A method for synthesizing tributyl citrate at low temperature using low-silicon beta-type molecular sieve membrane

Abstract

The present invention relates to the technical field of pervaporation-coupled esterification reactions, and provides a method for the low-temperature synthesis of tributyl citrate using a low-silicon β-type molecular sieve membrane. This method utilizes a low-silicon β-type molecular sieve membrane with high permeability and excellent acid resistance, coupled to the esterification reaction of citric acid and n-butanol. The pervaporation technique promptly removes the water byproduct produced by the reaction. This method not only reduces reaction temperature and energy consumption, but also significantly improves reaction conversion rate. It also simplifies production equipment and process flow, facilitating the continuous production of tributyl citrate.

Description

Technical Field

[0001] The invention relates to the technical field of pervaporation coupled esterification reaction, in particular to a method for synthesizing tributyl citrate at low temperature by using a low-silicon beta-type molecular sieve membrane. Background Art

[0002] Tributyl citrate is considered a new green and environmentally friendly plasticizer due to its advantages such as being non-toxic, odorless, highly thermally stable, and having a good plasticizing effect. It is widely used in the cosmetics, food, and medical industries. Traditionally, tributyl citrate is synthesized by the esterification reaction of citric acid with n-butanol using concentrated sulfuric acid or solid acid as a catalyst. The esterification reaction is a type of reversible chemical equilibrium reaction controlled by thermodynamics. To maximize the yield of the target product, tributyl citrate, this can usually be achieved by using an excess of the reactant n-butanol or by promptly removing the byproduct water. However, excess reactants not only increase the cost of raw materials and equipment, but also make subsequent separation difficult. Therefore, promptly removing water from the byproduct is an effective method to improve the conversion rate of the esterification reaction. Xiang Jueyi et al. ([J]. New Chemical Materials, 2022, 50, (8): 182-187) used a supported ionic liquid MImHSO4 / JQS to catalyze the synthesis of tributyl citrate. When the reaction temperature was 145°C, the reaction time was 9 hours, and the catalyst dosage was 6 wt%, the conversion rate of citric acid reached 91.1% when a water separator was used to remove the by-product water. This shows that traditional water removal methods such as adsorption distillation or water separators have disadvantages such as high reaction temperature, high energy consumption, or complex equipment.

[0003] Pervaporation (PV) membrane separation technology offers advantages such as high selectivity, energy conservation, environmental protection, and simple operation. Leveraging PV separation technology, coupled with zeolite membrane reactors for esterification reactions presents promising applications. Previous studies have reported the application of NaA, CHA, and ZSM-5 zeolite membrane reactors in esterification reactions, significantly improving reaction conversion by removing the byproduct water through the membranes. Li et al. ([J]. Ind Eng Chem Res, 2013, 52:6336-6342) employed PV technology to couple NaA zeolite membranes to the dehydration of acetic acid and n-propanol in the esterification reaction. At 373 K and an alcohol / acid molar ratio of 2:1, the final conversion of the reaction increased from 78.2% to 98.6% within 420 minutes. Hasegawa et al. ([J]. J Membr Sci, 2012, 415-416: 368-374) used CHA molecular sieve membranes in the esterification reaction of adipic acid and isopropyl alcohol using concentrated sulfuric acid as a catalyst. The membranes were placed in the gas phase to avoid direct contact with sulfuric acid, and the yield of fatty acid diisopropyl ester was increased from 56% to 98%.

[0004] It can be seen from the above reports that due to their low acid resistance, NaA and CHA molecular sieve membranes with low silicon-aluminum ratios need to avoid direct contact with acidic materials when used in esterification reactions in acidic systems. The membrane can only be placed in the gas phase, which often requires higher temperatures, resulting in unnecessary energy consumption and higher equipment requirements. Molecular sieve membranes with high silicon-aluminum ratios, such as ZSM-5 molecular sieve membranes, have good acid resistance. Xue Miao et al. ([J]. Membrane Science and Technology, 2018, 38(4):107-112) applied ZSM-5 molecular sieve membranes to the esterification reaction of acetic acid and isoamyl alcohol by pervaporation technology, removing the water produced by the reaction in situ, thereby breaking the reaction equilibrium and increasing the yield of the product isoamyl acetate. The results showed that when the reaction temperature was 100°C and the initial molar ratio of acetic acid to isoamyl alcohol was 3, the yield of isoamyl acetate reached 98.39%, and the flux of the molecular sieve membrane was 0.21 kg m -2 h -1 The lower permeation flux also limits the wider application of high silicon-aluminum ratio molecular sieve membranes. Summary of the Invention

