NL7907252A - METHOD FOR PREPARING TERFHTALIC ACID. - Google Patents

Description

1 - 1 - i vo 7670

Labofina S.A.

Brussels, Belgium »

Process for preparing terephthalic acid.

The invention relates to an oxidation process, more in particular to a process for the preparation of terephthalic acid by oxidation of p-toluic acid or mixtures of p-toluic acid with p-xylene and / or with partly oxidized derivatives thereof, 5 such as p-tolualdehyde.

Terephthalic acid is of great technical importance because it is increasingly used as a starting material for the preparation of high molecular weight resins, such as fiber and film-forming polyesters.

The prior art has many processes for the oxidation in the liquid phase of alkyl-substituted aromatics to aromatic carbonic acids. One of the first patents in this field is the United States patent, a one-step process for oxidizing alkyl aromatic compounds by molecular oxygen in the presence of a metal catalyst, a solvent such as acetic acid and optionally an oxyda -. tie initiator. However, even under severe conditions, the yield of dicarbonic acids is low. Thus, when oxidizing a mixture of xylenes with air in acetic acid containing cobalt and manganese acetate as a catalyst at 185-200 ° C, under a pressure of 50 at in the presence of diethyl ketone as an initiator, the yield of phthalic acid only 2% and the main reaction products were toluic acids along with other intermediate oxidation products.

A number of further patents disclose methods for oxidizing p-xylene in a step with improved yields to terephthalic acid. These patents are essentially. relating to the use of special activators, such as bromine-containing compounds (U.S. Pat. No. 2,833,816), ketones (U.S. Pat. No. 2,853,51 ^) or aldehydes (U.S. Oc. 790 7 2 5 2 ϊ Λ »2 patent 3,036 122). Although some of these methods are tech. applied nonetheless, they nevertheless have serious drawbacks. For example, serious corrosion problems occur when bromine-containing activators are used. When a ketone or an aldehyde is used, some of it is inevitably lost and the remainder is mainly converted into acetic acid, which must be recovered, purified and marketed for the process to be economically viable. However, despite these drawbacks, the use of an activator has been considered an essential requirement of the yield for preparing phthalic acid from xylenes.

In most cases, the use of a solvent is also claimed to be necessary. Low molecular fatty acids, especially acetic acid, are widely used for this purpose. The amount of solvent added should be sufficient to keep the reagents and reactor products in solution or at least in suspension without difficulty. In this way, the reaction mixture is easily stirred, the dispersion of oxygen is improved, the formation of by-products is minimized and the heat of reaction is easily removed by evaporation of solvent. However, under the generally used reaction conditions, a significant amount of solvent is lost through co-oxidation. In addition, the solvent should be separated from the other components of the reaction mixture, then purified and recycled. Obviously, this consumption of a portion of the solvent and these operations to recover the remainder will result in additional processing costs.

In order to avoid the above-mentioned major problems, it has been proposed to conduct the oxidation of p-xylene in the presence of a solvent. In this 'case it is of course necessary' 30.. operate at a temperature at least in the region of the melting point of p-toluic acid, i.e., about 180 ° C to obtain a liquid reaction mixture. The choice of temperature is therefore limited. Moreover, in the absence of a solvent, handling the reaction mixture as well as separating and purifying the terephthalic acid are difficult. Generally, the reaction 790 7 2 5 2 * 3 is carried out to the point where the terephthalic acid content of the mixture is not above 60% and it is preferably not above 50% by weight.

Above this point, "it becomes difficult to handle the oxidation reaction mixture as a slurry and therefore the reaction reaction is adversely affected" (see U.S. Pat. No. 3,883,58M ·

In the absence of a solvent, the removal of the heat of reaction presents another difficulty in industrial scale processes since extensive fouling takes place in the reactor, even when terephthalic acid is not present in large quantities. For example, in U.S. Patent 2,796,999 which relates to the oxidation of xylene to toluic acids in the presence of a solvent, it is stated, "that the main construction problem associated with cooling the xylene oxidation mixture is to prevent deposition of solids." US Pat. No. 3, 06,196 describes a two-stage process using water as a suspending agent for terephthalic acid. In the first stage, an alkyl aromatic compound, more particularly p-xylene, is oxidized by air in the absence of any additional solvent to 20 partially oxidized compounds, which are further oxidized in a second stage at a higher temperature at presence of significant amounts of water as a suspending agent. Bromine or a bromine-containing compound must. be present to promote exydation. However, very high temperatures in the range of 200-255 ° C, and more particularly of 225-250 ° C, are required to achieve conversion of these partially oxidized compounds to terephthalic acid. Consequently, the same or even more serious corrosion problems are necessarily encountered than those present in processes using acetic acid as a solvent. In addition, as stated in said patent specification, there are "significant losses of unreacted polyalkylaromatic compound due to degradation and other side reactions which tend to occur when such compounds are exposed to the higher temperatures considered necessary for efficient conversion. of the partial oxidation products prepared in the first step 7907252 ... 4% of the oxidation to aromatic polycarboxylic acids. " It is therefore clear from the teaching of this patent that the use of large amounts of water, even in the absence of a bromine promoter, does not give satisfactory results for oxidizing. p-xylene to 5 terephthalic acid in one step.

However, it has long been known that water is "a catalyst poison for oxidation reactions" (U.S. Pat. No. 2,696,999). Most widely believed, water adversely affects the rate of reaction by interfering with initialing. As a general rule, its presence is avoided as much as possible, regardless of whether a solvent and / or an activator are present. For example, U.S. Patent 3,0öt.OUU describes an improved technique for maintaining an end. (bromine-promoted) oxidation under substantially anhydrous conditions.

US Patent 3,519,684, which relates to an oxidation process using peracetic acid as a promoter, states that "preferably, substantially anhydrous conditions are used, although a water content can be tolerated up to about 1035 and a maximum water content not greater than 51 is preferred ... "In a continuous process of oxidizing xylenes in the absence of any promoter and wherein the partially oxidized intermediates are continuously recycled, water is removed from the liquid effluent prior to recycling of the latter "to include the water content of the" reaction mixture at ......

. Keep less than $ 15 and preferably less than $ 5 of the total reaction mixture "(U.S. Pat. No. 3,700,7311.

More recently, it has been found that the oxidation of p-xylene to terephthalic acid can unexpectedly be carried out in the presence of large amounts of water as a solvent, although in the absence of any bromine-containing activator previously considered an essential requirement. This method is described in copending U.S. Patent Application Ser. Hr. 764.9 & 1 and 785,827.

They include oxidizing 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 790 7 2 5 2 5 l. Spring 10 to about 220 ° C under a pressure sufficient to maintain at least some of the water in the liquid phase.

However, since the reciprocal solubility of p-xylene and water is low at the run temperature, such mixtures of water, p-xylene and p-toluic acid can separate in two phases: an aqueous phase and a organic phase, which is rich in hydrocarbon and also contains an important content of p-toluic acid, which is present in the reaction mixture. In this case, the oxidation reaction takes place mainly in the organic phase, where the concentration of water is relatively low. Therefore, the desired dissolving effect of water is partly lost. In addition, this phase separation causes important technical difficulties with regard to homogenization, oxygen dispersion and mass transfer effects.

