JP2003342226A - Method for producing glyoxylic acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an aqueous solution of glyoxylic acid having an excellent hue and storage stability. <P>SOLUTION: This method for producing glyoxylic acid comprises subjecting glyoxal, an alcohol and molecular oxygen as raw materials to an oxidative esterification reaction in the presence of a catalyst to give an oxidative esterification reaction solution containing a glyoxylic acid ester, obtaining a fraction containing the glyoxylic acid ester from the oxidative esterification reaction solution in a distillation process and subjecting the fraction to a hydrolysis reaction. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing glyoxylic acid by hydrolyzing a glyoxylic acid ester. Glyoxylic acid is a compound industrially useful as an intermediate raw material for various products such as pharmaceutical intermediates, cosmetics, fragrances and agricultural chemicals.

[0002]

As a method for producing glyoxylic acid by hydrolysis of glyoxylic acid ester, a method of countercurrently treating glyoxylic acid ester hemiacetal with steam in a cascade type reactor (JP-A-3-20239),
Method of reactive distillation in the presence of water (Japanese Patent Laid-Open No. 11-3353)
20) and the like.

[0003]

However, the glyoxylic acid ester is a glyoxylic acid ester obtained by an oxidative esterification reaction from glyoxal, alcohol and molecular oxygen, and the glyoxylic acid ester is hydrolyzed using the oxidative esterification reaction solution. When decomposed, there are many impurities in the obtained glyoxylic acid aqueous solution, and the aqueous solution is colored, or coloring occurs over time during storage, which is unsuitable as a raw material for pharmaceutical intermediates and fragrances. is there.

[0004]

Means for Solving the Problems The inventors of the present application have conducted extensive studies to achieve the above-mentioned object, and as a result, after performing a distillation step after an oxidative esterification reaction and then performing a hydrolysis reaction, coloring It was found that an aqueous glyoxylic acid solution having a small amount of storage and good storage stability was obtained, and the present invention was completed.

That is, according to the present invention, glyoxal, alcohol and molecular oxygen are used as raw materials, an oxidative esterification reaction is carried out in the presence of a catalyst, and the obtained liquid containing glyoxylic acid ester is hydrolyzed to give glyoxylic acid. A method of producing, wherein a step of distilling the reaction solution obtained by the oxidative esterification reaction is provided before hydrolysis, and the content of the glyoxylic acid ester distilled in the step of distilling is 99% by mass.
The present invention relates to a method for producing glyoxylic acid with little coloring, which comprises hydrolyzing the following fractions as raw materials.

Further, the present invention is characterized in that the distillation rate of the glyoxylic acid ester in the distillation step is 70 to 99% by mass.

Furthermore, the present invention is characterized in that the water contained in the oxidative esterification reaction liquid used in the distillation step is 3 mol times or less with respect to the alcohol.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing glyoxylic acid according to the present invention will be described in detail below. The present invention comprises (a) an oxidative esterification reaction, (b) a distillation step, (c)
A method for producing glyoxylic acid, which comprises three steps of a hydrolysis reaction.

(A) Oxidative esterification reaction In the oxidative esterification reaction, a glyoxylic acid ester is synthesized in the presence of a catalyst, using glyoxal, alcohol and molecular oxygen as raw materials.

Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butanol and ter.
t-butanol, hexanol, cyclohexanol,
Examples of the industrially available alkyl alcohol having 1 to 18 carbon atoms such as octanol, 2-ethylhexanol, lauryl alcohol, and stearyl alcohol.

Of the above examples, preferred are alkyl alcohols having 1 to 4 carbon atoms such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butanol and tert-butanol. .

The glyoxylic acid ester produced differs in the alkyl group of the ester group portion depending on the alcohol used. Specific examples thereof include glyoxylic acid methyl ester, glyoxylic acid ethyl ester, glyoxylic acid normal propyl ester, glyoxylic acid isopropyl ester, glyoxylic acid normal butyl ester, and glyoxylic acid tertiary butyl ester.

