Catalyst for preparing lactide through condensation of lactic acid as well as preparation method and application of catalyst

Abstract

The invention discloses a catalyst for preparing lactide through condensation of lactic acid and a preparation method and application thereof, the preparation method comprises the following steps: S1, adding an activator N-isopropyl-N-methylpropan-2-amine into a proline solution, then adding a zinc salt, reacting to obtain proline zinc, adding a catalyst, and reacting to obtain proline zinc; wherein the mass ratio of the N-isopropyl-N-methylpropane-2-amine to the proline is (4 to 6): 100, and the mass ratio of the N-isopropyl-N-methylpropane-2-amine to the proline is ( And S2, mixing and reacting zinc proline and the SAPO-11 molecular sieve in a solvent according to a mass ratio of 1: (0.8-1.2) to obtain the zinc proline / SAPO-11 composite material. According to the material, a commercialized and easily available SAPO-11 molecular sieve is used as a catalyst carrier, and zinc proline activated by N-isopropyl-N-methylpropan-2-amine is loaded. The material can catalyze lactic acid polymerization to prepare lactide. Compared with a traditional catalyst, the catalytic material has the advantages that the coking yield can be reduced to be within 5%, and a small amount of generated coke is not adhered to a container and is easy to clean.

Description

Technical Field

[0001] The present invention relates to the preparation of lactide, and in particular to a molecular sieve-loaded proline zinc material for preparing lactide by condensing lactic acid, a preparation method and an application thereof. Background Art

[0002] As a key intermediate in the synthesis of polylactic acid (PLA), the efficiency and stability of its synthesis process directly impact the production cost and quality of the PLA material. Currently, in the industrial production of lactide from lactic acid condensation, traditional catalysts such as metal oxides and organotin compounds, while exhibiting some catalytic activity, suffer from high coking yields, typically as high as 10% to 30%. Excessive coking not only reduces catalyst activity and shortens catalyst life, but also causes a large amount of difficult-to-clean coke to adhere to the inner walls of the reaction equipment, increasing equipment maintenance costs and downtime, severely hindering the large-scale industrial application of the lactide synthesis process.

[0003] Furthermore, during use, conventional catalysts form coke products that adhere tightly to equipment surfaces, making them difficult to completely remove using conventional physical or chemical cleaning methods. Long-term coke accumulation can also affect the sealing and safety of equipment, further limiting production efficiency and capacity expansion. Therefore, developing a new catalyst that can reduce coke yield and adhesion, thereby making it suitable for large-scale production, has become a key issue urgently needed in the lactide synthesis field. Summary of the Invention

[0004] In view of the problem of high coking yield in the process of preparing lactide by lactic acid condensation, the present invention provides a molecular sieve loaded proline zinc material for preparing lactide by lactic acid condensation, a preparation method and application thereof, using commercial and readily available SAPO-11 molecular sieve as a carrier, N -isopropyl- N -Proline zinc activated by methylpropane-2-amine, a new catalytic material was developed to effectively solve the coking problem of traditional catalysts and promote the development of lactide synthesis process towards scale and efficiency.

[0005] In order to achieve the above object, the present invention provides a method for preparing a molecular sieve-loaded proline zinc material for preparing lactide by lactic acid condensation, which comprises the following steps: S1. Add activator to proline solution N -isopropyl- N -methylpropane-2-amine, then add zinc salt to obtain proline zinc after reaction, wherein N -isopropyl- N The mass ratio of -methylpropan-2-amine to proline is (4~6):100; S2. Proline zinc and SAPO-11 molecular sieve are mixed and reacted in a solvent at a mass ratio of 1: (0.8-1.2) to obtain a proline zinc / SAPO-11 composite material.

[0006] N -isopropyl- N The optimal mass ratio of -methylpropan-2-amine to proline is 4:100. This ratio can fully activate the proline zinc and enhance the overall catalytic effect of the material.

[0007] The mass ratio of zinc proline and SAPO-11 molecular sieve is preferably 1:1. At this ratio, the catalytic performance and mechanical stability of the material are well balanced.

[0008] The molecular sieve-supported zinc proline material of the present invention uses commercial and readily available SAPO-11 molecular sieve as a catalyst carrier, and the supported N -isopropyl- N -Proline zinc activated by methylpropane-2-amine (CAS No.: 10342-97-9) can effectively catalyze the polymerization of lactic acid to produce lactide.