The invention now relates to a process for preparing terephthalic acid, comprising: a) oxidizing a substantially homogeneous liquid reaction mixture, comprising - at least one oxidizable terephthalic acid precursor selected from the group consisting of p-toluic acid and mixtures of p- ....... "20 toluic acid and an oxidisable compound selected from the group consisting of p-xylene, partially oxidized p-xylene derivatives and mixtures thereof; - an amount of at least 5% by weight of water, which it is possible to obtain a workable suspension, an amount of an oxidation catalyst, comprising -

at least one catalytically active metal compound selected from the group consisting of manganese compounds. cobalt compounds and mixtures thereof, which is sufficient to provide at least a minimum amount of M mmol of the catalytically active metal-30 compound, per kg of liquid reaction mixture, wherein M is defined by the equation (1I

M- y (x * A] + B x m

Cx + D v 1 where y represents the molar ratio of the water to p-toluic acid in the reaction mixture, 35x the molar ratio of mangaaii / total manganese + cobalt in 7907252

•'A

6

Particularly the catalyst mixture, i.e.

A = about 0.200, B = about 10.9, C = about ^ .35, and 5 D = about 0.072il ·, with a molecular oxygen-containing gas, and a reaction temperature of about 100-220 ° C and a pressure sufficient to maintain at least a portion of the water in the liquid phase at the reaction temperature and recovering a terephthalic acid-containing oxidized mixture for 10 h}.

By maintaining the catalyst concentration at least at the minimum value described above, an effective oxidation of each of the above starting material mixtures to terephthalic acid in a homogeneous liquid phase reaction mixture is achieved.

The drawing depicts a phase diagram of mixtures of p-xylene, p-toluic acid and water at a temperature of 85 ° C.

It is an important feature of the invention that water, the deleterious action of water, which is widely believed in the art, can be overcome when the oxidation is carried out under the above specific conditions. It is a further feature of the invention that it can be applied to the oxidation of various substrates, which may consist of p-toluic acid alone or mixed with p-xylene and / or partially oxidized derivatives, such as p-tolualdehyde. Another feature of the invention is that such substrates can be oxidized in a homogeneous aqueous solution if the amount of catalyst is selected in correlation with the ratio of the amounts of water and ρ-toluic acid present in the system. to be.

The method according to the invention can be carried out batchwise or continuously. The reactants are dissolved in water and the catalyst is added to the resulting solution.

Then oxygen is introduced into the mixture and the oxidation is carried out at a temperature between tko and 220 ° C under pressure. The pressure.

7907252 »t τ * * is adjusted so that the substantially liquid phase reaction mixture remains at the operating temperature. Tephthalic acid separates from the reaction mixture as a white crystalline precipitate. In a continuous process, this precipitate is continuously removed by any conventional solid-liquid separation method, such as filtration, centrifugation or settling and decanting, and the residual liquid, containing unreacted reactants and intermediate oxidation products, is recycled to the oxidation zone. . Fresh reaction components are continuously supplied to replenish the terephthalic acid thus withdrawn and to form by-products. Therefore, no addition of foreign chemicals is required. However, when carrying out the process of the invention, one or more organic solvents may be added to the reaction mixture which do not interfere with the reaction and are relatively inert under the process conditions. Examples of such compounds that can be used as a solvent mixed with water are benzoic acid and acetic acid. These compounds, which are used in small amounts during the reaction, can therefore be allowed to accumulate to some extent in the reaction mixture. However, the benefits deriving from this invention are obtained regardless of the presence of such compounds, which, when used, should not be present in the amount exceeding the amount of water in the system.

The amount of water used in the process of the invention may vary within wide limits, e.g. between 5 and about 80% by weight of the reaction mixture, depending on various factors. As already stated, an essential aspect of the invention is that the oxidation is carried out in a homogeneous aqueous system. As a result, the amount of water will be chosen to be substantially sufficient to provide a substantially homogeneous aqueous solution of the reactants, taking into account the other process variables, such as temperature and the relative amounts of the various oxidizing compounds. When the method e.g. is performed to oxidize p-toluic acid alone or mixed with 7907252 * - «- c '·. .

: 8 other oxygenated compounds, which are relatively. soluble in water, such as p-tolualdehyde, an amount of water sufficient to completely dissolve the p-toluic acid at the process temperature will be selected. Since the solubility of p-toluyl-5 acid in water increases steeply at elevated temperatures, the amount of water to be used can be decreased as the temperature is increased. As a general rule, however, the amount of water will not be less than 5% and preferably not less than 10% by weight based on the reaction mixture.

If p-xylene is also a component of the reaction mixture, the amount of water should not exceed the amount above which separation of. the mixture will be in two liquid phases. take place. This amount naturally depends on the amount of p-xylene in the mixture.

Figure 1 is a triangular phase diagram for mixtures of p-xylene, p-toluic acid and water at a temperature of 185 ° C (in wt%). In this' diagram it is. system in zone A a homogeneous solution and in zone B biphasic. As any expert can easily determine, the boundary line between these two zones is not highly temperature dependent. It can be seen that, in order to avoid the presence of an important organic phase, the amount of p-xylene should be limited. Therefore, when the oxidation of p-xylene according to the invention is carried out batchwise, p-xylene should be added gradually, either intermittently or continuously, to the reaction mixture at such a flow rate that the system remains in zone A. According to a preferred embodiment of the invention, the reaction. carried out by a continuous flow process, whereby unreacted p-xylene together with the intermediate oxidation products, i.e. essentially p-toluic acid, is continuous.

30 are recycled to the reaction zone. In this case, the molar ratio of p-toluic acid / p-xylene will be in the reaction mixture. in the stationary state, the range is between, about 3 and about 15, substantially. depending on the temperature .; in other words, the reaction mixture will be in the gray area of the. dia-35 grams. It can be seen that this region lies almost entirely in zone A, i.e.

7907252 9 / £ λf for the most part corresponds to a homogeneous solution.

From the "above" it appears that it is always possible to adjust the temperature and / or the amount of water used as a solvent in such a way that a homogeneous system is obtained. If the method according to the invention is carried out continuously, it is recommended to use the continuous method described in the co-pending US patent application Ser. Fr. 785,827.

However, other factors should also be taken into account TO. E.g. since terephthalic acid, the desired compound, is substantially insoluble in the reaction mixture, an amount of water sufficient to obtain a processable suspension should be added. However, there is no advantage in using such a large amount of water that more than e.g.

15 10 # terephthalic acid is dissolved at the working temperature. A further factor to be taken into account is the reaction rate; although in the process of the invention it is possible to carry out the oxidation reaction in an environment containing 80 wt.% water or even more, the presence of excess water * 20 may adversely affect the reaction rate. In addition, in some cases, part of the catalyst can be withdrawn from its catalytic function by forming a black compound which precipitates from the oxidation medium. Torch, for economic reasons, it is a disadvantage to lose part of the reactor capacity by using an excessive and unnecessary amount of water.

In general, for these various reasons, the amount of water present in the system will not exceed 75 wt%, and preferably will not exceed 60 wt% of the reaction mixture.

