Patent Description:
Dimethyl carbonate (DMC) is an important organic synthesis intermediate, and could be used to synthesize polycarbonate, carbamate, isocyanate etc. DMC is nontoxic, comprises functional groups such as methyl, carbonyl and the like in its structure, and can replace virulent dimethyl sulfate and phosgene to carry out methylation reaction to synthesize a plurality of downstream products with high additional values. Therefore, DMC has wide application prospect in the fields such as plastics, dyes, food additives, medicine, pesticides etc. DMC also has a high dielectric constant and can be used as electrolyte for lithium ion battery. In addition, DMC has the advantages of high oxygen content, high octane number, good gasoline/water distribution coefficient, low toxicity, rapid biodegradation etc, and is considered as a potential fuel oil additive. Therefore, the synthesis of dimethyl carbonate is becoming more and more important in the fields of petroleum and chemical industry.

At present, the synthetic methods of DMC disclosed in literatures are mainly as follows: (<NUM>) phosgenation: the defects thereof include long operation period, high toxicity of phosgene, serious corrosion of byproduct (hydrogen chloride) to devices and high chlorine content of product; (<NUM>) methanol oxidative carbonylation: poor stability of the catalyst and many oxygen-containing byproducts; (<NUM>) direct synthesis from methanol and carbon dioxide [<NUM>]: usual DMC yield of lower than <NUM>% limited by thermodynamics; (<NUM>) urea methanolysis: easy decomposition of urea to byproducts such as N-methylcarbamate and the like, and usual DMC yield of lower than <NUM>% limited by thermodynamics; (<NUM>) transesterification: transesterification of Ethylene Carbonate (EC) or Propylene Carbonate (PC) and methanol (MeOH), which has achieved industrial production, lower price of raw materials, mild reaction condition, simple process flow and no corrosion to devices and thus is a promising production method.

Currently, in the industrial ten-thousand ton/year of transesterification of EC and MeOH, sodium methylate is used as catalyst, a molar ratio of EC to MeOH as raw materials is <NUM>:<NUM>, and the catalyst is <NUM>. 0wt% of the mixed raw materials by weight. The mixed solution passes through atmospheric and reactive distillation column with the column bottom temperature of <NUM>-<NUM> and the column top temperature of <NUM>, an azeotrope of DMC and MeOH is taken from the column top of the distillation column, and high-purity DMC is obtained through high-low pressure separation. However, this process has some problems: sodium methylate participates in the reaction, is sensitive to water and extremely easy to inactivate, and thus cannot be reused; and the strongly alkaline solid waste generated by the inactivation of sodium methylate is difficult to separate from homogeneous reaction system and pollutes the environment. Literatures further disclosed other catalysts such as homogeneous soluble strong alkali (such as sodium hydroxide or potassium hydroxide), organic ammonium salt, alkali metal carbonate, ionic liquid, but these catalysts also have the problems of rapid deactivation, incapability of being reused or less reused times and the like, thereby limiting a large-scale industrial application thereof. Therefore, it is necessary to develop a new catalyst with high activity and high stability for the transesterification of EC and MeOH.

The Ionic Liquids (ILs) have the advantages of friendly-environment, low saturated vapor pressure, good thermal stability and the like, has good activity in terms of the transesterification of ethylene carbonate and methanol, and is a homogeneous catalyst which is potential to replace the traditional catalyst. Ma Chengming et al disclosed that four kinds of ionic liquids with the same cation and different anions (i.e., <NUM>-butyl-<NUM>-methylimidazolium bromide [BmIm]Br, <NUM>-butyl-<NUM>-methylimidazolium tetrafluoroborate[BmIm]BF<NUM>, <NUM>-butyl-<NUM>-methylimidazolium bicarbonate [BmIm]HCO<NUM>, and <NUM>-butyl-<NUM>-methylimidazolium hydroxide [BmIm]OH) were used for transesterification of EC and MeOH, and found that [BmIm]OH was most effective, wherein when a molar ratio of methanol to ethylene carbonate was <NUM>:<NUM>, the catalyst was <NUM>% of the total raw material by weight, and the reaction was carried out at <NUM> for <NUM>, EC conversion efficiency was <NUM>%, and DMC selectivity was <NUM> % but the activity of the catalyst decreased to <NUM>% after the catalyst was reused five times. Hye-Young Ju et al disclosed that under the condition that <NUM> wt% <NUM>-ethyl-<NUM>-methylimidazolium chloride EMImCl was used as catalyst and the reaction was carried out at <NUM> for <NUM>, EC conversion efficiency was <NUM>% and DMC selectivity was <NUM>%. Zhenzhen Yang synthesized an ionic liquid with <NUM>, <NUM>-diazabicyclooctane as cation, and found that the catalytic activity and stability were comprehensively best when the anion is OH-, and the EC conversion efficiency was <NUM>%, the DMC yield was <NUM>% when the reaction was carried out at <NUM> for <NUM> with <NUM> mol% catalyst; and after the catalyst was reused four times, the catalytic activity decreased to <NUM>%, and the yield decreased to <NUM>%. Compared with sodium methylate (an equilibrium conversion efficiency was reached when the reaction was carried out for <NUM>), the above ionic liquids have low activity and low DMC selectivity. And the inactivation phenomenon still exists for the disclosed ionic liquids during several reuses. The process for preparing dimethyl carbonate by catalytic distillation of ethylene carbonate and methanol needs to meet the requirements that an azeotrope of dimethyl carbonate and methanol is taken from the column top at <NUM>-<NUM>, the temperature of the column bottom was kept at about <NUM>-<NUM>, and it took <NUM>-<NUM> for raw materials to fall into the column bottom from the column top through the catalytic distillation column. Therefore, a heterogeneous catalyst to be developed requires to exhibit high catalytic activity at <NUM>-<NUM>, and a reaction equilibrium requires to be reached within <NUM>. However, the activity of the heterogeneous catalyst disclosed by literatures and patents is significantly lower than the requirement of industrial application.