[0009] The catalyst carrier in the present invention is specifically SAPO-11 molecular sieve, which has a regular pore structure and a large specific surface area, providing a good basis for the loading of active components.

[0010] Proline zinc serves as the core active substance, providing active sites for catalyzing the lactic acid condensation reaction.

[0011] use N -isopropyl- N -Methylpropane-2-amine is used to treat zinc proline, which is a key technology to reduce the coking reaction. The principle of coking is that lactic acid condenses too quickly around the active site of the catalyst, causing the generated lactide to decompose under heat before evaporation. Therefore, proper poisoning of the catalyst is conducive to the smooth progress of the reaction. Zinc catalysts are different from precious metal catalysts and cannot be poisoned by sulfides. Therefore, a nitride poisoning scheme is designed. In order to prevent the catalyst from being deeply poisoned and failing, a good effect is selected. N -isopropyl- N -Methylpropane-2-amine. The principle is that the isopropyl group in the molecule is a steric hindrance group, which can play a certain role in hindering the coordination of nitrogen and zinc, thereby preventing the binding from being too strong and causing deep poisoning and inactivation. The best effect is achieved by using two isopropyl groups with a small steric hindrance methyl group. It is worth noting that all nitrogen groups must be replaced by hydrocarbon groups, otherwise the catalyst will catalyze the dehydrogenation of amines to form imines, which will not only consume the active sites of the catalyst, but also have a stronger coordination ability and can poison the catalyst.

[0012] Preferably, in step S1, the proline isL -Proline.

[0013] Preferably, the activator N -isopropyl- N -Methylpropan-2-amine was added dropwise to the proline solution.

[0014] Preferably, in step S1, the zinc salt is one of zinc acetate, zinc chloride and zinc sulfate.

[0015] Preferably, in step S2, the pore size of the SAPO-11 molecular sieve is 0.5-0.75 nm.

[0016] Specifically, in step S2, the reaction is stirred at room temperature for 10 to 15 minutes.

[0017] The second aspect of the present invention provides a molecular sieve loaded with zinc proline material for preparing lactide by condensation of lactic acid obtained by the above-mentioned preparation method.

[0018] The third aspect of the present invention provides the use of the molecular sieve-loaded zinc proline material in the preparation of lactide by lactic acid condensation.

[0019] Specifically, the method for preparing lactide by condensing lactic acid is as follows: lactic acid is subjected to esterification reaction at a temperature of 140-160°C and a vacuum degree of -40-30 kPa for 3-4 hours; then a molecular sieve-loaded proline zinc material is added and mixed evenly; lactide is evaporated at a temperature of 200-220°C and a vacuum degree of -2-1 kPa; and finally, purification is performed.

[0020] Through the above technical solution, the present invention achieves the following beneficial effects: The present invention uses commercialized and readily available SAPO-11 molecular sieve as a catalyst carrier, and the loaded N -isopropyl- N Zinc proline activated with methylpropane-2-amine. This material catalyzes the polymerization of lactic acid to produce lactide. Compared to traditional catalysts, this catalytic material can reduce coking yields to less than 5%, and the small amount of coke that does form does not adhere to containers, making it easy to clean. This feature makes the lactide synthesis process developed using this catalyst more suitable for large-scale application. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the high-resolution fitting spectrum of the C 1s of the ZnPro / SAPO-11 sample XPS. The characteristic peaks at 284.80 eV, 286.09 eV, and 288.48 eV are attributed to the CC / C=C, CO, and C=O bonds of the material, respectively. Figure 2This is the high-resolution fitting spectrum of O 1s from the XPS of the ZnPro / SAPO-11 sample. The characteristic peaks at 530.16 eV, 531.32 eV, and 532.63 eV are attributed to the metal oxide, Al-O / C=O, and CO / Si-O bonds in the material, respectively. Figure 3 This is the high-resolution fitting spectrum of Si 2p from XPS of the ZnPro / SAPO-11 sample. The characteristic peaks at 102.01 eV and 103.03 eV are attributed to the Si-OH and Si-O bonds in the material, respectively. Figure 4 This is the Al 2p high-resolution fitting spectrum of the XPS of the ZnPro / SAPO-11 sample. The characteristic peak at 74.11 eV is attributed to the aluminum oxide in the material. Figure 5 This is the Zn Auger spectrum of the ZnPro / SAPO-11 sample XPS. The peak shape confirms that the Zn is mainly 2+ Zn ions; Figure 6 This is the infrared spectrum of the ZnPro / SAPO-11 sample. It can be concluded that the main chemical bonds in the sample are Si-O, CO, C=O, OH and Si-OH. DETAILED DESCRIPTION

