CN114805285A

CN114805285A - Method for preparing lactide from L-lactic acid

Info

Publication number: CN114805285A
Application number: CN202210633794.XA
Authority: CN
Inventors: 杨维冉; 王艳涛; 李娜
Original assignee: Nanchang University
Current assignee: Nanchang University
Priority date: 2022-06-07
Filing date: 2022-06-07
Publication date: 2022-07-29

Abstract

The invention discloses a method for preparing lactide from L-lactic acid, which comprises the steps of adding L-lactic acid and a catalyst into a round-bottom flask, then adding a solvent, installing an oil-water separator and a condenser tube, refluxing the solvent in the reaction process, and putting the solvent into an oil bath kettle preheated at the temperature of 120-160 ℃ for stirring reaction; and (3) cooling to room temperature after reacting for 1-6 h, and performing high-temperature reaction by using a fixed bed after constant volume. Detecting the concentration of the organic phase lactide; and carrying out reduced pressure distillation and concentration on the collected organic phase to obtain crude lactide. The reactant used by the invention can be prepared from biomass, and the source is green; the reaction time is short, and the reaction temperature is moderate; the reaction condition is mild, the reaction system is simple, no inflation body is needed, and the safety is high; the yield is higher; the post-treatment is simple; has important application value in industry.

Description

Method for preparing lactide from L-lactic acid

Technical Field

The invention belongs to the technical field of chemical synthesis, and particularly relates to a method for preparing lactide from L-lactic acid.

Background

In recent decades, biodegradable polymers have attracted increasing attention due to depletion of fossil fuel resources and pollution of plastic processing (Journal of Cleaner Production 165(2017) 157-167). The synthesis of stereopure lactide, particularly L-lactide, is the most desirable isomer for the synthesis of high quality polylactic acid (PLA). Polylactic acid (PLA) prepared from lactic acid is one of the most promising biopolymers. The environmental properties and good material properties (e.g. compostability and biocompatibility) of polylactic acid make it a benign alternative to petroleum-based polymers, with wide applications in packaging, electronics, textiles and medical materials (ind. The conventional route for lactide synthesis consists of two steps of polycondensation and depolymerization. Firstly, dehydrating and polycondensing lactic acid into a lactic acid oligomer under the conditions of certain temperature and vacuum degree; second, the lactic acid oligomer is depolymerized at high temperature in the presence of a catalyst to produce lactide. Finally, crude lactide (Ind. Eng. chem. Res.2018,57,7711-7716) is obtained by distillation under reduced pressure. The Lactic Acid (LA) can be prepared from renewable biomass such as corn, cassava, straw and the like, and has the advantages of wide source, low price and renewability. Most of lactic acid synthesized by microbial fermentation is L-LA (ChemSus chem 2016,9, 907-. At present, the production capacity of polylactic acid is mainly concentrated in overseas, lactide as an intermediate of the polylactic acid is high in price, and the lactic acid can be completely converted into the lactide through a fixed bed. There has been no report of the metal-free catalytic conversion of L-lactic acid to lactide using a fixed bed.

Disclosure of Invention

Aiming at overcoming the defects and difficulties in the prior art, the invention aims to provide a method for preparing lactide from L-lactic acid.

The invention is realized by the following technical scheme:

a method for preparing lactide from L-lactic acid, comprising the steps of:

s1, adding L-lactic acid and a catalyst into a round-bottom flask, then adding the solvent, installing an oil-water separator and a condenser tube, putting the mixture into a preheated oil bath, stirring and reacting, cooling to room temperature after the reaction is finished, and carrying out high-temperature reaction on a fixed bed after constant volume use;

s2, reacting for 1-6 h in the step S1, cooling to room temperature, and performing high-temperature reaction on the mixture in a fixed bed at constant volume; detecting the concentrations of the L-lactic acid and the lactide in the organic phase, and respectively taking a proper amount of filtrate to detect by using a high performance liquid chromatograph;

s3, carrying out reduced pressure distillation and concentration on the organic phase collected in the step S2 to obtain crude lactide.