The glyoxylic acid ester easily forms a glyoxylic acid ester hydrate upon contact with water, and exists stably. Also, when it comes into contact with alcohol, it easily forms glyoxylic acid ester alkyl hemiacetal,
Exists stably. These hydrates and alkyl hemiacetalizations are reversible reactions and can take any form depending on the coexisting substance with glyoxylic acid ester.
It may also be present as a mixture. However, in the hydrolysis reaction, glyoxylic acid ester, glyoxylic acid ester hydrate, and glyoxylic acid alkyl hemiacetal exhibit similar behavior and can be treated in the same manner.

Therefore, the glyoxylic acid ester in the present invention includes all of free glyoxylic acid ester, glyoxylic acid ester hydrate and glyoxylic acid ester alkyl hemiacetal.

The oxidative esterification reaction proceeds under any conditions of gas phase and liquid phase, and both can be adopted in the present invention.

As the oxidative esterification reaction by the gas phase reaction, for example, 1,2-diol ethylene glycol is subjected to gas phase oxidation (oxidative dehydrogenation) with a molecular oxygen-containing gas in the presence of a catalyst such as metallic silver. ) To obtain gaseous glyoxal (pre-reaction), and then add alcohol in the form of gas in the vaporization chamber to introduce it into the post-reactor connected to the pre-reactor. Examples thereof include a method of obtaining a glyoxylic acid ester by gas-phase oxidation (oxidative esterification) with a molecular oxygen-containing gas in the presence of a contained catalyst and the like. The generated reaction gas can be cooled or
By absorbing it in a solvent, it can be obtained as an oxidative esterification reaction liquid.

Also in the liquid phase reaction, glyoxal and alcohol are kept in the liquid phase, and molecular oxygen is allowed to coexist in the presence of a catalyst containing gold, palladium or the like to perform an oxidative esterification reaction to form a glyoxylic acid ester. You can also get it.

(B) Distillation step In the reaction liquid obtained from the above-mentioned glyoxal, alcohol and molecular oxygen by the oxidative esterification reaction in the presence of a catalyst, in addition to the target product, glyoxylic acid ester, , Glycolate and oxalate,
Further, by-products such as heavy components are included. When the reaction solution is hydrolyzed as it is, clear coloring is observed in the obtained glyoxylic acid aqueous solution. this is,
In some cases, a coloring component contained in the oxidative esterification reaction liquid is concentrated and colored, and in other cases, a by-product contained in the reaction liquid is changed into a coloring component by the hydrolysis step.

The coloring component and the component that causes coloring by the hydrolysis step are heavier than the glyoxylic acid ester, and by removing / reducing them before hydrolysis, the coloring of the product glyoxylic acid aqueous solution can be prevented. It can be remarkably reduced, and coloring over time can be suppressed.

The purpose of the distillation step in the present invention is to distill off the glyoxylic acid ester contained in the oxidative esterification reaction solution and remove the coloring component and the component that causes coloring in the hydrolysis step.

A general method can be used for the distillation step in the present invention. Examples of the distillation method include simple distillation, rectification using a multistage distillation column, natural circulation evaporation, forced circulation evaporation, and liquid film evaporation. The distillation step can be carried out in any of a batch system, a semi-batch system, and a continuous system.
Of the above examples, liquid film evaporation is preferable because the thermal history of the oxidative esterification reaction liquid can be reduced. Liquid film type evaporation includes forms such as a rising liquid film type, a falling liquid film type, and a stirring liquid film type, and any of them can be preferably implemented.

In the distillation step, it is not necessary to purify the glyoxylic acid ester with high purity. In order to obtain a high-purity glyoxylic acid ester, the utility cost and the equipment cost may increase, and the glyoxylic acid ester obtained as a fraction may decrease. Therefore, the following hydrolysis reaction may be carried out using a fraction containing water or alcohol, which has a lower boiling point than that of the glyoxylic acid ester. Moreover, since water is required in the hydrolysis step, it is not necessary to separate it in the distillation step. Therefore, the content of methyl glyoxylate in the fraction obtained in the distillation step is sufficient to be 99% by mass or less, and more preferably 97% by mass or less.

In the oxidative esterification reaction liquid, in addition to the glyoxylic acid ester, alcohol which is an unreacted raw material and water which is a by-product are present. In the presence of water, the glyoxylic acid ester hydrolyzes into glyoxylic acid and alcohol.

