Method for synthesizing cyclic carbonate by alcoholysis of urea catalyzed by quaternary phosphonium composite ionic liquid

Abstract

The present invention relates to a method for synthesizing cyclic carbonates by catalytic alcoholysis of urea with an ionic liquid and a metal salt in combination. The catalyst is a composite catalyst composed of a quaternary phosphonium ionic liquid and a metal salt, which catalyzes the synthesis of cyclic carbonates from urea and diols. The composite catalyst provided by the present invention is simple to prepare, low in cost, environmentally friendly, stable in performance and easy to separate, and can achieve efficient catalysis of the reaction under relatively mild conditions, and has high industrial application value.

Description

Technical Field

[0001] The present invention relates to a method for preparing cyclic carbonates, in which an ionic liquid and a metal salt are used to form a composite catalyst to catalyze the synthesis of cyclic carbonates from urea and diols, belonging to the field of green catalytic technology. Background Art

[0002] In recent years, due to the combustion of fossil fuels, the emissions of CO2 have been increasing continuously, causing global environmental and social problems. CO2 is a non-toxic, inexpensive, abundant and renewable C1 resource, which can be converted into a variety of useful high-value-added chemicals and fuels. In the resource utilization of CO2, the conversion of CO2 into cyclic carbonates is one of the effective ways to utilize CO2.

[0003] Cyclic carbonates are important organic synthesis intermediates and green solvents with excellent properties. They are the main raw materials for synthesizing dimethyl carbonate or diethyl carbonate by the transesterification method and an important product link for the development of polycarbonates. Currently, cyclic carbonates are mostly produced by the CO2 cycloaddition method in industry. This method has the advantages of short process flow and high product yield, but the selection of catalysts, subsequent separation and harsh reaction conditions are the difficulties affecting the reaction. The synthesis of cyclic carbonates from urea and diols is a new reaction route proposed in recent years. This method indirectly uses CO2 to overcome the high reaction energy barrier of CO2 conversion. Its raw materials are widely sourced, inexpensive and easily available, with low economic cost and mild reaction conditions. Since the physical properties of the raw materials and products are quite different, they are easy to separate, meeting the requirements of green chemistry.

[0004] At present, the research on the synthesis of cyclic carbonates by the alcoholysis of urea mainly focuses on finding suitable catalytic systems. The reported catalysts mainly include metal salts, metal oxides, supported catalysts, ionic liquids, etc. The literature (Chemical Engineering, 2015, 43(02): 63-67) investigated the catalytic performance of various zinc salts in the synthesis of propylene carbonate from urea and 1,2-propanediol. Among them, the yield of propylene carbonate synthesized by ZnBr2 at 160 °C for 3 h was 57.3%. The literature (Leather Science and Engineering, 2008, 18(06): 13-17) reported that in the alcoholysis reaction of urea and 1,2-propanediol catalyzed by the ionic liquid [Bmim]BF4 alone, the content of propylene carbonate generated was 20.3%; while in the co-catalytic reaction of the ionic liquid [Bmim]BF4 and supported zinc acetate, the highest content of propylene carbonate obtained was only 21.6%. The patent (CN201910591299.5) provides a method for synthesizing ethylene carbonate by the alcoholysis of urea catalyzed by a composite catalyst composed of an imidazole-based ionic liquid and a metal salt. Binary catalytic systems of 1-hexadecyl-3-methylimidazolium chloride ionic liquid / zinc chloride and 1-hexadecyl-3-methylimidazolium bromide ionic liquid / zinc chloride were designed and screened, and the yields of ethylene carbonate reached 92.2% and 92.4% respectively, and the catalytic activity was greatly improved. However, due to the easy decomposition of imidazole-based ionic liquids during long-term experiments, there are instability problems.

[0005] In summary, for the reaction of synthesizing cyclic carbonates by the alcoholysis of urea and diols, the currently studied single ionic liquid and metal salt catalysts have certain limitations. Therefore, carrying out the research on composite catalysts of ionic liquids and metal salts to find suitable catalytic systems is of great significance for realizing the industrial application of the efficient synthesis of cyclic carbonates by the alcoholysis of urea. Summary of the Invention

[0006] To solve the defects and deficiencies of the existing catalyst systems, the present invention provides a quaternary phosphonium ionic liquid and metal salt binary catalyst with simple preparation, environmental friendliness, low cost and excellent catalytic performance, which can achieve the efficient synthesis of cyclic carbonates under mild conditions.