<CIT> discloses a method for synthesizing dimethyl carbonate under the catalysis action of an ionic liquid. The bifunctional ionic liquid is adopted as a catalyst, and a transesterification reaction is performed between ethylene carbonate and methanol, wherein the use amount of the catalyst is <NUM>-<NUM> mol% of ethylene carbonate, the molar ratio of ethylene carbonate to methanol is <NUM>:(<NUM>-<NUM>), the reaction temperature is <NUM>-<NUM> DEG C, and the reaction time is <NUM>-<NUM>, so that dimethyl carbonate and ethylene glycol are synthesized with a high conversion rate and high selectivity. The method has the following advantages: (<NUM>) the bifunctional ionic liquid adopted as the catalyst has the effect of a homogeneous reaction and the characteristic of a heterogeneous catalyst, the shortcomings of conventional solid catalysts and the homogeneous catalysts are overcome, and high catalytic activity, easy separation, recyclability and no pollution to the environment are achieved; (<NUM>) since the bifunctional ionic liquid is applied to the reaction system, reactants can be activated under the synergistic action of anions and cations of the catalyst, higher catalytic activity is achieved, and synthesis operation of the catalyst is simple and easy.

<CIT> discloses a method of synthesizing diethyl carbonate by a basic ionic liquid as a catalyst, belonging to the technical field of catalytic synthesis of carbonic ester by ionic liquids. According to the method, propylene carbonate and ethanol are taken as raw materials or ethylene carbonate and ethanol are taken as raw materials, and the basic ionic liquid is selected as the catalyst, so that diethyl carbonate is synthesized through ester exchange. Based on deficiencies of conventional catalysts (solid acids and alkalis, resin, alkali salt and the like) and advantages of the ionic liquid, the invention provides the method of synthesizing diethyl carbonate by the basic ionic liquid as the catalyst. The method has the advantages of high catalyst activity (selectivity is close to <NUM>%, and yield is <NUM>-<NUM>%), short synthetic route, mild reaction condition, less equipment corrosion, environment-friendliness, good stability of catalyst, easiness in separation and recovery, repeated use in maintaining high activity and the like.

According to one aspect of the present disclosure, a method for preparing dimethyl carbonate is provided. Aiming at the problems such as weak alkalinity and poor nucleophilicity of ionic liquids, poor thermal stability and easy inactivation of ionic liquids with strong alkalinity and the like disclosed in the literatures and patents, a new ionic liquid with strong alkalinity and having special structure and high temperature-resistant and high stability is developed, and a method for preparing a heterogeneous catalyst with ionic liquid embedded in a riveting manner based on the homogeneous ionic liquid is further developed. A series of developed ionic liquids with strong alkalinity are used for synthesizing dimethyl carbonate and ethylene glycol by transesterification between ethylene carbonate and methanol, and exhibit extremely high reaction activity. A reaction equilibrium could be reached when the reaction is carried out at a temperature ranging from <NUM> to70°Cfor <NUM> even with <NUM> wt% catalyst. The alkaline ionic liquids still exhibit certain catalytic activity even at <NUM>.

The synthesized ionic liquids exhibit substantially unchanged catalytic activity and show higher stability, even after reused <NUM> times. The prepared ionic liquids with strong alkalinity directly replace soluble strong alkali such as sodium hydroxide or potassium hydroxide to facilitate hydrolysis of single solution of ethyl orthosilicate, tetrabutyl titanate or aluminum silicate and the like, or hydrolysis of any mixed solution of thereof. The ionic liquid is embedded, by means of inlay, in silica, titanium dioxide, aluminum oxide or single/multiple oxygen-containing compound framework(s) with specific structure(s), without introducing Na+ or K+ ions at all. The prepared heterogeneous catalyst does not need to be baked, and can be directly applied to transesterification after being dried/vacuum dried, and exhibits excellent catalytic activity. Because the ionic liquid matrix has a double-nitrogen ring structure and can be embedded in (like a double-headed mountaineering cone) the crystal lattice of -Si-O-, -Ti-O-, -Al-O-or oxygen-containing composite compound, the immobilized heterogeneous catalyst shows higher stability, and the active component of ionic liquid with strong alkalinity is not easy to lose.

A method for preparing dimethyl carbonate, characterized in that, the method comprises following steps: contacting a raw material comprising ethylene carbonate and methanol with a catalyst to produce the dimethyl carbonate; wherein the catalyst comprises an ionic liquid with an anion and a cation, wherein the anion and the cation both comprise a nitrogen-containing heterocycle, wherein the catalyst further comprises a support, and the ionic liquid is embedded in the support in a riveting manner.

Optionally, the cation has a structure represented by Formula I or Formula II;
<CHM>
the anion has a structure represented by Formula III, Formula IV or Formula V;
<CHM>
wherein, R<NUM> and R<NUM> independently from each other are C1-C6 alkyl, C2-C6 alkenyl or C3-C6 aryl.

Optionally, the catalyst is in a range from <NUM>% to <NUM>% of the raw material by weight.

Optionally, the catalyst is in a range from <NUM>% to5% of the raw material by weight.

Optionally, the catalyst is in a range from <NUM>% to0. <NUM>% of the raw material by weight. Optionally, the reaction temperature is in a range from <NUM> to <NUM>.

Optionally, the reaction temperature is in a range from <NUM> to <NUM>.

Optionally, a molar ratio of ethylene carbonate to ethanol in the raw material comprising ethylene carbonate and methanol is in a range from <NUM>: <NUM> to <NUM>: <NUM>.

