Method for catalytically synthesizing dimethyl carbonate by choline-based ionic liquid

Abstract

The invention relates to a method for catalytically synthesizing dimethyl carbonate by choline-based ionic liquid, and belongs to the technical field of chemical engineering. According to the method, propylene carbonate or ethylene carbonate and methanol are used as raw materials, amino acid choline or imidazole choline ionic liquid is used as a catalyst, dimethyl carbonate is synthesized through transesterification, the use amount of the ionic liquid is 0.1%-5% of the total mass of methanol and propylene (or ethylene) carbonate, the reaction pressure is 50-200 kPa, the reaction temperature is 60-95 DEG C, and the reaction time is 0.5-4 h. The ionic liquid catalyst provided by the invention has good catalytic activity and can be recycled, and the method for producing dimethyl carbonate has the advantages of simple process flow, mild reaction conditions, environmental friendliness, low production cost and the like, and is a novel method for easily realizing industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical engineering, and particularly relates to a method for preparing dimethyl carbonate by transesterification of methanol with propylene carbonate or ethylene carbonate. Background Art

[0002] Dimethyl carbonate (hereinafter referred to as DMC) is an important organic synthesis intermediate. The molecular structure of dimethyl carbonate contains functional groups such as carbonyl, methyl, and methoxy groups, so it has various reaction activities. In addition, due to its characteristics of safe use, convenience, less pollution, and easy transportation, dimethyl carbonate is regarded as a "green" chemical product and can replace highly toxic phosgene, dimethyl sulfate, etc. and be used in many chemical fields such as medicine, pesticides, and solvents. In recent years, dimethyl carbonate has been widely used in coating solvents, non-phosgene synthesis of polycarbonate, and lithium battery electrolytes, which has promoted the development of the dimethyl carbonate industry.

[0003] The traditional industrial method for synthesizing dimethyl carbonate mainly uses propylene oxide or ethylene oxide and carbon dioxide as raw materials, and under the action of a catalyst, a pressurized reaction is carried out to obtain propylene carbonate or ethylene carbonate, and then propylene carbonate or ethylene carbonate reacts with methanol by transesterification to prepare dimethyl carbonate, and propylene glycol or ethylene glycol is co-produced. At present, most of the catalysts for transesterification reactions are sodium methoxide. However, since sodium methoxide is sensitive to water, CO2, etc., it is extremely easy to form NaOH, Na2CO3, etc., resulting in catalyst deactivation and inability to be recycled. Therefore, industrially, a method of adding a small amount of water to the reaction product and passing CO2 for carbonization is adopted to make sodium methoxide crystallize out from the product in the form of sodium carbonate and be removed by filtration. And because a small amount of organic matter is contained in sodium carbonate, it becomes waste solid, increasing the treatment cost. Therefore, it is very necessary to develop a transesterification catalyst that can be recycled and has high catalytic activity.

