Method for preparing diester terephthalate by catalyzing depolymerization of polyethylene glycol terephthalate (PET)

Abstract

The invention discloses a method for preparing diester terephthalate by catalyzing polyethylene glycol terephthalate (PET) to depolymerize. The method comprises the following steps: treating PET or a composite system thereof in diester carbonate under the conditions of no catalyst or taking a salt substance as a catalyst to realize chemical depolymerization of PET, so as to obtain PET monomers of diester terephthalate and ethylene carbonate, and realize complete depolymerization of PET in a composite. According to the method disclosed by the invention, through the reaction of the diester carbonate and the PET, the depolymerization of the PET under a mild condition can be realized, and the related technology research and development of chemical recovery of the PET monomer in a PET composite system or a mixed system are further carried out. The method provided by the invention has the advantages of high efficiency, greenness, mild reaction conditions and the like, can catalyze PET depolymerization to generate the monomer terephthalic acid diester, and has higher industrial application value.

Description

Technical Field

[0001] The invention belongs to the technical field of waste plastic recycling, and specifically relates to a method for preparing terephthalate diester by catalyzing the depolymerization of polyethylene terephthalate (PET), and more specifically to a method for preparing terephthalate diester by catalyzing the depolymerization of PET without using a catalyst or salt substance. Background Art

[0002] Polyethylene terephthalate is a type of polymer formed by the polycondensation of terephthalic acid (ester) and ethylene glycol. It has excellent properties and has been widely used in various fields of production and life. With the rapid development of human society, the demand for PET has increased rapidly, and its waste volume has also increased accordingly. The annual output of PET in my country exceeds 50 million tons, and the impact of waste PET products on our ecological environment is becoming more and more serious. Therefore, it is urgent to solve this problem. Using chemical methods to convert it into useful substances is an important way to effectively solve this problem. At present, there are methods such as hydrolysis, alcoholysis, aminolysis, and hydrogenolysis that can depolymerize PET into its monomers and chemicals, providing an important means for its recycling. PET is usually used in combination with other polymer materials or inorganic materials, so there are often problems such as high reaction temperature and cross contamination in its depolymerization process, which makes the chemical recycling recovery cost of PET high and the technical implementation difficult. Therefore, it is urgent to use new methods to depolymerize PET without damaging the composite components and achieving effective separation.

[0003] The ester exchange method is a commonly used method in polyester depolymerization. The exchange reagents used are mainly alcohols, acids, and esters. However, the method of using carbonates for ester exchange has not been reported. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing terephthalic acid diester monomer by depolymerizing polyethylene terephthalate (PET).

[0005] The method for preparing terephthalic acid diester monomer by depolymerization of PET provided by the present invention comprises the following steps: treating PET or its composite system in carbonate diester without catalyst or with salt substances as catalyst to achieve chemical depolymerization of PET, obtain terephthalic acid diester monomer of PET and ethylene carbonate, and achieve complete depolymerization of PET in the composite.

[0006] In the above method, the salt substance is any one of an inorganic metal salt of a halogen anion, an ionic liquid of a halogen anion, and a composite system formed by them.

[0007] Wherein, the inorganic metal salt of the halogen anion is selected from at least one of: NaCl, KCl, ZnCl2, FeCl3, CuCl2, SnCl4, ZrCl4, CrCl3, NaBr, KBr, ZnBr2, FeBr3, CuBr2, SnBr4, ZrBr4, CrBr, NaI, KI, ZnI2, FeI3, CuI2, ZrI4, and CrI.

[0008] The cations of the ionic liquid of the halogen anion include, but are not limited to, imidazolyl cations, pyridyl cations, tetraalkylammonium cations, tetraalkylphosphonium cations, guanidinium cations, organic base cations, and the like.

[0009] The ionic liquid of the halogen anion is selected from at least one of 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-butyl-3-methylimidazolium bromide ([BMIM]Br), 1-butyl-3-methylimidazolium iodide ([BMIM]I), 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), 1-ethyl-3-methylimidazolium bromide ([EMIM]Br), and 1-ethyl-3-methylimidazolium iodide ([EMIM]I).

[0010] The carbonic acid diester may be at least one of dimethyl carbonate and diethyl carbonate.

[0011] The PET and its composite system include: at least one of all samples, physical objects, mixed systems or composite systems containing PET components.

