Preparation method and application of binary system for catalyzing copolymerization of carbon dioxide and cyclohexene oxide under supercritical condition

Abstract

The invention discloses a preparation method and application of a binary system for catalyzing copolymerization of carbon dioxide and cyclohexene oxide under a supercritical condition, and belongs to the technical field of catalysts, and the binary system is a composite catalyst obtained by combining heterogeneous double metal cyanide and homogeneous salen-Co (III) acetate according to a certain proportion. The supercritical carbon dioxide exists in a reaction system as a reaction monomer and a reaction solvent, the two catalysts generate a synergistic effect in a polymerization reaction, the double metal cyanide catalyst is a catalytic active center of the polymerization reaction, the salen-Co (III) acetate catalyst is a selective center of the polymerization reaction, and the two catalysts generate a synergistic effect in the polymerization reaction. And a high-activity and high-selectivity catalytic system can be obtained to prepare the poly (cyclohexyl carbonate). In the purification process of the polymerization product, the use of organic solvents such as methanol is harmful to the environment, and the method for purifying the polymerization product through supercritical carbon dioxide has potential application value.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts, and in particular relates to a preparation method and application of a binary system for catalyzing the copolymerization of carbon dioxide and cyclohexene oxide under supercritical conditions. Background Art

[0002] With the progress of society and the development of industry, carbon dioxide emissions are increasing. As the main source of greenhouse gases, the increase in carbon dioxide emissions has also caused a series of environmental problems, such as soil drying and glacier melting. At present, scientists are focusing on reducing carbon dioxide emissions through scientific means and using chemical methods to convert it into products with economic value. On the one hand, this reduces carbon dioxide emissions to nature, and on the other hand, it creates industrial value. Since Inoue et al. successfully used the ZnEt2 / H2O system to catalyze the synthesis of polycarbonate from carbon dioxide and propylene oxide (PO) for the first time in 1969 (Journal of Polymer Science Part B: Polymer Physics, 1969, 7, 287-292), the research focus in this field has become to find catalytic systems with higher activity and selectivity. Darensbourg et al. successfully catalyzed the polymerization of carbon dioxide with propylene oxide and cyclohexene oxide (CHO) using a heterogeneous zinc system (Journal of the American Chemical Society, 2002, 124, 7075-7083). Although the activity was only 370 g polymer / g catalyst (g poly / g cat), the carbonate chain content in the polymer product was between 70% and 95%.

[0003] In 2003, Coates et al. (Angewandte Chemie International Edition, 2003, 42, 5484-5487) first reported the use of salen-Co(III)X system to catalyze the polymerization of carbon dioxide and propylene oxide, and obtained an alternating copolymer product polypropylene carbonate (PPC) with a carbonate chain content of 99%. Since then, scientists have been committed to developing a salen-Co(III)X catalytic system with higher activity. In 2006, Coates et al. (Dalton Transactions, 2006, 237-249) further studied the polymerization of carbon dioxide and cyclohexene oxide catalyzed by the salen-Co(III)X system and found that the polymerization reaction pressure has a great influence on the selectivity of the syndiotactic alternating copolymer product. When the ortho / meta substituent of the (R,R')-salen-Co(III)X catalyst is tert-butyl, the catalytic system has the highest catalytic activity. Although the salen-Co(III)X catalytic system has the characteristics of high carbonate chain selectivity and regular spatial configuration of polymerization products, its relatively low catalytic activity limits its application.

[0004] As a representative of heterogeneous catalytic systems, double metal cyanide (DMC) catalysts have the characteristics of high catalytic activity. After Kruper and Smart et al. (The Dow Chemical Co. US Patent, 1983, 4, 704) successfully applied DMC catalysts to the homopolymerization of propylene oxide into polyethers, scientists began to explore its application in the alternating copolymerization of carbon dioxide and epoxides. The representative Zn-Co DMC catalyst can effectively catalyze the alternating copolymerization of carbon dioxide and ethylene oxide, propylene oxide, 1-butylene oxide and cyclohexene oxide to form polycarbonates, and has high catalytic activity. Although the high catalytic activity of DMC catalysts is one of the important indicators for achieving industrial production, it is still impossible to ignore the factors such as its low selectivity for carbonate segments in polymers and the presence of a large amount of polyethers.