Further, the molar ratio of the L-lactic acid to the catalyst in the step S1 is 0.01-10: 1; the molar volume ratio (mol/L) of the L-lactic acid to the solvent is 0.01-1: 2; the preheating temperature of the oil bath pot is 120-160 ℃.

Further, the solvent of step S1 is one or more of toluene, o-xylene, and m-xylene.

Further, the fixed bed reaction temperature of the step S2 is 210-250 ℃.

Further, the catalyst is one or more of sodium hydroxide, potassium hydroxide, 2-methylimidazole, cesium carbonate, sodium carbonate, potassium carbonate, sodium bicarbonate and ion exchange resin.

Compared with the prior art, the invention has the beneficial effects that:

(1) the invention provides a method for preparing lactide by biomass-based L-lactic acid, which has mild reaction conditions, simple reaction system and no need of any metal catalyst, only needs to add a certain amount of solvent into the L-lactic acid in an alkaline or acidic environment to convert the L-lactic acid into lactic acid oligomer, cools after the reaction is finished, carries out high-temperature reaction by using a fixed bed after the volume is fixed, and continuously depolymerizes the residual lactic acid oligomer into the lactide.

(2) The reactant used by the invention can be prepared from biomass, and the source is green; the reaction time is short, and the reaction temperature is moderate; an inflatable body is not needed, and the safety is high; the yield is high; the post-treatment is simple; has important application value in industry.

Drawings

FIG. 1 is a schematic diagram of the reaction of L-lactic acid to lactide according to the present invention.

Detailed Description

The present invention will be described below with reference to specific examples, but the present invention is not limited thereto.

The experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials are commercially available, unless otherwise specified.

Example 1.

1mmol L-lactic acid, 0.1g HND-580 and 30ml toluene are added into a 100ml round bottom flask in turn, put into an oil bath kettle preheated to 140 ℃ and stirred for reaction for 1 hour. Cooling to room temperature after reaction, adding acetonitrile to a constant volume of 250ml, performing high-temperature reaction by using a fixed bed at the temperature of 250 ℃ and the flow rate of 0.5ml/min, collecting an organic phase, and detecting to obtain 100% lactide. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

The lactide yield was measured and calculated according to the following method.

The lactide concentration detector is a Japanese Shimadzu ACQUITY UPLC HClass series high performance liquid chromatograph (PDA detector, 196nm wavelength, LC Column 250X 4.6mm liquid chromatography Column, mobile phase 30% acetonitrile, 70% ultrapure water).

Calculation of the yield of lactide:

the yield of lactide was 2 × (lactide molar amount/L-lactic acid charged molar amount) × 100%

Example 2.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added in sequence to a 100ml round bottom flask, and stirred in an oil bath preheated to 140 ℃ for reaction for 1 h. Cooling to room temperature after reaction, adding acetonitrile to constant volume of 250ml, performing high temperature reaction at 250 ℃ and a flow rate of 0.5ml/min by using a fixed bed, collecting an organic phase, and detecting to obtain the lactide yield of 72%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

Example 3.

Directly adding acetonitrile into 10mmol L-lactic acid to constant volume of 250ml, performing high temperature reaction at 250 deg.C and flow rate of 0.5ml/min with fixed bed, collecting organic phase, and detecting to obtain lactide with yield of 12%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

Example 4.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added in sequence to a 100ml round bottom flask, and stirred in an oil bath preheated to 140 ℃ for reaction for 1 h. Cooling to room temperature after reaction, adding acetonitrile to constant volume of 250ml, performing high temperature reaction at 250 ℃ and a flow rate of 0.3ml/min by using a fixed bed, collecting an organic phase, and detecting to obtain the lactide yield of 70%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

Example 5.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added in sequence to a 100ml round bottom flask, and stirred in an oil bath preheated to 140 ℃ for reaction for 1 h. After the reaction, the mixture is cooled to room temperature, acetonitrile is added to the mixture to reach a constant volume of 250ml, a fixed bed is used for high-temperature reaction at the temperature of 250 ℃ and the flow rate of 1ml/min, an organic phase is collected and detected, and the yield of the lactide is 66.6%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

Example 6.

