A silicon-based catalyst, its preparation method and application

Abstract

The present invention discloses a silicon-based catalyst, a preparation method thereof, and an application thereof. The catalyst has the following structural formula: #imgabs0# wherein R is any one of trifluoroacetate group, difluoroacetate group, trifluoromethanesulfonate group, and difluoromethanesulfonate group. The specific silicon-based catalyst provided by the present invention is simple to synthesize, has high catalytic activity, does not introduce foreign impurity ions, and has no side reactions. Moreover, the obtained lithium fluoroxalate phosphate product has high purity and excellent performance. In addition, the production process of lithium fluoroxalate phosphate proposed by the present invention does not require the use of highly toxic raw materials and can be carried out on a large scale for industrial production.

Description

Technical Field

[0001] The present invention belongs to the field of electrolyte materials, and specifically relates to a silicon-based catalyst. The present invention also relates to a preparation method and an application of the silicon-based catalyst. Background Art

[0002] In the prior art, lithium fluoroxalate phosphate is widely used as a lithium supplement additive in the electrolytes of lithium-ion batteries or lithium-ion capacitors. Currently, commonly used lithium fluoroxalate phosphates include lithium difluorobis(oxalate)phosphate and lithium tetrafluorobis(oxalate)phosphate. Lithium fluoroxalate phosphate can improve the high-temperature resistance of the electrolyte and can form a more stable solid electrolyte interface film (SEI film) on the positive electrode material, improving the cycle charge and discharge performance of its application in lithium-ion batteries or lithium-ion capacitors.

[0003] Currently, in the known and disclosed methods for synthesizing lithium fluoroxalate phosphate, the phosphorus pentafluoride method (see the patent application with the publication number CN109956976A) and the hydrogen fluoride method (see the patent application with the publication number CN112661791A) are usually included. These preparation and synthesis methods all require the use of toxic raw materials such as phosphorus pentafluoride, hydrogen fluoride, silicon tetrachloride, etc., and the content of chloride ions in the synthesized lithium fluoroxalate phosphate is relatively high, which has a greater impact on the performance of the subsequent electrolyte and is not conducive to large-scale production.

[0004] Therefore, there is an urgent need to develop a friendly technical solution for preparing lithium fluoroxalate phosphate. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a silicon-based catalyst, its preparation method and application. The provided specific silicon-based catalyst has simple synthesis, high catalytic activity, does not introduce foreign impurity ions, and has no side reactions; and the obtained lithium fluoroxalate phosphate product has high purity and excellent performance. In addition, the production process of lithium fluoroxalate phosphate proposed in this application does not require the use of highly toxic raw materials and can be carried out on a large scale for industrial production.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A silicon-based catalyst, the catalyst includes the following structural formula:

[0008] ; wherein, R is any one of trifluoroacetate group, difluoroacetate group, trifluoromethanesulfonate group, and difluoromethanesulfonate group.

[0009] Preferably, R is trifluoroacetate group or trifluoromethanesulfonate group.

[0010] Preferably, a preparation method of the silicon-based catalyst according to the above is provided, including the following operation steps:

[0011] S10), Dissolve boron tribromide, fluoroacetic acid or sulfonic acid in solvent A and then react to form an intermediate;

[0012] S20), Dissolve the intermediate obtained in the above step S10) and tetramethylsilane Si(CH3)4 in solvent B and then react to form a catalyst as the silicon-based catalyst;

[0013] Among them, the chemical formula of the boron tribromide is: ; The chemical formula of the fluoroacetic acid is: ; The chemical formula of the sulfonic acid is: ; Among them, n = 2 or n = 3;

[0014] In the step S10), when fluoroacetic acid is used, the boron tribromide and fluoroacetic acid react to form intermediate 1, and the chemical formula of the intermediate 1 is:

[0015] ;

[0016] In the step S20), the intermediate 1 and tetramethylsilane Si(CH3)4 react to form a catalyst 1 as the silicon-based catalyst, and the chemical formula of the catalyst 1 is:

[0017] ;

[0018] In the step S10), when sulfonic acid is used, the boron tribromide and sulfonic acid react to form intermediate 2, and the chemical formula of the intermediate 2 is:

[0019] ;

[0020] In the step S20), the intermediate 2 and tetramethylsilane Si(CH3)4 react to form a catalyst 2 as the silicon-based catalyst, and the chemical formula of the catalyst 2 is:

[0021] .

