JPH0149697B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for producing methyl lactate starting from vinyl acetate or vinyl propionate. More specifically, the present invention hydroformylates vinyl acetate or vinyl propionate to α-acetoxy- or propionyloxy-propionaldehyde, and oxidizes α-acetoxy- or propionyloxy-propionaldehyde to α-acetoxy- or propionyloxy-propionaldehyde. The present invention relates to a method for synthesizing methyl lactate by reacting with propionyloxy-propionic acid and then reacting the α-acetoxy- or propionyloxy-propionic acid with methanol. This method is shown by the following reaction formula.

However, this method includes the following problems.

1. Considering the current raw material situation, it is expected that it will be difficult to obtain the starting materials acetaldehyde and hydrocyanic acid in large quantities and at low cost.

2. Since highly toxic hydrocyanic acid and its derivatives are handled, waste treatment is required to prevent pollution.

3. The reaction (i) above requires a low temperature as a reaction condition, and the yield of the product acetaldehyde cyanohydrin is not necessarily high.

4. The reaction (ii) above generates intense heat and requires effective heat removal, which reduces operational stability. Furthermore, since sulfuric acid is used at high temperatures, a reactor made of special material is required.

5 In the reaction (iii) above, crystalline ammonium bisulfate, which is difficult to handle, is produced as a by-product in an equimolar ratio with methyl lactate. Furthermore, methyl methoxylactate,
By-products such as carbon monoxide and tar-like substances are noticeable. By coexisting large amounts of methanol and water in the system, the production of byproducts due to side reactions can be suppressed to an acceptable range, but this increases utility and increases the product price accordingly.

6. When looking at the total reaction, the yield of lactic acid is not necessarily high and the amount of steam consumed is large. Also, from a process standpoint, it is difficult to adopt a continuous method.

As a method for producing lactic acid that does not involve the above conventional method, vinyl carboxylate is hydroformylated to produce α-
acyloxypropionaldehyde, α-acyloxypropionaldehyde is oxidized with oxygen and α-
A method has been proposed in which lactic acid is synthesized by hydrolyzing α-acyloxypropionic acid and α-acyloxypropionic acid (see JP-A-51-143617). However, the method proposed in Japanese Unexamined Patent Publication No. 51-143617 has problems in points 1 to 3 below.

1 In the hydroformylation reaction of vinyl carboxylate, α-acyloxypropionaldehyde is separated from the reaction mixture by a distillation method. In this case, α-acyloxypropionaldehyde is decarboxylated under distillation conditions. , causing undesirable side reactions such as polycondensation reactions and oxidation reactions, and by repeating the rohydroformylation reaction and distillation separation of the product, the catalytic activity of the rhodium complex compound that is recycled decreases and high boiling point by-products are generated. Things accumulate.

2. Since α-acyloxypropionic acid obtained by oxygen oxidation of α-acyloxypropionaldehyde contains impurities, if it is subjected to hydrolysis as it is, the purity of lactic acid will decrease. α−
It is also possible to previously purify acyloxypropionic acid by distillation, but in this case, the yield of α-acyloxypropionic acid decreases due to its thermal instability.

3 The hydrolysis reaction mixture of α-acyloxypropionic acid contains, in addition to lactic acid, water, organic carboxylic acid derived from vinyl carboxylate, and unreacted α-acyloxypropionic acid. It is difficult to obtain it in high purity from the reaction mixture.

