Method for synthesizing ε-caprolactone

Abstract

The present invention relates to the technical field of the preparation of heterocyclic compounds and organic chemical raw materials. Disclosed is a method for synthesizing ε-caprolactone from cyclohexanone. The method comprises: converting cyclohexanone into an aqueous solution of a mixture of ε-caprolactone, 6-hydroxyhexanoic acid, and linear oligomers of 6-hydroxyhexanoic acid by means of oxidation, removing peroxides from the aqueous solution of the mixture, converting the mixture of ε-caprolactone, 6-hydroxyhexanoic acid, and linear oligomers of 6-hydroxyhexanoic acid into linear oligomers of 6-hydroxyhexanoic acid by means of polycondensation, and depolymerizing the linear oligomers of 6-hydroxyhexanoic acid into ε-caprolactone and / or an ε-caprolactone cyclic dimer. The present invention uses cyclohexanone and hydrogen peroxide as reaction starting materials and avoids centralized preparation of organic peroxides and the use of organic solvents throughout a route, thereby greatly reducing fire and explosion risks caused by the extensive use of organic peroxides and organic solvents. In addition, the polycondensation-depolymerization solution improves the yield of ε-caprolactone.

Description

A method for synthesizing ε-caprolactone Technical Field

[0001] This invention belongs to the field of heterocyclic compounds and organic chemical raw material manufacturing technology. More specifically, this invention relates to a green and safe method for synthesizing ε-caprolactone from cyclohexanone. Background Technology

[0002] ε-Caprolactone can be used to produce polycaprolactone and its copolymers, polycaprolactone polyols, and polycaprolactone-type polyurethanes. Current annual production exceeds 100,000 tons, yet demand still exceeds supply. Although ε-caprolactone can be synthesized from various raw materials such as cyclohexanone, 1,6-hexanediol, and 6-hydroxyhexanoate, for large-scale production, considering economic costs, most domestic and foreign companies have chosen cyclohexanone as the raw material, producing ε-caprolactone via the Baeyer-Villiger (BV) oxidation method.

[0003] The oxidants used in the BV oxidation method include peroxyacid, hydrogen peroxide, and molecular oxygen. Peroxyacid oxidation of cyclohexanone is currently the mainstream technical route for producing ε-caprolactone. However, due to the high chemical reactivity of peroxyacid, it decomposes violently upon heating, posing a risk of combustion and explosion upon contact with open flames, high heat, or exposure to friction, vibration, or impact. This route presents potential safety hazards in both oxygen source storage and the production process. Furthermore, the peroxyacid oxidation of cyclohexanone requires the use of organic solvents such as ethyl acetate and ethyl propionate, negatively impacting the route's environmental friendliness and further increasing the risk of combustion and explosion. In 2017, a pipeline explosion occurred at Daicel's ε-caprolactone production plant in Japan due to the accumulation of peracetic acid. The molecular oxygen route has attracted attention due to its relatively high environmental safety. It typically uses oxygen and aldehydes to generate peroxyacid in situ under the action of a catalyst, which is then used to further oxidize cyclohexanone to produce ε-caprolactone, with carboxylic acids as a byproduct. The main problem with this route is that it requires at least an equivalent amount of aldehyde and produces a large amount of carboxylic acid as a byproduct, resulting in low atom economy. In comparison, the oxygen source used in the hydrogen peroxide oxidation of cyclohexanone route is green and safe, and the production process also has high potential in terms of both greenness and safety.

[0004] In 2001, Corma et al. used Sn-beta molecular sieves prepared by hydrothermal method to catalyze the oxidation of cyclohexanone with hydrogen peroxide. The conversion rate of cyclohexanone in 1,4-dioxane solution reached 52%, and the selectivity of ε-caprolactone exceeded 98% (Nature, 2001, 412(6845), 423). Subsequently, a large number of inventions and reports on tin-catalyzed oxidation of cyclohexanone with hydrogen peroxide were published. Among them, the technology with better results is reported in Catal. Commun, 2007, 8(3), 305-309. The tin catalyst Sn-MMT supported on montmorillonite (MMT) prepared by ion exchange technology achieved a cyclohexanone conversion rate of up to 100% and a caprolactone selectivity of up to 100% in n-butanol solution with 30% H2O2 as oxidant. The route for directly synthesizing ε-caprolactone by oxidizing cyclohexanone with hydrogen peroxide is simple, but in order to prevent the hydrolysis of the generated ε-caprolactone caused by residual water in hydrogen peroxide, a large amount of organic solvent is inevitably used (the mass ratio is about 4 to 35 times that of cyclohexanone). This results in insufficient greenness, and the use of a large amount of organic solvent and post-processing also increases safety risks and economic costs.

