A hydrophobic gemini imidazole compound, a preparation method thereof and application thereof in removing lignin in corn stalks

Abstract

The application provides a hydrophobic gemini imidazole compound, a preparation method thereof and application of the hydrophobic gemini imidazole compound in removal of lignin in corn stalks. n -C m -C n im][A ‑ ]2, which has a strong interaction with hydrophobic lignin due to the multiple charges, large ion structure and hydrophobicity of the hydrophobic gemini imidazole compound, and can prevent dissolution and degradation of hemicellulose and cellulose while dissolving lignin in a solvent, and the reaction solution can be recycled, and the performance of dissolving lignin is almost not decreased after being reused for 5 times. The use amount of the hydrophobic gemini imidazole compound can be reduced due to the use of a high-boiling-point alcohol as a solvent. n -C m -C n im][A ‑ ]2, the whole separation process is simple, no toxic and harmful substances are generated, the recycling method is simple, and the energy consumption is low. Most of the lignin can be removed through one reaction, and high-quality holocellulose is obtained, and the problem that corn stalks are not easy to degrade is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of biomass component separation engineering, and particularly relates to a hydrophobic gemini imidazole compound, a preparation method thereof and application of the hydrophobic gemini imidazole compound in removal of lignin in corn stalks. BACKGROUND

[0002] More than 1 billion tons of straw are produced in China every year, and the straw mainly contains cellulose, hemicellulose and lignin, which can be converted into high-value chemicals and products. Due to the limitation of straw treatment technology, the main treatment method of these straws is returning to the field or used for papermaking, which not only wastes resources, but also easily causes serious field pest and environmental pollution problems.

[0003] In recent years, the separation of the three components in lignocellulose is an important research direction of green chemistry. Due to the formation of a dense complex protective layer structure of lignin and hemicellulose, lignocellulose is difficult to dissolve in conventional solvents, and it is difficult to separate cellulose, hemicellulose and lignin.

[0004] At present, the methods for separating the three components include physical method, chemical method and biological method, such as ultrasonic method, steam explosion pretreatment, alkali pretreatment, biological fermentation method and the like, but there are problems such as high equipment investment, high energy consumption, volatile, toxic and corrosive solvents used, long pretreatment cycle and the like, therefore, it is urgent to develop a green method for extracting cellulose from lignocellulosic biomass.

[0005] Due to excellent physical and chemical properties, especially strong solubility, ionic liquids composed of different cations and anions show broad application prospects in the field of biomass pretreatment. For example, [C4mim]Br can effectively dissolve cellulose, and the solubility of cellulose in it can reach 25wt%. However, when regenerating cellulose, water needs to be added to the ionic liquid. In order to recycle the ionic liquid, a large amount of water needs to be evaporated, which will cause great energy consumption. The high cost and high viscosity of ionic liquid limit its industrial application. In addition, the dissolution of cellulose in ionic liquid will cause its degradation, not only reducing the utilization rate of cellulose and increasing the operation difficulty of ionic liquid pretreatment of cellulose, but also reducing the quality of regenerated cellulose products. The presence of cellulose degradation products in ionic liquid will also affect its recycling. How to inhibit the degradation of cellulose in ionic liquid is an inevitable problem in the utilization of cellulose in ionic liquid. SUMMARY

[0006] The purpose of the present application is to provide a hydrophobic gemini imidazole compound and a preparation method thereof, and to provide 1,m-alkyl-di(3-alkylimidazole) ([C n -C m -Cn im][A - ]2), which is hydrophobic and has a simple and efficient preparation method.

[0007] The application further aims to provide an application of the hydrophobic gemini imidazole compound in removing lignin in corn stalks, and a single component in a high-boiling alcohol can dissolve lignin, and holocellulose is obtained in a high yield and high quality, active sites of cellulose are exposed, and the cellulose becomes suitable for fermentation to produce platform substances such as lactic acid and ethanol.

[0008] The specific technical scheme of the application is as follows:

[0009] The hydrophobic gemini imidazole compound is 1,m-alkyl-di(3-alkyl imidazole) [C n -C m -C n im][A - ]2, and the structural formula is:

[0010]

[0011] n=1, 2, 4, 8, 12, 16; m=1, 2, 4, 6;

[0012] Anion A - is tetrafluoroborate ([BF4 - ]) or hexafluorophosphate ([PF6 - ]);

[0013] The application provides a preparation method of the hydrophobic gemini imidazole compound.

