Method for synthesizing lactide by catalyzing lactic acid liquid phase one-step method

Abstract

The invention belongs to the field of synthesis of biodegradable materials, and particularly relates to a method for synthesizing lactide by catalyzing a lactic acid liquid phase one-step method. Specifically, lactide with high yield and high optical purity is obtained through one-step synthesis by catalyzing a raw material lactic acid with a modified catalyst. Aiming at the acidity range required for synthesizing lactide from lactic acid by a one-step method, the regulation and control of the acidity proportion in the H beta molecular sieve are realized in situ by innovatively utilizing a high-temperature water vapor treatment method, and compared with a post-treatment method for regulating and controlling the acidity proportion of the molecular sieve, the high-temperature water vapor treatment method is low in price and free of environmental pollution. The catalyst can improve the optical purity and selectivity of the lactide and reduce the purification difficulty of the lactide in the process of catalyzing the lactic acid liquid phase one-step method to synthesize the lactide, the liquid phase one-step method is operated at normal pressure / near normal pressure, the production condition is mild, the catalyst is easy to separate and recover, the production cost can be effectively reduced, and the method is suitable for industrial production. And the method has a relatively great industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of biodegradable material synthesis, specifically relating to a one-step liquid-phase synthesis method for lactide using catalytic lactic acid. Background Technology

[0002] In recent years, with the increasing prominence of global environmental problems and the growing pressure to control plastic pollution, the development of biodegradable materials has become one of the important directions for my country to achieve a green and low-carbon transformation. Currently, biodegradable materials such as polybutylene terephthalate (PBAT), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), and polylactic acid (PLA) are widely used in industries such as disposable products and express packaging. Among them, PLA, due to its excellent biocompatibility, complete degradability, and low carbon emission characteristics, is a sustainable polymer material with great application potential.

[0003] Lactide, a key intermediate in the synthesis of polylactic acid (PLA), has its molecular weight, properties, and applications largely determined by its optical and chemical purity. However, the highly selective preparation of lactide remains a bottleneck hindering the industrialization of PLA in my country. Currently, the preparation of high-purity lactide involves complex catalytic and separation steps, requiring extremely high control over catalyst activity, selectivity, and reaction conditions. Therefore, overcoming the key technologies for efficient lactide synthesis and purification is crucial for achieving independent control and sustainable development of the PLA industry chain.

[0004] Traditional catalysts such as organotin compounds and metal oxides are difficult to recover, prone to coking, and easily deactivated in the synthesis of lactide, increasing production costs. Molecular sieve catalysts, due to their unique pore characteristics and low diffusion pathways, can effectively reduce side reactions, thereby significantly improving the selectivity and yield of lactide. In recent years, several studies have reported the application of molecular sieves such as ZSM-5 and Hβ in the catalytic conversion of lactic acid to lactide. Molecular sieve catalytic systems offer advantages such as low cost, relatively mild reaction conditions, easy catalyst recovery, and simple operation, showing great promise for industrial application.

[0005] However, the surface acidity of the catalyst is crucial for improving lactide yield and selectivity. CN117548138B utilizes a method of covering the surface acidity of mesoporous Hβ molecular sieves with all-silica β molecular sieves to reduce surface Brønsted acidity, thereby reducing the increase in the molecular weight of lactic acid oligomers. However, this method can only achieve a maximum lactide selectivity of 75% and a lactic acid conversion rate of 97%, and does not mention optical purity indicators. Currently, traditional H-type molecular sieves, due to excessively strong Brønsted acidity, promote further condensation of lactic acid into dimers, trimers, and lactic acid oligomers with higher molecular weights, resulting in a low primary yield of lactide. Methods such as silanization modification, introducing all-silica β-covered surface active sites, and acid-base post-treatment to adjust the acidity of Hβ molecular sieves are not only costly to produce, but some methods inevitably generate acid and alkali waste liquids. Catalysts obtained through these methods have not yet fully met the goal of producing lactide with high yield, high optical purity, and high selectivity. Therefore, developing suitable catalysts is of great significance for promoting the industrialization of lactide. Summary of the Invention

[0006] To address the problems in the prior art, the present invention provides a one-step liquid-phase synthesis method for lactide from catalytic lactic acid.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for one-step liquid-phase synthesis of lactide from lactic acid using a modified catalyst is disclosed, which catalyzes the one-step synthesis of lactide from lactic acid with high yield and optical purity.

[0008] Furthermore, using lactic acid as a raw material, in the presence of an aprotic solvent, a modified catalyst is used to react at 130-150℃ for 4-7 hours to catalyze the conversion of lactic acid into lactide.

[0009] The ratio of the amount of raw materials to catalyst is in the range of 1:1 to 5:1.

[0010] The aprotic solvent is toluene or mesitylene.