The present inventors have conducted intensive research to develop a method for producing methyl lactate, which is a precursor thereof, in order to produce lactic acid industrially more advantageously than the above-mentioned methods, and as a result, they have arrived at the present invention. That is, according to the present invention, () vinyl acetate or vinyl propionate is treated with a mixed gas of hydrogen and carbon monoxide in the presence of a substantially water-insoluble rhodium complex compound and a trisubstituted phosphine in an organic solvent. After hydroformylation, α-acetoxypropionaldehyde or α-propionyloxypropionaldehyde is extracted and separated into an aqueous medium layer by subjecting the reaction mixture obtained in step () to an extraction operation with an aqueous medium, and a raffinate layer is obtained. is circulated to the hydroformylation reaction step of step (), and the aqueous medium layer containing α-acetoxypropionaldehyde or α-propionyloxypropionaldehyde obtained in step () is recycled while maintaining the temperature of the still liquid at about 90°C or lower. α-acetoxypropionaldehyde or α-propionyloxypropionaldehyde is separated from water by heating, and α-acetoxypropionaldehyde or α-propionyloxypropionaldehyde obtained in step () is oxidized in the presence of an oxidizing catalyst. is oxidized with oxygen gas or oxygen-containing gas in the liquid phase, and the α-acetoxypropionic acid or α-propionyloxypropionic acid obtained in step () is reacted with methanol in the presence of an acid catalyst to produce it. By obtaining methyl lactate by distillation, methyl lactate can be produced with high purity and high yield. According to the method of the present invention, separation of α-acetoxypropionaldehyde or α-propionyloxypropionaldehyde from a rhodium complex compound from a reaction mixture obtained by the hydroformylation reaction of vinyl acetate or vinylpropionate is performed by an extraction operation using an aqueous medium. Therefore, the catalytic activity of the rhodium complex compound can be stably maintained over a long period of time. Furthermore, the method of the present invention has excellent operational stability, the main raw materials vinyl acetate or vinyl propionate, carbon monoxide, hydrogen, and methanol can be obtained in large quantities at low cost, and special waste treatment is not required. It has the advantages that it is not necessary and that the reaction conditions for each reaction are mild.

Methyl lactate obtained by the method of the present invention can be used as a solvent as it is, but by further hydrolyzing it, it can be converted to lactic acid with high yield and high purity.

As the rhodium complex compound used in the hydroformylation reaction of vinyl acetate or vinyl propionate in the method of the present invention, any rhodium that has a hydroformylation catalytic ability under the hydroformylation reaction conditions and is substantially insoluble in an aqueous medium can be used. Complex compounds can be used. Many such rhodium complex compounds are already known, and most of these conventionally known rhodium complex compounds can be used in the method of the present invention. Specifically, HRh(CO)(PA ₃ ) ₃ (A: aryl group), RhCl
(PA ₃ ) ₃ , Rh (acac) ₃ (acac: acetylacetonate group), Rh (OAc) ₃ (OAc: acetic acid group), Rh ₄ (CO) ₁₂ ,
Rh ₆ (CO) ₁₆ , [Rh (CO) ₂ (PA ₃ ) ₂ ] ₂ , RhCl ₃・
Examples include 3H ₂ O, Rh ₂ O ₃ , etc. Among these,
HRh(CO)(PA ₃ ) type ₃ rhodium complex compounds are particularly preferred from the viewpoints of catalytic activity, solubility, ease of handling, and the like. The rhodium complex compound is usually used in a concentration range of 0.1 to 10 mmol per hydroformylation reaction solution. In the method of the present invention, a substantially water-insoluble organic solvent is used in consideration of the extraction step using an aqueous medium. There are many organic solvents that can be used, but the solubility of the catalyst components, loss of elution of the catalyst components into the aqueous medium layer, price, physical properties considering the subsequent separation process, and chemical Considering stability, hydroformylation reaction results, etc., aromatic hydrocarbons that may be substituted with lower alkyl groups, such as benzene, toluene, xylene, and ethylbenzene, and substituted or unsubstituted aromatic hydrocarbons, such as cyclohexane and methylcyclohexane, are used. Preferred examples include substituted saturated alicyclic hydrocarbons. Trisubstituted phosphine is represented by the general formula PR′R″R (R′ and R″ represent an aromatic hydrocarbon group, and R represents an aromatic hydrocarbon group or a saturated aliphatic hydrocarbon group having 3 or more carbon atoms). It will be done.

Examples of such trisubstituted phosphine include substituted or unsubstituted triarylphosphines such as triphenylphosphine, tritolylphosphine, and trinaphthylphosphine, and diphenylpropylphosphine, diphenylhexylphosphine, and the like. Examples include diarylalkylphosphine. Among these, substituted or unsubstituted triarylphosphines are particularly preferably used. The preferred amount of trisubstituted phosphine used is generally in the range of about 5 to about 50 mol per rhodium atom, and more precisely, it is practical to use it at a concentration of 10 to 150 mmol per hydroformylation reaction solution.