[0014] The gemini imidazole bromide is dissolved in water, sodium tetrafluoroborate or sodium hexafluorophosphate is added after complete dissolution, stirring reaction is performed, and finally methanol is added, and the product is separated and purified.

[0015] In the preparation method of the hydrophobic gemini imidazole compound, the stirring reaction is performed at room temperature, and the reaction time is 3-8 h; the amount of sodium tetrafluoroborate or sodium hexafluorophosphate is 2.4 times the amount of substance of the gemini imidazole bromide; the amount ratio of the gemini imidazole bromide to water is 1-4 mol / L; the amount of anhydrous methanol needs to ensure that the imidazole compound and sodium tetrafluoroborate or sodium hexafluorophosphate are completely dissolved; and the product separation and purification refers to filtration, and the product is recrystallized with petroleum ether for more than three times after filtration.

[0016] The preparation method of the gemini imidazole bromide is as follows:

[0017] 1) Sodium ethoxide is dissolved in ethanol, imidazole is added, bromoalkane is added dropwise, stirring and heating reflux reaction are performed under N2 protection, the reaction is completed, and separation and purification are performed to obtain alkyl imidazole;

[0018] 2) dissolving the alkyl imidazole in ethanol, adding dibromoalkane, stirring and heating to reflux under N2 protection, separating and purifying after the reaction to obtain the gemini imidazole bromide.

[0019] In step 1), the usage ratio of sodium ethoxide to ethanol is 0.04-0.08 g / mL;

[0020] In step 1), the molar ratio of imidazole to bromoalkane is 1.3:1-1.8:1;

[0021] In step 1), the stirring and heating to reflux reaction is carried out for 24-48 h, and the reflux reaction temperature is 60-75℃.

[0022] In step 1), the separating and purifying refers to: after the reaction, the upper clear liquid is taken, ethanol is removed by rotary evaporation, and the crude product is distilled under reduced pressure to obtain the alkyl imidazole;

[0023] In step 2), the usage ratio of alkyl imidazole to ethanol is 2-5 mol / L;

[0024] In step 2), the molar ratio of alkyl imidazole to dibromoalkane is 2.0:1-3.0:1;

[0025] In step 2), the stirring and heating to reflux reaction is carried out for 24-48 h, and the reflux reaction temperature is 85-100℃.

[0026] In step 2), the separating and purifying refers to: after the rotary evaporation of ethanol, the gemini imidazole bromide is recrystallized with petroleum ether for more than three times, and dried to constant weight.

[0027] The application provides the application of the hydrophobic gemini imidazole compound in removing lignin in corn stalks.

[0028] The hydrophobic gemini imidazole compound is dissolved in a high-boiling alcohol to obtain a treatment solution, corn stalks are added, heating reaction is carried out, and after the reaction, solid-liquid separation is carried out to obtain a solution containing lignin and holocellulose.

[0029] The high-boiling alcohol is any one of ethylene glycol, propylene glycol and butanediol.

[0030] The hydrophobic gemini imidazole compound is dissolved in a high-boiling alcohol at a concentration of 0.5-15 wt%;

[0031] The liquid-solid ratio of the treatment solution to corn stalks is 10 mL:1 g-30 mL:1 g.

[0032] The heating reaction is carried out at a reaction temperature of 90-150℃ for 90-240 min.

[0033] The corn stalks are dried, crushed into 20-100 mesh powder, and then processed;

[0034] Preferably, the concentration of the hydrophobic gemini imidazole compound in the treatment solution is 9.0-11.0 wt %, the heating reaction time is 3 h, the reaction temperature is 130° C., and white holocellulose can be obtained after solid-liquid separation.

[0035] Holocellulose is the general term for hemicellulose and cellulose.

[0036] According to the method provided by the present invention, the yield of holocellulose reaches more than 88%, and the lignin removal rate reaches more than 83%.