When the glyoxylic acid ester is hydrolyzed in the distillation step, the glyoxylic acid produced is not distilled out because of its low vapor pressure, but is extracted together with the heavy components.
In that case, the glyoxylic acid ester obtained as a distillate is reduced, which is not preferable. Therefore, it is preferable to carry out the distillation step under the condition that the hydrolysis of glyoxylic acid ester can be suppressed as much as possible.

The distillation conditions may be atmospheric pressure, reduced pressure or increased pressure. The temperature is preferably 200 ° C. or lower. The retention time of the oxidative esterification reaction liquid is preferably within 1 hour, more preferably within 20 minutes, even more preferably within 5 minutes.

The water / alcohol molar ratio in the oxidative esterification reaction liquid used in the distillation step is preferably 3 or less, more preferably 2 or less, and even more preferably 1 or less. If the molar ratio is greater than 3, the glyoxylic acid ester may be hydrolyzed during the distillation process, and the amount of glyoxylic acid that can be obtained as a product may decrease.

When the amount of water in the oxidative esterification reaction solution is large, a step of removing water may be provided in advance before the distillation step. Examples of the step of removing water include evaporation, distillation, adsorption and the like.

When water is removed by evaporation or distillation, an azeotropic agent such as benzene, toluene or cyclohexane may be used, or when the alcohol forms an azeotropic mixture with water such as ethanol, propanol or butanol. You may extract with those alcohols.

As the water adsorbent, molecular sieve,
Examples thereof include sodium sulfate and calcium chloride. Water may be adsorbed and removed by bringing these adsorbents into contact with the oxidative esterification reaction liquid. The adsorbent may be reused after removing water.

In the distillation step, it is desirable to remove components heavier than glyoxylic acid ester as much as possible. However, if the distilling rate, which is the ratio of the distilled glyoxylic acid ester to the supplied glyoxylic acid ester, is reduced, the yield of glyoxylic acid in the product decreases, which is not economical. On the other hand, when the distillation rate of the glyoxylic acid ester is too large, the heavy component is not sufficiently separated, and the heavy component is mixed in the fraction, so that the coloring may not be sufficiently reduced.
Therefore, the distillation rate of glyoxylic acid ester is 70 to 99.
It is preferably in the range of mass%, more preferably 8
It is 0 to 98 mass%.

(C) Hydrolysis reaction The hydrolysis reaction in the present invention is not particularly limited as long as it can hydrolyze a glyoxylic acid ester to produce glyoxylic acid. The hydrolysis of the ester proceeds with either an acid catalyst or a base catalyst.

In the case of hydrolysis using an acid catalyst, the ratio of water to glyoxylic acid ester varies depending on the type of glyoxylic acid ester and reaction conditions, but is generally equimolar to 200-fold molar, and preferably 2 times mole to 10
It is 0 times the mole. If it is less than this molar amount, water may be insufficient, and if it is more than 200 times by mole, the concentration of glyoxylic acid after the reaction is low, and it may be costly to concentrate.

The reaction temperature is suitably 30 ° C to 300 ° C, preferably 50 ° C to 250 ° C. The reaction pressure can be any of atmospheric pressure, increased pressure, and reduced pressure.

The form of the hydrolysis reaction may be any of continuous type, batch type and semi-batch type, and is not particularly limited.

When the hydrolysis is carried out under acidic conditions, the reaction is an equilibrium reaction, and the conversion rate is governed by the equilibrium. Therefore, if it is desired to increase the conversion rate, it is necessary to move the equilibrium to the product side. As means for improving the conversion rate, there are methods such as increasing the amount of water used and performing hydrolysis while extracting alcohol and glyoxylic acid produced by reactive distillation or extraction from the system.

When the hydrolysis is carried out by reactive distillation, the alcohol produced as a by-product may be extracted from the top of the column for the reaction. When alcohol has a lower boiling point than water, such as methanol, methanol can be concentrated and distilled by distillation. When the alcohol is an alcohol that is azeotropic with water, such as ethanol, propanol, butanol, etc., it can be distilled with an azeotropic composition, but in order to move the equilibrium to the product side, it is necessary to use water in the bottom liquid. It is necessary to control the raw material composition so that the ratio of alcohol is larger than that of the azeotropic composition. Reactive distillation is batch type, semi-batch type,
It can be implemented in any form of continuous type.