[0022] The following is a detailed description of the specific embodiments of the present invention in conjunction with the examples. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0023] Example 1 Preparation of zinc proline (ZnPro): In a 250 mL round-bottom flask with a magnetic stirrer, add L -Proline (2.50 g, purchased from Shanghai Macklin Reagent, purity 98%, CAS number: 147-85-3), then added methanol (50 mL), and stirred under magnetic stirring at room temperature until completely dissolved. Add 0.1 g of activator dropwise N -isopropyl- N 2-Methylpropane-2-amine (purchased from Anaiji Chemical, 97% purity, CAS number: 10342-97-9). After magnetic stirring for 10 minutes, add anhydrous zinc acetate (2.00 g, purchased from Shanghai Macklin Reagent, 99% purity, CAS number: 557-34-6). Continue stirring at room temperature for 45 minutes, filter, and vacuum dry at 80°C for 3 hours. Grind to obtain 1.5 g of zinc proline powder.

[0024] Preparation process of zinc proline / SAPO-11 composite material: 0.7g of the above zinc proline (ZnPro) and 0.7g of SAPO-11 molecular sieve (Si / Al ratio 0.5, pore size 0.75 nm, specific surface area 200m 2 / g, purchased from Liyuan New Technology Chemical Materials Business Department, Yicheng District, Zaozhuang City) was mixed with 20 mL of deionized water, stirred at room temperature for 10 minutes, filtered, and vacuum-dried at 70°C for 3 hours to obtain the catalyst proline zinc / SAPO-11 composite material (ZnPro / SAPO-11). XPS and IR tests were performed on the composite material, as shown in Figure 2. Figures 1-6 As shown in the figure, it is determined that the composite material contains organic components (such as CC, C=C, etc.) and inorganic carriers (such as Al-O, Si-O, etc.).

[0025] Lactic acid condensation experiment: Add 50 g of lactic acid to a 250 mL three-necked flask, activate magnetic stirring, and heat. Raise the temperature to 150°C, adjust the vacuum to -40 kPa, and allow the esterification reaction to proceed for 4 hours. Evaporate the generated water. Add 0.5 g of catalyst and stir thoroughly for 5 minutes. Then, adjust the temperature to 220°C and the vacuum to -1 kPa to evaporate the lactide into a collection flask. Add an equal amount of ethyl acetate and heat to 50°C to dissolve the lactide. Vacuum distill the lactide-ethyl acetate solution at 45°C to remove the ethyl acetate. Add an equal amount of deionized water at 0-5°C, mix thoroughly, and stir for 5 minutes. Filter the mixture, and rinse the filter cake three times with deionized water at 0-5°C. Transfer the filter cake to a vacuum drying oven and dry it at 40°C for 6 hours to obtain lactide. The yield is 72%. The optical purity of lactide was calculated by measuring the product's optical rotation with a polarimeter and comparing it to a standard sample (according to the reference method: ACS Sustainable Chem. Eng. 2022, 10, 7658−7663). The result was 97.4%. After cooling, the coke in the reaction flask was scraped (mixed with catalyst; compared to other methods, gently scraping can remove the coke), dried, and weighed. The weight of the coke was subtracted from the weight of the catalyst (0.1 g), resulting in a coke weight of 0.28 g. The coke yield was calculated using the following formula: Coking rate = (coking weight ÷ lactic acid weight used) × 100% = 2.8% The catalyst containing coke could be directly used in the next reaction, achieving a lactide yield of 71% and an optical purity of 97.2%, demonstrating the catalyst's adaptability. Even after 10 cycles, the material maintained a yield exceeding 70%. In contrast, conventional catalysts experience a gradual decrease in activity and product yield due to excessive coke covering the catalytic sites.

[0026] Example 2 Other conditions were the same as in Example 1, and different catalyst supports were used to prepare the materials. The results are shown in Table 1: Table 1 Experimental results of materials prepared using different catalyst supports

[0027] As shown in the table above, SAPO-11 (No. 1, Example 1) achieved the best results. The role of the support is to fully utilize the active centers of the catalyst. Lactide can also be produced by catalyzing the reaction using only activated ZnPro without a support, but the yield is significantly lower and coking increases.