[0005] There is currently no complete technical route for the high-yield preparation of ε-caprolactone using cyclohexanone and hydrogen peroxide as raw materials without the use of organic solvents. Existing technologies can oxidize cyclohexanone with hydrogen peroxide to prepare cyclohexanone peroxide, and then catalyze the conversion of cyclohexanone peroxide into poly-6-hydroxyhexanoic acid (Journal of Qingdao University of Science and Technology (Natural Science Edition), 2020, 41(5), 28-32). However, due to the large amount of cyclohexanone peroxide intermediates involved, the safety of this technology needs to be verified. Furthermore, it is unknown how to remove residual peroxides in the prepared poly-6-hydroxyhexanoic acid and whether it can be further efficiently depolymerized. In addition, existing technologies can catalyze the oxidation of cyclohexanone with hydrogen peroxide to prepare 6-hydroxyhexanoic acid (China Refining & Petrochemical (English Edition), 2018, 20(03), 1-6). However, the amount of water solvent used is large and the solution concentration is low, which is not conducive to the removal of residual peroxides and the preparation and depolymerization of linear oligomers of 6-hydroxyhexanoic acid. Moreover, there are no reports on the further conversion and utilization of the aqueous solution of 6-hydroxyhexanoic acid. Summary of the Invention

[0006] One object of the present invention is to solve the above-mentioned problems and / or defects, and to provide advantages that will be described later.

[0007] To achieve these and other advantages according to the present invention, a method for the green and safe synthesis of ε-caprolactone from cyclohexanone is provided, comprising: oxidizing cyclohexanone to an aqueous solution of a mixture of ε-caprolactone, 6-hydroxyhexanoic acid, and linear oligomers of 6-hydroxyhexanoic acid; removing peroxides from the aqueous solution of the mixture; polycondensing the mixture of ε-caprolactone, 6-hydroxyhexanoic acid, and linear oligomers of 6-hydroxyhexanoic acid to linear oligomers of 6-hydroxyhexanoic acid; and depolymerizing the linear oligomers of 6-hydroxyhexanoic acid to convert them into ε-caprolactone and / or ε-caprolactone cyclic dimers.

[0008] Preferably, the process specifically includes the following steps:

[0009] Step 1: Using modified molecular sieve as a catalyst and hydrogen peroxide as an oxidant, cyclohexanone aqueous solution is oxidized into a mixture of ε-caprolactone, 6-hydroxyhexanoic acid and linear oligomers of 6-hydroxyhexanoic acid in aqueous solution.

[0010] Step 2: Solid-liquid separation, separating the modified molecular sieve and the mixed aqueous solution;

[0011] Step 3: Remove residual peroxides from the aqueous solution of the mixture by heating, ultrasound, or catalytic decomposition;

[0012] Step 4: Distill the aqueous mixture after removing peroxides to remove water, and continuously increase the temperature and / or decrease the pressure to further remove incompletely converted cyclohexanone and convert the mixture of ε-caprolactone, 6-hydroxyhexanoic acid and 6-hydroxyhexanoic acid linear oligo / oligomers into 6-hydroxyhexanoic acid linear oligo / oligomers; wherein, the water and cyclohexanone removed by distillation are recycled to Step 1;

[0013] Step 5: The linear oligomer of 6-hydroxyhexanoic acid is converted into ε-caprolactone or / and ε-caprolactone cyclic dimer through cyclization depolymerization under catalytic, heating and reduced pressure conditions. The ε-caprolactone cyclic dimer is then hydrolyzed into 6-hydroxyhexanoic acid and 6-hydroxyhexanoic acid linear oligomers and recycled to Step 4, or directly recycled to the reactor in Step 5 to be converted into oligomers through ring-opening polymerization.

[0014] Preferably, in step one, the modified molecular sieve is a hydrogen-type β-molecular sieve that has undergone dealumination treatment or a tin mesoporous molecular sieve that has undergone tin ion loading treatment.

[0015] The silicon to aluminum ratio in the hydrogen-type β-zeolite after dealumination is 30~600:1.

[0016] Preferably, the tin mesoporous molecular sieve treated with tin sub-ion loading is one of tin β molecular sieve catalyst, Sn-USY molecular sieve catalyst and Sn-MCM-41 molecular sieve catalyst.

[0017] Preferably, in step one, the hydrogen peroxide concentration is ≥10wt%; in the cyclohexanone aqueous solution, the mass ratio of water to cyclohexanone is 2~10:1, the molar ratio of cyclohexanone to hydrogen peroxide is 0.8~1.2:1, and the mass ratio of cyclohexanone to catalyst is 20~200:1.

[0018] Preferably, in step one, the oxidation conversion temperature is 60~100℃, the reaction time is 1~60h, the cyclohexanone conversion rate is ≥85%, and the hydrogen peroxide utilization rate is ≥95%.

[0019] Preferably, in step two, the solid-liquid separation method is decantation, filtration, and / or centrifugation.

[0020] Preferably, in step three, the method for removing peroxides from the aqueous mixture by heating includes: heating to boiling for a boiling time ≥ 10 min;

[0021] Methods for removing peroxides from aqueous mixtures by ultrasound include ultrasonic treatment time ≥10 min;

[0022] The catalytic decomposition removal of peroxides from a mixture aqueous solution involves adding 0.1~1.0 wt% of metal oxides or metal ions to the mixture aqueous solution, with a catalytic decomposition time ≥20 min. The metal oxides include manganese dioxide and iron oxide, and the metal ions include iron ions and copper ions.

