bromine must be present to accelerate the oxidation. Nevertheless, very high temperatures in the range from 200 to 275 ° C., in particular from 225 to 250 ° C., are necessary in order to achieve the conversion of these partially oxidized products into terephthalic acid. Therefore, the same or worse corrosion problems occur than in processes in which acetic acid is used as a solvent. As further stated in the cited patent, there are often significant losses of unreacted aromatic poly-alkyl compound through decomposition and other side reactions when these compounds are exposed to the higher temperatures that are required for effective conversion of the partially oxidized, produced in the first stage of the oxidation Products in aromatic polycarboxylic acid are necessary ». This patent thus clearly teaches that the use of large amounts of water, even in the presence of a bromine accelerator, does not provide satisfactory results for the oxidation of p-xylene in terephthalic acid in one step.

In fact, it has long been known that water is “a catalyst poison in the oxidation reaction” (cf. US Pat

2,696,499). According to popular belief, water has an adverse effect on the rate of reaction by disrupting initiation. As a general rule, its presence is avoided as much as possible, whether a solvent and / or activator is present or not. For example, US Pat. No. 3,064,044 describes an improved process for keeping a final (bromine-accelerated) oxidation under practically anhydrous conditions. In the US PS

3,519,684, which relates to an oxidation process using peracetic acid as an accelerator, states that "almost anhydrous conditions are preferably used, although a water content up to about 10% can be tolerated and a maximum water content of not more than 5% is preferred ». In a continuous process for the oxidation of xylenes in the absence of any accelerator in which partially oxidized intermediates are continuously recycled, water is removed from the liquid effluent before it is recycled in order to keep the water content in the reaction mixture below 15%, preferably below 5% of the total reaction mixture »To hold (US Pat. No. 3,700,731).

Surprisingly, it has recently been found that the oxidation of p-xylene in terephthalic acid can be carried out in the presence of substantial amounts of water as a solvent, but in the absence of any brominated activator which was previously considered an essential requirement. This process is described in German patent application P 2 745 918-3 and comprises the oxidation of p-xylene in the liquid phase by an oxygen-containing gas in the presence of p-toluic acid, water and a heavy metal salt as a catalyst at a temperature of about 140 to 220 ° C under sufficient pressure to keep at least part of the water in the liquid phase. However, since the mutual solubility of p-xylene and water is low at the working temperature, mixtures of water, p-xylene and p-toluic acid can separate into two phases: an aqueous phase and an organic phase that is rich in hydrocarbons and also a significant proportion which may contain p-toluic acid present in the reaction mixture. In this case, the oxidation reaction takes place essentially in the organic phase, where the water concentration is relatively low. The desired solvent effect of the water is therefore partially lost. Furthermore, this phase separation gives rise to technical difficulties with regard to homogenization, oxygen dispersion and mass transfer.

The aim of the present invention is now to provide a process for the preparation of terephthalic acid, by which terephthalic acid is obtained in high yield and good purity and the above-mentioned disadvantages of the prior art are avoided. Furthermore, this requires

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Process according to the invention is not a highly corrosion-resistant system and can be carried out in conventional stainless steel devices; in addition, the process according to the invention can be carried out in the presence of substantial amounts of water without the presence of any additional solvent.

In the process according to the invention for the production of terephthalic acid, a reaction mixture can be oxidized, which is essentially a homogeneous aqueous solution which mainly consists of the terephthalic acid precursors to be oxidized, water and an oxidation catalyst dissolved therein. The method does not require the use of an accelerator, e.g. a bromine compound, in addition to the oxidation catalyst, the terephthalic acid can easily be recovered from the oxidized reaction mixture at relatively moderate temperatures, and the catalyst and oxidation intermediates can be recovered and reused for oxidation.

The process of the invention for the production of terephthalic acid can be carried out batchwise or continuously on an industrial scale at a relatively low cost, and the amounts of catalyst, water and p-toluic acid required for the effective oxidation of the starting materials can easily be calculated.

The above objectives are achieved with the process according to the invention for the production of terephthalic acid, which is characterized in that a) a practically homogeneous, liquid reaction mixture is composed of:

- At least one oxidizable terephthalic acid precursor from the group of p-toluic acid or mixtures thereof with an oxidizable compound from the group of p-xylene, partially oxidized p-xylene derivatives and mixtures thereof;

a sufficient amount of water to achieve a workable slurry of at least 5% by weight;

- An amount of an oxidation catalyst from at least one catalytically active metal compound from the group of manganese compounds, cobalt compounds and mixtures thereof, which are sufficient to create a minimum amount of M millimoles of catalytically active metal compound per kg of liquid reaction mixture, where M is defined by the following equation (1) :

in which y represents the molar ratio of water to p-toluic acid in the reaction mixture;

x represents the molar ratio of manganese to the total amount of manganese and cobalt in the catalyst preparation; A has a value of at least approximately 0.200, B has a value of at least approximately 10.9, C has a value of at least approximately 4.35 and D has a value of at least approximately 0.0724, with a gas containing molecular oxygen a reaction temperature of 140 to 220 ° C and a sufficient pressure to keep at least part of the water at the reaction temperature in the liquid phase, and b) the oxidized mixture containing terephthalic acid is obtained.

By keeping the catalyst concentration at least at the minimum value defined above, an effective oxidation of each mixture of starting materials mentioned above in terephthalic acid is achieved in a homogeneous reaction mixture in the liquid phase.

The attached drawing is a phase diagram for mixtures of p-xylene, p-toluic acid and water at 185 ° C.

It is an important feature of the present invention that the deleterious effects of water generally accepted in the prior art can be overcome if the oxidation is carried out under the above specific conditions. Another feature of the present invention is its applicability to the oxidation of various substrates, which may consist of p-toluic acid alone or in a mixture with p-xylene and / or partially oxidized derivatives such as p-tolualdehyde. In addition, according to the invention, these substrates can be oxidized in a homogeneous, aqueous solution if the amount of catalyst is selected in correlation to the ratio between the amounts of water and p-toluic acid present in the system.

