CN114591285B - Method for improving lactide yield - Google Patents
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Abstract
The invention relates to the field of lactide preparation, and aims to provide a method for improving the yield of lactide. The method is that when polylactic acid (PLA) oligomer is depolymerized to prepare lactide, polyethylene glycol (PEG) -b-PLA segmented copolymer is added to participate in the reaction, and the addition amount accounts for 0.01-5% of the mass of PLA oligomer. According to the invention, the PEG-b-PLA segmented copolymer is added during the depolymerization reaction of the PLA oligomer, so that the system viscosity of the oligomer in the high-temperature depolymerization process is reduced, and the improvement of the yield and purity of lactide is effectively realized. The PEG-b-PLA segmented copolymer adopted by the invention has the advantages of simple synthesis, low price and low technical cost, and can realize large-scale industrial production.
Description
Technical Field
The invention relates to the field of lactide preparation, in particular to a method for improving the yield of lactide.
Background
The disposable plastic product has high usage amount and low recycling rate, so that the disposable plastic product has serious pollution to soil and marine environment, causes harm to ecological environment, and also seriously damages the health of human beings. The management of plastic contamination has become a common public consensus that various measures or policies have been taken to limit the use of non-biodegradable plastics in a wide variety of disposable plastic products.
Polylactic acid (PLA) is the most widely used biodegradable plastic with highest cost performance in the world at present, has excellent mechanical property and biodegradability, and is widely applied to packaging films, disposable tableware, textile fibers, medical devices and the like.
Polylactic acid (PLA) is currently produced mainly using a "two-step process", namely: lactide is first prepared from lactic acid, and then lactide is ring-opening polymerized to prepare high molecular weight PLA. Among them, lactide is a key monomer for synthesizing high molecular weight PLA. The main process route for preparing lactide is as follows: 1. lactic acid is dehydrated and condensed to prepare PLA oligomer; 2. lactide is prepared by depolymerizing PLA oligomers. Therefore, it is important to improve the yield and purity of lactide. However, the existing process route of lactide has the problem of low yield and purity. For example, the yield and purity of lactide is typically between 70% and 80% due to the presence of coking-prone and meso-prone lactide during depolymerization of PLA oligomers.
Therefore, finding a suitable method to increase the yield and purity of lactide produced by depolymerizing PLA oligomers is of great importance.
Disclosure of Invention
The invention aims to solve the technical problem of overcoming the defects in the prior art and providing a method for improving the yield of lactide.
In order to solve the technical problems, the invention adopts the following solutions:
the method for improving the lactide yield is characterized in that when PLA oligomer is subjected to depolymerization reaction to prepare lactide, polyethylene glycol (PEG) -b-PLA segmented copolymer is added to participate in the reaction, wherein the addition amount of the polyethylene glycol (PEG) -b-PLA segmented copolymer accounts for 0.01-5% of the mass of the PLA oligomer;
the chemical formula of the PEG-b-PLA block copolymer is shown as follows:
wherein m represents the polymerization degree of PEG, and the value range is 2-250; n represents the polymerization degree of PLA and has a value range of 1-150; b in the block copolymer designation indicates that PEG is block copolymerized with PLA.
In the PEG-b-PLA block copolymer, the molecular weight of PEG or PLA is preferably 100 to 10000.
As a preferred embodiment of the present invention, the depolymerization reaction includes: and (3) adding the PLA oligomer and the PEG-b-PLA segmented copolymer into a cracking reactor, and carrying out depolymerization reaction for 0.5-6 h at 170-250 ℃ and 50-200 Pa to obtain lactide.
As a preferred embodiment of the present invention, the PLA oligomer is prepared by the following steps: adding lactic acid and a catalyst into a polycondensation reaction kettle, and mechanically stirring and uniformly mixing; then heating to 100-140 ℃, and carrying out normal pressure polycondensation for 0.5-4 h under the protection of nitrogen; and then the temperature is raised to 160-180 ℃ and polycondensation is carried out for 0.5-6 h under 50-2000 Pa, thus obtaining PLA oligomer.
As a preferred embodiment of the present invention, the catalyst is stannous octoate (Sn (Oct) 2 ) Stannous chloride (SnCl) 2 ) Tetraisopropyl titanate (TBT), zinc oxide (ZnO), antimony oxide (Sb) 2 O 3 ) Any one of the following.
In a preferred embodiment of the present invention, the mass of the catalyst is 0.01 to 1% of the mass of lactic acid.
