JPS5921849B2 - Production method of benzaldehyde - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the production of benzaldehyde by liquid phase oxygen oxidation of toluene.

In particular, the present invention relates to a method for producing benzaldehyde with high selectivity. It is well known that benzaldehyde is produced when toluene is oxidized with oxygen in the liquid phase.

However, in the conventional method, the main product is benzoic acid, and the selectivity for benzaldehyde is only about 10 to 20% even when the conversion rate of toluene is low. The present inventors researched a method for increasing the selectivity of benzaldehyde and found that when the reaction is carried out in a carboxylic acid solvent in the presence of a catalyst containing cobalt or cobalt and manganese and bromine,
It has been found that increasing the concentration of catalytic metal in the reaction solution improves the selectivity of benzaldehyde.

It has also been found that in this case, a sufficient reaction rate can be obtained when the atomic ratio of bromine to the catalyst metal atoms is 0.4 or less. The present invention was completed based on such knowledge, and provides a method for producing benzaldehyde with good selectivity even when the reaction rate of toluene is high.

More specifically, the present invention relates to an improved method for oxidizing toluene with molecular oxygen in a carboxylic acid solvent in the presence of a catalyst consisting of a heavy metal and bromine by a conventional method.

The reaction is carried out in a carboxylic acid solvent, and usually an aliphatic carboxylic acid such as acetic acid or an aromatic carboxylic acid such as benzoic acid is used.

In particular, it is preferable to use benzoic acid produced in the reaction as a solvent. In the method of the present invention, the reaction is carried out in a state with a high concentration of catalyst metal, so it is economically necessary to recover the catalyst. However, when benzoic acid is used as a solvent, unreacted toluene, benzyl alcohol, and benzaldehyde are removed from the reaction solution. It is possible to recover the benzoic acid by distillation, distill off a portion of the benzoic acid, and circulate the remaining benzoic acid, benzyl benzoate, and the distillation residue containing the catalyst as it is to the reaction system. The carboxylic acid solvent is 0.5 to 5 times (weight ratio) to toluene, especially 1 to 3 times
It is preferable to use twice (weight ratio). The catalyst in the process of the invention is cobalt and bromine or a combination of cobalt, manganese and bromine.

Although the reaction is difficult to proceed with the combination of manganese and bromine, when a small amount of cobalt is used in combination with manganese, manganese comes to exhibit the same catalytic effect as cobalt. Since cobalt is significantly more expensive than manganese, it is economically advantageous to use manganese as a catalyst in combination with cobalt. Cobalt is preferably used in combination with manganese in an atomic ratio of 0.1 or more. Cobalt or cobalt and manganese are added to the reaction solution in an amount of 1000 ppm or more as metals.

It is preferably added in an amount of 3,000 to 50,000 ppm. As shown in FIG. 1, the catalytic metal concentration has a close relationship with the benzaldehyde selectivity, and generally the higher the catalytic metal concentration, the better the benzaldehyde selectivity. The catalyst metal is usually added to the reaction solution as a salt of an aliphatic carboxylic acid such as acetic acid or propionic acid, an alicyclic carboxylic acid such as naphthenic acid, an aromatic carboxylic acid such as benzoic acid, or acetylacetone.

It can also be added as a hydroxide, oxide, or the like. Most preferably it is added as the acetate or benzoate salt. Bromine, the other component of the catalyst, is used in an atomic ratio of 0.4 or less to cobalt or cobalt and manganese.

Bromine has the following equilibrium in the reaction solution: Br+R-COOH==:=±R-COO-+HBrThe generated hydrogen bromide is easily gasified and escapes from the reaction solution.

Therefore, when a high concentration of catalytic metal is used as in the method of the present invention, if bromine is present in an amount equivalent to the catalytic metal, a large amount of bromine escapes, which impairs economic efficiency. The presence of high concentrations of bromine also promotes corrosion of the equipment and the oxidation of benzaldehyde to benzoic acid. The inventors investigated the effects of bromine and found that the reaction is difficult to proceed in the absence of bromine, but if a small proportion of bromine is present relative to the catalyst metal, the reaction proceeds satisfactorily. It has been found that benzaldehyde can be obtained with selectivity.