[0005] Compared with traditional organic solvents, supercritical carbon dioxide has unique physical and chemical properties such as low density, strong diffusivity and strong solubility. Many studies have used supercritical carbon dioxide as a solvent to synthesize different polymers. In early studies, supercritical carbon dioxide has been used as both a reaction monomer and a reaction solvent to prepare aliphatic polycarbonates. Beckman et al. (Macromolecules, 1997, 30, 368-372) successfully catalyzed the polymerization of carbon dioxide and propylene oxide under supercritical conditions using zinc glutarate (ZnGA) catalyst, and the catalytic activity was increased by 10~20% compared with non-supercritical conditions. Compared with the copolymer of propylene oxide and carbon dioxide, the copolymer of cyclohexene oxide and carbon dioxide, polycyclohexyl carbonate (PCHC), has a higher glass transition temperature (Tg). At present, there are relatively few studies on the polymerization of carbon dioxide and cyclohexene oxide under supercritical conditions. Summary of the invention

[0006] In view of the deficiencies of the prior art, the present invention provides a preparation method and application of a binary system for catalyzing the copolymerization of carbon dioxide and cyclohexene oxide under supercritical conditions, and the method can realize the preparation of a binary composite catalyst composed of a heterogeneous double metal cyanide and a homogeneous salen-Co (III) acetate. Supercritical carbon dioxide is present in the reaction system as both a reaction monomer and a reaction solvent, and the two catalysts produce a synergistic effect in the polymerization reaction, the double metal cyanide catalyst is the catalytic active center of the polymerization reaction, and the salen-Co (III) acetate catalyst is the selective center of the polymerization reaction, and a highly active and highly selective catalytic system can be obtained to prepare polycyclohexyl carbonate.

[0007] To achieve the above object, the present invention provides the following technical solutions: The present invention provides a method for preparing a binary system for catalyzing the copolymerization of carbon dioxide and cyclohexene oxide under supercritical conditions, comprising the following steps: M1[M2(CN)6] is dissolved in an aqueous solution to prepare a solution, and zinc salt A is dissolved in a mixed solution of water and organic solvent B1 to prepare a solution. Under strong stirring, the M1[M2(CN)6] solution is added dropwise to the zinc salt A solution at a rate of 1 to 10 drops / minute. After the addition is completed, the mixed solution is allowed to continue to react for 1 to 3 hours while maintaining strong stirring. The obtained white suspension is washed with a mixed solution of water and organic solvent B1 under strong stirring, and the mixed solution is filtered. The obtained solid is washed 1 to 3 times with organic solvent B1 at 30 to 90 °C to achieve the purpose of dehydration, and then placed in a vacuum oven at 50 to 90 °C for 12 to 36 hours. The obtained light yellow solid is ground into powder to obtain a double metal cyanide (DMC) catalyst. Salen-Co(II) is dissolved in dichloromethane to prepare a solution. Carboxylic acid C is dissolved in dichloromethane to prepare a solution. Under the protection of dry oxygen, the prepared carboxylic acid C solution is slowly dripped into the salen-Co(II) solution, the stirring speed is maintained at 2000-4000 rpm, and the reaction is carried out for 1-3 h. Then, the obtained solution is filtered and dried to obtain a dark green solid powder. The obtained dark green solid powder is washed 1-5 times with a mixed solution of diethyl ether and n-hexane, and then the obtained solid is vacuum dried at 40-80 °C for 12-36 h, and the obtained solid is ground into powder to obtain a salen-Co(III) acetate catalyst. Under the protection of nitrogen, DMC: salen-Co(II)=1:1-10:1 is dispersed and dissolved in an organic solvent C. The mixed solution is stirred and reacted for 3-10 h at 40-80 °C, and the stirring rate is 500-3000 rpm. The obtained gray powder is washed with n-hexane for 1 to 5 times, and then placed in a vacuum oven and vacuum dried at 40 to 80 ° C for 12 to 36 hours. The dried solid is ground into powder to obtain the double metal cyanide / salen-Co (III) acetate composite catalyst.

[0008] Furthermore, the metal M1 is one or more of potassium, sodium and lithium.

[0009] Furthermore, the metal M2 is one or more of iron, cobalt, nickel, manganese, molybdenum, chromium, tin, aluminum, vanadium, and copper.

