CN114805285A - Method for preparing lactide from L-lactic acid - Google Patents

Abstract

The invention discloses a method for preparing lactide from L-lactic acid. In the method, L-lactic acid and a catalyst are added into a round-bottomed flask, then a solvent is added, and an oil-water separator and a condenser are installed. The solvent was refluxed, put into a preheated oil bath at 120-160°C, and stirred for reaction; cooled to room temperature after 1-6 hours of reaction, and a fixed bed was used for high-temperature reaction after constant volume. Detect the lactide concentration of the organic phase; the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The reactant used in 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 conditions are mild, the reaction system is simple, no gas is required, and the safety is high; the yield is high; the post-processing is simple; industrially It has very important application value.

Description

A kind of method for preparing lactide from L-lactic acid

technical field

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

Background technique

Biodegradable polymers have attracted increasing attention in recent decades due to the depletion of fossil fuel resources and the pollution of plastic disposal (Journal of Cleaner Production 165 (2017) 157-167). Synthesis of stereopure lactide, especially L-lactide, which is the most ideal 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 eco-friendly properties and good material properties (such as compostability and biocompatibility) of PLA make it a benign alternative to petroleum-based polymers, with a wide range of applications in packaging, electronics, textiles, and medical materials (Ind.Eng Chem. Res. 2017, 56, 4867-4877). The conventional route for lactide synthesis consists of two steps, polycondensation and depolymerization. In the first step, lactic acid is dehydrated and polycondensed into lactic acid oligomers under certain temperature and vacuum conditions; second, under the conditions of high temperature and the presence of catalysts, lactic acid oligomers are depolymerized to form lactide. Finally, crude lactide was obtained by distillation under reduced pressure (Ind.Eng.Chem.Res.2018,57,7711-7716). Lactic acid (LA) can be used as raw materials from renewable biomass such as corn, cassava, and straw, and it has a wide range of sources, is inexpensive and renewable. Most of the lactic acid synthesized by microbial fermentation is L-LA (ChemSusChem 2016, 9, 907–921), and LA can be converted into various high-value chemicals, such as acetaldehyde, 2,3-pentanedione, acrylic acid, pyruvic acid, Propionic acid, lactide, 1,2-propanediol and biodegradable polylactic acid polymers (Chem. Rec. 2020, 20, 1–22). At present, the production capacity of polylactic acid is mainly concentrated overseas, and the price of lactide as an intermediate of polylactic acid is relatively high, and lactic acid can be completely converted into lactide through a fixed bed. There is no report on the production of lactide from the metal-free catalytic conversion of L-lactic acid using a fixed bed.

SUMMARY OF THE INVENTION

In view of the deficiencies and problems in the prior art, the present invention aims to provide a method for preparing lactide from L-lactic acid.

The present invention is achieved through the following technical solutions:

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

S1. Add L-lactic acid and a catalyst into a round-bottomed flask, then add a solvent, install an oil-water separator and a condenser, put it into a preheated oil bath to stir the reaction, and cool down to room temperature after the reaction is completed. Fixed bed for high temperature reaction;

S2, cooling to room temperature after 1～6h reaction in step S1, and then carrying out a high temperature reaction with a fixed bed to constant volume; detecting the concentrations of L-lactic acid and lactide in the organic phase, respectively taking an appropriate amount for filtration and detecting with a high performance liquid chromatograph;

S3. The organic phase collected in step S2 is subjected to vacuum distillation and concentration steps to obtain crude lactide.

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

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

Further, the reaction temperature of the fixed bed in step S2 is 210-250°C.

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 beneficial effects of the present invention include:

(1) the invention provides a kind of method for preparing lactide with biomass-based L-lactic acid, the reaction conditions of the present invention are mild, the reaction system is simple, does not need any metal catalyst, only needs L-lactic acid in an alkaline or acidic environment , adding a certain amount of solvent to convert L-lactic acid into lactic acid oligomers, cooling after the reaction, and then using a fixed bed for high temperature reaction after the reaction is completed, and the remaining lactic acid oligomers continue to be depolymerized into lactide.