In the method of the present invention, the hydroformylation reaction of vinyl acetate or vinyl propionate is usually carried out in consideration of reaction temperature, selectivity to α-acetoxy- or propionyloxy-propionaldehyde, catalyst activity life, equipment cost, etc. Temperature 50~120℃, absolute partial pressure of carbon monoxide 4~50Kg/
cm ² , reaction pressure 25 to 150 Kg/cm ² , and a molar ratio of hydrogen to carbon monoxide of 0.5 to 5. The hydroformylation reaction can be carried out continuously or batchwise in a stirred reactor or bubble column reactor. During the reaction, the accumulation of reaction heat is suppressed and α-
To improve the selectivity to acetoxy- or propionyloxy-propionaldehyde and prevent the accumulation of high-boiling by-products, vinyl acetate or vinyl propionate can be continuously fed into the reaction system. It is advantageous to carry out the reaction while keeping the concentration of acetate or vinyl propionate within a certain range. The concentration of α-acetoxy- or propionyloxy-propionaldehyde in the reaction mixture also depends on the accumulation of high-boiling byproducts, the amount of rhodium complex compounds and trisubstituted phosphine eluted into the aqueous medium layer, and the concentration of α-acetoxy- or propionyloxy-propionaldehyde in the aqueous medium layer. From the viewpoint of extraction efficiency of α-acetoxy- or propionyloxy-propionaldehyde, it is advantageous to maintain the amount in the range of about 0.5 to about 3 mol per reaction mixture.

The reaction mixture containing α-acetoxy- or propionyloxy-propionaldehyde, which is the target product obtained in the hydroformylation reaction step of vinyl acetate or vinyl propionate (step ()), is extracted with an aqueous medium in step (). is applied, whereby the α-acetoxy- or propionyloxy-propionaldehyde is extracted and separated into the aqueous medium layer. In the process of the invention, the aqueous medium is water or a small amount (for example 5-10
%) is used. As the extraction device, a stirring type extraction column, an RDC type extraction column, a perforated plate column, etc., which are known per se, can be used. Considering various points such as the amount of trisubstituted phosphine eluted into the aqueous medium layer, an RDC type extraction column is most preferable. According to detailed studies by the present inventors, the extraction rate of α-acetoxy- or propionyloxy-propionaldehyde into the aqueous medium layer, the elution amount of rhodium complex compounds and trisubstituted phosphine into the aqueous medium layer, Contact efficiency between the medium layer and the hydroformylation reaction mixture layer, extraction temperature, α-acetoxy- or propionyloxy-propionaldehyde concentration in the reaction mixture, volume ratio of aqueous medium to reaction mixture, gas phase during extraction. The higher the contact efficiency between the aqueous medium layer and the hydroformylation reaction mixture layer, the lower the extraction temperature, and the larger the volume ratio of the aqueous medium to the hydroformylation reaction mixture, the higher the α
It was observed that the extraction rate of -acetoxy- or propionyloxy-propionaldehyde into the aqueous medium layer increased, and the amount of rhodium complex compound and trisubstituted phosphine eluted into the aqueous medium layer tended to decrease. The extraction temperature is selected from a range of about 5 to 40°C. The volume ratio of the aqueous medium to the hydroformylation reaction mixture depends on the α-acetoxy- or propionyloxy-propionaldehyde concentration in the reaction mixture.
When the amount is about 0.5 to about 3 mol per mole, it is preferably selected from the range of 0.3 to 3. The extraction operation with an aqueous medium in step () is performed using substantially oxygen-free inert gases such as nitrogen gas, helium gas and argon gas or hydrogen/monoxide having a partial pressure of carbon monoxide of at least 0.1 Kg/cm2 or ^more . It is preferable to conduct this under an atmosphere of a carbon mixed gas or a hydrogen/carbon monoxide mixed gas diluted with the above-mentioned inert gas and having a carbon monoxide partial pressure of 0.1 Kg/cm ² or more. Elution of rhodium complex compounds can be suppressed. Although the extraction operation can be carried out in a batch manner, it is industrially advantageous to carry out the extraction operation in a continuous manner.

The raffinate layer containing the catalyst component obtained in step () is recycled to the hydroformylation reaction step and reused.

In this case, if necessary, a part of the raffinate layer can be separately taken out, subjected to a known catalyst activation treatment, and then recycled to the hydroformylation reaction step.

