A method for efficiently preparing high optical purity propylene lactone

Abstract

The application discloses a method for efficiently preparing high-optical-purity lactide, which comprises the following steps: uniformly mixing a high-molecular-weight organic acid catalyst with a lactic acid aqueous solution, and then dehydrating and polymerizing the lactic acid into lactic acid oligomers under heating and reduced pressure; and then depolymerizing and cyclizing the lactic acid oligomers under high temperature and low pressure, so as to collect the lactide; wherein the high-molecular-weight organic acid catalyst is a polymer with maleic anhydride or acrylic acid as an acidic center, has the advantages of high catalytic activity, low volatilization, low equipment corrosion and the like, can replace traditional metal salt catalysts such as tin or zinc, and can catalyze the efficient dehydration of lactic acid under high temperature and reduced pressure to prepare the lactide product with low metal salt impurity content and high optical purity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polylactic acid degradable materials, and particularly relates to a method for efficiently preparing low-metal-salt-impurity-content and high-optical-purity lactide. BACKGROUND

[0002] The annual output of non-degradable polymer materials based on petroleum has exceeded 100 million tons worldwide. Only a small part of plastic products is recycled after use. The stacking of a large amount of plastic product waste can easily cause soil and water pollution, and seriously endanger the health and survival of human beings and other organisms. Therefore, renewable biomass resources and biodegradable materials are increasingly valued by people. As a new type of bio-based material, polylactic acid (PLA) can be converted into carbon dioxide and water through microbial decomposition after use, without generating other harmful substances. PLA has gradually developed into one of the mainstream biodegradable materials, and is widely used in disposable household products, packaging materials, clothing, automobiles and medical fields, and has a very large market growth space.

[0003] PLA can be prepared by one-step polymerization of lactic acid under self-catalysis. However, under the process condition, with the increase of the polymerization degree of lactic acid, the carboxyl content in the lactic acid material is greatly reduced, and the self-catalytic ability is rapidly decreased, which easily causes the molecular weight of lactic acid polymer to be low, and is not conducive to the subsequent depolymerization reaction, and seriously limits the large-scale application of PLA. The commonly used production method in the industry is to first prepare lactide from lactic acid by dehydration, and then to obtain PLA with high molecular weight from lactide by ring-opening polymerization (usually referred to as two-step method). Therefore, the synthesis of lactide is one of the important links of the large-scale industrialization of PLA, and the related technical research has attracted widespread attention of domestic and foreign scholars.

[0004] Traditional lactide production processes generally use metal salts of aluminum, zinc or tin as catalysts, among which the most commonly used are various inorganic or organic acid salts of metallic tin. Chinese patent CN 1806919A discloses a method for preparing cyclic ester from lactic acid using stannous benzoate as catalyst, which has the characteristics of low catalyst dosage, low cracking temperature, and reusability, but the efficiency of the catalyst needs to be further improved. Chinese patent CN114349733A discloses a method for continuously preparing high-optical-purity L-lactide, which uses tin salt as the main catalyst and hydroxyl organic carboxylic acid as the cocatalyst. The use of the cocatalyst not only improves the stability of the main catalyst, but also effectively alleviates the plugging of the equipment by crude lactide, ensuring long-term operation of the device. Chinese patent CN116060088A discloses a catalyst for the dehydration of lactic acid to lactide and a method for synthesizing the same. The catalyst is prepared by preparing a Fe3O4@SiO2@Sn-SPP@meso-SiO2 core-shell catalyst, and the catalyst is used for the dehydration and cyclization of lactic acid to prepare lactide. The catalyst has the advantages of mild reaction conditions, high lactide yield and purity, easy and fast recovery of the catalyst, and good stability of the catalyst for repeated use. Chinese patent CN 114957197A discloses a method for synthesizing lactide in a one-step process under gas phase and normal pressure conditions. Compared with the two-step process for preparing lactide in current industrial production, the process route is simpler, and the tin-containing metal oxide catalyst used can be recycled.

[0005] It should be noted that when using metal salt catalysts represented by tin to catalyze the preparation of lactide from lactic acid, although the catalyst activity is relatively high, a certain amount of catalyst metal impurities will inevitably be introduced into the lactide product, and the presence of metal impurities will significantly reduce the optical purity of the lactide product, limiting the application of lactide in the synthesis of food contact or direct entry type polylactic acid materials and medical materials. Chinese patent CN 108610323A uses a solid acid molecular sieve catalyst to synthesize lactide, which has a relatively high optical purity of the lactide product, but the activity of the catalyst is low. Traditional inorganic acids such as sulfuric acid and hydrochloric acid can also be used as catalysts for the preparation of lactide from lactic acid, but small-molecule inorganic or organic acids are volatile and can easily contaminate the lactide product. In addition, small-molecule inorganic acids are prone to cause serious corrosion of the reactor and pipeline equipment due to their strong acidity, which in turn leads to more metal impurities in the product, and iron and nickel metal impurities can cause configuration transformation of lactic acid or lactide, which seriously affects the optical purity of the lactide product.