[0011] The modified catalyst is obtained by granulating a commercial Hβ catalyst and placing it in a fixed-bed reactor, then controlling the acidity ratio of the Hβ molecular sieve in situ through high-temperature steam treatment, thereby obtaining the modified Hβ molecular sieve.

[0012] Furthermore, after granulating the commercial Hβ catalyst, it is placed in a fixed-bed reactor and gasified in the presence of nitrogen for 2-12 hours to obtain a catalyst with in-situ control of the acidity ratio of the Hβ molecular sieve.

[0013] To elaborate further, the commercial Hβ catalyst is granulated, placed in a quartz tube, and then placed in a fixed-bed reactor; the temperature is set to 250-400℃, and once the set temperature is reached, the flow rate is 5-20 mL / min. g cat Nitrogen gas is introduced into the reactor at a flow rate of 0.5-2 mL / min. g cat Water is introduced into the gasifier at a flow rate of 100°C. After gasification, the water enters the reactor. The catalyst is treated with steam for 2-12 hours to obtain a catalyst (Hβ-mnx, where m is temperature, n is water flow rate, and x is steam treatment time) that can be used to regulate the acidity ratio of Hβ molecular sieve in situ.

[0014] The water vapor that has been fully vaporized in the gasifier is obtained by introducing deionized water into the fixed-bed gasifier through a horizontal flow pump, and fully vaporizing the deionized water at 120°C in the presence of nitrogen, and then introducing the resulting water vapor into the fixed-bed reactor.

[0015] The Hβ molecular sieve catalyst has an acidity ratio of 1.67-1.82.

[0016] The present invention has the following advantages: In this invention, during the preparation of the Hβ molecular sieve catalyst, the proportion of Hβ molecular sieve is controlled in situ through high-temperature steam treatment without altering the molecular sieve structure. This allows for successful control of the Hβ acidity ratio without changing the silica-alumina ratio of the molecular sieve. Furthermore, treating the catalyst with water is environmentally friendly and a commonly used industrial method, effectively reducing costs and showing broad prospects for industrial application. Simultaneously, in the one-step liquid-phase process, reactive distillation is used to promptly remove the water generated during the reaction, breaking the equilibrium constraint, promoting the forward reaction, reducing side reactions, and improving the lactic acid conversion rate and lactide yield. Attached Figure Description

[0017] Figure 1 The above is the XRD pattern of the in-situ regulated Hβ molecular sieve acid ratio catalyst treated with high-temperature steam according to the present invention. Figure 2 The Py-IR diagram of the in-situ regulated Hβ molecular sieve acid ratio catalyst treated with high-temperature steam according to this invention. Figure 3 This is a SEM image of the in-situ regulated Hβ molecular sieve acid ratio catalyst treated with high-temperature steam according to the present invention. Detailed Implementation

[0018] To illustrate the invention more clearly, the following embodiments are provided, but the invention is not limited thereto.

[0019] This invention utilizes high-temperature steam treatment to in-situ regulate the acidity ratio of Hβ molecular sieve catalysts. This allows for the in-situ control of the Brønsted and Lewis acidity ratios without altering the Hβ molecular sieve structure, effectively improving the yield and optical purity of lactide. Simultaneously, in a one-step liquid-phase process, reactive distillation rapidly removes water generated during the reaction from the raw materials, promoting the forward reaction and reducing side reactions, thus enhancing the yield, optical purity, and selectivity of lactide.

[0020] This invention proposes an in-situ method to achieve the acidity ratio of Hβ molecular sieves through high-temperature steam treatment. This method not only uses environmentally friendly water as a solvent, but also features a simple apparatus and low catalyst processing costs. The catalyst preparation method is as follows: The commercial Hβ catalyst was granulated and placed in a quartz tube in a fixed-bed reactor. The temperature was set at 250-400℃. After reaching the set temperature, the catalyst was fed at a rate of 5-20 mL / min·g. cat Nitrogen gas is introduced into the reactor at a flow rate of 0.5-2 mL / min. g cat Water is fed into the gasifier at a flow rate of 100°C. After gasification, the water enters the reactor. The catalyst is treated with steam for 2-12 hours to obtain a catalyst (Hβ-mnx, where m is temperature, n is water flow rate, and x is steam treatment time) that can be used to regulate the acidity ratio of Hβ molecular sieve in situ.

[0021] Example 1 In-situ regulated Hβ molecular sieve acid ratio catalysts were prepared under different treatment conditions according to the above description: The Hβ molecular sieve catalyst (purchased from Tianjin Yuanli Chemical Co., Ltd. as raw material) was granulated and placed in a fixed-bed reactor with the temperature set at 300℃ and the gasifier temperature set at 120℃, while the deionized water flow rate was 1 mL / min·g. cat The N2 flow rate was 10 mL / min·g cat And the Hβ-300-1-3h catalyst was obtained by treating the reactor at 300℃ for 3 hours (see [link]). Figure 1 Catalysts with different treatment temperatures, water flow rates, and treatment times were synthesized using similar methods.

