JPH10306091A - Method for producing lactide - Google Patents

Abstract

(57) An object of the present invention is to provide a method for producing a lactide which can obtain a lactide product having high optical purity without requiring a high temperature and a high degree of vacuum. SOLUTION: While blowing steam 3 into a depolymerization reactor 1, a lactic acid oligomer is depolymerized under appropriate depolymerization temperature and pressure conditions. Water vapor flow rate is 1m ³ of lactic acid oligomer
Per 0.1 to 30 mol / s. The generated lactide vapor is condensed in the condenser 5 and collected in the receiver 6. Racemization can be suppressed, and depending on the optical purity of the raw material lactic acid oligomer,
A colorless lactide of high optical purity is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing lactide, which is a cyclic dimer of lactic acid, by depolymerizing a lactic acid oligomer, and more particularly, to a method for producing lactide at a relatively low temperature. Regarding Lactide is particularly useful as a raw material for producing polylactic acid, which is a biodegradable polymer.

[0002]

2. Description of the Related Art Conventionally, in the production of lactide, lactic acid is dehydrated and condensed to obtain a lactic acid oligomer having a relatively low molecular weight as an intermediate, and then the lactic acid oligomer is depolymerized by heating in the presence of a catalyst under reduced pressure. In this way, lactide is produced, and the lactide is taken out of the reaction system as a vapor.

[0003] There are many known examples of this lactide production method. For example, DE 267 826 describes a method for producing lactide in which lactic acid is heated to 200 ° C. in a vacuum and distilled. Also, German Patent No. 12
No. 34703 describes a method for producing L-lactide from an aqueous L-lactic acid solution in the presence of a titanium tetraalkoxide catalyst.

Further, Japanese Patent Application Laid-Open No. 63-101378 discloses a method for producing lactide on an industrial scale,
Japanese Patent Publication No. 7-50091 discloses a method for continuously producing lactide from low-molecular-weight lactic acid oligomers by using thin-film distillation.

However, according to these conventional methods, a considerably high degree of vacuum is required for distilling out lactide, so that a high vacuum apparatus is indispensable. The degree of vacuum depends on the reaction temperature, but generally needs to be 1.3 × 10 ³ Pa or less. A higher degree of vacuum was required to increase the lactide production efficiency. In addition, if the system is operated in a high vacuum system, clogging of the piping is likely to occur, which is not suitable for an actual plant.

On the other hand, in order to distill out lactide without making the degree of vacuum so high, it is necessary to raise the reaction temperature. However, when the reaction temperature is increased, racemization occurs, and as a raw material lactic acid oligomer, L- which has a high optical purity is obtained.
Even when a lactic acid oligomer or a D-lactic acid oligomer is used, the optical purity of the obtained lactide is reduced. The decrease in optical purity has been a very serious problem when lactide having high optical purity is required.

[0007]

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to obtain a lactide product having a high optical purity without requiring a high temperature and a high degree of vacuum. It is to provide a manufacturing method of.

[0008]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that the above object can be achieved by depolymerizing a lactic acid oligomer while blowing steam into a depolymerization reaction system. Has been completed.

That is, the method for producing lactide of the present invention is a method for producing lactide by depolymerization of a lactic acid oligomer, in which steam is blown into the depolymerization reaction system,
This is a method in which lactic acid oligomers are depolymerized under appropriate depolymerization temperature and pressure conditions to distill off the generated lactide.
Hereinafter, the present invention will be described in detail.

In the present invention, while saturating steam or heated steam is blown into the depolymerization reaction system, the lactic acid oligomer is heated to an appropriate depolymerization temperature for depolymerization. By heating the lactic acid oligomer while blowing water vapor in this way, a sufficiently large contact interface area is generated between the lactic acid oligomer and the water vapor, and the generated lactide is easily vaporized with the water vapor. That is, since the partial pressure of lactide in the gas phase is small due to the presence of water vapor, lactide is easily vaporized. Therefore, in the present method, it is possible to generate lactide and distill lactide vapor at a lower temperature and a higher pressure than in the conventional case where no steam flow is present.

