CN114957705B

CN114957705B - Lignin demethylation modification method

Info

Publication number: CN114957705B
Application number: CN202210794529.XA
Authority: CN
Inventors: 陈小红; 孙润仓; 柴澜芳; 李伟; 郑谦; 肖领平
Original assignee: Dalian Polytechnic University
Current assignee: Dalian Polytechnic University
Priority date: 2022-07-05
Filing date: 2022-07-05
Publication date: 2023-06-13
Anticipated expiration: 2042-07-05
Also published as: CN114957705A

Abstract

The invention relates to a method for demethylating and modifying lignin, which belongs to the technical field of materials. After demethylation modification, the number of lignin phenolic hydroxyl groups can be increased by 171.67%, so that the reactivity of lignin is obviously enhanced, and the molecular structure of lignin can be reserved to a large extent. The method has important significance in improving the reactivity of lignin, realizing the high-value utilization of lignin and relieving the pressure of petroleum resources.

Description

Lignin demethylation modification method

Technical Field

The invention relates to the field of lignin, in particular to a novel lignin demethylation modification method.

Background

Due to the massive consumption of fossil fuels and their limited storage, alternative energy and chemical resources are urgently needed. The pollution of lignin to the environment mainly exists in papermaking wastewater. The wood is used for papermaking, and the waste papermaking liquid is subjected to alkali recovery, so that lignin is combusted and converted into heat energy, and the waste papermaking liquid does not pollute the environment. And the environment is seriously polluted for the papermaking waste liquid which is difficult to recycle alkali. Therefore, development of lignin and cellulose rich in natural plant resources has become a research hotspot. Biomass is a promising alternative, also a renewable resource for fuels and chemicals.

Lignin is a renewable biomass resource with mass fraction inferior to that of cellulose in plant kingdom, is a main component forming plant cell walls, and its basic structure is a three-dimensional reticular polyphenol polymer composed of phenylpropane units with aromatic characteristics, and contains various functional groups such as hydroxyl and methoxy.

A great deal of research on lignin to replace phenol for preparing phenolic resin at home and abroad is available, but the lignin has a relatively low hydroxyl content due to the large molecular weight, and the reactivity is far lower than that of the ligninPhenol has fewer active sites. In addition, the lignin molecular structure contains a large amount of methoxy groups, so that the steric hindrance of lignin is increased, and further, the hydroxyl groups on part of lignin are deactivated, and the activity of lignin is further reduced. Therefore, the methoxy on the lignin is subjected to demethylation reaction, so that the steric hindrance in the lignin can be reduced, and the hydroxyl content can be increased, thereby achieving the purpose of improving the reactivity of the lignin. Demethylation of lignin, cleavage of stable ether linkages between benzene ring and methyl group, typically requires special reagents (mostly toxic, expensive and unstable) and/or harsh conditions (e.g., high temperature and strong acidity). For example, molten pyridine hydrochloride is a classical reagent for cleavage of arylmethyl ethers, the cleavage temperature is up to 200 ℃, a large amount of pyridine hydrochloride is consumed and high pressure equipment is used, the process requirements are high, and serious cleavage and polycondensation side reactions exist during the reaction. Zhao and Q.Mei et al selectively demethylate lignin using concentrated HBr and HI, but with a small increase in phenolic hydroxyl content. Iodine reagents such as iodocyclohexane and trimethylsilyl iodide are also effective for demethylation. For example, X.Li and J.Tian et al employ AlI ₃ Lignin and lignin model compounds can be demethylated at or below room temperature, but such agents are costly and can also result in serious damage to the lignin molecular structure. Overall, existing methods have mainly the following problems: firstly, the increase of the phenolic hydroxyl content is small, usually about 10-80%, and the reactivity of lignin is difficult to be obviously improved; second, the requirements on production costs or production processes are high, or severe reaction conditions are required, or expensive demethylating reagents are required; third, existing methods, while demethylating lignin, can severely disrupt the molecular structure of lignin itself, such as cleavage of flexible β -O-4 aryl ether linkages, and even lead to disruption of benzene ring structure, which is somewhat contrary to the original intent of developing lignin utility.

Disclosure of Invention

In order to solve the problems that high-pressure equipment is needed in the existing lignin demethylation process, the process requirement is high, the increase of the hydroxyl content is small, the molecular structure of lignin is damaged and the like, the invention provides a lignin demethylation modification method, which is used for carrying out demethylation reaction at normal pressure and lower temperature, no unpleasant smell exists, and the obtained lignin is black powder, the phenolic hydroxyl content can be improved by 171.67% at the maximum, and the total hydroxyl content can be improved by 79.89% at the maximum.

The technical scheme of the invention is as follows: a lignin demethylation modification method, which comprises the following steps of: (0.1-1): (0.5-2): (10-100) adding lignin, a phase transfer reagent, a modifier and an organic solvent into a reaction bottle, then adjusting the temperature to 60-135 ℃ to carry out demethylation reaction, and obtaining the purified demethylated lignin after acid precipitation, washing and drying after reacting for 1-20 h.