[0023] Preferably, in step four, the conditions for removing moisture are atmospheric pressure and a distillation temperature of 100~150℃.

[0024] By continuously increasing the temperature and / or decreasing the pressure to ultimately maintain ≤3kPa and 160~220℃, the average degree of polymerization of the prepared 6-hydroxyhexanoic acid linear oligomers / oligomers is 5~100.

[0025] Preferably, in step five, the heating reaction temperature is 180~240℃, and the decompression pressure condition is ≤1kPa.

[0026] The present invention has the following beneficial effects: Since the ε-caprolactone synthesis route provided by the present invention uses cyclohexanone and hydrogen peroxide as reaction raw materials and the entire route does not concentrate on the preparation of organic peroxides or use organic solvents, it greatly reduces the risk of combustion and explosion caused by the large-scale use of organic peroxides and organic solvents, thus having higher safety and being more conducive to large-scale industrial production.

[0027] Since the catalyst, water, incompletely converted cyclohexanone, and byproduct ε-caprolactone cyclic dimer in the ε-caprolactone synthesis route provided by this invention can all be recycled, the overall yield of the route is high, material loss is low, and emissions of "three wastes" are minimal, thus resulting in better economic cost and greener performance, which is conducive to improving product competitiveness.

[0028] Since the target product of step one in the ε-caprolactone synthesis route provided by this invention is a mixture of ε-caprolactone, 6-hydroxyhexanoic acid and their linear oligomers, compared with using only 6-hydroxyhexanoic acid as the target product (which requires a large amount of water as a solvent to promote the hydrolysis of ε-caprolactone and 6-hydroxyhexanoic acid linear oligomers to improve the selectivity of 6-hydroxyhexanoic acid), efficient conversion can be achieved at a higher concentration. Therefore, under the same equipment scale and time, the production / processing capacity of the oxidation conversion in step one, the solid-liquid separation in step two, the removal of residual peroxides in step three, and the removal of water in step four can be increased, thereby improving the efficiency of the entire route and reducing the energy consumption of the entire route.

[0029] Since the residual peroxide is removed in step three of the ε-caprolactone synthesis route provided by this invention, the oxygen generated by the decomposition of the peroxide can be collected and treated in a concentrated manner. This also avoids the residual peroxide from further oxidizing the 6-hydroxyhexanoic acid linear oligomer in the subsequent step four, which would cause its terminal hydroxyl group to be converted into a terminal carboxyl group. This would improve the rate and yield of the cyclization depolymerization reaction in step five (cyclization depolymerization is mainly a "biting back" or "zipper-connecting" reaction mechanism of the terminal hydroxyl group. If the terminal hydroxyl group is converted into a terminal carboxyl group, the reaction will be blocked).

[0030] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0031] Figure 1 shows the 1H NMR spectrum (CDCl3, 400M) of the aqueous mixture obtained in step one of Example 1.

[0032] Figure 2 shows the proton NMR spectrum (CDCl3, 400M) of the polymer obtained in step four of Example 1.

[0033] Figure 3 is a gas chromatogram of the depolymerization product (prepared as a 10% CH2Cl2 solution) obtained in step five of Example 1. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0035] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0036] The residual peroxide content was detected by indirect iodometric titration and peroxide test paper. The cyclohexanone conversion rate was calculated by nuclear magnetic resonance hydrogen spectroscopy. The target product selectivity was calculated by gas chromatography. The hydrogen peroxide utilization rate was calculated by combining the hydrogen peroxide feed ratio, residual peroxide content and cyclohexanone conversion rate. The yield of depolymerization product was calculated by the mass change in the collection bottle before and after the reaction. Example 1

[0037] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0038] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.0245g dealuminated hydrogen-form β molecular sieve catalyst (0.5wt%), 5.1g 30% hydrogen peroxide solution (45mmol), and 4.9g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 60℃ for 10h, and reflux using a spherical condenser.

[0039] Step 2: After the reaction is complete, the mixture is filtered to obtain a β-molecular sieve catalyst and an aqueous solution containing ε-caprolactone, 6-hydroxyhexanoic acid and linear oligomers of 6-hydroxyhexanoic acid.

[0040] Step 3: Purification of peroxides: Peroxide residue was 5.45 wt%. 0.1 wt% manganese dioxide was added to the aqueous solution of the mixture, and the reaction was carried out at 100℃ for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone was 85.6% (theoretical value 90.0%), the utilization rate of hydrogen peroxide was 95.1%, and the selectivity of the target product was 94.6%.

[0041] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 100℃ for 4 hours and then reacted under reduced pressure (400Pa) at 160℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 21.

[0042] Step 5, Depolymerization reaction: The depolymerization reaction of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 180℃ under reduced pressure (50Pa) for 6 hours under the catalysis of 0.5mol% MgCl2.

