A modification method for demethylation of lignin

Abstract

The invention discloses a modification method for demethylation of lignin, comprising the following steps: heating hexamethyldisilane and elemental iodine to react to form trimethylsilyl iodide, then directly adding lignin, sodium sulfite solution and solvent, controlling the temperature to react, and post-treating the reaction solution to obtain demethylated lignin. The invention has simple process operation, mild reaction conditions, low production cost, and after demethylation modification, the total hydroxyl and phenolic hydroxyl contents of lignin are increased, the reaction activity is significantly enhanced, and the invention has industrial application value.

Description

Technical Field

[0001] The invention relates to a modification method for demethylating lignin, in particular to a method for demethylating lignin with simple operation and low cost, belonging to the technical field of biomaterials. Background Art

[0002] Lignocellulose is mainly composed of cellulose, hemicellulose and lignin. Among them, cellulose and hemicellulose have been widely used in the fields of sugar production, pulp and paper making, and bioethanol fuel. However, these industrial processes produce a large amount of lignin by-products. Currently, only a small part of them is used to manufacture high-value-added products such as additives, dispersants, adhesives and surfactants. Most of them are regarded as industrial waste and directly incinerated, resulting in resource waste and environmental pollution. Lignin has a complex three-dimensional network structure, composed of guaiacyl (G), p-hydroxyl (H) and syringyl (S) groups, and contains rich functional group structures such as phenolic hydroxyl, alcoholic hydroxyl, methoxy, and carboxyl. There are a large number of methoxy groups in the molecular structure of lignin. The presence of methoxy groups increases the steric hindrance of lignin, which in turn affects the activity of lignin, making its industrial application very limited. In recent years, the methods of lignin modification that have been reported include phenolization, hydroxymethylation, demethylation, and amination. Demethylation is the process of converting methoxy groups on lignin into hydroxyl groups, which can not only reduce the steric hindrance inside lignin, but also increase the mass fraction of hydroxyl groups to enhance the reaction activity of lignin, which has great practical significance.

[0003] Currently, typical lignin demethylation reagents include BBr 3 , HBr / HI, ethyl mercaptan and n-dodecyl mercaptan, etc. According to research reports, BBr 3 It is very active, and the reaction needs to be carried out at extremely low temperatures (-78℃~0℃), and the reaction conditions are harsh; halogen acids such as HBr / HI are used as catalysts, and the acid strength is high, resulting in high corrosion in the conversion process and difficulty in separating the products; the combination of thiols and inorganic bases has a better effect of removing methyl groups, and the reaction conditions are relatively mild, but the odor is strong, which threatens human health. Therefore, finding a lignin demethylation modification method with simple operation, mild reaction conditions, and good demethylation effect is a technical problem that needs to be solved at present. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a modification method for demethylation of lignin. The method adopts trimethyl iodosilane and sodium sulfite as dual catalysts to carry out demethylation reaction of lignin. The reaction conditions are mild, the catalytic efficiency is high, the cost is low, the effect is good, and it is more suitable for industrial production.

[0005] The specific technical solutions of the present invention are as follows:

[0006] A modification method for demethylating lignin, the method comprising the following steps:

[0007] Step 1, heating hexamethyldisilane and elemental iodine to react to form trimethylsilyl iodide;

[0008] Step 2, adding lignin, sodium sulfite solution and solvent to the reaction solution of step 1, and controlling the temperature to react;

[0009] Step 3, post-treating the reaction solution of step 2 to obtain demethylated lignin.

[0010] Further, in step 1, the mass ratio of hexamethyldisilane to elemental iodine is 1.1 to 1.7:1, for example, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, preferably 1.4:1.

[0011] Furthermore, in step 1, elemental iodine is added to hexamethyldisilane in batches. The purpose of adding in batches is to make the reaction more moderate and thorough. There is no special requirement for the number of batches.

[0012] Furthermore, in step 1, after hexamethyldisilane is added to the reaction container, preferably nitrogen is first introduced to purge and remove oxygen in the reaction container, and then elemental iodine is added in batches.

[0013] Furthermore, in step 1, elemental iodine is added to hexamethyldisilane at 55-65°C.

[0014] Furthermore, in step 1, after adding elemental iodine, the temperature is raised to 90-120°C for reaction, for example, 90°C, 100°C, 110°C, 120°C, preferably 100°C; the reaction time is 1-3h, for example, 1h, 1.5h, 2h, 2.5h, 3h, preferably 2h.

