JPS6123781B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is described in Japanese Patent No. 235515 (see Japanese Patent Publication No. 32-2765), Japanese Patent No. 286284 (see Japanese Patent Publication No. 7022-1977), and Patent No. 470886 (see Japanese Patent Publication No. 40-23493). As in, p-xylene, p-
one or more compounds selected from the group consisting of methyl toluate and p-tolualdehyde,
When dimethyl terephthalate is produced by oxidation and methyl esterification, the crude dimethyl terephthalate separated from the esterification reaction mixture is purified by recrystallization, etc., and it itself has little utility value. The present invention relates to a method for recovering effective components from purified residues. Here, the term "active ingredient" refers to a substance that forms dimethyl terephthalate through oxidation and/or esterification, such as p-xylene, methyl p-toluate, and methyl p-(methoxymethyl)-benzoate. It refers to compounds that can be separated relatively easily from dimethyl isophthalate and dimethyl phthalate. As methods for recovering useful compounds from such purification residues, only the following two or three methods have been proposed. For example, a method for obtaining methyl p-formylbenzoate by reacting the purified residue with acidic sodium sulfite (JP-A-50-71640 and JP-A-Sho.
50-71641), but this method is complicated and difficult to implement industrially using acidic sodium sulfite. A method has also been proposed in which the active ingredient is recovered by simply heating (see the specification of Japanese Patent Publication No. 46-6412). This method recovers a small amount of active ingredient. Furthermore, the main component increased by this method is dimethyl terephthalate, which is difficult to separate from the purified residue after the reaction. The present inventors conducted intensive research to find a method that is more advantageous than these methods and can recover a larger amount of active ingredients, and found that the purified residue was brought into contact with hydrogen in the presence of a hydrogenation catalyst. In particular, it has been found that active ingredients such as methyl p-toluate can be recovered in large quantities, leading to the present invention. That is, the present invention provides 1 selected from the group consisting of methyl p-toluate and p-tolualdehyde.
Or when producing dimethyl terephthalate by esterifying with methanol a reaction mixture obtained by oxidizing two or more compounds in the liquid phase with molecular oxygen or molecular oxygen-containing gas in the presence of a heavy metal catalyst. The crude dimethyl terephthalate separated from the reaction mixture is purified to obtain high-purity dimethyl terephthalate.By contacting the purified residue obtained in the purification step with hydrogen in the presence of a hydrogenation catalyst, the purified residue is purified. This is a method of recovering components. The purification residue targeted by the present invention is, for example, as shown below, oxidized one or more compounds selected from the group consisting of p-xylene, methyl p-toluate, and p-tolualdehyde, and then An oxidation-esterification byproduct remaining in the recrystallization mother liquor obtained after distilling the ester mixture obtained by esterification to obtain crude dimethyl terephthalate, which is further purified by recrystallization. It is a concentrate of (a) When producing dimethyl terephthalate from p-xylene by the so-called four-step process described in Patent No. 235515, p-xylene is first oxidized in a liquid phase with an oxygen-containing gas in the presence of a heavy metal catalyst. This was then esterified with methanol to obtain an oxidation product containing p-toluic acid as the main component, and the resulting methyl p-toluate was again oxidized and esterified in the same manner as above to yield dimethyl terephthalate. How to get it. (b) When producing dimethyl terephthalate from p-xylene by the so-called two-step process described in Patent No. 286284, which is an improved method of (a), first, p-xylene and methyl p-toluate are is subjected to liquid phase oxidation with oxygen-containing gas in the presence of a heavy metal catalyst to obtain p-toluic acid and monomethyl terephthalate, which is then esterified with methanol to mainly produce methyl p-toluate and dimethyl terephthalate. A method of obtaining an ester mixture as a component and separating it into dimethyl terephthalate and methyl p-toluate. (c) An improved two-stage method of (b) above, in which the mixture of p-xylene and methyl p-toluate is partially oxidized in the liquid phase with a molecular oxygen-containing gas in the first stage. Then, in the second and subsequent stages, p-xylene is further added to perform oxidation continuously, the resulting oxidation product is esterified with methanol, and dimethyl terephthalate is separated from the reaction mixture thus obtained. The method according to No. 318382, etc. (d) p by the method described in Patent No. 470886
- When producing dimethyl terephthalate from tolualdehyde, first, a mixture of p-tolualdehyde and methyl p-toluate is oxidized in a liquid phase with an oxygen-containing gas in the presence of a heavy metal catalyst, and p-