When the starting alcohol, such as butanol or hexanol, forms two liquid phases with water, hydrolysis and extraction can be combined to shift the equilibrium to the product side. When the raw material alcohol forms two liquid phases with water, the glyoxylic acid ester has low solubility in water and is mainly dissolved in the organic phase (alcohol phase). On the other hand, glyoxylic acid has a low solubility in alcohol and is easily soluble in water. Therefore, in the fraction containing the alcohol and glyoxylic acid ester obtained in the distillation step,
By adding water in an amount such that the organic phase and the aqueous phase are separated into two phases, the resulting glyoxylic acid is extracted from the organic phase to the aqueous phase at the same time, resulting in hydrolysis in the organic phase. The reaction equilibrium can be transferred to the product side.

A hydrophobic solvent may be added to the fraction obtained in the distillation step to form a two liquid phase. Examples of the hydrophobic solvent include benzene, toluene, cyclohexane, hexane, octane and methyl isobutyl ketone.

In addition, in hydrolysis under acidic conditions, the equilibrium can be moved to the product side by a method of adding a basic compound while maintaining the pH of the reaction solution acidic. This is because glyoxylic acid forms a salt with a basic compound, which reduces the concentration of free glyoxylic acid and shifts the equilibrium of the hydrolysis reaction.

When the basic compound is added, it is preferable to stir the reaction solution to reduce the bias of pH in the reaction solution. If the stirring is not performed, the pH around the added basic compound is high, and there is a possibility that a side reaction will proceed in that portion. The method of stirring the reaction solution is not particularly limited, and examples thereof include a method using a stirring blade, a method in which the reaction solution is forcedly circulated by a reboiler, a method in which an inert gas is bubbled, and the like.

When the hydrolysis step is carried out under acidic conditions, it does not matter whether a catalyst is used or not. The glyoxylic acid produced by the hydrolysis of the glyoxylic acid ester functions as an acid catalyst, so that the hydrolysis proceeds without adding an additional acid catalyst.

However, an acid catalyst may be added separately in order to accelerate the reaction rate or to relax the reaction conditions. As the acid catalyst, for example, specifically,
Examples thereof include hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, heteropolyacid, p-toluenesulfonic acid, acetic acid, glyoxylic acid, acidic ion exchange resin, zeolite, clay and the like.

When hydrolysis is carried out in two liquid phases,
The reaction rate can be accelerated by using a catalyst that can exist on the organic phase side.

The hydrolysis reaction can also be carried out using a base catalyst. Examples of the base catalyst include hydroxides, carbonates, hydrogen carbonates of alkali metals and alkaline earth metals,
Examples include alkoxide and basic ion exchange resins,
Specific examples include sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, sodium methoxide, potassium methoxide, and the like. However, when performing hydrolysis using a base catalyst, if the reaction solution becomes too basic, glyoxylic acid ester and glyoxylic acid cause a Cannizzaro reaction, resulting in glycolic acid ester, oxalic acid ester, glycolic acid, and oxalic acid. May be generated, leading to a decrease in selectivity. Therefore, when carrying out the hydrolysis reaction under basic conditions, it is preferable to carry out while maintaining the pH of the reaction solution at 11 or less.

When hydrolyzing using a base catalyst, the ratio of water to glyoxylic acid ester varies depending on the type of glyoxylic acid ester and reaction conditions, but is generally equimolar to 200-fold molar, and preferably 2 times mole-1
It is 00 times the mole. Below this mole, water will be insufficient and 200
When the amount is more than twice the molar amount, the concentration of glyoxylic acid after the reaction is low, and it may be costly to concentrate.

The reaction temperature is suitably 10 ° C to 200 ° C, preferably 30 ° C to 150 ° C. The reaction pressure can be any of atmospheric pressure, increased pressure, and reduced pressure.

The form of the hydrolysis reaction may be any of continuous type, batch type and semi-batch type, and is not particularly limited.