[0010] Furthermore, the organic solvent B1 is one or more of acetone, methyl ethyl ketone, methyl propyl ketone, ethylene glycol, isopropanol, n-butanol, isobutanol, tert-butanol, glycol dimethyl ether, diglycol dimethyl ether, ethylene glycol methyl ether, or one or more of their derivatives.

[0011] Furthermore, the organic solvent B2 is one or more of cyclohexane, methylcyclohexane, toluene, polyether, polysulfide, polyethylene glycol, polypropylene glycol, polyethylene glycol-polypropylene glycol-polyethylene glycol, polyformaldehyde, polyester, polyamide, polyvinyl alcohol, or one or more of their derivatives.

[0012] Furthermore, the carboxylic acid C is one or more of formic acid, acetic acid, propionic acid, or one or more of their derivatives.

[0013] The present invention also provides the use of the composite catalyst prepared by the above method in synthesizing carbon dioxide-based polycarbonate, comprising the following steps: Before the start of the copolymerization reaction, the feed pipeline was cleaned several times and dried to remove the residual solvent. In a three-necked flask, 0.01-5% of the double metal cyanide / salen-Co(III) acetate composite catalyst by weight of the reaction monomer was dispersed and dissolved in cyclohexene oxide, and the obtained mixed solution was ultrasonically stirred for 0.5-3 h to completely disperse the double metal cyanide / salen-Co(III) acetate composite catalyst in the cyclohexene oxide solution for standby use. Then, the polymerization reactor was evacuated three times, the mixed solution was added to the reactor, and then cyclohexene oxide was added. The temperature and pressure of the reaction system were raised to 50-80 °C and 10-20.6 MPa, and stabilized for 10-30 minutes. The stirring paddle was turned on and the speed was 100-300 rpm for 10-20 h. After the reaction, the reactor was cooled to 25 °C with dry ice and the pressure was released. After opening the reactor, dichloromethane was used to dissolve the obtained polymerization product, and methanol was used to precipitate the polymerization product. The obtained polymerization product was then washed three times with methanol and placed in a vacuum oven and dried at 55 °C to constant weight.

[0014] Compared with the prior art, the present invention has the following beneficial effects: DMC and salen-Co(III) acetate catalysts have a synergistic effect to obtain a highly active and highly selective catalytic system for the preparation of alternating copolymer products PCHC. Since DMC catalyst has the characteristics of high activity and low selectivity, and salen-Co(III) acetate has the characteristics of high selectivity and low activity, the authors predict that DMC catalyst is the catalytic active center of the polymerization reaction and salen-Co(III) acetate catalyst is the selective center of the polymerization reaction. The use of DMC / salen-Co(III) acetate binary catalytic system greatly shortens the catalytic induction period of carbon dioxide and cyclohexene oxide polymerization. Since the coordination ring-opening insertion process of cyclohexene oxide is the rate-determining step of the polymerization reaction, the introduction of DMC catalyst overcomes the extremely high Gibbs free energy required for the coordination ring-opening insertion of cyclohexene oxide. In addition, supercritical carbon dioxide exists as both a reaction monomer and a reaction solvent in the polymerization reaction, which promotes the dissolution of salen-Co(III) acetate in the supercritical CO2 / CHO phase, which is beneficial to the coordination of carbon dioxide and salen-Co(III) acetate catalyst. Therefore, the acceleration of the activation rate of carbon dioxide and cyclohexene oxide also greatly shortens the catalytic induction period. Supercritical carbon dioxide promotes mass transfer and heat transfer in the polymerization system, so the activity of DMC / salen-Co(III) acetate binary system in catalyzing the polymerization of carbon dioxide and cyclohexene oxide is greatly improved. In the purification process of the polymer product, the use of organic solvents such as methanol is harmful to the environment. The method of purifying the polymer product with supercritical carbon dioxide only needs a simple pressure release process to recover the DMC / salen-Co(III) acetate binary system catalyst, which has potential application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0016] Figure 1 This is a SEM image of the double metal cyanide / salen-Co(III) acetate composite catalyst provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0017] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0018] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0019] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0020] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present application description and examples are exemplary only.