[0007] To achieve the above object, the present invention adopts the following solutions: A composite catalyst of an ionic liquid and a metal salt for catalytic synthesis of cyclic carbonates, which is characterized by including the following steps: Using urea and diol as raw materials, with a molar ratio of 1:(1-6), at a reaction temperature of 130 °C - 170 °C, a reaction pressure of 5 - 30 kPa, and a reaction time of 2 - 6 h, catalytically prepare cyclic carbonates.

[0008] In the above solution, the diol is ethylene glycol, propylene glycol or butylene glycol, and the cyclic carbonate is ethylene carbonate, propylene carbonate or butylene carbonate;

[0009] In the above solution, the structure of the quaternary phosphonium ionic liquid is as follows:

[0010]

[0011] In the said structure, R1, R2, R3, and R4 are any one of substituted or unsubstituted C1-C8 alkyl groups; the anion X - is F - , Cl - , Br - , I - , OH - , BF4 - , PF6 - , NTf2 - , HCOO - , CH3COO - , C6H5COO - , Ala - , Lys - , Gly - , Suc - , TsO - any one of;

[0012] In the above solution, the metal salt is any one of metal halides, acetates, carbonates, sulfates, and phosphates, preferably metal halides.

[0013] In the above solution, the metal salt is any one of zinc salts, copper salts, aluminum salts, and iron salts, preferably zinc salts.

[0014] In the above solution, the molar ratio of urea to diol is 1:(1-6), for example, it can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, and preferably 1:4.

[0015] In the above solution, the molar ratio of the ionic liquid to the metal salt is (0.1-2):1, for example, it can be 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, and preferably 0.1:1.

[0016] In the above solution, the molar ratio of the metal salt to urea is (0.05-0.15):1, for example, it can be 0.05:1, 0.06:1, 0.07:1, 0.08:1, 0.09:1, 0.10:1, 0.11:1, 0.12:1, 0.13:1, 0.14:1, 0.15:1, and preferably 0.05:1.

[0017] In the above solution, the reaction temperature is 130°C - 170°C, for example, it can be 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, 165°C, 170°C, and preferably 160°C.

[0018] In the above solution, the reaction pressure is 5 - 30 kPa, for example, it can be 5 kPa, 6 kPa, 7 kPa, 8 kPa, 9 kPa, 10 kPa, 11 kPa, 12 kPa, 13 kPa, 14 kPa, 15 kPa, 16 kPa, 17 kPa, 18 kPa, 19 kPa, 20 kPa, 21 kPa, 22 kPa, 23 kPa, 24 kPa, 25 kPa, 26 kPa, 27 kPa, 28 kPa, 29 kPa, 30 kPa, and preferably 15 kPa.

[0019] In the above solution, the reaction time is 2 - 6 h, for example, it can be 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h, and preferably 3 h.

[0020] Compared with the existing technology, the characteristics of this invention are as follows:

[0021] (1) The quaternary phosphonium ionic liquid and metal salt composite catalyst provided by this invention is simple to prepare, environmentally friendly, low - cost, easy to separate and has good stability, providing ideas for the design of new catalysts in the reaction of urea alcoholysis to synthesize cyclic carbonates.

[0022] (2) Compared with the existing single ionic liquid and metal salt catalysts used for preparing cyclic carbonates, the composite catalyst used in this invention catalyzes the reaction synergistically, has high catalytic activity, can efficiently catalyze the synthesis of cyclic carbonates from urea and diols under relatively mild conditions, and the selectivity reaches more than 95%.

[0023] (3) The reaction process of this invention is simple to operate, does not require the addition of extra solvents, is convenient for subsequent process treatment, and realizes industrial application. Detailed implementation mode

[0024] The technical solution of this invention will be introduced in detail in combination with the following specific examples. However, the listed examples are only for further understanding of this invention and are not limited thereto.

