Accurate temperature control on / off type microcapsule catalyst and application thereof in synthesis of 3-mercaptopropionic acid

Abstract

The invention discloses a precise temperature control on / off type microcapsule catalyst and application thereof in synthesis of 3-mercaptopropionic acid, and belongs to the technical field of catalytic materials. The catalyst adopts a core-shell structure design, an inner core is a beta-cyclodextrin-quaternary ammonium salt ionic liquid inclusion compound, and a shell layer is a polyamide-poly (N-isopropylacrylamide) copolymer; through the swelling / shrinkage behavior of the shell layer at a specific temperature (35 + / -2 DEG C) and the dissociation balance of the inner core inclusion compound, the precise temperature control on / off regulation reaction is realized. In a low-temperature interval of 35-37 DEG C, the catalyst keeps high activity, the acrylic acid conversion rate is greater than or equal to 95%, and the mercaptopropionic acid selectivity is greater than When the temperature is higher than 45 DEG C, the catalytic activity is obviously reduced (the conversion rate is less than or equal to 10%), and reaction stop control can be realized. The catalyst circulation can realize the start and stop of the reaction through temperature control, and after 10 times of use, the conversion rate of acrylic acid is greater than or equal to 92%, the selectivity of mercaptopropionic acid is greater than or equal to 95%, the catalyst is suitable for a fixed bed continuous reaction system, the production operation is simplified, the production cost is reduced, and the catalyst has good industrial application prospects.

Description

Technical Field

[0001] This invention belongs to the field of catalytic materials and chemical reaction engineering technology, specifically relating to a precise temperature-controlled on / off microcapsule catalyst and its application in the synthesis of 3-mercaptopropionic acid. Background Technology

[0002] 3-Mercaptopropionic acid, as an important fine chemical intermediate, is widely used in pharmaceuticals, cosmetics, and polymer materials. Its traditional synthesis mainly relies on the addition reaction of acrylic acid and hydrogen sulfide. Currently, commonly used catalysts for this reaction include inorganic bases (such as NaOH), organic amine compounds, and quaternary ammonium salt ionic liquids. However, existing technologies suffer from problems such as difficulty in catalyst recovery, product separation difficulties, high energy consumption, and serious pollution.

[0003] To address the aforementioned issues, temperature-responsive catalytic materials have become a research hotspot. Existing temperature-responsive catalysts are mostly designed based on thermosensitive polymers such as poly(N-isopropylacrylamide) (PNIPAM). However, single thermosensitive materials suffer from drawbacks such as insufficient shell stability (e.g., poor acid corrosion resistance) and a single response mechanism (relying solely on shell swelling / contraction), making them unsuitable for the weakly acidic, sulfur-containing atmosphere of the acrylic acid-hydrogen sulfide reaction. Therefore, developing a microencapsulated catalyst that combines high stability, precise temperature-controlled on / off switching, and excellent catalytic performance is of great significance for the efficient synthesis of mercaptopropionic acid. Summary of the Invention

[0004] The purpose of this invention is to provide a precisely temperature-controlled on / off microcapsule catalyst and its application in the synthesis of 3-mercaptopropionic acid, so as to solve the problems of uncontrollable activity, poor cycle stability and low mass transfer efficiency of existing catalysts.

[0005] This invention provides a precise temperature-controlled on / off microcapsule catalyst, employing a core-shell structure design, wherein: The core is an inclusion complex formed by β-cyclodextrin and quaternary ammonium salt ionic liquid; The shell is a polyamide-polyN-isopropylacrylamide copolymer; The catalyst's shell is temperature-responsive, with a minimum critical dissolution temperature of 30-40°C, enabling the "on / off" regulation of catalytic activity through temperature changes.

[0006] The molar ratio of β-cyclodextrin to quaternary ammonium salt ionic liquid is 1:1 to 1:5, preferably 1:1-2.5; the quaternary ammonium salt ionic liquid is selected from at least one of tetrabutylammonium bis(trifluoromethanesulfonyl)imide salt, tetramethylammonium bis(trifluoromethanesulfonyl)imide salt, tetraethylammonium bis(trifluoromethanesulfonyl)imide salt, and tributylmethylammonium bis(trifluoromethanesulfonyl)imide salt.

[0007] The β-cyclodextrin-quaternary ammonium salt ionic liquid inclusion complex is prepared by dissolving β-cyclodextrin in deionized water to form a saturated solution, adding quaternary ammonium salt ionic liquid, stirring at a certain temperature, slowly cooling, and then filtering, washing and drying.

