CN111116938B - Method for efficiently extracting lignin from lignocellulose raw material - Google Patents
Method for efficiently extracting lignin from lignocellulose raw material Download PDFInfo
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Abstract
The invention belongs to the technical field of lignin separation and extraction, and particularly relates to a method for efficiently extracting lignin from a lignocellulose raw material. The method comprises the following steps: placing the lignocellulose raw material in a Fenton reaction pretreatment system, and treating for 6-72 hours; then, the lignin in the raw materials is separated and extracted by a wood grinding method or an alkaline method; the Fenton reaction pretreatment system is a mixed solution composed of 3-9 mmol/L of ferrous compound, 0.2-1.6 mol/L of peroxide and 0-0.128 mol/L of sulfhydryl compound. The optimized Fenton catalytic reaction is utilized to promote the separation of lignin in the lignocellulose raw material, so that the lignin with a more complete structure and more concentrated molecular weight distribution is obtained, and the extraction rate of the lignin can be effectively improved.
Description
Technical Field
The invention belongs to the technical field of lignin separation and extraction, and particularly relates to a method for efficiently extracting lignin from a lignocellulose raw material.
Background
With the increasing exhaustion of petrochemical resources, the crisis of energy and chemical supply becomes more prominent, and the search for renewable resources to prepare energy and chemicals to replace petrochemical resources is urgently needed. Biomass resources represented by crop straws and forest residues are rich in lignocellulose raw materials, can be degraded and converted into raw materials such as cellulose, hemicellulose, lignin and the like after treatment, and are considered to be the most promising renewable resources for replacing petrochemical resources at present. At present, cellulose and hemicellulose polysaccharide carbohydrate can be subjected to enzyme hydrolysis to obtain monosaccharide platform compounds, and further subjected to microbial fermentation or chemical catalytic conversion to prepare biomass energy sources and fine chemicals such as ethanol, butanol, xylitol, succinic acid, furfural and lactic acid.
The lignin in the lignocellulose raw material is a natural polymer material formed by connecting aromatic monomers through covalent bonds, and is the only renewable resource of aromatic compounds in the nature. In the early biorefinery, the comprehensive utilization of carbohydrates such as cellulose and hemicellulose is taken as a target, lignin is considered as a main obstacle influencing the conversion of the carbohydrates, and the lignin is usually removed to the maximum extent by adopting severe pretreatment means such as high temperature, high pressure, strong acid and strong alkali, so as to realize the high-efficiency enzymatic hydrolysis conversion of the carbohydrates. In the severe pretreatment process, the beta-O-4 active connecting bond in the lignin is broken to form an active segment, and the active segment is polymerized again to finally form the more stable polycondensation lignin connected by the C-C bond. The lignin is more difficult to obtain aromatic monomers by depolymerization due to the destruction of the original structure, and the prior researches show that the monomer yield and selectivity of lignin depolymerization products are positively correlated with the retention ratio of beta-O-4 bonds in the lignin.
With the increase of the acceptance of lignin value by people, researchers worldwide are searching for efficient strategies for realizing high-value conversion of lignin. Given that structural changes in lignin are mainly derived from the retention ratio of β -O-4 bonds, there are two main types of current strategies: one is called passive protection and the other is called active protection. The passive protection method takes the reduction of the severity of pretreatment conditions as a means, adopts a high-efficiency solvent, and extracts under milder conditions to obtain a low-condensation lignin product for later catalytic conversion. The lignin obtained by the method retains a high proportion of beta-O-4 bonds, is beneficial to high-value conversion in the later period, but has very low extraction efficiency, and limits the large-scale application of the lignin. In order to solve the problem, researchers use formaldehyde to protect the active hydroxyl of lignin in advance, and the protection can realize the retention of the beta-O-4 bond of the lignin in a high proportion under relatively severe extraction conditions, so that the extraction efficiency is improved. But the protection of formaldehyde increases the raw material investment on one hand and also increases the difficulty for the depolymerization and separation in the later period; the active protection method adopts catalytic hydrogenolysis passivation method to the object by using active fragments formed in the lignin extraction process to realize in-situ depolymerization of lignin, obtain high-selectivity lignin monomer and oligomer, and simultaneously retain the quality of carbohydrate. This approach is considered to be a very promising strategy to achieve full lignin component utilization. Although the method has the advantages, the problems related to the use of noble metal catalysts, the deactivation of the catalysts, the recycling, the complex operation process and the like are still to be solved, and the method is limited to the laboratory research stage at present.