The process according to the invention can be carried out batchwise or continuously. The reactants are dissolved in water and the catalyst is added to the solution obtained. Then oxygen is introduced into this mixture and the oxidation takes place at a temperature between 140 and 220 ° C. under pressure. The pressure is adjusted so that the reaction mixture is kept practically in the liquid phase at the working temperature. The terephthalic acid separates from the reaction mixture as a white crystalline precipitate. In the continuous process, this precipitate is continuously removed by conventional solid / liquid separation processes, e.g. Filtration, centrifugation or settling and decanting are removed and the remaining liquid containing the unconverted reactants and oxidation intermediates is returned to the oxidation zone. Fresh reactants are added continuously to compensate for the terephthalic acid so withdrawn and the formation of by-products. It is therefore not necessary to add external chemicals. Nevertheless, when carrying out the process according to the invention, one or more organic solvents which do not interfere with the reaction and are relatively inert under the working conditions can be added to the reaction mixture. The compounds which can be used as solvents in a mixture with water are, for. B. benzoic acid and acetic acid. These compounds, which are formed in small amounts during the reaction, can therefore accumulate to some extent in the reaction mixture. However, the advantages of the present invention are also achieved regardless of the presence of these compounds which, when used, should not be present in the system in an amount above the amount of water.

The amount of water expediently used in the process according to the invention can be used in a wide range, e.g. vary between 5 and 80% by weight of the reaction mixture, depending on different factors. As already mentioned, it is an essential feature of the present invention that the oxidation takes place in a homogeneous aqueous system. Therefore, the amount of water is mainly chosen so that it is sufficient to give a practically homogeneous, aqueous solution of the reactants, taking into account other process variables, such as temperature and relative amounts of the various compounds to be oxidized. E.g. the method is applied to the oxidation of p-toluic acid alone or in a mixture with other oxygenated, relatively water-soluble compounds, such as p-tolylaldehyde, then such an amount of water is selected that is necessary for complete dissolving

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solution of p-toluic acid at the working temperature is sufficient. Since the solubility of p-toluic acid increases sharply with increased temperature, the amount of water to be used can be reduced with increased temperature. In general, however, the amount of water should not be less than 5%, preferably not less than 10% by weight, based on the reaction mixture.

If p-xylene is present as a component of the reaction mixture, the amount of water should not be above a value above which the mixture can be separated into two liquid phases. This amount of course depends on the amount of p-xylene in the mixture. 1 is a triangular phase diagram for mixtures of p-xylene, p-toluic acid and water at a temperature of 185 ° C. (in% by weight). In this diagram, the system is a homogeneous solution in zone A and two-phase in zone B. As one skilled in the art can easily determine, the boundary line between the two zones does not vary significantly with temperature. In order to avoid the presence of an essential organic phase, the amount of p-xylene must of course be limited. If the oxidation of p-xylene takes place batchwise according to the process according to the invention, then p-xylene should be added progressively, batchwise or continuously, to the reaction mixture at such a rate that the system is kept in zone A. According to a preferred embodiment of the present invention, the reaction takes place in a continuous flow process in which unreacted p-xylene together with the oxidation intermediates, i.e. essentially p-toluic acid is continuously returned to the reaction zone. In this case, the molar ratio of p-toluic acid / p-xylene in the reaction mixture under steady-state conditions is between about 3 and 15, which essentially depends on the temperature; in other words, the reaction mixture is in the gray area of the diagram of Fig. 1. As can be seen, this area is almost entirely in zone A, i.e. it corresponds to the largest part to homogeneous solutions. As can be seen from what has been said above, it is always possible to adjust the temperature and / or the amount of water to be used as solvent according to the invention in such a way that a homogeneous system is obtained. If the process according to the invention is carried out continuously, the process described in German patent application P 2 745 918-3, which is hereby incorporated into the present application, is used.

However, there are other factors to consider. Since the terephthalic acid desired as the product is practically insoluble in the reaction mixture, a sufficient amount of water must be used to obtain a workable slurry. However, there are no advantages from using such large amounts of water that e.g. more than 10% of the terephthalic acid are dissolved at working temperature. Another factor to consider is the response rate; Although it is possible according to the invention to carry out the oxidation in a medium of up to 80% by weight of water or even more, the presence of excessive amounts of water can impair the reaction rate. In some cases, part of the catalyst can also be prevented from its catalytic function by the formation of a black compound which separates from the oxidation medium. It is also impractical for economic reasons to lose part of the reactor capacity by using an excessive and useless amount of water. For these reasons, the water level present in the system is usually no more than 75

% By weight, preferably not more than 60% by weight, of the reaction mixture.

The oxidation is expediently carried out at a temperature of at least 140 ° C. Below this temperature it is difficult to achieve a reaction mixture in the form of a homogeneous solution. In contrast, working above 220 ° C leads to increased overoxidation, undesirable side reactions and corrosion problems; In most cases, the reaction temperature is between 150 and 190 ° C. io The pressure is set as a function of temperature. Sufficient superatmospheric pressure must be used to keep the reaction mixture in a liquid state at the working temperature. A pressure above this value is usually appropriate to ensure active oxidation. Usually the pressure is between 500,000 and 4,000,000 Pa.

The oxidation catalyst used in the process according to the invention is a preparation made from a heavy metal compound which comprises at least one metal compound from the 2o group of manganese or cobalt compounds or mixtures thereof, provided that it is at least partially soluble in the aqueous reaction mixture or can be a soluble one or at least partially soluble compound with one of the reactants in this mixture. The metal compounds which can be used in the catalyst preparation are expediently salts, with particular preference being given to the salts of carboxylic acids, such as the acetates, naphthenates, toluates etc. In addition to the cobalt and / or manganese compounds, the catalyst preparation can comprise compounds, in particular carboxylic acid salts, of other metals. which are commonly used in oxidation catalysts.

An essential aspect of the present invention is that the minimum concentration of the catalyst required to ensure an oxidation in a homogeneous, aqueous system depends on the respective amounts of water and p-toluic acid in this system. In fact, in a homogeneous aqueous solution of p-toluic acid and partially p-xylene and / or partially oxidized derivatives thereof, the oxidation cannot take place if the amount of active catalyst is below the critical concentration M in millimoles of the metal compound per kg of reaction mixture according to the following Equation (1) is:

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M =

y (x + A) + Bx Cx + D

(1)

so here y = molar ratio of water to p-toluic acid x = relative amount of manganese in the metal catalyst in mole parts, based on the total amount of manganese and cobalt therein, i.e.

Mn

Mn + Co

60

A B C D

about 0.200 about 10.9 about 4.35 about 0.0724

65 The quantities A, B, C, D given above are the result of test determinations and are therefore subject to measurement errors. It was calculated that these values with their fluctuations (error ranges)

642343

6

A = 0.200 ± 0.031

B = 10.9 ± 1.3

C = 4.35 ± 0.17

D = 0.0724 + 0.0117

be.