[0021] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0022] Example 1: Preparation of double metal cyanide / salen-Co(III) acetate composite catalyst Dissolve 120 mg K3[Co(CN)6] in 20 mL aqueous solution to prepare a solution, and dissolve 480 mg ZnCl2 in 60 mL water and 30 mL tert-butanol mixed solution to prepare a solution. Under strong stirring, add K3[Co(CN)6] solution drop by drop into ZnCl2 solution at a rate of 3 drops / minute. After the addition is completed, maintain strong stirring to allow the mixed solution to continue to react for 2 h. The resulting white suspension is washed with a mixed solution of 50 mL water and 50 mL tert-butanol under strong stirring until the potassium ion detection reagent Na3Co(NO2)6 can no longer detect K in the mixed solution. + The obtained solid was washed once with 100 mL of tert-butyl alcohol at 50 °C to remove water, and then dried in a vacuum oven at 80 °C for 24 h. The obtained light yellow solid was ground into powder to obtain a double metal cyanide (DMC) catalyst, which was sealed and stored in a desiccator.

[0023] In a 1 L round-bottom flask, 2.32 g (4.0 mmol) of salen-Co(II) was dissolved in 300 mL of dichloromethane to prepare a solution. 0.24 g of CH3COOH was dissolved in 40 mL of dichloromethane to prepare a solution. Under the protection of dry oxygen, 40 mL of the prepared CH3COOH solution was slowly dripped into 300 mL of salen-Co(II) solution, and the stirring speed was kept at 3000 rpm for 1 h. Then, the obtained solution was filtered and dried to obtain a dark green solid powder. The obtained dark green solid powder was washed three times with a mixed solution of 50 mL of diethyl ether and 50 mL of n-hexane, and then the obtained solid was vacuum dried at 60 °C for 24 h, and the obtained solid was ground into powder to obtain the salen-Co(III) acetate catalyst, which was sealed and stored in a desiccator.

[0024] Under nitrogen protection, DMC: salen-Co(II)=3:1 (3.20 g DMC and 1.00 gsalen-Co(III) acetate) was dispersed and dissolved in 200 mL toluene. The mixed solution was stirred and reacted for 6 h at 50 °C with a stirring rate of 1000 rpm. The obtained gray powder was washed 3 times with 100 mL n-hexane, then placed in a vacuum oven and dried at 60 °C for 24 h for use. The dried solid was ground into powder to obtain a double metal cyanide / salen-Co(III) acetate composite catalyst, which was sealed and stored in a desiccator.

[0025] The SEM images obtained in this embodiment are as follows Figure 1 As shown in the figure, it can be seen that the Zn-Co DMC particles have a typical lamellar structure. The edge length of the DMC catalyst particles is about 200~400 nm and the thickness is 10 nm; while the salen-Co(III) acetate has a square structure with a side length of about 20 nm. The unidimensional lamellar structure of the DMC / salen-Co(III) acetate composite catalyst greatly increases its specific surface area, so it has more exposed active centers.

[0026] Example 2: Synthesis of CO2-based polycarbonate using DMC / salen-Co(III) acetate composite catalyst In a 50 mL three-necked flask, 30 mg of the DMC / salen-Co(III) acetate composite catalyst prepared in Example 1 was dispersed and dissolved in 10 mL of cyclohexene oxide, and the obtained mixed solution was ultrasonically stirred for 1 h to completely disperse the DMC / salen-Co(III) acetate composite catalyst in the cyclohexene oxide solution for standby use. Then, the polymerization reactor was evacuated three times, 10 mL of the mixed solution was added to the reactor, and then 10 mL of cyclohexene oxide was added. The temperature and pressure of the reaction system were raised to 60 ° C and 2000 psi (13.8 MPa) and stabilized for 15 minutes. The stirring paddle was turned on and the speed was 200 rpm for 12 h. After 12 h, the reactor was cooled to 25 ° C and depressurized using dry ice. After opening the reactor, 50 mL of dichloromethane was used to dissolve the obtained polymer product, and 200 mL of methanol was used to precipitate the polymer product, and then the obtained polymer product was washed three times with 50 mL of methanol, and placed in a vacuum oven and dried at 55 °C to constant weight. The catalyst activity was 656.5 g polym / g cat, and the selectivity was 92.9%.