Therefore, bromine is used in an atomic ratio of 0.4 or less to the catalyst metal. In particular, it is preferable to use a ratio of 0.06 to 0.30. Bromine is usually added to the reaction solution as hydrogen bromide or a salt such as sodium bromide, potassium bromide, ammonium bromide, or the like. It is also possible to use organic bromine compounds. If cobalt bromide or manganese bromide is used, bromine and a portion of the catalyst metal can be added at the same time. The reaction is carried out by blowing oxygen or oxygen diluted with an inert gas, especially air, into the reaction solution according to the conventional method of liquid phase oxygen oxidation reaction.

The reaction proceeds easily at temperatures above 100°C, but
It is usually carried out at a temperature of 120 to 220°C, particularly 140 to 200°C. The reaction pressure is usually 20 atmospheres or less, particularly 10 atmospheres or less. Since this reaction is an exothermic reaction, it is preferable to operate so that the heat of reaction is removed by the latent heat of vaporization of toluene, and therefore the reaction conditions are preferably selected so that toluene is easily vaporized. When a predetermined reaction rate is reached, the reaction solution is distilled to recover unreacted toluene, and then benzaldehyde and by-produced benzyl alcohol, benzoic acid, etc. are recovered.

When benzoic acid is used as the reaction solvent, the reaction solution is flashed to evaporate some of the toluene, benzaldehyde, benzyl alcohol and benzoic acid, and the evaporation residue containing the catalyst is recycled to the reactor. The evaporated fraction is then rectified and separated into each component. According to the present invention, benzaldehyde can be produced from toluene with high selectivity.

EXAMPLES Next, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples.

Examples 1 to 3, Comparative Examples 1 to 5 Internal volume 10 equipped with a stirrer, gas inlet, and condenser
To a 0ml titanium reactor was added 13.2f7 toluene, 26.4y1 benzoic acid, and cobalt acetate, manganese acetate and sodium bromide.

The reaction was carried out at 160°C while stirring and blowing air into the reactor. After the reaction was completed, the reactor was cooled and the reaction solution was analyzed. The results are shown in Table 1. Further, the results in Table 1 are organized into the relationship between catalyst metal concentration and benzaldehyde selectivity and are shown in FIG.

Examples 4 to 7 Toluene 757, acetic acid or benzoic acid 75f7, and cobalt acetate (9000 pp as cobalt based on the reaction solution) were placed in a titanium reactor with an internal volume of 300 m1 equipped with a stirrer, gas inlet, condenser, and liquid outlet.
m) and sodium bromide (1/1 of cobalt as bromine)
0 atomic ratio) was prepared.

20f of oxidizing gas in the reactor! /Hr, and the reaction was carried out at 160° C. with stirring until no oxygen was absorbed.

During the reaction, about 5 ml of reaction liquid was drawn out from the liquid outlet and its composition was analyzed. Table 2 shows the conversion rate of toluene and the selectivity of the reaction product when the yield of benzaldehyde is maximum for each example. Further, for Example 4, changes in the composition of the reaction solution as the reaction progresses are shown in FIG.

As is clear from FIG. 2, the by-produced benzyl benzoate is ultimately oxidized to benzoic acid, so it does not pose an obstacle to the circulation of the catalyst liquid.

[Brief explanation of the drawing]

FIG. 1 shows an example of the relationship between the concentration of catalyst metal in the reaction solution and the selectivity of benzaldehyde. FIG. 2 shows an example of changes in the composition of the reaction solution as the reaction progresses. 1...Toluene, 2...
...benzoic acid, 3...benzaldehyde, 4.
...Benzyl alcohol, 5...Benzyl benzoate.

Claims (1)

[Scope of Claims] 1. In producing benzaldehyde by oxidizing toluene with molecular oxygen in a carboxylic acid solvent in the presence of cobalt or a catalyst containing cobalt and manganese and bromine, the amount of cobalt and manganese in the reaction solution is Total amount 1000p
pm or more and the atomic ratio of bromine to the total amount of cobalt and manganese atoms is 0.4 or less. 2. The method according to claim 1, characterized in that the atomic ratio of cobalt to manganese is 0.1 or more. 3. The method according to claim 1 or 2, wherein the carboxylic acid solvent is benzoic acid.