Therefore, the value of M calculated using equation (1) for a given value of y and x must be viewed as an estimate of the actual critical concentration, which is statistically within the error range centered around M. As can be seen by those skilled in the art, this range of error (variation) depends on the values of y and x used in the calculation of M, but can be calculated for each particular case from the above data by converting the Taylor formula. So z. B. the critical catalyst concentration within

21.46 ± 0.81 if y = 70 and x = 1.0 and 2.74 + 0.45 if y = 1 and x = 0.5

The critical catalyst concentration defined above is the lowest concentration that can be used under given conditions. The process cannot be carried out for lower quantities. In practice, therefore, it is advantageous to use a larger amount of catalyst than this minimum. In fact, the rate of oxidation increases with increased catalyst concentration. However, catalyst concentrations above 40 millimoles of metal compound per kg of reaction mixture are not advantageous for economic reasons. The catalyst is expediently used in an amount which is between a value slightly above the minimum M calculated from equation (1) and 30 millimoles per kg of reaction mixture.

As can be seen from equation (1), the critical catalyst concentration increases as the water concentration increases and the p-toluic acid concentration decreases. In other words, the oxidation cannot take place in such an aqueous solution if the concentration of p-toluic acid is not above a critical value which increases with a reduced catalyst concentration. So not only the catalyst, but also the p-toluic acid is essential so that the oxidation can take place in the presence of substantial amounts of water. Due to the combined action of catalyst and p-toluic acid, both of which are used in appropriate amounts according to the invention, p-xylene can therefore be oxidized in terephthalic acid without having to resort to costly and / or corrosive activators, such as brominated compound. This effect of p-toluic acid is a completely unexpected aspect of the present invention, since other carboxylic acids, even acids with a similar structure, such as benzoic acid, do not show these properties.

From equation (1) it can also be seen that the critical minimum catalyst concentration, which is necessary to ensure oxidation in a homogeneous aqueous system, depends on the relative proportions of the manganese and cobalt compounds within the catalyst used. As can be seen, manganese is significantly more effective than cobalt. So that e.g. oxidation in a 50% by weight aqueous p-toluic acid solution (molar ratio of water to p-toluic acid = 7.56), the required minimum catalyst concentration is 4.5 or 20.9 millimoles / kg, depending on whether manganese or cobalt is the only one Catalyst is used. For practical reasons, however, a mixture of the two metals is expediently used, since cobalt usually has a favorable effect on the reaction rate. It was further found that when a mixture of manganese and cobalt compounds is used as the catalyst, less overoxidation takes place and usually higher yields of terephthalic acid are achieved than if each metal is used individually. The combustion ratio (molar ratio of the released carbon dioxide per absorbed oxygen) has, for. B. generally a value between 0.06 and 0.09 in the former case, while values of approximately 0.15 were found in the second case. In fact, the beneficial effect of the combined use of manganese and cobalt as a catalyst is a typical feature and a further advantage of the present invention: if the reaction takes place in a two-phase system instead of a homogeneous solution as in the present process, then combustion ratios regularly become 0 , 12 or more, regardless of whether manganese and / or cobalt is used. In most cases, a value of x in equation (1) between about 0.1 and about 0.9 is advantageously used in order to ensure active oxidation in a homogeneous aqueous medium and high yields of terephthalic acid.

For the same practical reason as above, it can also be advantageous if the catalyst contains, in addition to cobalt and / or manganese, a further metal component, such as Nikkei, lead or cerium. Although these other metals are not essential for the oxidation to take place in a homogeneous aqueous medium, they provide some practical improvement with respect to e.g. product purity and response speed.

It is a surprising feature of the present invention that the minimum catalyst concentration to be used to ensure oxidation in aqueous medium can be calculated from equation (1) for each composition of the reaction mixture; and this equation is independent of such important work variables as temperature. This is evident from the results of Table 1, in which the values of M obtained experimentally over a wide range of conditions are compared with the values calculated from equation (1). Most of these results were obtained using the following experimental procedure.

In a corrosion-resistant 1 liter autoclave, which was equipped with a mechanical stirrer, heating jacket, gas inlet pipe, vent and measuring pump for the introduction of a liquid, the various components of the oxidation mixture, i.e. p-toluic acid, water and catalyst introduced. This mixture was then heated while stirring and passing air. After the start of the oxidation, an aqueous solution of the catalyst with the same catalyst concentration as the initial reaction mixture was gradually introduced into the mixture. This progressively diluted the reaction mixture with water without varying the catalyst concentration. P-Xylene can also be present in the initial mixture, provided its concentration is such that after the reaction mixture has been diluted with the aqueous catalyst solution, there is no phase separation.

The extent of the oxidation is determined by continuously measuring the oxygen content of the exhaust gas using an oxygen analyzer. As long as the concentration of p-toluic acid in the system is sufficiently high to ensure effective oxidation at the chosen catalyst concentration, the reaction proceeds regularly at a rate that progressively decreases with dilution. However, once the critical concentration of p-toluic acid is reached, the reaction rate quickly drops to almost zero. Then the introduction is interrupted and the reaction mixture is analyzed. The molar ratio of water to p

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Toluic acid is obviously the critical y value corresponding to the given concentration M of the catalyst, or conversely M is the minimum catalyst concentration to be used to ensure oxidation in a reaction mixture in which the ratio between water and p-toluic acid is given as y. In fact, M still depends on the composition of the catalyst, which can be expressed as the molar ratio of manganese to the total amount of manganese and cobalt in the catalyst preparation used (x in equation 1).

Of course, these determinations are subject to experimental errors, so that for a given reaction mixture (for given y and x) different values of Mi are to be expected, which are statistically distributed around the actual value with a standard deviation 6. The values of Mi thus determined for different yi and Xj are shown in the following table. From this data it can be estimated that the standard deviation of Mi from the value M calculated according to equation (1) is 0.70. Furthermore, any test value Mi that does not differ from the calculated value by more than two standard deviations, i.e. by more than 1.4 than are considered to be in accordance with equation (1).

It is clear from the results in Table 1 that

1.Equation (1) applies to every value of y and x (compare e.g. experiments 1 and 7 for y and experiments 7 and 19 for x);

2.Equation (1) is independent of the presence or absence of p-xylene (see e.g. experiments 8 and 13);

3. Equation (1) is independent of the temperature (compare e.g. experiments 3 and 9);

4. Equation (1) is independent of the presence of metals other than manganese and cobalt (compare e.g. experiments 3 and 5).