[0037] The inventors discovered that hemicellulose is highly susceptible to degradation. Dissolved hemicellulose at high temperatures degrades into small molecules. When a stripping agent is added, the small molecules cannot be regenerated, resulting in hemicellulose waste. Furthermore, the presence of water significantly reduces the ability of ionic liquids to pretreat lignocellulose. Therefore, by designing ionic liquids to be hydrophobic, the adverse effects of water can be overcome while also reducing the viscosity of the ionic liquids. Because ionic liquids homopolymerize through the α-O-4' and β-O-4' bonds, releasing aromatic compounds, they dissolve lignin, significantly reduce the stress resistance of biomass, and enhance the enzymatic hydrolysis of fermentable sugars. Therefore, through ionic liquid design and process engineering, more energy-efficient technologies for dissolving and separating lignocellulose can be developed. The cellulose dissolving ability of ionic liquids depends largely on the hydrogen bond acceptance of the anion. Furthermore, the chemical structure of the imidazolium cation also significantly influences cellulose dissolution. The dissolution rate of cellulose in ionic liquids (ILs) decreases with increasing alkyl chain length or cation symmetry, due to increased viscosity and decreased hydrogen bond acidity. Designing the imidazolium ionic liquid into a gemini structure can increase the number of charges, thereby increasing the polarity of the imidazole compound and the ability to dissolve lignin. By changing the chain length of the gemini imidazole compound, its ability to dissolve cellulose can be adjusted to prevent excessive dissolution of cellulose. By changing the anion of the gemini imidazole compound, its hydrophobicity is adjusted to reduce its ability to dissolve cellulose. The hydrophobic gemini imidazole compound provided by the present invention, through strong electrostatic action or the hydrophobic part of the surfactant combined with the hydrophobic part of the lignin, achieves efficient dissolution of lignin while preventing the dissolution of cellulose and hemicellulose, and efficiently separates holocellulose from lignin in corn stalks, obtains high-quality holocellulose in high yield, and provides a platform material for subsequent biomass preparation of lactic acid, etc.

[0038] Compared with the prior art, the present invention utilizes a hydrophobic gemini imidazole compound [C n -C m -C n im][A -The multi-charge, large ion structure and hydrophobicity of [C1-C2-C1im][BF4]2 produce strong interaction with hydrophobic lignin, and can prevent the dissolution and degradation of hemicellulose and cellulose while dissolving lignin, and the reaction solution can be recycled, and the performance of dissolving lignin is almost not decreased after being reused for 5 times. Since high-boiling alcohol is used as the solvent, the use amount of [C1-C2-C1im][BF4]2 can be reduced, and thus the cost is relatively low. n -C m -C n im][A - ]2, and thus the cost is relatively low. The whole separation process is simple, no toxic and harmful substances are produced, the recycling method is simple, and the energy consumption is low. Most of the lignin can be removed through one reaction, and high-quality holocellulose is obtained, and the problem that corn straw is not easy to degrade is solved, and the technology has significant scientific significance and economic value. The hydrophobic gemini imidazole compound has simple preparation method and low cost. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 Digital photo of cellulose sample after treatment by [C1-C2-C1im][BF4]2;

[0040] Figure 2 SEM spectrum of residues of corn straw raw material (a, b, c) and 4.0 wt% [C1-C2-C1im][BF4]2 / EG (d, e, f) after treatment at 150 DEG C for 3 h. DETAILED DESCRIPTION

[0041] To make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in a clear and complete manner in combination with the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0042] In the following examples, the test materials and reagents used in the examples, unless otherwise specified, can be obtained from commercial channels.

[0043] In the embodiments, the specific technologies or conditions not specified can be carried out according to the technologies or conditions described in the literature in the art or according to the product instructions.

[0044] The straw separation method provided by the present application is simple to operate, and can retain a large amount of holocellulose while dissolving and separating lignin in corn straw. By efficiently removing lignin in the straw, high-purity holocellulose is obtained, which provides a platform material for subsequent holocellulose biomass conversion, and has important significance for corn straw biomass resource utilization and environmental protection.

[0045] Example 1

[0046] A method for preparing a hydrophobic gemini imidazolium compound [C1-C2-C1im][BF4]2, the specific steps are as follows:

[0047] S1: 18.38 g (0.224 mol) of methyl imidazole was dissolved in 50 mL of ethanol, then 15.04 g (0.080 mol) of 1,2-dibromoethane (MW: 187.86 g / mol) was added, and the reaction was carried out at 85°C for 48h under nitrogen protection. After the reaction was completed, the sample was rotary evaporated to remove ethanol, then recrystallized with anhydrous diethyl ether for three times, and vacuum dried to obtain [C1-C2-C1im]Br2, with a yield of 88.1%.

[0048] S2: 9.30 g (0.0287 mol) of [C1-C2-C1im]Br2 (MW: 323.5 g / mol) and 7.56 g (0.0689 mol) of NaBF4 (MW: 109.79 g / mol) were dissolved in 10 mL of water, stirred at room temperature for 3.0h, then 15.1 g of anhydrous methanol was added, the product was filtered, and recrystallized with petroleum ether for three times, with a product yield of 65.1%.