[0008] The mass ratio of the structural units of polyamide to polyN-isopropylacrylamide is 1:1 to 4:1, preferably 2-3:1; the shell thickness is 150-200 nm; and the overall particle size of the microcapsule catalyst is 100-500 μm.

[0009] The microcapsule catalyst is obtained by interfacial polymerization combined with initiation polymerization. First, a uniform water-in-oil emulsion is formed by emulsification, and then an azo initiator is used to initiate the copolymerization reaction of N-isopropylacrylamide and polyamide.

[0010] A method for preparing the aforementioned precise temperature-controlled on / off microcapsule catalyst, characterized by comprising the following steps: S1. Preparation of β-cyclodextrin-quaternary ammonium salt ionic liquid inclusion complexes; S2. Using the inclusion compound, diamine compound, and base as the aqueous phase, and organic solvent, surfactant, acyl chloride compound, and N-isopropylacrylamide monomer as the oil phase, the aqueous phase and the oil phase are mixed and emulsified to form a water-in-oil emulsion; S3. Add an initiator to the emulsion to carry out interfacial polymerization and free radical copolymerization reaction; S4. After the reaction is complete, the solid product is separated, washed, and dried to obtain the microcapsule catalyst.

[0011] In step S2, the aqueous phase further includes a pH adjustment step, adjusting the pH to 8-9; the diamine compound is hexamethylenediamine; the base is anhydrous sodium carbonate; the mass ratio of the inclusion complex, the diamine compound, and the base is (8-10):(2.5-4.5):1. The mass-to-volume ratio of the inclusion complex to deionized water is 1:8-12 (g / mL), preferably 1:9-11 (g / mL).

[0012] The inclusion complex was dispersed in deionized water, and hexamethylenediamine and anhydrous sodium carbonate were added. The mixture was stirred until completely dissolved, and the pH was adjusted to 8-9 to obtain an aqueous phase. The stirring temperature was 30-70℃, preferably 40-65℃, and the stirring time was 1-4 h, preferably 1.5-2.5 h.

[0013] In step S2, in the oil phase, the organic solvent is cyclohexane, the surfactant is Span-80, and the acyl chloride compound is adipic acid chloride; the mass ratio of the organic solvent, the surfactant, the acyl chloride compound, and the N-isopropylacrylamide monomer is (70-100):1:(2-6):1.

[0014] The mass ratio of cyclohexane, Span-80, adipicoyl chloride, and N-isopropylacrylamide is 70-100:1:2-6:1.

[0015] In some preferred embodiments, in step S2, the microcapsules are prepared by: dropping the aqueous phase into the oil phase and stirring to form a water-in-oil emulsion; adding an azo initiator after heating and reacting for a period of time; collecting the product by vacuum filtration, washing it sequentially with ethanol and deionized water, drying it under vacuum, and then sieving it to obtain the microcapsule catalyst.

[0016] The rate of aqueous phase addition is 1-3 mL / min.

[0017] Azo initiators are one or more of azobisisobutyronitrile (AIBN), dimethyl azobisisobutyrate (AIBME), azobiscyanopentanoic acid (ACVA), and azobiscyclohexylformonitrile (ACCN).

[0018] The mass ratio of azo initiator to N-isopropylacrylamide is 1:8-12.

[0019] In step S3, the initiator is an azo initiator selected from at least one of azobisisobutyronitrile, dimethyl azobisisobutyrate, azodicyanovalerate, and azobiscyclohexylformonitrile; the mass ratio of the initiator to the N-isopropylacrylamide monomer is 1:8 to 1:12.

[0020] The precise temperature-controlled on-off microcapsule catalyst prepared by the method of this invention has a minimum critical dissolution temperature (LCST) of 30-40℃, a thickness of 150-200 nm, and a pore size of 15-20 nm at low temperatures (<35℃) and 5-10 nm at high temperatures (>35℃). The particle size of the microcapsule catalyst is 100-500 μm.

[0021] Application of the aforementioned precise temperature-controlled on-off microcapsule catalyst in the catalytic addition reaction of acrylic acid with hydrogen sulfide to prepare 3-mercaptopropionic acid.

[0022] A method for synthesizing 3-mercaptopropionic acid, using the aforementioned precise temperature-controlled on / off microcapsule catalyst, in a fixed-bed reactor, allows acrylic acid and hydrogen sulfide to undergo an addition reaction in the presence of the catalyst; The reaction temperature was 34-37℃, the reaction pressure was 0.1-0.5 MPa, the molar ratio of acrylic acid to hydrogen sulfide was 1:1.2 to 1:2, and the feed space velocity was 1-2.5 h⁻¹. -1 .