Disclosure of Invention
In order to solve the technical problems, the invention aims to provide a method for efficiently extracting lignin from a lignocellulose raw material, which promotes the separation of the lignin in the lignocellulose raw material by utilizing an optimized fenton catalytic reaction, so that the lignin with a more complete structure and more concentrated molecular weight distribution is obtained, and the extraction rate of the lignin can be effectively improved.
In order to achieve the technical purpose, the invention adopts the following technical scheme:
the method for efficiently extracting lignin from a lignocellulose raw material comprises the following specific steps: placing the lignocellulose raw material in a Fenton reaction pretreatment system, and treating for 6-72 hours; then, the lignin in the raw materials is separated and extracted by a wood grinding method or an alkaline method;
the Fenton reaction pretreatment system is a mixed solution consisting of 3-9 mmol/L of ferrous compound, 0.2-1.6 mol/L of peroxide and 0-0.128 mol/L of sulfhydryl compound;
the ferrous compound is FeCl2、FeCl2Hydrate of (2), FeSO4、FeSO4Any one or a mixture of two or more of the hydrates of (a); the peroxide is any one or a mixture of hydrogen peroxide and sodium peroxide; the mercapto compound is one or more of thioglycolic acid, mercaptopropionic acid, thioglycolate and mercaptopropionate.
Preferably, the ferrous compound in the pretreatment system is 6.25mmol/L, the peroxide is 0.88mol/L, and the sulfhydryl compound is 0.032-0.128 mol/L.
Further preferably, the ferrous compound is FeCl2A hydrate of (a); the peroxide is hydrogen peroxide; the mercapto compound mercaptoacetic acid.
Further preferably, the treatment time of the lignocellulosic feedstock in the fenton pretreatment system is 12-20 h.
Preferably, the addition amount of the lignocellulose raw material in the pretreatment system is 1 g: 5-30 mL; the lignocellulose raw material is a wood raw material or crop straws.
The method for extracting the lignin by the wood grinding method comprises the following specific steps:
(1) placing the lignocellulose raw material subjected to Fenton pretreatment into a ball mill for ball milling for 1-5 h;
(2) extracting for 1-5 h at 95-105 ℃ by using dioxane after the ball milling is finished, wherein the mass-volume ratio of the raw material obtained in the step (1) to the dioxane is 1 g: 10-30 mL;
(3) concentrating the extracting solution to a certain volume, and then dropwise adding the extracting solution into 2-5 times of ethanol; after removing the precipitate, concentrating the filtrate again to a certain volume;
(4) and adding the concentrated solution into an acid solution with the volume 5-15 times that of the concentrated solution, and after the precipitation is completed, carrying out solid-liquid separation to obtain a precipitate, namely the lignin.
The alkali method for extracting the lignin comprises the following specific steps:
(1) placing the lignocellulose raw material subjected to Fenton pretreatment in a strong alkali solution, and reacting for 1-10 hours at 25-75 ℃; the mass volume ratio of the lignocellulose raw material to the strong alkali solution is 1 g: 5-20 mL;
the strong base is sodium hydroxide, potassium hydroxide or calcium hydroxide; the concentration of the alkali liquor is 0.025-0.2M;
(2) and (3) removing the precipitate through suction filtration, then adding the filtrate into an acid solution with the volume of 2-15 times, and after the precipitate is completely precipitated, carrying out solid-liquid separation to obtain the precipitate, namely the lignin.
Preferably, the acid solution is a hydrochloric acid solution with a pH value of 1.5-3.