[0012] The method is operated as follows: placing a sample containing PET (PET or its composite system) in a carbonic acid diester without a catalyst or adding a salt substance as a catalyst, heating the reaction for a certain time, cooling, and separating to obtain terephthalic acid diester monomer and ethylene carbonate.

[0013] The molar ratio of the carbonic acid diester to the PET structural unit may be 1:1 to 25:1, specifically 10:1 to 15:1, 10:1, 15:1, 20:1;

[0014] The molar content of the salt substance in the carbonic acid diester is 1% to 20%, specifically 1%;

[0015] Add salt substances as catalysts, the reaction temperature is 50 to 250°C, and the reaction time is 0.5 to 36 hours;

[0016] In the absence of a catalyst, the reaction temperature is 170 to 250° C. and the reaction time is 15 to 36 hours.

[0017] The carbonic acid diester is dimethyl carbonate, and the prepared terephthalic acid diester is dimethyl terephthalate.

[0018] The carbonic acid diester is diethyl carbonate, and the prepared terephthalic acid diester is diethyl terephthalate.

[0019] The method further comprises the following operations: filtering and separating the unreacted solids; cooling to precipitate the terephthalic acid diester, filtering to obtain the terephthalic acid diester; further, collecting the excess carbonic acid diester and the generated ethylene carbonate in sequence by conventional vacuum distillation method, and recovering the catalyst.

[0020] Through the above method, the products obtained are terephthalic acid diester and ethylene carbonate.

[0021] The recovered catalyst can be reused, and the obtained ethylene carbonate can be used to react with methanol or ethanol to generate carbonic acid diester required for depolymerization of PET.

[0022] Regarding the separation of the products after the reaction, it also includes adopting corresponding separation methods according to the different catalysts used.

[0023] The present invention can realize the depolymerization of PET under mild conditions through the reaction of carbonic acid diester and PET, and further carry out the research and development of related technologies for chemical recovery of PET monomer in PET composite system or mixed system. The method provided by the present invention has the advantages of high efficiency, greenness, mild reaction conditions, etc., can catalyze the depolymerization of PET to generate its monomer terephthalic acid diester, and has strong industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The solid nuclear magnetic resonance spectroscopy of the polyester-cotton blended product (composition: 65% PET, 35% cotton) in Example 5 of the present invention and the residue after the reaction 13 C spectrum. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below in conjunction with specific embodiments, and the examples provided are only for illustrating the present invention, rather than for limiting the scope of the present invention. The examples provided below can be used as a guide for further improvements by those of ordinary skill in the art, and do not constitute a limitation of the present invention in any way.

[0026] The experimental methods in the following examples, unless otherwise specified, are all conventional methods, and are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials, reagents, etc. used in the following examples, unless otherwise specified, can all be obtained from commercial channels.

[0027] Example 1, ionic liquid N 4444 Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by Cl

[0028] 0.96 g PET powder sample, 0.27 g tetrabutylammonium chloride (N 4444 Cl), 9g of dimethyl carbonate were added to a 25mL round-bottom flask and stirred at 120°C for 2 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.93g of dimethyl terephthalate (yield 95.9%) was obtained. 1 H NMR (400MHz, CDCl3, 25℃): δ / ppm=8.10 (s, 4H), 3.95 (s, 6H); 13 C NMR (101 MHz, CDCl3, 25°C): δ / ppm = 166.57, 134.21, 129.84, 52.69). The filtrate was distilled under reduced pressure to obtain 0.43 g of ethylene carbonate (yield 97.7%). 1 H NMR (400MHz, CDCl3, 25℃): δ / ppm=4.57 (s, 4H); 13 C NMR (101MHz, CDCl3, 25°C): δ / ppm=155.12, 64.40. ).

[0029] Example 2: Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by metal salt ZnCl2

[0030] 0.96 g of PET powder sample, 0.13 g of zinc chloride (ZnCl2), and 9 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 120°C for 12 hours. Then, dimethyl terephthalate was cooled, precipitated, filtered and dried to obtain 0.92 g of dimethyl terephthalate (yield 94.8%). The filtrate was distilled under reduced pressure to obtain 0.42 g of ethylene carbonate (yield 95.5%).