Optionally, the molar ratio of ethylene carbonate to ethanol in the raw material comprising ethylene carbonate and methanol is in a range from <NUM>: <NUM> to1: <NUM>.

Optionally, the molar ratio of ethylene carbonate to ethanol in the raw material comprising ethylene carbonate and methanol is in a range from <NUM>: <NUM> to <NUM>: <NUM>.

Optionally, the upper limit of the molar ratio of ethylene carbonate to ethanol in the raw material comprising ethylene carbonate and methanol is <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, or <NUM>: <NUM>, while the lower limit thereof is <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, or <NUM>:<NUM>. Optionally, the reaction is carried out for a reaction time ranging from <NUM> minute to <NUM>.

Optionally, a chemical equilibrium of the reaction is reached within <NUM> when the reaction temperature is at <NUM>.

Optionally, a chemical equilibrium is reached when the reaction time is <NUM> and the reaction temperature is <NUM>.

Optionally, R<NUM> and R<NUM> independently from each other are -CH<NUM>, -CH<NUM>CH<NUM>, - (CH<NUM>)<NUM>CH<NUM>, or- (CH<NUM>)<NUM>CH<NUM>.

The catalyst further comprises a support, and the ionic liquid is embedded in the support in a riveting manner.

Optionally, the ionic liquid is embedded in the framework of the support.

Optionally, at least one action of coupling and hydrogen bonding exists between the support and the ionic liquid.

Optionally, a weight ratio of the ionic liquid to the support in the catalyst is in a range of <NUM>~<NUM>: <NUM>~<NUM>.

Optionally, the support is at least one selected from silica, titanium dioxide and aluminum oxide, and the ionic liquid is embedded in the framework of the support during the preparation of the support.

Optionally, the catalyst further comprises a structure regulator, and the ionic liquid is embedded in a composite framework of the structure regulator and the support. The structure regulator has a valence of <NUM>, <NUM> or <NUM>, and is used to construct a framework structure of composite oxide(s) together with the support, so as to improve the environment where the ionic liquid is embedded, thereby enhancing acting force between the ionic liquid and the composite oxide of support.

Optionally, the structure regulator comprises at least one selected from Mg, Ca, Ba, La, Ce, Zr, Fe, Zn, Li, Cs, and Al.

Optionally, the weight content of the structure regulator in the catalyst is in a range from <NUM>% to <NUM>%.

Optionally, the ionic liquid is prepared by a method comprising the following steps: a1) adding an alkali into solution I comprising an ionic liquid anion source, and reacting to obtain ionic liquid anion metal salt; and a2) dissolving the ionic liquid anion metal salt in a solvent, then adding an ionic liquid cation salt, and reacting to obtain the ionic liquid.

Optionally, in step a1), the solvent in solution I is at least one selected from ethanol, benzene, toluene and xylene; the alkali is an organic alkali or an inorganic alkali, wherein the organic alkali comprises sodium methylate, sodium ethylate or sodium tert-butoxide, potassium methylate, potassium ethylate or potassium tert-butoxide, and the inorganic alkali comprises sodium hydroxide or potassium hydroxide; and the ionic liquid anion metal salt is at least one selected from the ionic liquid anion sodium salts and the ionic liquid anion kalium salts.

Optionally, in step a1), the concentration of the ionic liquid anion source in the solution I is in a range from <NUM>/mL to <NUM>/mL; and a molar ratio of the ionic liquid anion source to the alkali is in a range from <NUM> to <NUM>.

Optionally, in step a1), the ionic liquid anion source comprises imidazole, pyrrole or morpholine.

Optionally, in step a1), reaction conditions are as follows: the reaction temperature is in a range from <NUM> to <NUM> and the reaction time is in a range from <NUM> to <NUM>.

Optionally, step a1) further comprises: after the reaction is completed, removing the solvent in the reaction system to obtain imidazole anion salt, pyrrole anion salt or morpholine anion salt.

Optionally, in step a2), the solvent comprises a water-carrying agent which is at least one selected from ethanol, benzene, toluene and xylene; and the ionic liquid cation salt is at least one selected from <NUM>-R<NUM>-<NUM>-methyl-imidazolium bromide, <NUM>-R1-<NUM>-methylimidazolium iodide, N-methyl-N-R<NUM>-morpholinium bromide and N-methyl-N-R<NUM>-morpholinium iodide.

Optionally, in step a2), the ratio of the ionic liquid anion metal salt to the solvent is in a range of <NUM>~<NUM>: <NUM>~<NUM>/mL.

Optionally, in step a2), reaction conditions are as follows: the reaction temperature is in a range from <NUM>~<NUM> and the reaction time is in a range from12h to <NUM>.

Optionally, step a2) further comprises: after the reaction is completed, removing the solvent in the reaction system to obtain the ionic liquid.

Optionally, a method for preparing the catalyst comprises step b): adding water into a mixture comprising a support precursor and the ionic liquid, and carrying out hydrolysis to obtain the catalyst.

Optionally, in step b), the support precursor comprises at least one selected from ethyl orthosilicate, tetrabutyl titanate, aluminum isopropoxide, and sodium aluminate.

Optionally, the mixture in step b) further comprises a solvent which is <NUM>%~<NUM>% of the mixture by weight, and the solvent comprises at least one selected from methanol, ethanol, propanol, butanol, methyl acetate and ethyl acetate.

Optionally, in step b), a molar ratio of the support precursor, the ionic liquid and water is in a range of <NUM>~<NUM>: <NUM>~<NUM>: <NUM>~<NUM>.

Optionally, in step b), the hydrolysis temperature is in a range from <NUM> to <NUM>.