Suitably, the oxidation reaction is carried out at a temperature of at least 100 ° C. Below this temperature, it is difficult to have the reaction mixture as a homogeneous solution. On the other hand, operation above 220 ° C would result in increased over-oxidation, undesired side reactions and corrosion problems. In most cases, the reaction temperature will be between about 150 and about 190 ° C.

7907252 ». 10.

The pressure is set as a function of the temperature. A pressure should be used which is sufficiently high above atmospheric pressure to maintain the reaction mixture in a liquid state at operating temperature. A pressure above this value is generally suitable for imparting active oxidation. Generally, the pressure will be between about 5 and 0 kg / cm.

The oxidation catalyst used in the process of the invention may be a heavy metal compound comprising. at least one metal compound selected from the group of manganese-10 compounds, cobalt compounds and mixtures thereof, provided that it is at least partially soluble In the aqueous reaction mixture or capable of soluble or at least partially soluble compound with one of the reactants in form the mixture. The metal compounds that can be used in the catalyst mixture are suitably salts. In particular, the carboxylic acid salts are preferred, e.g. acetates, naphthanates, toluates, and the like.

In addition to cobalt and / or manganese compounds, the cat alysate mixture may contain compounds, in particular carboxylic acid salts, of other metals which are common as oxidation catalysts. An essential aspect of the present invention is that the minimum concentration of catalyst necessary to ensure oxidation in a homogeneous aqueous system depends on the respective amounts of water and. p-toluic acid in the system. Of course you can. in a homogeneous aqueous solution of p-toluic acid and optionally p-xylene and / or partly oxidized derivatives thereof, oxidation does not take place if the amount of active catalyst is less than a critical concentration of M. in rmol of the metal compound / kg reaction mixture, given by the equation below (11.

30 m y '(tc' + 'Ai + B X, y

C x + D u L

where y is the molar ratio of water to p-toluic acid, x is the amount of manganese in the metal catalyst in mill with respect to the total amount of manganese and cobalt therein, -.----:

Mn + Co A - about 0.200; 35 B = about 10.9; 7907252 to C * about 4.35; and D = about 0.0724.

The above amounts of A, B, C and D are the results of experimental Assays and consequently subject to Assay Errors. It has been calculated that these amounts with their confidence limits are: A = 0.200 + 0.031; B = 10.9 +1.3; C = 4.35 + 0.17; and D = 0.0724 + 0.0117.

Consequently, the value of M as Calculated 10 using Equation '(ll for a' data 'value of y + x should be considered an estimate of the actual critical concentration »which is located in terms of statistics within a confidence interval all around M. As will be appreciated by the skilled person, this confidence interval depends on the values of y ear used in the Calculation of M, but for each special case it can be Calculated from the Above Numbers using Taylor's formula, Thus, for example, the critical catalyst concentrations will be between the confidence intervals 21.46 "+ 0.81 if y = 70 and x = 1.0 20 and 2.74 + 0.4? If y = 1 eni = 0 , 5.

The critical catalyst concentration as described above is the lowest concentration that can be used under the given conditions. The process is not feasible at lower quantities. It is of course preferred in practice to use an amount of catalyst which is higher than this minimum.

Of course, the oxidation rate will increase as the catalyst concentration increases. However, catalyst concentrations higher than about 40 mmole metal compound / kg reaction mixture are not advantageous for economic reasons. Suitably, the catalyst will be used in an amount comprised between a value slightly higher than the minimum value of M as Calculated from equation (1}) and about 30 mmol / kg of reaction mixture.

As can be seen from equation (11), the critical concentration of the catalyst increases as the concentration of water increases and the concentration of p-toluic acid decreases.

790725Z

c 12 '

In other words, oxidation cannot take place in such an aqueous solution unless the concentration of p-toluic acid is above the critical value, which increases as the concentration of catalyst decreases. Therefore, not only the catalyst, but also the p-toluic acid are essential for the oxidation to proceed in the presence of large amounts of water. In fact, due to the combined action of the catalyst, it is the p-toluic acid, both used in the correct amounts according to the invention, it is. possibly oxidize p-xylene to terephthalic acid without resorting to the use of expensive and / or corrosive activators, such as brominated compounds. This effect of p-tolynic acid is a completely unexpected aspect of the invention since other carboxylic acids, even acids of similar structure, such as benzoic acid, do not exhibit the same property.

Furthermore, it appears from equation (1) that the critical minimum concentration of the catalyst, which is necessary for the oxidation to take place in a homogeneous aqueous system, also depends on the mutual amounts of manganese and cobalt compounds in the catalyst used. It can be seen that manganese is much more effective than cobalt. Bil e.g. oxidation to take place in a 50 wt.% aqueous solution of p-toluic acid (molar ratio of water to p-toluic acid is 7.561, then the minimum concentration of catalyst required is bt5 or 20.9 mmol / kg, depending on the whether manganese or cobalt is used as a single catalyst, however, for practical reasons it is advantageous to use a mixture of both metals, since cobalt generally has a favorable effect on the reaction rate. , a mixture of manganese and cobalt compounds is used as a catalyst, less overoxidation takes place and generally higher yields of terephthalic acid are obtained than when each metal is used separately, for example the combustion ratio (the molar ratio of developed carbon dioxide / absorbed oxygen) in generally a value between 0.06 and 0.09 in the first case, while values of about 0.15 in the second case are observed enomen. In fact, this beneficial effect of using 7907252 13 simultaneously using manganese and cobalt as a catalyst is a characteristic feature and a further advantage of the present invention. When the reaction is carried out in a two-phase system instead of a homogeneous solution, as in the present process, combustion ratios of 0.12 or even more are observed regularly, regardless of whether manganese or cobalt or mixtures thereof are used. In most cases, a value of x in equation (1) between about 0.1 and about 0.9 will be advantageously used to obtain an active oxidation in a homogeneous aqueous environment and high yields of terephthalic acid.

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

A surprising feature of the present invention - is that the minimum concentration of catalyst to be used to obtain oxidation in an aqueous medium can be calculated from equation (1) for each composition of the reaction mixture, and this comparison is independent of such important process variables as the temperature, this is illustrated by the results in Table A, comparing values of M, which have been determined experimentally, over a wide range of conditions, with the values, calculated from equation (1]. Most of these results were obtained by the following experimental method.

The various components for the present oxidation reaction mixture, namely p-toluic acid, are introduced into an 11 corrosion-resistant autoclave, equipped with a mechanical stirrer, a heating jacket, a gas supply tube, a discharge pipe and a metering pump for injecting a liquid. , water and the catalyst. This mixture is then heated with stirring and air passage. Once oxidation has started, an aqueous solution of 7907252 1U. Discharge of the catalyst with the same catalyst concentration as the starting mixture is gradually injected into the mixture. As a result, the reaction mixture is gradually diluted with water without any change in the catalyst concentration. p-Xylene can also handle.

Being present in the starting mixture, Mts its concentration is such that no phase separation takes place and diluting the reaction mixture with the aqueous catalyst solution.