[0023] The reaction is stopped by adjusting the reaction temperature to above 45°C to put the catalyst in a "closed" state, and the reaction is restarted by adjusting the reaction temperature to below 35°C to put the catalyst back in a "closed" state. The catalyst can be recycled at least 10 times, and in the 10th cycle, the acrylic acid conversion rate is not less than 80% and the 3-mercaptopropionic acid selectivity is not less than 90%.

[0024] Core-shell structure design: Utilizing the inclusion effect between cyclodextrin and quaternary ammonium salt cations, a "molecular lock" catalytic active site regulation system can be constructed: the quaternary ammonium salt cation, as a key component of the catalytic active site, can temporarily "seal" the active site inside the cyclodextrin cavity through its specific inclusion effect, preventing premature exposure at non-target temperatures; polyamides possess excellent acid and sulfur resistance, and poly-N-isopropylacrylamide, as a typical temperature-responsive polymer, contains both hydrophilic amide groups and hydrophobic isopropyl groups in its molecular chain: when the temperature is below its lower critical solution temperature (LCST), the amide groups form hydrogen bonds with water molecules, the molecular chain is in an extended state, and the shell exhibits high swelling capacity, providing a channel for the release of active sites; when the temperature is above the LCST, the hydrogen bonds break, the molecular chain coils due to hydrophobic interactions, and the shell rapidly contracts, which can further regulate the exposure degree and diffusion rate of the active site.

[0025] Using the technical solution of the present invention, the catalyst achieves an acrylic acid conversion rate of ≥85%, preferably ≥95%, and a 3-mercaptopropionic acid selectivity of ≥92%, preferably ≥98%, in a single use. The catalyst can be recycled at least 10 times, and in the 10th cycle, the acrylic acid conversion rate is ≥80%, preferably ≥92%, and the 3-mercaptopropionic acid selectivity is ≥90%, preferably ≥95%.

[0026] Beneficial effects 1. Dual temperature response: At low temperatures (<35℃), the shell swells (pore size 15-20nm), the inclusion complex is stable, and the reactants easily diffuse to the active sites, resulting in high catalytic activity; at high temperatures (>35℃), the shell shrinks (pore size 5-10 nm), the degree of dissociation of the inclusion complex increases (from 15% to 62%), the number of active sites decreases, and reaction start-up and shutdown control is achieved.

[0027] 2. High stability and recyclability: The polyamide shell is resistant to acid and sulfur, which can effectively protect the core, making the catalyst's cycle life much longer than that of traditional supported solid base catalysts.

[0028] 3. Compatible with fixed-bed systems: The 100-500 μm particle size design ensures stable bed pressure drop (0.1-0.5 MPa), preventing flow deviation, and allowing feed space velocities of 1-2.5 h⁻¹. -1 This meets the needs of continuous industrial production.

[0029] 4. High product purity: The catalyst has high selectivity for mercaptopropionic acid, which can reduce the cost of subsequent separation. Attached Figure Description

[0030] Figure 1 Temperature response performance test of catalyst C1.

[0031] Figure 2 Cyclic stability test of catalyst C1. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below through embodiments.

[0033] To achieve the above objectives, the present invention adopts the following technical solution: (1) Preparation method of inclusion complex: β-cyclodextrin is dissolved in deionized water to form a saturated solution, quaternary ammonium salt ionic liquid is added, stirred at a certain temperature and then slowly cooled, and the inclusion complex is obtained after filtration, washing and drying.

[0034] (2) Preparation of temperature-responsive microcapsules S1: Aqueous phase preparation: The inclusion complex was dispersed in deionized water, hexamethylenediamine and anhydrous Na2CO3 were added, stirred until completely dissolved, and the pH was adjusted to 8-9 to obtain the aqueous phase; S2. Preparation of oil phase: Cyclohexane and Span-80 were mixed, adipyl chloride and N-isopropylacrylamide monomer were added, and the mixture was stirred until completely dissolved to obtain the oil phase; S3. Preparation of microcapsules: The aqueous phase is dropped into the oil phase and stirred to form a water-in-oil emulsion; after heating, an initiator is added and reacted for a certain period of time. The product is then collected by vacuum filtration, washed with ethanol and deionized water in sequence, dried under vacuum, and sieved to obtain the microcapsule catalyst.