The prior art regarding the choice of the metal compound to be used for the oxi-5 dation of p-xylene and its partially oxidized derivatives in terephthalic acid is somewhat confusing. In general, however, cobalt is described as the best catalyst, especially in the absence of a brominated activator, and manganese as less active or even inactive. Manganese has been shown to be inactive for the oxidation of p-toluic acid (N. Ohta et al. Chem. Abstr. 56, 8620 [1962]) or even shown to be an inhibitor (VN Aleksandrow et al., Kinet. Katal. 15, 505 [ 1974]). In other circumstances, manganese shows catalytic activity, but the terephthalic acid is discolored if the water content is more than 10% by weight of the solvent or the manganese content in the reaction mixture is high. It was therefore completely unexpected that manganese in the process according to the invention has the exceptional activity to allow oxidation in the presence of large amounts of water and that terephthalic acid is obtained in high yield as a white crystalline precipitate, which is particularly suitable for further purification up to the purity required for the production of polyester fibers (= fiber quality; «fiber-grade») is suitable.

Mean in the table below

(1) = an equimolecular mixture of manganese and nickel acetate was used as catalyst. The calculation of x and M was made without considering the nickel

(2) = the value was obtained without diluting the reaction mixture. The critical ratio of water to p-toluic acid was achieved spontaneously by consuming the latter and forming the former due to the oxidation.

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Table I

No.

Temp.

Reaction mixture; % By weight p-xylene p-toluic acid

Water y

X

M

determined calculated

1

185

0.3

9.7

90.0

70.22

1.00

20.1

21.5

2nd

'185

0.5

20.0

79.5

08/30

1.00

10.1

10.6

3rd

185

1.5

45.7

52.8

8.76

1.00

5.1

4.8

4th

170

0.4

10.6

89.0

63.54

1.00

20.0

19.7

5

170

0.6

23.5

75.9

24.38

1.00

9.0

9.1

(1)

(1)

6

170

1.4

43.5

55.1

9.60

1.00

5.1

5.1

7

170

0.1

61.3

38.6

4.76

1.00

3.7

3.8

8th

170

11.6

76.6

11.8

1.17

1.00

3.1

2.8

(2)

9

160

1.3

42.6

56.1

9.95

1.00

'5.1

5.2

10th

170

1.3

44.4

54.3

9.25

0.79

5.2

5.1

11

170

0.3

23.0

76.7

25.21

0.50

10.1

10.3

12

170

0.2

10.8

89.0

62.34

0.46

21.7

22.2

13

170

0.0

41.8

58.2

10.53

0.46

5.5

5.8

14

160

0.5

43.9

55.5

9.57

0.46

5.5

5.5

15

170

0.5

24.3

75.2

23.46

0.33

10.1

10.6

16

170

0.3

27.0

72.7

20.39

0.25

10.1

10.2

17th

170

0.4

April 30

69.2

17.23

0.17

10.1

10.1

18th

170

0.5

35.3

64.2

13.77

0.09

10.1

10.7

19th

170

5.4

60.5

34.1

4.27

0.00

11.7

11.8

20th

170

0.8

74.9

24.3

2.45

0.00

6.1

6.8

21st

170

0.0

81.4

18.6

1.73

0.00

3.9

4.8

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The following examples illustrate the present invention.

example 1

In a corrosion-resistant 1 liter autoclave equipped with a mechanical stirrer, heating jacket, gas inlet pipe and venting were introduced:

p-xylene p-toluic acid various oxygenated p-xylene derivatives

water

Manganese acetate

Cobalt acetate

45.0 g 187.5 g

4.5 g 63.0 g 1.50 millimoles 1.74 millimoles

The reactor was pressurized with air to 2,000,000 Pa and the above mixture was stirred and admitted with air at a flow rate of 921 / hour. heated.

In the above feed, the molar ratio of water to p-toluic acid (y) was (63.0 x 136.14) / (18.02 x 187.5) = 2.54, and the molar ratio Mn / Mn + Co in the catalyst (x) was 1.50 /

(1.50 + 1.74) = 0.46. Using equation (1), the minimum catalyst concentration required in this case to ensure oxidation was calculated as follows:

M =

2.54 (0.46 + 0.200) +10.9 (0.46) 4.35 (0.46) +0.0724

= 3.2 millimoles / kg

In this example, the catalyst concentration was actually 10.8 millimoles / kg, i.e. about three times the minimum amount. In fact, the reaction started spontaneously after heating and was active; therefore the temperature rose rapidly and was kept at 170 ° C by controlled cooling.

After a reaction time of 180 minutes, the oxygen absorption was 40.91 (measured at room temperature and 1 at pressure). The air supply was cut off and the reactor was gradually depressurized in order to recover unreacted p-xylene by stripping with water. Finally the reactor was cooled and opened. The precipitate contained therein was filtered, washed with water and dried under vacuum at about 80 ° C.

Then it was analyzed by a combination of methods including acidimetry, polarography and vapor phase chromatography. It was determined that 89.4% of the imported p-xylene and 23.1% of the imported solder p-toluic acid had been converted into the following products:

. Terephthalic acid 90.2 g

4-carboxybenzaldehyde 8.8 g other intermediates 3.8 g

15 heavy by-products 2.2 g

Considering that 4-carboxybenzaldehyde and other intermediates can be recycled in a continuous process and the substantial portion thereof ultimately converted to terephthalic acid, it can be estimated that the terephthalic acid yield in such a continuous process is greater than 90 Mol%, based on the amount of p-xylene consumed.

In another process carried out under the same conditions, the hot reaction mixture was filtered, the cake obtained was further washed on the filter with hot water and dried under vacuum. Analysis of the dried product showed the following composition (in% by weight): 30 terephthalic acid 85.6

P-toluic acid 9.6

4-carboxybenzaldehyde 3.6

The color of this sample was determined by measuring the optical density of a solution of the same in dilute ammonia according to the method of US Pat. No. 3,354,802, using a 5 cm cell instead of a 4 cm cell. The optical density values thus obtained are shown in Table II below and were compared with the optical density values obtained with a sample of 40 commercial terephthalic acid of 99 +% purity.

Table II

raw sample optical density at (m | i)

Origins of purity

% By weight 340 380 400

manufactured according to the invention 85.6 0.428 0.102 0.076

commercial product 99+ 0.726 0.183 0.111

As can be seen, the raw sample obtained according to the invention, after simple filtering and superficial washing, has better color properties than a commercially available terephthalic acid sample of a substantially higher purity. The raw sample is therefore particularly suitable for further cleaning for fiber quality.