[0005] The present invention aims to overcome at least one of the problems of the above-mentioned prior art and provides a method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane with excellent acid resistance and hydrophilicity. This method uses pervaporation membrane separation technology to continuously remove the byproduct water produced in situ, breaking the traditional reaction equilibrium. It can achieve a high reaction conversion rate and a product tributyl citrate yield close to 100% at a lower reaction temperature. At the same time, it reduces reaction energy consumption, reduces equipment requirements, is simple to operate, and facilitates continuous production. The objectives of the present invention are achieved based on the following technical solutions:

[0006] The present invention provides a method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane, comprising the following steps:

[0007] S1. Synthesis of low-silicon β-type molecular sieve membrane: Various raw materials and deionized water are mixed and stirred, and aged to obtain a membrane synthesis solution; induction seed crystals and a seeded support are added, and the mixture is placed in a reactor for hydrothermal crystallization reaction. After crystallization, the mixture is washed to neutrality and dried to obtain a low-silicon β-type molecular sieve membrane with a silicon-aluminum ratio of 1.5 to 4.5;

[0008] S2. Assembly of the membrane reactor: seal one end of the low-silicon β-type molecular sieve membrane and connect the other end to a vacuum line to assemble a membrane assembly, which is then placed in a reactor; add n-butanol, citric acid, and a catalyst into the reactor so that the reaction solution submerges the low-silicon β-type molecular sieve membrane, with the outside of the molecular sieve membrane serving as the reaction side and the inside serving as the permeation side;

[0009] S3. Synthesis of tributyl citrate: After the reaction solution is stirred evenly, the temperature is raised to 60-100°C for esterification reaction. The vacuum pipeline uses a vacuum pump to provide a pressure difference, and the material measured by the molecular sieve membrane permeation is collected by low-temperature condensation.

[0010] The β-type zeolite membrane has a three-dimensional twelve-ring cross-pore structure and excellent thermal stability, good acid stability and chemical stability. It has broad application prospects in the fields of organic separation and membrane catalysis. Among them, the low-silicon β-type zeolite membrane has excellent acid resistance and hydrophilicity, and shows great potential in the dehydration of organic butanol. It has potential prospects in the esterification reaction of citric acid and n-butanol to produce tributyl citrate.

[0011] The current process of synthesizing tributyl citrate by the esterification reaction of citric acid and n-butanol has the following problems: high reaction temperature, high energy consumption, complex equipment and operation, large amount of catalyst, and the use of additional steps such as water separators. This study explores the use of low-silicon β-type molecular sieve membrane coupling in the production process of tributyl citrate. At a lower operating temperature and less catalyst dosage, the byproduct water produced by the esterification reaction is removed in situ through pervaporation technology, breaking the traditional esterification reaction balance and improving the conversion rate of citric acid and the yield of tributyl citrate. The present invention has the advantages of low temperature and high efficiency, energy saving and environmental protection, simple equipment and operation, and provides a theoretical basis for the continuous industrial production of tributyl citrate.

[0012] Preferably, the synthesis of the low-silicon β-type molecular sieve membrane in step S1 specifically includes the following steps:

[0013] Various raw materials and deionized water are mixed and stirred, and aged to obtain a membrane synthesis solution. The molar ratio of the raw materials is expressed in the form of oxides as follows: 20-60SiO2: 0.5-1Al2O3: 5-30Na2O: 0.1-2NaF: 200-1200H2O; 0.03wt%-0.5wt% of low-silicon β-type seed crystals used in the synthesis solution are added to the synthesis solution, and a seeding support is added, and the two are placed together in a reactor for hydrothermal crystallization reaction at a crystallization temperature of 90-180°C and a time of 12-72h. After the crystallization is completed, the membrane is washed to neutrality and dried to obtain a low-silicon β-type molecular sieve membrane.