According to the invention, the Fenton catalytic reaction is applied to the separation and extraction of lignin in the lignocellulose raw material, the lignocellulose raw material is treated in a Fenton pretreatment system for a period of time, and the lignin in the lignocellulose raw material is extracted under mild conditions (including an alkaline method and a wood grinding method), and specific implementation results show that the extraction rate of the lignin in the raw material after Fenton pretreatment is higher than that under the condition of no pretreatment, and the obtained lignin has a larger molecular weight and a more complete structure; by performing infrared characterization analysis on the obtained lignin, the inventor finds that Fenton pretreatment not only can not influence the chemical structure of the lignin, but also can promote the separation of the lignin, so that the lignin with more complete molecular structure and functional groups is obtained; and proton nuclear magnetic resonance spectrum of lignin structure1H-NMR analysis results show that the lignin obtained after pretreatment has H, G, S characteristic peaks and belongs to a typical herbaceous lignin structure, and the pretreatment does not destroy beta-O-4 connecting bonds of the lignin, basically retains the original lignin structure and has little influence on the lignin skeleton structure.
In addition, in order to further improve the catalytic efficiency of the Fenton reaction, the invention also considers the synergistic effect of the thioglycolic acid on the Fenton reaction; the results show that after addition of thioglycolic acid, FentonThe pretreatment time is shortened from 72h to 16h, and the effective components in the corn straws can be retained to a greater extent, so that the method is an efficient and mild pretreatment system; after condition optimization and the pretreatment system with thioglycolic acid is used for treatment, the sugar content in the corn straw enzymatic hydrolysate is improved by 43.46% compared with a control group without thioglycolic acid, and is improved by 186.92% compared with a blank group without pretreatment; when the pretreatment system is used for extracting lignin, the extraction rate of the lignin in the corn straws can reach 87.06 percent; the improvement is 32.94 percent compared with the treatment group without adding thioglycolic acid under the same condition, and the improvement is 228.16 percent compared with the blank control without pretreatment. Relative GPC, FTIR,1H-NMR characterization analysis results show that compared with the traditional Fenton reaction system, the Fenton reaction system added with thioglycolic acid can promote the separation of lignin, and simultaneously retains a high proportion of beta-O-4 bonds in the lignin, so that natural lignin molecules with more complete molecular structures and better quality are obtained, the later-stage high-value conversion of the lignin is facilitated, and the technical problem of low extraction rate of high-quality lignin in the prior art is solved.
Drawings
FIG. 1 illustrates the effect of different pretreatment conditions on the degradation effect of corn stover;
FIG. 2 is a graph showing the molecular weight distribution of lignin extracted from corn stover under different pretreatment conditions;
FIG. 3 FT-IR spectra of lignin extracted from corn stover obtained under different pretreatment conditions;
FIG. 4 of three lignin samples1H-NMR spectrum analysis.
Detailed Description
The present invention will be described in further detail with reference to specific examples.
1. Test materials
The relevant experiments of the inventor show that the pretreatment system can obviously improve the degradation efficiency of lignocellulose in the wood raw materials such as eucalyptus, beech, pine, birch or poplar, and crop straws such as corn, wheat, sorghum, cotton or bagasse after the pretreatment; in the following examples, the inventor only takes corn stalks as an example to explain the effect, and the used corn stalks are produced in the consolidation city of Henan province; crushing the corn straws, sieving the crushed corn straws with a 60-mesh sieve, and drying the crushed corn straws for later use;
thioglycolic acid (TGA) used was purchased from Aladdin; the other various starting materials are all common commercial products or are obtained by methods known to the person skilled in the art or disclosed in the prior art.