[0022] Preferably, the step S10) or the step S20) is carried out under stirring at room temperature, where the temperature range of the room temperature is 20 - 35°C, preferably 20 - 30°C, and further preferably 20 - 25°C; and / or, the reaction time of the step S10) or the step S20) is 1 - 5 hours, preferably 2 - 4 hours.

[0023] Preferably, the molar ratio of the fluoroacetic acid or sulfonic acid to the boron tribromide is 1.2 - 5.0:1, more preferably 2.0 - 4.0:1, and further preferably 3.0 - 4.0:1; and / or, the molar ratio of the intermediate 2 to the tetramethylsilane Si(CH3)4 is 3.0 - 7.0:1, more preferably 4.0 - 6.0:1, and further preferably 4.5 - 5.0:1.

[0024] Preferably, the solvent A used in the step S10) includes one or a mixture of two of dichloromethane and chloroform, preferably including dichloromethane; and / or, the solvent B used in the step S20) includes one or a mixture of two of dichloromethane and chloroform, preferably including chloroform.

[0025] Preferably, an application of the silicon-based catalyst as described above is used as a catalyst for preparing lithium fluoroxalate phosphate, wherein the lithium fluoroxalate phosphate is used as a lithium supplement additive for a lithium-ion battery or a lithium-ion capacitor.

[0026] Preferably, lithium hexafluorophosphate and oxalic acid are added to a carbonate solvent, and the silicon-based catalyst is added, and a reaction is carried out to obtain a lithium fluoroxalate phosphate solution; then the solvent in the lithium fluoroxalate phosphate solution is separated, and after recrystallization and drying, a lithium fluoroxalate phosphate solid is obtained, and the Cl - content in the lithium fluoroxalate phosphate solid is not higher than 1 ppm.

[0027] Preferably, the feeding molar ratio of lithium hexafluorophosphate, oxalic acid, and the silicon-based catalyst is 1:1.0 - 3.0:1.0 - 2.5, more preferably 1:1.0 - 3.0:0.6 - 2.0.

[0028] Preferably, the temperature of the reaction is 20 - 50 °C, and / or, the time of the reaction is 2 - 6 hours; the carbonate solvent is selected from any one or a mixture of several of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0029] When the specific silicon-based catalyst provided by the present application is used as a catalyst for preparing lithium fluoroxalate phosphate, its catalytic mechanism is mainly manifested as follows: The silicon-based catalyst can preferably replace the fluorine atoms of lithium hexafluorophosphate to generate gaseous SiF4. During the reaction, the continuous elimination of SiF4 enables the reaction between oxalic acid and lithium hexafluorophosphate to proceed continuously in the forward direction. Then, during the process of the reaction between oxalic acid and lithium hexafluorophosphate to form lithium fluoroxalate phosphate, the fluorosulfonic acid or acetate group in the silicon-based catalyst can quickly detach from the phosphorus atom as an excellent leaving group, allowing oxalic acid to coordinate to the phosphorus atom to ensure the completion of the reaction to form lithium fluoroxalate phosphate. Moreover, the specific silicon-based catalyst provided by the present application is simple to synthesize, has high catalytic activity, does not introduce foreign impurity ions, and has no side reactions. In addition, the obtained lithium fluoroxalate phosphate product has high purity and excellent performance. Furthermore, the production process of lithium fluoroxalate phosphate proposed in the present application does not require the use of highly toxic raw materials and can be carried out on a large scale for industrial production. Detailed implementation manners

[0030] This embodiment provides a silicon-based catalyst, and the catalyst has the following structural formula:

[0031] ; where R is any one of trifluoroacetate group, difluoroacetate group, trifluoromethanesulfonate group, and difluoromethanesulfonate group; preferably, R is trifluoroacetate group or trifluoromethanesulfonate group.