[0028] Example 3 Other conditions were the same as in Example 1, and different catalyst active components were used. The results are shown in Table 2: Table 2 Experimental results of catalyst preparation using different catalyst active components

[0029] As shown in the table above, zinc proline (ZnPro) performs best as the core active material (No. 1, Example 1). Its core function is to act as a Lewis acid to catalyze the dehydration condensation reaction of lactic acid molecules. While other active catalyst components also exhibit catalytic effects, they are inferior to zinc proline in terms of product yield, optical purity, and coking rate. The unique structure of zinc proline enhances its catalytic activity, and its nitrogen complex coordinates zinc, improving catalytic performance.

[0030] Example 4 Other conditions were the same as in Example 1, and catalysts were prepared using different activators. The results are shown in Table 3: Table 3 Experimental results of catalysts prepared using different activators

[0031] As shown in the table above, using N -isopropyl- N 2-Methylpropane-2-amine (No. 1, Example 1) was used to treat zinc proline, achieving the best coking efficiency. While other activators achieved good lactide yield and optical purity, they performed poorly in terms of coking efficiency. If the nitrogen atom is hydrogenated, the activator will rapidly decompose to form imines, poisoning the catalyst metal and resulting in a decrease in catalytic reaction yield (Table 3, No. 5).

[0032] Example 5 Other conditions were the same as in Example 1, and catalysts were prepared using SAPO-11 molecular sieve and zinc proline at different mass ratios. The results are shown in Table 4: Table 4 Experimental results of catalyst preparation using different mass ratios of SAPO-11 molecular sieve and zinc proline

[0033] As shown in the table above, the optimal mass ratio of SAPO-11 molecular sieve to zinc proline is 100% (No. 4, Example 1). Under these conditions, both lactic acid conversion and lactide selectivity are superior to those under other conditions. This demonstrates that a 100% mass ratio of SAPO-11 molecular sieve to zinc proline achieves a good balance between catalytic performance and mechanical stability.

[0034] Example 6 Other conditions were the same as in Example 1. Different weight ratios of N-isopropyl-N-methylpropan-2-amine to proline were used in the preparation of zinc proline. The results are shown in Table 5: Table 5 Experimental results of different weight ratios of activator and proline during catalyst preparation

[0035] As shown in the table above, when preparing zinc proline, N -isopropyl- N The optimal weight ratio of 2-methylpropan-2-amine to proline is 4% (No. 4, Example 1). The lactide yields varied significantly at different weight ratios, indicating that this ratio fully activates the zinc proline and enhances the overall catalytic effect of the material.

[0036] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0037] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0038] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing a molecular sieve-loaded proline zinc material for preparing lactide by lactic acid condensation, characterized in that: The steps include: S1. Add activator to proline solution N -isopropyl- N -methylpropane-2-amine, then add zinc salt to obtain proline zinc after reaction, wherein N -isopropyl- N The mass ratio of -methylpropan-2-amine to proline is (4~6):100; S2. Proline zinc and SAPO-11 molecular sieve are mixed and reacted in a solvent at a mass ratio of 1: (0.8-1.2) to obtain a proline zinc / SAPO-11 composite material.

2. The preparation method according to claim 1, characterized in that In step S1, the proline is L -Proline.

3. The preparation method according to claim 1, characterized in that In step S1, the activator N -isopropyl- N -Methylpropan-2-amine was added dropwise to the proline solution.

4. The preparation method according to claim 1, characterized in that In step S1, the zinc salt is one of zinc acetate, zinc chloride and zinc sulfate.

5. The preparation method according to claim 1, wherein In step S2, the pore size of the SAPO-11 molecular sieve is 0.5-0.75 nm.

6. The preparation method according to claim 1, wherein In step S2, the reaction is stirred at room temperature for 10 to 15 minutes.

7. The molecular sieve-loaded zinc proline material for preparing lactide by lactic acid condensation obtained by the preparation method according to any one of claims 1 to 6.

8. Use of the molecular sieve-supported zinc proline material according to claim 7 in the preparation of lactide by lactic acid condensation.

9. The use according to claim 8, characterized in that The method for preparing lactide by condensing lactic acid is as follows: lactic acid is subjected to esterification reaction at a temperature of 140-160°C and a vacuum of -40-30 kPa for 3-4 hours; then, a molecular sieve-loaded proline zinc material is added and mixed evenly; lactide is evaporated at a temperature of 200-220°C and a vacuum of -2-1 kPa; and finally, purification is performed.

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