A method for preparing a heteroatom porous beta molecular sieve and its application in preparing propiolactone

Abstract

This invention discloses a method for preparing heteroatom porous beta molecular sieves, comprising the following steps: (1) mixing deionized water, tetrahydrofuran, sodium oxide, sodium aluminate, and zwitterionic polymer PAAn- b - PILm is mixed and stirred evenly, and tetraethyl orthosilicate is added dropwise. The mixture is stirred at 35-45℃ for 8-16 hours; (2) it is transferred to a reaction vessel lined with polytetrafluoroethylene, crystallized at 75-85℃ for 36-60 hours, and then crystallized at 130-150℃ for 8-16 hours; then dried and calcined. At the same time, the present invention also discloses the Sn-beta molecular sieve catalyst for preparing lactide using the beta molecular sieve and its preparation method and its application in the preparation of lactide. The reaction conditions of the present invention are mild and suitable for high concentrations of lactic acid raw materials, which reduces the production cost and produces lactide with high purity, thus solving the problems existing in the two-step preparation of lactide.

Description

Technical Field

[0001] This invention belongs to the field of lactide preparation technology, specifically relating to a method for preparing heteroatom porous beta molecular sieves and their application in lactide preparation. Background Technology

[0002] Polylactic acid (PLA) is a linear aliphatic polyester with excellent mechanical strength, biocompatibility, renewability, and biodegradability. It is hailed as one of the most promising green and environmentally friendly bio-based polymer materials and an ideal alternative to petroleum-based polymers. To date, PLA has found significant applications in agriculture, forestry, environmental protection, medicine, and life sciences, such as as packaging materials, 3D printing materials, surgical sutures, and scaffolds for fixing human bone structures.

[0003] Ring-opening polymerization of lactide monomers can yield PLA with molecular weights reaching hundreds of thousands or even millions and a narrow molecular weight distribution. PLA products made from this method exhibit better anti-aging properties and stronger mechanical properties; therefore, lactide is crucial for PLA synthesis. Currently, lactide synthesis mainly involves a two-step reaction (referred to as the "two-step method"): the first step involves dehydrating lactic acid under high temperature and low pressure conditions to form oligolactic acid; the second step involves depolymerizing the oligolactic acid under the action of a metal salt catalyst to obtain lactide monomers. This industrial production method has long been monopolized by foreign countries, as evidenced by representative patents such as US1095205A, US5053522A, WO9509879A1, EP98203427A, and US6005067A. Until recently, domestically developed two-step methods for preparing lactide have been successively developed. For example, Academician Chen Xuesi of the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, has been deeply involved in the research for many years and has mastered the whole process of "lactic acid-lactide-polylactic acid". The organic guanidine catalyst proposed in patent CN103193759A can be used to prepare lactide. The copper salt and polyselenoether catalyst proposed in patent CN113234055A are used for the synthesis of lactide. However, the above-mentioned lactide synthesis is a two-step reaction. Although this process is mature, the two-step reaction also has the following shortcomings: (1) The reaction conditions of this process are harsh. It usually needs to be carried out at high temperature (greater than 210°C) and high vacuum (7mm). (1) The process is carried out in the Hg system, which greatly increases energy consumption and has high requirements for equipment; (2) The temperature of this process is high, which will cause lactide to racemize at high temperature, resulting in low optical purity of crude lactide, affecting the product quality of lactide. The crude lactide needs to be refined and purified in the future, which further increases the cost; (3) Organometallic catalysts (such as stannous octoate) are used in the depolymerization process. In this system, the catalyst is not easy to remove from the system, and a certain amount of tin reagent has a certain toxic effect on the environment and human body, which does not conform to the concept of green environmental protection.

[0004] In recent years, there have been documents ( Science 2015, 349 , 78-80; Angew. Chem. (2018, 130, 3128–3132, etc.) reported a one-step catalytic synthesis of lactide from lactic acid or methyl lactate (referred to as "one-step method"). However, this method is heavily dependent on the concentration of the lactic acid substrate (50). wt (The optimal percentage is low), which limits the application of this method. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing heteroatom porous beta molecular sieves, and also provides the application of the beta molecular sieve in the preparation of lactide, including a porous Sn-beta molecular sieve for preparing lactide prepared using the beta molecular sieve, its preparation method, and its application in the preparation of lactide.

[0006] A method for preparing heteroatom porous beta molecular sieves includes the following steps:

[0007] (1) Deionized water, tetrahydrofuran, sodium oxide, sodium aluminate and zwitterionic polymer PAAn- b - PILm is mixed and stirred evenly, and tetraethyl orthosilicate is added dropwise to it. Stir at 35-45℃ for 8-16 hours.