[0032] Preferably, this embodiment provides a preparation method of the silicon-based catalyst as described above, including the following operation steps:

[0033] S10): Dissolve boron tribromide and fluoroacetic acid or sulfonic acid in solvent A and then react to generate an intermediate; where the chemical formula of boron tribromide is: ; the chemical formula of fluoroacetic acid is: ; the chemical formula of sulfonic acid is: ; where n = 2 or n = 3;

[0034] Preferably, for the sake of the reaction effect, this step S10) is carried out under stirring conditions at room temperature, where the temperature range of room temperature is 20 - 35 °C, preferably 20 - 30 °C, and more preferably 20 - 25 °C; and / or, the reaction time of this step S10) is 1 - 5 hours, preferably 2 - 5 hours, more preferably 2 - 4 hours, and further more preferably 2 - 3 hours;

[0035] Preferably, to facilitate the reaction effect, in this step S10), the molar ratio of fluoroacetic acid or sulfonic acid to boron tribromide is 1.2 - 5.0:1, more preferably 2.0 - 4.0:1 when considered separately, further preferably 3.0 - 4.0:1, and still further preferably 3.2 - 3.5:1; it should be noted that in the specific implementation of this application, fluoroacetic acid or sulfonic acid is preferably used. Of course, in other embodiments, a mixture of fluoroacetic acid and sulfonic acid can also be used as needed, and the molar ratio of this mixture to boron tribromide can refer to the preferred molar ratio of fluoroacetic acid or sulfonic acid to boron tribromide;

[0036] Preferably, to facilitate the dissolution effect, the solvent A used in this step S10) includes one or a mixture of two of dichloromethane and chloroform, preferably includes dichloromethane, and more preferably: the solvent A is dichloromethane;

[0037] Preferably, in this step S10), when fluoroacetic acid is used, boron tribromide and fluoroacetic acid react to form intermediate 1, and the chemical formula of intermediate 1 is:

[0038] The corresponding specific reaction process is as follows:

[0039]

[0040] In this step S10), when sulfonic acid is used, boron tribromide and sulfonic acid react to form intermediate 2, and the chemical formula of intermediate 2 is:

[0041] The corresponding specific reaction process is as follows:

[0042] ;

[0043] S20), After dissolving the intermediate obtained in the above step S10) and tetramethylsilane Si(CH3)4 through solvent B, a catalyst serving as a silicon-based catalyst is formed by reaction;

[0044] Preferably, to facilitate the reaction effect, this step S20) is carried out under stirring conditions at room temperature, where the temperature range of room temperature is 20 - 35 °C, preferably 20 - 30 °C, and further preferably 20 - 25 °C; and / or, the reaction time of this step S20) is 1 - 5 hours, preferably 2 - 4 hours, and more preferably 2 - 3 hours;

[0045] Preferably, to facilitate the reaction effect, in this step S20), the molar ratio of intermediate 2 to tetramethylsilane Si(CH3)4 is 3.0 - 7.0:1, more preferably 4.0 - 6.0:1, and further preferably 4.5 - 5.0:1; preferably, to facilitate the dissolution effect, the solvent B used in this step S20) includes one or a mixture of two of dichloromethane and chloroform, preferably includes chloroform, and more preferably: the solvent B uses chloroform.

[0046] In this step S20), intermediate 1 reacts with tetramethylsilane Si(CH3)4 to generate catalyst 1 as a silicon-based catalyst, and the chemical formula of catalyst 1 is:

[0047] ; The specific reaction process corresponding thereto is as follows:

[0048] ;

[0049] In this step S20), intermediate 2 reacts with tetramethylsilane Si(CH3)4 to generate catalyst 2 as a silicon-based catalyst, and the chemical formula of catalyst 2 is:

[0050] ; The specific reaction process corresponding thereto is as follows:

[0051]

[0052] Preferably, this embodiment also provides an application of the silicon-based catalyst as described above. The silicon-based catalyst provided in this embodiment is used as a catalyst for preparing lithium fluoroxalate phosphate, wherein lithium fluoroxalate phosphate is used as a lithium supplement additive for lithium-ion batteries or lithium-ion capacitors.