The lactic acid oligomer used as a raw material in the present invention is not particularly limited, but has a weight average molecular weight of 400 to 3
It has a relatively low molecular weight of about 0000. This lactic acid oligomer can be produced by a conventionally known method, that is, by subjecting a lactic acid monomer to dehydration condensation under an appropriate heating temperature and reduced pressure conditions. For example, a lactic acid monomer having a weight average molecular weight of about 400 to 30,000 can be dehydrated by heating (generally, 120 to 170 ° C.) under reduced pressure (generally, about 6 × 10 ^{2 to} 1.5 × 10 ⁴ Pa) to dehydrate lactic acid monomer. Obtainable.

In the present method, a lactic acid oligomer can be obtained from a lactic acid monomer as described above, and the depolymerization reaction can be carried out without removing the lactic acid oligomer.

In this method, it is preferable that the steam is blown at a flow rate of 0.1 to ³⁰ mol / s per 1 m ³ of the lactic acid oligomer. With such a water vapor flow rate, a sufficiently large contact interface area is generated between the lactic acid oligomer and the water vapor, so that lactide is easily vaporized and lactide generation is promoted. When the steam flow rate is less than 0.1 mol / s per 1 m ^{3 of the} lactic acid oligomer, the lactide yield decreases and the lactide production rate becomes slow. This is because a sufficient amount of steam is required to distill with lactide. On the other hand, if the steam flow rate is higher than 30 mol / s per 1 m ^{3 of} lactic acid oligomer, the reaction is not hindered, but the reactor becomes unnecessarily large and the cost becomes high, which is not preferable. A more preferable steam flow rate is 0.3 to ² per 1 m ^{3 of} lactic acid oligomer.
It is 0 mol / s.

In the present method, the steam is preheated to a temperature equal to or close to the temperature in the depolymerization reaction system.
It is preferable to blow in this preheated steam.
By preheating the steam, it is possible to prevent the temperature inside the reactor from decreasing, so that the temperature inside the reactor can be easily controlled.

In the present method, the suitable temperature in the depolymerization reaction system is about 130 to 260 ° C., preferably 14
0 to 250 ° C, more preferably 150 to 200 ° C.
° C. When the temperature in the depolymerization reaction system is lower than 130 ° C., the reaction is difficult to proceed, while when the temperature in the depolymerization reaction system is higher than 260 ° C., the optical purity of lactide which may cause racemization is lowered. To do. In addition, it is not preferable because coloring occurs.

In the present method, there is theoretically no upper limit of the pressure in the depolymerization reaction system, but it is preferable to keep the pressure low practically. Normally, the pressure is set to atmospheric pressure (1.013 × 1
0 ⁵ Pa) or less, preferably 1.5 × 10 ⁴
More preferably, it is Pa or less. The reason is that the higher the pressure in the reaction system, the larger the amount of steam required, the lower the lactide concentration in the steam, and the lower the lactide collection efficiency. On the other hand, as for the lower limit of the pressure in the depolymerization reaction system, the lower the pressure, the more desirable in terms of reactivity. However, as described above, a high vacuum device is required to achieve a low pressure, and the high vacuum If you drive in the system,
It is not suitable for an actual plant because the pipes are likely to be clogged. Therefore, in this method, it is appropriate to set the pressure in the depolymerization reaction system to 6.0 × 10 ^{2 to} 1.013 × 10 ⁵ Pa, and 1.3 × 10 ^{3 to} 1.5 × 10 ⁴ P
a is preferred.

In the method of the present invention, a depolymerization catalyst can generally be used. Although the catalyst for depolymerization is not particularly limited, it is usually group IA, I
A catalyst comprising a metal or a metal compound selected from the group consisting of Group IIA, Group IVA, Group IIB, Group IVB and Group VA can be used.