[0008] (2) Transfer to a polytetrafluoroethylene-lined reactor, crystallize at 75-85℃ for 36-60h, then crystallize at 130-150℃ for 8-16h; then dry and calcine to obtain heteroatom porous beta molecular sieve.

[0009] Among them, the zwitterionic polymer PAAn- b The structural formula for -PILm is as follows:

[0010] ,

[0011] n=45, m=12, or n=45, m=23, or n=23, m=12.

[0012] Preferably, the amount of tetraethyl orthosilicate is calculated as SiO2, the amount of sodium aluminate is calculated as Al2O3, and the amounts of tetraethyl orthosilicate, sodium aluminate, and zwitterionic polymer PAAn- are... b The molar ratio of PILm, sodium oxide, tetrahydrofuran, and deionized water is 1:(0.005-0.045):(0.15-0.75):(0.01-0.5):(1-8):(1-8).

[0013] Preferably, n=45 and m=23, i.e., zwitterionic polymer PAA 45 -b -PIL 23 .

[0014] Preferably, the drying conditions are drying at 80-90℃ for 6-8 hours; the calcination conditions are calcination at 500-600℃ for 5-7 hours.

[0015] A Sn-beta molecular sieve catalyst for the preparation of lactide comprises a support and Sn supported on the support, wherein Sn accounts for 1-10% of the weight of the catalyst, and the support is a beta molecular sieve prepared by the preparation method of the present invention.

[0016] The method for preparing the Sn-beta molecular sieve catalyst for lactide production includes the following steps:

[0017] (a) The beta molecular sieve was refluxed with concentrated nitric acid for 12-18 h, centrifuged, washed with distilled water, and dried to obtain the dealuminized beta-molecular sieve;

[0018] (b) The dealuminated beta molecular sieve was added to a mixed solution of ethanol and tin tetrachloride, refluxed for 12-18 h, dried, and calcined to obtain the Sn-beta molecular sieve catalyst.

[0019] Preferably, the drying in step (b) is performed at 95-105°C for 5-8 hours; the calcination is performed at 500-600°C for 5-7 hours.

[0020] The method for preparing lactide using the Sn-beta molecular sieve catalyst described in this invention involves mixing lactic acid with an organic solvent, adding the Sn-beta molecular sieve catalyst, heating under reflux at 120-180°C for 1-4 hours under inert gas protection, filtering, recovering the Sn-beta molecular sieve catalyst, and concentrating the filtrate to obtain lactide.

[0021] Preferably, the mass ratio of lactic acid to Sn-beta molecular sieve catalyst is 1:(0.01-0.5), the mass ratio of lactic acid to organic solvent is 1:(1-20), and the organic solvent is benzene, toluene, xylene, or mesitylene.

[0022] Preferably, the lactic acid is L-lactic acid or D-lactic acid, and the lactic acid is added in the form of a 50-90 wt% aqueous solution or 98 wt% anhydrous lactic acid.

[0023] In the preparation of lactide, the Sn-beta molecular sieve catalyst recovered by filtration is washed and soaked with an organic solvent (for example, a mixed solution of acetonitrile and methanol) and then dried, and can be reused.

[0024] The zwitterionic polymer PAAn- of the present inventionb -PILm can be prepared using existing technologies, for example, according to the literature "Cubosomes from hierarchical self-assembly of poly(ionic liquid) blockcopolymers" (… Nature Communications , 2017, 8 It was prepared using the method disclosed in , 14057.

[0025] Advantages of this invention:

[0026] The present invention provides mild reaction conditions, is suitable for high-concentration lactic acid raw materials, reduces production costs, and produces high-purity lactide, thus solving the problems existing in the two-step lactide preparation process. Detailed Implementation

[0027] The zwitterionic polymer PAA used in the embodiments of the present invention 45 - b -PIL 23 The structural formula is as follows:

[0028] ,

[0029] Where n=45 and m=23, the zwitterionic polymer PAA 45 - b -PIL 23 The literature "Cubosomes from hierarchical self-assembly of poly(ionic liquid) block copolymers" was adopted. Nature Communications , 2017, 8 It was prepared using the method disclosed in , 14057.