[0015] Furthermore, in step 2, the lignin used in the present invention can be various types of lignin, such as hardwood lignin, softwood lignin, etc. Softwood lignin includes pine lignin, spruce lignin, cypress lignin and other commercially available lignins.

[0016] Furthermore, in step 2, the mass ratio of lignin to sodium sulfite is 6 to 10:1, for example, 6:1, 7:1, 8:1, 9:1, 10:1, preferably 8:1.

[0017] Furthermore, in step 2, sodium sulfite is added in the form of an aqueous solution, and there is no special requirement for the concentration of the sodium sulfite solution, for example, it can be 3wt%, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, etc., and any range therebetween.

[0018] Furthermore, in step 2, the mass ratio of lignin to hexamethyldisilane is 1:3-5, for example, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, preferably 1:3.5.

[0019] Further, in step 2, the solvent includes at least one of dimethyl sulfoxide (DMSO), acetonitrile, ethanol, dimethylformamide (DMF), tetrahydrofuran (THF), dichloromethane, carbon tetrachloride, etc., preferably acetonitrile. The role of the solvent is to dissolve trimethyl iodosilane, which is conducive to the reaction. The amount of the solvent can be adjusted according to actual needs, such as the mass ratio of the solvent and lignin can be 10~20:1, such as 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1.

[0020] Further, in step 2, the reaction temperature is 60-100°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, preferably 70°C; the reaction time is 1-3h, for example, 1h, 1.5h, 2h, 2.5h, 3h, preferably 1.5h.

[0021] Further, in step 3, the post-treatment step includes: cooling the reaction solution of step 2 to room temperature, and then performing acid precipitation, washing, and drying to obtain demethylated lignin. Acid precipitation refers to adding acid to the reaction solution to precipitate demethylated lignin. The acid can be selected from inorganic acids such as hydrochloric acid and sulfuric acid, and the concentration of the acid is 0.5-1.5 mol / L. Washing is performed with distilled water, and drying is generally performed at 50-70°C.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The present invention uses hexamethyldisilane and iodine as raw materials to prepare trimethyl iodosilane, with mild reaction conditions, no by-products, and no need to separate trimethyl iodosilane to directly carry out the next step of lignin demethylation reaction, which is easy to operate. In addition, the smoke phenomenon caused by direct use of trimethyl iodosilane is further avoided, and the cost is reduced.

[0024] 2. The present invention utilizes the nucleophilicity of sodium sulfite as a demethylation catalyst to replace part of trimethyl iodosilane. The dual catalysts can increase the reaction rate, improve the demethylation effect, and reduce the cost of raw materials.

[0025] 3. The overall process of the present invention is easy to operate, has mild reaction conditions, no high temperature or high pressure, improved safety, and is suitable for industrial production.

[0026] 4. The process of the present invention is simple to operate, has mild reaction conditions, and has low production cost. After demethylation of lignin, the total hydroxyl content and phenolic hydroxyl content are significantly improved, and the reaction activity is significantly enhanced, which has industrial application value. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with examples, but the following examples are only exemplary and do not limit the contents thereof.

[0028] In the following examples and comparative examples, the lignin used is pine wood lignin purchased from Shanghai Changfa New Materials Co., Ltd., and its phenolic hydroxyl content is 3.42 mmol / g.

[0029] In the following examples and comparative examples, the test method for the phenolic hydroxyl content in lignin and demethylated lignin is the Folin phenol method.

[0030] The calculation formula for the increase rate of the phenolic hydroxyl content of the demethylated lignin is: (phenolic hydroxyl content of lignin after demethylation - phenolic hydroxyl content before demethylation) / phenolic hydroxyl content before demethylation × 100%.

[0031] Example 1

[0032] The reaction device such as a 1L three-necked flask was first evacuated and then purged with nitrogen three times, 42g of hexamethyldisilane was added, and the mixture was stirred and heated to 60°C. 30g of elemental iodine was added in batches, and the temperature was slowly raised to 100°C, and refluxed. After about 2h, the temperature was lowered to 70°C, and then 13.83g of lignin, 11.5g of 15% sodium sulfite solution and 221.3g of acetonitrile were added, and the temperature was maintained for 1.5h and cooled to room temperature. An appropriate amount of 1mol / L dilute hydrochloric acid was added to the reaction solution for acid precipitation, and the precipitated solid was washed with water and then dried at 60°C to obtain demethylated lignin. It was determined that the phenolic hydroxyl content of the demethylated lignin was increased by 120% compared with that before demethylation.