As those belonging to Group IA, for example, alkali metal hydroxides (eg sodium hydroxide, potassium hydroxide, lithium hydroxide, etc.), alkali metal and weak acid salts (eg sodium lactate, sodium acetate). , Sodium carbonate, sodium octylate, sodium stearate, potassium lactate, potassium acetate, potassium carbonate, potassium octylate, etc.), alkali metal alkoxides (eg, sodium methoxide, potassium methoxide, sodium ethoxide, potassium ethoxide, etc.) )
And the like. Examples of the group IIIA group include aluminum ethoxide, aluminum isopropoxide, aluminum oxide, and aluminum chloride. Examples of those belonging to the IVA group include organotin-based catalysts (for example, tin lactate, tin tartrate, tin dicaprylate, tin dilaurate, tin dipalmitate, tin distearate, tin dioleate, tin α-naphthoate) , Tin β-naphthoate, tin octylate, etc.), powdered tin, tin oxide, tin halide, and the like. Those belonging to Group IIB include, for example, zinc dust, zinc halide, zinc oxide, organic zinc compounds and the like. Examples of the compounds belonging to Group IVB include titanium-based compounds such as tetrapropyl titanate and zirconium-based compounds such as zirconium isopropoxide. Examples of compounds belonging to Group VA include antimony compounds such as antimony trioxide and bismuth compounds such as bismuth (III) oxide.

Among these, a catalyst comprising tin or a tin compound is preferred from the viewpoint of activity, and tin octylate is particularly preferred. These catalysts are used in an amount of 0.05 to 20% by weight, preferably 0.1% by weight, based on the lactic acid oligomer as the raw material.
It is about 5 to 10% by weight.

Next, an apparatus for carrying out the method of the present invention will be described. In this method, a device for increasing the gas-liquid contact area known as a depolymerization reactor can be used. For example, a reactor such as a bubble stirring tower or a spray tower is preferable. Apparatus designed for steam distillation widely used in industry can also be used.

FIG. 1 is a schematic diagram showing a typical example of a batch type apparatus. In FIG. 1, the depolymerization reactor (1) is
This is a vertical reactor having a stirrer (2) and has a temperature control function. In this method, a raw material lactic acid oligomer is charged into this reactor (1). Alternatively, a lactic acid monomer may be charged into the reactor (1) to be dehydrated and condensed to obtain a lactic acid oligomer, and the lactic acid oligomer may be used as a raw material.

The conduit (3) is for supplying steam into the reactor (1). The steam is preferably preheated by a heating device (not shown) and fed into the reactor (1) via a conduit (3).

The conduit (4) is for sending lactide vapor and steam generated in the reactor (1) out of the reactor (1) and leading it to the first condenser (5). This conduit (4) is either heated or insulated. The condensate in the first condenser (5) is collected in the first receiver (6). The vapor not condensed in the first condenser (5) passes through the second condenser through the conduit (7).
The condensate is led to (8) and collected in the second receiver (9). The second condenser (8) has a vacuum pump if necessary.
(10) is connected so that the pressure inside the reaction system can be reduced.

In the present method, it is desirable to set the temperature of the refrigerant supplied to the first condenser (5) to a temperature at which lactide is condensed and water is not condensed in accordance with the operating pressure in the depolymerization reaction system. Further, it is desirable to set the temperature of the refrigerant supplied to the second condenser (8) to a temperature at which water condenses in accordance with the operating pressure in the depolymerization reaction system. By setting in this way, the lactide is mainly condensed from the vapor mixture of lactide and water in the first condenser (5) and collected in the first receiver (6), and the second condenser (8). The water can be mainly condensed with and can be collected with the second receiver (9).

According to the method of the present invention, since the depolymerization of the lactic acid oligomer is performed while blowing steam into the depolymerization reaction system, a sufficiently large contact surface area is generated between the lactic acid oligomer and the steam, and the generated lactide is produced. It is easy to vaporize.
For this reason, in the present method, it is possible to generate lactide and distill lactide vapor at a lower temperature and a higher pressure than in the conventional case where no steam flow is present, and it causes coloring and racemization. Lactide products of high optical purity can be obtained without the need.

That is, as a raw material lactic acid oligomer,
When an L-lactic acid oligomer or a D-lactic acid oligomer is used, a lactide product having high optical purity can be obtained according to their optical purity. Also, no high vacuum device is required.

[0027]

EXAMPLES The present invention will be described in more detail below with reference to examples. The weight average molecular weight of the lactic acid oligomer was determined by GPC (gel permeation chromatography) under the following conditions.
It was determined by comparison with a polystyrene standard sample. Detector: RID-6A, Pump: LC-9A, Column oven: CT
O-6A, Column: Shim-pack GPC-801C, -804C, -806C, -8
025C in series (manufactured by Shimadzu Corporation) Mobile phase: chloroform, flow rate: 1 ml / min, sample amount: 200 μl (dissolved in chloroform so that sample concentration becomes 0.5 w / w%), column temperature: 40
° C.