[0030] Example 1

[0031] A method for preparing heteroatom porous beta molecular sieves includes the following steps:

[0032] (1) Deionized water, tetrahydrofuran, sodium oxide, sodium aluminate and zwitterionic polymer PAA 45 - b -PIL 23 Mix thoroughly, add tetraethyl orthosilicate dropwise, and stir at 40°C for 12 hours;

[0033] (2) Transfer to a reaction vessel lined with polytetrafluoroethylene, crystallize at 80°C for 48 hours, then crystallize at 140°C for 12 hours; then dry at 80°C for 8 hours and calcine at 550°C for 6 hours to obtain heteroatom porous beta molecular sieve.

[0034] In this context, the amount of tetraethyl orthosilicate is calculated as SiO2, and the amount of sodium aluminate is calculated as Al2O3. Tetraethyl orthosilicate, sodium aluminate, and PAA... 45 - b -PIL 23 The molar ratios of sodium oxide, tetrahydrofuran, and deionized water, as well as the designations of the resulting catalysts, are shown in Table 1.

[0035] Table 1. Molar ratio of raw materials and corresponding catalysts

[0036] .

[0037] Example 2

[0038] A method for preparing heteroatom porous beta molecular sieves includes the following steps:

[0039] (1) Deionized water, tetrahydrofuran, sodium oxide, sodium aluminate and zwitterionic polymer PAA 45 - b -PIL 23 Mix thoroughly, add tetraethyl orthosilicate dropwise, and stir at 35°C for 16 hours;

[0040] (2) Transfer to a reaction vessel lined with polytetrafluoroethylene, crystallize at 75°C for 60 h, then crystallize at 130°C for 16 h; then dry at 90°C for 6 h and calcine at 500°C for 7 h to obtain heteroatom porous beta molecular sieve, labeled as beta-7.

[0041] In this context, the amount of tetraethyl orthosilicate is calculated as SiO2, and the amount of sodium aluminate is calculated as Al2O3. The molar ratios of each raw material are as follows: tetraethyl orthosilicate, sodium aluminate, PAA 45 - b -PIL 23 The molar ratio of sodium oxide, tetrahydrofuran, and deionized water is 1:0.005:0.15:0.01:1:1.

[0042] Example 3

[0043] A method for preparing heteroatom porous beta molecular sieves includes the following steps:

[0044] (1) Deionized water, tetrahydrofuran, sodium oxide, sodium aluminate and zwitterionic polymer PAA 45 - b -PIL23 Mix thoroughly, add tetraethyl orthosilicate dropwise until dissolved, and stir at 45°C for 8 hours;

[0045] (2) Transfer to a polytetrafluoroethylene-lined reactor, crystallize at 85°C for 36 hours, then crystallize at 150°C for 8 hours; then dry at 80°C for 8 hours and calcine at 600°C for 5 hours to obtain heteroatom porous beta molecular sieve, labeled as beta-8.

[0046] In this context, the amount of tetraethyl orthosilicate is calculated as SiO2, and the amount of sodium aluminate is calculated as Al2O3. The molar ratios of each raw material are as follows: tetraethyl orthosilicate, sodium aluminate, PAA 45 - b -PIL 23 The molar ratio of sodium oxide, tetrahydrofuran, and deionized water is 1:0.005:0.15:0.5:1:8.

[0047] Example 4

[0048] A Sn-beta molecular sieve catalyst is composed of a beta molecular sieve support and Sn supported on the support, wherein Sn accounts for 1% of the weight of the catalyst, the support is the beta molecular sieve labeled Beta-1 in Example 1, and the catalyst is labeled Sn-beta-1@1.

[0049] The preparation method of the Sn-beta molecular sieve catalyst is as follows:

[0050] (a) The beta molecular sieve was refluxed with concentrated nitric acid for 12 h, centrifuged, washed with distilled water, and dried at 100 °C for 6 h to obtain the dealuminated beta-molecular sieve.

[0051] (b) The dealuminated beta molecular sieve was added to a mixed solution of tetrahydrofuran and anhydrous tin tetrachloride, refluxed for 12 h, dried at 100 °C for 6 h, and calcined at 550 °C for 6 h to obtain the Sn-beta molecular sieve catalyst, labeled as Sn-beta-1@1.

[0052] Example 5

[0053] Sn accounts for 5% of the catalyst weight, and the rest is the same as in Example 4. The resulting catalyst is labeled Sn-beta-1@5.

[0054] Example 6

[0055] Sn accounts for 10% of the catalyst weight, and the rest is the same as in Example 4. The resulting catalyst is labeled Sn-beta-1@10.

[0056] Example 7

[0057] A Sn-beta molecular sieve catalyst is composed of a beta molecular sieve support and Sn supported on the support, wherein Sn accounts for 2% of the weight of the catalyst, the support is the beta molecular sieve labeled Beta-1 in Example 1, and the catalyst is labeled Sn-beta-1@2.