Description of the principles of the invention
In the traditional process, the PLA oligomer has higher system viscosity and easy carbonization in the high-temperature depolymerization process, so that the lactide yield is lower and the meso-rotation is serious, and the PEG has better lubricity and flexibility, but the PEG and PLA have poorer compatibility, so that the compatibility between the PEG and the PLA oligomer can be improved through the form of a segmented copolymer formed by the PEG and the PLA, thereby effectively reducing the viscosity of the PLA oligomer in the high-temperature depolymerization process, and further improving the yield and purity of lactide preparation.
In the prior art, the improvement of the related parameters of the process flow (such as patent US4983745 and US 5801255) is often adopted by the person skilled in the art to improve the yield and purity of lactide, but the key problems of easy coking and easy meso-dissolution of lactide in the process of depolymerizing and preparing lactide of PLA oligomer cannot be solved. The invention breaks the inertia thinking, and reduces the system viscosity of the PLA oligomer in the high-temperature depolymerization process by adding the PEG-b-PLA block copolymer into the PLA oligomer. In the traditional production process, the system viscosity of the depolymerization process of the PLA oligomer is usually about 0.8 Pa.s, and the system viscosity can be reduced to about 0.2 Pa.s by adding the block copolymer. The viscosity of the depolymerization system is greatly reduced, so that the heat and mass transfer efficiency in the depolymerization reactor is improved, carbonization of PLA oligomer is avoided, the depolymerization reaction can be more efficiently carried out, and finally the yield and purity of lactide are improved.
Furthermore, the preparation of lactide involves: lactic acid polycondensation to produce PLA oligomers, and then depolymerization of the PLA oligomers to lactide. The present invention only emphasizes the yield and purity of lactide produced by depolymerizing PLA oligomers. If high molecular weight PLA is to be produced, the lactide still needs to be further purified, nor is the invention particularly concerned with this operation.
The manner of obtaining the PEG-b-PLA block copolymer is simpler and less costly than the process flow optimization commonly adopted by those skilled in the art. Thus, the present invention is more innovative relative to other improved techniques.
Compared with the prior art, the invention has the beneficial effects that:
1. according to the invention, the PEG-b-PLA segmented copolymer is added during the depolymerization reaction of the PLA oligomer, so that the system viscosity of the oligomer in the high-temperature depolymerization process is reduced, and the improvement of the yield and purity of lactide is effectively realized.
2. The PEG-b-PLA segmented copolymer adopted by the invention has the advantages of simple synthesis, low price and low technical cost, and can realize large-scale industrial production.
Detailed Description
The invention will be described in further detail in connection with the following detailed description, but the scope of the invention as claimed is not limited to the examples.
The raw materials used in the present invention are described below: lactic acid was purchased from Henan Jindan lactic acid technologies Co.Ltd. Catalyst Sn (Oct) 2 、SnCl 2 、TBT、ZnO、Sb 2 O 3 All purchased from Sigma oreq (Sigma-Aldrich). Monohydroxy-terminated PEG100, PEG1000, PEG10000, all available from aladine (aladin) company, the numbers following the drug name represent the number average molecular weight of the polymer. The PEG-b-PLA block copolymer can be prepared by means of a monohydroxy terminated PEG-initiated lactide ring-opening polymerization (as described in The Journal of Physical Chemistry B2015,119,6471-6480 document) and can also be commercially available (as the PEG-b-PLA block copolymer product of Guangzhou water science and technology Co., ltd.), and the invention is not particularly limited.
In the present invention, the materials and types of the polycondensation reaction vessel and the cleavage reactor are not particularly limited, and they can be directly purchased and used by a known method.
The following examples will enable those skilled in the art to more fully understand the present invention and are not intended to limit the same in any way.
Example 1:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature is raisedPolycondensation was carried out at 2000Pa for 1h to 170℃to give PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 0.01 percent of the mass of the oligomer into a cracking reactor, respectively heating the molecular weight of the PEG and the PLA in the segmented copolymer to 100 and 100, and depolymerizing the segmented copolymer at 50Pa for 2 hours to obtain the product lactide 1.
Example 2:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 0.1 percent of the mass of the oligomer into a cracking reactor, respectively heating the molecular weight of the PEG and the PLA in the segmented copolymer to 100 and 100, and depolymerizing the segmented copolymer at 50Pa for 2 hours to obtain the product lactide 2.