Optionally, in step b), the upper limit of the temperature of the hydrolysis is <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>, while the lower limit thereof is <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>.

Optionally, in step b), after the hydrolysis, aging step is carried out to obtain the catalyst, wherein the aging temperature is in a range from <NUM> to <NUM> and the aging time is in a range from <NUM> to24 h.

Optionally, in step b), the mixture comprises a structure regulator precursor which comprises at least one selected from corresponding acetate, silicate, hydrochloride and nitrate of the structure regulator.

Optionally, step b) further comprises: mixing the support precursor, the structure regulator precursor with the solvent, adding the ionic liquid, mixing uniformly, and then adding water to carry out hydrolysis to obtain the catalyst.

Optionally, in step b), a weight ratio of the support precursor, the structure regulator precursor, the solvent, the ionic liquid and water is in a range of <NUM>~<NUM>: <NUM>~<NUM>: <NUM>~<NUM>: <NUM>~<NUM>: <NUM>~<NUM>.

As an embodiment, the catalyst is a homogeneous ionic liquid with strong alkalinity.

A method for preparing the catalyst of homogeneous ionic liquid with strong alkalinity comprises the following steps:.

The imidazole-based, pyrrole-based or morpholine-based ionic liquid has the following structure, wherein the cation is <NUM>-R-<NUM>-methylimidazolium or N-methyl-N-R-morpholinium, and R is a normal or isomeric structure of alkane, alkene or arene; and the anion is imidazole, pyrrole or morpholine anion. <CHM>
R = -CH<NUM>, -CH<NUM>CH<NUM>, -(CH<NUM>)<NUM>CH<NUM>, -(CH<NUM>)<NUM>CH<NUM> or other structure of alkane, alkene and arene.

As an embodiment, the catalyst is a heterogeneous catalyst with the ionic liquid embedded in a riveting manner. In the method for preparing heterogeneous catalyst with the ionic liquid embedded in a riveting manner:.

In the heterogeneous catalyst, the content of the active ionic liquid is in a range from <NUM>% to30%, the content of the support is in a range from <NUM>% to60%, and the content of the structure regulator/structure stabilizer is in a range from10% to60%.

In the present disclosure, "[EmIm]Im" refers to <NUM>-ethyl-<NUM>-methylimidazolium imidazole salt.

In the present disclosure, "[EmIm]py" refers to <NUM>-ethyl-<NUM>-methylimidazolyl pyrrole salt.

In the present disclosure, "EC" refers to ethylene carbonate.

In the present disclosure, "DMC" refers to dimethyl carbonate.

In the present disclosure, "EG" refers to ethylene glycol.

In the present disclosure, "HEMC", namely intermediate product <NUM>, refers to "methanol glycol carbonate".

In the present disclosure, "intermediate product <NUM>" refers to "diethylene glycol carbonate".

In the present disclosure, C1-C6 refers to the number of carbon atoms contained. For example, the term "C1-C6 alkyl" refers to alkyl containing <NUM>-<NUM> carbon atoms.

In the present disclosure, the "alkyl" is a group formed from an alkane compound molecule by losing any one of hydrogen atoms. The alkane compound comprises straight-chain alkanes, branched-chain alkanes, cycloalkanes and branched-chain cycloalkanes.

In the present disclosure, the "alkenyl" is a group formed from an olefin compound molecule by losing any one of hydrogen atoms.

In the present disclosure, an "aryl" is a group formed from an aromatic compound molecule by losing one hydrogen atom on an aromatic ring; for example, the p-tolyl is formed from toluene by losing the hydrogen atom at the para-position with respect to the methyl group on the phenyl ring.

The beneficial effects achieved by the present disclosure are as follows:.

The present disclosure will be described in detail below with reference to examples, but is not limited to these examples.

Unless otherwise specified, the raw material and catalyst in the examples of the present disclosure are commercially available.

In the examples of the present disclosure, 1HNMR analysis, 13CNMR analysis, infrared spectrum analysis, SEM analysis, and XRD analysis are all conventional operations, and those skilled in the art can operate according to the instructions of the instrument.

The conversion efficiency and selectivity in the examples of the present disclosure are calculated as follows.

In the examples of the present disclosure, the conversion efficiency and selectivity are calculated based on the molar number of carbon.

Conversion efficiency of methanol is not involved in the examples. <MAT> <MAT> <MAT>.

A series of typical and commercialized ionic liquids were selected, and the following acidic ionic liquids were researched in terms of the effects on the transesterification of Ethylene Carbonate (EC) and methanol (MeOH): <NUM>-ethyl-<NUM>-methylimidazolium ethyl sulfate ([Emim]C<NUM>H<NUM>SO<NUM>), <NUM>-ethyl-<NUM>-methylimidazolium hexafluoroantimonate ([Emim]SbF<NUM>), <NUM>-ethyl-<NUM>-methylimidazolium p-methylbenzenesulfonate([Emim] ToS), <NUM>-butyl-<NUM>-methylimidazolium tetrafluoroborate ([Bmim]BF<NUM>), <NUM>-ethyl-<NUM>-methylimidazolium tetrafluoroborate ([Emim]BF<NUM>), <NUM>-butyl-<NUM>-methylimidazolium hexafluorophosphate ([Bmim]PF<NUM>), <NUM>-ethyl-<NUM>-methylimidazolium hexafluorophosphate ([Emim]PF<NUM>), <NUM>-butyl-<NUM>-methylimidazolium chloride ([Bmim]Cl), <NUM>-ethyl-<NUM>-methylimidazolium chloride ([Emim]Cl), <NUM>,<NUM>-dimethylimidazolium chloride ([Mmim]Cl), <NUM>-butyl-<NUM>-methylimidazolium bromide ([Bmim]Br), <NUM>-ethyl-<NUM>-methylimidazolium bromide ([Emim]Br), <NUM>-butyl-<NUM>-methylimidazolium iodide ([Bmim]I) and <NUM>-ethyl-<NUM>-methylimidazolium iodide ([Emim]I), <NUM>,<NUM>-dimethylimidazolium iodide ([Mmim]I). The results were shown in table <NUM>. The reaction conditions were as follows: EC: MeOH = <NUM>:<NUM>, reaction temperature was <NUM>, catalyst content was <NUM> wt%, and reaction time is <NUM>.