2. Test method
(1) Degreasing treatment
Wrapping corn stalks with filter paper, placing the corn stalks in a Soxhlet extractor, and performing Soxhlet extraction with ethanol to remove lipids in the raw materials; drying the degreased raw materials in a fume hood, drying the degreased raw materials in an oven, and storing the dried raw materials in a dryer for later use;
(2) pretreatment of
Weighing 10g of the degreased corn straws, placing the degreased corn straws in a 200mL Fenton reaction pretreatment system, and after a period of treatment time, cleaning and drying a sample for later use;
the pretreatment system consists of 12.5mM FeCl prepared at present2 .4H2O solution and 1.76M H2O2The solution is prepared according to the equal volume, and a certain amount of thioglycolic acid is also added into a pretreatment system according to the experimental design;
(3) extraction of lignin
Taking 20g of corn straws before and after Fenton pretreatment, respectively placing the corn straws in 200mL of NaOH solutions with different concentrations, and reacting for 2h at different temperatures; filtering to remove precipitate, adjusting pH to 2 with 3% hydrochloric acid solution, centrifuging after precipitation is complete, and lyophilizing to obtain the final product;
(4) acetylation of lignin
When the molecular weight of the lignin is measured, the lignin needs to be acetylated, and the method comprises the following specific steps: 50mg of purified lignin is dissolved in 3mL of mixed solution of DMSO and NMS (2:1, v/v), and the mixture is reacted for 24 hours at room temperature in the dark; adding 1mL of acetic anhydride to react for 1.5h, and adding a small amount of ethanol into the reaction system to remove the excessive acetic anhydride; slowly adding the solution into a hydrochloric acid solution with 10 times of volume and pH value of 2 until a precipitate is separated out; washing the precipitate with ethanol until no acetic anhydride smell exists, centrifuging, and freeze-drying to obtain acetylated lignin for molecular weight determination; the purification of the lignin adopts a classical purification method of Beckmann lignin.
3. Analytical method
(1) Lignin molecular weight determination
The weight-average molecular weight (Mw) and the number-average molecular weight (Mn) of the lignin samples were determined by Agilent PL-GPC model 220 gel permeation chromatography under the following specific analytical conditions:
weighing 4mg of acetylated lignin sample, dissolving the acetylated lignin sample in 2mL of chromatographic grade tetrahydrofuran, taking a polystyrene standard sample as a reference molecular mass, taking tetrahydrofuran as an eluent, and detecting an ultraviolet absorption value at 254 n m, wherein the sample amount is 10 mu L;
(2) infrared spectroscopic analysis of lignin
Uniformly mixing KBr powder and a small amount of lignin sample in a ratio of 1:100, grinding and tabletting, analyzing by using a Fourier infrared spectrometer, wherein the scanning wavelength range is 4000-500cm-1The number of sample scans was 32, and the resolution was 2cm-1；
(3) Lignin proton nuclear magnetic resonance spectrum1H-NMR analysis
Dissolving 50mg lignin sample in 0.5mL DMSO-d6, transferring the solution into 5mm sample tube, and placing into probe of NMR spectrometer1H-NMR measurements were carried out using a reverse probe, the operating frequency of the measurements being 500MHz relative to DMSO-d6 (2.5 ppm).
Example 1
In order to investigate the influence of the fenton reaction on cellulose, lignin and hemicellulose in lignocellulosic feedstocks, the inventors selected two fenton pretreatment systems for investigation: system 1 is 12.5mM FeCl2 .4H2O solution and 1.76M H2O2The solution is 200ml of mixed solution which is composed of the components according to the volume ratio of 1: 1; the system 2 is a mixed solution formed by adding 0.008M thioglycolic acid into the system 1;
weighing 10g of corn straws, respectively placing the corn straws in the two pretreatment systems, and carrying out vibration treatment for 16h at 25 ℃; after treatment, carrying out suction filtration, washing and drying on the corn straw sample, and determining the content of cellulose, hemicellulose and lignin in the sample; taking corn straws which are not pretreated as a control group, setting three groups of parallel tests in the tests, and taking an average value of test results; the results of measuring the content of cellulose, hemicellulose and lignin in the corn stalk samples under different conditions are shown in table 1.
TABLE 1 determination of the composition of corn stover after treatment under different conditions
From the table above, it can be seen that certain loss of cellulose, hemicellulose and lignin in the corn stalks occurs after 16h of pretreatment, but through calculation, the retention rate of cellulose in the corn stalks after 16h of treatment in the system 2 is 98.4%, the retention rate of hemicellulose is 91.1%, the retention rate of lignin is 94.26%, and the retention rates of all three elements are higher than that of the system 1 without thioglycolic acid;
the pretreatment process can cause the loss of lignocellulose in the corn straws, which is inevitable, but after the corn straws are pretreated by using the system 2, the retention rate of all the three elements in the sample exceeds 90%, and the retention rate of lignin reaches 94.26%, so compared with the Fenton pretreatment of the system 1, the Fenton reaction system added with thioglycolic acid is a milder pretreatment system, and can retain the effective components in the corn straws to a greater extent.