Example 3:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively heating the molecular weight of the PEG and the PLA in the segmented copolymer to 100 and 100, and depolymerizing the mixture at the temperature of 210 ℃ for 2 hours under 50Pa to obtain the product lactide 3.
Example 4:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 5% of the mass of the oligomer into a cracking reactor, wherein the molecular weight of PEG and PLA in the segmented copolymer is respectively 100 and 100, the temperature is increased to 210 ℃, and depolymerizing is carried out for 2 hours under 50Pa, so as to obtain the product lactide 4.
Example 5:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively increasing the temperature to 210 ℃ and depolymerizing for 2 hours under 50Pa to obtain the product lactide 5, wherein the molecular weights of the PEG and the PLA in the segmented copolymer are 1000 and 1000.
Example 6:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively increasing the temperature to 210 ℃ and depolymerizing for 2 hours under 50Pa to obtain the product lactide 6, wherein the molecular weights of the PEG and the PLA in the segmented copolymer are 10000 and 10000.
Example 7:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively heating the molecular weight of the PEG and the PLA in the segmented copolymer to 100 and 10000, and depolymerizing the mixture at 50Pa for 2 hours to obtain the product lactide 7.
Example 8:
firstly, adding lactic acid and ZnO accounting for 0.4 percent of the mass of the lactic acid into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively heating the molecular weight of the PEG and the PLA in the segmented copolymer to 100 and 100, and depolymerizing the segmented copolymer at 50Pa for 2 hours to obtain the product lactide 8.
Example 9:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid Sb 2 O 3 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively heating the molecular weight of the PEG and the PLA in the segmented copolymer to 100 and 100, and depolymerizing the segmented copolymer at the temperature of 210 ℃ for 2 hours under 50Pa to obtain the product lactide 9.
Example 10:
firstly, lactic acid, 0.01% Sn (Oct) by mass of lactic acid is added 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 100 ℃, and carrying out normal-pressure polycondensation for 0.5h under the protection of nitrogen; then the temperature was raised to 160℃and polycondensation was carried out at 50Pa for 0.5h to give PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1 percent of the mass of the oligomer into a cracking reactor, respectively heating the PEG and PLA in the segmented copolymer to 170 ℃ for depolymerization for 6 hours at 100Pa to obtain the product lactide 9.
Example 11:
firstly, adding lactic acid and TBT accounting for 1% of the mass of the lactic acid into a polycondensation reaction kettle, mechanically stirring and mixing, heating to 140 ℃, and carrying out normal-pressure polycondensation for 4 hours under the protection of nitrogen; then the temperature was raised to 180℃and polycondensation was carried out at 1000Pa for 6 hours to obtain a PLA oligomer. And then, transferring the PLA oligomer and the PEG-g-PLA segmented copolymer accounting for 1% of the mass of the oligomer into a cracking reactor, wherein the molecular weight of PEG and PLA in the segmented copolymer is respectively 100 and 100, the temperature is increased to 250 ℃, and depolymerization is carried out for 0.5h under 200Pa, thus obtaining the product lactide 11.
Comparative example 1:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding into a polycondensation reaction kettle for mechanical stirringAfter mixing, the temperature is raised to 120 ℃, and the polycondensation is carried out for 2 hours under the protection of nitrogen at normal pressure; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And transferring the PLA oligomer into a cracking reactor, heating to 210 ℃, and depolymerizing for 2 hours under 50Pa to obtain the product lactide.
Comparative example 2:
firstly, lactic acid accounting for 0.4 percent of the mass of the lactic acid and SnCl 2 Adding the mixture into a polycondensation reaction kettle, mechanically stirring and mixing, and then heating to 120 ℃ and carrying out normal-pressure polycondensation for 2 hours under the protection of nitrogen; then the temperature was raised to 170℃and polycondensation was carried out at 2000Pa for 1 hour to obtain a PLA oligomer. And then, transferring PLA oligomer and PEG accounting for 1% of the weight of the oligomer into a cracking reactor, wherein the molecular weight of the PEG is 100 respectively, heating to 210 ℃, and depolymerizing for 2 hours under 50Pa to obtain the product lactide.
The current process route for preparing lactide is only lactic acid-PLA oligomer-lactide, while comparative examples 1 and 2 employ the optimized process parameter conditions in the current process route.