[0053] Preferably, in this embodiment, lithium hexafluorophosphate and oxalic acid are added to a carbonate solvent, a silicon-based catalyst is added, and a lithium fluoroxalate phosphate solution is obtained by reaction; then the solvent in the lithium fluoroxalate phosphate solution is separated, and after recrystallization and drying, a lithium fluoroxalate phosphate solid is obtained. The Cl - content in the lithium fluoroxalate phosphate solid is not higher than 1 ppm; preferably, in this embodiment, the feeding molar ratio of lithium hexafluorophosphate, oxalic acid, and the silicon-based catalyst is 1:1.0 - 3.0:1.0 - 2.5, more preferably 1:1.0 - 3.0:0.6 - 2.0; preferably, in this embodiment, the reaction temperature is 20 - 50 °C, and / or the reaction time is 2 - 6 hours; the carbonate solvent is selected from any one or a mixture of several of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0054] Based on the above-described embodiments, the following further describes the embodiments of the present invention in detail with specific examples.

[0055] First of all, it should be noted that the sources of the reagent raw materials used in the following specific embodiments and comparative examples of the present invention are as follows:

[0056] Boron tribromide, trifluoromethanesulfonic acid, difluoroacetic acid, and trifluoroacetic acid were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0057] Difluoromethanesulfonic acid was purchased from Alfa Chemistry;

[0058] Unless otherwise specified, the remaining reagent raw materials are all ordinary commercially available products.

[0059] The test methods used in the specific embodiments and comparative examples of the present invention are as follows:

[0060] The structure of the product was determined by nuclear magnetic resonance, and the spectrometer used was Bruker AVANCE II 500 MHz;

[0061] It should be understood that the following specific embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. Embodiment

[0062] S10) Preparation of intermediate 1:

[0063] Boron tribromide (250.5 g, 1.0 mol) and trifluoroacetic acid (399.0 g, 3.5 mol) were dissolved in 800 g of dichloromethane, and the reaction was carried out at room temperature of 20 °C for 2 hours. The waste gas generated during the reaction was introduced into an alkaline solution for absorption to obtain intermediate 1 (339.4 g, 0.97 mol), and the structural formula is as follows:

[0064]

[0065] The structural characterization of the above intermediate 1 is as follows:

[0066] 19 F NMR (500 MHz, Methanol-d): δ -80.13—80.71 (m, 9F).

[0067] S20) Intermediate 1 (339.4 g, 0.97 mol) and tetramethylsilane (17.1 g, 0.194 mol) were dissolved in 800 g of chloroform, and the reaction was carried out at room temperature of 20 °C for 2 hours to obtain catalyst 1 (92.7 g, 0.193 mol), and the structural formula is as follows:

[0068]

[0069] The structural characterization of the above catalyst 1 is as follows:

[0070] 19¹⁹F NMR (500 MHz, Methanol-d): δ -80.13—80.71 (m, 9F).

[0071] Step S30): Synthesis of lithium tetrafluoroxalate phosphate, and the specific operation process is as follows:

[0072] Oxalic acid (90.0 g, 1.0 mol) and lithium hexafluorophosphate (152.0 g, 1.0 mol) were added to 1000 g of DMC (dimethyl carbonate) solvent for dissolution, and then transferred into a reaction kettle; the catalyst 1 (360.1 g, 0.75 mol) obtained in step S20) of this example was dissolved in 1000 g of DMC, and then added dropwise to the reaction kettle within 1 hour. After the addition was completed, the reaction continued for 1 hour. After the reaction was completed, most of the solvent was removed under a vacuum environment of -0.095 MPaG and 90 °C to obtain the remaining material;

[0073] Then, dichloromethane with a mass 1.5 times that of the remaining material was added for recrystallization. After filtration and drying, a solid (185.8 g, 0.92 mol) was obtained. After nuclear magnetic analysis, it was lithium tetrafluoroxalate phosphate with a purity of 99.7%. The characterization results are as follows:

[0074] 19 ¹⁹F NMR (500 MHz, Methanol-d): δ -58.0 (m, 1F), δ -60.2 (m, 1F), δ -74.6 (m, 1F), δ -76.6 (m, 1F);