[0006] Therefore, in view of the above problems existing in the current production process of preparing lactide from lactic acid, it is necessary to develop a method for efficiently preparing lactide with high optical purity. SUMMARY

[0007] In view of the deficiencies of the prior art, the present application provides a method for efficiently preparing high-optical-purity lactide, which uses a high-molecular-weight organic acid catalyst instead of a traditional metal salt catalyst such as aluminum, zinc or tin, which has the advantages of high catalytic activity, low volatility under high-temperature and reduced-pressure conditions, and low corrosion of equipment. This method can not only avoid the introduction of metal salt catalyst impurities into the product, but also achieve efficient conversion of lactic acid, thereby obtaining lactide with high optical purity and low metal salt impurity content.

[0008] To achieve the above-mentioned object, the present application adopts the following technical solutions:

[0009] A method for efficiently preparing high-optical-purity lactide, which uniformly mixes an aqueous lactic acid solution with a high-molecular-weight organic acid catalyst, then dehydrates and polymerizes the lactic acid under heating and reduced pressure to generate lactic acid oligomers, and then depolymerizes and cyclizes under high temperature and low pressure to obtain lactide with low metal salt impurity content and high optical purity. The high-molecular-weight organic acid catalyst is a polymer with maleic anhydride or acrylic acid as the acidic center.

[0010] Further, the lactic acid used can be one of D-lactic acid and L-lactic acid, or a mixture of the two, which can be a product of fermentation of grain crops such as corn, or a lactic acid product obtained by fermentation of non-edible biomass waste such as straw.

[0011] Further, the mass concentration of the aqueous lactic acid solution is 20% to 100%.

[0012] Further, the amount of the high-molecular-weight organic acid catalyst used is 0.6% to 6.0% of the mass of the aqueous lactic acid solution used.

[0013] Further, the content of maleic anhydride or acrylic acid in the high-molecular-weight organic acid catalyst is not less than 0.6wt%, and is preferably 1.0wt% to 15.0wt%.

[0014] Further, the weight-average molecular weight of the high-molecular-weight organic acid catalyst is 400 to 20,000 g / mol, and is preferably 400 to 5,000 g / mol.

[0015] Further, the high-molecular-weight organic acid catalyst is specifically at least one of maleic anhydride supported on polyethylene or polypropylene, or an acidic polymer such as polymaleic anhydride or polyacrylic acid.

[0016] Further, the reaction temperature for lactic acid dehydration polymerization is 80 to 190℃, the absolute pressure is 0.4 to 0.8 bar, and the reaction time is 0.5 to 2.0 h.

[0017] Furthermore, the reaction temperature for depolymerization and cyclization is 190-240℃, the absolute pressure is 0.005-0.2 bar, and the reaction time is 0.4-4.5 min.

[0018] Furthermore, the optical purity of the obtained lactide is over 98%, and its main optical configuration is consistent with that of the lactic acid used. It can be L,L-lactide, D,D-lactide, L,D-lactide, or racemic lactide; the content of metal salt impurities, represented by metallic tin or zinc, is less than 3 ppm.

[0019] The significant advantages of this invention are:

[0020] The high molecular weight organic acid catalyst used in this invention is a high molecular weight Brønsted acid. The acidic center in the high molecular weight organic acid catalyst can ionize hydrogen ions under the action of water, and this process is reversible. Therefore, it can catalyze the polymerization of lactic acid and the formation of lactide.

[0021] Unlike traditional metal salt catalysts, this high molecular weight organic acid catalyst can not only achieve efficient catalytic dehydration of lactic acid to prepare lactide, but also, thanks to its high molecular weight, it is not easily volatilized under high temperature and reduced pressure depolymerization conditions, thus avoiding contamination of lactide products.

[0022] Meanwhile, since the catalyst used in this invention is a weak acid, it will not cause significant corrosion to the equipment, thus avoiding serious metal contamination of the lactide product caused by equipment corrosion or the use of metal catalysts.