[0004] Ionic liquids (ILs) have the advantages of being green and environmentally friendly, having a low saturated vapor pressure, and good thermal stability. They have good activity for the transesterification of propylene carbonate or ethylene carbonate with methanol and are a promising homogeneous catalyst to replace traditional catalysts. For example, CN104043480A discloses the synthesis of dimethyl carbonate by transesterification using a quaternary ammonium salt ionic liquid as a catalyst. The catalyst dosage is 0.1% - 2.5% of the mass of propylene carbonate, the pressure is 0.1 - 0.2 MPa, the temperature is 60 - 90 °C, and the time is 1 - 2 h. However, the equilibrium conversion rate of propylene carbonate is only 23.1%. CN103980124B discloses using a quaternary ammonium salt ionic liquid as a catalyst to react and separate methanol, propylene carbonate, and the ionic liquid catalyst in a reactive distillation column. The separation and purification of propylene glycol in the bottom material of the column consists of three distillation columns: a light component removal column, a propylene glycol product column, and a methanol recovery column; the separated catalyst is returned to the reactive distillation system for reuse. CN102126927B discloses a heterogeneous catalytic method for preparing ethylene glycol and carbonate by reacting styrene resin-supported ionic liquid catalyst at a reaction temperature of 333 - 433 K, a reaction time of 0.25 - 4 hours, and a molar ratio of ethylene carbonate to fatty alcohol of 1:4 - 1:32. CN105237336B discloses a method for synthesizing dimethyl carbonate and co-producing ethylene glycol by transesterification using a supported ionic liquid catalyst. The supported ionic liquid uses a chlorine-containing resin particle as a carrier and an ionic liquid with a hydroxyl anion structural unit paired with an imidazole cation structural unit as the catalytic active center, and is prepared by chemical bonding to obtain a functionalized supported ionic liquid catalyst for catalyzing the transesterification of ethylene carbonate and methanol. In a flask with a reflux condenser, the catalyst dosage is 8% of EC, and the reaction is carried out at 80 °C for 8 h. The EC conversion rate actually reaches 99.4%. After the catalyst is recycled 5 times, the selectivity drops to 80%. However, there is a problem with this patent, that is, in the case of reflux, the transesterification reaction actually does not have an equilibrium reaction! This is inconsistent with the common sense that the transesterification reaction is a reversible reaction. CN102126957B discloses a method for preparing carbonate and ethylene glycol using an ionic liquid catalyst. A halogenated pyridine or halogenated aminopyridine-type ionic liquid catalyst is used for pressurized reaction at a reaction temperature of 373 - 453 K, a reaction time of 0.25 - 6 h, and a molar ratio of ethylene carbonate to fatty alcohol of 1:4 - 1:32. Since pressurized conditions are used, it can be seen that the activity of the catalyst is not very high. CN111018710A discloses a method for preparing dimethyl carbonate, using an ionic liquid as a transesterification catalyst. The ionic liquid is an imidazole type, pyrrole type, or morpholine base or salt, with a content of 0.3%, and the reaction reaches equilibrium in 5 min at 68 - 70 °C. CN111001438A discloses a catalyst for synthesizing dimethyl carbonate by transesterification and its application. The transesterification reaction uses an ionic liquid as a catalyst, with Li + 、Na +, K + One or more of them are mixed as cations, and the anion is CF3COO - , C3F7COO - , CF3SO3 - , (C4F9SO2)N - , (C2F5SO2)N - , C8H 17 SO4 - A mixture of one or more of them. Liu Ju et al. (Fine Chemicals 2020, 37(7): 1438-1446) disclosed the use of highly thermally stable strong basic ionic liquids in the synthesis of dimethyl carbonate, and studied the effect of a series of imidazole-based ionic liquids such as 1-ethyl-3-methylimidazolium imidazolate on the transesterification of ethylene carbonate (EC) and methanol (MeOH) to synthesize dimethyl carbonate (DMC). When the catalyst dosage was 0.3% of the total mass of the reaction raw materials, the reaction reached equilibrium in 1 min, the selectivity of DMC was 100%, and the yield was 88.70%. However, after testing by the inventors, this ionic liquid was still prone to deactivation and could not achieve the purpose of reuse. Ma Chengming et al. (Fine Petrochemicals, 2012, 29(6): 58-62) used 1-butyl-3-methylimidazolium-based ionic liquids as catalysts for the transesterification of ethylene carbonate and methanol, and found that [Bmim]OH had the best catalytic effect. JU et al. (Reaction Kinetics and Catalysis Letters, 2007, 90(⒈): 3-9) studied the use of 1-ethyl-3-methylimidazolium chloride ([Emim]Cl) as a catalyst. At a reaction temperature of 160 °C (under pressure) and a catalyst addition amount of 0.89% of the total mass of the raw materials, when the reaction time was ⒈ h, the conversion rate of EC reached 80.4%. YANG et al. (Tetrahedron Letters, 2010, 51(21): 2931-2934) synthesized an ionic liquid with 1,4-diazabicyclo[2.2.2]octane as the cation, and found that when the anion was OH -When the catalytic activity and stability of the ionic liquid are the best, at a reaction temperature of 70 °C, the catalyst addition amount is 1% of the amount of substance of EC, and the reaction time is 6 h, the conversion rate of EC is 90%. LEE et al. (Research on Chemical Intermediates, 2016, 42(1): 109-121) found on the basis of the research of YANG et al. (Tetrahedron Letters, 2010, 51(21): 2931-2934) that when this ionic liquid is loaded on mesoporous foam, at n(EC)∶n(MeOH)=1∶15, the catalyst content is 1% of the amount of substance of EC, and the reaction is carried out at 100 °C for 4 h, the yield of DMC reaches 77%. WANG et al. (Catalysis Science & Technology, 2012, 2(3): 600-605) reported that for 1,3-dimethylimidazole-2-carboxylate at n(EC):n(MeOH)=1:10, the catalyst dosage is 1% of the amount of substance of EC, and the reaction is carried out at 110 °C for 80 min, the yield of DMC is 81%. XU et al. (Applied Catalysis A: General, 2013, 464 / 465: 357-363) reported that for 1-(triethoxysilyl)propyl-3-methylimidazole hydroxide at 65 °C, n(EC)∶n(MeOH)= 1:10, and the reaction is carried out for 5 h, the yield of DMC is 86.1%.

