Method for preparing crude lactide by using compound catalytic system

Abstract

The invention belongs to the technical field of high polymer material preparation, and particularly relates to a method for preparing crude lactide by using a compound catalytic system. The method comprises the following steps: S1, respectively adding a stannous main catalyst and a sulfonic acid cocatalyst into a reaction system containing lactic acid, and carrying out dehydration and oligomerization reaction by controlling the reaction temperature and pressure to obtain lactic acid oligomers with different molecular weights; s2, further performing cracking reaction on the lactic acid oligomer under the action of a compound catalytic system to obtain crude lactide; wherein the stannous main catalyst accounts for 0.01-1.0% of the mass of the lactic acid, and the sulfonic acid cocatalyst accounts for 0.01-1.5% of the mass of the lactic acid. According to the method, the problems of long dehydration retention time and low reaction efficiency in the dehydration process of the traditional single tin catalyst are solved, the strong protonic acid is introduced to facilitate the dehydration reaction, shorten the retention time and efficiently remove water among lactic acid molecules, the prepared crude lactide has lower free acid content, and a large amount of cost is saved for the subsequent purification of the crude lactide.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer material preparation, and particularly relates to a method for preparing crude lactide by utilizing a composite catalytic system. Background Art

[0002] Polylactic acid (PLA) is a biodegradable polymer material with excellent processing properties and promising market applications. It plays a significant role in supporting national policies to combat plastic pollution. Currently, commercial PLA production typically uses lactic acid as a raw material through a two-step process. This involves dehydrating and oligomerizing the lactic acid to form oligomers. These oligomers are then further cracked to produce crude lactide. This crude lactide is then purified through distillation and crystallization techniques to produce polymer-grade lactide. Finally, lactide is ring-opening polymerized to produce a range of polylactic acids with varying molecular weights.

[0003] The two-step process of producing polylactic acid requires dehydration, oligomerization and cracking to obtain crude lactide (see Appendix Figure 1 ), currently, stannous catalysts such as stannous chloride or stannous octoate are commonly used commercially. However, in the process of obtaining oligomers, tin catalysts are primarily effective in the cracking process, but their efficiency in the lactic acid dehydration condensation process is relatively average, resulting in a long dehydration reaction time. In addition to increasing the energy consumption for lactide production, prolonged exposure to high temperatures can lead to racemization reactions on the oligomer molecular chains, resulting in low-quality lactide products. Furthermore, the molecular weight obtained with existing dehydration systems is generally low, resulting in an insufficient degree of esterification. Residual lactic acid is drawn into the collection system under high temperature and high vacuum during the cracking process, resulting in a generally high free acid content in the crude lactide obtained, typically exceeding 300-400 meq / kg. This creates many problems for subsequent purification.

[0004] To address the deficiencies of the prior art, the present invention improves existing crude lactide preparation technology, particularly by screening and compounding the catalytic systems used in the dehydration and cracking processes. This effectively addresses the problems of long residence time and high reaction temperature during the dehydration process, resulting in insufficient molecular weight of the produced oligomers, and a high free acid content in the final crude lactide. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention can be achieved through the following technical solutions:

[0006] A method for preparing crude lactide using a composite catalytic system, the method comprising the following steps:

[0007] S1, adding a stannous main catalyst and a sulfonic acid co-catalyst to a reaction system containing lactic acid, respectively, and obtaining lactic acid oligomers of different molecular weights through dehydration and oligomerization by controlling the reaction temperature and pressure;

[0008] S2. further subjecting the lactic acid oligomers to a cracking reaction under the action of a composite catalytic system to obtain crude lactide;

[0009] The mass ratio of the stannous main catalyst to the lactic acid is 0.01-1.0%, and the mass ratio of the sulfonic acid co-catalyst to the lactic acid is 0.01-1.5%.

[0010] Furthermore, the stannous main catalyst is at least one of stannous chloride, stannous octoate, stannous sulfate, stannous pyrophosphate and dibutyltin dilaurate.

[0011] Furthermore, the sulfonic acid co-catalyst is at least one of p-toluenesulfonic acid, p-aminobenzenesulfonic acid, p-hydroxybenzenesulfonic acid and isethionic acid.

[0012] Furthermore, the dehydration reaction conditions in step S1 are: reaction temperature of 120-180° C., reaction time of 1-10 h, and vacuum pressure of 200-20000 Pa.

[0013] Furthermore, in step S1, the molecular weight of the lactic acid oligomer is controlled to be 800 to 3000 g / mol.

[0014] Furthermore, the cracking reaction conditions in step S2 are: reaction temperature of 190-230° C., reaction time of 0.5-6 h, and vacuum pressure of 50-500 Pa.

