Method for preparing maleate by selective catalytic oxidation of lignin

Abstract

The invention discloses a method for preparing maleate by selective catalytic oxidation of lignin. The method uses a heteropolyacid functionalized ionic liquid as a catalyst, and an aqueous alcohol solution as a reaction medium to achieve high-efficiency selective catalytic conversion and ring opening oxidation of biomass lignin at a reaction temperature of 110-160 C. and an oxygen pressure of 0.5-1.0 MPa for 1-6 h. The selectivity of maleate is higher than 47.83%. The yield and selectivity of a single chemical derived from the depolymerization of lignin in the present invention are much higher than the prior art, and the ionic liquid catalyst exhibits outstanding advantages such as availability of recovery and recycling through simple temperature adjustment; the reaction conditions of the present invention are mild, and the process is green and safe, easy to operate, and available for batch and continuous production. The invention provides a new green way for preparing bulk chemicals like maleate from reproducible raw materials such as lignin.

Claims

1. A method for preparing maleate by selective catalytic oxidation of lignin, comprising: adding lignin, a heteropolyacid functionalized ionic liquid, and an aqueous alcohol solution to a autoclave, and reacting at 110-160 C. under 0.5-1.0 MPa oxygen atmosphere for 1-5 h; after the reaction, centrifuging the reaction solution, conducting liquid-liquid separation to obtain maleate and the ionic liquid catalyst; wherein the heteropolyacid functionalized ionic liquid consists of a cation and an anion; the cation consists of at least one member selected from a alkyl or alkanesulfonate imidazole cation or a pyridine, and the anion consists of at least one member selected from a-phosphotungstate, a phosphomolybdate, a silicotungstate, and a silicomolybate.

2. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 1, wherein the cation has an alkyl chain length of C1-C6.

3. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 1, wherein the heteropolyacid functionalized ionic liquid is prepared by the following steps: (1) weighing an equimolar amount of N-alkylimidazole and butane sultone and reacting at 30-50 C. for 12-24 h; after the reaction, washing with diethyl ether and drying under vacuum to obtain a white solid internal salt; wherein the N-alkylimidazole has a carbon chain length of C1-C6; (2) dissolving the phosphotungstic acid in deionized water and stirring until completely dissolved; weighing a carbonate with half of the molar amount of phosphotungstic acid, slowly adding it to the phosphotungstic acid solution, stirring at room temperature to obtain a homogeneous solution; removing the solvent, and drying under vacuum for 12-48 h to obtain a heteropolyacid salt; (3) weighing an equimolar amount of the heteropolyacid salt and the internal salt, completely dissolving them in deionized water to prepare two solutions respectively, adding the inner salt solution dropwise to the heteropolyacid salt solution with stirring, then stirring at room temperature for 12-48 h; (4) after the reaction, removing the solvent by rotary evaporation, and drying the obtained solid under vacuum to obtain a heteropolyacid functionalized ionic liquid.

4. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 3, wherein said drying under vacuum of steps (1) and (2) is conducted at 40-60 C.; said drying the obtained solid under vacuum in step (4) is conducted at 60-80 C. in a vacuum drying oven for 12-48 h.

5. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 3, wherein the lignin is obtained by sufficiently drying a lignin raw material, pulverizing it to 40-60 mesh, washing the soluble component and the ash thereof with deionized water and sufficiently drying for use; mixing 1 part by mass of H.sub.2SO.sub.4 and 10-20 parts by mass of the treated lignin raw material thoroughly, adding 50-200 parts by mass of a 75% by volume aqueous ethanol solution, introducing an inert atmosphere, keeping the reaction temperature at 100-120 C., and after the reaction, conducting separation and vacuum drying; and the lignin raw material consists of at least one member selected from bagasse, cassava, corn cobs, and corn stover.

6. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 3, wherein said stirring at room temperature lasts for 12-24 h.

7. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 1, wherein the alcohol in the aqueous alcohol solution is methanol, ethanol, n-propanol or isopropanol, and the volume concentration of the aqueous alcohol solution is 10-100%.

8. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 1, wherein the aqueous alcohol solution is used in an amount of 10-80 mL per gram of lignin, and the heteropolyacid functionalized ionic liquid is used in an amount of 0.5-3 mmol per gram of lignin.

9. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 1, wherein the ionic liquid catalyst is used after recovery.

10. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 2, wherein the heteropolyacid functionalized ionic liquid is prepared by the following steps: (1) weighing an equimolar amount of N-alkylimidazole and butane sultone and reacting at 30-50 C. for 12-24 h; after the reaction, washing with diethyl ether and drying under vacuum to obtain a white solid internal salt; wherein the N-alkylimidazole has a carbon chain length of C1-C6; (2) dissolving phosphotungstic acid in deionized water and stirring until completely dissolved; weighing a carbonate with half of the molar amount of the phosphotungstic acid, slowly adding it to the phosphotungstic acid solution, stirring at room temperature to obtain a homogeneous solution; removing the solvent, and drying under vacuum for 12-48 h to obtain a heteropolyacid salt; (3) weighing an equimolar amount of the heteropolyacid salt and the internal salt, completely dissolving them in deionized water to prepare two solutions respectively, adding the inner salt solution dropwise to the heteropolyacid salt solution with stirring, then stirring at room temperature for 12-48 h; (4) after the reaction, removing the solvent by rotary evaporation, and drying the obtained solid under vacuum to obtain a heteropolyacid functionalized ionic liquid.

11. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 10, wherein said drying under vacuum of steps (1) and (2) is conducted at 40-60 C.; said drying the obtained solid under vacuum in step (4) is conducted at 60-80 C. in a vacuum drying oven for 12-48 h.

12. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 10, wherein the lignin is obtained by sufficiently drying a lignin raw material, pulverizing it to 40-60 mesh, washing the soluble component and the ash thereof with deionized water and sufficiently drying for use; mixing 1 part by mass of H.sub.2SO.sub.4 and 10-20 parts by mass of the treated lignin raw material thoroughly, adding 50-200 parts by mass of a 75% by volume aqueous ethanol solution, introducing an inert atmosphere, keeping the reaction temperature at 100-120 C., and after the reaction, conducting separation and vacuum drying; and the lignin raw material consists of at least one member selected from bagasse, cassava, corn cobs, and corn stover.

13. The method for preparing maleate by selective catalytic oxidation of lignin according to claim 10, wherein said stirring at room temperature lasts for 12-24 h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the recycling performance of the 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid catalyst in Example 1.

(2) FIG. 2 is a GC-MS diagram of the lignin catalytic oxidation product in Example 1.

(3) FIG. 3 is a mass spectrum of diethyl maleate, the main product of lignin catalytic oxidation in Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) In order to better understand the present invention, the present invention will be further described below in conjunction with the embodiments, but the embodiments of the present invention are not limited thereto.

Example 1

(5) 1. Preparation of Ionic Liquid BSmimCuPW.sub.12O.sub.40:

(6) (1) Weighing an equimolar amount of N-methylimidazole and 1,4-butane sultone and reacting at 50 C. for 24 h; after the reaction, washing with diethyl ether and drying under vacuum at 60 C. to obtain a white solid internal salt;

(7) (2) dissolving the phosphotungstic acid in deionized water, stirring until the phosphotungstic acid is completely dissolved; weighing basic copper carbonate with half of the molar amount of phosphotungstic acid, slowly adding to the phosphotungstic acid solution, stirring at room temperature for 24 h to obtain a light blue solution; then removing the solvent by a rotary evaporator at 80 C., conducting vacuum drying at 60 C. for 12 h to obtain a heteropolyacid CuHPW.sub.12O.sub.40;

(8) (3) weighing an equimolar amount of the heteropolyacid CuHPW.sub.12O.sub.40 and the inner salt prepared in step (1), dissolving the heteropolyacid CuHPW.sub.12O.sub.40 and the inner salt in deionized water, respectively, then dropwise adding the inner salt solution to the heteropolyacid CuHPW.sub.12O.sub.40 solution with stirring, and allowing them to react at room temperature for 48 h;

(9) (4) after the reaction, removing the solvent by rotary evaporation to obtain a blue solid, drying under vacuum at 60 C. for 48 h to obtain the heteropolyacid functionalized ionic liquid, namely 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40).

(10) 2. Preparation of Bagasse Lignin:

(11) (1) Pretreatment of agricultural waste: after the bagasse is sufficiently dried, pulverizing it to below 60 mesh by mechanical pulverization, washing the soluble component and ash thereof with deionized water and sufficiently drying the bagasse for use;

(12) (2) Extraction of organic dissolvable lignin: mixing 1 part by mass of H.sub.2SO.sub.4 and 15 parts by mass of bagasse thoroughly, introducing an inert atmosphere, and keeping the reaction temperature at 120 C.; after the reaction, conducting separation and vacuum drying to obtain high purity bagasse lignin.

(13) 3. Method for Selective Catalytic Oxidation of Lignin:

(14) Accurately weighing 0.25 g bagasse lignin, 20 mL 80% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The catalyst was used for the catalyst cycle performance test, and after the catalyst was used for 5 times, no significant decrease of the catalytic activity was observed (FIG. 1). The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate is calculated by the ratio of the mass difference between the lignin raw material and unreacted lignin to the mass of the lignin raw material. The product of catalytic oxidation of lignin was qualitatively analyzed by GC/MS (FIG. 2 and FIG. 3) and quantified by gas chromatography. The selectivity of diethyl maleate is obtained from the ratio between the mass obtained by gas chromatography and the mass of all products. After the test, it was found that the lignin conversion rate was 90.7%, and the yield and selectivity of diethyl maleate were 153.60 mg/g and 59.32%, respectively.