[Example] (Synthesis of lactic acid oligomer) L-lactic acid (PH90, manufactured by PURAC) having an optical purity of 98.5% was dehydrated at a temperature of 160 ° C and a pressure of 1.3 × 10 ³ Pa. A lactic acid oligomer having a weight average molecular weight of about 1500 was synthesized. The optical purity of the obtained lactic acid oligomer was 98.0%.

(Depolymerization of lactic acid oligomer) A reactor having the same structure as shown in FIG. 1 was used. Temperature: 160 ° C., pressure: 2.1 × 10 ³ Pa in a 500 ml capacity flask (1)
The temperature of the first condenser (5) was set to 80 ° C., and the temperature of the second condenser (8) was set to 0 ° C. Flask (1)
405 g of the lactic acid oligomer obtained above was charged, and 3 g of tin octylate was added as a catalyst, and the blowing of steam preheated to 160 ° C. was started. After about 2 hours, lactide was still distilled, but operation was stopped. As a condensate in the first condenser (5), 54.1 g of colorless lactide was obtained, and the optical purity of this lactide was 9
It was 7.7%. Also, the water vapor supplied during this period is
It was 24.5 g.

[0030]

According to the lactide production method of the present invention, as described above, since the lactic acid oligomer is depolymerized while injecting steam into the depolymerization reaction system, the temperature is lower than that in the conventional method. The depolymerization operation can be performed at a higher pressure. As a result, lactide having a high optical purity can be efficiently produced without causing coloring or racemization, particularly when a lactic acid oligomer having a high optical purity is used as a raw material. Also, the method of the present invention,
Since it does not require a high vacuum device, it is a very suitable method for an actual plant.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a batch type apparatus configuration for carrying out the method of the present invention.

[Explanation of symbols]

 (1) -Depolymerization reactor (2) -Stirrer (3) (4) (7) -Conduit (5) -First condenser (6) -First receiver (8) -Second condenser (9) )-Second receiver (10)-Vacuum pump

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsushi Kawamoto 1 Nishinokyo Kuwaharacho, Nakagyo-ku, Kyoto-shi, Kyoto Inside Shimazu Seisakusho Sanjo Plant (72) Inventor Hitomi Ohara 1 Nishinokyo-kuwaharacho, Nakagyo-ku, Kyoto, Kyoto (72) Inventor Hideo Noda 2-9-7 Minami-Nanamatsu-cho, Amagasaki City, Hyogo Prefecture Kansai Chemical Machinery Manufacturing Co., Ltd.

Claims (8)

[Claims] 1. A method for producing a lactide by depolymerizing a lactic acid oligomer, which is produced by depolymerizing a lactic acid oligomer under appropriate depolymerization temperature and pressure conditions while blowing steam into the depolymerization reaction system. Distill lactide,
A method for producing lactide. 2. The weight average molecular weight of the lactic acid oligomer is 40.
The method for producing lactide according to claim 1, wherein the lactide is 0 to 30,000. 3. The method for producing lactide according to claim 1, wherein steam is blown at a flow rate of 0.1 to ³⁰ mol / s per m ^{3 of} lactic acid oligomer. 4. The temperature in the depolymerization reaction system is from 130 to 260.
The method for producing lactide according to any one of claims 1 to 3, which is at 0 ° C. 5. The steam according to claim 1, wherein steam is preheated to a temperature equal to or close to the temperature in the depolymerization reaction system, and the steam preheated is blown. A method for producing the lactide described. 6. The pressure in the depolymerization reaction system is 1.013 × 1.
The method for producing lactide according to any one of claims 1 to 5, wherein the lactide is 0 ⁵ Pa or less. 7. The lactic acid oligomer according to claim 1, wherein the lactic acid is dehydrated and condensed at an appropriate heating temperature and reduced pressure to obtain a lactic acid oligomer, and the lactic acid oligomer is depolymerized. A method for producing lactide. 8. The lactic acid oligomer is an L-lactic acid oligomer, a D-lactic acid oligomer, a DL-lactic acid oligomer, or a lactic acid oligomer having any D, L ratio.
Item 7. A method for producing lactide according to any one of items 7.