α-acetoxy- or propionyloxy-propionaldehyde obtained in step () is heated by heating the aqueous medium layer containing α-acetoxy- or propionyloxy-propionaldehyde while maintaining the temperature of the still liquid at about 90°C or less.
Alternatively, propionyloxy-propionaldehyde is separated from water (step ()). In this case, the water is generally obtained as a distillate, but the α-acetoxy- or propionyloxy-propionaldehyde can be obtained as a distillate or as a distillation residue. By keeping the temperature of the still liquid below approximately 90℃, α can be reduced during heating.
It is possible to prevent the purity and yield of -acetoxy- or propionyloxy-propionaldehyde from decreasing. It goes without saying that it is desirable that the α-acetoxy- or propionyloxy-propionaldehyde obtained in step () contains as little water as possible in consideration of the next oxidation reaction, but α-acetoxy- or propionyloxy-propion If the aldehyde contains less than an acceptable amount of water (usually less than 2 in molar ratio to α-acetoxy- or propionyloxy-propionaldehyde), it can be directly subjected to the oxidation reaction.

α-acetoxypropionaldehyde or α-propionyloxypropionaldehyde obtained in step () is oxidized in step () with oxygen gas or oxygen-containing gas in the presence of an oxidation catalyst. Examples of oxidation catalysts include copper salts, iron salts, nickel salts, cobalt salts, and manganese salts, but from the viewpoint of reaction selectivity, copper salts,
Iron and nickel salts are preferred. Specific examples of oxidation catalysts include cuprous halides, cupric halides, cuprous carboxylates, cupric carboxylates, cuprous sulfates, cupric sulfates, cupric nitrates, and cupric halides. monoferrous, ferric halides, ferrous carboxylates,
Examples include ferric carboxylate, ferric sulfate, nickel carboxylate, nickel sulfate, nickel halide, cobaltous carboxylate, cobaltous sulfate, manganous carboxylate, and manganese sulfate. Among these, considering various points such as solubility of the metal salt in the reaction mixture, catalytic activity, and corrosiveness to the reaction equipment, cupric carboxylate, ferrous carboxylate, ferric carboxylate, and carboxylic acid Nickel is particularly preferred. As carboxylic acids forming these carboxylic acid salts, lower carboxylic acids such as acetic acid, propionic acid, and butyric acid are considered.

These metal salts may be used alone or in combination of two or more. Copper salts, iron salts and nickel salts are used in amounts of 0.1 to 50 mmol per reaction mixture;
Cobalt salts and manganese salts are used in amounts of 0.01 to 1.0 mmol per reaction mixture. The reaction is carried out at a reaction pressure of 1 to about 20 atm and a reaction temperature of 40 to 100°C.
The reaction is carried out under the following conditions using a stirred reaction tank or a bubble-type reaction tank while blowing oxygen gas, air, or a nitrogen/oxygen containing gas in an arbitrary ratio. As the reaction solvent, acetic acid, propionic acid, methyl acetate, α-acetoxy- or propionyloxy-propionic acid, and mixtures thereof in arbitrary proportions can be used, but among these, the product α-acetoxy- or propionyloxy - It is most preferable in relation to the subsequent steps to allow the propionic acid to also function as a solvent.
When α-acetoxy- or propionyloxy-propionic acid is used as a solvent, there is no problem even if it contains, for example, a small amount of acetic acid.
This reaction, like the usual oxygen oxidation reaction of aldehydes, is accompanied by extreme heat generation, so it is necessary to control the reaction temperature. Alternatively, the aldehyde concentration in the reaction mixture may be maintained at a low concentration by intermittent addition.

The α-acetoxypropionic acid or α-propionyloxypropionic acid produced in step () is converted into methyl lactate by directly reacting it with methanol in the presence of an acid catalyst without distilling it from the reaction mixture (step ()).
In step (), α-acetoxy- or propionyloxy-propionic acid, which is thermally stable, is subjected to the reaction without being separated by distillation.
Thermal decomposition of -acetoxy- or propionyloxy-propionic acid can be suppressed. It should be noted that α-
If acetoxy- or propionyloxy-propionaldehyde remains unreacted, it will react with methanol and produce lactic aldehyde and acetol as by-products, so the oxidation reaction in step () should be delayed as much as possible. It is better to leave it there. Examples of the acid catalyst include sulfuric acid, deoxyacid, benzenesulfonic acid, p-toenesulfonic acid, and proton type ion exchange resin. The acid catalyst is used in an amount of 1 to 1000 milliequivalents per reaction mixture in terms of proton concentration. Methanol is used in a molar ratio of 3 to 7 to α-acetoxy- or propionyloxy-propionic acid. The reaction temperature is selected from the range of 50 to 150°C.
The reaction varies depending on the form of the acid catalyst used, but
When the catalyst is liquid in the reaction system, the reaction is carried out in a stirred reaction tank, and when an ion exchange resin is used, the reaction is carried out in a stirred reaction tank or in a packed column filled with an ion exchange resin. The most preferred reaction mode is methanol and α in the reaction medium containing the acid catalyst.
-acetoxy- or propionyloxy-propionic acid is supplied continuously or intermittently, and the reaction is carried out while distilling the produced methyl lactate out of the reaction system. This method improves the yield of methyl lactate and improves utility. You can save money. In this case, the distillate contains methyl acetate or methyl propionate, water, α-acetoxy or propionyloxy-methyl propionate, and unreacted methanol in addition to methyl lactate. The distillate can be separated into its respective fractions by treating the distillate in a gaseous state or by leading the distillate in a gaseous state to a distillation column and treating it. In addition, methyl lactate can also be obtained by distillation from the reaction mixture after the reaction. The separated methyl α-acetoxy- or propionyloxy-propionate can be recycled to the reaction of step () and reused.