Example 2
The results in example 1 show that the addition of thioglycolic acid can improve the retention rate of the three elements in the corn stalks after Fenton pretreatment; next, the inventors examined the effect of the amount of thioglycolic acid added on the effect of the pretreatment system.
Respectively putting the corn straws into a pretreatment system with the addition amount of thioglycollic acid of 0.008M, 0.016M, 0.032M, 0.064M and 0.128M for treatment for 16 h; after treatment, cleaning and drying the corn straw sample, and then carrying out enzymolysis saccharification treatment; the specific enzymolysis process is as follows:
weighing 0.5g of corn straw samples treated under different conditions, placing the corn straw samples in a 50mL triangular flask, adding 20mL of 0.05M citric acid-sodium citrate buffer solution, wherein the pH value of the buffer solution is 4.8; adding cellulase according to the amount of 20FPU/g dry material, stirring, and performing shake enzymolysis in a shaking table at 50 deg.C and 180 r/min for 72 h; adding an amount of ampicillin to prevent growth of the microorganism;
after enzymolysis, taking a proper amount of enzymolysis liquid, diluting by 10 times with water, measuring the sugar concentration in the enzymolysis liquid by using HPLC, and converting the sugar concentration into sugar content (unit g/100g represents the amount of sugar generated by 100g of straws); in the process, the corn straws which are treated for 72 hours by the system 1 in the embodiment 1 are used as a control group, and the corn straws which are not pretreated are used as a blank group; a comparison of the sugar content in the enzymatic saccharification liquid of corn stover under different treatment conditions is shown in FIG. 1.
As can be seen from FIG. 1, the sugar content in the corn straw enzymatic saccharification liquid treated for 16 hours with different thioglycolic acid addition amounts is higher than that of the control group, which indicates that the catalytic efficiency of a Fenton reaction system can be improved by adding thioglycolic acid; the sugar content in the pretreated corn straw enzymatic hydrolysate is gradually increased along with the increase of the addition amount of thioglycolic acid in the pretreatment system, and when the addition amount of thioglycolic acid in the pretreatment system is 0.064M, the sugar content in the pretreated corn straw enzymatic hydrolysate is the maximum and reaches 32.68g/100g, which is 43.46% higher than that of a control group and 186.92% higher than that of a blank group;
in the Fenton catalytic reaction, the reaction capable of generating free radicals is participated by ferrous ions, and the reduction process of the ferric ions generated in the process to the ferrous ions is the biggest bottleneck for limiting the effect of the Fenton reaction; the thioglycolic acid can be combined with ferric ions generated in the fenton reaction to form an iron-sulfur complex, and under the condition of the presence of oxygen, the complex can be rapidly decomposed into ferrous ions and thioglycolate, so that the conversion speed of the ferric ions to the ferrous ions is accelerated, the concentration of the ferrous ions in the reaction system is effectively increased, and the speed of the fenton catalytic reaction is improved, which is the main reason that the fenton catalytic efficiency is remarkably improved after the thioglycolic acid is added in the embodiment.
Example 3
The results show that the pretreatment system with added thioglycolic acid is used for treating for 16h, and the sugar content in the corn straw enzymatic hydrolysate is higher than that in the condition of not adding thioglycolic acid for treating for 72h, which indicates that the addition of thioglycolic acid can obviously improve the Fenton catalytic efficiency and promote the degradation of cellulose in the lignocellulose raw material; then, the inventor investigates the influence of different pretreatment systems on the separation and extraction of lignin in the lignocellulose raw material;
respectively placing 20g of corn straws before and after pretreatment in 200mL of NaOH solutions with the concentrations of 0.025M, 0.075M and 0.1M, and reacting for 2h at the temperature of 25 ℃, 50 ℃ and 75 ℃; filtering to remove precipitate, slowly adding the filtrate into 10 times of hydrochloric acid solution with pH of 2, centrifuging after the precipitate is completely precipitated, and obtaining lignin from the obtained precipitate; washing and drying the precipitate, weighing and measuring the extraction rate of the lignin obtained under different conditions; taking corn straws which are not pretreated as a blank group, setting three groups of parallel tests in the tests, and taking an average value of test results; the extraction rates of the obtained lignin under different conditions are shown in Table 2.