(2) The reactant used in the present invention can be prepared from biomass, and the source is green; the reaction time is short, the reaction temperature is moderate; no inflator is required, and the safety is high; the yield is high; the post-processing is simple; value.

Description of drawings

Fig. 1 is the reaction schematic diagram of preparing lactide from L-lactic acid of the present invention.

Detailed ways

The present invention will be described below through specific embodiments, but the present invention is not limited thereto.

The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources unless otherwise specified.

Example 1.

Add 1 mmol L-lactic acid, 0.1 g HND-580, 30 ml toluene into a 100 ml round-bottomed flask in sequence, and put them into an oil bath preheated to 140 °C and stir the reaction for 1 h. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out on a fixed bed, the temperature was set to 250°C, the flow rate was 0.5ml/min, the organic phase was collected and detected, and the lactide was 100%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

The detection and calculation of the yield of lactide were carried out according to the following method.

The detection instrument of lactide concentration is Japan Shimadzu ACQUITY UPLC HClass series high performance liquid chromatograph (PDA detector, 196nm wavelength, LC Column 250×4.6mm liquid chromatography column, mobile phase is 30% acetonitrile, 70% ultra pure water).

Yield calculation of lactide:

Yield of lactide=2*(molar amount of lactide/molar amount of input L-lactic acid)×100%

Example 2.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added to a 100ml round-bottomed flask in sequence, and the reaction was stirred in an oil bath preheated to 140°C for 1 hour. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out in a fixed bed at a temperature of 250°C and a flow rate of 0.5ml/min. The organic phase was collected and detected. The yield of lactide was 72%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

Example 3.

10mmol L-lactic acid was directly added with acetonitrile to make the volume to 250ml, and then the high temperature reaction was carried out in a fixed bed, the temperature was 250°C, the flow rate was 0.5ml/min, the organic phase was collected and detected, and the yield of lactide was 12%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

Example 4.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added to a 100ml round-bottomed flask in sequence, and the reaction was stirred in an oil bath preheated to 140°C for 1 hour. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out in a fixed bed at a temperature of 250°C and a flow rate of 0.3ml/min. The organic phase was collected and detected. The yield of lactide was 70%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

Example 5.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added to a 100ml round-bottomed flask in sequence, and the reaction was stirred in an oil bath preheated to 140°C for 1 hour. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out in a fixed bed at a temperature of 250°C and a flow rate of 1ml/min. The organic phase was collected and detected. The yield of lactide was 66.6%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

Example 6.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added to a 100ml round-bottomed flask in sequence, and the reaction was stirred in an oil bath preheated to 140°C for 2h. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out on a fixed bed at a temperature of 250°C and a flow rate of 0.5ml/min. The organic phase was collected and detected. The yield of lactide was 73.1%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

Example 7.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added to a 100ml round-bottomed flask in sequence, and the reaction was stirred in an oil bath preheated to 140°C for 1 hour. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out in a fixed bed at a temperature of 220°C and a flow rate of 0.5ml/min. The organic phase was collected and detected. The yield of lactide was 59.5%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

Example 8.

10mmol L-lactic acid, 0.1g HND-580, 30ml toluene were added to a 100ml round-bottomed flask in sequence, and the reaction was stirred in an oil bath preheated to 140°C for 1 hour. After the reaction, it was cooled to room temperature, and acetonitrile was added to make the volume to 250ml, and then the high temperature reaction was carried out in a fixed bed at a temperature of 270°C and a flow rate of 0.5ml/min. The organic phase was collected and detected. The yield of lactide was 51%. Then, the collected organic phase is subjected to vacuum distillation and concentration steps to obtain crude lactide. The above reaction formula is shown in Figure 1.

The above description only expresses the preferred embodiments of the present invention, and the description thereof is relatively specific and detailed, but should not be construed as a limitation on the scope of the patent of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications, improvements and substitutions can be made, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims (5)

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;

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

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.

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