Hereinafter, the method of the present invention will be specifically explained with reference to Examples, but the present invention is not limited in any way by these Examples.

Example 1 Synthesis of α-acetoxypropionaldehyde and separation of α-acetoxypropionaldehyde from the reaction mixture The synthesis reaction of α-acetoxypropionaldehyde and the water extraction operation were repeated using the following reaction apparatus. In addition, throughout each experimental operation, the mixing of air into the system was avoided as much as possible, and distilled water and toluene were used after displacing and removing dissolved oxygen with nitrogen gas.

Reactor: A stainless steel autoclave with contents 1 equipped with a thermometer, a stirring device, a liquid feed pump, a liquid extraction port, and a gas inlet and outlet was used.

Extraction device: thermometer, stirring device, liquid inlet,
Contents 1 with liquid extraction port and gas inlet/outlet
A four-necked flask was used as an extraction device.
The extractor is directly connected to the autoclave by a pipe.

HRh(CO) [P(C ₆ E ₅ ) ₃ ] ₃ 918 mg (1.0 mmol) and triphenylphosphine 2620 mg (10
420 ml of toluene solution containing 1 mmol) was washed twice with 420 ml of distilled water at room temperature under an atmosphere of a mixed gas of hydrogen and carbon monoxide (H ₂ /CO molar ratio 2/1) while stirring twice. Afterwards, it was placed in the autoclave mentioned above. After sufficiently replacing the inside of the autoclave with a mixed gas of hydrogen and carbon monoxide (H ₂ /CO molar ratio 2/1), the pressure was increased to 30 Kg/cm ² (G) with the same mixed gas, and the autoclave was placed in an oil bath. It was heated until the room temperature was constant at 70°C. Start stirring at a speed of 600 rpm and add 71 g of vinyl acetate.
(830 mmol) was introduced continuously over 1.5 hours. Off gas flow rate is 20/hour, 70℃,
The reaction was carried out under stirring under the condition of 30 Kg/cm ² (G).
Low-boiling compounds (vinyl acetate, propionaldehyde, toluene, etc.) entrained in the off-gas were collected in a trap in a dry ice-acetone bath. After the vinyl acetate feed was completed, the reaction was further accelerated by continuing stirring for an additional 2 hours under the same conditions. The conversion rate of vinyl acetate at 3.5 hours after the start of the reaction was determined from the analysis of the reaction mixture by gas chromatography.
90%, α- based on converted vinyl acetate
The selectivity to acetoxypropionaldehyde is
It turned out to be 95%. After cooling the reaction mixture to room temperature, a mixed gas of hydrogen and carbon monoxide (H ₂ /CO molar ratio 2/1) is generated using internal pressure.
was pumped into the above extractor, which was replaced with Pour 125 ml of nitrogen-substituted distilled water and heat at 20℃.
By stirring for 20 minutes at a speed of 500 rpm.
α-acetoxypropionaldehyde in toluene solution (reaction mixture) was extracted with water. After the aqueous layer obtained by standing was taken out of the system, 125 ml of distilled water was added again to the toluene solution, water extraction was performed under the same conditions, and the aqueous layer was taken out of the system. Through these two extraction operations, 95% of α-acetoxypropionaldehyde was extracted into the aqueous layer side. The rhodium concentration in the extracted aqueous layer (determined by atomic absorption spectrometry) is
The concentration of phosphorus compounds (determined by colorimetric analysis) was 2 ppm as phosphorus atoms.
The raffinate layer, a toluene solution containing catalyst components, was pumped into the reaction autoclave using the pressure of a mixed gas of hydrogen and carbon monoxide. 70
C., 30 Kg/cm ² (G), and 600 rpm, vinyl acetate was continuously added at a rate of 48 g/hour for 80 minutes while stirring, and the reaction was continued by intermittent stirring for 2 hours. After the reaction, the reaction mixture was extracted with water in the same manner as above under the same conditions. The raffinate was again pumped into the reaction autoclave to carry out a hydroformylation reaction of vinyl acetate. This method combines the hydroformylation reaction and water extraction of vinyl acetate.
Repeated 10 times. As a result, a total of about 3.2 extracted aqueous layers were obtained. 3rd, 6th and 10th
The conversion rates of vinyl acetate in the second round were 91%, 90%, and 91%, respectively, and the rhodium concentrations in the extracted aqueous layer were calculated in terms of atoms, respectively.
0.05ppm, 0.06ppm and 0.06ppm, and these values did not change at all depending on the number of repetitions.