The organic solvent is one or more of N, N-dimethylformamide, dichloroethane, dichloromethane, tetrahydrofuran, dimethyl sulfoxide and N, N-dimethylacetamide.

The modifier is one or more of anhydrous aluminum chloride and anhydrous aluminum bromide.

The phase transfer reagent is one or more of cetyl tributyl phosphonium bromide, tetrabutyl ammonium fluoride, tetrabutyl ammonium chloride and tetrabutyl ammonium iodide.

The acid precipitating reagent is one or more of hydrochloric acid, sulfuric acid and nitric acid.

The lignin used is any one of needle wood lignin, poplar lignin, herbaceous lignin, industrial alkali lignin, lignin sulfonate, sulfate lignin, hydrolytic lignin, hydrothermal lignin and organic solvent lignin in wood hydrolysis industry and papermaking industry wastes.

The mass ratio of the lignin to the phase transfer agent to the modifier to the organic solvent is preferably 1:0.15:1.5:20.

the temperature of the demethylation reaction is preferably 80 to 115 ℃.

The time for the demethylation reaction is preferably 3 to 6 hours.

The beneficial effects are that:

1. the invention provides a method for demethylating and modifying lignin, which has low process requirements and can be carried out at normal pressure and lower temperature without using high-pressure equipment.

2. The invention carries out demethylation modification on rich lignin resources in China, improves the phenolic hydroxyl content by 171%, greatly improves the reactivity of lignin, and can replace phenol in a higher proportion.

3. The molecular structure of the demethylated lignin obtained by modification by the method can be reserved to a greater extent.

Drawings

FIG. 1 is a nuclear magnetic hydrogen spectrum of lignin and lignin after demethylation.

FIG. 2 is a two-dimensional nuclear magnetic pattern of lignin and lignin after demethylation.

Detailed Description

The following non-limiting examples will enable those of ordinary skill in the art to more fully understand the invention and are not intended to limit the invention in any way.

In the following examples, industrial alkali lignin was purchased from Shandong Longli biotechnology Co., ltd, hydrothermal lignin was purchased from Shanghai Dong Sheng New Material Co., ltd, and poplar alkali lignin was self-extracted lignin by the following extraction method: 100g of poplar absolute dry wood chips are taken and immersed in 15% sodium hydroxide solution, and the mixture is steamed and kept at 170 ℃ for 2 hours.

Example 1

According to the mass ratio of 1:0.15:1.5: adding industrial alkali lignin, hexadecyl tributyl phosphonium bromide, aluminum trichloride and N, N-dimethylformamide into a reaction bottle, raising the reaction temperature to 115 ℃, reacting for 20 hours, adding a hydrochloric acid solution with the concentration of 2M to adjust the pH value to 2, washing the precipitate, and drying in a vacuum drying oven to constant weight to obtain the modified lignin.

200mg of modified lignin was taken and mixed with pyridine: acetic anhydride (1:1) was acetylated at room temperature for 72 h. According to the measurement, the aromatic hydroxyl content of the modified lignin product is increased by 55.67%, and the aliphatic hydroxyl content is reduced by 9.35%.

Example 2

According to the mass ratio of 1:0.15:1.5: adding industrial alkali lignin, hexadecyl tributyl phosphonium bromide, aluminum trichloride and N, N-dimethylformamide into a reaction bottle, raising the reaction temperature to 80 ℃, reacting for 10 hours, adding a hydrochloric acid solution with the concentration of 2M to adjust the pH value to 2, washing the precipitate, and drying in a vacuum drying oven to constant weight to obtain the modified lignin.

200mg of modified lignin was taken and mixed with pyridine: acetic anhydride (1:1) was acetylated at room temperature for 72 h. The aromatic hydroxyl content of the modified lignin product of this example was increased by 19.81% and the aliphatic hydroxyl content was reduced by 2.34%.

Example 3

According to the mass ratio of 1:0.15:1.5: and 20, adding industrial alkali lignin, tetrabutylammonium bromide, aluminum trichloride and N, N-dimethylformamide into a reaction bottle, raising the reaction temperature to 115 ℃, reacting for 10 hours, adding a hydrochloric acid solution with the concentration of 2M to adjust the pH value to 2, washing the precipitate, and drying in a vacuum drying oven to constant weight to obtain the modified lignin. 200mg of modified lignin was taken and mixed with pyridine: acetic anhydride (1:1) was acetylated at room temperature for 72 h. The aromatic hydroxyl content of the modified lignin product of this example was increased by 124.52% and the aliphatic hydroxyl content was increased by 24.56%.

Example 4

According to the mass ratio of 1:0.15:1.5:20, adding hydrothermal lignin, tetrabutylammonium bromide, aluminum trichloride and N, N-dimethylformamide into a reaction bottle, raising the reaction temperature to 115 ℃, reacting for 10 hours, adding a hydrochloric acid solution with the concentration of 2M to adjust the pH value to 2, washing the precipitate, and drying in a vacuum drying oven to constant weight to obtain the modified lignin.