[0027] Example 3: Synthesis of CO2-based polycarbonate using DMC / salen-Co(III) acetate composite catalyst In a 50 mL three-necked flask, 30 mg of the DMC / salen-Co(III) acetate composite catalyst prepared in Example 1 was dispersed and dissolved in 10 mL of cyclohexene oxide, and the obtained mixed solution was ultrasonically stirred for 1 h to completely disperse the DMC / salen-Co(III) acetate composite catalyst in the cyclohexene oxide solution for standby use. Then, the polymerization reactor was evacuated three times, 10 mL of the mixed solution was added to the reactor, and then 10 mL of cyclohexene oxide was added. The temperature and pressure of the reaction system were raised to 70 °C and 2000 psi (13.8 MPa) and stabilized for 15 minutes. The stirring paddle was turned on and the speed was 200 rpm for 12 h. After 12 h, the reactor was cooled to 25 °C and depressurized using dry ice. After opening the reactor, 50 mL of dichloromethane was used to dissolve the obtained polymer product, and 200 mL of methanol was used to precipitate the polymer product, and then the obtained polymer product was washed three times with 50 mL of methanol, and placed in a vacuum oven and dried at 55 °C to constant weight. The catalyst activity was 450.3 g polym / gcat, and the selectivity was 94.8%.

[0028] Example 4: Synthesis of CO2-based polycarbonate using DMC / salen-Co(III) acetate composite catalyst In a 50 mL three-necked flask, 30 mg of the DMC / salen-Co(III) acetate composite catalyst prepared in Example 1 was dispersed and dissolved in 10 mL of cyclohexene oxide, and the obtained mixed solution was ultrasonically stirred for 1 h to completely disperse the DMC / salen-Co(III) acetate composite catalyst in the cyclohexene oxide solution for standby use. Then, the polymerization reactor was evacuated three times, 10 mL of the mixed solution was added to the reactor, and then 10 mL of cyclohexene oxide was added. The temperature and pressure of the reaction system were raised to 55 ° C and 2000 psi (13.8 MPa) and stabilized for 15 minutes. The stirring paddle was turned on and the speed was 200 rpm for 12 h. After 12 h, the reactor was cooled to 25 ° C and depressurized using dry ice. After opening the reactor, 50 mL of dichloromethane was used to dissolve the obtained polymer product, and 200 mL of methanol was used to precipitate the polymer product, and then the obtained polymer product was washed three times with 50 mL of methanol, and placed in a vacuum oven and dried at 55 °C to constant weight. The catalyst activity was 246.2 g polym / g cat, and the selectivity was 92.7%.

[0029] Example 5: Synthesis of CO2-based polycarbonate using DMC / salen-Co(III) acetate composite catalyst In a 50 mL three-necked flask, 30 mg of the DMC / salen-Co(III) acetate composite catalyst prepared in Example 1 was dispersed and dissolved in 10 mL of cyclohexene oxide, and the obtained mixed solution was ultrasonically stirred for 1 h to completely disperse the DMC / salen-Co(III) acetate binary catalyst in the cyclohexene oxide solution for standby use. Then, the polymerization reactor was evacuated three times, 10 mL of the mixed solution was added to the reactor, and then 10 mL of cyclohexene oxide was added. The temperature and pressure of the reaction system were raised to 60 ° C and 3000 psi (20.6 MPa) and stabilized for 15 minutes. The stirring paddle was turned on and the speed was 200 rpm for 12 h. After 12 h, the reactor was cooled to 25 ° C and depressurized using dry ice. After opening the reactor, 50 mL of dichloromethane was used to dissolve the obtained polymer product, and 200 mL of methanol was used to precipitate the polymer product, and then the obtained polymer product was washed three times with 50 mL of methanol, and placed in a vacuum oven and dried at 55 °C to constant weight. The catalyst activity was 434.2 g polym / g cat, and the selectivity was 93.5%.