The extracted water layer in each round was extracted under nitrogen gas atmosphere.
Stored at 5°C.

Separation of α-acetoxypropionaldehyde from the extracted aqueous layer 500 ml of the extracted aqueous layer (aqueous solution containing about 2 mol/α-acetoxypropionaldehyde) obtained as a result of the above repeated experiment was replaced with nitrogen gas in advance. It was placed in a glass vacuum distillation apparatus. Fractional distillation was carried out over a period of approximately 2 hours while maintaining the temperature of the can liquid at 60 to 65°C and varying the degree of vacuum. As a result, α-
115 g of acetoxypropionaldehyde was obtained.

In addition, no other impurities were observed in the α-acetoxypropionaldehyde fraction except for water and a small amount of acetic acid. In the same manner, the fractional distillation operation of α-acetoxypropionaldehyde was repeated five times under the same conditions for each of the remaining extracted aqueous layers, each containing 500 ml. The distillation yield of α-acetoxypropionaldehyde in the fractional distillation was 90 to 95%, and almost no high-boiling by-products were observed in the distillation residue. The separated α-acetoxypropionaldehyde was stored at 5°C after purging with nitrogen gas.

Synthesis of α-acetoxypropionic acid. Includes thermometer, stirring device, reflux condenser, liquid metering feed pump and gas inlet.
A 300 ml four-necked flask is charged with 50 ml of an α-acetoxypropionic acid solution containing 12.5 mg (0.05 mmol) of nickel acetate tetrahydrate and 8% by weight of water synthesized separately, and the internal temperature becomes 55°C. The mixture was stirred until the nickel acetate was completely dissolved. Thereafter, stirring was started at 55° C. and 800 rpm while blowing oxygen gas at a rate of 10/hour. α containing 8% by weight of water obtained by the above distillation separation operation using a liquid metering feed pump.
The oxygen oxidation reaction of α-acetoxypropionaldehyde was carried out by continuously adding -acetoxypropionaldehyde at a rate of 12 ml/hour over a period of 8 hours. After 8 hours, the feed of α-acetoxypropionaldehyde was stopped, and stirring was continued for an additional 2 hours at a temperature of 55° C. and a speed of 800 rpm to drive the reaction. The conversion rate of α-acetoxypropionaldehyde was measured by gas chromatography at the time when the feed of α-acetoxypropionaldehyde was stopped (8 hours after the start of the reaction) and at the end of the reaction (10 hours after the start of the reaction). It was 98%. When the concentration of carbon dioxide contained in the off-gas was measured by gas chromatography, the production rate of carbon dioxide was 3% based on the converted α-acetoxypropionaldehyde. The reaction solution after the oxidation reaction was placed in a 200 ml reactor equipped with a thermometer, oxygen gas inlet (installed at the bottom of the reactor), and off-gas outlet, and heated in an oil bath until the internal temperature was constant at 80°C. It was warm. Inject oxygen gas from the oxygen gas inlet 3
The temperature was maintained at 80°C for 1.5 hours while introducing at a flow rate of /hour. After 1.5 hours had elapsed, the reaction solution was analyzed by gas chromatography, and it was found that the conversion rate of α-acetoxypropionaldehyde was 99.5% or more. After the reaction solution was cooled to room temperature, the amount of active oxygen in a portion of the solution was determined by iodometric titration and found to be 0.01%.