In the case of base-catalyzed hydrolysis, the product is obtained as the salt of glyoxylic acid. Glyoxylate can release glyoxylic acid by contacting it with a strong acid such as hydrochloric acid or sulfuric acid.

Glyoxylic acid or glyoxylate can be produced by hydrolyzing the glyoxylic acid ester as described above. In the present invention, glyoxylic acid includes free glyoxylic acid and glyoxylate.

The produced glyoxylic acid easily becomes a glyoxylic acid hydrate when contacted with water. Also, when it comes into contact with alcohol, it easily becomes an alkyl glyoxylate hemiacetal.

This hydration or alkyl hemiacetalization is an equilibrium reaction, and when water is present in the system after completion of hydrolysis, glyoxylic acid in the composition is partially or wholly glyoxylic acid according to equilibrium. Present as a hydrate.

If alcohol is present in the system,
The glyoxylic acid in the composition is partly or wholly present as a glyoxylic acid alkyl hemiacetal according to the equilibrium.

Therefore, glyoxylic acid in the method for producing glyoxylic acid of the present invention may take the form of glyoxylic acid hydrate or glyoxylic acid alkyl hemiacetal in addition to free glyoxylic acid depending on the composition and concentration of the coexisting substances. There is.

The glyoxylic acid obtained as described above may be used as a solution or may be concentrated to give a solid. Further, it may be purified, if necessary, depending on the storage method, the transfer method, and the purpose thereafter. Purification can be carried out by a known method such as extraction or recrystallization.

The glyoxylic acid obtained by the present invention is an industrially useful compound as an intermediate raw material for various products such as pharmaceutical intermediates, cosmetics, fragrances and agricultural chemicals.

[0056]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Example 1 (Oxidative Esterification Reaction) An adiabatic reactor filled with 0.8 g of metallic silver (catalyst) having a particle size of 20 to 30 mesh was charged with 6% by volume of ethylene glycol preliminarily vaporized, air and A raw material gas prepared by mixing nitrogen gas (oxygen concentration 7% by volume) was used at a reaction temperature of 440 ° C. and a space velocity (SV) of 830,000.
A reaction gas containing glyoxal was obtained by continuously supplying gas phase oxidation (oxidative dehydrogenation) under the condition of hr ⁻¹ (ethylene glycol-based glyoxal yield 81
Mol%). Next, the above reaction gas (glyoxal concentration 4% by volume), pre-evaporated methanol (20% by volume) and air were mixed in a heat exchange reactor filled with a catalyst composed of ferric phosphate. Mixed gas (oxygen concentration 5% by volume) with a reaction temperature of 250 ° C. and a space velocity (SV) of 2,000
The reaction gas containing methyl glyoxylate was obtained by performing continuous gas-phase oxidation (oxidative esterification) under the condition of ⁰ hr ⁻¹ (ethylene glycol-based methyl glyoxylate yield 61 mol%).

Both vapor phase oxidations were carried out continuously. Then, by condensing and collecting the reaction gas, methyl glyoxylate (22% by mass), methanol (45% by mass), water (25%)
An oxidative esterification reaction liquid containing 3% by mass of methyl glycolate and 3% by mass of methyl glycolate was obtained.

(Distillation Step) The oxidative esterification reaction liquid was distilled using a thin film evaporator (stirring liquid film type). The water / methanol molar ratio in the oxidative esterification reaction liquid was 0.99. Pressure 0.02MPa, heat medium temperature 17
0 ° C, rotation speed 350 rpm, feed speed 300 m / h
The operation was performed at 1 to obtain a distillate containing methanol, water and methyl glyoxylate. The concentration of methyl glyoxylate in the distillate was 23% by mass. The distillation rate of methyl glyoxylate at that time was 96%.

(Hydrolysis reaction) Hydrolysis was performed using the distillate obtained in the evaporation step. A 1-liter flask equipped with a 5-stage glass Oldershaw distillation column was charged with 546 g of the above-mentioned distillate and 354 g of water. At this time, the molar ratio of water to methyl glyoxylate was 20. The flask was heated and reactive distillation was performed for 6 hours while extracting a mixture of methanol and water from the top of the column at a reflux ratio of 0.5.
After the reaction, when the bottom liquid was analyzed, the yield of glyoxylic acid based on methyl glyoxylate was 98 mol%, and the concentration of glyoxylic acid in the bottom liquid was 46% by mass.
The Hazen color number of the bottom liquid was 180.