[0014] Preferably, the aging method used in step S1 includes water bath static aging method, water bath stirring aging method, ultrasonic dispersion static aging method, ultrasonic dispersion stirring aging method or oven static aging method; the aging conditions are aging at 45-105°C for 2-10h.

[0015] Preferably, the support in step S1 is tubular, including porous alumina, porous stainless steel, porous silicon carbide and porous hollow fiber.

[0016] Preferably, the silicon-aluminum ratio of the low-silicon β-type molecular sieve membrane in step S1 is 2 to 3.5.

[0017] Preferably, the molar ratio of n-butanol to citric acid in step S2 is 1-6.

[0018] Preferably, the catalyst in step S2 includes one or more of concentrated sulfuric acid, p-toluenesulfonic acid, sodium bisulfate, and Sn-Beta zeolite.

[0019] Preferably, the amount of the catalyst used in step S2 is 0.01 wt% to 0.3 wt% of the total mass of the reactants.

[0020] Preferably, the effective membrane area to reaction liquid mass ratio in step S2 is 0.005 to 0.1 m 2 / kg.

[0021] Preferably, the reaction temperature of the esterification reaction in step S3 is 70-100° C.; and the reaction time is 8-15 h.

[0022] Preferably, the vacuum pump in step S3 maintains the vacuum degree in the membrane reactor device at ≤200 Pa.

[0023] The present invention can achieve at least one of the following beneficial effects:

[0024] The present invention systematically studies the effects of different catalysts, different reaction temperatures and different molar ratios of n-butanol and citric acid on the conversion rate and yield of a tributyl citrate synthesis reaction. By regulating the amount of the catalyst, the reaction time and the mass ratio of the effective membrane area to the reaction liquid, the present invention achieves a high conversion rate and a high yield of the esterification reaction. The reaction temperature can be lowered, the process flow of the esterification reaction can be simplified, and the continuous production of tributyl citrate can be promoted.

[0025] The present invention provides a method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane with high permeability, high acid resistance and hydrophilicity. The synthesis temperature is reduced from the conventional temperature of more than 130°C to 100°C, which reduces energy consumption. In addition, the amount of catalyst used can be greatly reduced, and the operating equipment and process flow are simplified, thereby achieving continuous production of tributyl citrate. In addition, the low-silicon β-type molecular sieve membrane can be reused through a simple regeneration method. After participating in up to 10 esterification reactions, the low-silicon β-type molecular sieve membrane layer still maintains its original morphology and good symbiosis, and has good cycle performance. The membrane reactor used in the present invention to synthesize tributyl citrate using the low-silicon β-type molecular sieve membrane can also be applied to other organic carboxylic acid ester reaction systems, such as ethyl acetate reaction, isoamyl acetate reaction, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A diagram of the apparatus for preparing tributyl citrate using the low-silicon β-type molecular sieve membrane employed in the present invention;

[0027] Figure 2 This is an electron microscope image of the surface of the low-silicon β-type molecular sieve membrane prepared by the present invention;

[0028] Figure 3 This is an electron microscope image of the surface of the low-silicon β-type molecular sieve membrane after multiple esterification reactions of the present invention;

[0029] Figure 4 This is a graph showing the change in citric acid conversion rate over time in Example 1 of the present invention;

[0030] Figure 5 This is a graph showing the change in citric acid conversion over time in Example 2 of the present invention;

[0031] Figure 6 This is a graph showing the change in citric acid conversion over time in Example 3 of the present invention;

[0032] Figure 7 This is a graph showing the change in citric acid conversion over time in Example 4 of the present invention;

[0033] Figure 8 This is a graph showing the change in citric acid conversion over time in Example 5 of the present invention;

[0034] Figure 9 This is a graph showing the change in citric acid conversion rate over time in Comparative Example 1 of the present invention;

[0035] Explanation of the accompanying symbols: 1. Reactor, 2. Low-silicon β-type molecular sieve membrane, 3. Condenser, 4. Stirring oil bath, 5. Buffer tank, 6. Vacuum pump, 7. Liquid nitrogen tank, 8. Cold trap, 9. Three-way valve, 10. Vacuum gauge. DETAILED DESCRIPTION