[0025] Example 1

[0026]

[0027] First, weigh 1.80 g of urea and 7.44 g of ethylene glycol (the molar ratio of urea to ethylene glycol is 1:4) and add them to a 25 ml three-necked flask equipped with a reflux condenser. Gradually heat with magnetic stirring in a constant-temperature oil bath until the urea is completely dissolved in the ethylene glycol. Then weigh 0.08 g of tetrabutylphosphonium bromide and 0.54 g of zinc bromide (the molar ratio of ionic liquid to metal salt is 0.1:1, and the molar ratio of metal salt to urea is 0.08:1), and add them to the three-necked flask in sequence. Continue heating to 160 °C. At the same time, use a vacuum pump to control the reaction pressure, always maintaining it at 15 kPa, and continue the reaction for 3 h. After the reaction is completed, cool the mixture in the system to room temperature, and the pressure returns to normal pressure. Use gas chromatography to quantitatively analyze the product composition. The calculated yield of ethylene carbonate is 76.4%, and the selectivity is 98.7%.

[0028] Example 2

[0029] The amount of tetrabutylphosphonium bromide used is 0.16 g, and the amount of zinc bromide used is 0.54 g (the molar ratio of ionic liquid to metal salt is 0.2:1, and the molar ratio of metal salt to urea is 0.08:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 71.7%, and the selectivity is 94.7%.

[0030] Example 3

[0031] The amount of tetrabutylphosphonium bromide used is 0.05 g, and the amount of zinc bromide used is 0.34 g (the molar ratio of ionic liquid to metal salt is 0.1:1, and the molar ratio of metal salt to urea is 0.05:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 80.3%, and the selectivity is 98.8%.

[0032] Example 4

[0033] The amount of tetrabutylphosphonium bromide used is 0.10 g, and the amount of zinc bromide used is 0.34 g (the molar ratio of ionic liquid to metal salt is 0.2:1, and the molar ratio of metal salt to urea is 0.05:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 78.8%, and the selectivity is 99.1%.

[0034] Example 5

[0035] The amount of tetrabutylphosphonium bromide used is 0.15 g, and the amount of zinc bromide used is 0.34 g (the molar ratio of ionic liquid to metal salt is 0.3:1, and the molar ratio of metal salt to urea is 0.05:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 72.2%, and the selectivity is 98.8%.

[0036] Example 6

[0037] The amount of tetrabutylphosphonium bromide used is 0.20 g, and the amount of zinc bromide used is 0.34 g (the molar ratio of ionic liquid to metal salt is 0.4:1, and the molar ratio of metal salt to urea is 0.05:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 71.5%, and the selectivity is 98.7%.

[0038] Example 7

[0039] The amount of tetrabutylphosphonium bromide used is 0.25 g, and the amount of zinc bromide used is 0.34 g (the molar ratio of ionic liquid to metal salt is 0.5:1, and the molar ratio of metal salt to urea is 0.05:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 68.6%, and the selectivity is 97.9%.

[0040] Example 8

[0041] The amount of tetrabutylphosphonium bromide used is 0.12 g, and the amount of zinc bromide used is 0.41 g (the molar ratio of ionic liquid to metal salt is 0.2:1, and the molar ratio of metal salt to urea is 0.06:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 73.2%, and the selectivity is 95.9%.

[0042] Example 9

[0043] The amount of tetrabutylphosphonium bromide used is 0.18 g, and the amount of zinc bromide used is 0.41 g (the molar ratio of ionic liquid to metal salt is 0.3:1, and the molar ratio of metal salt to urea is 0.06:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 76.6%, and the selectivity is 98.8%.

[0044] Example 10

[0045] The amount of tetrabutylphosphonium bromide used is 0.31 g, and the amount of zinc bromide used is 0.41 g (the molar ratio of ionic liquid to metal salt is 0.5:1, and the molar ratio of metal salt to urea is 0.06:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 74.6%, and the selectivity is 98.8%.

[0046] Example 11

[0047] The amount of tetrabutylphosphonium bromide used is 0.15 g, and the amount of zinc bromide used is 0.68 g (the molar ratio of ionic liquid to metal salt is 0.15:1, and the molar ratio of metal salt to urea is 0.10:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 71.5%, and the selectivity is 95.6%.

[0048] Example 12

[0049] The dosage of tetrabutylphosphonium bromide is 0.20 g, and the dosage of zinc bromide is 0.68 g (the molar ratio of ionic liquid to metal salt is 0.2:1, and the molar ratio of metal salt to urea is 0.10:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 72.7%, and the selectivity is 98.8%.