200mg of modified lignin was taken and mixed with pyridine: acetic anhydride (1:1) was acetylated at room temperature for 72 h. The aromatic hydroxyl content of the modified lignin product of this example was increased by 61.26% and the aliphatic hydroxyl content was increased by 52.83%.

Example 5

According to the mass ratio of 1:0.15:1.5:20 adding poplar alkali lignin, hexadecyl tributyl phosphonium bromide, aluminum trichloride and N, N-dimethylformamide into a reaction bottle, raising the reaction temperature to 115 ℃, after 4 hours of reaction, adding a hydrochloric acid solution with the concentration of 2M to adjust the pH value to 2, washing the precipitate, and then drying in a vacuum drying box to constant weight to obtain the modified lignin.

200mg of modified lignin was taken and mixed with pyridine: acetic anhydride (1:1) was acetylated at room temperature for 72 h. The aromatic hydroxyl content of the modified lignin product of this example was increased by 171.67% and the aliphatic hydroxyl content was reduced by 7.80%.

Example 6

According to the mass ratio of 1:0.3:0.75: adding industrial alkali lignin, hexadecyl tributyl phosphonium bromide, aluminum trichloride and N, N-dimethylformamide into a reaction bottle, raising the reaction temperature to 115 ℃, reacting for 4 hours, adding a hydrochloric acid solution with the concentration of 2M to adjust the pH value to 2, washing the precipitate, and drying in a vacuum drying oven to constant weight to obtain the modified lignin.

200mg of modified lignin was taken and mixed with pyridine: acetic anhydride (1:1) was acetylated at room temperature for 48 h. The aromatic hydroxyl content of the modified lignin product of this example was increased by 51.41% and the aliphatic hydroxyl content was reduced by 3.51%.

Table 1 shows the hydroxyl content of the original lignin (industrial alkali lignin, poplar alkali lignin, hydrothermal lignin) and examples 1-6.

Determination of hydroxyl content: acetylation is carried out on lignin before and after modification, and the change of the hydroxyl content is judged through the change of the nuclear magnetic resonance hydrogen spectrum on the acetoxy content.

Table 1.

The results in Table 1 show that lignin type, reaction temperature, reaction time, reaction solvent, phase transfer reagent type and amount, etc. all have great influence on the improvement of phenolic hydroxyl group content. The reaction time is too short, which results in incomplete demethylation reaction, and too long, which results in aggravation of side reactions such as depolymerization reaction and condensation reaction, and the like, but reduces the hydroxyl content improving effect, and simultaneously results in destruction of lignin molecular structure. Therefore, the optimal reaction time is about 3 to 6 hours. The aromatic hydroxyl group content increasing effect of example 5 was most remarkable, and it was as high as 171.7%.

FIG. 1 is a nuclear magnetic resonance spectrum of lignin and lignin after demethylation, wherein lignin is industrial alkali lignin, and demethylated lignin is demethylated modified lignin prepared in example 3. In the spectrogram, delta=2.4-2.04 is an acetylated aromatic hydroxyl proton absorption peak, the intensity of an absorption peak after demethylation modification is enhanced, delta=2.04-1.6 is an acetylated aliphatic hydroxyl proton absorption peak, and the intensity of an absorption peak after demethylation modification is enhanced.

FIG. 2 is a two-dimensional nuclear magnetic resonance diagram of lignin and lignin after demethylation, wherein lignin is industrial alkali lignin, and demethylated lignin is demethylated modified lignin prepared in example 1. The lignin molecular structure after demethylation modification is mostly reserved by using aluminum trichloride as a lignin sample of a demethylating reagent.

Table 2 shows the two-dimensional nuclear magnetic quantification results of the original lignin industrial alkali lignin and the demethylated modified lignin prepared in example 1.

Determination of beta-O-4 content: the beta-O-4 content is calculated from the Aalpha signal.

TABLE 2

The results in table 2 show that: the lignin structure of example 1 can be retained to a greater extent, with β -O-4 linkages being retained around 75.52%.

Claims

1. A lignin demethylation modification method, which is characterized by comprising the following steps of: 0.15:1.5:20, adding lignin, a phase transfer reagent, a modifier and an organic solvent into a reaction bottle, carrying out demethylation reaction at 115 ℃ for 4-10 hours, and obtaining purified demethylated lignin after acid precipitation, washing and drying after the reaction is finished;

the modifier is anhydrous aluminum chloride;

the organic solvent is N, N-dimethylformamide;

the phase transfer catalyst used is cetyl tributyl phosphonium bromide or tetrabutyl ammonium bromide;

the lignin is one of industrial alkali lignin, poplar alkali lignin and hydrothermal lignin.

2. The method according to claim 1, characterized in that: the reagent used for acid precipitation is one or more of hydrochloric acid, sulfuric acid and nitric acid.