An oxidation product containing toluic acid and monomethyl terephthalate as main components is obtained, which is then esterified with methanol to obtain an ester mixture containing p-methyl toluate and dimethyl terephthalate as main components, which is then mixed with dimethyl terephthalate. A method for separating methyl p-toluate. In these methods, by-products remain in the recrystallization mother liquor obtained when the crude dimethyl terephthalate obtained from the ester mixture by distillation or the like is further purified by recrystallization. This refinery residue is dimethyl phthalate, which is produced by the oxidation and esterification of ortho and/or meta isomers contained in raw materials such as p-xylene, methyl p-toluate, and p-tolualdehyde used in the oxidation reaction. and/or contains intermediate products such as methyl p-formylbenzoate and dimethyl acetal of methyl p-formylbenzoate in addition to dimethyl isophthalate. Since these isomers and intermediate products have similar molecular weights and boiling points to dimethyl terephthalate, it is very difficult to separate them from dimethyl terephthalate by distillation. Since this purified residue still contains a considerable amount of dimethyl terephthalate, recrystallization is repeated and dimethyl terephthalate is recovered to obtain a purified residue containing less dimethyl terephthalate. This contains methyl p-formylbenzoate, dimethyl acetal of methyl p-formylbenzoate, and some dimethyl terephthalate, and it is thought that it can be recovered by recycling it to the oxidation process, but in reality, if it is recycled, phthalate Dimethyl and dimethyl isophthalate accumulate, and dimethyl phthalate cannot be recycled to the oxidation step because it has the effect of lowering the oxidation yield. Therefore, the above-mentioned refining residue was conventionally disposed of as an unavoidable loss in the production of dimethyl terephthalate or was used as fuel, but according to the present invention, isophthalic acid such as methyl p-toluate can be produced from such refining residue. This makes it possible to recover active ingredients that can be easily separated from dimethyl and dimethyl phthalate. The purified residue targeted by the present invention is not only concentrated during separation and purification of dimethyl terephthalate as described above, but also such by-products even if separated and concentrated from dimethyl terephthalate by other methods. It can be used in the same way as long as it includes things. Although the details of the composition of the purification residue targeted by the present invention are not clear, examples include dimethyl phthalate, dimethyl isophthalate, dimethyl terephthalate, trimethyl trimellitate, methyl p-formylbenzoate, and p-formylbenzoic acid. It contains a wide variety of complex compounds, including methyl dimethyl acetal, and in addition to these, it also contains large amounts of high-boiling byproducts and compounds with unknown structures. By contacting with hydrogen according to the present invention, methyl p-formylbenzoate and dimethyl acetal of methyl p-formylbenzoate in the above-mentioned purified residue can be converted into methyl p-toluate or p-
It can be converted to methyl (methoxymethyl)benzoate and separated from other substances simply by distillation. Furthermore, according to this method, it is possible to recover a larger amount of active ingredients such as methyl p-toluate than would be produced simply by reducing methyl p-formylbenzoate and dimethyl acetal of methyl p-formylbenzoate.
A larger amount of the active ingredient can be recovered compared to the method of simply heating as described in the above-mentioned Japanese Patent Publication No. 46-6412. The method of the present invention can be carried out either batchwise or continuously. For this reactor, any method may be used as long as the hydrogen and by-product concentrate can come into contact safely and nuclear hydrogenation does not substantially occur, but for example, a granular hydrogenation catalyst may be used. Fixed catalyst permeate packed column reactors are preferred. In the present invention, the reaction pressure varies depending on the type of catalyst and the presence or absence of additives, but the partial pressure of hydrogen is 1
It is desirable to carry out at ~100Kg/ ^cm2 , preferably 1 to 35Kg/ ^cm2 . In general, hydrogen partial pressures exceeding 35 Kg/cm ² , particularly 100 Kg/cm ² are undesirable because aromatic hydrogenation tends to occur. In the present invention, it is advantageous to carry out the treatment at a temperature of 80 to 350°C, preferably 120 to 330°C. If the temperature is low, the reaction will be slow, and if the temperature is too high, undesirable side reactions will occur and the amount of active ingredient recovered will be reduced. Although it is not necessary to use a solvent in the present invention, for example, a solvent such as alcohols such as methanol, lower fatty acids such as acetic acid, or methyl benzoate may be used. In particular, when methanol is used as a solvent, p-toluic acid, which is produced in small amounts when reacted without a solvent, can be simultaneously esterified and easily recovered by appropriately selecting conditions within the above pressure and temperature range. I can do it. Of course, it is also possible to react without a solvent and then treat with methanol. The amount of solvent used here is industrially preferably 1% by weight or more in terms of the concentration of the purified residue. Although it is not necessary to add an acid in the present invention, for example, if a lower fatty acid such as acetic acid is used as an additive or solvent, methyl p-(methoxymethyl)benzoate is further reduced to methyl p-toluate. It becomes an easily separable component. The treatment of the by-product concentrate in the present invention is carried out in the presence of a hydrogenation catalyst, and preferred hydrogenation catalysts include palladium, rhodium nickel and copper-chromium catalysts, among which nickel and copper-chromium catalysts are preferred. Particular preference is given to palladium, especially metallic palladium. When palladium is used as a hydrogenation catalyst, it is preferably supported on a support, and it is particularly advantageous to use it supported on activated carbon or alumina. In this case palladium is 0.01 by weight