[0050] Example 13

[0051] The dosage of tetrabutylphosphonium bromide is 0.51 g, and the dosage of zinc bromide is 0.68 g (the molar ratio of ionic liquid to metal salt is 0.5:1, and the molar ratio of metal salt to urea is 0.10:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 57.5%, and the selectivity is 95.7%.

[0052] Example 14

[0053] The dosage of tetrabutylphosphonium bromide is 0.15 g, and the dosage of zinc bromide is 1.01 g (the molar ratio of ionic liquid to metal salt is 0.1:1, and the molar ratio of metal salt to urea is 0.15:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 73.2%, and the selectivity is 98.2%.

[0054] Example 15

[0055] The dosage of tetrabutylphosphonium bromide is 0.31 g, and the dosage of zinc bromide is 1.01 g (the molar ratio of ionic liquid to metal salt is 0.2:1, and the molar ratio of metal salt to urea is 0.15:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 66.0%, and the selectivity is 97.9%.

[0056] Example 16

[0057] The dosage of tetrabutylphosphonium bromide is 0.46 g, and the dosage of zinc bromide is 1.01 g (the molar ratio of ionic liquid to metal salt is 0.3:1, and the molar ratio of metal salt to urea is 0.15:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 61.3%, and the selectivity is 98.2%.

[0058] Example 17

[0059] The dosage of ethylene glycol is 3.72 g (the molar ratio of urea to ethylene glycol is 1:2). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 61.2%, and the selectivity is 95.1%.

[0060] Example 18

[0061] The dosage of ethylene glycol is 5.58 g (the molar ratio of urea to ethylene glycol is 1:3). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 74.5%, and the selectivity is 98.9%.

[0062] Example 19

[0063] The amount of ethylene glycol used was 9.30 g (the molar ratio of urea to ethylene glycol was 1:5). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 79.4%, and the selectivity was 98.2%.

[0064] Example 20

[0065] The amount of ethylene glycol used was 11.16 g (the molar ratio of urea to ethylene glycol was 1:6). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 74.7%, and the selectivity was 92.5%.

[0066] Example 21

[0067] The reaction temperature was 140 °C. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 55.8%, and the selectivity was 84.8%.

[0068] Example 22

[0069] The reaction temperature was 150 °C. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 71.5%, and the selectivity was 98.6%.

[0070] Example 23

[0071] The reaction temperature was 170 °C. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 76.2%, and the selectivity was 98.7%.

[0072] Example 24

[0073] The reaction time was 2 h. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 74.8%, and the selectivity was 98.5%.

[0074] Example 25

[0075] The reaction time was 4 h. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 70.7%, and the selectivity was 98.5%.

[0076] Example 26

[0077] The reaction time was 5 h. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 64.2%, and the selectivity was 98.5%.

[0078] Example 27

[0079] The reaction time was 6 h. Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 60.8%, and the selectivity was 97.6%.

[0080] Example 28

[0081] The reaction pressure was 20 kPa, and other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 54.0%, and the selectivity was 90.5%.

[0082] Example 29

[0083]

[0084] The diol was 9.13 g of 1,2 - propanediol (the molar ratio of urea to 1,2 - propanediol was 1:4). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 88.3%, and the selectivity was 98.8%.

[0085] Example 30

[0086] The ionic liquid was 0.05 g of tetraethylphosphonium bromide (the molar ratio of ionic liquid to metal salt was 0.1:1, and the molar ratio of metal salt to urea was 0.08:1). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 73.9%, and the selectivity was 98.6%.

[0087] Example 31

[0088] The ionic liquid was 0.07 g of tributylethylphosphonium bromide (the molar ratio of ionic liquid to metal salt was 0.1:1, and the molar ratio of metal salt to urea was 0.08:1). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 76.5%, and the selectivity was 97.9%.

[0089] Example 32

[0090] The ionic liquid was 0.14 g of tetraoctylphosphonium bromide (the molar ratio of ionic liquid to metal salt was 0.1:1, and the molar ratio of metal salt to urea was 0.08:1). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 76.9%, and the selectivity was 98.8%.