[0030] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a double metal cyanide / salen-Co(III) acetate composite catalyst, characterized in that: The following steps are involved: M1[M2(CN)6] is dissolved in an aqueous solution to prepare a solution, and zinc salt A is dissolved in a mixed solution of water and organic solvent B1 to prepare a solution. Under strong stirring, the M1[M2(CN)6] solution is added dropwise to the zinc salt A solution at a rate of 1 to 10 drops / minute. After the addition is completed, the mixed solution is allowed to continue to react for 1 to 3 hours while maintaining strong stirring. The obtained white suspension is washed with a mixed solution of water and organic solvent B1 under strong stirring, and the mixed solution is filtered. The obtained solid is washed 1 to 3 times with organic solvent B1 at 30 to 90 °C to achieve the purpose of dehydration, and then placed in a vacuum oven at 50 to 90 °C for 12 to 36 hours. The obtained light yellow solid is ground into powder to obtain a double metal cyanide (DMC) catalyst. Salen-Co(II) is dissolved in dichloromethane to prepare a solution. Carboxylic acid C is dissolved in dichloromethane to prepare a solution. Under the protection of dry oxygen, the prepared carboxylic acid C solution is slowly dripped into the salen-Co(II) solution, the stirring speed is maintained at 2000~4000 rpm, and the reaction is carried out for 1~3 hours. Then, the obtained solution is filtered and dried to obtain a dark green solid powder. The obtained dark green solid powder is washed 1~5 times with a mixed solution of diethyl ether and n-hexane, and then the obtained solid is vacuum dried at 40~80 °C for 12~36 hours, and the obtained solid is ground into powder to obtain a salen-Co(III) acetate catalyst. Under the protection of nitrogen, DMC: salen-Co(II)=1:1~10:1 is dispersed and dissolved in an organic solvent C. The mixed solution is stirred and reacted for 3~10 hours at 40~80 °C, and the stirring rate is 500~3000 rpm. The obtained gray powder is washed with n-hexane for 1 to 5 times, and then placed in a vacuum oven and vacuum dried at 40 to 80 ° C for 12 to 36 hours. The dried solid is ground into powder to obtain the double metal cyanide / salen-Co (III) acetate composite catalyst.

2. The method for preparing a double metal cyanide / salen-Co (III) acetate composite catalyst as claimed in claim 1, characterized in that The metal M1 is one or more of potassium, sodium and lithium.

3. The method for preparing a double metal cyanide / salen-Co (III) acetate composite catalyst as claimed in claim 1, characterized in that The metal M2 is one or more of iron, cobalt, nickel, manganese, molybdenum, chromium, tin, aluminum, vanadium and copper.

4. The method for preparing a double metal cyanide / salen-Co (III) acetate composite catalyst as claimed in claim 1, characterized in that The organic solvent B1 is one or more of acetone, methyl ethyl ketone, methyl propyl ketone, ethylene glycol, isopropanol, n-butanol, isobutanol, tert-butanol, glycol dimethyl ether, diglycol dimethyl ether, ethylene glycol methyl ether, or one or more of their derivatives.

5. The method for preparing a double metal cyanide / salen-Co (III) acetate composite catalyst as claimed in claim 1, characterized in that The organic solvent B2 is one or more of cyclohexane, methylcyclohexane, toluene, polyether, polysulfide, polyethylene glycol, polypropylene glycol, polyethylene glycol-polypropylene glycol-polyethylene glycol, polyformaldehyde, polyester, polyamide, polyvinyl alcohol, or one or more of their derivatives.

6. The method for preparing a double metal cyanide / salen-Co (III) acetate composite catalyst as claimed in claim 1, characterized in that The carboxylic acid C is one or more of formic acid, acetic acid, propionic acid, or one or more of their derivatives.

7. Use of the composite catalyst prepared by the method of claim 1 in synthesizing carbon dioxide-based polycarbonate, characterized in that: The following steps are involved: Before the start of the copolymerization reaction, the feed pipeline was cleaned several times and dried to remove the residual solvent. In a three-necked flask, 0.01-5% of the double metal cyanide / salen-Co(III) acetate composite catalyst by weight of the reaction monomer was dispersed and dissolved in cyclohexene oxide, and the obtained mixed solution was ultrasonically stirred for 0.5-3 h to completely disperse the double metal cyanide / salen-Co(III) acetate composite catalyst in the cyclohexene oxide solution for standby use. Then, the polymerization reactor was evacuated three times, the mixed solution was added to the reactor, and then cyclohexene oxide was added. The temperature and pressure of the reaction system were raised to 50-80 °C and 10-20.6 MPa, and stabilized for 10-30 minutes. The stirring paddle was turned on and the speed was 100-300 rpm for 10-20 h. After the reaction, the reactor was cooled to 25 °C with dry ice and the pressure was released. After opening the reactor, dichloromethane was used to dissolve the obtained polymerization product, and methanol was used to precipitate the polymerization product. The obtained polymerization product was then washed three times with methanol and placed in a vacuum oven and dried at 55 °C to constant weight.

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