As seen from table <NUM>: the activities of the conventional and commercialized catalysts were not high, and under the condition that the optimal content of the ionic liquid [Mmim]I was <NUM>% of the reaction raw material by weight and reaction time is <NUM> , EC conversion efficiency of <NUM>%, DMC selectivity of <NUM>%, and DMC yield of only <NUM>% were achieved.

The specific preparation method of the <NUM>-ethyl-<NUM>-methylimidazolium imidazole salt is as follow: <NUM>. 5mol imidazole was dissolved in <NUM> ethanol solvent, and equimolar potassium ethoxide was added into a three-neck flask, and they were vigorously stirred for <NUM> at <NUM>; after the reaction was completed, rotary evaporation under reduced pressure was carried out at <NUM> in an oil bath for <NUM> to remove the solvent ethanol and the product ethanol, and the remains were placed in a vacuum drying oven and dried for <NUM> until no weight change occurred to obtain the imidazolium potassium salt; <NUM>. 5mol commercially available [EmIm]Br was added into a three-neck flask, and under the condition that a water-carrying agent ethanol was used as solvent, equimolar imidazolium potassium was added therein, and they were stirred and reacted at room temperature for <NUM>; after the reaction was completed, the resulting white solid KBr was removed by filteration, rotary evaporation under reduced pressure was carried out at <NUM> in an oil bath for <NUM> to remove the solvent, and then the remains were placed in a vacuum drying oven and dried for <NUM> until no weight change occurred, to obtain viscous liquid with light yellow, i.e. [EmIm]Im, of which nuclear magnetic resonance spectrums are shown in <FIG>. 1HNMR spectrum of synthesized [EmIm]Im ionic liquid in <FIG> shows the followings: [EmIm]Im 1HNMR (<NUM>, DMSO-d6) δ (ppm): <NUM> (d, J = <NUM>, <NUM>, NCHN), <NUM> (d, J = <NUM>, <NUM>, NCH), <NUM> (s, <NUM>, NCH), <NUM> (s, <NUM>, NCHCHN), <NUM> (q, J = <NUM>, <NUM>, NCH<NUM>CH<NUM>), <NUM> (s, <NUM>, NCH<NUM>), <NUM> (t, J = <NUM>, <NUM>, NCH<NUM>CH<NUM>). 13CNMR spectrum of synthesized [EmIm]Im ionic liquid in <FIG> shows the followings: <NUM>C NMR (<NUM>, DMSO-d<NUM>) δ (ppm): <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s). Therefore, the synthesized ionic liquid was determined to be <NUM>-ethyl-<NUM>-methylimidazolium imidazole salt.

The specific preparation method of the <NUM>-ethyl-<NUM>-methylimidazolium pyrrole salt is as follow: <NUM>. 5mol pyrrole was dissolved in <NUM> ethanol solvent, equimolar potassium ethoxide was added into a three-neck flask, and they were vigorously stirred for <NUM> at <NUM>; after the reaction was completed, rotary evaporation under reduced pressure was carried out for <NUM> at <NUM> in an oil bath to remove the ethanol solvent and the product ethanol, and the remains were placed into a vacuum drying oven and dried until no weight change occurred for <NUM> to obtain the pyrrole potassium; <NUM>. 5mol commercially available [EmIm]Br was added into a three-necked flask, and equimolar pyrrole potassium was added therein, and under the condition that a water-carrying agent ethanol is used as solvent, they were stirred at room temperature for <NUM>; after the reaction was completed, the resulting white solid KBr was removed by filtration, rotary evaporation under reduced pressure was carried out at <NUM> in an oil bath for <NUM> to remove the solvent, and the remains were placed in a vacuum drying oven and dried for <NUM> until no weight change occurred, to obtain viscous liquid with light yellow [EmIm]py, of which nuclear magnetic resonance spectrums are shown in <FIG>. 1HNMR spectrum of synthesized [EmIm]py ionic liquid in <FIG> shows the followings: [EmIm]Py<NUM>H NMR (<NUM>, DMSO-d<NUM>) δ (ppm): <NUM> (s, <NUM>, NCHN), <NUM> (s, <NUM>, NCHCHN), <NUM> (s, <NUM>, NCHCHN), <NUM> (d, J = <NUM>, <NUM>, NCHCH), <NUM> (d, J = <NUM>, <NUM>, CHCHCH), <NUM> (q, J = <NUM>, <NUM>,NCH<NUM>CH<NUM>), <NUM> (s, <NUM>, NCH<NUM>), <NUM> (t, J=<NUM>, <NUM>, NCH<NUM>CH<NUM>). 13CNMR spectrum of synthesized [EmIm]py ionic liquid in <FIG> shows the followings: <NUM>C NMR (<NUM>, DMSO-d<NUM>) δ (ppm) <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s), <NUM> (s). Therefore, the synthesized ionic liquid was determined to be <NUM>-ethyl-<NUM>-methylimidazolium pyrrole salt.