[0043] As shown in Figure 1, the aqueous solution of the mixture obtained in step one consists of ε-caprolactone, 6-hydroxyhexanoic acid, linear oligomers of 6-hydroxyhexanoic acid, and incompletely converted cyclohexanone. The proportions of the four components, calculated based on the integral area ratio, are 4.3%, 49.3%, 26.6%, and 14.4%, respectively.

[0044] As shown in Figure 2, step four yielded a well-defined 6-hydroxyhexanoic acid oligomer without obvious impurities.

[0045] As shown in Figure 3, the yield of ε-caprolactone was 93.2% and the yield of ε-caprolactone cyclic dimer was 4.9% according to the area normalization method. Example 2

[0046] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0047] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.049g dealuminated hydrogen-form β molecular sieve catalyst (1.0wt%), 6.8g 30% hydrogen peroxide solution (60mmol), and 4.9g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 60℃ for 20h, and reflux using a spherical condenser.

[0048] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 4.42 wt% of peroxide remaining.

[0049] Step 3: Purification of peroxides: Add 0.5 wt% copper oxide to the aqueous solution of the mixture and react at 80°C for 20 min to complete the purification of peroxides. Cyclohexanone conversion rate was 99.2%, hydrogen peroxide utilization rate was 99.5%, and target product selectivity was 97.2%.

[0050] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 100℃ for 6 hours and then reacted under reduced pressure (200Pa) at 160℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 25.

[0051] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 180℃ under reduced pressure (150 Pa) for 4 h under 2 mol% MgBr2 catalysis. The yield of ε-caprolactone was 88.6%, and the yield of ε-caprolactone cyclic dimer was 7.3%. Example 3

[0052] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0053] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.147g Sn-β molecular sieve catalyst (1.0wt%) treated with stannous ions, 7.7g 30% hydrogen peroxide solution (68mmol), and 14.7g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 65℃ for 30h, and reflux using a spherical condenser.

[0054] Step 2: After the reaction is complete, the catalyst and the aqueous solution of the mixture are separated by filtration, with 4.75 wt% of peroxide remaining.

[0055] Step 3: Purification of peroxides: The aqueous solution of the mixture was ultrasonically treated for 30 minutes to remove peroxides, achieving a cyclohexanone conversion rate of 98.7%, a hydrogen peroxide utilization rate of 99.5%, and a target product selectivity of 98.1%.

[0056] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 120℃ for 6 hours and then reacted under reduced pressure (200Pa) at 180℃ for 8 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 34.

[0057] Step 5, Depolymerization Reaction: The linear oligomer of 6-hydroxyhexanoic acid was depolymerized at 220℃ under reduced pressure (300 Pa) for 2 hours under 1 mol% ZnCl2 catalysis. The yield of ε-caprolactone was 82.1%, and the yield of ε-caprolactone cyclic dimer was 13.6%. The prepared ε-caprolactone cyclic dimer was added to polycaprolactone condensate and kept at 180℃ under nitrogen atmosphere and normal pressure for 2 hours (the ε-caprolactone cyclic dimer was obtained by ring-opening polymerization to obtain PCL oligomers), followed by depolymerization. The yield of ε-caprolactone was 91.0%, and the yield of ε-caprolactone cyclic dimer was 8.3%. Example 4

[0058] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0059] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.245g Sn-β molecular sieve catalyst (5.0wt%) treated with stannous ions, 8.5g 30% hydrogen peroxide solution (75mmol), and 14.7g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 65℃ for 40h, and reflux using a spherical condenser.

[0060] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 4.21 wt% peroxide residue.

[0061] Step 3: Purification of peroxides: The mixture aqueous solution was heated at 80°C for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone was 99.2%, the utilization rate of hydrogen peroxide was 99.4%, and the selectivity of the target product was 98.6%.

[0062] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 140℃ for 8 hours and then reacted under reduced pressure (400Pa) at 180℃ for 8 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 45.

[0063] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out under reduced pressure (400 Pa) at 230 °C for 8 h catalyzed by 2 mol% Zn(OAc)2. The yield of ε-caprolactone was 71.5%, and the yield of ε-caprolactone cyclic dimer was 25.6%. Example 5

[0064] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0065] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.049g Sn-MCM-41 molecular sieve catalyst (1.0wt%) treated with stannous ions, 5.7g 30% hydrogen peroxide solution (50mmol), and 24.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 70℃ for 50h, and reflux using a spherical condenser.

[0066] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 3.76 wt% of peroxide remaining.

[0067] Step 3: Purification of peroxides: Add 0.2 wt% manganese dioxide to the aqueous solution of the mixture and react for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.9%, the utilization rate of hydrogen peroxide is 99.0%, and the selectivity of the target product is 97.8%.

[0068] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 150℃ for 8 hours and then reacted under reduced pressure (400Pa) at 200℃ for 6 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 48.

[0069] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 230℃ under reduced pressure (300 Pa) for 10 h under 1 mol% SnCl2 catalysis. The yield of ε-caprolactone was 62.3%, and the yield of ε-caprolactone cyclic dimer was 33.1%. Example 6

[0070] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0071] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.098g dealuminated hydrogen-form β molecular sieve catalyst (2.0wt%), 6.0g 30% hydrogen peroxide solution (53mmol), and 24.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 70℃ for 60h, and reflux using a spherical condenser.