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0037] A preferred embodiment of the present invention provides an apparatus for preparing tributyl citrate using a low-silicon β-type molecular sieve membrane, comprising a reactor 1, a low-silicon β-type molecular sieve membrane 2, a condenser 3, a stirring oil bath 4, a buffer tank 5, a vacuum pump 6, a liquid nitrogen tank 7, a cold trap 8, a three-way valve 9, and a vacuum gauge 10, wherein:

[0038] One end of the low-silicon β-type molecular sieve membrane 2 is closed, and the other end is connected to a vacuum pipeline to be assembled into a membrane assembly. The membrane assembly is placed in a reactor 1, and the reactor 1 is placed in a stirring oil bath 4 for heating. The reactor 1 adopts a three-way reactor, one end of the three-way reactor is closed, one end is for the membrane assembly to pass through, and one end is connected to a condenser 3; n-butanol, citric acid and a catalyst are added to the reactor 1 to immerse the low-silicon β-type molecular sieve membrane 2 in the reaction liquid, the outside of the low-silicon β-type molecular sieve membrane is the reaction side, and the inside is the permeation test; the vacuum pipeline is connected to one end of a cold trap 8 and a vacuum gauge 10 through a three-way valve 9, and the other end of the cold trap 8 is connected to a vacuum pump 6, and the vacuum pipeline provides a pressure difference with the vacuum pump 6, and the substance permeated by the low-silicon β-type molecular sieve membrane 2 is condensed at low temperature in the cold trap 8 through a liquid nitrogen tank 7, and then collected for detection and analysis.

[0039] In a preferred embodiment of the present invention, the seeds added to the synthesis solution are the same as the seeds coated on the support, and the low-silicon β-type seeds used are purchased from, and the molar ratio thereof is approximately: 23SiO2:1Al2O3:10Na2O:8NaF:460H2O.

[0040] The following is a specific example of preparing tributyl citrate using a low-silicon β-type molecular sieve membrane.

[0041] Example 1

[0042] 1. Preparation of low-silicon β-type molecular sieve membrane:

[0043] Sodium hydroxide, aluminum hydroxide, sodium fluoride, silica sol, and water were used as raw materials and vigorously stirred to prepare a milky white membrane synthesis solution. The molar ratio of the solution, expressed as oxides, was 30SiO₂:1Al₂O₃:9Na₂O:12NaF:800H₂O. The membrane synthesis solution was then ultrasonically dispersed and aged for 8 hours at 70°C. Nanoscale low-silicon β-zeolite seeds were then loaded onto the surface of a porous alumina support using a hot-dip method. The seeded support was then oven-dried at 75°C for 4 hours. The dried, seeded support was then placed in the aged membrane synthesis solution and subjected to a secondary hydrothermal crystallization at 150°C for 36 hours. After crystallization, the surface and interior of the tube were washed with deionized water to remove any alkali or particulate matter. The membrane was then dried at 70°C for 6 hours before use. The resulting low-silicon β-zeolite membrane had a Si / Al ratio of 2.7.

[0044] Six low-silica β-type molecular sieve membranes were used in a pervaporation experiment to test their pervaporation performance (75°C) in a n-butanol / water (90 / 10 wt%) system. The experimental results are shown in Table 1, indicating that the prepared membrane has a high permeation flux and an excellent separation factor, and is suitable for the dehydration process of the esterification reaction of citric acid and n-butanol.

[0045] Table 1

[0046]