(15) The method of the invention discloses obtaining maleate directly from the catalytic oxidation of lignin for the first time. Compared with the current lignin treating process, it has advantages of mild reaction conditions, green and safe reaction process, high conversion rate of raw material, high product yield and selectivity (currently catalytic depolymerization products of lignin are mainly phenolic compounds, the highest yield of which is 23%, and single compound selectivity of which is less than 30%), easy recovery of the catalyst with high recovery rate and good recyclability, and availability of batch and continuous production.

Example 2

(16) The difference between Example 2 and Example 1 is:

(17) 1. Preparation of Ionic Liquid 1-(4-sulfobutyl)-3-ethylimidazolium phosphotungstate Nickel salt BSeimNiPW.sub.12O.sub.40:

(18) (1) Weighing an equimolar amount of N-ethylimidazole and 1,4-butane sultone and reacting at 30 C. for 18 h; after the reaction, washing with diethyl ether and drying under vacuum at 50 C. to obtain a white solid internal salt;

(19) (2) dissolving the phosphotungstic acid in deionized water, stirring until the phosphotungstic acid is completely dissolved; weighing Nickel carbonate with half of the molar amount of phosphotungstic acid, slowly adding to the phosphotungstic acid solution, stirring at room temperature for 18 h; then removing the solvent by a rotary evaporator at 70 C., conducting vacuum drying at 50 C. for 48 h to obtain a heteropolyacid NiHPW.sub.12O.sub.40;

(20) (3) weighing an equimolar amount of the heteropolyacid NiHPW.sub.12O.sub.40 and the inner salt prepared in step (1), dissolving the heteropolyacid NiHPW.sub.12O.sub.40 and the inner salt in deionized water, respectively, then dropwise adding the inner salt solution to the heteropolyacid NiHPW.sub.12O.sub.40 solution with stirring, and allowing them to react at room temperature for 48 h;

(21) (4) after the reaction, removing the solvent by rotary evaporation to obtain a solid, and drying under vacuum at 70 C. for 36 h to obtain the heteropolyacid functionalized ionic liquid BSeimNiPW.sub.12O.sub.40;

(22) 3. Method for Selective Oxidation of Bagasse Lignin:

(23) Accurately weighing 0.5 g bagasse lignin, 50 mL 70% (volume concentration) aqueous ethanol solution, 1.8 mmol 1-(4-sulfobutyl)-3-ethylimidazolium phosphotungstate Nickel salt ionic liquid (BSeimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 77.9%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of dimethyl maleate were 71.61 mg/g and 58.84%, respectively.

Example 3

(24) The difference between Example 3 and Example 1 is:

(25) 1. Preparation of Ionic Liquid 1-(4-sulfobutyl)-3-methylimidazolium Phosphotungstate Manganese Salt BSmimMnPW.sub.12O.sub.40:

(26) (1) Weighing an equimolar amount of N-methylimidazole and 1,4-butane sultone and reacting at 50 C. for 24 h; after the reaction, washing with diethyl ether and drying under vacuum at 60 C. to obtain a white solid internal salt;

(27) (2) dissolving the phosphotungstic acid in deionized water, stirring until the phosphotungstic acid is completely dissolved; weighing Manganese carbonate with half of the molar amount of phosphotungstic acid, slowly adding to the phosphotungstic acid solution, stirring at room temperature for 24 h; then removing the solvent by a rotary evaporator, conducting vacuum drying at 80 C. for 30 h to obtain a heteropolyacid MnHPW.sub.12O.sub.40;

(28) (3) weighing an equimolar amount of the heteropolyacid MnHPW.sub.12O.sub.40 and the inner salt prepared in step (1), dissolving the heteropolyacid NiHPW.sub.12O.sub.40 and the inner salt in deionized water, respectively, then dropwise adding the inner salt solution to the heteropolyacid MnHPW.sub.12O.sub.40 solution with stirring, and allowing them to react at room temperature for 48 h;

(29) (4) after the reaction, removing the solvent by rotary evaporation to obtain a solid, and drying under vacuum at 70 C. for 36 h to obtain the heteropolyacid functionalized ionic liquid BSmimMnPW.sub.12O.sub.40;

(30) 3. Method for Selective Oxidation of Bagasse Lignin:

(31) Accurately weighing 0.25 g corn stover lignin, 20 mL 30% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-ethylimidazolium phosphotungstate Manganese salt ionic liquid (BSmimMnPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 1.0 MPa, and reacting at 140 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 80.9%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 87.65 mg/g and 67.36%, respectively.