[0072] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 3.52 wt% peroxide residue.

[0073] Step 3: Purification of peroxides: Add 1.0 wt% silver powder to the aqueous solution of the mixture and react for 30 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 99.3%, the utilization rate of hydrogen peroxide is 98.7%, and the selectivity of the target product is 98.9%.

[0074] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 100℃ for 12 hours and then reacted under reduced pressure (200Pa) at 200℃ for 6 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 18.

[0075] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 240℃ under reduced pressure (500 Pa) for 12 h under the catalysis of 1 mol% Bu2Sn(OMe)2. The yield of ε-caprolactone was 66.7%, and the yield of ε-caprolactone cyclic dimer was 31.5%. Example 7

[0076] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0077] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.147g Sn-β molecular sieve catalyst (3.0wt%) treated with stannous ions, 7.7g 30% hydrogen peroxide solution (68mmol), and 24.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 75℃ for 10h, and reflux using a spherical condenser.

[0078] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 2.81 wt% peroxide residue.

[0079] Step 3: Purification of peroxides: Add 0.2 wt% ferric oxide to the aqueous solution of the mixture and react for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.6%, the utilization rate of hydrogen peroxide is 99.5%, and the selectivity of the target product is 97.9%.

[0080] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 120℃ for 12 hours and then reacted under reduced pressure (200Pa) at 220℃ for 4 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 26.

[0081] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 180℃ under reduced pressure (150 Pa) for 4 h under the catalysis of 0.5 mol% Ti(OiPr)4. The yield of ε-caprolactone was 88.6%, and the yield of ε-caprolactone cyclic dimer was 7.3%. Example 8

[0082] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0083] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.245g dealuminated hydrogen-form β molecular sieve catalyst (5.0wt%), 8.5g 30% hydrogen peroxide solution (75mmol), and 24.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 75℃ for 20h, and reflux using a spherical condenser.

[0084] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 2.60 wt% of peroxide remaining.

[0085] Step 3: Purification of peroxides: The mixture aqueous solution was ultrasonically treated for 60 min to remove peroxides, with a cyclohexanone conversion rate of 99.5%, hydrogen peroxide utilization rate of 99.4%, and target product selectivity of 98.8%.

[0086] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 120℃ for 18 hours and then reacted under reduced pressure (400Pa) at 220℃ for 4 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 42.

[0087] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 220℃ under reduced pressure (50 Pa) for 8 h under 1 mol% MgCl2 catalysis. The yield of ε-caprolactone was 89.8%, and the yield of ε-caprolactone cyclic dimer was 5.6%. Example 9

[0088] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0089] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.098g Sn-USY molecular sieve catalyst (2.0wt%) treated with stannous ions, 5.7g 30% hydrogen peroxide solution (50mmol), and 39.2g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 80℃ for 30h, and reflux using a spherical condenser.

[0090] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 2.42 wt% peroxide residue.

[0091] Step 3: Purification of peroxides: The aqueous solution of the mixture was heated at 100°C for 30 min to complete the purification of peroxides. The conversion rate of cyclohexanone was 98.9%, the utilization rate of hydrogen peroxide was 99.3%, and the selectivity of the target product was 97.8%.

[0092] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 140℃ for 10 hours and then reacted under reduced pressure (400Pa) at 160℃ for 8 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 68.

[0093] Step 5, Depolymerization Reaction: The linear oligomer of 6-hydroxyhexanoic acid was depolymerized at 200℃ under reduced pressure (100 Pa) for 6 h under 1 mol% MgBr2 catalysis. The yield of ε-caprolactone was 90.1%, and the yield of the dimer was 6.9%. The prepared ε-caprolactone cyclic dimer was added to the depolymerization reaction flask during the depolymerization step and kept at 200℃ under nitrogen atmosphere and normal pressure for 40 min (ring-opening polymerization yielded PCL oligomers), followed by the depolymerization reaction. The yield of ε-caprolactone was 91.8%, and the yield of the ε-caprolactone cyclic dimer was 8.6%. Example 10

[0094] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0095] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.098g dealuminated hydrogen-form β molecular sieve catalyst (2.0wt%), 6.2g 30% hydrogen peroxide solution (55mmol), and 39.2g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 80℃ for 40h, and reflux using a spherical condenser.

[0096] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 2.12 wt% of peroxide remaining.

[0097] Step 3: Purification of peroxides: Add 0.5 wt% manganese dioxide to the aqueous solution of the mixture and react for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.1%, the utilization rate of hydrogen peroxide is 99.2%, and the selectivity of the target product is 97.5%.

[0098] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 140℃ for 18 hours and then reacted under reduced pressure (400Pa) at 160℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 79.

