JPH07138253A - Production of lactide - Google Patents

Abstract

PURPOSE:To obtain in high yield lactide of high optical purity through such processes that lactic acid is heated and concentrated and then dehydrated under reflux to produce a lactic acid oligomer which is then depolymerized in the presence of a catalyst into lactide which is then distilled. CONSTITUTION:Firstly, lactic acid (pref. >=99% in optical purity produced by fermentation process) is heated pref. at 130-140 deg.C under normal pressures and concentrated, and then dehydrated under reflux to produce a lactic acid oligomer. Next, this oligomer is heated pref. at 190-210 deg.C under a reduced pressure of 10-50Torr to conduct a depolymerization and the objective lactide produced is distilled. It is recommended that the above-mentioned refluxing process be conducted while gradually raising temperature at 135+ or -3 deg.C in the 1st stage, at 150+ or -3 deg.C in the 2nd stage and at 160+ or -3 deg.C in the 3rd stage.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing L- or D-lactide having high optical purity.

[0002]

2. Description of the Related Art Lactide is a cyclic diester formed by dehydration of two lactic acid molecules and is a raw material for polylactic acid. As a method for producing lactide, for example, JP-A-63-101378 is used.
In the gazette, first, lactic acid is dehydrated and concentrated to obtain an intermediate oligomer, which is added with a tin-based catalyst and heated at about 200 ° C. in a high vacuum for decomposition and cyclic dimerization to give lactide. The manufacturing method of distilling off is described.

[0003]

[Chemical 1] However, in such a production method, in addition to lactide which is a cyclic dimer, the reaction solution contains a large amount of a linear oligomer having a low degree of polymerization. Therefore, when distilling lactide, these oligomers distill together with lactide, and the optical purity and yield of the product lactide decrease. On the other hand, to prevent this,
When the degree of dehydration and concentration of lactic acid is increased, lactic acid is distilled out at the time of concentration, the yield of lactide is reduced, and the optical purity is also reduced.

An object of the present invention is to provide a method for producing lactide with high optical purity in high yield.

[0005]

Means for Solving the Problems The present invention includes the following (i) to
Step (iii): (i) heating and concentrating lactic acid, (ii) dehydrating the concentrated lactic acid while refluxing to produce a lactic acid oligomer, and (iii) using the lactic acid oligomer as a catalyst. The present invention provides a method for producing lactide, which comprises a step of depolymerizing in the presence and distilling off. Next, each of these steps will be described in detail.

(I) Concentration Step In the first step of the present invention, lactic acid is concentrated to produce an oligomer. The lactic acid used may be either L-lactic acid or D-lactic acid. As such lactic acid, any one produced by a conventionally known method is used.
Especially, the purity of 80% or more produced by the fermentation method and the optical purity of 9
It is preferably 9% or more. Such lactic acid is heated at a temperature of 130 to 140 ° C. under normal pressure to perform simple distillation, and water is distilled off until no gas is generated.

(Ii) Refluxing step The mixed solution containing the oligomer of lactic acid obtained in the above step (ii) as a main component is totally refluxed while dehydrating, preferably under reduced pressure. The reduced pressure is preferably 10 to 100 Torr. The reflux is performed at a temperature of 130 to 170 ° C. for 3 to 4 hours. It is preferable that the reflux temperature be gradually raised in stages. For example, it is preferable that the first stage is 135 ± 3 ° C., the second stage is 150 ± 3 ° C., and the third stage is 160 ± 3 ° C. By such refluxing, lactic acid becomes a 6 to 10-mer linear oligomer, and the difference in boiling point from lactide becomes large, which facilitates separation. The refluxing step may be performed in multiple stages, or the temperature may be continuously and gradually increased to perform heating until reflux is stopped. Lactide is a needle-shaped crystal with a melting point of 95 to 100 ° C., and 105 ° C. to prevent clogging of the reflux pipe and the pipe.
Keep it warm.

(Iii) Distillation step Next, the highly pure oligomer obtained in step (ii) is subjected to cyclic dimerization. That is, a cyclic dimerization (transesterification reaction) catalyst is added, and the mixture is preferably heated under reduced pressure to distill off lactide. As the catalyst used here, any conventionally known catalyst can be used, and for example, tin octylate, antimony trioxide, zinc oxide or the like can be used. Heating is performed under reduced pressure of 10 to 50 Torr, preferably 1
Perform at 90-210 ° C. Further, the yield of lactide is further improved by using a high vacuum and high temperature apparatus.