The degree of oxidation is determined by continuously measuring the oxygen content of the exhaust gas using an oxygen analyzer. Provided that the concentration of p-toluic acid in the system is sufficiently high to ensure effective oxidation at the selected catalyst concentration, the reaction proceeds regularly at a rate that gradually decreases due to dilution. However, once the critical concentration of p-toluic acid is reached, the reaction rate drops rapidly to about 0. Then the injection is stopped and the reaction mixture is analyzed. The mole ratio of water to p-toluic acid thus determined is of course the critical y value, corresponding to the given concentration M of the catalyst or vice versa M is the minimum concentration of catalyst, which must be used to obtain oxidation in the reaction mixture, in which the ratio between the water and the p-toluic acid is given as y. In fact, M is still dependent on the catalyst composition, which can be expressed as the molar ratio of the manganese to the total amount of manganese and cobalt in the catalyst composition used (x in: equation 1].

Of course, such determinations are subject to experimental errors, so that for a given reaction mixture (for a given y and x), different values are expected, which will be statistically distributed around the true value by a standard deviation <T. Values of which are thus determined for different y. and x. are listed in Table A below.

✓

From this data it can be determined that the standard deviation of the value M as calculated in equation (1) is 0.70. As a result, any experimental value that does not deviate from the calculated 35, with more than two standard deviations, i.e., by more than 7907252 than 1, U, will be considered consistent with equation (1).

It is clear from the results reported in Table A that: 1. Equation (1} is valid for any value of y 5 and x (compare eg tests No. I and No. VU for y and runs No. VII and No. XIX for x).

2. Equation (1) is independent of the presence or absence of p-xylene (compare, e.g., No. VTII and No. XIH).

3. Comparison (1} is independent of the temperature (compare, e.g., experiments UI and IX).

4. Comparison (1] is independent of the presence of metals other than manganese and cobalt (compare e.g. tests H and Vj.

790 7 2 5 2 ï. ... * 16 -

TABLE · A

Reaction mixture (wt. $) M__

Tempe- pr-tolu water y. x calculated _rature xylene ylic acid_nomen_ I 185 0.3 9.7 90.0 70.22 1.00 20.1 21.5 5 II 185 0.5 20.0 79.5 30.08 1.00 10, 1 10.6 III 185 1.5 45.7 52.8 8.76 1.00 5.1 4.8 iv 170 0.4 10.6 89.0 63.5b 1.00 20.0 19.7 V 170 0.6 23.5 75.9 24.38 1.00 9.0 9.1 (1) (1) 10 VI 170 1.4 43.5 55.1 9.60 1.00 5.1 5.1 VII 170 0.1 61.3 38.6 4.76 1.00 3.7 3.8 VIII 170 11.6 76.6 11.8 1J7 1.00 3.1 2.8 (2) IX 160 1.3 42.6 56.1 9.95 1.00 5.1 5.2 15 X 170 1.3 44.4 54.3 9.25 0.79 5.2 5.1 XI 170 0 .3 23.0 76.7 25.21 0.50 10.1 10.3 XII 170 0.2 10.8 89.0 62.34 0.46 21.7 22.2 XIII 170 0.0 41, 8 58.2 10.53 0.46 5.5 5.8 XIV 160 0.5 43.9 55.5 9.57 0.46 5.5 5.5 20 XV 170 0.5 24.3 75, 2 23.46 0.33 10.1 10.6 XVI 170 0.3 27.0 72.7 20.39 0.25 10.1 10.2 XVII 170 0.4 30.4 69.2 17.23 0.17 10.1 10.1 XVIII 170 0.5 35.3 64.2 13.77 0.09 10.1 10.7 XIX 170 5.4 60.5 34.1 4.27 0.00 11 .7 11.8 25 XX 170 0.8 74.9 24.3 2.45 0.00 6.1 6.8 XXI 170 0.0 81.4 18.6 1.73 0.00 3.9 4 , 8 (1) An equimolar mixture of manganese and nickel acetate was used ast as a catalyst. The calculation of x and M is done without taking nickel into account.

30 (2). Value obtained without diluting the reaction mixture. The critical water / p-toluic acid ratio was reached spontaneously by consumption of the latter and production of the former due to the oxidation.

790 7 2 5 2 f

IT

The teaching of the prior art, with regard to the choice of. the metal compound to be used for oxidizing p-xylene and its partial oxidation derivatives to terephthalic acid is slightly heated. However, as a general rule, cobalt is considered to be the best catalyst, especially in the absence of a brominated activator, and manganese is considered to be less active, if not inactive. For the oxidation of p-toluic acid, manganese has been shown to be inactive (TT. Ohta et al., Chem. Abstr. 56, 8620g, 1962) or even acts as a retarder (YN Aleksandrov et al., 10 Kinet. Katal. _1_5, 505 »197 * 0 · Under other conditions, manganese exhibits catalytic activity, but discoloration of the terephthalic acid occurs when the water content is greater than 10% by weight of the solvent or when the manganese content in the reaction mixture is high. very unexpectedly, that in the process of the invention manganese has excellent activity for conducting the oxidation reaction in the presence of large amounts of water, and terephthalic acid is obtained in high yield as a white crystalline precipitate, which is particularly suitable for further purification to a degree required for the preparation of polyester scythe (fiber grade}.

The invention will now be further elucidated by means of the examples below.

Example I

Into a corrosion resistant autoclave of 1 1 contents equipped with a mechanical stirrer, a heating jacket, a gas supply tube and a discharge pipe, were placed: p-xylene ** 5.0 g p-toluic acid 187.5 g different oxygenated p - 30 xylene derivatives k-, 5 g water 63.0 g manganese acetate 1.50 millimoles cobalt acetate 1, T * J millimoles.

2

The reactor was pressurized to 20 kg / cm @ 3 with air and the above described mixture was heated with stirring and supplying air at a flow rate of 92 L / hr.

In the "batch described above, the molar ratio of water / p-toluic acid (y) (63.0 x 136.15) / (18.02 x 187.5) - 2.5 * + and the molar ratio Mn / Mn + Co in the catalyst (x): 1.50 / (1.50 + 5 1.7 * 0 = 0.1 + 6) Using equation (1), the minimum concentration of catalyst required in this case for obtaining oxidation calculated as follows: 2.5 * + (0.1 + 6 + 0.200) + 10.9 (0.1 + 6) M = * +, 35 (0,) + 6) + 0.072 * + = 3.2

In fact, in the present example, the catalyst concentration was 10.8 mmol / kg, i.e. about three times the minimum amount. Naturally, the reaction started spontaneously with heating and took place actively; as a result. the temperature increased rapidly, it was maintained at 170 ° C by control cooling.

After a reaction period of 180 minutes, the oxygen absorption was 1 + 0.9 l (determined at room temperature and a pressure of 1 at). The air supply was then cut off and the pressure gradually released 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 in vacuo at about 80 ° C. It was then analyzed by a combination of methods, including acidimetry, polarography and vapor phase chromatography. It was thus determined that 89 * +% of the supplied p-xylene and 23.1% of the supplied p-toluic acid had been converted into the following products: 25 terephthalic acid 90.2 g 1 + -carboxybenzaldehyde 8.8 g other intermediates 3.8 g heavy by-products 2.2 g

Taking that into account. 1+ -carboxybenzaldehyde and other intermediates would in fact be recycled in a continuous process and most of them would eventually be converted into terephthalic acid, it can be estimated that the yield of terephthalic acid in such a continuous process is more than 90 mol% 7907252 19 based on the p-xylene consumed.