[0023] In addition, lactide products prepared using this high molecular weight organic acid catalyst have higher optical purity, which can solve the problems of yellowing and decreased optical purity that are common with traditional metal salt catalysts. Attached Figure Description

[0024] Figure 1 The image shows the liquid chromatogram of the product obtained in Example 1.

[0025] Figure 2 This is a gas chromatogram for optical purity analysis of the product obtained in Example 1.

[0026] Figure 3 The image shows the liquid chromatogram of the product obtained in Example 2.

[0027] Figure 4 This is a gas chromatogram for optical purity analysis of the product obtained in Example 2. Detailed Implementation

[0028] A method for efficiently preparing high optical purity lactide includes the following steps:

[0029] (1) 100 parts of a lactic acid aqueous solution with a mass concentration of 20% to 100% is added into a reaction kettle, and 0.6-6.0 parts of a high molecular weight organic acid catalyst is added, and the reaction is carried out at 80-190℃, absolute pressure 0.4-0.8 bar for 0.5-2.0 h to generate lactic acid oligomers;

[0030] (2) Then the reaction temperature is raised to 190-240℃, the reaction absolute pressure is adjusted to 0.005-0.2 bar, and the reaction is carried out for 0.4-4.5 min to depolymerize the lactic acid oligomers to generate lactide with low metal salt impurity content and high optical purity.

[0031] The high molecular weight organic acid catalyst is a polymer with maleic anhydride or acrylic acid as the acidic center, with a weight average molecular weight of 400-20000 g / mol and a maleic anhydride or acrylic acid content of not less than 0.6wt%. It can be at least one of maleic anhydride (M1) supported by polyethylene or polypropylene, or acid polymers such as polymaleic anhydride (M2), polyacrylic acid (M3), etc.

[0032] The polyethylene or polypropylene supported maleic anhydride is prepared by melt extrusion process, and the specific steps are as follows: the polyethylene or polypropylene resin is mixed uniformly with maleic anhydride and initiator in a high-speed mixer, and then melt extruded in a twin-screw extruder; the length-diameter ratio of the barrel of the twin-screw extruder is 44, and the reaction temperature is 220℃; the extrudate is water-cooled, air-dried, cut and dried to obtain the product. The amount of maleic anhydride is 1%-5% of the mass of the resin; the initiator used is one or more of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butyl peroxide) hexane, and tert-butyl peroxide-3,5,5-trimethyl hexanoate, and the addition amount is 0.4% of the mass of the resin.

[0033] In order to make the content of the application more convenient to understand, the following examples will further illustrate the method for efficiently preparing high optical purity lactide according to the application. The examples are implemented on the premise of the technical solutions of the application, and detailed implementation methods and specific operation steps, technical parameters, etc. are given in the examples and comparative examples, but the protection scope of the application is not limited to the examples. Unless otherwise specified, the experimental methods in the examples are conventional operations in the art, and the experimental materials or reagents used in the examples can be obtained from the market.

[0034] The analysis of the reaction product was performed using an Agilent 1260 liquid chromatograph equipped with a C18 column and a SPD10 UV-Vis detector. Two solvents were used in the elution program: a) water / acetonitrile 95 / 5 (v / v, with 2 mL of 85% phosphoric acid / L); b) 100% acetonitrile (with 2 mL of 85% phosphoric acid / L). The elution program was as follows:

[0035] 0-1 min: 100 / 0 (v / v);

[0036] 1-15 min: linear adjustment to 20 / 80 (v / v);

[0037] 15-20 min: 20 / 80 (v / v);

[0038] 20-30 min: return to 100 / 0 (v / v).

[0039] The water / acetonitrile phase can be used both as a sample solvent and as an eluent mobile phase, allowing the complete dissolution and separation of the polar hydrophilic lactic acid, lactic acid oligomers (L n A, n < 5) and lactide and macromolecular lactic acid polymers (L n A, n > 5).

[0040] The optical purity of the lactide was analyzed using gas chromatography. An Agilent model 7820A gas chromatograph was used, equipped with a CycloSil-B chiral column (30 m x 0.25 mm x 0.25 pm) and an FID detector. During the analysis, the injection port temperature was set to 240 °C, the initial column temperature in the oven was 65 °C, maintained for 5 minutes, then heated to 220 °C at a rate of 15 °C / min and maintained for 20 minutes.

[0041] Example 1

[0042] A method for the efficient preparation of high optical purity lactide, comprising the following steps:

[0043] (1) 100 parts by weight of an aqueous solution of L-lactic acid with a mass concentration of 82% were added to a reaction kettle, and 2.3 parts of polypropylene-supported maleic anhydride (M1, with a maleic anhydride loading of about 8.6 wt% measured by acid-base titration and a weight average molecular weight of about 18700 g / mol measured by GPC) were added, and the reaction was carried out at 185 °C and an absolute pressure of 0.5 bar for 1.5 h to produce lactic acid oligomers.