[0099] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 190℃ under reduced pressure (100 Pa) for 6 h under the catalysis of 2 mol% ZnCl2. The yield of ε-caprolactone was 83.2%, and the yield of ε-caprolactone cyclic dimer was 13.9%. Example 11

[0100] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0101] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.147g Sn-MCM-41 molecular sieve catalyst (3.0wt%) treated with stannous ions, 7.7g 30% hydrogen peroxide solution (68mmol), and 39.2g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 85℃ for 50h, and reflux using a spherical condenser.

[0102] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 1.82 wt% peroxide residue.

[0103] Step 3: Purification of peroxides: Add 1.0 wt% copper oxide to the aqueous solution of the mixture and react for 30 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.6%, the utilization rate of hydrogen peroxide is 99.5%, and the selectivity of the target product is 98.2%.

[0104] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 150℃ for 12 hours and then reacted under reduced pressure (500Pa) at 180℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 86.

[0105] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 180℃ under reduced pressure (100 Pa) for 10 h under 1 mol% Zn(OAc)2 catalysis. The yield of ε-caprolactone was 73.6%, and the yield of ε-caprolactone cyclic dimer was 22.4%. Example 12

[0106] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0107] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.245g dealuminated hydrogen-form β molecular sieve catalyst (5.0wt%), 8.5g 30% hydrogen peroxide solution (75mmol), and 39.2g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 85℃ for 60h, and reflux using a spherical condenser.

[0108] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 1.72 wt% of peroxide remaining.

[0109] Step 3: Purification of peroxides: Add 0.1 wt% silver powder to the aqueous solution of the mixture and react for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.9%, the utilization rate of hydrogen peroxide is 98.8%, and the selectivity of the target product is 97.8%.

[0110] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 150℃ for 14 hours, and then reacted under reduced pressure (200Pa) at 180℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 89.

[0111] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 200℃ under reduced pressure (150 Pa) for 10 h under the catalysis of 2 mol% SnCl2. The yield of ε-caprolactone was 65.4%, and the yield of ε-caprolactone cyclic dimer was 31.5%. Example 13

[0112] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0113] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.0245g Sn-USY molecular sieve catalyst (0.5wt%) treated with stannous ions, 6.0g 30% hydrogen peroxide solution (53mmol), and 49.0g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 90℃ for 1h, and reflux using a spherical condenser.

[0114] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 1.64 wt% of peroxide remaining.

[0115] Step 3: Purification of peroxides: The aqueous solution of the mixture was ultrasonically treated for 30 minutes to remove peroxides. The cyclohexanone conversion rate was 98.3%, the hydrogen peroxide utilization rate was 98.5%, and the target product selectivity was 98.4%.

[0116] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 100℃ for 18 hours and then reacted under reduced pressure (300Pa) at 200℃ for 6 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 95.

[0117] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 220℃ under reduced pressure (150 Pa) for 12 h under the catalysis of 1 mol% Bu2Sn(OMe)2. The yield of ε-caprolactone was 64.9%, and the yield of ε-caprolactone cyclic dimer was 29.6%. Example 14

[0118] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0119] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.049g Sn-β molecular sieve catalyst (1.0wt%) treated with stannous ions, 6.8g 30% hydrogen peroxide solution (60mmol), and 49.0g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 90℃ for 5h, and reflux using a spherical condenser.

[0120] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 1.52 wt% peroxide residue.

[0121] Step 3: Purification of peroxides: The mixture aqueous solution was reacted at 80℃ for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone was 98.9%, the utilization rate of hydrogen peroxide was 99.6%, and the selectivity of the target product was 98.3%.

[0122] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 100℃ for 18 hours and then reacted under reduced pressure (400Pa) at 200℃ for 6 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 97.

[0123] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 230℃ under reduced pressure (200 Pa) for 12 h under 1 mol% Ti(OiPr)4 catalysis. The yield of ε-caprolactone was 74.3%, and the yield of ε-caprolactone cyclic dimer was 21.4%. Example 15

[0124] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0125] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.147g Sn-MCM-41 molecular sieve catalyst (3.0wt%) treated with stannous ions, 8.5g 30% hydrogen peroxide solution (75mmol), and 49.0g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 95℃ for 10h, and reflux using a spherical condenser.

[0126] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 1.36 wt% of peroxide remaining.

[0127] Step 3: Purification of peroxides: Add 0.5 wt% ferric oxide to the aqueous solution of the mixture and react for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 99.6%, the utilization rate of hydrogen peroxide is 99.4%, and the selectivity of the target product is 98.9%.

[0128] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 120℃ for 4 hours, and then reacted under reduced pressure (400Pa) at 160℃ for 4 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 8.

[0129] Step 5, Depolymerization Reaction: The linear oligomer of 6-hydroxyhexanoic acid was depolymerized at 240℃ under reduced pressure (200 Pa) for 4 hours under 1 mol% MgCl2 catalysis. The yield of ε-caprolactone was 90.4%, and the yield of the dimer was 5.1%. The prepared ε-caprolactone cyclic dimer was mixed with water at a ratio of 1:5 and hydrolyzed at 140℃ for 2 hours to obtain 6-hydroxyhexanoic acid and its linear oligomer. This mixture was then added to an aqueous solution of the mixture, and the polycondensation-depolymerization reactions of steps 4 and 5 were carried out. The yield of ε-caprolactone was 90.6%, and the yield of the ε-caprolactone cyclic dimer was 7.8%. Example 16

[0130] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0131] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.0245g dealuminated hydrogen-type β molecular sieve catalyst (0.5wt%), 5.7g 30% hydrogen peroxide solution (50mmol), and 73.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 95℃ for 20h, and reflux using a spherical condenser.