In another process performed under the same conditions, the hot reaction mixture was filtered; the resulting cake was further washed on the filter with hot water and dried in vacuo. Analysis of the dried product showed that it consisted (in wt%) of tereffalic acid 85.6% p-toluic acid 9.6% 4-carboxylic enzaldehyde 3.6%. The color of this sample was determined by measuring the optical density of its solution in dilute ammonia, according to the method described in U.S. Pat. No. 3,354,802 except that a 5 cm cell is used instead of a 4 cm cell. The values of the optical densities thus obtained are shown in Table B and were compared with the values of the optical density obtained with a commercial terephthalic acid sample of 99% purity.

TABLE · B

Raw sample Optical density at (ayu):

Origin_Purity (wt. $ 3 340_380_400 20 prepared according to the invention 85.6 0.428 0.102 0.076 commercial product 99+ 0.726 0.183 0.111

It can be seen that the crude sample obtained by the present process by simple filtration and surface washing had better coloring properties than a commercial terephthalic acid sample of much greater purity. This raw sample is therefore particularly suitable for further cleaning up to the fiber quality. Example II

The experiment of Example I was repeated with the exception that a mixture of water 33.0 g 7907252 acetic acid 30.0 g was used instead of 63.0 g water as solvent

After 180 minutes of reaction, the oxygen absorption was 90.7 l, i.e. approximately the same amount as in Example I.

The reaction mixture was then treated and analyzed as described in Example 1. It was thus determined that 88.0% of the supplied p-xylene and 23.5% of the supplied p-toluic acid had been converted to the following products: terephthalic acid 89.4 g 4-carboxybenzaldehyde 6.5 g 10 other intermediates 4.8 g heavy by-products 3.2 g

According to the same method as in Example 1, it can be estimated that in a continuous process operating under the same conditions as used in the present example, the yield of terephthalic acid would be about 90 mol% based on the p-xylene consumed.

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

Example III

The experiment of Example II was repeated except that formic acid replaced the acetic acid. The solvent used in this case therefore consisted of a mixture of: water 33.0 g of formic acid 30.0 g

After 180 minutes of reaction, the oxygen absorption was 36.7 l. According to the same analytical method as in the previous examples, it was determined that 88.0% of the supplied p-xylene and 15.6% of the supplied p-toluic acid were converted into the vol - 30 products: terephthalic acid 66.7 g 4-carboxybenzaldehyde 6.8 g other intermediates 7.0 g heavy by-products 3.7 g 790 7 2 5 2 ί 21 .-

By the same method as in the previous example, it can be estimated that in a continuous process operating under the same conditions as the present example, the yield of terephthalic acid would be about 85 moles based on the xylene consumed.

t

Although these results are not as good as those of the previous examples, they show that large amounts of formic acid can be tolerated in the reaction mixture without seriously affecting the yield and the reaction rate. This is particularly unexpected, as formic acid has always been described in the prior art as a strong retardant of oxidation reactions. Since formic acid is always produced in such reactions, generally elaborate and expensive methods have been designed to prevent it from accumulating in the reaction mixture. In the present process, formic acid is burned as evidenced by the fact that the carbon dioxide amount developed in this test was 6.0 L instead of 3.8 L as determined in the test of Example 1. Consequently, in the present process no special treatment required to remove formic acid uxt the reaction mixture.

Example IV

The experiment of Example I was repeated except that cobalt was used as the sole catalyst. The actual charge was the following: 25 p-xylene ^ 5.0 g p-toluic acid 190.1 g various oxygenated derivatives 1.9 g of water 63.0 g of 30 cobalt acetate 3.8 millimoles

In this batch y was 2.50 and consequently x was of course 0.00. As a result, the limit amount was cobalt ahead. obtaining oxidation in this case M * «’ "Cgff ·! = 6.9 millimoles / kg.

790 72 5 2 22

The concentration of cobalt acetate in the starting mixture was 11.6 mmol / kg, i.e. z. about twice the limit amount. After 180 minutes of reaction, the amount of oxygen absorbed was U1.5. The reaction mixture was then treated and analyzed as previously described. It was thus determined that 90.8% of the supplied p-xylene and 19.5% of the supplied p-toluic acid had been converted into the following products: terephthalic acid 7, 2 g of U-carboxybenzaldehyde, 6.8 g of 10, intermediates 1.5 g. heavy by-products 3.7 g

By comparing these results with those of Example I, it can be seen that less terephthalic acid was produced, although the oxygen absorption was slightly higher. By estimating as in the previous example, the yield of terephthalic acid obtained in the continuous process would be a value of 81% instead of 90. This difference clearly shows the increase in yield obtained by using a catalyst consisting of a mixture of manganese and cobalt instead of only cobalt-20. Example Y

The test of Example Γ was repeated except that manganese was used as the sole catalyst. The batch composition was as follows: p-xylene U5.0 g 25 p-toluic acid 182.8 g various oxygenated derivatives 9.2 g water 63.0 g manganese acetate 3.00 millimole 30 olη this batch was y * 2.60 and of course x 1.00. As a result, the limit amount of manganese for obtaining oxidation was: '2.60 (1.00 + 0.200) + ,10.9 M i1.35. 6.07 g / l = 3.2 ml / mol

In the present example, the manganese concentration was 7907252 23 concentration 10.0 mmol / kg, i.e. z. about three times the limiting amount of Example 1. After 180 minutes of reaction, the oxygen absorption was 25.3 l, i.e. considerably lower than in the previous examples. The reaction mixture was then treated and analyzed as already described. It was thus determined that 70.2% of the supplied p-xylene and 5.8% of the supplied p-toluic acid had been converted into the following products: terephthalic acid 31.2 g of 1-carboxybenzaldehyde 5.9 g of 10 other intermediates 6 .3 g heavy by-products 5.1 g

When comparing these results with those of Examples I and II, it is clear that less reactants were consumed here and less terephthalic acid was produced.

In addition, the yield of terephthalic acid in a continuous process calculated from these numbers as in the previous examples is only 69%. Therefore, although manganese is clearly much more effective than cobalt, it is to process the oxidation in the presence of water (as evidenced by the lower values calculated for M

20 compared to the value compared for cobalt only), there is a clear advantage in both the rate and yield of the reaction to use manganese combined with cobalt as in Example I.

EXAMPLE 25 The experiment of Example I was repeated with the exception that in this case manganese was used in combination with nickel. The batch composition was as follows: p-xylene -5.0 g of p-toluic acid 182.8 g of 30 different oxygenated derivatives 9 g of water 63.0 g of manganese acetate 1.50 millimoles of nickel acetate 1, 50 millimoles 35 In this batch, y was 2.60 and x 1.00 was as in the previous example 790 7252 -2k, therefore M was 3.2 mmol / kg (nickel not taken into account in the calculation of M). In the present example, the manganese concentration was 5.0 mmol / kg, i.e., 1.8 greater than the limit amount. Oxidation naturally took place actively.

For about 180 minutes, the oxygen absorption was 29.2 L and the reaction mixture was analyzed as already described. It was found that 79.1% of the supplied p-xylene and 3.3% of the supplied p-toluic acid had been converted into the following products: terephthalic acid 35.6 g 10 k-carboxybenzaldehyde 6.6 g other intermediates 2.6 g heavy by-products 3.2 g

Comparing these results with those of the previous example, it can be seen that slightly more oxygen was absorbed and more terephthalic acid was produced than if manganese was used alone. concentration twice as high as in the present example. This shows that metals other than cobalt combined with manganese can be used to increase the reaction rate.