[0033] Example 2-3

[0034] Demethylated lignin was produced according to the method of Example 1, except that the mass ratio of hexamethyldisilane to iodine was changed, and the other reaction conditions remained unchanged. The results are shown in Table 1 below:

[0035]

[0036] Embodiment 4-7

[0037] Demethylated lignin was produced according to the method of Example 1, except that the reaction temperature and reaction time of hexamethyldisilane and iodine were changed, and the other reaction conditions remained unchanged. The results are shown in Table 2 below:

[0038]

[0039] Embodiment 8-11

[0040] Demethylated lignin was produced according to the method of Example 1, except that the mass ratio of lignin to sodium sulfite and the mass ratio of lignin to hexamethyldisilane were changed, and the other reaction conditions remained unchanged. The results are shown below:

[0041]

[0042] Examples 12-15

[0043] Demethylated lignin was produced according to the method of Example 1, except that the reaction temperature and reaction time of demethylation were changed, and the other reaction conditions remained unchanged. The results are shown in Table 4 below:

[0044]

[0045] Examples 16-17

[0046] Demethylated lignin was produced according to the method of Example 1, except that the demethylation solvent was changed and the other reaction conditions remained unchanged. The results are shown in Table 5 below:

[0047]

[0048] Comparative Example 1

[0049] The reaction device such as a 1L three-necked flask was first evacuated and then purged with nitrogen three times, 42g of hexamethyldisilane was added, and the mixture was stirred and heated to 60°C. 30g of elemental iodine was added in batches, and the temperature was slowly raised to 100°C, and refluxed. After about 2 hours, the temperature was lowered to 70°C, and then 13.83g of lignin and 221.3g of acetonitrile were added. The temperature was maintained for 1.5 hours and cooled to room temperature. An appropriate amount of 1mol / L dilute hydrochloric acid was added to the reaction solution for acid precipitation, and the precipitated solid was washed with water and then dried at 60°C to obtain demethylated lignin. It was determined that the phenolic hydroxyl content of the demethylated lignin increased by 75%.

[0050] Comparative Example 2

[0051] Add 13.83g of lignin and 11.5g of 15% sodium sulfite solution into a 1L three-necked flask, heat to 70°C, keep the temperature for 1.5h, and cool to room temperature. Add an appropriate amount of 1mol / L dilute hydrochloric acid to the reaction solution for acid precipitation, wash the precipitated solid with water, and then dry it at 60°C to obtain demethylated lignin. It was determined that the phenolic hydroxyl content of the demethylated lignin increased by 58%.

Claims

1. A modification method for demethylation of lignin, characterized in that: The following steps are involved: Step 1, heating hexamethyldisilane and elemental iodine to react to form trimethylsilyl iodide; Step 2, adding lignin, sodium sulfite solution and solvent to the reaction solution of step 1, and controlling the temperature to react; Step 3, post-treating the reaction solution of step 2 to obtain demethylated lignin; In step 1, the mass ratio of hexamethyldisilane to elemental iodine is 1.1-1.7:1; In step 1, after iodine is completely added, the temperature is raised to 90-100°C and reacted for 1-3 hours; In step 2, the mass ratio of lignin to sodium sulfite is 6-10:1, and the mass ratio of lignin to hexamethyldisilane is 1:3-3.5; In step 2, the reaction temperature is 60-70°C and the reaction time is 1-1.5h.

2. The modification method according to claim 1, characterized in that: In step 1, elemental iodine is added to hexamethyldisilane in batches.

3. The modification method according to claim 1, characterized in that: In step 1, elemental iodine is added to hexamethyldisilane at 55-65°C.

4. The modification method according to claim 1, characterized in that: In step 2, the solvent includes dimethyl sulfoxide, acetonitrile, ethanol, dimethylformamide, tetrahydrofuran, dichloromethane, and carbon tetrachloride.

5. The modification method according to claim 4, characterized in that: In step 2, the solvent is acetonitrile.

6. The modification method according to claim 1, characterized in that: In step 2, the mass ratio of solvent to lignin is 10-20:

1.

7. The modification method according to claim 1, characterized in that: In step 2, the reaction temperature is 70°C.

8. The modification method according to claim 1, characterized in that: In step 3, the post-treatment step includes: cooling the reaction solution to room temperature, and then performing acid precipitation, washing, and drying to obtain demethylated lignin.

Images