[0015] A crude lactide is prepared by the above-mentioned method for preparing crude lactide using a composite catalytic system.

[0016] The crude lactide described above is used for industrial preparation of polymerization-grade lactide.

[0017] The present invention provides a method for preparing crude lactide using a composite catalytic system. This method addresses the long dehydration residence time and low reaction efficiency associated with traditional single tin catalysts during dehydration. The introduction of a strong proton acid facilitates the dehydration reaction, shortens the residence time, and efficiently removes water between lactic acid molecules. The resulting crude lactide has a low free acid content, significantly reducing costs for subsequent purification of the crude lactide. Furthermore, the composite catalytic system provided by the present invention is simple, practical, economical, and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present disclosure or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0019] Figure 1 This is a simplified flow chart of the process for preparing lactide by dehydration and cracking of lactic acid in the present invention;

[0020] Figure 2 This is the H-NMR spectrum of the crude lactide in the present invention. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0022] A method for preparing crude lactide using a composite catalytic system, the method comprising the following steps:

[0023] S1. Adding a stannous main catalyst and a sulfonic acid co-catalyst to a reaction system containing lactic acid, respectively, and controlling the reaction temperature and pressure to obtain lactic acid oligomers of different molecular weights through dehydration and oligomerization reactions; the stannous main catalyst is at least one of stannous chloride, stannous octoate, stannous sulfate, stannous pyrophosphate, and dibutyltin dilaurate; the sulfonic acid co-catalyst is at least one of p-toluenesulfonic acid, p-aminobenzenesulfonic acid, p-hydroxybenzenesulfonic acid, and isethionic acid; the dehydration reaction conditions are: reaction temperature of 120-180° C., reaction time of 1-10 hours, and vacuum pressure of 200-20,000 Pa; the molecular weight of the obtained lactic acid oligomers is controlled to be 800-3,000 g / mol;

[0024] S2. The lactic acid oligomers are further subjected to a cracking reaction under the action of a composite catalytic system to obtain crude lactide; the cracking reaction conditions are: reaction temperature of 190-230° C., reaction time of 0.5-6 hours, and vacuum pressure of 50-500 Pa.

[0025] The mass ratio of the stannous main catalyst to the lactic acid is 0.01-1.0%, and the mass ratio of the sulfonic acid co-catalyst to the lactic acid is 0.01-1.5%.

[0026] Specifically, several examples and comparative examples are used to verify

[0027] Example 1: Composite catalytic system for the preparation of crude lactide

[0028] Weigh 200g of lactic acid into a 500mL round-bottom flask, add 1g of p-toluenesulfonic acid and 1g of stannous chloride, and place the round-bottom flask in an oil bath. Connect a condenser to the flask via a distillation head. This condenser is connected to a collection flask via a tail pipe. The other end of the tail pipe is connected to a cold trap, which is then connected to a vacuum pump. Once the experimental setup is complete, check for leaks. After preparation is complete, raise the oil bath temperature to 160°C. Once the reaction temperature is reached, turn on the vacuum pump and maintain the pressure at approximately 5000 Pa according to the reading on the connected vacuum gauge. After 3 hours of reaction, turn off the vacuum pump and weigh the acid water collected in the collection flask to calculate the amount of acid water. Weigh the substrate in the reaction flask to calculate the mass of the oligomer and take approximately 2g of oligomer for molecular weight testing. Continue the cracking reaction with the remaining oligomer. Replace the original condenser with an air condenser and wrap a heating tape around it. Lactide is heat-sensitive and will rapidly cool and solidify inside the air condenser when the temperature drops below 95°C, potentially blocking the tube. After the air is condensed, replace it with a new collection flask. Connect the cold trap and vacuum pump in the same manner as for dehydration. Check the reaction system for leaks. After some inspections are completed, the oil bath temperature is raised to 195°C, the vacuum pump is turned on, the pressure is controlled at about 200 Pa, and the cracking reaction is carried out. When no sample can be collected in the collection bottle, the vacuum pump and the oil bath are turned off, and the collected sample is weighed. The collected sample is crude lactide (characterized by nuclear magnetic hydrogen spectrum, attached Figure 2 ), the crude lactide was vacuum-packed and stored in an aluminum foil bag before being used for gas chromatography (chiral column) and potentiometric titration to evaluate the composition and acid value of the lactide. The specific results are shown in Table 1.

[0029] Example 2: Preparation of Crude Lactide Using p-Toluenesulfonic Acid-Stannoic Acid

[0030] The basic operating steps were the same as those in Example 1, except that the composite catalyst system was replaced with 1 g of p-toluenesulfonic acid and 1 g of stannous octoate. Crude lactide was obtained through the above dehydration and cracking reactions, and tested and evaluated. The specific results are shown in Table 1.