[0047] 2. Synthesis of tributyl citrate

[0048] like Figure 1 As shown, a certain amount of n-butanol and citric acid are weighed in a molar ratio of 4 and added to a three-way reactor equipped with a stirrer and a condenser (the other opening of the three-way reactor is closed), and heated to 50°C. After the citric acid is completely dissolved, the initial acid value X0 of the esterification is determined by acid-base neutralization titration; continue heating to the reaction temperature of 100°C, maintain the reaction temperature by a constant temperature oil bath, add 0.1wt% of p-toluenesulfonic acid as a catalyst; at the same time, the prepared low-silicon β-type molecular sieve membrane is directly placed into the reaction solution of the esterification reaction as a membrane reactor, and start timing. The acid value Xn is measured every hour. When the acid value remains basically unchanged, stop the reaction and determine the final acid value. During the reaction, the effective area of the molecular sieve membrane is completely immersed in the reaction solution, and the ratio of the effective membrane area to the mass of the initial reaction solution is 0.025m 2 The permeate was collected every hour using a cold trap and analyzed for component content using a gas chromatograph (GC-2014C, Shimadzu) equipped with a TCD detector. The conversion of the reactant citric acid and the yield of the product tributyl citrate were calculated based on the acid value and chromatographic analysis results.

[0049] After the reaction is complete, the product, tributyl citrate, is purified and isolated. The molecular sieve membrane is removed and rinsed with running deionized water until neutral. After drying at 70°C for 6 hours, it can be reused. The progress of the esterification reaction is expressed as the citric acid conversion rate: citric acid conversion rate = (amount of citric acid before reaction - amount of citric acid after reaction) / amount of citric acid before reaction * 100%.

[0050] Scanning electron microscope (SEM) was used to characterize the synthesized low-silicon β-type zeolite membrane and the zeolite membrane after 10 esterification reactions. Figure 2 It can be seen that the surface of the low-silicon β-type molecular sieve membrane synthesized by the present invention has good interaction and symbiosis, and the membrane layer is continuous and dense; Figure 3 It can be seen that after participating in multiple esterification reactions, the low-silicon β-type zeolite membrane layer still maintains its original morphology and good symbiosis.

[0051] The curve diagram of the conversion rate of the reactant citric acid in the esterification reaction of Example 1 over time is as follows Figure 4 shown.

[0052] Example 2

[0053] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction are the same as those in Example 1, except that the molar ratio of n-butanol to citric acid in the esterification reaction is adjusted to 3:1. The curve of the conversion rate of citric acid in the esterification reaction over time is shown in FIG. Figure 5 shown.

[0054] Example 3

[0055] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction are the same as those in Example 1, except that the catalyst in the esterification reaction is adjusted to Sn-Beta zeolite. The curve of the conversion rate of citric acid in the esterification reaction over time is shown in FIG. Figure 6 shown.

[0056] Example 4

[0057] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction are the same as those in Example 1, except that the amount of catalyst used in the esterification reaction is adjusted to 0.05 wt %. The curve of the conversion rate of citric acid in the esterification reaction over time is shown in FIG. Figure 7 shown.

[0058] Example 5

[0059] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction are the same as those in Example 1, except that the reaction temperature in the esterification reaction is adjusted to 90°C. The curve of the conversion rate of citric acid in the esterification reaction over time is shown in FIG. Figure 8 shown.

[0060] Example 6

[0061] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction were the same as those in Example 1, except that the molar ratio of n-butanol to citric acid in the esterification reaction was adjusted to 5:1.

[0062] Example 7

[0063] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction were the same as those in Example 1, except that the amount of the catalyst in the esterification reaction was adjusted to 0.15 wt %.

[0064] Example 8

[0065] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction were the same as those in Example 1, except that the reaction temperature in the esterification reaction was adjusted to 70°C.

[0066] Example 9

[0067] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction were the same as those in Example 1, except that the ratio of the effective membrane area of the molecular sieve membrane to the mass of the initial reaction solution was 0.050 m 2 / kg.

[0068] Example 10

[0069] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction were the same as those in Example 1, except that the ratio of the effective membrane area of the molecular sieve membrane to the mass of the initial reaction solution was 0.010 m 2 / kg.

[0070] Example 11

[0071] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction are the same as those in Example 1, except that the SiO2:Al2O3 ratio in the synthetic raw materials of the molecular sieve membrane is 40:1, and the silicon-aluminum ratio of the obtained molecular sieve membrane is about 3.3.

[0072] Example 12

[0073] The preparation steps of the molecular sieve membrane and the synthesis conditions of the esterification reaction are the same as those in Example 1, except that the SiO2:Al2O3 ratio in the synthetic raw materials of the molecular sieve membrane is 20:1, and the silicon-aluminum ratio of the obtained molecular sieve membrane is about 2.4.