[0035] S4. Application: Microcapsule catalysts are packed in a fixed-bed reactor and a mixture of acrylic acid and hydrogen sulfide is introduced to carry out the reaction.

[0036] Example 1: (1) Preparation of a β-cyclodextrin-tetrabutylammonium bis(trifluoromethanesulfonyl)imide salt inclusion complex I1 5 g of β-cyclodextrin was dissolved in 100 mL of deionized water at 60 °C and stirred until completely dissolved to form a saturated solution. 3 g of tetrabutylammonium bis(trifluoromethanesulfonyl)imide salt was added and stirred at 60 °C for 2 h. After stirring, the solution was slowly cooled to room temperature (25 °C) and allowed to stand for 1 h. The precipitate was collected by vacuum filtration and washed with anhydrous ethanol (50 mL × 3) to remove unencapsulated tetrabutylammonium bis(trifluoromethanesulfonyl)imide salt. The precipitate was dried in a vacuum drying oven at 40 °C for 8 h to obtain β-cyclodextrin-quaternary ammonium salt ionic liquid inclusion complex I1.

[0037] (2) Preparation of precision temperature-controlled switch type microcapsule C1 S1. Preparation of aqueous phase: Weigh 5 g of inclusion compound I1 prepared in step (1), disperse it in 50 mL of deionized water, add 2 g of hexamethylenediamine and 0.5 g of anhydrous Na2CO3, stir magnetically for 30 min until completely dissolved, and test the pH with pH paper to find that it is 8.5, thus obtaining the aqueous phase; S2. Preparation of oil phase: Measure 100 g of cyclohexane, add 1 g of Span-80, and stir for 10 min until dissolved; add 2.5 g of adipic acid chloride and 1 g of N-isopropylacrylamide monomer, and continue stirring for 20 min until completely dissolved to obtain the oil phase; S3. Preparation of microcapsules: The aqueous phase was placed in a constant pressure dropping funnel and added dropwise to the oil phase (placed in a 500 mL three-necked flask) at a rate of 2 mL / min, while mechanical stirring was started at 600 rpm. After the addition was complete, stirring was continued for 30 min to form a milky white water-in-oil emulsion. The three-necked flask was placed in a 50 ℃ water bath, and 0.1 g of AIBN initiator was added. The reaction was carried out under nitrogen protection for 3 h. After the reaction was completed, the mixture was cooled to room temperature and the microcapsule product was collected by vacuum filtration. The product was washed with anhydrous ethanol (50 mL × 3) to remove residual Span-80 and unreacted monomers. Then it was washed with deionized water (50 mL × 2) to remove ethanol and Na2CO3. The product was dried in a vacuum drying oven at 30 ℃ for 12 h and sieved with a standard sieve (100-150 mesh) to obtain microcapsule catalyst C1 with a particle size of 100-500 μm.

[0038] (3) Application of microencapsulated catalyst C1 in fixed bed The microcapsule catalyst C1 (100 g) prepared in step (2) was filled into a fixed-bed reactor with an inner diameter of 20 mm and a height of 300 mm, with a filling density of 0.65 g / cm³. 3 The bed height was 200 mm. The reactor temperature was raised to 34°C, and a mixture of acrylic acid (99% purity) and hydrogen sulfide (99.9% purity) was introduced at a molar ratio of 1:1.2. The reaction pressure was 0.2 MPa, and the space velocity was 1.5 h⁻¹. -1 The reaction lasted for 5 hours, with an acrylic acid conversion rate of 96.2% and a selectivity of 98.4% for 3-mercaptopropionic acid. The reaction was then paused to adjust the product collection device, and the reactor temperature was raised to 45 °C. At this point, the acrylic acid conversion rate dropped to 9%, and the reaction was essentially terminated. After the adjustment was completed, the temperature was lowered to 34 °C, and the catalyst activity recovered after 10 minutes.

[0039] Example 2 Temperature response performance test of catalyst C1 ( Figure 1 ) The microcapsule catalyst C1 from step (2) of the example was placed in a fixed-bed reactor, and a mixture of acrylic acid and hydrogen sulfide (molar ratio 1:1.2) was introduced at 25°C, 30°C, 32°C, 34°C, 36°C, 38°C, 40°C, and 45°C, respectively, with a reaction pressure of 0.2 MPa and a space velocity of 1.5 h⁻¹. -1 After 5 hours of reaction, the conversion rate of acrylic acid and the selectivity of 3-mercaptopropionic acid were measured.