Example VII

Into the same autoclave as in the previous examples were charged: p-xylene ^ + 5.0 g p-toluic acid 218.5 g 25 different oxygenated derivatives 6.5 g water 30.0 g manganese acetate 0.52 millimoles cobalt acetate 0.5-5 millimole 30 Ih this batch was the molar ratio of the water to the p-toluic acid (yl 1.0U and the mole fraction manganese in the catalyst (x) 0.006. By using equation (1) the minimum concentration of catalyst to be used in the present case to obtain oxidation therefore M = 2.7 35 mmol / kg. In fact, in the present example, the catalyst concentration was 790 7 2 5 2 25 concentration 0 , 9T mmol - for 300 g of initial mixture or 3 »2 mmol / kg, ie only 0.5 mmol greater than the limit amount, nevertheless, oxygen absorption started spontaneously and took place actively, as a result the temperature increased rapidly and was maintained at 170 ° C 5 by controlled cooling.

After 395 minutes of reaction, 62.9 L of oxygen had been absorbed. The air supply was then interrupted and the same method as in Example 1 was used to determine the composition of the reaction mixture. Thus, it was determined that 98.8 # 10 of the supplied p-xylene and 29.2% of the supplied p-toluic acid had been converted into the following products: terephthalic acid 102.5 g of U-carboxybenzaldehyde 6.6 g of other intermediates k96 g 15 heavy by-products 3.6 g

Estimating from these numbers as in Example 1 the yield of terephthalic acid that would be obtained at a con is obtained to be a value of $ 80, i.e., $ 10 less than the yield estimated in Example I. The main difference between the two tests is that in example Γ the water content of the batch was 21% by weight compared to only 10% in the present case.

The yield difference obtained can therefore be considered as a further illustration of the advantage of carrying out the oxidation of p-xylene in the presence of large amounts of water, according to the present invention.

Comparative example

The same amounts of compounds as in the previous example were autoclaved except that only 0.30 moles of manganese acetate and 0.35 moles of cobalt acetate were used. The total metal catalyst concentration was therefore 2.2 mmol / kg, i.e. 0.5 mmol less than the limit amount calculated in the previous example.

By heating the mixture in the presence of a stream of air under the same conditions as in the previous example, the oxygen absorption started immediately, but suddenly dropped to negligible levels after about 70 minutes. The oxygen absorption was only 6.7 l.

Example VIII

In the same autoclave as in example I, p-tolualdehyde 1 + 5.0 g of p-toluic acid were charged 187.5 g of various oxygenated derivatives U, 5 g of water 63.0 g of manganese acetate 1.50 millimoles of cobalt acetate 1, 7 ^ millimoles

As can be seen, this mixture was very close to that of the batch used in Examples I - V, except that the p-xylene was replaced with p-tolualdehyde. The molar ratio of water to p-toluic acid was again 2.5 ^. Therefore, using equation (1), the minimum concentration of catalyst to be used to obtain oxidation was M 3 2 mmole / kg.TT 20 In fact in the present example, the concentration of the catalyst was 10.8 mmol / kg, dw z. about three times higher.

This mixture was heated in the presence of air as in the previous examples. Acidification or absorption started spontaneously at about 30 ° C. The temperature was allowed to increase and maintained at 170 ° C by regular cooling. After 180 minutes of reaction, the oxygen absorption was 25.6 1. The air supply was then stopped and 180 cm 3 of n-heptane was injected into the reactor to extract unreacted p-tolualdehyde with continued stirring and heating. The resulting mixture was then cooled and the reactor opened. The precipitate contained therein was filtered, washed with n-heptane, dried in vacuo at 50 ° C, washed again with water and finally dried under vacuum at about 70 ° C.

It was then analyzed. " by the same method as in the previous examples. The filtrate and wash water were combined and the heptane extract was separated by decanting. A proportional portion of this extract was analyzed by vapor phase chromatography to determine unreacted p-tolualdehyde. The remaining part was evaporated to dryness in vacuo and the residue was analyzed by the same method as the first precipitate.

From these different analyzes it was determined that 99.9a of the p-tolualdebyde fed and 10.7% of the p-toluic acid supplied had been converted into the following products: terephthalic acid 58.1 g U-carboxybenzaldehyde 7.1 g 10 other intermediates 6.1 g heavy by-products 5 Λ g

Comparative Example I

The same amounts of compound as in the previous example were autoclaved with the exception that only 0.3 mmol of both manganese and cobalt acetate were used. The total catalyst concentration was therefore 2.0 million / kg, i.e., 1.2 mmol was less than the limit amount as calculated by equation (1].

The mixture was heated in the presence of air under the same conditions as in the previous example. After 180 minutes of reaction, the oxygen absorption was only 9.0 l. The reaction mixture was then treated and analyzed as in the previous example. It was thus determined that the supplied p-tolualdehyde was completely converted into the following products: terephthalic acid 2.3 g p-toluic acid 31.7 g 4-carboxybenzaldehyde 3.8 g other intermediates 0.9 g heavy by-products, 2 It can be seen that in the present case p-tolualdehyde has been substantially converted to p-toluic acid, without significant terephthalic acid formation.

Comparative Example II

In this example, p-tolualdehyde was used as the sole substrate to be oxidized. The actual batch was as follows: 7907252% ».28 p-tolualdehyde 225» 0 g water 75 »0 g manganese acetate 1.50 millimole cobalt acetate 1.50 millimole 5 When heating this mixture in the presence of air under the same conditions as in example VIII, oxygen absorption started in the same manner, but then noticeably decreased.

After about 20 minutes reaction at 170 ° C, the oxygen absorption was 29.6. By treating and analyzing the reaction mixture obtained as described above, it was found that 99.% of the p-tolualdehyde fed was converted into the following products: terephthalic acid k, 6 g p-toluic acid 171, + g 1 + -carboxybenzaldehyde k, 6 g 15 other intermediates 1.3 g heavy by-products 1.3 g

It can be seen again here that p-toluic acid was the main product of the oxidation of p-tolualdehyde and only minor amounts of terephthalic acid were formed. This result aptly illustrates that in order to oxidize p-tolualdehyde in terephthalic acid of the invention, p-toluic acid must be present in sufficient amount from the start of the reaction.

Example IX

The test of Example VIH was repeated except that 3.18 mmol of cobalt acetate was used as the sole catalyst. By using equation (1), it can be calculated that the minimum cobalt concentration to be used in this case is M = 7.0 mmol / kg. In fact, the concentration of cobalt catalyst in the present example was 11.6 mmol / kg.