Calculation of lactide yield:
let m be fed into the depolymerization reactor 1 g PLA oligomer and m 2 g PEG-b-PLA block copolymer, e.g. mass of lactide after depolymerization of m 3 g, the yield of lactide is m 3 /m 1 *100％。
Testing of lactide purity:
quantitative analysis of lactide content was performed using a gas chromatograph-mass spectrometer (Aglient 7820-5977B). The column temperature is raised to 250 ℃ from 50 ℃ at a speed of 15 ℃/min by adopting a temperature programming mode, the temperatures of the sample injector and the FID detector are respectively maintained at 250 ℃ and 280 ℃, and nitrogen is used as carrier gas.
In the following table, the yields and purities of lactide prepared in comparative examples 1 and 2 and examples 1 to 11 are counted.
| Yield (%) | Purity (%) | |
| Comparative example 1 | 81 | 73 |
| Comparative example 2 | 83 | 75 |
| Example 1 | 81 | 74 |
| Example 2 | 86 | 85 |
| Example 3 | 95 | 97 |
| Example 4 | 94 | 97 |
| Example 5 | 92 | 93 |
| Example 6 | 68 | 71 |
| Example 7 | 74 | 76 |
| Example 8 | 93 | 95 |
| Example 9 | 92 | 94 |
| Example 10 | 85 | 88 |
| Example 11 | 67 | 79 |
As can be seen from comparing comparative example 1 with examples 1-4, the addition of PEG-b-PLA block copolymer can significantly improve the yield and purity of lactide, which can be increased from 81% and 73% of comparative example 1 up to 95% and 97% of example 3. The yields and purities of lactide in example 1 and comparative example 1 do not differ much because the PEG-b-PLA block copolymer added in example 1 is too little, while the yields and purities of lactide in examples 3 and 4 are substantially identical because further additions of PEG-b-PLA have not been able to further increase the yields and purities of lactide. Comparing comparative examples 1 and 2, although a certain amount of PEG was added in comparative example 2, the yield and purity of lactide were not significantly changed because of the poor compatibility of PEG with PLA oligomer, and comparing comparative example 2 and example 3, it is apparent that adding PEG-b-PLA block copolymer is a method of improving the yield and purity of lactide more effectively than directly adding PEG, and the preferable addition amount of PEG-b-PLA is 1%. In addition, as is clear from comparison of examples 3 and 5 to 7, in the PEG-b-PLA block copolymer, the molecular weight of PEG and the molecular weight of PLA are preferably 100, because too high molecular weights of PEG and PLA do not favor the movement of molecular chains and lower the viscosity of the system.
Finally, it should be noted that the above list is only specific embodiments of the present invention. Obviously, the invention is not limited to the above embodiments, but many variations are possible. All modifications directly derived or suggested to one skilled in the art from the present disclosure should be considered as being within the scope of the present invention.
Claims (5)
1. A method for improving the yield of lactide is characterized in that when polylactic acid (PLA) oligomer is subjected to depolymerization reaction to prepare lactide, polyethylene glycol (PEG) -b-PLA segmented copolymer is added to participate in the reaction, and the addition amount is 0.1-5% of the mass of the PLA oligomer;
the chemical formula of the block copolymer is shown as follows:
wherein m represents the polymerization degree of PEG, and the value range is 2-250; n represents the polymerization degree of PLA and has a value range of 1-150; b in the block copolymer designation indicates that PEG is block copolymerized with PLA; controlling the values of m and n to ensure that the molecular weight of PEG or PLA in the PEG-b-PLA segmented copolymer is between 100 and 10000.
2. The method of claim 1, wherein the depolymerization reaction comprises:
and (3) adding the PLA oligomer and the PEG-b-PLA segmented copolymer into a cracking reactor, and carrying out depolymerization reaction for 0.5-6 h at 170-250 ℃ and 50-200 Pa to obtain lactide.
3. The method of claim 1, wherein the PLA oligomer is prepared by a process comprising:
adding lactic acid and a catalyst into a polycondensation reaction kettle, and mechanically stirring and uniformly mixing; then heating to 100-140 ℃, and carrying out normal pressure polycondensation for 0.5-4 h under the protection of nitrogen; and then the temperature is raised to 160-180 ℃ and polycondensation is carried out for 0.5-6 h under 50-2000 Pa, thus obtaining PLA oligomer.
4. A method according to claim 3, wherein the catalyst is any one of stannous octoate, stannous chloride, tetraisopropyl titanate, zinc oxide, antimony oxide.
5. A method according to claim 3, wherein the mass of the catalyst is 0.01 to 1% of the mass of lactic acid.
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