[0132] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 1.12 wt% of peroxide remaining.

[0133] Step 3: Purification of peroxides: Add 0.2 wt% manganese dioxide to the aqueous solution of the mixture and react for 30 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 99.7%, the utilization rate of hydrogen peroxide is 99.6%, and the selectivity of the target product is 98.6%.

[0134] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 120℃ for 6 hours and then reacted under reduced pressure (400Pa) at 200℃ for 4 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 17.

[0135] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 180℃ under reduced pressure (150 Pa) for 4 h under 1 mol% MgBr2 catalysis. The yield of ε-caprolactone was 89.4%, and the yield of ε-caprolactone cyclic dimer was 6.8%. Example 17

[0136] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0137] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.049g Sn-MCM-41 molecular sieve catalyst (1.0wt%) treated with stannous ions, 6.0g 30% hydrogen peroxide solution (53mmol), and 73.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 100℃ for 1h, and reflux using a spherical condenser.

[0138] Step 2: After the reaction is complete, the β molecular sieve catalyst and the mixed aqueous solution are obtained by filtration and separation, with 1.08 wt% peroxide residue.

[0139] Step 3: Purification of peroxides: Add 0.5 wt% copper oxide to the aqueous solution of the mixture and react for 10 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.2%, the utilization rate of hydrogen peroxide is 99.3%, and the selectivity of the target product is 97.9%.

[0140] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 140℃ for 8 hours and then reacted under reduced pressure (400Pa) at 200℃ for 8 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 28.

[0141] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 200℃ under reduced pressure (300 Pa) for 6 h under 1 mol% ZnCl2 catalysis. The yield of ε-caprolactone was 84.1%, and the yield of ε-caprolactone cyclic dimer was 11.8%. Example 18

[0142] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0143] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.098g dealuminated hydrogen-form β molecular sieve catalyst (2.0wt%), 6.2g 30% hydrogen peroxide solution (55mmol), and 73.5g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 100℃ for 5h, and reflux using a spherical condenser.

[0144] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 0.86 wt% peroxide residue.

[0145] Step 3: Purification of peroxides: Add 1.0 wt% silver powder to the aqueous solution of the mixture and react for 30 min to complete the purification of peroxides. The conversion rate of cyclohexanone is 98.6%, the utilization rate of hydrogen peroxide is 99.2%, and the selectivity of the target product is 98.1%.

[0146] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 140℃ for 10 hours and then reacted under reduced pressure (400Pa) at 220℃ for 8 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 34.

[0147] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out under reduced pressure (400 Pa) at 220 °C for 6 h catalyzed by 1 mol% Zn(OAc)2. The yield of ε-caprolactone was 72.8%, and the yield of ε-caprolactone cyclic dimer was 21.9%. Example 19

[0148] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0149] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.147g Sn-β molecular sieve catalyst (3.0wt%) treated with stannous ions, 7.7g 30% hydrogen peroxide solution (68mmol), and 98.0g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 100℃ for 10h, and reflux using a spherical condenser.

[0150] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 0.76 wt% peroxide residue.

[0151] Step 3: Purification of peroxides: The mixture aqueous solution was ultrasonically treated for 60 minutes to remove peroxides. The conversion rate of cyclohexanone was 99.2%, the utilization rate of hydrogen peroxide was 99.5%, and the selectivity of the target product was 98.7%.

[0152] Step 4, Polycondensation reaction: Under N2 protection, the product was heated in an oil bath at 150℃ for 12 hours and then reacted under reduced pressure (200Pa) at 220℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 42.

[0153] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 230℃ under reduced pressure (300 Pa) for 8 h under catalysis of 0.5 mol% SnCl2. The yield of ε-caprolactone was 63.2%, and the yield of ε-caprolactone cyclic dimer was 33.6%. Example 20

[0154] A green and safe method for synthesizing ε-caprolactone from cyclohexanone includes the following steps:

[0155] Step 1, Oxidation reaction: Add 4.9g cyclohexanone (50mmol), 0.245g Sn-USY molecular sieve catalyst (5.0wt%) treated with stannous ions, 8.5g 30% hydrogen peroxide solution (75mmol), and 98.0g deionized water to a 100mL round-bottom flask, heat and stir in an oil bath at 100℃ for 20h, and reflux using a spherical condenser.

[0156] Step 2: After the reaction is complete, the β molecular sieve catalyst and the aqueous solution of the mixture are obtained by filtration and separation, with 1.21 wt% of peroxide residue.

[0157] Step 3: Purification of peroxides: The aqueous solution of the mixture was heated at 100℃ for 30 min to remove impurities from the peroxides. The conversion rate of cyclohexanone was 99.7%, the utilization rate of hydrogen peroxide was 98.9%, and the selectivity of the target product was 98.6%.