Here, too, oxygen absorption started at a low temperature. Nevertheless, it was heated to maintain a temperature of 170 ° C. After about 210 minutes of reaction, the oxygen absorption was 20.8 1. The reaction mixture was then treated and analyzed as in the previous example. Thus, it was determined that 99.9% of the p-tolualdehyde supplied and 6.0% of the p-toluic acid supplied 790 7 2 5 2 29 fern converted to the following products: terephthalic acid 9.1 g 4- carboxybenzaldehyde 6.9 g other intermediates 6.5 g 5 heavy by-products 6.3 g

Comparative example

The same autoclave as in the previous examples were charged: p-tolualdehyde 69.6 g 10 p-toluic acid 123.0 g various oxygenated derivatives 3.0 g water · 105.0 g cobalt acetate 3.1) 8 millimoles 15 amount of cobalt catalyst used as above (11.6 mmol / kg], but here the mole ratio of water / p-toluic acid was 6.1) -5 instead of 2.5¼. The minimum concentration of cobalt catalyst to be used in this case is therefore M = 17.8, i.e. 6.2 mmol more than actually present in the batch. This mixture was heated in the presence of air.

Here, too, absorption of sunlight took place immediately. However, after 1) 5 minutes it dropped to a negligible level; nevertheless, heating was continued at 170 ° C, after 300 minutes of reaction, oxygen absorption was only 10.0 l. The reaction mixture was then treated and analyzed as described in Example VIII. Thus, it was determined that 90.1% of the p-tolualdehyde fed had been converted to the following products: terephthalic acid 3.7 g p-toluic acid 53.5 g 30 l -carboxybenzaldehyde * 3.3 g other intermediates 0.7 g heavy by-products 6.6 g

It is seen that p-tolualdehyde has been substantially converted to p-toluic acid, without significant terephthalic acid formation.

7907252 \ t 30

Example X.

In the same autoclave as in the previous examples were charged: p-toluic acid 21 ^, 3 g of 5 different oxygenated derivative and 10.7 g of water 75.0 g of cobalt acetate 1.7 µ millimole of manganese acetate 1.50 millimole In this example, no p-xylene nor p-tolualdehyde was used. The molar ratio of water to p-toluic acid (y) was 2.6k, and the manganese mole fraction in the catalyst (x) was Q, k6 as in Example 1. The minimum concentration of catalyst to be used as calculated from equation (1) is M - 3.3 mmol / kg.

In fact, in the present example, the concentration of catalyst is 3.2kmmol per 300g reaction mixture or 10.8mmol / kg, i.e. about three times the limit.

This mixture was heated in the presence of air under the same conditions as in Example Γ. In this case too ----------------- 20 the reaction started spontaneously. After 180 minutes of reaction, 19.1 k of oxygen had been absorbed and the reaction was stopped by cooling. The autoclave was then opened and the precipitate contained therein was treated and analyzed as described in example. I. It was thus determined that 33.2% of the supplied p-toluic acid had been converted into the following products: terephthalic acid 75.5 g h-carboxybenzaldehyde 6.1 g other intermediates 1.8 g heavy by-products 0.1 g 30 The net yield of terephthalic acid based on converted p-toluic acid is 87 mol #. However, discounting the intermediate oxidation products, such as .beta.-carboxybenzaldehyde, the actual yield of terephthalic acid as would be obtained in a continuous process recycling intermediates can be estimated at 97 moles.

7 9 0 7 2 5 2 Λ *.

31

This example clearly demonstrates that according to the invention p-toluic acid can be effectively oxidized to terephthalic acid in high yield even in the presence of p-xylene or other readily oxidizable compounds as a promoter.

5 Example XI

The same autoclave as in the previous examples were charged: p-xylene 3.0 g p-toluic acid 142.6 g 10 different oxygenated derivatives 4.4 g water 150.0 g manganese acetate 1.05 millimole cobalt acetate 1.05 millimole 15 Therefore in this example, the amount of water was 50 parts by weight of the original mixture: y was 7.95 and x was of course 0.50. By using equation (1}, the minimum amount of catalyst to be used is II = 4.9. In fact, in the present example, the catalyst concentration was 7.0 mmol / kg, 20 dvz 2.1 mmol greater than the limit quantity.

Air was supplied to the reactor at a flow rate of 110 l / h under a pressure of 20 kg / cm and the mixture was heated with stirring. When the temperature was about 170 ° C, a small amount of t-butyl hydroperoxide was added to aid in starting the reaction. As a result, oxygen absorption took place immediately. The temperature then rose rapidly and was maintained at 185 ° C by controlled cooling. After 300 minutes of reaction, the acidity or absorption was 22.5 l. The reaction was then quenched and the resulting mixture treated and analyzed as in Example 1. Thus, it was determined that 95.0% of the supplied p-xylene and 40.3% of the supplied p-toluic acid were converted into the following products: terephthalic acid 53.5 g of 4-carboxybenzaldehyde 5.0 g of other intermediates 1, 6 g heavy by-products 1.6 g 7907252 - '32 1 Γ

Comparative Example I

The same experiment as in the previous example was accurately repeated except that half the amount of each metal catalyst was used. The total concentration of catalyst was therefore 3.5 mmol / kg, that is, 1-Λ mmol smaller than the limit amount.

This mixture was heated in the presence of air under exactly the same conditions as in the above example. No oxygen absorption took place at all upon maintaining this mixture at 185 ° C for 160 minutes despite two successive additions of t-butyl hydroperoxide to aid in reaction reaction.

Considering the results obtained in the above example, wherein active and strong oxidation took place in the presence of large amounts of water, it was clear that such oxidation is only feasible if the correct amount of catalyst is used as described according to the invention.

Comparative Example II

The same experiment as in Example XI was repeated except that part of the p-toluic acid was replaced by benzoic acid. The batch composition was as follows: p-xylene 3.0 g p-toluic acid UU, 5 g 25 benzoic acid 93.9 g various oxygenated compounds 2.0 g water 156.6 g manganese acetate 1.05 millimole 30 cobalt acetate 1 0.05 millimole

In. this example was y 26.59; therefore, when using equation (1J), the minimum concentration of catalyst to be used to obtain oxidation of the above mixture is 10.7 mmoles / kg, ie 3.7 mmoles more than the amount actually present. If, on the other hand, benzoic acid is the same had effect as 7907252 "* * 33 p-toluic acid for obtaining oxidation in aqueous medium, this should be taken into account when calculating y and M. In this case we would have: · y = (156.6 / 18.02) / (1.5 / 136.15 + 93.9 / 122.12) = 7.93.5, ie the same value as calculated for y in Example XI, in which active oxidation took place.

The above mixture was heated to 185 ° C under the same conditions as in Example XI and here again some t-butyl hydroperoxide was added to help start the reaction. No significant oxidation occurred even with continued heating for 180 minutes. This shows that at least benzoic acid does not have the same activity as p-toluic acid to promote oxidation in the presence of water as carried out in the process of the invention.

15 Comparative example UI

The same test as in Example XI. repeated except that acetic acid for some of it. p-toluic acid was substituted. The actual batch composition was as follows: -— ..............—-- 20 p-xylene 3.0 g p-toluic acid 52.7 g acetic acid 55.2 g different oxygenated compounds 1.3 g 25 water 187.8 g manganese acetate 1.05 millimole cobalt acetate 1.05 millimole

In this example, y was 26.92, so that M was 1.8, i.e., about the same as in the previous example. Here again, if acetic acid had the same promoting effect as p-toluic acid on the oxidation in aqueous medium, it should be taken into account when calculating y and M. In this case we would have: y = (187.2 / 18.02) / (52.7 / 136.15 + 55.2 / 60.05) = 7.95, ie exactly the same values as in Example XI.