[0005] It can be seen that some ionic liquids used as catalysts for transesterification reactions have low activity. Compared with sodium methoxide catalysts, the reaction time is longer, and some even require pressurization; although some ionic liquids have high activity when used for the first time, they also have disadvantages such as easy deactivation and inability to be reused. Summary of the Invention

[0006] The object of the present invention is to overcome the defects existing in the prior art and propose an ionic liquid that can be recycled and has high catalytic activity as a catalyst for transesterification reactions.

[0007] Technical solution for achieving the object of the present invention: According to domestic and foreign research and production practices, it is found that for the transesterification reaction of propylene carbonate or ethylene carbonate with methanol, under the action of a strong alkaline catalyst, the reaction rate is relatively fast and the conversion rate of propylene carbonate or ethylene carbonate is relatively high.

[0008] The inventor found that in the molecular structure of choline-based ionic liquids, due to the positive charge being distributed on one nitrogen atom and the surrounding four carbons in the choline cation, the molecule has good thermal stability and relatively strong alkalinity, and has good catalytic performance for transesterification reactions.

[0009] The present invention provides a method for synthesizing dimethyl carbonate through transesterification, which comprises the following main steps: using methanol and propylene (or ethylene) carbonate as raw materials and preparing dimethyl carbonate under the action of a catalyst ionic liquid, and is characterized by: The ionic liquid is one having the structural formula shown in Formula I or II: [Cho] + [AA] - [Cho] + [n-Im] - Formula I Formula II Among them, [Cho] + is the choline cation, with the structural formula ; [AA] - is an amino acid anion, AA is independently selected from one of alanine (Ala), valine (Val), leucine (Leu), isoleucine (Ile), phenylalanine (Phe), methionine (Met), proline (Pro), tryptophan (Trp), serine (Ser), threonine (Thr), cysteine (Cys), asparagine (Asn), glutamine (Gln), tyrosine (Tyr), glycine (Gly), aspartic acid (Asp), glutamic acid (Glu), lysine (Lys), arginine (Arg), and histidine (His); [n-Im] - It is an anion of an imidazolium compound, wherein n is one of 0, 2, and 4, i.e., one of imidazole, 2-methylimidazole, and 4-methylimidazole.

[0010] The amount of the catalyst ionic liquid used is 0.1%-5% of the total mass of methanol and propylene (or ethylene) carbonate.

[0011] The molar ratio of methanol to propylene carbonate (or) vinyl carbonate is 1:4 to 1:14.

[0012] The temperature of the transesterification reaction is 50-75°C.

[0013] The pressure of the transesterification reaction is 50-200 kPa.

[0014] The residence time of the transesterification reaction is 0.5 to 4 hours.

[0015] By adopting the above technical solution, the equilibrium conversion rate of propylene carbonate or ethylene carbonate can reach 65-75%. By adopting the reactive distillation process solution, the conversion rate of propylene carbonate or ethylene carbonate can reach more than 99.8%, and the yield of dimethyl carbonate can reach more than 99%. DETAILED DESCRIPTION

[0016] In this application, EC refers to ethylene carbonate, PC refers to propylene carbonate, ME refers to methanol, EG refers to ethylene glycol, PG refers to 1,2 - propylene glycol, and DMC refers to dimethyl carbonate.

[0017] The present invention will be further described below in conjunction with embodiments, but these embodiments do not limit the protection scope of the present invention.

[0018] Example 1 In a four - necked flask equipped with reflux and stirring, add 1 mol of PC, 10 mol of methanol, and the catalyst choline alanine [Cho][Ala]. The addition amount is 1.5% of the total of PC + methanol. The reaction temperature is 68 °C. After reacting for 60 min, a sample is taken for analysis.

[0019] Example 2 The catalyst is choline valine [Cho][Val], and the addition amount is 1.0%. Other conditions are the same as in Example 1. Example 3

[0020] [[ID={18}]]The catalyst is choline arginine [Cho][Arg], and the addition amount is 0.7%. Other conditions are the same as in Example 1.

[0021] Example 4

[0022] The catalyst is choline lysine [Cho][Lys], and the addition amount is 1.2%. Other conditions are the same as in Example 1.