[0031] Example 3, ionic liquid N 4444 Preparation of dimethyl terephthalate by recycling PET plastic bottles catalyzed by Cl

[0032] 0.96 g of PET plastic bottle fragments, 0.27 g of tetrabutylammonium chloride (N 4444 Cl), 9g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 120°C for 4 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried to obtain 0.93g of dimethyl terephthalate (yield 95.9%). The filtrate was distilled under reduced pressure to obtain 0.43g of ethylene carbonate (yield 97.7%).

[0033] Example 4, ionic liquid N 4444 Preparation of methyl terephthalate from PET textiles by Cl-catalyzed recycling

[0034] 0.97 g of PET textile (composition: 100% PET) chips, 0.28 g of tetrabutylammonium chloride (N 4444 Cl), 8g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 140°C for 1 hour. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.94g of dimethyl terephthalate (yield 95.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.41g of ethylene carbonate (yield 92.2%).

[0035] Example 5, Ionic Liquid N 4444 Preparation of dimethyl terephthalate by recycling polyester-cotton fabrics catalyzed by Cl

[0036] 1.48 g of PET textile (composition: 65% polyester, 35% cotton fiber) fragments, 0.38 g of tetrabutylammonium chloride (N 4444 Cl), 10g of dimethyl carbonate was added to a 25mL round-bottom flask and stirred at 130°C for 2 hours. Then, the unreacted solid was separated by filtration, washed, and dried to obtain 0.52g of solid, the composition of which was cellulose ( Figure 1 ). Subsequently, the filtrate was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.93 g of dimethyl terephthalate (yield 96.0%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.42 g of ethylene carbonate (95.5%).

[0037] Example 6, Ionic Liquid N 4444 Preparation of dimethyl terephthalate by recycling polyester-cotton fabrics catalyzed by Cl

[0038] 2.02 g of PET textile (composition: 47.5% polyester, 47.5% cotton fiber, 5% spandex) fragments, 0.27 g of tetrabutylammonium chloride (N 4444 Cl), 9g of dimethyl carbonate was added to a 25mL round-bottom flask and stirred at 130°C for 10 hours. Then, the unreacted solid was separated by filtration, washed, and dried to obtain 1.06g of solid, which consisted of cellulose and spandex. Subsequently, the filtrate was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.94g of dimethyl terephthalate (yield 96.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.43g of ethylene carbonate (yield 97.7%).

[0039] Example 7, Ionic Liquid N 4444 I Catalytic depolymerization of PET to prepare dimethyl terephthalate

[0040] 0.97 g PET powder sample, 0.37 g tetrabutylammonium iodide (N 4444I), 8g of dimethyl carbonate was added to a 25mL round-bottom flask and stirred at 110°C for 2 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.45g of dimethyl terephthalate (yield 45.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.22g of ethylene carbonate (yield 49.5%).

[0041] Example 8, Ionic Liquid N 4444 Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by Br

[0042] 0.95 g PET powder sample, 0.32 g tetrabutylammonium bromide (N 4444 Br), 9g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 120°C for 2 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried to obtain 0.69g of dimethyl terephthalate (yield 71.9%). The filtrate was distilled under reduced pressure to obtain 0.30g of ethylene carbonate (yield 68.9%).

[0043] Example 9, ionic liquid P 4444 Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by Cl

[0044] 0.97 g PET powder sample, 0.29 g tetrabutylphosphine chloride (P 4444 Cl), 8g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 120°C for 2 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.90g of dimethyl terephthalate (yield 91.8%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.37g of ethylene carbonate (yield 83.2%).

[0045] Example 10, Ionic Liquid P 4444 Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by Br

[0046] 0.96 g of PET powder sample, 0.40 g of tetrabutylphosphonium bromide (P 4444 Br), 9g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 120°C for 2 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.76g of dimethyl terephthalate (yield 78.4%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.36g of ethylene carbonate (yield 81.8%).

[0047] Example 11, ionic liquid P 4444 I Catalytic depolymerization of PET to prepare dimethyl terephthalate

[0048] 0.97 g PET powder sample, 0.36 g tetrabutylphosphine iodide (P4444 I) 7g of dimethyl carbonate was added to a 25mL round-bottom flask and stirred at 140°C for 2 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.47g of dimethyl terephthalate (yield 48.0%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.23g of ethylene carbonate (yield 51.7%).