The same operation was repeated five more times under the same conditions for the remaining α-acetoxypropionaldehyde. The average conversion rate of α-acetoxypropionaldehyde at the end of the reaction (10 hours after the start of the reaction) was 98%, and that of α-acetoxypropionaldehyde after 1.5 hours while maintaining the internal temperature at 80°C was 99.5% or more. It was hot.

Synthesis of methyl lactate A four-necked flask with a content of 100 ml was equipped with a thermometer, a stirrer, a nitrogen gas inlet/outlet, a liquid metering feed pump (the feed inlet was installed at the bottom of the flask), and a distillate collector connected to a condenser. 10 ml of the oxidation reaction mixture of α-acetoxypropionaldehyde obtained above and 150 mg of 97% sulfuric acid were charged. While slowly introducing nitrogen gas,
The flask was immersed in an oil bath adjusted to 145°C.
When the internal temperature exceeds 100°C, mix the oxidation reaction mixture of α-acetoxypropionaldehyde preheated to 40 to 50°C with methanol (methanol/reaction mixture weight ratio = 17/20) was introduced continuously at a rate of 28 ml/hour. Immediately after the introduction of the liquid, distillation of a distillate consisting of methyl acetate, methanol, water, methyl lactate, and methyl α-acetoxypropionate began. Continuous reaction was carried out under these conditions for 15 hours. The amount of liquid in the reactor was kept approximately constant throughout the reaction, but occasionally the liquid could be reduced by removing the flask from the oil bath for a very short time or by briefly turning off the feed liquid. We made fine adjustments to the endogenous amount. Distillate (approx.
400ml) was analyzed by gas chromatography and Karl Fischer method, and it was found that the following compounds were contained.

Methanol; 93.6g Methyl acetate; 92.6g Water; 39.2g Methyl lactate; 127.2g Methyl α-acetoxypropionate; 24.0g Methyl methoxylactate; 0.48g From this, the yield of methyl lactate based on the charged α-acetoxypropionic acid is 87%. It is calculated as follows.

Furthermore, analysis by gas chromatography revealed that only trace amounts of carbon monoxide were produced as a by-product.

Separation of methyl lactate from the reaction mixture 375 ml of the distillate obtained in the above experimental procedure was charged into a distillation apparatus equipped with a packed column having 20 theoretical plates, and acetic acid was first separated by fractional distillation under normal pressure. Methyl and methanol were almost completely distilled off. Next, fractional distillation was carried out under a pressure of 70 mmHg, and finally about 90 g of methyl lactate was obtained as a fraction (main fraction) with a boiling point of 75 to 77°C. When the isolated methyl lactate was analyzed by gas chromatography, it was clear and colorless, containing no other impurities, and had an extremely pleasant odor.

Example 2 80 g (800 mmol) of vinyl propionate instead of vinyl acetate, 3040 mg (10 mmol) of tritolylphosphine instead of triphenylphosphine, distilled water purged with nitrogen as the extraction solvent
500ml (total volume ratio of water to reaction mixture 1/1)
The hydroformylation reaction and extraction operation of vinyl propionate were repeated a total of 10 times in the same manner as in Example 1, except that .

As a result, a total of about 5.7 extracted aqueous layers were obtained.
The conversion rate of vinyl propionate, the selectivity of α-propionyloxypropionaldehyde, the extraction rate of α-propionyloxypropionaldehyde, and the rhodium concentration in the extracted aqueous layer are each average.
They were 88%, 92%, 90%, and 0.03ppm.

Of the extracted aqueous layer (aqueous solution containing about 1 mol/α-propionyloxypropionaldehyde)
500ml was charged into the vacuum distillation apparatus used in Example 1(ii), and fractional distillation was carried out under reduced pressure for about 2 hours while maintaining the bottom liquid temperature at 75°C. α-propionyloxypropionaldehyde containing about 5% water by weight is
63g was obtained. In the same manner, fractional distillation experiments of α-propionyloxypropionaldehyde were conducted eight times under the same conditions using 500 ml each of the remaining extracted aqueous layers, and the average distillation yield was 90%.