TABLE 2 comparison of the extraction rates of lignin from corn stover under different conditions
In the table above, F is a corn stalk sample treated for 16h in the system 1 of example 1; f + T is a corn straw sample processed for 16h by the system 2, and the addition amount of thioglycollic acid in the system 2 is 0.064M;
it can be seen that, compared with the blank group without pretreatment, under the conditions of different extraction temperatures and alkali concentrations, the extraction rate of lignin in the corn straws treated by the system 1 is higher than that under the conditions without pretreatment; compared with the system 1, the extraction rate of the lignin after the system 2 added with the thioglycolic acid is higher: taking the extraction temperature of 75 ℃ and the NaOH concentration of 0.1M as an example, after the system 2 added with the thioglycolic acid is used for treatment, the extraction rate of lignin in the corn straws can reach 87.06 percent, which is 32.94 percent higher than that of the system 1 not added with the thioglycolic acid and 228.16 percent higher than that of a blank group not subjected to pretreatment.
Example 4
The results show that the Fenton pretreatment system added with thioglycolic acid can obviously improve the extraction rate of lignin; in order to further reveal the accelerating effect of the pretreatment system added with thioglycolic acid on lignin separation and extraction, the inventors performed characterization analysis on the molecular structure, including the main chain structure, the side chain structure, the molecular weight and the like of the obtained lignin by means of GPC, FTIR, nuclear magnetic hydrogen spectrum and the like.
The alkaline extraction process of lignin is simple, the extraction efficiency is high, but the natural structure of lignin is easily damaged in the extraction process, so in the aspect of lignin structure analysis, the lignin extraction is carried out by adopting a wood grinding method with milder extraction conditions, and the specific process is as follows:
ball-milling 30g of corn straw samples under different pretreatment conditions for 2 hours by using a ball mill, wherein the ball-milling frequency is 500 r/min; in order to prevent the raw materials from being oxidized in the ball milling process, the ball mill is set to perform ball milling for three minutes, and the batch time is one minute; after the ball milling is finished, 20g of ball-milled sample is extracted by 96% dioxane at 100 ℃ for 2 hours (carried out twice, each time is 1 hour), and the load of the dioxane is 1g of straw/20 mL of dioxane; mixing the extractive solutions, concentrating the filtrate to 60ml with rotary evaporator, slowly adding the concentrated extractive solution into 95% ethanol 3 times the volume of the filtrate, and stirring to obtain precipitate as hemicellulose; filtering, removing precipitate, and concentrating the filtrate to 60ml again; dripping the concentrated solution into 10 times of hydrochloric acid solution with the pH value of 2.0, centrifuging, and freeze-drying the precipitate to obtain a crude extract of lignin;
purifying the crude lignin extract by a Beckmann lignin purification method before analysis and measurement, and then performing acetylation treatment; the specific characterization analysis results are as follows:
1. molecular weight of lignin
Measuring the molecular weight of lignin samples extracted under different conditions by an Agilent PL-GPC 220 type gel permeation chromatograph, wherein the molecular weight distribution curve is shown in figure 2;
integrating the obtained lignin molecular weight distribution curve to obtain the number average molecular weight M of the obtained ligninnWeight average molecular weight MwAnd are andpolydispersity index (M)w/Mn) Are shown in Table 3.