Table <NUM> shows the data of the synthesized ionic liquid in the laboratory for transesterification of EC and MeOH, and the preparation method of the ionic liquid in table <NUM> is the same as that of <NUM>-ethyl-<NUM>-methylimidazolium imidazole salt in example <NUM>, except that corresponding raw materials were changed. The evaluation conditions of the catalyst were as follows: a reaction temperature is <NUM>, a reaction time is <NUM>, a molar ratio of the raw material is EC:MeOH =<NUM>:<NUM>, and the catalyst is <NUM>% of the raw material by weight.

The reaction conditions were as follows: a mole ratio of EC to MeOH in the raw material was <NUM>:<NUM>, the amount of the catalyst was <NUM> wt%, and the reaction temperature was <NUM>.

As seen from table <NUM>, it is clear that the ionic liquid with strong alkalinity exhibits excellent catalytic activity, DMC and EG selectivity reaches <NUM>%, and expect for <NUM>-ethyl-<NUM>-methylimidazolium methoxide and <NUM>-ethyl-<NUM>-methylimidazolium hydroxide, all the rest ionic liquids approached equilibrium conversion efficiency of about <NUM>% under the reaction conditions.

<NUM>,<NUM>-dimethylimidazolium pyrrole was used as catalyst, the reaction temperature was <NUM>, the catalyst is <NUM>% of the total raw materials by weight, a molar ratio of EC to MeOH (methanol) was <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM> and <NUM>:<NUM> respectively, and the reaction time was <NUM>. The effects of different molar ratios of raw material on the transesterification reaction of EC and MeOH are shown in <FIG>. The reaction mechanism of EC and MeOH is as follow: Firstly, ring opening reaction of EC and MeOH generated an intermediate HEMC, and transesterification of HEMC and MeOH generated the target DMC. However, when strongly alkaline catalyst <NUM>, <NUM>-dimethylimidazolium pyrrole salt was used, even if the MeOH content was very low (EC: MeOH = <NUM>: <NUM>) and the DMC yield was only <NUM>%, the formation of the intermediate product HEMC and other byproducts were still not detected, indicating that the EC ring opening was the rate controlling step during the transesterification of EC and MeOH. At reaction temperature, it was not limited by thermodynamic equilibrium that transesterification of HEMC (intermediate <NUM>, methanol glycol carbonate) and MeOH generated DMC. DMC yield increased rapidly to <NUM>% when EC: MeOH = <NUM>:<NUM>, and DMC yield continued increasing as MeOH content increased, and DMC yield increased to <NUM>% when EC: MeOH =<NUM>:<NUM>, and DMC yield increased with small extent to <NUM>% when a molar ratio of MeOH to EC was <NUM>:<NUM>. The transesterification of EC and MeOH is a reversible process, and the conversion of EC could be promoted by increasing the proportion of raw material MeOH, so that the reaction could continue in the positive reaction direction, and thus the DMC yield could increase.

N-methyl-N-ethylmorpholinium morpholine salt was used as catalyst, which is <NUM>% of the total raw materials by weight. The DMC selectivity and DMC yield vary with temperature and reaction time, which are shown in <FIG>. The DMC yield was <NUM> % when the reaction was carried out at <NUM> for <NUM>. Although the EC conversion efficiency was low at the time, the DMC selectivity was still higher than <NUM>%. When the reaction was carried out at <NUM> for <NUM>, the EC conversion efficiency increased to <NUM>%, the DMC selectivity was <NUM>%, and the DMC yield was <NUM>%, indicating that N-methyl-N-ethylmorpholinium morpholine salt still had a relatively strong ability of promoting transesterification of HEMC and MeOH even at low temperature. The yield was basically the same as that of [Mmim]I (the ionic liquid was <NUM>% of the total raw material by weight) which had the best effects among the commercialized ionic liquids. The above data fully show that N-methyl-N-ethylmorpholinium morpholine salt ionic liquid still exhibits relatively high transesterification activity even at <NUM>. As the reaction temperature gradually increased, the DMC yield increased and the DMC selectivity had been maintained as <NUM>%, without generating any intermediate product. The DMC yield could reach <NUM>% when the reaction was carried out at <NUM> for <NUM>. However, DMC yield reached <NUM>% when the reaction was carried out at <NUM> (reflux temperature) for <NUM>, and DMC yield was basically the same as that for <NUM> when the reaction was continued to <NUM>, indicating that a chemical equilibrium was basically reached when the reaction was carried out at the reflux temperature for <NUM> and the activity of N-methyl-N-ethylmorpholinium morpholine ionic liquid was extremely high, the product selectivity was <NUM>%, and no intermediate product and byproduct were generated at all.

<NUM>-ethyl-<NUM>-methylimidazolium hydroxide and N-methyl-N- ethylmorpholinium pyrrole salt were respectively used as catalysts to investigate their stabilities. Under the condition of <NUM> wt% ionic liquid, EC:MeOH (molar ratio) = <NUM>:<NUM>, the reaction was carried out at <NUM> and ended after <NUM>. The resulting mixture was sampled and analyzed. Then rotary evaporation under reduced pressure -<NUM> MPa was carried out at <NUM>-<NUM> for <NUM>, and the remaining ionic liquid was recycled for next cycle. <FIG> shows the relationship between the DMC yield and the reuse times of <NUM>-ethyl-<NUM>-methylimidazolium hydroxide (i. e, "<NUM>-ethyl-<NUM>-methylcholine" in <FIG>) ionic liquid and N-methyl-N-ethylmorpholinium pyrrole ionic liquid. The DMC yield was <NUM>% when the <NUM>-ethyl-<NUM>-methylimidazolium alkali was used for the first time, but the DMC yield decreased to <NUM>% when the catalyst was reused once, which was only about a quarter of the initial activity. DMC yield continued decreasing as the reuse number increased, and the DMC yield was only <NUM>% when the catalyst was reused four times. The above results fully show that [Emim]OH has poor stability, can only be used once, and deactivates quickly. In contrast, the synthesized N-methyl-N-ethylmorpholinium pyrrole ionic liquid was reused <NUM> times, the DMC yield maintained at <NUM>%-<NUM>%, the DMC selectivity was <NUM>%, no deactivation occurred, and excellent stability was exhibited with only <NUM>. 3wt% thereof. It can be seen that N-methyl-N-ethylmorpholinium pyrrole has not only good low-temperature catalytic activity, product selectivity of <NUM>%, but also excellent thermal stability and reuse performance.