Example 2

Example 1 was repeated using the following mixture as solvent instead of 63.0 g of water:

Water 33.0 g

Acetic acid 30.0 g

After 180 minutes of reaction, the oxygen absorption was 39.7 g, i.e. about the same amount as in the example above. Then the reaction mixture was treated and analyzed as in Example 1. It was found that 88.0% ss of the introduced p-xylene and 23.5% of the introduced p-toluic acid had been converted into the following products: terephthalic acid 89.4 g

4-carboxybenzaldehyde 6.5 g other intermediates 4.8 g

60 heavy by-products 3.2 g

Following the same procedure as in Example 1, it was estimated that in a continuous process under the conditions used in this example, the terephthalic acid yield would be about 90 mole percent based on the p-xylene consumed.

As can be seen, these results are practically identical to those of the example above, in which water was used as the sole component of the solvent.

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Example 3

Example 2 was repeated using formic acid instead of acetic acid. Therefore, the solvent used in this case was the following mixture: water 33.0 g

Formic acid 30.9 g

After 180 minutes of reaction, the oxygen absorption was 36.71. Following the same analysis procedure of the above examples, it was found that 88.0% of the introduced p-xylene and 15.6% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 66.7 g

4-carboxybenzaldehyde 6.8 g other intermediates 7.0 g heavy by-products 3.7 g

Following the procedure of the above example, it can be estimated that in a continuous process under the present conditions the terephthalic acid yield would be about 85 mole percent based on the p-xylene consumed.

Although these results are not quite as good as those of the previous examples, they show that large amounts of formic acid can be tolerated in the reaction mixture without serious damage to the yield and reaction rate. This is particularly unexpected because formic acid has always been described as a strong inhibitor of oxidation reactions. Since formic acid is always formed in such reactions, complicated and costly procedures usually have to be provided to avoid formic acid accumulation in the reaction mixture. In the present process, formic acid is burned, which is confirmed by the fact that in this experiment carbon dioxide was released in an amount of 6.01 instead of 3.8 1 according to Example 1. Therefore, in the process of the present invention, no special process for removing formic acid from the reaction mixture is necessary.

Example 4

Example 1 was repeated using cobalt as the only catalyst. The feed was as follows:

p-xylene 45.0 g p-toluic acid 190.1 g various oxygenated derivatives 1.9 g

Water 63.0 g

Cobalt acetate 3.48 millimoles

In this feed, u = 2.50 and x was obviously = 0.00. Therefore, the cobalt limit to ensure oxidation in this case was:

M =

2.50 (0.200) 0.0724

= 6.9 millimoles / kg

The concentration of cobalt acetate in the initial mixture was thus 11.6 millimoles / kg, i.e. about twice the limit. After 180 minutes of reaction, the oxygen absorption was 41.5 1. The reaction mixture was then treated and analyzed as above. It was found that 90.8% of the introduced p-xylene and 19.5% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 74.2 g

4-carboxybenzaldehyde 6.8 g other intermediates 1.5 g heavy by-products 3.7 g

A comparison of these results with those of Example 1 shows that less terephthalic acid was formed, although the oxygen absorption was somewhat higher. By estimating as in the examples above, a yield of terephthalic acid can actually be expected in a continuous process of 81% instead of 90%. This difference clearly shows the advantage of using a mixture of manganese and cobalt as a catalyst instead of cobalt alone.

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Example 5

Example 1 was repeated using manganese as the only catalyst. The feed had the following composition:

p-xylene 45.0 g p-toluic acid 182.8 g various oxygenated derivatives 9.2 g

Water 63.0 g

Manganese acetate 3.00 millimoles

In this feed, y = 2.60 and x was obviously = 1.00. Therefore, the limit amount of manganese to ensure oxidation was:

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M =

2.60 (1.00 + 0.200) +10.9 4.35 + 0.0724

3.2 millimoles / kg

In the present example, the manganese concentration was 10.0 millimoles / kg, i.e. about three times the limit as in Example 1.

After 180 minutes of reaction, the oxygen absorption was 25.3 g. H. was noticeably lower than in the examples above. Then the reaction mixture was treated and analyzed as above 35. It was found that 70.2% of the introduced p-xylene and 5.8% of the introduced p-toluic acid had been converted into the following products: terephthalic acid 31.2 g

4-carboxybenzaldehyde 5.9 g

40 other intermediates 6.3 g heavy by-products 5.1 g

A comparison of these results with those of Examples 1 and 2 clearly shows that here fewer reactants were consumed and less terephthalic acid was formed. Furthermore, the terephthalic acid yield in a continuous process from this data, as estimated above, is only 69%. Therefore, although manganese is significantly more effective than cobalt in providing oxidation in the presence of water (as shown by the lower calculated value for M compared to the value calculated for cobalt alone), there is a distinct advantage in reaction rate and yield in using manganese in combination with cobalt as in Example 1.

Example 6

Example 1 was repeated using manganese combined with nickel; the feed was as follows:

p-xylene 45.0 g p-toluic acid 182.8 g various oxygenated derivatives 9.2 g

Water 63.0 g

Manganese acetate 1.5 millimoles

Nickel acetate 1.50 millimoles

With this feed, y = 2.60 and x = 1.00 as in the previous example; thus M = 3.2 millimoles kg (with nickel not being60 in the calculation of M

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was taken into account). In the present example, the manganese concentration was 5.0 millimoles / kg and was thus 1.8 above the limit; the oxidation was very active.

After 180 minutes the oxygen absorption was 29.21 and the reaction mixture was analyzed as above. It was found that 79.1% of the imported p-xylene and 3.3% of the imported p-toluic acid had been converted into the following products:

Terephthalic acid 35.6 g

4-carboxybenzaldehyde 6.6 g other intermediates 2.6 g heavy by-products 3.2 g

A comparison of these results with those of the previous example shows that slightly more oxygen was absorbed and more terephthalic acid was formed than when using manganese alone in a double concentration of the value of the present example. This shows that metals other than cobalt can be used in combination with manganese to improve the amount of the reaction.

Example 7

In the autoclave used in the above examples were introduced:

p-xylene 45.0 g p-toluic acid 218.5 g various oxygenated derivatives 6.5 g

Water 30.0 g

Manganese acetate 0.52 millimoles

Cobalt acetate 0.45 millimoles

In this feed, the molar ratio of water to p-toluic acid (y) = 1.04 and the mole fraction of manganese in the catalyst (x) was 0.46. According to equation (1), the minimum amount of catalyst to be used for oxidation in this case is M = 2.7 millimoles / kg. Indeed, in this example, the catalyst concentration was 0.97 millimoles per 300 grams of starting mixture or 3.2 millimoles / kg, i.e. it was only 0.5 millimoles above the limit. Nevertheless, oxygen absorption started spontaneously and was active; therefore the temperature rose rapidly and was kept at 170 ° C. by controlled cooling.