[0044] (2) The reaction temperature was then raised to 235 °C, and the absolute pressure of the reaction was adjusted to 0.008 bar, and the reaction was carried out for 4.5 min to obtain the lactide product.

[0045] The product was analyzed by liquid chromatography, and the results showed that the content of lactide was 78.3%, the content of L-lactic acid was 14.1%, and the content of lactic acid oligomer was about 7.6%. The product was analyzed by gas chromatography, and the results showed that the content of L,L-lactide was 98.2%, and the remaining 1.8% was meso-lactide (i.e. L,D-lactide), and no D,D-lactide was detected. In addition, the product was analyzed by ICP, and the results showed that it did not contain metal impurities such as tin and zinc.

[0046] Example 2

[0047] The amount of catalyst in Example 1 was increased to 3.5 parts, and the other conditions were the same as in Example 1.

[0048] The product was analyzed by liquid chromatography, and the results showed that the content of lactide was 90.4%, the content of L-lactic acid was 5.7%, and the content of lactic acid oligomer was about 3.9%. The product was analyzed by gas chromatography, and the results showed that the content of L,L-lactide was 96.9%, and the remaining 3.1% was meso-lactide (i.e. L,D-lactide), and no D,D-lactide was detected. In addition, the product was analyzed by ICP, and the results showed that it did not contain metal impurities such as tin and zinc.

[0049] Example 3

[0050] A method for efficiently preparing high-optical-purity lactide, comprising the following steps:

[0051] (1) 100 parts of an L-lactic acid aqueous solution with a mass concentration of 80% was added to a reaction kettle, and 4.0 parts of a polymaleic anhydride aqueous solution (M2, wherein the solid content of the polymaleic anhydride was about 48 wt%, and the weight average molecular weight was about 690 g / mol measured by GPC) was added, and the reaction was carried out at 165°C and an absolute pressure of 0.5 bar for 1.3 h to generate lactic acid oligomer.

[0052] (2) Then the reaction temperature was raised to 230°C, the reaction absolute pressure was adjusted to 0.005 bar, and the reaction was carried out for 2.5 min to obtain the lactide product.

[0053] The product was analyzed by liquid chromatography, and the results showed that the content of lactide was 91.7%, the content of L-lactic acid was 2.7%, and the content of lactic acid oligomer was about 5.6%. The product was analyzed by gas chromatography, and the results showed that the content of L,L-lactide was 98.1%, and the remaining 1.9% was meso-lactide (i.e. L,D-lactide), and no D,D-lactide was detected. In addition, the product was analyzed by ICP, and the results showed that it did not contain metal impurities such as tin and zinc.

[0054] Example 4

[0055] A method for efficiently preparing high-optical-purity lactide, comprising the following steps:

[0056] (1) 100 parts of L-lactic acid aqueous solution with a mass concentration of 80% was added into a reaction kettle, and 2.0 parts of polyacrylic acid aqueous solution (M3, wherein the solid content of polyacrylic acid is about 55 wt%, and the weight average molecular weight is about 1920 g / mol measured by GPC) was added, and the reaction was carried out at 165℃ and 0.5 bar absolute pressure for 1.3 h to generate lactic acid oligomers.

[0057] (2) Then the reaction temperature was raised to 230℃, the reaction absolute pressure was adjusted to 0.005 bar, and the reaction was carried out for 2.5 min to obtain the lactide product.

[0058] The liquid chromatography analysis showed that the content of lactide in the product was 90.1%, the content of L-lactic acid was 5.2%, and the content of lactic acid oligomers was about 4.7%. The gas chromatography analysis showed that the content of L,L-lactide in the product was 98.8%, and the remaining 3.2% was meso-lactide, and no D,D-lactide was detected. In addition, the ICP analysis showed that the product did not contain metal impurities such as tin and zinc.

[0059] The comparison of the determination results of Examples 1-4 showed that when a high molecular weight organic acid catalyst was used, lactic acid could be efficiently catalyzed to prepare lactide, the content of lactide in the product was as high as 91.7%, the optical purity was more than 98%, and the prepared lactide did not contain metal salt impurities such as tin due to the absence of metal salt in the catalyst.

[0060] Comparative Example 1

[0061] A method for preparing lactide, comprising the following steps:

[0062] (1) 100 parts of L-lactic acid aqueous solution with a mass concentration of 80% was added into a reaction kettle, and the reaction was carried out at 165℃ and 0.5 bar absolute pressure for 2.0 h to generate lactic acid oligomers.