A single silica obtained by hydrolyzing ethyl orthosilicate was used as support, and <NUM>-ethyl-<NUM> methylimidazolium imidazole ionic liquid is embedded therein in a riveting manner to prepare a heterogeneous catalyst with silica. <NUM> ethyl orthosilicate and <NUM> ethanol were added to a conical flask respectively, when they were heated to <NUM>, about <NUM> <NUM>-ethyl-3methylimidazolium imidazole ionic liquid was immediately added to them, and after even mixing thereof, <NUM> deionized water was added therein. Among them, <NUM>-ethyl-<NUM> methylimidazolium imidazole ionic liquid not only plays a role of alkali catalysis to facilitate the hydrolysis of ethyl orthosilicate, but also serves as active center of transesterification reaction. After adding deionized water, the system gelled quickly. The gelled system was aged at <NUM> for <NUM>, then dried at <NUM> for <NUM>, washed three times with <NUM> ethanol, and vacuum dried at <NUM> for <NUM>, to obtain an immobilized ionic liquid catalyst. The physical photo of the catalyst prepared in embedding-riveting manner was shown in <FIG>. As seen from the physical photo, the catalyst prepared in embedding-riveting manner has apparent color of light yellow and has a crunchy texture.

A fixed-bed reactor was used to evaluate the prepared heterogeneous catalyst. Reaction conditions were as follows: a molar ratio of ethylene carbonate to methanol in the reaction raw materials was <NUM>:<NUM>, the reaction temperature was <NUM>-<NUM>, the filling weight of catalyst was <NUM>, and weight hourly space velocity of the reaction raw materials was <NUM>-<NUM>, that is, feeding weight per hour for the raw material was <NUM>, and the continuous evaluation time for each catalyst was not more than <NUM>. Table <NUM> shows the activity of the catalyst that was obtained by embedding <NUM> <NUM>-ethyl-<NUM> methylimidazolium imidazole ionic liquid at <NUM>-<NUM>. The catalysts with SiO<NUM> as embedding support, which were prepared by hydrolysis at different temperatures, all exhibited DMC selectivity higher than <NUM>%, but EC conversion efficiency of the reaction raw materials had significant differences. When the catalyst was hydrolyzed at <NUM>, EC conversion efficiency was up to <NUM>%. When the catalyst was hydrolyzed at <NUM>, EC conversion efficiency was only <NUM>%, which was very low. When the catalyst was hydrolyzed at <NUM>, EC conversion efficiency was <NUM>%,which was moderate.

Under a single silica obtained by hydrolyzing ethyl orthosilicate was used as support and hydrolysis was carried out at <NUM>, <NUM> <NUM>-ethyl-<NUM>-methylimidazolium pyrrole salt, <NUM> <NUM>-ethyl-<NUM>-methyl imidazolium morpholine salt, <NUM> <NUM>-butyl-<NUM>-methylimidazolium imidazole salt, <NUM> <NUM>,<NUM>-dimethylimidazolium pyrrole salt, <NUM> N-methyl-N-ethylmorpholinium morpholine salt and <NUM> N-methyl-N-butylmorpholinium pyrrole salt were embedded therein, respectively. A fixed-bed reactor was used to evaluate the prepared heterogeneous catalyst. Reaction conditions were as follows: a molar ratio of ethylene carbonate to methanol in the reaction raw material was <NUM>:<NUM>, the reaction temperature was <NUM>-<NUM>, the filling weight of the catalyst was <NUM>, and weight hourly space velocity of the reaction raw materials was <NUM>-<NUM>, that is, the feeding weight per hour for the raw material was <NUM>, and the continuous evaluation time for each catalyst was not more than <NUM>. Table <NUM> shows the activities of catalysts with different ionic liquids embedded.

A single silica obtained by hydrolyzing ethyl orthosilicate was used as support, and different ionic liquids were embedded therein to obtain the catalysts. The catalysts all exhibited excellent capability in the transesterification of EC and MeOH, with EC conversion efficiency of above <NUM>% and DMC selectivity of above <NUM>%. Among them, <NUM>,<NUM>-dimethylimidazolium pyrrole salt showed the best catalytic efficiency, with EC conversion efficiency of <NUM>% and DMC selectivity of <NUM>%.

Under a single silica obtained by hydrolyzing ethyl orthosilicate was used as support and <NUM>,<NUM>-dimethylimidazolium pyrrole salt was used as active component, the effect of the prepared catalyst with reduced amount of the active component on the transesterification efficiency was studied. The catalyst was prepared as follow: <NUM> ethyl orthosilicate and <NUM> ethanol were added into a conical flask, when they were heated to <NUM>, <NUM> <NUM>,<NUM>-dimethylimidazolium pyrrole salt ionic liquid was immediately added therein, after even mixing, <NUM> deionized water was added therein, and the system gelled quickly; the gelled system was aged at <NUM> for <NUM>, and dried at <NUM> for <NUM>, then washed three times with <NUM> ethanol, and vacuum dried at <NUM> for <NUM> to obtain an immobilized catalyst.