[0075] The Cl ion concentration in the lithium tetrafluoroxalate phosphate solid was determined by potentiometric titration to be <1 ppm. Example

[0076] S10): Preparation of intermediate 2:

[0077] Boron tribromide (250.5 g, 1.0 mol) and trifluoromethanesulfonic acid (480.2 g, 3.2 mol) were dissolved in 800 g of dichloromethane and reacted at room temperature of 25 °C for 2 hours. The waste gas generated during the reaction was introduced into an alkali solution for absorption to obtain intermediate 2 (448.8 g, 0.98 mol), and its structural formula is as follows:

[0078]

[0079] The structural characterization of the above intermediate 2 is as follows:

[0080] 19 ¹⁹F NMR (500 MHz, Methanol-d): δ -80.13—80.71 (m, 9F).

[0081] S20) Dissolve intermediate 2 (448.8 g, 0.98 mol) and tetramethylsilane (19.4 g, 0.22 mol) in 800 g of chloroform, and react for 1 hour at room temperature of 25 °C to obtain catalyst 2 (137.35 g, 0.22 mol), whose structural formula is as follows:

[0082]

[0083] The structural characterization of the above catalyst 2 is as follows:

[0084] 19 19F NMR (500 MHz, Methanol-d): δ -80.13—80.71 (m, 9F).

[0085] S30) Synthesis of lithium difluorobis(oxalato)phosphate, and the specific operation process is as follows:

[0086] Add oxalic acid (198 g, 2.2 mol) and lithium hexafluorophosphate (152.0 g, 1.0 mol) to 1000 g of DMC (dimethyl carbonate) for dissolution, then transfer it into a reaction kettle. Then dissolve catalyst 2 (568.26 g, 1.4 mol) obtained in step S20) of this example in 1000 g of DMC, and then drop it into the reaction kettle within 2 hours. After the dropping is completed, continue to react for 1 hour. After the reaction is completed, remove most of the solvent under a vacuum environment of -0.095 MPaG and 90 °C to obtain the remaining material;

[0087] Subsequently, add dichloromethane for recrystallization with 1.5 times the mass of the remaining material, and obtain a solid (226.7 g, 0.9 mol) after filtration and drying. It is determined by NMR analysis that this solid is lithium difluorobis(oxalato)phosphate with a purity of 99.5%. The NMR characterization results are as follows:

[0088] 19 19F NMR (500 MHz, Methanol-d): δ -63.4 (d, 1F), δ -65.3 (d, 1F);

[0089] It is determined by potentiometric titration that the Cl ion concentration in this lithium difluorobis(oxalato)phosphate solid is <1 ppm.

[0090] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention.

[0091] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment contains only an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A preparation method of a silicon-based catalyst, characterized in that, The silicon-based catalyst includes the following structural formula: ; wherein, R is any one of trifluoroacetate group, difluoroacetate group, trifluoromethanesulfonate group, and difluoromethanesulfonate group; The preparation method includes the following operating steps: S10): Boron tribromide, fluoroacetic acid or sulfonic acid are dissolved through solvent A and then react to generate an intermediate; S20): The intermediate obtained in the above step S10) and tetramethylsilane Si(CH3)4 are dissolved through solvent B and then react to generate a catalyst as the silicon-based catalyst; Among them, the chemical formula of the boron tribromide is: ; the chemical formula of the fluoroacetic acid is: ; the chemical formula of the sulfonic acid is: ; among them, n = 2 or n = 3; In the step S10), when fluoroacetic acid is used, boron tribromide and fluoroacetic acid react to generate intermediate 1, and the chemical formula of intermediate 1 is: ； In the step S20), intermediate 1 and tetramethylsilane Si(CH3)4 react to generate catalyst 1 as the silicon-based catalyst, and the chemical formula of catalyst 1 is: ； In the step S10), when sulfonic acid is used, boron tribromide and sulfonic acid react to generate intermediate 2, and the chemical formula of intermediate 2 is: ； In the step S20), intermediate 2 and tetramethylsilane Si(CH3)4 react to generate catalyst 2 as the silicon-based catalyst, and the chemical formula of catalyst 2 is: 。 2. The preparation method of the silicon-based catalyst according to claim 1, characterized in that, R is trifluoroacetate or trifluoromethanesulfonate.