[0049] Example 2

[0050] A method for preparing a hydrophobic gemini imidazolium compound [C2-C2-C2im][PF6]2, the specific steps are as follows:

[0051] S1: 21.53 g (0.224 mol) of ethyl imidazole was dissolved in 50 mL of ethanol, then 15.04 g (0.080 mol) of 1,2-dibromoethane (MW: 187.86 g / mol) was added, and the reaction was carried out at 85°C for 48h under nitrogen protection. After the reaction was completed, the sample was rotary evaporated to remove ethanol, then recrystallized with anhydrous diethyl ether for three times, and vacuum dried to obtain [C2-C2-C2im]Br2, with a yield of 88.1%.

[0052] S2: 13.20 g (0.0348 mol) of [C2-C2-C2im]Br2 (MW: 379.5 g / mol) was dissolved in 20 mL of water, then 14.03 g (0.0835 mol) of NaPF6 (MW: 167.95 g / mol) was slowly added, stirred at room temperature for 3.0h, the product was filtered, and recrystallized with petroleum ether for three times, with a product yield of 72.3%.

[0053] Example 3

[0054] The application of a hydrophobic gemini imidazole compound [C1-C2-C1im][BF4]2 in removing lignin in corn stalks, mainly has the following steps:

[0055] 1) Dry corn stalks are pulverized to 60 mesh.

[0056] 2) [C1-C2-C1im][BF4]2 and corn stalks are added to a 250 mL round-bottom flask, then glycol is added, so that the concentration of [C1-C2-C1im][BF4]2 is 12.00 wt%, and the liquid-solid ratio is 25 mL:1 g.

[0057] 3) The above mixture is reacted at 150°C for 3h, and after separation, a reaction liquid and a stalk residue are obtained.

[0058] 4) The stalk residue is washed with anhydrous ethanol several times, the washing liquid is combined with the reaction liquid, and after rotary evaporation, a reused reaction liquid is obtained.

[0059] 5) The solid residue is washed with anhydrous ethanol and distilled water several times in turn, and is dried at 100°C to obtain holocellulose.

[0060] 6) The above steps 1)-5) are repeated 4 times using the reused reaction liquid as the treatment solution.

[0061] The holocellulose detection shows that after 5 times of treatment of corn stalks by the hydrophobic gemini imidazole compound [C1-C2-C1im][BF4]2, the yield of holocellulose is 93.1%, 92.8%, 92.6%, 92.2% and 91.9% respectively, and the lignin removal rate is 91.1%, 90.8%, 90.9%, 90.2% and 90.5% respectively.

[0062] Example 4

[0063] The application of a hydrophobic gemini imidazole compound [C2-C2-C2im][BF4]2 in removing lignin in corn stalks, mainly has the following steps:

[0064] 1) 13.20 g (0.0348 mol) [C2-C2-C2im]Br2 (MW: 379.5 g / mol) is dissolved in 20 mL water, then 9.167 g (0.0835 mol) NaBF4 (MW: 109.79 g / mol) is slowly added, stirred at room temperature for 3.0 h, the product is filtered, and the product is recrystallized with petroleum ether three times, and the product yield is 88.3%.

[0065] 2) [C2-C2-C2im][BF4]2 and 80 mesh corn stalks were added to a 250 mL round bottom flask, followed by the addition of propylene glycol to give a [C2-C2-C2im][BF4]2 concentration of 13.00 wt% and a liquid to solid ratio of 22 mL: 1 g.

[0066] 3) The above mixture was reacted at 120°C for 2 h, and after separation, a reaction liquid and a stalk residue were obtained.

[0067] 4) The stalk residue was washed several times with anhydrous ethanol, and the wash liquid was combined with the reaction liquid. After rotary evaporation, a reused reaction liquid was obtained.

[0068] 5) The solid residue was washed several times with anhydrous ethanol and distilled water, and was dried at 100°C to obtain holocellulose.

[0069] 6) The above steps 1) to 5) were repeated four times using the reused reaction liquid as the treatment solution.

[0070] The holocellulose was detected, and the results showed that after the corn stalks were treated with the hydrophobic gemini imidazole compound [C2-C2-C2im][BF4]2 for five times, the holocellulose yields were 96.2%, 95.8%, 95.6%, 95.2% and 94.9%, and the lignin removal rates were 92.6%, 92.3%, 91.9%, 91.6% and 91.5%, respectively.