After 395 minutes of reaction, 62.91 oxygen was absorbed. Then the air supply was stopped and the procedure of Example 1 was used to determine the composition of the reaction mixture. It was found that 98.8% of the imported p-xylene and 29.24% of the imported p-toluic acid had been converted into the following products:

Terephthalic acid 102.5 g

4-carboxybenzaldehyde 6.6 g other intermediates 4.6 g heavy by-products 3.6 g

From this data, a terephthalic yield was estimated according to Example 1 in a continuous process with recycling of the intermediate products of 80%, i.e. 10% below the yield estimated in Example 1. The main difference between the two tests was that in Example 1 the water content of the feed was 21% by weight compared to only 10% by weight in the present case. The different yield obtained in this way can thus be seen as a further illustration of the advantage of carrying out the oxidation of p-xylene in the presence of substantial amounts of water by the process according to the invention.

Comparative Example The same amounts of compounds as in the previous example were introduced into the autoclave, but only 0.30 mol of manganese acetate and 0.35 mol of cobalt acetate were used. The total concentration of the metal catalyst was therefore 2.2 millimoles / kg, i.e. was 0.5 millimoles lower than the limit calculated in the previous example.

After heating the mixture in the presence of an air stream under the same conditions as in the previous example, the oxygen absorption started similarly, but suddenly dropped to a negligible value after about 70 minutes; it was only 6, 7 1.

Example 8

The following were introduced into the autoclave of Example 1:

p-Tolualdehyde 45.0 g p-toluic acid 187.5 g various oxygenated

Derivatives 4.5 g

Water 63.0 g

Manganese acetate 1.50 millimoles

Cobalt acetate 1.74 millimoles

As can be seen, this mixture is very similar to the mixture introduced in Examples 1 to 5, using p-tolualdehyde instead of p-xylene. The molar ratio of water to p-toluic acid was again 2.54. By using equation (1), the minimum catalyst concentration to be used for oxidation is M = 3.2 millimoles / kg. In fact, in this example, the catalyst concentration was 10.9 millimoles / kg. was about 3 times as high.

This mixture was heated in the presence of air as in the examples above. Oxygen absorption started spontaneously at around 30CC. The temperature was allowed to rise and was kept at 170 ° C by controlled cooling. After 180 minutes of reaction, the oxygen absorption was 25.6 1. The air supply was cut off and 180 cc of n-heptane was introduced into the reactor to extract unreacted p-tolualdehyde with constant stirring and heating. The resulting mixture was then cooled and the reactor opened. The precipitate contained therein was filtered, washed with n-heptane, dried under vacuum at 50 ° C, washed again with water and finally dried under vacuum at about 70 ° C. The analysis was carried out as in the examples above. The filtrate and washing material were combined and the heptane extract was separated by decanting. An aliquot of this extract was analyzed by vapor phase chromatography to determine the unreacted p-toluene alde hyd portion. The remainder was evaporated to dryness in vacuo and the residue was analyzed using the same procedure as the first precipitate.

From these different analyzes, it was found that 99.9% of the introduced p-tolualdehyde and 10.7% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 58.4 g

4-carboxybenzaldehyde 7.1 g other intermediates 6.1 g heavy by-products 5.4 g

Comparative Example 1 The same amounts of compounds as in the previous example were introduced into the autoclave, only 0.3 millimoles of manganese and cobalt acetate being used. The total catalyst concentration was therefore 2.0 millimoles /

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kg, i.e. was 1.2 millimoles lower than the limit calculated by equation (1).

This mixture was heated under the same conditions as in the previous example in the presence of air. After 180 minutes of reaction, the oxygen absorption was only 9.01. Then the reaction mixture was treated and analyzed as in the previous example. It was found that the introduced p-tolualdehyde was completely converted into the following products:

Terephthalic acid

2.3 g p-toluic acid

31.7g

4-carboxybenzaldehyde

3.8 g of other intermediates

By-products weighing 0.9 g

2.0 g

As can be seen, the p-

■ Tolual

dehydrogenated essentially into p-toluic acid without substantial formation of terephthalic acid.

Comparative Example 2 Here, p-tolualdehyde was used as the only substrate to be oxidized. The feed was as follows: p-tolualdehyde 225 g

Water 75 g

Manganese acetate 1.50 millimoles

Cobalt acetate 1.50 millimoles

After heating this mixture in the presence of air as in Example 8, oxygen absorption started similarly, but then slowed down significantly. After about 240 minutes of reaction at 170 ° C, the oxygen absorption was 29.61. By treating and analyzing the reaction mixture obtained as above, it was found that 99.7% of the introduced p-tolualdehyde had been converted into the following products:

Terephthalic acid 4.6 g p-toluenic acid 171.4 g

4-carboxybenzaldehyde 4.6 g other intermediates 1.3 g heavy by-products 1.3 g

As can be seen, p-toluic acid was again the main product from the oxidation of p-toluene aldehyde and only small amounts of terephthalic acid were formed. This clearly shows that for the oxidation of p-tolualdehyde in terephthalic acid according to the present process, p-toluic acid must be present in a sufficient amount from the start of the reaction.

Example 9

Example 8 was repeated using 3.48 millimoles of cobalt acetate as the only catalyst. According to equation (1), the minimum cobalt concentration to be used in this case is M = 7.0 millimoles / kg. In fact, in the present example, the concentration of the cobalt catalyst was 11.6 millimoles / kg.

Again, the low temperature oxygen absorption started. Nevertheless, heat was applied to keep the temperature at 170 ° C. After about 210 minutes of reaction, the oxygen absorption was 20.8 1. The reaction mixture was treated and analyzed as above. It was found that 99.9% of the introduced p-tolual dehyde and 6.0% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 49.1 g

4-carboxybenzaldehyde 6.9 g other intermediates 6.5 g heavy by-products 6.3 g

Comparative Example 4 In the same autoclave were introduced: p-tolualdehyde 69.6 g p-toluic acid 123.0 g various oxygenated derivatives 3.0 g

Water 105.50 g

Cobalt acetate 3.48 millimoles

Thus, the same amount of cobalt catalyst as above (11.6 millimoles / kg) was used, but the molar ratio of water to p-toluic acid was 6.45 instead of 2.54. Therefore, the minimum cobalt catalyst concentration to be used in this case was M = 17.8, i.e. 6.2 millimoles more than was actually present in the feed.

