Patent Application
20230338936
This patent application claims the benefit and priority of Chinese Patent Application No. 202011480932.2 filed on Dec. 15, 2020, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.
The present disclosure belongs to the technical field of L-lactide preparation, and specifically relates to a stannous salt complex catalyst and a method for producing L-lactide by using the same.
At present, the commonly used cracking catalysts for preparing lactide are mainly metals such as zinc, tin and their metal salts, among which the most widely used are stannous salts, including stannous octoate, stannous chloride, and stannous sulfate. However, stannous salt is very easy to be oxidized, especially under the conditions of cracking reaction (>200° C.), which will be rapidly oxidized and decomposed to generate tetravalent tin salt, thereby losing catalytic activity, and tin salt will also cause the racemization of lactide, which increases the amount of meso-lactide. At the same time, it will cause the color of lactide and residual liquid to deepen.
CN101903370B discloses a stannous phosphite as a cracking catalyst for synthesizing lactide. The reducibility of phosphorous acid prevents the oxidation of stannous ions into tin ions, thereby reducing the conversion rate of meso-lactide and improving the hue of lactide. However, phosphite is more toxic and can generate highly toxic phosphine under acidic conditions, which is less safe. CN110156745A discloses a composite catalyst of zinc compound and / or tin compound and alkali metal compound, but the addition of alkali metal compound cannot improve the oxidation of stannous salt to tin salt. GB2331986A discloses the compound use of stannous octoate and phosphite stabilizer for preparing lactide. However, phosphite, as a stabilizer, does not coordinate with stannous salt, which could not completely prevent stannous salt from being oxidized to stannous salt, only improves the hue of lactide, but does not reduce the conversion rate of meso-lactide.
The purpose of the present disclosure is to provide a method for preparing a stannous salt complex catalyst and a method for producing lactide using the same. The preparation process of the catalyst provided by the present disclosure is simple and pollution-free, has low requirements on equipment, and is convenient for industrialization. The catalyst has high stability at high temperature, stannous salt will not be oxidized into tetravalent tin salt, and the L-lactide produced by using the catalyst provided by the present disclosure has a good hue, a small content of meso-lactide and a good hue of the residual liquid.
In order to achieve the above purpose, the present disclosure adopts the following technical schemes:
In some embodiments, the stannous salt is one or more selected from the group consisting of stannous isooctanoate, stannous chloride, stannous sulfate, stannous oxalate and stannous phosphate.
In some embodiments, the phosphite is one or more selected from the group consisting of triphenyl phosphite, tridecyl phosphite, diphenyl phosphite, and diisodecyl pentaerythritol diphosphite.
In some embodiments, the solvent is one or two or more of benzene, toluene, ethyl acetate, acetone and xylene.
In some embodiments, the mass ratio of stannous salt to phosphite is 5: 1, and the mass ratio of stannous salt to solvent is 2: 1.
The present disclosure provides a method for preparing the stannous salt complex catalyst described in above technical schemes, wherein comprising the following steps:
The present disclosure provides a method for producing L-lactide using the stannous salt complex catalyst, wherein comprising the following steps:
In some embodiments, the addition amount of the stannous salt complex catalyst is 0.5-5 % of the mass of the oligomeric lactic acid with a degree of polymerization of 8-25.
In some embodiments, the third reactor is a cage evaporation reactor, a scraper type film evaporator, a falling film evaporator or a rising film evaporator.
In some embodiments, the crude lactide is purified by one or several methods of melt crystallization, rectification and solvent recrystallization.
The beneficial effects of the present disclosure are as follows: the present disclosure uses the coordination reaction of stannous salt and phosphite to prepare a catalyst, which can significantly improve the stability of the catalyst at high temperature, and can effectively inhibit the oxidation of divalent stannous ions to tetravalent tin ions; it can effectively reduce the conversion rate of meso-lactide, increase the yield of L-lactide, and obtain nearly white L-lactide. In the crude lactide produced after the depolymerization reaction, the content of L-lactide is more than or equal to 95 %; the content of meso-lactide is less than or equal to 3.8 %, while ensuring that the color of the residual liquid is close to milky white.
The present disclosure will be further described below in conjunction with specific embodiments, but the present disclosure is not limited to these embodiments only.
100 g of stannous isooctanoate was added to 50 g of toluene, dissolved under reflux at 110° C. for 30 min, 20 g triedecyl phosphite was added to the mixed solution of stannous isooctanoate and toluene, and the reflux reaction was continued for 2 h. After the reaction, toluene in the reaction system was extracted under vacuum and recycled. The resulting light yellow viscous liquid was the stannous salt complex catalyst.
A L-lactic acid aqueous solution with a mass concentration of 90 % was added to the first reactor at a rate of 15 kg/h. The first reactor was equipped with a rectifying tower. The reaction temperature was kept at 120° C., the vacuum degree was -0.08 MPa, the time was 2 h, the degree of polymerization of the oligomeric lactic acid was controlled to 6-8, and the reaction product was continuously transported to the second reactor by a pump.
The second reactor was also equipped with a rectification device. The oligomeric lactic acid was further polymerized at a temperature of 160° C. and a vacuum degree of -0.1 MPa. The degree of polymerization of the oligomeric lactic acid was controlled to 15-20. The reaction product was continuously transported to the third reactor by a pump.
The oligomeric lactic acid from the second reactor and the continuously injected stannous salt complex catalyst were mixed in a static mixer, and then added to the rising film evaporator with an evaporation area of 0.8 m2 at a rate of 13 kg/h, wherein the amount of stannous salt complex catalyst used was 1% of the mass of oligomeric lactic acid, the reaction temperature of the evaporator was 230° C., and the pressure was 500 Pa. The crude lactide was continuously generated, and condensed and collected in the lactide storage tank. The unevaporated heavy components were introduced into the residual liquid collection tank. The components of the crude lactide are shown in Table 1.
The residual liquid produced by cracking was mixed with oligomeric lactic acid and stannous salt complex catalyst in a static mixer by continuous injection, and then re-added to the rising film evaporator. After a month of circulating operation, the components of crude lactide are shown in Table 2.
The catalyst used in the Comparative Example is stannous isooctanoate, and the amount and the method for producing lactide are the same as the Example.
It can be seen from Table 1 that after using the stannous salt complex catalyst, the purity of L-lactide is significantly improved, while the amount of meso-lactide is significantly reduced, and the color of the crude lactide and the residual liquid is good. It can be seen from Table 2 that the purity of L-lactide in the Example is reduced to a certain extent after all the residual liquid is recycled for one month, but the purity of L-lactide in the Comparative Example is reduced more obviously, the color of crude lactide is darker, and the color of the residual liquid turns brown. It can be seen that the stannous salt complex catalyst provided by the present disclosure can increase the purity of L-lactide, reduce the amount of meso-lactide produced, and improve the color of lactide and residual liquid.
The description of the above embodiments is only used to help understand the method and core idea of the present disclosure. It should be pointed out that for the persons skilled in the art, without departing from the principle of the present disclosure, several improvements and modifications can be made to the present disclosure, and these improvements and modifications also fall within the protection scope of the claims of the present disclosure. Various modifications to these embodiments are obvious to the persons skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to the embodiments shown in this application, but should conform to the widest scope consistent with the principles and novel features disclosed in this application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202011480932.2 | Dec 2020 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2021/113174 | 8/18/2021 | WO |