[0023] Example 5

[0024] The catalyst is choline histidine [Cho][His], and the addition amount is 0.7%. Other conditions are the same as in Example 1.

[0025] Example 6

[0026] The catalyst is choline glycine [Cho][Gly], and the addition amount is 0.8%. Other conditions are the same as in Example 1.

[0027] Example 7

[0028] The catalyst is choline asparagine [Cho][Asn], and the addition amount is 0.8%. Other conditions are the same as in Example 1.

[0029] Example 8

[0030] The catalyst is [Cho][Leu], and the addition amount is 2.0%. Other conditions are the same as in Example 1.

[0031] Example 9 Other conditions are the same as in Example 1. Add 1 mol of EC, 12 mol of methanol, and the catalyst choline arginine [Cho][Arg], and the addition amount is 1.0%.

[0032] Example 10

[0033] Other conditions are the same as in Example 9, and the amount of the catalyst imidazolium choline [Cho][IM] added is 1.0%.

[0034] Example 11

[0035] In a laboratory reactive distillation column with a diameter of 50 mm and a height of 2.0 m, θ-type 3*4 mm metal packing is installed. The distillation column is divided into three sections: the upper rectifying section, the middle reaction section, and the lower stripping section. The catalyst [Cho][IM] is dissolved in part of methanol (the [Cho][IM] accounts for 1% of the total feed of ME and PC), and is continuously fed separately from between the rectifying section and the reaction section with PC (ME:PC = 4:1, mol). The remaining methanol (ME:PC = 8:1, mol) is continuously fed into the column from above the stripping section. Under atmospheric pressure operation, the reflux ratio at the top of the column is controlled between 1 and 3. The azeotrope of methanol and dimethyl carbonate is continuously distilled out from the top of the column at a temperature of 64 - 64.5 °C, the temperature at the bottom of the column is 80 - 85 °C, the liquid level at the bottom of the column is controlled to be stable, and continuous discharging is carried out. After the top and the bottom of the column are stabilized, samples are taken for analysis respectively. The content of DMC in the azeotrope at the top of the column is 29.6% (wt), the conversion rate of PC at the bottom of the column is 99.7%, and the selectivity of DMC is 99.8%.

[0036] Example 12

[0037] The catalyst imidazolium choline [Cho][IM] with an addition amount of 1.0%, other conditions are the same as in Example 1. After each reaction, the organic liquid is removed by rotary evaporation under reduced pressure, and the catalyst is left. PC and methanol are added to react again, and the catalyst is recycled until the 10th cycle of use of the catalyst. Comparative Example:

[0038] Other conditions are the same as in Example 1, and the amount of sodium methoxide added as the catalyst is 0.7%.

[0039] The results of the above examples and comparative examples are shown in Table 1.

[0040] Table 1

Claims

1. A method for catalytic synthesis of dimethyl carbonate with choline-based ionic liquid, the main steps of which are: using methanol and propylene carbonate or ethylene carbonate as raw materials, and preparing dimethyl carbonate through transesterification reaction under the catalytic action of choline-based ionic liquid, characterized in that: The ionic liquid is one having the formula I or II shown below: [Cho] + [AA] - [Cho] + [n-Im] - Formula I Formula II Among them, [Cho] + is a choline cation, [AA] - is an amino acid anion, and AA is independently selected from one of alanine (Ala), valine (Val), leucine (Leu), isoleucine (Ile), phenylalanine (Phe), methionine (Met), proline (Pro), tryptophan (Trp), serine (Ser), threonine (Thr), cysteine (Cys), asparagine (Asn), glutamine (Gln), tyrosine (Tyr), glycine (Gly), aspartic acid (Asp), glutamic acid (Glu), lysine (Lys), arginine (Arg), histidine (His); [n-Im] - is an imidazole compound anion, and n is one of 0, 2, 4, namely one of imidazole, 2-methylimidazole, 4-methylimidazole.

2. The method according to claim 1, wherein The dosage of the catalyst ionic liquid is 0.1% - 5.0% of the total mass of methanol and propylene carbonate or ethylene carbonate.

3. The method according to claim 1, characterized in that, The molar ratio of propylene carbonate or ethylene carbonate to methanol is 1:4 - 1:

14.

4. The method according to claim 1, characterized in that, The temperature of the transesterification reaction is 60 - 95 °C.

5. The method according to claim 1, wherein, The pressure of the transesterification reaction is 50 - 200 kPa.

6. The method according to claim 1, wherein The residence time of the transesterification reaction is 0.5 - 4 h.

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