Under the single silica obtained by hydrolyzing ethyl orthosilicate was used as support and even <NUM> <NUM>,<NUM>-dimethylimidazolium pyrrole salt ionic liquid was embedded therein, the catalyst obtained showed EC conversion efficiency of <NUM>% and DMC selectivity of <NUM>%.

Under titanium dioxide obtained by hydrolyzing tetrabutyl titanate was used as support and hydrolysis was carried out at <NUM>, <NUM> <NUM>-ethyl-<NUM>-methylimidazolium pyrrole salt, <NUM> <NUM>-ethyl-<NUM>-methylimidazolium morpholine salt, <NUM> <NUM>-butyl-<NUM>-methylimidazolium imidazole salt, <NUM> <NUM>,<NUM>-dimethylimidazolium pyrrole salt, <NUM> N-methyl-N-ethylmorpholinium morpholine salt and <NUM> N-methyl-N-butylmorpholinium pyrrole salt were embedded in an embedding-riveting manner, respectively. A fixed-bed reactor was used to evaluate the prepared heterogeneous catalyst. Reaction conditions were as follows: a molar ratio of ethylene carbonate to methanol in the reaction raw material was <NUM>:<NUM>, the reaction temperature was <NUM>-<NUM>, the filling weight of the catalyst was <NUM>, weight hourly space velocity of the raw material was <NUM>-<NUM>, that is, feeding weight per hour for the raw material was <NUM>, and the continuous evaluation time for each catalyst was not more than <NUM>. Table <NUM> shows the activities of catalysts with different ionic liquids embedded.

Under different ionic liquids were embedded-riveted in titanium dioxide as support obtained by hydrolyzing tetrabutyl titanate, the obtained catalysts exhibited significantly lower catalytic activity than those with different ionic liquids embedded-riveted in silica as support, with EC conversion efficiency of <NUM>%-<NUM>%, and DMC selectivity (except for <NUM>,<NUM>-dimethylimidazolium pyrrole salt) less than <NUM>%.

An oxide obtained by hydrolyzing ethyl orthosilicate and tetrabutyl titanate was used as support, and <NUM>-ethyl-<NUM>-methylimidazolium pyrrole salt ionic liquid was embedded therein. <NUM> ethyl orthosilicate and <NUM> ethanol were added to a conical flask, <NUM> tetrabutyl titanate was added therein, then the structure regulator (<NUM> zinc acetate, <NUM> magnesium acetate and <NUM> sodium aluminate) was added therein, and after even mixing, <NUM> deionized water was added. The resulting mixture was aged at <NUM> for <NUM>, dried at <NUM> for <NUM>, then washed three times with <NUM> ethanol, and vacuum dried at <NUM> for <NUM> to obtain immobilized ionic liquid catalyst. A fixed-bed reactor was used to evaluate the prepared heterogeneous catalyst. Reaction conditions were as follows: a molar ratio of ethylene carbonate to methanol in the reaction raw material was <NUM>:<NUM>, the reaction temperature was <NUM>-<NUM>, the filling weight of the catalyst was <NUM>, and weight hourly space velocity of the reaction raw material is <NUM>-<NUM>, that is, feeding weight for the raw material was <NUM>, and the continuous evaluation time for each catalyst was not more than <NUM>. The EC conversion efficiency was <NUM>%, and the DMC selectivity was <NUM>%, significantly higher ethylene carbonate transesterification efficiency than those of various catalysts with ionic liquids embedded in tetrabutyl titanate alone, and the DMC yield is <NUM> times higher than those of the catalysts with ionic liquids embedded in tetrabutyl titanate alone.

<FIG> showed infrared spectrums of the catalysts obtained by impregnating silica as support with <NUM> <NUM>-ethyl-<NUM>-methyl-imidazolium imidazole salt and the catalysts obtained by embedding silica as support with <NUM>, <NUM> and <NUM> <NUM>-ethyl-<NUM>-methyl-imidazolium imidazole salt, respectively. The strong absorption peak at <NUM>-<NUM> was the stretching vibration characteristic peak of Si-O-Si, the absorption peak at <NUM>-<NUM> was the stretching vibration peak of alcoholic O-H group, the absorption peak at <NUM>-<NUM> was the characteristic absorption peak of C=C on the imidazole ring, and the characteristic absorption peak of the imidazole ring was at <NUM>-<NUM>. It showed that the developed method for preparing immobilized ionic liquid catalyst in an embedding manner was reliable and feasible, and the active component ionic liquid was determined to be embedded in the heterogeneous catalyst. The height of the characteristic peak of imidazole in the prepared catalyst increased strictly and regularly in accordance with the law of embedding weight of <NUM>, <NUM> and <NUM>. The catalyst prepared by impregnating with <NUM> ionic liquid also showed that characteristic peak intensity of imidazole thereof was higher than that shown by the catalysts obtained by embedding <NUM> ionic liquid, but lower than that shown by the catalysts obtained by embedding <NUM> ionic liquid, which was substantially consistent with their catalytic activity.

<FIG> showed SEM images of the catalysts obtained by impregnating silica as support with <NUM> <NUM>-ethyl-<NUM>-methyl-imidazolium imidazole salt and the catalysts obtained by embedding <NUM>, <NUM> and <NUM> <NUM>-ethyl-<NUM>-methyl-imidazolium imidazole salt in silica as support, respectively.

Claim 1:
A method for preparing dimethyl carbonate comprising the following steps:
contacting a raw material comprising ethylene carbonate and methanol with a catalyst to produce dimethyl carbonate;
wherein the catalyst comprises an ionic liquid with an anion and a cation;
wherein the anion and the cation both comprise a nitrogen-containing heterocycle;
wherein the catalyst further comprises a support, and the ionic liquid is embedded in the support in a riveting manner.