[0022] Depend on Figure 1 The XRD pattern shows that the catalyst with in-situ controlled acidity ratio of Hβ molecular sieve obtained after high-temperature steam treatment still has the characteristic peak of β and the topology remains unchanged. Figure 2 The Py-IR chromatogram is shown in Table 1. The total acidity ranges from 675 to 1156 μmol / g, and the BAS / LAS ratio ranges from 1.67 to 1.81. Figure 3The image shows SEM images of Hβ and Hβ-300-1-3h. As can be seen from the image, their morphology is approximately spherical, with a size of about 100 nm.

[0023] Table 1

[0024] Comparative Example 1 Weigh 0.20g of the raw material Hβ catalyst described in Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 130℃. Stir the mixture at 500r / min for 6h.

[0025] Comparative Example 2 Weigh 0.20g of Hβ-300-1-3h catalyst from Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 130℃. Stir the mixture at 500r / min for 3h.

[0026] Example 2 Weigh 0.20g of Hβ-300-1-3h catalyst from Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 130℃. Stir the mixture at 500r / min for 6h.

[0027] Example 3 Weigh 0.20g of Hβ-300-1-3h catalyst from Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 140℃. Stir the mixture at 500r / min for 6h.

[0028] Example 4 Weigh 0.20g of Hβ-300-1-3h catalyst from Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 150℃. Stir the mixture at 500r / min for 6h.

[0029] Example 5 Weigh 0.20g of Hβ-300-1-12h catalyst from Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 140℃. Stir the mixture at 500r / min for 6h.

[0030] Example 6 Weigh 0.20g of Hβ-300-1-12h catalyst from Example 1, 0.40g of 50wt% L-lactic acid, and 20mL of toluene into a single-necked flask equipped with a phase separator and place it in an oil bath at 150℃. Stir the mixture at 500r / min for 6h.

[0031] Table 2 summarizes the reaction conditions, lactide selective yield, and optical purity of the above-mentioned examples and comparative examples. Table 2

[0032] As can be seen from the above examples, the acidity ratio of the Hβ molecular sieve itself is adjusted in situ after high-temperature steam treatment, which is conducive to the occurrence of cyclization reaction. At the same time, the pore shape selectivity of Hβ itself is used to inhibit the occurrence of racemization reaction, thereby improving the selectivity of lactide.

[0033] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for one-step synthesis of lactide via catalytic liquid-phase lactic acid, characterized in that: Using a modified catalyst, lactic acid was synthesized in one step to obtain lactide with high yield and optical purity.

2. The method according to claim 1, characterized in that, Using lactic acid as a raw material, in the presence of an aprotic solvent, a modified catalyst is used to react at 130-150℃ for 4-7 hours to catalyze the conversion of lactic acid into lactide.

3. The method according to claim 1, characterized in that, The ratio of the amount of raw materials to catalyst is in the range of 1:1 to 5:

1.

4. The method according to claim 2, characterized in that, The aprotic solvent is toluene or mesitylene.

5. The method according to claim 1 or 2, characterized in that, The modified catalyst is obtained by granulating a commercial Hβ catalyst and placing it in a fixed-bed reactor, then controlling the acidity ratio of the Hβ molecular sieve in situ through high-temperature steam treatment, thereby obtaining the modified Hβ molecular sieve.

6. The method according to claim 5, characterized in that, After granulation, the commercial Hβ catalyst is placed in a fixed-bed reactor and gasified for 2-12 hours in the presence of nitrogen to obtain a catalyst with in-situ control of the acidity ratio of Hβ molecular sieve.

7. The method according to claim 6, characterized in that, The commercial Hβ catalyst was granulated and placed in a fixed-bed reactor, with the temperature set at 250-400℃. After reaching the set temperature, the catalyst was fed at a rate of 5-20 mL / min. g cat Nitrogen gas is introduced into the reactor at a flow rate of 0.5-2 mL / min. g cat Water is introduced into the gasifier at a flow rate of 100°C. After gasification, the water enters the reactor. The catalyst is treated with steam for 2-12 hours to obtain a catalyst (Hβ-mnx, where m is temperature, n is water flow rate, and x is steam treatment time) that can be used to regulate the acidity ratio of Hβ molecular sieve in situ.

8. The method according to claim 7, characterized in that, The water vapor that is fully vaporized in the fixed-bed gasifier is obtained by introducing deionized water into the fixed-bed gasifier through a horizontal flow pump, and fully vaporizing the deionized water at 120°C in the presence of nitrogen, and then introducing the obtained water vapor into the fixed-bed reactor.

9. The method according to claim 5, characterized in that, The Hβ molecular sieve catalyst has an acidity ratio (BAS / LAS) of 1.67-1.82.

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