[0049] Example 12: Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by ionic liquid BMImCl

[0050] 1.0 g of PET powder sample, 0.20 g of 1-butyl-3-methylimidazolium chloride (BMImCl), and 8 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 110° C. for 6 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.81 g of dimethyl terephthalate (yield 80.2%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.38 g of ethylene carbonate (yield 82.9%).

[0051] Example 13: Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by metal salt NaCl

[0052] 0.94 g of PET powder sample, 0.06 g of sodium chloride (NaCl), and 8 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 180° C. for 24 hours. Then, dimethyl terephthalate was cooled, precipitated, filtered and dried to obtain 0.78 g of dimethyl terephthalate (yield 82.1%). The filtrate was distilled under reduced pressure to obtain 0.32 g of ethylene carbonate (yield 74.3%).

[0053] Example 14, Ionic Liquid N 4444 Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by Cl

[0054] 0.95 g PET powder sample, 0.36 g tetrabutylammonium chloride (N 4444 Cl), 9g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 80°C for 12 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.30g of dimethyl terephthalate (yield 42.2%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.14g of ethylene carbonate (yield 38.5%).

[0055] Example 15, Ionic Liquid N 4444 Preparation of dimethyl terephthalate by depolymerization of PET catalyzed by Cl

[0056] 0.95 g PET powder sample, 0.25 g tetrabutylammonium chloride (N 4444Cl), 7g of dimethyl carbonate were added into a 25mL round-bottom flask and stirred at 80°C for 24 hours. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.55g of dimethyl terephthalate (yield 57.3%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.24g of ethylene carbonate (yield 55.1%).

[0057] Example 16, Ionic Liquid N 4444 Preparation of diethyl terephthalate by depolymerization of PET catalyzed by Cl

[0058] 0.94 g PET powder sample, 0.26 g tetrabutylammonium chloride (N 4444 Cl), 10g of diethyl carbonate was added to a 25mL round-bottom flask, and stirred at 130°C for 4 hours. Then, it was cooled, diethyl terephthalate precipitated, filtered and dried to obtain 0.96g of diethyl terephthalate (yield 88.3%). 1 H NMR (400MHz, CDCl3, 25℃): δ / ppm=8.10 (s, 4H), 4.41 (q, 4H, J=7.3), 1.41 (t, 6H, J=7.0); 13 CNMR (101 MHz, CDCl3, 25°C): δ / ppm=166.02, 134.41, 129.69, 61.60, 14.51. The filtrate was distilled under reduced pressure to obtain 0.33 g of ethylene carbonate (yield 76.6%).

[0059] Example 17, Ionic Liquid N 4444 Preparation of diethyl terephthalate by depolymerization of PET catalyzed by Cl

[0060] 0.99 g PET powder sample, 0.27 g tetrabutylammonium chloride (N 4444 Cl), 12g of diethyl carbonate were added to a 25mL round-bottom flask and stirred at 140°C for 1 hour. Then, the mixture was cooled, diethyl terephthalate was precipitated, filtered and dried, and 0.98g of diethyl terephthalate (yield 85.6%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.40g of ethylene carbonate (yield 88.2%).

[0061] Example 18, Ionic Liquid P 4444 Preparation of diethyl terephthalate by depolymerization of PET catalyzed by Cl

[0062] 0.97 g of PET powder sample, 0.46 g of tetrabutylphosphonium chloride (P 4444Cl), 12g of diethyl carbonate were added to a 25mL round-bottom flask and stirred at 160°C for 1 hour. Then, the mixture was cooled, diethyl terephthalate was precipitated, filtered and dried, and 1.05g of diethyl terephthalate (yield 91.7%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.42g of ethylene carbonate (yield 92.6%).

[0063] Example 19: Preparation of dimethyl terephthalate by recycling polyester-cotton textiles catalyzed by ZnCl2

[0064] 1.48g of PET textile (composition: 65% polyester, 35% cotton fiber) fragments, 0.15g of ZnCl2, and 8g of dimethyl carbonate were added to a 25mL round-bottom flask and stirred for reaction at 150°C for 5 hours. Then, the unreacted solid was separated by filtration, washed, and dried to obtain 0.52g of solid, the component of which was cellulose. Subsequently, the filtrate was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.94g of dimethyl terephthalate (yield 96.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.43g of ethylene carbonate (yield 97.7%).