As described above, if a process of refluxing at a predetermined temperature and dehydration is added between the conventional two processes of lactic acid concentration and lactide distillation, the amount of oligomers having a high degree of polymerization increases and the lactide can be easily isolated. Became. In the present invention, distilling lactic acid can be prevented by concentrating and refluxing lactic acid, the optical purity of lactide does not decrease, and the yield of lactide improves.

[0010]

EXAMPLES Next, the present invention will be described more specifically based on examples.

[Example 1] L- of 99% or more of optical purity
L-lactide was synthesized from lactic acid (90% by weight) as follows.

35 kg of the raw material was put into a glass reactor (50 L), the raw material lactic acid was heated to a liquid temperature of 135 ° C., and simple distillation was carried out under normal pressure to concentrate for 30 minutes. Then, under reduced pressure (13-7
Total reflux was performed at 0 Torr). The liquid temperature was set in the following three stages, and the process proceeded to the next stage as soon as it reached each stage.

First stage: Liquid temperature set at 135 ° C. Second stage: Liquid temperature set at 150 ° C. Third stage: Liquid temperature set at 160 ° C. Then, tin octylate was added to the catalyst under reduced pressure (10 to 10 ° C.).
The lactide was distilled off by simple distillation at a liquid temperature of 200 ° C. at 35 Torr). The condenser temperature was set to 105 ° C. to prevent pipe clogging. During the above operation, stirring at 300 rpm was continued. The results are shown in Table 1.

[Example 2] L- with 99% or more of optical purity
L-lactide was synthesized from lactic acid (90% by weight) as follows.

35 kg of the raw material was put into a glass reactor (50 L), the raw material lactic acid was heated to a liquid temperature of 135 ° C., and simple distillation was carried out under normal pressure to concentrate for 30 minutes. Then, under reduced pressure (20-7
(0 Torr) The liquid temperature was set to 200 ° C. to carry out total reflux.
Then, the catalyst was added with tin octylate under reduced pressure (10-
Lactide was distilled off by simple distillation at a liquid temperature of 200 ° C. at 25 Torr). The condenser temperature was set to 105 ° C. to prevent pipe clogging. During the above operation, stirring at 300 rpm was continued. The results are shown in Table 1.

[Example 3] L- with 99% or more of optical purity
L-lactide was synthesized from lactic acid (90% by weight) as follows.

35 kg of the raw material was put into a glass reactor (50 L), the raw material lactic acid was heated to a liquid temperature of 135 ° C., and simple distillation was carried out under normal pressure to concentrate for 30 minutes. Then, under reduced pressure (30-6
(0 Torr) The liquid temperature was set to 160 ° C. to carry out total reflux.
Then, the catalyst was added with tin octylate under reduced pressure (10-
The lactide was distilled off by simple distillation at a liquid temperature of 200 ° C. at 20 Torr). The condenser temperature was set to 105 ° C. to prevent pipe clogging. During the above operation, stirring at 300 rpm was continued. The results are shown in Table 1.

[Table 1] ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Test No Purity (%) Yield (%) Optical purity (%) ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Example 1 80.4 77 .7 99.1 〃 2 85.9 82.9 91.7 〃 3 58.5 56.0 99.5 ━━━━━━━━━━━━━━━━━━━━━━━ ━━━━━━━━━━━━ As is clear from Table 1, according to the production method of the present invention, L-lactic acid (90% by weight) having an optical purity of 99% or more was used to obtain an optical purity of 9%.
9.0% or more of L-lactide is obtained in a high yield of 70% or more with respect to the theoretical yield.

[0019]

According to the production method of the present invention, lactide having high optical purity can be obtained in high yield.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hitomi Ohara, No. 1 Kuwabara-cho, Nishinokyo, Nakagyo-ku, Kyoto City Stock Company Shimadzu Sanjo Factory

Claims (2)

[Claims] 1. The following steps (i) to (iii): (i) heating and concentrating lactic acid, (ii) dehydrating the concentrated lactic acid while refluxing to produce a lactic acid oligomer, And (iii) a method for producing lactide, which comprises depolymerizing the lactic acid oligomer in the presence of a catalyst and distilling it off. 2. The following first to third steps in the step (ii):
The method for producing lactide according to claim 1, wherein the reflux is carried out under stepwise temperature conditions. First stage: Liquid temperature 135 ± 3 ° C Second stage: Liquid temperature 150 ± 3 ° C Third stage: Liquid temperature 160 ± 3 ° C