[0074] Comparative Example 1

[0075] The synthesis conditions and steps of the esterification reaction were the same as those in Example 1, except that the low-silicon β-type molecular sieve membrane reactor was not coupled to the esterification reaction. The curve of the conversion rate of citric acid in the esterification reaction over time is shown in FIG. Figure 9 shown.

[0076] Table 2 shows the conversion rates of the reactants in the esterification reaction under the conditions of Examples 1 to 12 and Comparative Example 1 for 12 hours.

[0077] Table 1

[0078] Conversion rate of citric acid (%) Example 1 97.58 Example 2 85.76 Example 3 79.77 Example 4 91.75 Example 5 89.13 Example 6 98.46 Example 7 98.24 Example 8 72.47 Example 9 98.91 Example 10 85.44 Example 11 86.72 Example 12 89.55 Comparative Example 1 69.75

[0079] As can be seen from the data in Table 2, the method provided by the present invention has a high conversion rate of tributyl citrate. Compared with the esterification reaction using a low-silicon β-type molecular sieve membrane reactor coupled to the esterification reaction, the citric acid conversion rate without the molecular sieve membrane reactor decreased from 97.58% to 69.75%, indicating that the method of synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane can significantly improve the reaction conversion rate and has significant advantages.

[0080] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane, characterized in that: The following steps are involved: S1. Synthesis of low-silicon β-type molecular sieve membrane: Various raw materials and deionized water are mixed and stirred, and aged to obtain a membrane synthesis solution. The raw material molar ratio is expressed in the form of oxides as follows: 20-60SiO2: 0.5-1Al2O3: 5-30Na2O: 0.1-2NaF: 200-1200H2O; 0.03wt%-0.5wt% of the synthesis solution amount of low-silicon β-type seed crystals are added to the synthesis solution, and a seed support is added, and the whole is placed in a reactor for hydrothermal crystallization reaction. The crystallization temperature is 90 to 180° C. for 12 to 72 hours. After the crystallization is completed, the membrane is washed to neutrality and dried to obtain a low-silicon β-type molecular sieve membrane with a silicon-aluminum ratio of 2.4 to 3.

3. The aging methods used include water bath static aging, water bath stirring aging, ultrasonic dispersion static aging, ultrasonic dispersion stirring aging or oven static aging. The aging conditions are aging at 45 to 105° C. for 2 to 10 hours. The support is tubular and includes porous alumina, porous stainless steel, porous silicon carbide and porous hollow fiber. S2. Assembly of membrane reactor: seal one end of low-silicon β-type molecular sieve membrane and connect the other end to vacuum line to assemble into membrane assembly, and place the membrane assembly into reactor; add n-butanol, citric acid and catalyst into the reactor so that the reaction liquid immerses the low-silicon β-type molecular sieve membrane, with the outside of the molecular sieve membrane as the reaction side and the inside as the permeation side; the effective membrane area to reaction liquid mass ratio is 0.005-0.1m 2 / kg; S3. Synthesis of tributyl citrate: After the reaction solution is stirred evenly, the temperature is raised to 90-100°C for esterification reaction. The vacuum pipeline uses a vacuum pump to provide a pressure difference, and the material measured by the molecular sieve membrane permeation is collected by low-temperature condensation.

2. The method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane according to claim 1, characterized in that: The molar ratio of n-butanol to citric acid in step S2 is 1-6.

3. The method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane according to claim 1, characterized in that: The catalyst in step S2 includes one or more of concentrated sulfuric acid, p-toluenesulfonic acid, sodium bisulfate, and Sn-Beta zeolite.

4. The method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane according to claim 1, characterized in that: The amount of the catalyst used in step S2 is 0.01 wt% to 0.3 wt% of the total mass of the reactants.

5. The method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane according to claim 1, characterized in that: The reaction time of the esterification reaction in step S3 is 8 to 15 hours.

6. The method for synthesizing tributyl citrate at low temperature using a low-silicon β-type molecular sieve membrane according to claim 1, characterized in that: In step S3, the vacuum pump maintains the vacuum degree in the membrane reactor device at ≤200 Pa.

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