790 7 2 5 2 34

The above described mixture was heated to 185 ° C under the same conditions as in Example XI and here again some t-butyl hydroperoxide was added to help start the reaction. After 300 minutes of reaction, the oxygen absorption was only 5 3.8 l. The result once again shows that the promoting effect of p-toluic acid, as shown in the process of the invention, is not exhibited by other carboxylic acids, ie it is specific for p-toluic acid.

Example XII

In the same autoclave as in the previous examples were charged: 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 millimole

Here y = 4.33 and since manganese was used as the sole catalyst, x = 1.00. Therefore, when using equation (1), the minimum concentration of manganese to be used in this case was M = 3.6 mmol / kg. In fact, in the present example, the concentration of manganese is 6.0 mmol / kg, i.e., clearly above the limit amount.

This mixture was heated in the presence of air.

Under exactly the same conditions as in Example XI, except that the temperature was maintained at 170 ° C instead of 185 ° C. After 305 minutes of reaction, the acidity or absorbance was 21.2. When treating and analyzing the resulting reaction mixture as in the previous examples, it was determined that about 96% of the supplied p-xylene and 29.9% of the supplied 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 35 heavy by-products 1.2 g 7907252 ï 35

Comparative example

The same amounts of compounds as in the previous example were charged to the reactor except that only 0.60 mmol of manganese acetate was used as the catalyst. Its concentration in the mixture was therefore 2.0 mmol / kg, i.e. z. 1.6 mmol less than the limit amount. As a result, no oxygen absorption occurred upon heating the mixture for 60 minutes in the presence of air under the same conditions as in the previous example, even after adding t-butyl-10 hydroperoxide to aid start-up.

Example XIII

Into the same autoclave as in the previous examples were charged: p-xylene 6.0 g 15 p-toluic acid 218.3 g various oxygenated derivatives 2.2 g water 73.5 g cobalt acetate 3.8-8 millimoles. example cobalt used as the sole catalyst, ie x = 0. On the other hand, y was 2.5 ^. By using equation (1), the minimum concentration to be used in this case was cobalt, M is 7.0 mmol / kg. In fact, in the present example, the concentration of cobalt was 11.6 mmol / kg.

Air was supplied to the reactor at a flow rate of 90 l / h under a pressure of 20 kg / cm and the mixture was heated to a temperature of 170 ° C. The reaction started spontaneously while heating and was continued for 2h0 minutes. At the end of the reaction, the amount of oxygen absorbed was 29.8 l. When treating and analyzing the reaction mixture as described in Example 1, it was determined that 09.3% of the p-xylene fed and 37.0% of the supplied p-toluic acid were converted into the following products: terephthalic acid 81.5 g 35 it-carboxybenzaldehyde 6.8 g 7907252 - 36 5 *

t I

-other intermediates 1.2 g heavy by-products 3.0 g

Comparative example

The same amounts of compounds as in the previous example were charged to the reactor except that only 0.90 mmol of cobalt acetate was used as the catalyst. Its concentration in the mixture was therefore 3.0 mmol / kg, i.e., 1.0 mmol less than the limit amount. As a result, no oxygen absorption took place when this mixture was heated for 2 minutes in the presence of air under the same conditions as in the previous example, even after repeated additions of t-butyl hydroperoxide.

Those skilled in the art will recognize that different variations are possible within the scope of the invention, e.g. in carrying out the process, the reaction mixture can be added. one or more foreign compounds are added, used as an activator in other methods, e.g. an aldehyde or ketone, or also a bromine-containing compound. Such additives may even be advantageous with respect to e.g. the reaction speed.

i 7907252

Claims (16)

Process for the preparation of terephthalic acid, characterized in that a) is oxidized in a substantially homogeneous liquid reaction mixture, comprising 5. at least one oxidizable terephthalic acid precursor, selected from the group consisting of p-toluic acid and mixture of p-toluic acid and an oxidisable compound selected from the group consisting of p-xylene, partially oxidized p-xylene derivatives and mixtures thereof, 10. an amount of at least 5% by weight of water sufficient to provide a processable suspension, - an amount of an oxidation catalyst, comprising at least one catalytically active metal compound selected from the group consisting of manganese compounds, cobalt compounds and mixtures thereof, sufficient to provide at least a minimum amount of M mmol of rfp kaf-aiyti sr-ha ^tive: metal compound per kg of the liquid reaction mixture, where M is defined by the equation (1} rliul m (j 20 C * + D where y represents the molar ratio water / p-toluic acid in the reaction mixture, x the molar ratio manganese / total manganese + cobalt Mh m the catalyst mixture, i.e. - + - A = about 0.200 B = about 10.9 C = about U, 35 and D = about 0.072 ^, with a molecular oxygen-containing gas at a reaction temperature of from about 140 to about 220 ° C and under a pressure sufficient to maintain at least a portion of the liquid phase water at the reaction temperature and recover a terephthalic acid-containing end oxidized mixture. 2. Process according to claim 1, characterized in that A = about 0.200 + 0.031 B = about 10.9 + 1.3 C = about 1 +, 35 + 0.17 and D = about 0.0721 + + 0.0117 Process according to claim 1, characterized in that the amount of water in the liquid reaction mixture is 5-80% by weight of the reaction mixture. 10 1+. Process according to claim 3, characterized in that the amount of water in the liquid reaction mixture is 5-75 wt. % of the reaction mixture. Process according to claim 3, characterized in that the amount of water in the liquid reaction mixture is 10-75% by weight of the reaction mixture. Process according to claim 3, characterized in that the amount of water in the liquid reaction mixture is 10-60% by weight of the reaction mixture. 7. Process according to claim 1, characterized in that -20 ~ the reaction mixture further contains 0-100% based on the amount of water of an inert organic solvent. Process according to claim 7S, characterized in that the organic solvent contains acetic acid. 9. Process according to claim 1, characterized in that the reaction temperature is about 150 to about 190 ° C. Process according to claim 1, characterized in that the reaction pressure is 5-1 + 0 kg / cm. 11. Process according to claim 1, characterized in that the amount of oxidation catalyst is sufficient to provide an amount of catalytically active compounds lying between the minimum amount as defined in claim 1 and about 1 + 0 mmol / kg reaction mixture. 12. Process according to claim 1, characterized in that the amount of oxidation catalyst is sufficient to provide an amount of catalytically active compounds lying between 7907252 - -¾ A- the minimum as defined in claim 1 and about 30 mmol / kg reaction mixture el. 13- Process according to claim 1, characterized in that the catalyst contains a mixture of a manganese compound and cobalt compound-5 "bond. 1U. Process according to claim 13, characterized in that x is 0.1 to about 0.9. 15. A method according to claim 1, characterized in that the catalytically active metal compound is a metal salt of a carboxylic acid. Process according to claim 1, characterized in that the catalyst further contains a compound of a metal selected from the group consisting of nickel, lead and cerium. 17. Process according to claim 1, characterized in that the molar ratio p-toluic acid / p-xylene is about 3-15. 7907252 / p-TA / \ 1 85 * C / p-Xyt./1—------^H2$ \ 7 9 0 7 2,5 2 # LABOPIXA S.A.