10mmol L-lactic acid, 0.1g HND-580 and 30ml toluene are added into a 100ml round bottom flask in turn, put into an oil bath kettle preheated to 140 ℃ and stirred for reaction for 2 hours. After the reaction, the mixture is cooled to room temperature, acetonitrile is added to the mixture to reach a constant volume of 250ml, a fixed bed is used for high-temperature reaction at the temperature of 250 ℃ and the flow rate of 0.5ml/min, an organic phase is collected and detected, and the yield of the lactide is 73.1%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

Example 7.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added in sequence to a 100ml round bottom flask, and stirred in an oil bath preheated to 140 ℃ for reaction for 1 h. After the reaction, the mixture is cooled to room temperature, acetonitrile is added to the mixture to reach a constant volume of 250ml, the mixture is subjected to high-temperature reaction by a fixed bed at a temperature of 220 ℃ and a flow rate of 0.5ml/min, an organic phase is collected and detected, and the yield of the lactide is 59.5%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

Example 8.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added in sequence to a 100ml round bottom flask, and stirred in an oil bath preheated to 140 ℃ for reaction for 1 h. Cooling to room temperature after reaction, adding acetonitrile to constant volume of 250ml, performing high temperature reaction at 270 ℃ and a flow rate of 0.5ml/min by using a fixed bed, collecting an organic phase, and detecting to obtain the lactide yield of 51%. Then the collected organic phase is subjected to reduced pressure distillation and concentration steps to obtain crude lactide, and the reaction formula is shown in figure 1.

The foregoing merely represents preferred embodiments of the invention, which are described in some detail and detail, and therefore should not be construed as limiting the scope of the invention. It should be noted that, for those skilled in the art, various changes, modifications and substitutions can be made without departing from the spirit of the present invention, and these are all within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A method for producing lactide from L-lactic acid, characterized by: the method comprises the following steps:

s1, adding L-lactic acid and a catalyst into a round-bottom flask, then adding the solvent, installing an oil-water separator and a condenser tube, putting the mixture into a preheated oil bath, stirring and reacting, cooling to room temperature after the reaction is finished, and carrying out high-temperature reaction by using a fixed bed after constant volume;

s2, reacting for 1-6 h in the step S1, cooling to room temperature, and performing high-temperature reaction on the mixture in a fixed bed at constant volume; detecting the concentrations of the L-lactic acid and the lactide in the organic phase, respectively filtering the organic phase by proper amounts and detecting the organic phase by a high performance liquid chromatograph;

2. A process for the preparation of lactide from L-lactic acid according to claim 1, characterised in that: in the step S1, the molar ratio of the L-lactic acid to the catalyst is 0.01-10: 1; the molar volume ratio of the L-lactic acid to the solvent is 0.01-1: 2; the preheating temperature of the oil bath pot is 120-160 ℃.

3. A process for the preparation of lactide from L-lactic acid according to claim 1, characterized in that: in the step S2, the reaction temperature of the fixed bed is 210-250 ℃.

4. A process for the preparation of lactide from L-lactic acid according to claim 1, characterized in that: the solvent in the step S1 is one or more of toluene, o-xylene and m-xylene.

5. A process for the preparation of lactide from L-lactic acid according to claim 1, characterized in that: the catalyst is one or more of sodium hydroxide, potassium hydroxide, 2-methylimidazole, cesium carbonate, sodium carbonate, potassium carbonate, sodium bicarbonate and ion exchange resin.