[0065] Example 20, Ionic Liquid N 4444 Preparation of diethyl terephthalate by depolymerization of polyester-cotton textile catalyzed by Cl

[0066] 1.48 g of PET textile (composition: 65% polyester, 35% cotton fiber) fragments, 0.50 g of N 4444 Cl, 15g of diethyl carbonate were added to a 25mL round-bottom flask and stirred at 150°C for 3 hours. Then, the unreacted solid was separated by filtration, washed, and dried to obtain 0.52g of solid, whose component was cellulose. Subsequently, the filtrate was cooled, diethyl terephthalate was precipitated, filtered and dried, and 0.95g of diethyl terephthalate (yield 97.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.41g of ethylene carbonate (yield 93.2%).

[0067] Example 21: Treatment of PET in dimethyl carbonate without catalyst

[0068] 0.96g of PET powder sample and 9g of dimethyl carbonate were added to a 25mL pressure-resistant reactor, stirred and reacted at 180°C for 24 hours, and the solid matter completely disappeared. Then, the mixture was cooled, dimethyl terephthalate was precipitated, filtered and dried, and 0.94g of dimethyl terephthalate (yield 96.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.41g of ethylene carbonate (yield 93.2%).

[0069] Comparative Example: Treatment of PET in dimethyl carbonate without catalyst

[0070] 0.96g PET powder sample and 9g dimethyl carbonate were added to a 25mL round-bottom flask and stirred at 120°C for 10 hours. Then, the mixture was cooled, filtered and dried to recover 0.96g of solid sample. The liquid was detected by H-NMR spectrum and no product was found. This indicates that the reaction cannot occur under the reaction conditions.

[0071] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that the present invention can be further improved. In a word, according to the principles of the present invention, the application is intended to include any changes, uses or improvements to the present invention, including departure from the disclosed scope in the application, and changes made with conventional techniques known in the art.

Claims

1. A method for preparing terephthalic acid diester monomers by depolymerization of polyethylene terephthalate (PET), comprising the following steps: treating PET or its composite system in a carbonate diester without a catalyst or with a salt substance as a catalyst to achieve chemical depolymerization of PET to obtain terephthalic acid diester monomers and ethylene carbonate, and achieving complete depolymerization of PET in the composite.

2. The method according to claim 1, characterized in that The salt substance is any one of an inorganic metal salt of a halogen anion, an ionic liquid of a halogen anion, and a composite system formed by them.

3. The method according to claim 1, characterized in that The inorganic metal salt of the halogen anion is selected from at least one of NaCl, KCl, ZnCl2, FeCl3, CuCl2, SnCl4, ZrCl4, CrCl3, NaBr, KBr, ZnBr2, FeBr3, CuBr2, SnBr4, ZrBr4, CrBr, NaI, KI, ZnI2, FeI3, CuI2, ZrI4, and CrI; The cation of the ionic liquid of the halogen anion is selected from at least one of an imidazolyl cation, a pyridyl cation, a tetraalkylammonium cation, a tetraalkylphosphine cation, a guanidinium cation and an organic base cation.

4. The method according to claim 1, characterized in that: The carbonic acid diester is at least one of dimethyl carbonate (DMC) and diethyl carbonate (DEC).

5. The method according to claim 1, characterized in that The PET and its composite system include: at least one of all samples, physical objects, mixed systems or composite systems containing PET components.

6. The method according to claim 1, characterized in that The method is operated as follows: placing PET or its composite system in carbonic acid diester, heating and reacting for a certain period of time without catalyst or adding salt substances as catalyst, cooling, and separating to obtain terephthalic acid diester monomer and ethylene carbonate.

7. The method according to claim 6, characterized in that The molar ratio of carbonic acid diester to PET structural unit is 1:1 to 25:1; The molar content of the salt substance in the carbonic acid diester is 1% to 20%.

8. The method according to claim 6, characterized in that Add salt substances as catalysts, the reaction temperature is 50 to 250°C, and the reaction time is 0.5 to 36 hours; In the absence of a catalyst, the reaction temperature is 170 to 250° C. and the reaction time is 15 to 36 hours.

9. The method according to claim 6, characterized in that The method further comprises the following operations: filtering and separating the unreacted solids; then cooling to precipitate the terephthalic acid diester, filtering to obtain the terephthalic acid diester; further, sequentially collecting the excess carbonic acid diester and the generated ethylene carbonate through a conventional vacuum distillation method, and recovering the catalyst.

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