[0058] The preparation method of the Sn-beta molecular sieve catalyst is as follows:

[0059] (a) The beta molecular sieve was refluxed with concentrated nitric acid for 18 h, centrifuged, washed with distilled water, and dried at 95 °C for 8 h to obtain the dealuminated beta-molecular sieve.

[0060] (b) The dealuminated beta molecular sieve was added to a mixed solution of tetrahydrofuran and anhydrous tin tetrachloride, refluxed for 18 h, dried at 95 °C for 8 h, and calcined at 500 °C for 7 h to obtain the Sn-beta molecular sieve catalyst, labeled as Sn-beta-1@2.

[0061] Example 8

[0062] A Sn-beta molecular sieve catalyst is composed of a beta molecular sieve support and Sn supported on the support, wherein Sn accounts for 8% of the weight of the catalyst, the support is the beta molecular sieve labeled Beta-1 in Example 1, and the catalyst is labeled Sn-beta-1@8.

[0063] The preparation method of the Sn-beta molecular sieve catalyst is as follows:

[0064] (a) The beta molecular sieve was refluxed with concentrated nitric acid for 16 h, centrifuged, washed with distilled water, and dried at 105 °C for 5 h to obtain the dealuminated beta-molecular sieve.

[0065] (b) The dealuminated beta molecular sieve was added to a mixed solution of tetrahydrofuran and anhydrous tin tetrachloride, refluxed for 16 h, dried at 105 °C for 5 h, and calcined at 600 °C for 5 h to obtain the Sn-beta molecular sieve catalyst, labeled as Sn-beta-1@8.

[0066] Example 9

[0067] A method for preparing lactide involves weighing 100 g of 90 wt% L-lactic acid aqueous solution and mixing it with 200 mL of o-xylene. 10 g of the Sn-beta molecular sieve catalyst Sn-beta-1@1 described in Example 4 is added. A water separator and reflux device are installed, and the mixture is heated to reflux at 150°C for 3 hours under argon protection. After the reaction is complete, the reaction solution is filtered, the catalyst is recovered, and the filtrate is directly concentrated. The crude product is recrystallized from ethyl acetate to obtain lactide. Liquid chromatography analysis is performed, and the results are shown in Table 2.

[0068] Example 10

[0069] A method for preparing lactide involves weighing 100 g of 80 wt% L-lactic acid aqueous solution and mixing it with 100 mL of toluene. 5 g of the Sn-beta molecular sieve catalyst Sn-beta-1@10 described in Example 6 is added. A water separator and reflux device are installed, and the mixture is heated under argon protection at 140°C for 3 hours under reflux. After the reaction is complete, the reaction solution is filtered, the catalyst is recovered, and the filtrate is directly concentrated. The crude product is recrystallized from ethanol to obtain lactide. Liquid chromatography analysis is performed, and the results are shown in Table 2.

[0070] Example 11

[0071] A method for preparing lactide involves weighing 100 g of 98 wt% anhydrous L-lactic acid and mixing it with 200 mL of mesitylene. 5 g of the Sn-beta molecular sieve catalyst Sn-beta-1@10 described in Example 6 is added. A water separator and reflux device are installed, and the mixture is heated to reflux at 180°C for 3 hours under argon protection. After the reaction is complete, the reaction solution is filtered, the catalyst is recovered, and the filtrate is directly concentrated. The crude product is recrystallized from ethyl acetate to obtain lactide. Liquid chromatography analysis is performed, and the results are shown in Table 2.

[0072] Example 12

[0073] The catalyst obtained by filtration and recovery in Example 11 was soaked in acetonitrile and methanol for 1 hour, and then dried in an oven at 100°C for 6 hours. The experiment of Example 11 was repeated, and the catalyst was recycled 7 times. The analysis results are shown in Table 2.

[0074] Example 13

[0075] A method for preparing lactide involves weighing 100 g of 50 wt% L-lactic acid aqueous solution and mixing it with 50 mL of toluene. 0.5 g of the Sn-beta molecular sieve catalyst Sn-beta-1@10 described in Example 6 is added. A water separator and reflux device are installed, and the mixture is heated under argon protection at 140°C for 4 hours under reflux. After the reaction is complete, the reaction solution is filtered, the catalyst is recovered, and the filtrate is directly concentrated. The crude product is recrystallized from ethyl acetate to obtain lactide. Liquid chromatography analysis is performed, and the results are shown in Table 2.