Separately prepare 50 ml of an α-propionyloxypropionic acid solution containing 5% by weight of water and a nickel catalyst, and add 13ml/hour of α-propionyloxypropionaldehyde containing 5% by weight of water obtained by the above fractional distillation operation. The oxidation reaction of α-propionyloxypropionaldehyde was repeated a total of four times in the same manner as in Example 1(iii) except that the addition was carried out continuously at a rapid rate. At the end of the reaction (10 hours after the start of the reaction), the average conversion rate of α-propionyloxypropionaldehyde was 97%, and the carbon dioxide production rate was 4% on average based on the converted α-propionyloxypropionaldehyde.

Further, the average conversion rate of α-propionyloxypropionaldehyde after 1.5 hours while maintaining the internal temperature at 80°C was 99.5% or more. A mixed solution of the oxidation reaction solution of α-propionyloxypropionaldehyde obtained in this way and methanol (methanol/reaction solution weight ratio = 1/1) was mixed with 25
Methyl lactate was synthesized in the same manner as in Example 1(iv) except that the mixture was introduced continuously at a rate of ml/hour. The yield of methyl lactate based on the charged α-propionyloxypropionic acid was 88%. The distillate containing methyl lactate was charged into the distillation apparatus used in Example 1(v), and the same fractional distillation procedure for methyl lactate was used to finally obtain about 75 g of methyl lactate fraction with a boiling point of 75-77°C/70 mmHg. I got it.

Example 3 The experiment in (i) of Example 1 was repeated five times to obtain about 1.6 aqueous extract layers. 500 ml of the extracted aqueous layer was charged into the vacuum distillation apparatus used in Example 1(ii), and water and acetic acid were distilled off over 2 hours while changing the degree of vacuum at a bottom liquid temperature of 70°C. Ta. As a result, 105 g of α-acetoxypropionaldehyde containing 3% by weight of water and a trace amount of acetic acid was obtained as a bottom liquid. The same operation was repeated twice for each 500 ml of the remaining extracted aqueous layer, and about 100 g of α-acetoxypropionaldehyde containing an average of 3% by weight of water and a trace amount of acetic acid was obtained each time as a bottom liquid.

A catalyst solution was prepared by dissolving 26 mg (0.15 mmol) of ferrous acetate in 50 ml of separately synthesized α-acetoxypropionic acid, and the addition rate of α-acetoxypropionaldehyde containing 3% by weight of water at a reaction temperature of 65°C. The oxidation reaction of α-acetoxypropionaldehyde was repeated a total of two times in the same manner as in Example 1(iii) except that the reaction rate was changed to 11 ml/hour.

α- at the end of the reaction (10 hours after the start of the reaction)
The average conversion rate of acetoxypropionaldehyde and carbon dioxide production rate are 96% and 2.5%, respectively.
It was hot. Further, the average conversion rate of α-acetoxypropionaldehyde after 1.5 hours while maintaining the internal temperature at 80°C was 99%. A mixture of the thus obtained oxidation reaction solution of α-acetoxypropionaldehyde and methanol (methanol/
Methyl lactate was synthesized in the same manner as in Example 1(iv), except that the reaction solution (weight ratio = 8/7) was continuously introduced at a rate of 22 ml/hour. Preparation α
The yield of methyl lactate based on -acetoxypropionic acid was 90%. The distillate containing methyl lactate was charged into the distillation apparatus used in Example 1(v), and methyl lactate was fractionally distilled in the same manner until the boiling point was 75-77.
Approximately 76 g of methyl lactate fraction was obtained at a temperature of 70 mmHg.

Claims (1)

[Claims] 1. Hydroformylation of vinyl acetate or vinyl propionate with a mixed gas of hydrogen and carbon monoxide in the presence of a substantially water-insoluble rhodium complex compound and a trisubstituted phosphine in an organic solvent. , 2. α-acetoxy-
Alternatively, propionyloxy-propionaldehyde is extracted and separated into an aqueous medium layer, the raffinate layer is recycled to the hydroformylation reaction step of step 1, and 3. an aqueous medium containing α-acetoxy- or propionyloxy-propionaldehyde obtained in step 2. α-acetoxy- or propionyloxy-propionaldehyde is separated from water by heating the layer while keeping the temperature of the distillate liquid below about 90°C, and 4. α-acetoxy- or propionyloxy obtained in step 3. - oxidizing propionaldehyde with oxygen gas or oxygen-containing gas in the liquid phase in the presence of an oxidation catalyst; 5. oxidizing the α-acetoxy- or propionyloxy-propionic acid obtained in step 4 with methanol in the presence of an acid catalyst; A method for producing methyl lactate, which comprises: reacting and obtaining the produced methyl lactate by distillation.