[0031] Example 3: Preparation of Crude Lactide Using p-Hydroxybenzenesulfonic Acid-Stannoic Acid

[0032] The basic operating steps were the same as those in Example 1, except that the composite catalyst system was replaced with 1 g of p-hydroxybenzenesulfonic acid and 1 g of stannous octoate. Crude lactide was obtained through the above-mentioned dehydration and cracking reactions, and tested and evaluated. The specific results are shown in Table 1.

[0033] Comparative Example 1: Commercial stannous chloride catalyst used in the preparation of crude lactide

[0034] The basic operating steps were the same as those in Example 1, except that the composite catalyst system was replaced with a single 1 g stannous chloride catalyst. Crude lactide was obtained through the above dehydration and cracking reactions, and tested and evaluated. The specific results are shown in Table 1.

[0035] Comparative Example 2: Commercial stannous octoate catalyst used for crude lactide preparation

[0036] The basic operating steps were the same as those in Example 1, except that the composite catalytic system was replaced with 1 g of stannous octoate catalyst. Crude lactide was obtained through the above dehydration and cracking reactions, and tested and evaluated. The specific results are shown in Table 1.

[0037] Comparative Example 3: Use of p-toluenesulfonic acid in the preparation of crude lactide

[0038] The basic operating steps were the same as those in Example 1, except that the composite catalytic system was replaced with 1 g of p-toluenesulfonic acid catalyst. Crude lactide was obtained through the above dehydration and cracking reactions, and tested and evaluated. The specific results are shown in Table 1.

[0039] Table 1

[0040]

[0041] As can be found in Table 1, Comparative Examples 1 and 2 all directly use commercial stannous chloride or stannous octoate catalysts for the preparation of crude lactide, and their final free acid content is more than 400meq / kg. When stannous chloride or stannous octoate are compounded with sulfonic acid catalysts in Examples 1-3, the free acid content in crude lactide can be reduced to below 25meq / kg, which shows the excellent performance of the compounded catalyst in dehydration cracking, and can significantly reduce the acid value of crude lactide. Further comparison of Comparative Example 3 shows that if only sulfonic acid catalysts are used, its free acid content reaches 762meq / kg. Therefore, comprehensive comparison can show that the use of the compounded catalytic system is very beneficial for the preparation of crude lactide with low free acid content.

[0042] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0043] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the invention as claimed.

Claims

1. A method for preparing crude lactide using a composite catalytic system, characterized in that: The method comprises the following steps: S1, adding a stannous main catalyst and a sulfonic acid co-catalyst to a reaction system containing lactic acid, respectively, and obtaining lactic acid oligomers of different molecular weights through dehydration and oligomerization by controlling the reaction temperature and pressure; S2. further subjecting the lactic acid oligomers to a cracking reaction under the action of a composite catalytic system to obtain crude lactide; The mass ratio of the stannous main catalyst to the lactic acid is 0.01-1.0%, and the mass ratio of the sulfonic acid co-catalyst to the lactic acid is 0.01-1.5%.

2. The method for preparing crude lactide using a composite catalytic system according to claim 1, characterized in that: The stannous main catalyst is at least one of stannous chloride, stannous octoate, stannous sulfate, stannous pyrophosphate and dibutyltin dilaurate.

3. The method for preparing crude lactide using a composite catalytic system according to claim 1, characterized in that: The sulfonic acid co-catalyst is at least one of p-toluenesulfonic acid, p-aminobenzenesulfonic acid, p-hydroxybenzenesulfonic acid and isethionic acid.

4. The method for preparing crude lactide using a composite catalytic system according to claim 1, characterized in that: The dehydration reaction conditions in step S1 are: reaction temperature of 120-180° C., reaction time of 1-10 h, and vacuum pressure of 200-20000 Pa.

5. The method for preparing crude lactide using a composite catalytic system according to claim 1, characterized in that: In step S1, the molecular weight of the lactic acid oligomer is controlled to be 800 to 3000 g / mol.

6. The method for preparing crude lactide using a composite catalytic system according to claim 1, characterized in that: The cracking reaction conditions in step S2 are: reaction temperature of 190-230° C., reaction time of 0.5-6 h, and vacuum pressure of 50-500 Pa.

7. A crude lactide, characterized in that: The crude lactide is prepared by the method for preparing crude lactide using a composite catalyst system according to any one of claims 1 to 6.

8. The crude lactide according to claim 7, characterized in that: The crude lactide is used for industrial preparation of polymerization-grade lactide.

Images