[0040] The results showed that the conversion rate was highest at 34 ℃ (96.2%), and the conversion rate decreased significantly above 38 ℃, which is in line with the temperature response design. Selectivity: Under reaction conditions of 34℃, the composition of the product was determined by gas chromatography (GC-2014, Shimadzu). The selectivity of mercaptopropionic acid was 98.4%, and the content of by-products (such as thiodipropionic acid) was only 1.6%.

[0041] Cyclic stability test ( Figure 2 ): Under the reaction conditions of 34℃, after each reaction, the temperature was raised to 45℃ to collect the product, and then cooled to 34℃ for the next batch of reaction. After 10 cycles, the acrylic acid conversion rate was 92% and the selectivity was 95.6%.

[0042] Example 3 Preparation of C2 microencapsulated catalyst with precise temperature control and its application in the synthesis of 3-mercaptopropionic acid The difference from Example 1 is that the quaternary ammonium salt ionic liquid added in step (1) is tetraethylammonium bis(trifluoromethanesulfonyl)imide salt. After reacting it in a fixed-bed reactor at 34°C for 5 h, the acrylic acid conversion rate was measured to be 91.6%, and the selectivity for 3-mercaptopropionic acid was 95.8%.

[0043] Example 4 Preparation of C3 microencapsulated catalyst with precise temperature control and its application in the synthesis of 3-mercaptopropionic acid The difference from Example 1 is that the azo initiator added in step (2) is azodicyclohexylformonitrile (ACCN). After reacting in a fixed-bed reactor at 34°C for 5 h, the conversion rate of acrylic acid was measured to be 88.7%, and the selectivity of 3-mercaptopropionic acid was 92.6%.

[0044] Example 5 Preparation of C1 microencapsulated catalyst with precise temperature control and its application in the synthesis of 3-mercaptopropionic acid The difference from Example 1 is that in step (3), the molar ratio of acrylic acid to hydrogen sulfide mixed raw materials is 1:1.5, and after reacting at 34°C for 5 h, the conversion rate of acrylic acid is 97.8% and the selectivity of 3-mercaptopropionic acid is 98.9%.

[0045] Comparative Example 1 Preparation of C4 microencapsulated catalyst with precise temperature control and its application in the synthesis of 3-mercaptopropionic acid The difference from Example 1 is that no quaternary ammonium salt ionic liquid was added in step (1). After reacting in a fixed-bed reactor at 34°C for 5 h, the conversion rate of acrylic acid was 14.7%, and the selectivity of 3-mercaptopropionic acid was 21.4%.

[0046] Comparative Example 2 Preparation of C4 microencapsulated catalyst with precise temperature control and its application in the synthesis of 3-mercaptopropionic acid Unlike Example 1, no azo initiator was added in step (1). After reacting in a fixed-bed reactor at 34°C for 5 h, the acrylic acid conversion rate was measured to be 32.7%, and the selectivity for 3-mercaptopropionic acid was 51.4%.

[0047] Comparative Example 3 Unlike Example 1, only tetrabutylammonium bis(trifluoromethanesulfonyl)imide salt was used as a catalyst to catalyze the reaction of acrylic acid and hydrogen sulfide in a high-pressure reactor to synthesize 3-mercaptopropionic acid. The temperature of the high-pressure reactor was raised to 34°C, and a mixture of acrylic acid (99% purity) and hydrogen sulfide (99.9% purity) was introduced at a molar ratio of 1:1.2. The reaction pressure was 0.2 MPa, and the reaction lasted for 5 h. The conversion rate of acrylic acid was 54.9%, and the selectivity of 3-mercaptopropionic acid was 38.6%.

[0048] The core of this invention's precision temperature-controlled microcapsule catalyst utilizes a β-cyclodextrin-quaternary ammonium salt ionic liquid inclusion complex. The inclusion of the cyclodextrin cavity with the quaternary ammonium salt cation enables temperature-controlled release of the catalytically active sites. The shell layer is composed of a copolymer of polyamide and poly(N-isopropylacrylamide). The polyamide provides excellent acid and sulfur resistance, while PNIPAM imparts temperature-responsive solubility / shrinkage properties to the shell. Together, they achieve a dual temperature response mechanism. The narrow response temperature range allows the reaction to be stopped by increasing the temperature and resumed by decreasing it. In the low-temperature range of 34-38℃, the catalyst maintains high activity, with acrylic acid conversion ≥95% and mercaptopropionic acid selectivity ≥98%. Above 45℃, the catalytic activity significantly decreases (conversion ≤10%), allowing for controlled reaction start-up and shutdown. Furthermore, it can be recycled multiple times, simplifying production operations, reducing production costs, and demonstrating promising industrial application prospects.