3. The preparation method of the silicon-based catalyst according to claim 1, characterized in that, The step S10) or the step S20) is carried out under stirring at room temperature, where the temperature range of the room temperature is 20 - 35 °C; and / or, the reaction time of the step S10) or the step S20) is 1 - 5 hours.

4. The preparation method of the silicon-based catalyst according to claim 3, characterized in that, The temperature range of the room temperature is 20 - 30 °C.

5. The preparation method of the silicon-based catalyst according to claim 3, characterized in that, The temperature range of the room temperature is 20 - 25 °C.

6. The preparation method of the silicon-based catalyst according to claim 3, characterized in that, The reaction time of the step S10) or the step S20) is 2 - 4 hours.

7. The preparation method of the silicon-based catalyst according to claim 1, characterized in that, The molar ratio of the fluoroacetic acid or sulfonic acid to the boron tribromide is 1.2 - 5.0:1; and / or, the molar ratio of the intermediate 2 to the tetramethylsilane Si(CH3)4 is 3.0 - 7.0:

1.

8. The preparation method of the silicon-based catalyst according to claim 7, characterized in that, The molar ratio of the fluoroacetic acid or sulfonic acid to the boron tribromide is 2.0 - 4.0:

1.

9. The preparation method of the silicon-based catalyst according to claim 7, characterized in that, The molar ratio of the fluoroacetic acid or sulfonic acid to the boron tribromide is 3.0 - 4.0:

1.

10. The preparation method of the silicon-based catalyst according to claim 7, characterized in that, The molar ratio of the intermediate 2 to the tetramethylsilane Si(CH3)4 is 4.0 - 6.0:

1.

11. The preparation method of the silicon-based catalyst according to claim 7, characterized in that, The molar ratio of the intermediate 2 to the tetramethylsilane Si(CH3)4 is 4.5 - 5.0:

1.

12. The preparation method of the silicon-based catalyst according to claim 1, characterized in that, The solvent A used in the step S10) includes one or a mixture of two of dichloromethane and chloroform; and / or, the solvent B used in the step S20) includes one or a mixture of two of dichloromethane and chloroform.

13. The preparation method of the silicon-based catalyst according to claim 12, characterized in that, The solvent A used in the step S10) includes dichloromethane.

14. The preparation method of the silicon-based catalyst according to claim 12, characterized in that, The solvent B used in the step S20) includes chloroform.

15. An application of a silicon-based catalyst prepared by the method for preparing a silicon-based catalyst according to any one of claims 1-14, characterized in that, As a catalyst for preparing lithium fluoroxalate phosphate, where the lithium fluoroxalate phosphate is used as a lithium supplement additive for a lithium-ion battery or a lithium-ion capacitor.

16. The application of the silicon-based catalyst according to claim 15, characterized in that, Lithium hexafluorophosphate and oxalic acid are added to a carbonate solvent, and the silicon-based catalyst is added, and a lithium fluoroxalate phosphate solution is obtained through reaction; then the solvent in the lithium fluoroxalate phosphate solution is separated, and after recrystallization and drying, a lithium fluoroxalate phosphate solid is obtained, and the Cl - content in the lithium fluoroxalate phosphate solid is not higher than 1 ppm.

17. The use of the silicon-based catalyst according to claim 16, characterized in that, The molar ratio of lithium hexafluorophosphate, oxalic acid, and the silicon-based catalyst in the feed is 1:1.0 - 3.0:1.0 - 2.

5.

18. The application of the silicon-based catalyst according to claim 17, characterized in that, The molar ratio of lithium hexafluorophosphate, oxalic acid, and the silicon-based catalyst in the feed is 1:1.0 - 3.0:0.6 - 2.

0.

19. The application of the silicon-based catalyst according to claim 17, wherein The temperature of the reaction is 20 - 50 °C, and / or the time of the reaction is 2 - 6 hours; the carbonate solvent is selected from any one or a mixture of several of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

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