TABLE 3 comparison of molecular weight distribution of lignin extracted from corn stover obtained under different pretreatment conditions
In the above table, F is a sample set of 16h processed by the system 1 in example 1; f + T is a sample group processed for 16h by the system 2, wherein the addition amount of thioglycolic acid in the system 2 is 0.032M; the blank group is a corn straw sample group which is not pretreated;
it can be seen that the number average molecular weight M of the obtained lignin after Fenton treatmentnAnd a weight average molecular weight MwAre all larger than the corn straws which are not pretreated; the number average molecular weight and the weight average molecular weight of the lignin obtained after the pretreatment system with the thioglycolic acid is treated are slightly lower than those of the lignin sample obtained without the thioglycolic acid, but the polydispersity is lower than that of the lignin sample obtained without the thioglycolic acid, so that the lignin obtained after the pretreatment system with the thioglycolic acid is treated is more concentrated in molecular weight distribution and more uniform in components.
2. Lignin FT-IR
Fourier transform infrared spectroscopy is an important tool for researching the physical and chemical structure and functional group change of lignin, and can detect information of various functional groups and chemical bonds in lignin, such as information of important functional groups related to the main structure of lignin, such as hydroxyl, carbonyl, methoxy and the like. Due to the complexity of the lignin structure and the uncertainty and irregularity of the connection between the monomers, the infrared spectrum of the lignin is difficult to analyze by using the traditional radical theory, the attribution of each functional group in the lignin is an empirical result obtained by various researches, and the attribution of common infrared characteristic peaks of the lignin in the literature is listed in table 4; using FT-IR technique at 4000--1The lignin samples obtained under different conditions were tested within the range, and the test results are shown in fig. 3.
TABLE 4 common infrared characteristic peak assignment of lignin
As can be seen from the above table, the characteristic absorption of the lignin functional groups is mainly concentrated at 800-1800cm-1A fingerprint area of;
as can be seen from FIG. 3, the lignin obtained by pretreatment has a significant change in this region, mainly represented by 1707cm-1Increase in C = O stretching vibration absorption Peak of non-conjugated carbonyl ketone and carbonyl group, 1513cm-1The absorption intensity of the stretching vibration absorption peak of the benzene ring skeleton is obviously enhanced, and 1656cm-1C = O telescopic vibration of carbonyl conjugated aromatic ketone, 1239cm-1Condensation of an aromatic nucleus associated with the syringyl nucleus C = O, 1123cm-1Guaiacyl and syringyl C-C, C-O stretching vibration and 833cmPurple lower fragrant of-1The C-H stretching vibration is obviously increased; the results show that the Fenton reaction pretreatment process does not substantially damage the chemical structure and functional groups of lignin in the corn straws, and the main framework structure of the lignin is kept; it can also be seen from the figure that the thioglycolic acid added pretreatment conditions resulted in lignin with a greater amplitude of the relevant fingerprint region, i.e. a higher degree of separation of the resulting lignin, compared to the pretreatment conditions without thioglycolic acid addition.
3. Lignin proton nuclear magnetic resonance spectrum1H-NMR analysis
1H-NMR is a means for analyzing the hydrogen-containing structure of lignin, in which hydrogen in the macromolecular structure of lignin has hydrogen on the benzene ring skeleton and hydrogen on the side chain of the benzene ring (ether bond C between benzene rings)α，β，γHydrogen, phenolic hydroxyl, alcoholic hydroxyl) and the like, and the different hydrogen-containing structures of the lignin can be quantitatively analyzed according to the proportional relationship between the peak area and the number of hydrogen atoms, namely the three types of lignin1The results of H-NMR analysis are shown in FIG. 4;
as can be seen from fig. 4, all three samples had a characteristic peak of lignin H, G, S, typical of herbal lignin structure. Syringyl phenylpropane (S) and guaiacyl phenylpropane at chemical shift of 6.7-6.79ppm(G) The proton generated on the aromatic ring on the structural unit indicates that the S unit and the G unit content in the two samples are equivalent; 7.5ppm is hydrogen in p-hydroxyphenyl group, 4.17ppm is the peak of signal from H linked to carbon in aromatic ether linkageβAnd HγThe generated figure shows that the lignin obtained by pretreatment has beta-O-4 bonds and the absorption intensity is consistent with that of the untreated lignin, and the beta-O-4 bonds of the lignin are not basically damaged in the pretreatment process; in addition, 3.72ppm is a strong signal peak of wood methoxyl group, 1.51ppm is a proton signal peak of methyl and methylene in carbonyl, and the aliphatic proton signal peak of lignin side chain is between 0.86 and 1.24ppm, and the sites can also show that the lignin extracted from the corn straw after pretreatment basically keeps the original lignin structure, which further shows that the pretreatment of the invention does not cause significant influence on the main structure of the lignin.