[0063] (2) Then the reaction temperature was raised to 230℃, the reaction absolute pressure was adjusted to 0.005 bar, and the reaction was carried out for 4.5 min to obtain the lactide product.

[0064] The liquid chromatography analysis showed that the content of lactide in the product was 27.9%, the content of L-lactic acid was 1.4%, and the content of lactic acid oligomers was about 70.7%. The gas chromatography analysis showed that the content of L,L-lactide in the product was 96.8%, and the remaining was meso-lactide, and no D,D-lactide was detected. In addition, the ICP analysis showed that the product did not contain metal salt catalyst impurities such as tin.

[0065] From the comparative example 1, it can be seen that the lactic acid dehydration to prepare lactide is only carried out under the self-catalysis without additional catalyst, and the lactide content in the product after reaction is only 27.9%. It is possible that with the increase of the polymerization degree of lactic acid, the carboxyl content in the lactic acid material is greatly reduced, the self-catalysis ability is rapidly decreased, the molecular weight of the lactic acid polymer is low, and the subsequent depolymerization reaction is not conducive to the subsequent depolymerization reaction.

[0066] Comparative example 2

[0067] A method for preparing lactide, comprising the following steps:

[0068] (1) 100 parts of L-lactic acid aqueous solution with a mass concentration of 80% by weight are added into a reaction kettle, and 1.1 parts of stannous octoate is added, and the reaction is carried out at 165℃ and an absolute pressure of 0.5 bar for 1.3 h to generate lactic acid oligomer.

[0069] (2) Then the reaction temperature is raised to 235℃, the reaction absolute pressure is adjusted to 0.005 bar, and the reaction is carried out for 25 min to obtain lactide product.

[0070] It can be seen from the liquid chromatography analysis that the lactide content in the product is 83.2%, the L-lactic acid content is 1.9%, and the lactic acid oligomer content is about 14.9%. It can be seen from the gas chromatography analysis that the L,L-lactide content in the product is 92.1%, and the remaining 7.9% is meso-lactide, and no D,D-lactide is detected. In addition, it can be seen from the ICP analysis that the tin impurity content in the product is 37 ppm.

[0071] As can be seen from the comparative example 2, when stannous octoate is used as a catalyst, the lactide content in the product reaches 83.2%, but the optical purity is only 92.1%, so it can be seen that the use of metal tin salt catalyst will significantly reduce the optical purity of the lactide product. In addition, the tin salt impurity content of 37 ppm is detected in the crude lactide product, and the use of the obtained metal catalyst will also cause a certain degree of metal pollution to the lactide product.

[0072] The above only describes the preferred embodiments of the present application, and any changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A method for efficiently preparing high optical purity propiolactone, characterized by, After the lactic acid aqueous solution is uniformly mixed with the high molecular weight organic acid catalyst, lactic acid dehydration polymerization into lactic acid oligomers occurs under heating and reduced pressure, and then depolymerization and cyclization under high temperature and low pressure, so as to obtain lactide with low metal salt impurity content and high optical purity; The high molecular weight organic acid catalyst is a polymer with maleic anhydride or acrylic acid as the acidic center, specifically at least one of polyethylene or polypropylene supported maleic anhydride, or polymaleic anhydride, polyacrylic acid; The preparation steps of the polyethylene or polypropylene supported maleic anhydride are as follows: after the polyethylene or polypropylene resin is uniformly mixed with maleic anhydride and an initiator in a high-speed mixer, a twin-screw extruder is used for melt extrusion.

2. The method of claim 1, wherein, The mass concentration of the lactic acid aqueous solution is 20%-100%.

3. The method of claim 1, wherein, The amount of the high molecular weight organic acid catalyst is 0.6%-6.0% of the mass of the lactic acid aqueous solution used.

4. The method of claim 1, wherein, The content of maleic anhydride or acrylic acid in the high molecular weight organic acid catalyst is not less than 0.6%.

5. The method of claim 1, wherein, The weight average molecular weight of the high molecular weight organic acid catalyst is 400-20,000 g / mol.

6. The method of claim 1, wherein, The reaction temperature for lactic acid dehydration polymerization is 80-190℃, the absolute pressure is 0.4-0.8 bar, and the reaction time is 0.5-2.0 h.

7. The method of claim 1, wherein, The reaction temperature for depolymerization and cyclization is 190-240℃, the absolute pressure is 0.005-0.2 bar, and the reaction time is 0.4-4.5 min.

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