This mixture was heated in the presence of air. Again, the oxygen absorption was immediate, but fell to a negligible level after 45 minutes; nevertheless, the temperature was raised to 170 ° C. After 300 minutes of reaction, the oxygen absorption was only 10.01. The reaction mixture was treated and analyzed as in Example 8. It was found that 90.1% of the imported p-tolualdehyde had been converted into the following products:

Terephthalic acid 3.7 g p-toluic acid 53.5 g

4-carboxybenzaldehyde 0.7 g other intermediates 0.7 g heavy by-products 6.6 g

As can be seen, p-tolualdehyde was essentially converted to p-toluic acid without substantial terephthalic acid formation.

Example 10

In the above autoclave were introduced: p-toluic acid 215.3 g various oxygenated derivatives 10.7 g

Water 75.0 g

Cobalt acetate 1.74 millimoles

Manganese acetate 1.50 millimoles

Thus, no p-xylene or p-tolualdehyde was used in this example. The molar ratio of water to p-toluic acid (y) was 2.64 and the mole fraction of manganese in this catalyst (x) was 0.46 as in Example 1. The minimum catalyst concentration to be used according to equation (1) is M = 3.3 millimoles / kg. In fact, in this example it was 3.24 millimoles for 300 g reaction mixture or 10.8 millimoles / kg, i.e. a little three times more than the limit.

This mixture was heated in the presence of air under the conditions of Example 1. Again the reaction started spontaneously. After 180 minutes of reaction, 19.41 oxygen was absorbed and the reaction was stopped by cooling. The autoclave was opened and the precipitate contained therein was treated and analyzed as in Example 1. It was determined that 33.2% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 75.5 g

4-carboxybenzaldehyde 6.1 g other intermediates 1.8 g heavy by-products 0.1 g

The net yield of terephthalic acid, based on the converted p-toluic acid, was 87 mol%. Taking into account the oxidation intermediates, such as 4-carboxybenzaldehyde, the actual estimated tere

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Yield of phthalic acid in a continuous process with recycling of the intermediates at 97 mol%.

This example clearly shows that p-toluic acid can be converted efficiently and in high yield into terephthalic acid by the process according to the invention, even in the absence of p-xylene or any other easily oxidizable compound as an accelerator.

Example 11

In the above autoclave were introduced:

p-xylene 3.0 g p-toluic acid 142.6 g various oxygenated derivatives 4.4 g

Water 150.0 g

Manganese acetate 1.05 millimoles

Cobalt acetate 1.05 millimoles

Thus in this example the water content was 50% by weight of the initial mixture; y was 7.95 and x was obviously 0.50. According to equation (1), the minimum catalyst concentration to be used was M = 4.9. In fact, in the present example it was 7.0 millimoles / kg, i.e. 2.1 millimoles above the limit.

Air was introduced into the reactor at a flow rate of 1101 / hour. introduced under a pressure of 20 kg / cm 2 and the mixture heated with stirring. At a temperature of about 170 ° C, a tiny amount of tert-butyl hydroperoxide was added to help start the reaction. The temperature then increased rapidly and was kept at 185 ° C. by controlled cooling.

After 300 minutes of reaction, the oxygen absorption was 23.51. The reaction was stopped and the mixture obtained was treated and analyzed as in Example 1. It was found that 95.0% of the imported p-xylene and 40.3% of the imported p-toluic acid were converted into the following products:

Terephthalic acid 53.5 g

4-carboxybenzaldehyde 5.0 g other intermediates 1.6 g heavy by-products 1.6 g

Comparative Example 5 The above example was repeated using half the amount of each metal catalyst. Therefore, the total catalyst concentration was 3.5 millimoles / kg, i.e. was 1.4 millimoles below the limit.

This mixture was heated in the presence of air as in the example above. There was no oxygen absorption at all while the mixture was held at 185 ° C for 160 minutes, although tert-butyl hydroperoxide was added twice in succession to help start the reaction.

By comparison with the results of the previous example, in which an active and substantial oxidation had taken place in the presence of large amounts of water, it becomes clear that such an oxidation is only possible if, as stated according to the invention, the correct amount of catalyst is used.

Comparative Example 6 Example 11 was repeated using benzoic acid for part of the p-toluic acid. The total feed was as follows:

p-xylene 3.0 g p-toluic acid 44.5 g

Benzoic acid 93.9 g various oxygenated compounds 2.0 g

Water 156.6 g

Manganese acetate 1.05 millimoles

Cobalt acetate 1.05 millimoles

In this example, y = 26.59; According to equation (1) s, the minimum catalyst concentration to be used for an oxidation of the above mixture was 10.7 millimoles / kg, i.e. 3.7 millimoles more than was actually present. However, if benzoic acid had the same effect as p-toluic acid in ensuring oxidation in an aqueous medium, this should be taken into account when calculating y and M. In this case, y = (156.5 / 18j02) / (44.5 / 136.15 + 93.0 / 122.12) = 7.93, i.e. the value would be the same as calculated for y in Example 11 where active oxidation occurred. 15 The above mixture was heated to 185 ° C under the conditions of Example 11, again adding some tertiary butyl hydroperoxide to aid initiation. However, even after continued heating for 180 minutes, there was no appreciable oxidation. This shows that benzoic acid does not have at least the same activity as p-toluic acid to accelerate oxidation in the presence of water by the method of the present invention.

25 Comparative Example 7

Example 11 was repeated using acetic acid for part of the p-toluic acid. The feed was as follows:

p-xylene 3.0 g

3o p-toluic acid 52.7 g

Acetic acid 55.2 g various oxygenated compounds 1.3 g

Water 187.8 g

35 manganese acetate 1.05 millimoles

Cobalt acetate 1.05 millimoles

In this example, y = 26.92, so M = 10.8, i.e. about the same as in the previous comparative example. If, in turn, acetic acid had the same accelerating effect 40 as p-toluic acid for oxidation in an aqueous medium, this would have to be taken into account when calculating y and M. In this case, y = (187.2 / 18.02) / (52.7 / 136.15 + 55.2 / 60.05 = 7.95), i.e. exactly as in Example 11.

45 The above mixture was heated to 185 ° C under the conditions of Example 11 and again some tertiary butyl hydroperoxide was added to aid initiation. After 300 minutes of reaction, the oxygen absorption was only 3.81. This again shows that the beneficial effect of p-toluic acid according to the process of the invention is not shown by other carboxylic acids and is therefore specific for p-toluic acid.