[0091] Example 33

[0092] The ionic liquid was 0.13 g of tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide (the molar ratio of ionic liquid to metal salt was 0.1:1, and the molar ratio of metal salt to urea was 0.08:1). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 80.7%, and the selectivity was 99.1%.

[0093] Example 34

[0094] The ionic liquid was 0.07 g of tetrabutylphosphonium hydroxide (the molar ratio of ionic liquid to metal salt was 0.1:1, and the molar ratio of metal salt to urea was 0.08:1). Other operating conditions were the same as in Example 1. The yield of ethylene carbonate was 69.0%, and the selectivity was 97.7%.

[0095] Example 35

[0096] The ionic liquid is 0.08 g of tetrabutylphosphonium tetrafluoroborate (the molar ratio of the ionic liquid to the metal salt is 0.1:1, and the molar ratio of the metal salt to urea is 0.08:1). Other operating conditions are the same as in Example 1. The yield of ethylene carbonate is 77.5%, and the selectivity is 98.7%.

[0097] Comparative Example 1

[0098] The ionic liquid is 0.05 g of tetrabutylphosphonium bromide (the molar ratio of the ionic liquid to urea is 0.005:1). The metal salt is not added. Other operating conditions are the same as in Example 3. The yield of ethylene carbonate is 14.7%, and the selectivity is 42.0%.

[0099] Comparative Example 2

[0100] The ionic liquid is 0.51 g of tetrabutylphosphonium bromide (the molar ratio of the ionic liquid to urea is 0.05:1). The metal salt is not added. Other operating conditions are the same as in Example 3. The yield of ethylene carbonate is 12.2%, and the selectivity is 34.1%.

[0101] Comparative Example 3

[0102] 0.34 g of zinc bromide is added (the molar ratio of the metal salt to urea is 0.05:1). The ionic liquid is not added. Other operating conditions are the same as in Example 3. The yield of ethylene carbonate is 62.3%, and the selectivity is 89.8%.

[0103] The quaternary phosphonium ionic liquid and metal salt composite catalytic system provided by the present invention for catalyzing the alcoholysis reaction of urea and diol enhances the solubility of the metal salt in the diol by adding a small amount of ionic liquid. The reaction process is simple to operate, without the need to add an additional solvent, which simplifies the subsequent process treatment. Under the synergistic catalytic action of the ionic liquid and the metal salt, the catalytic performance of the catalyst is improved, enabling the reaction to achieve high selectivity of the product under relatively mild conditions.

Claims

1. A method for preparing cyclic carbonates by urea alcoholysis, characterized in that, Using a composite catalyst of quaternary phosphonium ionic liquid and metal salt, cyclic carbonates are prepared by catalytic reaction of urea and diol under the feed ratio of urea to diol of 1:(1 - 6), ionic liquid to metal salt of (0.1 - 2):1, and metal salt to urea of (0.05 - 0.15):1; The diol is ethylene glycol or 1,2 - propanediol, and the cyclic carbonate is ethylene carbonate or propylene carbonate; The structure of the quaternary phosphonium ionic liquid is as follows: Among them, in the structure of the quaternary phosphonium ionic liquid, R1, R2, R3, and R4 are any one of C1-C8 alkyl groups; the anion X - is F - 、Cl - 、Br - 、I - 、OH - 、BF4 - any one of; The metal salt is zinc bromide; The reaction temperature for the alcoholysis of urea and diol is 140°C - 170°C; The reaction pressure for the alcoholysis of urea and diol is 5 - 30 kPa.

2. The preparation method according to claim 1, wherein The reaction time for the alcoholysis of urea and diol is 2 - 6 h.

3. The preparation method according to claim 1, characterized in that, The specific preparation steps are as follows: First, urea and diol are added to a reaction vessel in a molar ratio of 1:4 and mixed. The temperature is raised by heating, and magnetic stirring is carried out to completely dissolve urea in diol. Then, the quaternary phosphonium ionic liquid and metal salt are successively added to the reaction vessel in a molar ratio of 0.1:1, and the molar ratio of metal salt to urea is 0.05:

1. Heating is continued until the temperature reaches 160°C, and at the same time, the reaction pressure is controlled at 15 kPa by using a vacuum pump. The reaction is carried out for 3 h to synthesize cyclic carbonate.

Images