In conclusion, compared with the traditional Fenton reaction system, the Fenton reaction system added with thioglycolic acid can promote the separation of lignin, and simultaneously retains a high proportion of beta-O-4 bonds in the lignin, so that natural lignin molecules with more complete molecular structures and better quality are obtained, the later-stage high-value conversion of the lignin is facilitated, and the technical problem of low extraction rate of high-quality lignin in the prior art is solved.
Claims (8)
1. The method for efficiently extracting lignin from a lignocellulose raw material is characterized by comprising the following steps of: placing the lignocellulose raw material in a Fenton reaction pretreatment system, and treating for 6-72 hours; then, the lignin in the raw materials is separated and extracted by a wood grinding method or an alkaline method;
the Fenton reaction pretreatment system is a mixed solution consisting of 3-9 mmol/L of ferrous compound, 0.2-1.6 mol/L of peroxide and 0.032-0.128 mol/L of sulfhydryl compound;
the ferrous compound is FeCl2、FeCl2Hydrate of (2), FeSO4、FeSO4Any one or a mixture of two or more of the hydrates of (a); the peroxide is any one or a mixture of hydrogen peroxide and sodium peroxide; the mercapto compound is thioglycolic acid, mercaptopropionic acid, thioglycolic ester, or mercaptopropionateAny one or a mixture of two or more.
2. The method of claim 1, wherein: 6.25mmol/L of ferrous compound, 0.88mol/L of peroxide and 0.032-0.128 mol/L of sulfhydryl compound in the pretreatment system.
3. The method of claim 2, wherein: the ferrous compound is FeCl2﹒4H2O; the peroxide is hydrogen peroxide; the mercapto compound mercaptoacetic acid.
4. The method of claim 3, wherein: the processing time of the lignocellulose raw material in a Fenton pretreatment system is 12-20 h.
5. The method of any of claims 1 to 4, wherein: the addition amount of the lignocellulose raw material in the pretreatment system is 1 g: 5-30 mL; the lignocellulose raw material is a wood raw material or crop straws.
6. The method as claimed in claim 1, wherein the wood grinding method comprises the following specific steps:
(1) placing the lignocellulose raw material subjected to Fenton pretreatment into a ball mill for ball milling for 1-5 h;
(2) extracting for 1-5 h at 95-105 ℃ by using dioxane after the ball milling is finished, wherein the mass-volume ratio of the raw material obtained in the step (1) to the dioxane is 1 g: 10-30 mL;
(3) concentrating the extracting solution to a certain volume, and then dropwise adding the extracting solution into 2-5 times of ethanol; after removing the precipitate, concentrating the filtrate again to a certain volume;
(4) and adding the concentrated solution into an acid solution with the volume 5-15 times that of the concentrated solution, and after the precipitation is completed, carrying out solid-liquid separation to obtain a precipitate, namely the lignin.
7. The method of claim 1, wherein the alkaline extraction of lignin comprises the following steps:
(1) placing the lignocellulose raw material subjected to Fenton pretreatment in a strong alkali solution, and reacting for 1-10 hours at 25-75 ℃; the mass volume ratio of the lignocellulose raw material to the strong alkali solution is 1 g: 5-20 mL;
the strong base is sodium hydroxide, potassium hydroxide or calcium hydroxide, and the concentration of the base is 0.025-0.2M;
(2) and (3) removing the precipitate through suction filtration, then adding the filtrate into an acid solution with the volume of 2-15 times, and after the precipitate is completely precipitated, carrying out solid-liquid separation to obtain the precipitate, namely the lignin.
8. The method of claim 6 or 7, wherein: the acid solution is a hydrochloric acid solution with the pH value of 1.5-3.
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