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Example 12 In the above autoclave was introduced:

p-xylene 6.0 g p-toluic acid 183.3 g various oxygenated derivatives 5.7 g

Water 105.0 g

Manganese acetate 1.80 millimoles

Here y = 4.33 and, since manganese was used as the only catalyst, x = 1.00. According to equation (1), the minimum manganese concentration to be used in this case was M = 3.6 millimoles / kg. In fact, it was 6.0 millimoles / kg, well above the limit.

This mixture was heated in the presence of air under the conditions of Example 11, but the

Temperature was kept at 170 ° C instead of 185 ° C. After 305 minutes of reaction, the oxygen absorption was 21.21. After treatment and analysis of the reaction mixture as in the above examples, it was found that about 96% of the introduced p-xylene and 29.9% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 44.8 g

4-carboxybenzaldehyde 14.2 g other intermediates 0.9 g heavy by-products 1.2 g

Comparative Example 8 The same amounts of compounds as in the previous example were introduced into the reactor using only 0.60 millimoles of manganese acetate as the catalyst. Therefore the catalyst concentration in the mixture was 2.0 millimoles / kg, i.e. 1.6 millimoles lower than the limit. Therefore, after heating the mixture for 60 minutes in the presence of air under the conditions of the previous example, there was no oxygen absorption even after adding tert-butyl hydroperoxide to aid initiation.

Example 13

The above autoclave was loaded with the following materials:

p-xylene 6.9 g p-toluic acid 218.3 g various oxygenated derivatives 2.2 g

Water 73.5 g

Cobalt acetate 3.48 millimoles

In this example, cobalt was used as the only catalyst, so x was 0 while y was 2.54. According to equation (1), the cobalt min642 to be used was 343

minimum concentration in this case M = 7.0 millimoles / kg. In fact, cobalt was present at a concentration of 11.6 millimoles / kg.

Air was at a flow rate of 901 / hour. s introduced into the reactor at a pressure of 2,000,000 Pa and the mixture heated to 170 ° C. During the heating the reaction started spontaneously and continued for 240 minutes. At the end of the reaction, the oxygen absorbed was 29.81. After treatment and analysis of the reaction mixture as in Example 1, it was found that 89.3% of the introduced p-xylene and 37.0% of the introduced p-toluic acid had been converted into the following products:

Terephthalic acid 81.5g

15 4-carboxybenzaldehyde 6.8 g other intermediates 1.2 g heavy by-products 3.0 g

Comparative Example 9 20 The same amounts of compounds as in the previous example were introduced into the reactor, but only 0.90 millimoles of cobalt acetate were used as the catalyst. Therefore, its concentration in the mixture was 3.0 millimoles / kg, i.e. was 4.0 millimoles below the limit of 25. Therefore, after heating this mixture for 240 minutes in the presence of air under the conditions of the previous example, even after repeated addition of tert-butyl hydroperoxide, there was no oxygen absorption.

Of course, in the process according to the invention, the modifications familiar to the person skilled in the art can be carried out. According to the invention, the reaction mixture can, for. B. one or more external compounds used in other processes as activators, such as aldehydes or ketones or even bromine-containing compounds, can be added. These additives can e.g. B. be favorable for the reaction rate.

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Claims (11)

642 343 PATENT CLAIMS 1. Process for the preparation of terephthalic acid, characterized in that a) a practically homogeneous, liquid reaction mixture - At least one oxidizable terephthalic acid precursor from the group of p-toluic acid and mixtures thereof with an oxidizable compound from the group of p-xylene, partially oxidized p-xylene derivatives and mixtures thereof; - A sufficient amount of at least 5% by weight of water to achieve a workable slurry; an amount of an oxidation catalyst composed of at least one catalytically active metal compound from the group of manganese and cobalt compounds and their mixtures, which is sufficient to create at least a minimum amount of M millimoles of the catalytically active metal compound per kg of liquid reaction mixture, where M is represented by the following equation (1 ) is defined: M = y (x + A) + Bx C x + D (1) in which y = molar ratio of water / p-toluic acid in the reaction mixture x = molar ratio of manganese to the total amount of manganese and cobalt in the catalyst preparation A is at least approximately equal to 0.200 B at least approximately equal to 10.9 C at least approximately equal to 4.35 D at least approximately equal to 0.0724 with a molecular oxygen-containing gas at a reaction temperature of 140 to 220 ° C and a sufficient pressure to keep at least part of the water at the reaction temperature in the liquid phase is oxidized and b) an oxidized, the mixture containing terephthalic acid is obtained. 2. The method according to claim 1, characterized in that A = 0.200 ± 0.031 B = 10.9 ± 1.3 C = 4.35 ± 0.17 and D = 0.0724 ± 0.0117. 3. The method according to claim 1, characterized in that the amount of water in the liquid reaction mixture makes up 5 to 80 wt .-% of the latter. 4. The method according to claim 3, characterized in that the amount of water in the liquid reaction mixture Makes up 10 to 75 wt .-% of the reaction mixture. 5. The method according to claim 3, characterized in that the amount of water in the liquid reaction mixture makes up 10 to 60 wt .-% of the reaction mixture. 6. The method according to claim 1, characterized in that the reaction mixture still contains 0 to 100%, based on the amount of water, of an inert organic solvent. 7. The method according to claim 6, characterized in that the organic solvent is acetic acid. 8. The method according to claim 1, characterized in that the reaction temperature is between 150 to 190 ° C. 9. The method according to claim 1, characterized in that the reaction pressure is 500,000 to 4,000,000 Pa. 10. The method according to claim 1, characterized in that the amount of the oxidation catalyst is sufficient to provide an amount of catalytically active compounds between the minimum amount defined in claim 1 and 40 millimoles per kg of reaction mixture. 11. The method according to claim 1, characterized in that the amount of the oxidation catalyst is sufficient to provide an amount of catalytically active compounds between the minimum amount defined in claim 1 and 30 millimoles per kg of reaction mixture. 30 12. The method according to claim 1, characterized in that the catalyst consists of a mixture of a manganese and a cobalt compound. 13. The method according to claim 12, characterized in that x is 0.1 to 0.9. 14. The method according to claim 1, characterized in that the catalytically active metal compound is a metal salt of a carboxylic acid. 15. The method according to claim 1, characterized in that the catalyst contains a further compound of a metal from the group of nickel, lead and cerium. 16. The method according to claim 1, characterized in that the molar ratio of p-toluic acid to p-Xy-45 loi is between 3 to 15.