[0049] The embodiments described above are merely preferred embodiments of the present invention, but the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, several modifications and improvements can be made to the technical solution of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. Precise temperature controlled on / off microcapsule catalyst characterized in that, Adopt a core-shell structure design, wherein: The inner core is a clathrate formed by β-cyclodextrin and quaternary ammonium salt ionic liquid; The shell layer is polyamide-poly N-isopropyl acrylamide copolymer; The shell layer of the catalyst has temperature responsiveness, and the lower critical solution temperature is 30-40℃, and the "on / off" regulation of catalytic activity can be realized by temperature change.

2. The precision temperature-controlled on / off microencapsulated catalyst according to claim 1, wherein, The molar ratio of the β-cyclodextrin to the quaternary ammonium salt ionic liquid is 1:1 to 1:5; the quaternary ammonium salt ionic liquid is selected from at least one of tetrabutylammonium bis-trifluoromethanesulfonimide, tetramethylammonium bis-trifluoromethanesulfonimide, tetraethylammonium bis-trifluoromethanesulfonimide, and tributylmethylammonium bis-trifluoromethanesulfonimide.

3. The precision temperature-controlled on / off microencapsulated catalyst according to claim 1, wherein, In the shell layer, the mass ratio of the structural units of polyamide and poly N-isopropyl acrylamide is 1:1 to 4:1; the thickness of the shell layer is 150-200 nm; and the overall particle size of the microcapsule catalyst is 100-500 μm.

4. A method for preparing the precise temperature-controlled on / off microencapsulated catalyst according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: S1. Preparing a β-cyclodextrin-quaternary ammonium salt ionic liquid clathrate; S2. Mixing and emulsifying an aqueous phase comprising the clathrate, a diamine compound, and a base, and an oil phase comprising an organic solvent, a surfactant, an acyl chloride compound, and N-isopropyl acrylamide monomers to form a water-in-oil emulsion; S3. Adding an initiator to the emulsion to perform interfacial polymerization and free radical copolymerization; S4. After the reaction is completed, separating, washing, and drying the solid product to obtain the microcapsule catalyst.

5. The preparation method according to claim 4, characterized in that, In step S2, the aqueous phase further comprises a pH adjusting step, and the pH is adjusted to 8-9; the diamine compound is hexanediamine; the base is anhydrous sodium carbonate; and the mass ratio of the clathrate, the diamine compound, and the base is (8-10):(2.5-4.5):

1.

6. The preparation method according to claim 4, characterized in that, In step S2, in the oil phase, the organic solvent is cyclohexane, the surfactant is Span-80, and the acyl chloride compound is adipoyl chloride; and the mass ratio of the organic solvent, the surfactant, the acyl chloride compound, and the N-isopropyl acrylamide monomers is (70-100):1:(2-6):

1.

7. The preparation method according to claim 4, characterized in that, In step S3, the initiator is an azo initiator selected from at least one of azobisisobutyronitrile, dimethyl azobis-2-isobutyrate, azobis-dicyanopentanoic acid, and azobis-cyclohexyl nitrile; and the mass ratio of the initiator to the N-isopropyl acrylamide monomers is 1:8 to 1:

12.

8. Use of the precise temperature-controlled on / off type microcapsule catalyst according to any one of claims 1-3 in the catalytic addition reaction of acrylic acid and hydrogen sulfide to prepare 3-mercapto propionic acid.

9. A method of synthesizing 3-mercaptopropionic acid, characterized by, The precise temperature-controlled on / off type microcapsule catalyst according to any one of claims 1-3 is used to make acrylic acid and hydrogen sulfide undergo an addition reaction in the presence of the catalyst in a fixed bed reactor; wherein the reaction temperature is 34-37°C, the reaction pressure is 0.1-0.5 MPa, the molar ratio of acrylic acid to hydrogen sulfide is 1:1.2 to 1:2, the raw material space velocity is 1-2.5 h -1 .

10. The method of synthesizing 3-mercapto propionic acid according to claim 9, wherein, The catalyst is in the "off state to stop the reaction by adjusting the reaction temperature to above 45℃, and the catalyst returns to the "on state to restart the reaction by adjusting the reaction temperature to below 35℃.

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