The bottom liquid was heated at 85 ° C. for 6 hours. As a result, the Hazen color number was 180 and stable with no change.

Comparative Example 1 The oxidative esterification reaction liquid of Example 1 was directly used for the hydrolysis reaction without distillation. (Hydrolysis reaction) A 1 L flask equipped with a 5-stage glass Oldershaw distillation column was charged with 530 g of the oxidative esterification reaction liquid of Example 1 and 356 g of water. At this time, the molar ratio of water to methyl glyoxylate was 20. The flask was heated and reactive distillation was performed for 6 hours while extracting a mixture of methanol and water from the top of the column at a reflux ratio of 0.5. After the reaction, the bottom liquid was analyzed to find that the yield of glyoxylic acid based on methyl glyoxylate was 97 mol%.
And the concentration of glyoxylic acid in the bottom liquid is 45% by mass.
Met. The Hazen color number of the bottom liquid was 1000 or more.

Example 2 (Distillation step) The distillation operation of Example 1 was conducted under a pressure of 0.007M.
The same procedure was performed except that Pa and the feed rate were 200 ml / hour. The concentration of methyl glyoxylate in the distillate is 2
It was 4% by mass. At that time, the distillation rate of methyl glyoxylate was 98.5%.

(Hydrolysis reaction) Hydrolysis was performed using the distillate obtained in the evaporation step. 532 g of the above-mentioned distillate and 354 g of water were charged into a 1 L flask equipped with a five-stage glass Oldershaw distillation column. At this time, the molar ratio of water to methyl glyoxylate was 20. The flask was heated and reactive distillation was performed for 6 hours while extracting a mixture of methanol and water from the top of the column at a reflux ratio of 0.5.
After the reaction, when the bottom liquid was analyzed, the yield of glyoxylic acid based on methyl glyoxylate was 98 mol%, and the concentration of glyoxylic acid in the bottom liquid was 45% by mass.
The Hazen color number of the bottom liquid was 220.

The obtained bottom liquid was heated at 85 ° C. for 6 hours. As a result, the Hazen color number was 250, which was a degree where coloring was slightly dark.

[0066]

The method for producing glyoxylic acid of the present invention comprises:
After the oxidative esterification reaction, a distillation step is performed and further hydrolyzed to obtain an aqueous glyoxylic acid solution having an excellent hue. Further, the glyoxylic acid aqueous solution obtained in the present invention has excellent storage stability and can reduce coloration over time.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideyuki Baba             5-8 Nishiomitabicho Suita City, Osaka Prefecture             Within Nippon Shokubai (72) Inventor Kimio Ariyoshi             5-8 Nishiomitabicho Suita City, Osaka Prefecture             Within Nippon Shokubai F-term (reference) 4H006 AA02 AC45 AC46 AC48 AD11                       BA05 BA19 BA35 BA60 BB31                       BC31 BD70 BE30 BE60 BQ10                       BS10                 4H039 CA62 CA66 CC20 CC30 CD10                       CD30

Claims (3)

[Claims] 1. A method for producing glyoxylic acid by performing oxidative esterification reaction in the presence of a catalyst using glyoxal, alcohol and molecular oxygen as raw materials, and hydrolyzing the obtained liquid containing glyoxylic acid ester. In addition, before the hydrolysis, a distillation step of the reaction solution obtained by the oxidative esterification reaction is provided, and a fraction in which the content of the glyoxylic acid ester distilled in the distillation step is 99% by mass or less is A method for producing glyoxylic acid with little coloring, which comprises hydrolyzing as a raw material. 2. The method for producing glyoxylic acid with little coloring according to claim 1, wherein the distillation rate of the glyoxylic acid ester in the distillation step is 70 to 99% by mass. 3. The method for producing glyoxylic acid with little coloring according to claim 1, wherein the water contained in the oxidative esterification reaction liquid to be subjected to the distillation step is 3 mol times or less with respect to the alcohol. .