It is more preferable to carry it in a range of 15% to 15%. Further, when a nickel catalyst is used as a hydrogenation catalyst, for example, a nickel-phosphorous catalyst prepared as follows is particularly effective. When nickel chloride and sodium hypophosphite are dissolved in water and heated to the boiling point, a black powder is produced while generating hydrogen. This is washed with water and dried to obtain a catalyst (method described in Industrial Chemistry Magazine 71, 626 (1968)). The method of the present invention will be described in detail below with reference to Examples. Example 1 A mixture of p-xylene and methyl p-toluate in the presence of cobalt acetate and manganese acetate,
Liquid phase oxidation with air at 165℃ and 4Kg/cm ² G
- An oxidation product containing toluic acid and monomethyl terephthalate as main components was obtained, which was then esterified with methanol to obtain an ester mixture containing p-methyl toluate and dimethyl terephthalate as main components (JP-B No. 27574 (method described in the specification). This ester mixture was distilled to obtain crude dimethyl terephthalate, which was recrystallized from methanol to purify dimethyl terephthalate. By-products having boiling points close to dimethyl terephthalate remain in the recrystallization mother liquor generated at this time. When the methanol of this recrystallization mother liquor is distilled off, a residue remains, which still contains a considerable amount of dimethyl terephthalate. This residue was further recrystallized several times from methanol to reduce the amount of dimethyl terephthalate as much as possible. The purified residue thus obtained contained 12.4% dimethyl terephthalate, 41.7% dimethyl isophthalate, and 41.7% dimethyl phthalate, and 4.8% methyl p-formylbenzoate and 14.3% dimethyl acetal of methyl p-formylbenzoate. there was. p-
Xylene is 0.7%, methyl p-toluate is 0.5%
It contained only 200 g of this purified residue was placed in a stainless steel autoclave equipped with a refluxer, a stirrer, and a gas inlet, and 1 g of 5% palladium/carbon powder was added, and the mixture was stirred at high speed at 250°C and a pressure of 10 Kg/cm ² G. Hydrogen gas was blown in so that the flow rate at the outlet was 100 c.c./min. Reaction was allowed to take place in this state for 0.5 hour. After the reaction was completed, the product was taken out and its weight and composition were analyzed. The product was 201g, of which p-
xylene 1.2%, methyl p-toluate 12.2%,
The concentration of methyl p-(methoxymethyl)benzoate was 11.8%. The contents of dimethyl terephthalate, dimethyl isophthalate, and dimethyl phthalate did not change. 100g of this product was placed in a three-necked flask with a rectifying tube and distilled under reduced pressure at 60mmHg.
I got g. This fraction contains 1.2 g of p-xylene, p-
It contained 13.1 g of methyl toluate and 8.3 g of methyl p-(methoxymethyl)benzoate. Examples 2 and 3 50 g of the purified residue obtained in exactly the same manner as in Example 1 and 150 g of methanol were placed in the same autoclave as in Example 1, 1 g of 5% palladium/carbon powder was added, and the temperature was 150°C and the pressure was 10 Kg/cm. While stirring at a high speed of ² G, hydrogen gas was blown in so that the flow rate at the outlet was 100 c.c./min, and the reaction was allowed to proceed for 0.5 hours. After the reaction was completed, the product was taken out, methanol was distilled off, and then analyzed in the same manner as in Example 1. (Example-2)
The results are shown in Table-1. Further, an experiment was carried out in exactly the same manner as in Example 2, except that 10 g of acetic acid was added. (Example 3) The results are also shown in Table 1.

[Table] Example 4 200 g of the purified residue obtained in the same manner as in Example 1 was placed in the same autoclave as in Example 1, 1 g of 50% nickel/diatomaceous earth was added, the temperature was 200°C, the pressure was
While stirring at high speed at 10Kg/cm ² G, the flow rate at the outlet is 100%.
Hydrogen gas was blown in at a rate of cc/min, and the reaction was allowed to proceed for 1 hour. The reaction product was removed and analyzed for its weight and composition. The product was 203 g, of which 1.0% p-xylene, 13.5% methyl p-toluate, and 9.2% methyl p-(methoxymethyl)benzoate. The contents of dimethyl phthalate, dimethyl isophthalate, and dimethyl phthalate remained unchanged.

Claims (1)

[Scope of Claims] 1. One or more compounds selected from the group consisting of p-xylene, methyl p-toluate, and p-tolualdehyde are mixed with molecular oxygen or When producing dimethyl terephthalate by esterifying a reaction mixture obtained by oxidation with a molecular oxygen-containing gas with methanol, the crude dimethyl terephthalate separated from the esterification reaction mixture is purified to a high purity. A method for recovering an active ingredient from a purification residue, which comprises contacting the purification residue obtained in a purification step for obtaining dimethyl terephthalate with hydrogen in the presence of a hydrogenation catalyst. 2. The method according to claim 1, in which palladium is used as the hydrogenation catalyst. 3. The method according to claim 1, in which the contact is carried out in the presence of methanol. 4. The method according to claim 1, wherein the purified residue is brought into contact with methanol after the contact. 5. The method according to claim 1, in which the contact is carried out in the presence of a lower fatty acid.