[0076] Example 14

[0077] A method for preparing lactide involves weighing 100 g of 50 wt% L-lactic acid and mixing it with 100 mL of benzene. 25 g of the Sn-beta molecular sieve catalyst Sn-beta-1@10 described in Example 6 is added. A water separator and reflux device are installed, and the mixture is heated under argon protection at 140°C for 1 hour under reflux. After the reaction is complete, the reaction solution is filtered, the catalyst is recovered, and the filtrate is directly concentrated. The crude product is recrystallized from ethyl acetate to obtain lactide. Liquid chromatography analysis is performed, and the results are shown in Table 2.

[0078] Comparative Example 1

[0079] The beta molecular sieve in Example 6 was replaced with a beta molecular sieve (Nankai University Catalyst Factory, catalyst model: NKF-6-25H) to prepare a Sn-beta molecular sieve catalyst, labeled as Sn-beta-D1@10. The obtained catalyst was then subjected to experiments in Examples 11 and 12, and the analysis results are shown in Table 2.

[0080] Table 2 Results of lactide synthesis using porous Sn-beta molecular sieves

[0081] .

Claims

1. A method for preparing heteroatom porous beta molecular sieves, characterized in that: Includes the following steps: (1) Deionized water, tetrahydrofuran, sodium oxide, sodium aluminate and zwitterionic polymer PAAn- b - PILm is mixed and stirred evenly, and tetraethyl orthosilicate is added dropwise to it. Stir at 35-45℃ for 8-16 hours. (2) Transfer to a polytetrafluoroethylene-lined reactor, crystallize at 75-85℃ for 36-60h, then crystallize at 130-150℃ for 8-16h; then dry and calcine to obtain heteroatom porous beta molecular sieve. Among them, the zwitterionic polymer PAAn- b The structural formula for -PILm is as follows: ， n=45, m=12, or n=45, m=23, or n=23, m=12.

2. The method for preparing heteroatom porous beta molecular sieve according to claim 1, characterized in that: The amount of tetraethyl orthosilicate is calculated as SiO2, and the amount of sodium aluminate is calculated as Al2O3. Tetraethyl orthosilicate, sodium aluminate, and the zwitterionic polymer PAAn- b The molar ratio of PILm, sodium oxide, tetrahydrofuran, and deionized water is 1:(0.005-0.045):(0.15-0.75):(0.01-0.5):(1-8):(1-8).

3. The method for preparing heteroatom porous beta molecular sieve according to claim 2, characterized in that: The drying conditions are drying at 80-90℃ for 6-8 hours; the calcination conditions are calcination at 500-600℃ for 5-7 hours.

4. A Sn-beta molecular sieve catalyst for the preparation of lactide, characterized in that: It consists of a support and Sn loaded on the support, wherein Sn accounts for 1-10% of the weight of the catalyst, and the support is a beta molecular sieve prepared by the preparation method according to any one of claims 1-3.

5. The method for preparing the Sn-beta molecular sieve catalyst for lactide production according to claim 4, characterized in that: Includes the following steps: (a) The beta molecular sieve was refluxed with concentrated nitric acid for 12-18 h, centrifuged, washed with distilled water, and dried to obtain the dealuminized beta-molecular sieve; (b) The dealuminated beta molecular sieve was added to a mixed solution of tetrahydrofuran and tin tetrachloride, refluxed for 12-18 h, dried, and calcined to obtain the Sn-beta molecular sieve catalyst.

6. The method for preparing the Sn-beta molecular sieve catalyst for lactide production according to claim 5, characterized in that: The drying in step (b) is performed at 95-105℃ for 5-8 hours; the calcination is performed at 500-600℃ for 5-7 hours.

7. A method for preparing lactide using the Sn-beta molecular sieve catalyst according to claim 4, characterized in that: Lactic acid is mixed with an organic solvent, and the Sn-beta molecular sieve catalyst is added. The mixture is heated under reflux at 120-180°C for 1-4 hours under inert gas protection. After filtration, the Sn-beta molecular sieve catalyst is recovered, and the filtrate is concentrated to obtain lactide.

8. The method for preparing lactide according to claim 7, characterized in that: The mass ratio of lactic acid to Sn-beta molecular sieve catalyst is 1:(0.01-0.5), and the mass ratio of lactic acid to organic solvent is 1:(1-20), wherein the organic solvent is benzene, toluene, xylene, or mesitylene.

9. The method for preparing lactide according to claim 8, characterized in that: The lactic acid is L-lactic acid or D-lactic acid, and it is added in the form of a 50-90 wt% aqueous solution or 98 wt% anhydrous lactic acid.

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