[0158] Step 4, Polycondensation reaction: Under N2 protection, the mixture was heated in an oil bath at 150℃ for 18 hours and then reacted under reduced pressure (300Pa) at 220℃ for 10 hours. The oxidation product was dehydrated and polycondensed to prepare a linear oligomer of 6-hydroxyhexanoic acid with a degree of polymerization of 36.

[0159] Step 5, Depolymerization reaction: The depolymerization of the linear oligomer of 6-hydroxyhexanoic acid was carried out at 250℃ under reduced pressure (100 Pa) for 10 h under the catalysis of 0.5 mol% Ti(OiPr)4. The yield of ε-caprolactone was 73.2%, and the yield of ε-caprolactone cyclic dimer was 22.5%.

[0160] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0161] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for synthesizing ε-caprolactone, characterized in that, Includes the following steps: Step 1: Using modified molecular sieve as a catalyst and hydrogen peroxide as an oxidant, cyclohexanone aqueous solution is oxidized into a mixture of ε-caprolactone, 6-hydroxyhexanoic acid and linear oligomers of 6-hydroxyhexanoic acid in aqueous solution. Step 2: Solid-liquid separation, separating the modified molecular sieve and the mixed aqueous solution; Step 3: Remove residual peroxides from the aqueous solution of the mixture by heating, ultrasound, or catalytic decomposition; Step 4: Distill the aqueous mixture after removing peroxides to remove water, and continuously increase the temperature and / or decrease the pressure to further remove incompletely converted cyclohexanone and convert the mixture of ε-caprolactone, 6-hydroxyhexanoic acid and 6-hydroxyhexanoic acid linear oligo / oligomers into 6-hydroxyhexanoic acid linear oligo / oligomers; wherein, the water and cyclohexanone removed by distillation are recycled to Step 1; Step 5: The linear oligomer of 6-hydroxyhexanoic acid is converted into ε-caprolactone or / and ε-caprolactone cyclic dimer through cyclization depolymerization under catalytic, heating and reduced pressure conditions. The ε-caprolactone cyclic dimer is then hydrolyzed to convert into 6-hydroxyhexanoic acid and 6-hydroxyhexanoic acid linear oligomers and recycled to Step 4, or directly recycled to the reactor in Step 5 to be converted into oligomers through ring-opening polymerization. In step one, the modified molecular sieve is a hydrogen-type β molecular sieve that has undergone dealumination treatment or a tin mesoporous molecular sieve that has undergone tin ion loading treatment. The silicon to aluminum ratio in the hydrogen-type β-zeolite after dealumination is 30~600:

1.

2. The method for synthesizing ε-caprolactone according to claim 1, characterized in that, Tin mesoporous molecular sieves treated with tin ion loading are one of the following: tin β molecular sieve catalyst, Sn-USY molecular sieve catalyst, and Sn-MCM-41 molecular sieve catalyst.

3. The method for synthesizing ε-caprolactone as described in claim 1, characterized in that, In step one, the hydrogen peroxide concentration is ≥10wt%; in the cyclohexanone aqueous solution, the mass ratio of water to cyclohexanone is 2~10:1, the molar ratio of cyclohexanone to hydrogen peroxide is 0.8~1.2:1, and the mass ratio of cyclohexanone to catalyst is 20~200:

1.

4. The method for synthesizing ε-caprolactone according to claim 1, characterized in that, In step one, the oxidation conversion temperature is 60~100℃, the reaction time is 1~60h, the cyclohexanone conversion rate is ≥85%, and the hydrogen peroxide utilization rate is ≥95%.

5. The method for synthesizing ε-caprolactone according to claim 1, characterized in that, In step two, the solid-liquid separation method is decantation, filtration, and / or centrifugation.

6. The method for synthesizing ε-caprolactone according to claim 1, characterized in that, In step three, the method for removing peroxides from the aqueous mixture by heating includes: heating to boiling, with a boiling time ≥ 10 min; Methods for removing peroxides from aqueous mixtures by ultrasound include ultrasonic treatment time ≥10 min; The catalytic decomposition removal of peroxides from a mixture aqueous solution involves adding 0.1~1.0 wt% of metal oxides or metal ions to the mixture aqueous solution, with a catalytic decomposition time ≥20 min. The metal oxides include manganese dioxide and iron oxide, and the metal ions include iron ions and copper ions.

7. The method for synthesizing ε-caprolactone according to claim 1, characterized in that, In step four, the conditions for removing moisture are atmospheric pressure and a distillation temperature of 100~150℃. By continuously increasing the temperature and / or decreasing the pressure to ultimately maintain ≤3kPa and 160~220℃, the average degree of polymerization of the prepared 6-hydroxyhexanoic acid linear oligomers / oligomers is 5~100.

8. The method for synthesizing ε-caprolactone according to claim 1, characterized in that, In step five, the heating reaction temperature is 180~240℃, and the pressure condition for decompression is ≤1kPa.

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