[0071] Example 5

[0072] An application of a hydrophobic gemini imidazole compound [C2-C2-C2im][PF6]2 in removing lignin from corn stalks, mainly included the following steps:

[0073] 1) Dry corn stalks were crushed to 40 mesh.

[0074] 2) [C2-C2-C2im][PF6]2 and corn stalks were added to a 250 mL round bottom flask, followed by the addition of ethylene glycol to give a [C2-C2-C2im][PF6]2 concentration of 15.00 wt% and a liquid to solid ratio of 20 mL: 1 g.

[0075] 3) The above mixture was reacted at 150°C for 2 h, and after separation, a reaction liquid and a stalk residue were obtained.

[0076] 4) The solid residue was washed several times with anhydrous ethanol and distilled water, and was dried at 100°C to obtain holocellulose.

[0077] The holocellulose was detected, and the results showed that after the corn stalks were treated with the hydrophobic gemini imidazole compound [C2-C2-C2im][PF6]2, the holocellulose yield was 89.2%, and the lignin removal rate was 83.6%.

[0078] Example 6

[0079] A hydrophobic gemini imidazolium compound [C 16 -C2-C 16 im][BF4]2application in removing lignin from corn stover, mainly including the following steps:

[0080] 1) 32.76 g (0.112 mol) of hexadecyl imidazole (MW: 292.5 g / mol) was dissolved in 50 mL of ethanol, then 10.33 g (0.055 mol) of 1,2-dibromoethane (MW: 187.86 g / mol) was added, and the reaction was carried out at 90°C for 48 h under nitrogen protection. After the reaction was completed, the sample was rotary evaporated to remove ethanol, then recrystallized with anhydrous diethyl ether for three times, and vacuum dried to obtain [C 16 -C2-C 16 im]Br2, with a yield of 88.1%.

[0081] 2) 22.14 g (0.0287 mol) of [C 16 -C2-C 16 im]Br2(MW: 771.5 g / mol) and 7.56 g (0.0689 mol) of NaBF4(MW: 109.79 g / mol) were dissolved in 10 mL of water, stirred at room temperature for 3.0 h, then 38.1 g of anhydrous methanol was added, the product was filtered, and recrystallized with petroleum ether for three times, with a product yield of 79.1%.

[0082] 3) [C 16 -C2-C 16 im][BF4]2and 60-mesh corn stover were added to a 250 mL round-bottom flask, then propylene glycol was added, and the concentration of [C 16 -C2-C 16 im][BF4]2was 12.00 wt%, and the liquid-solid ratio was 22 mL: 1 g.

[0083] 4) The above mixture was reacted at 150°C for 3.0 h, and after separation, a reaction liquid and a stover residue were obtained.

[0084] 5) The stover residue was washed with anhydrous ethanol several times, and the washing liquid was combined with the reaction liquid, and after rotary evaporation, a reused reaction liquid was obtained.

[0085] 6) The solid residue was washed with anhydrous ethanol and distilled water alternately several times, and dried at 100°C to obtain holocellulose.

[0086] 7) The above steps 1) to 5) were repeated four times using the reused reaction liquid as the treatment solution.

[0087] The holocellulose detection shows that after the corn stalk is treated for 5 times by the hydrophobic gemini imidazole compound [C2-C2-C2im][BF4]2, the holocellulose yield is 89.6%, 88.8%, 89.3%, 89.1% and 88.9% respectively, and the lignin removal rate is 87.6%, 87.3%, 87.7%, 87.5% and 86.9% respectively.

[0088] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is apparent that those skilled in the art can easily make various modifications to the embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art without departing from the scope of the invention should be within the scope of protection of the invention.

Claims

1. Use of a hydrophobic gemini imidazole compound for the removal of lignin from corn stover, characterized in that, The specific application method is: The hydrophobic gemini imidazole compound is dissolved in a high-boiling alcohol to obtain a treatment solution, and then corn stalks are added for heating reaction, and after the reaction is completed, solid-liquid separation is performed to obtain a lignin-containing solution and holocellulose; The high-boiling alcohol is any one of ethylene glycol, propylene glycol and butanediol; The hydrophobic gemini imidazole compound is any one of , , or .

2. Use according to claim 1, characterized in that, The hydrophobic gemini imidazole compound is dissolved in a high-boiling alcohol at a concentration of 0.5-15wt%; the liquid-solid ratio of the treatment solution to corn stalks is 10mL:1g-30mL:1g.

3. Use according to claim 1 or 2, characterized in that, The heating reaction is performed at a reaction temperature of 90-150℃ for 90-240min.

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