Example 4

(32) The difference between Example 4 and Example 1 is:

(33) 1. Preparation of Ionic Liquid 1-(4-sulfobutyl)-3-butylimidazolium Phosphotungstate Sodium Salt BSbimNa.sub.2PW.sub.12O.sub.40:

(34) (1) Weighing an equimolar amount of N-butylimidazole and 1,4-butane sultone and reacting at 40 C. for 24 h; after the reaction, washing with diethyl ether and drying under vacuum at 60 C. to obtain a white solid internal salt;

(35) (2) dissolving the phosphotungstic acid in deionized water, stirring until the phosphotungstic acid is completely dissolved; weighing Sodium carbonate with half of the molar amount of phosphotungstic acid, slowly adding to the phosphotungstic acid solution, stirring at room temperature for 24 h; then removing the solvent by a rotary evaporator, conducting vacuum drying at 60 C. for 36 h to obtain a heteropolyacid Na.sub.2HPW.sub.12O.sub.40;

(36) (3) weighing an equimolar amount of the heteropolyacid Na.sub.2HPW.sub.12O.sub.40 and the inner salt prepared in step (1), dissolving the heteropolyacid Na.sub.2HPW.sub.12O.sub.40 and the inner salt in deionized water, respectively, then dropwise adding the inner salt solution to the heteropolyacid Na.sub.2HPW.sub.12O.sub.40 solution with stirring, and allowing them to react at room temperature for 48 h;

(37) (4) after the reaction, removing the solvent by rotary evaporation to obtain a solid, and drying under vacuum at 80 C. for 14 h to obtain the heteropolyacid functionalized ionic liquid BSbimNa.sub.2PW.sub.12O.sub.40;

(38) 3. Method for Selective Oxidation of Bagasse Lignin:

(39) Accurately weighing 0.25 g bagasse lignin, 20 mL 70% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-butylimidazolium phosphotungstate Sodium salt ionic liquid (BSbimNa.sub.2PW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 81.3%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 53.62 mg/g and 77.50%, respectively.

Example 5

(40) The difference between Example 5 and Example 1 is:

(41) Method for Selective Oxidation of Bagasse Lignin:

(42) Accurately weighing 1.0 g bagasse lignin, 20 mL 80% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.5 MPa, and reacting at 150 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 83.2%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 38.38 mg/g and 48.33%, respectively.

Example 6

(43) The difference between Example 6 and Example 1 is:

(44) Method for Selective Oxidation of Bagasse Lignin:

(45) Accurately weighing 0.25 g bagasse lignin, 20 mL 80% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 4 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 88.3%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 133.41 mg/g and 60.18%, respectively.

Example 7

(46) The difference between Example 7 and Example 1 is:

(47) Method for Selective Oxidation of Bagasse Lignin:

(48) Accurately weighing 0.25 g bagasse lignin, 20 mL 80% (volume concentration) aqueous ethanol solution, 1.5 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 6 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 93.1%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 142.31 mg/g and 61.25%, respectively.

Example 8

(49) The difference between Example 8 and Example 1 is:

(50) Method for Selective Oxidation of Bagasse Lignin:

(51) Accurately weighing 0.25 g cassava lignin, 20 mL 100% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 82.3%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 95.71 mg/g and 47.83%, respectively.

Example 9

(52) The difference between Example 9 and Example 1 is:

(53) Method for Selective Oxidation of Bagasse Lignin:

(54) Accurately weighing 0.25 g dealkali lignin, 20 mL 100% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 92.3%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 162.22 mg/g and 73.21%, respectively.

Example 10

(55) The difference between Example 10 and Example 1 is:

(56) Method for Selective Oxidation of Bagasse Lignin:

(57) Accurately weighing 0.25 g bagasse lignin, 20 mL 100% (volume concentration) aqueous ethanol solution, 0.9 mmol 1-(4-sulfobutyl)-3-methylimidazolium phosphotungstate copper salt ionic liquid (BSmimCuPW.sub.12O.sub.40) in a 100 mL autoclave, sealing it, replacing the atmosphere with high purity oxygen for 5 times, pressurizing to 0.8 MPa, and reacting at 160 C. for 5 h. After the reaction, cooling the mixture, and centrifuging to obtain the ionic liquid catalyst, which is then vacuum-dried at 60 C. for 24 hours. The ionic liquid supernate was made up to 25 mL, and 10 mL of the made up solution was added to 25 mL of deionized water to precipitate unreacted lignin. The lignin conversion rate was 96.5%. Another 10 mL of the made up solution was used for composition analysis in the same manner as in Example 1. The yield and selectivity of diethyl maleate were 147.33 mg/g and 57.15%, respectively.

(58) The embodiments of the present invention are not limited to the above-described embodiments, and any other changes, modifications, substitutions, combinations, and simplifications that are made without departing from the spirit and scope of the present invention should be equivalent and within the scope of the invention.