CN111116936A - Lignin separation and extraction method based on FeOCl Fenton reaction - Google Patents
Lignin separation and extraction method based on FeOCl Fenton reaction Download PDFInfo
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Abstract
The invention belongs to the technical field of lignocellulose pretreatment, and particularly relates to a lignin separation and extraction method based on FeOCl Fenton reaction. The method comprises the following steps: placing the lignocellulose raw material in a Fenton-like pretreatment system consisting of FeOCl, and treating for 1-12 h; the addition amount of the lignocellulose raw material is 1g: 5-30 mL; extracting lignin in the raw materials by an alkaline method or a wood grinding method; the effective components of the Fenton-like pretreatment system consisting of FeOOL consist of the following components: 0.2-3.5g/L of FeOCl and 0.09-1.55mol/L of hydrogen peroxide; the pH value of the pretreatment system is 3-8. The method has the advantages that the method can promote the separation and extraction of lignin in the lignocellulose raw material by treating the lignocellulose raw material by using an FeOCl Fenton pretreatment system, and the obtained lignin has high molecular weight, more complete structure and more uniform components.
Description
Technical Field
The invention belongs to the technical field of lignocellulose pretreatment, and particularly relates to a lignin separation and extraction method based on FeOCl Fenton reaction.
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. Lignocellulosic biomass represented by crop straws and forest residues is rich in cellulose and hemicellulose to replace traditional resources and lignin, and is considered to be the most promising renewable resource for replacing petrochemical resources at present. Wherein, the cellulose and hemicellulose polysaccharide carbohydrate can be hydrolyzed by enzyme to obtain monosaccharide platform compound, and can be further prepared into biomass energy sources and fine chemicals such as ethanol, butanol, xylitol, succinic acid, furfural, lactic acid and the like by microbial fermentation or chemical catalytic conversion.
Early biorefinery, with the aim of comprehensive utilization of carbohydrates, lignin is considered to be a main obstacle influencing carbohydrate conversion, and often adopts severe pretreatment means such as high temperature, high pressure, strong acid, strong alkali and the like to remove lignin to the maximum extent so as to realize efficient enzymatic hydrolysis conversion of carbohydrates.
In view of the fact that structural changes of lignin mainly originate from the reserved ratio of β -O-4 bonds, the current strategies mainly comprise two types, namely a passive protection method and an active protection method, the passive protection method is used for extracting lignin products with low polycondensation under milder conditions by means of reducing the severity of pretreatment conditions, the lignin obtained by the method is used for later catalytic conversion, the lignin retains a high proportion of β -O-4 bonds, is beneficial to later high-value conversion, but has very low extraction efficiency and limits application of the lignin, and in order to solve the problem, the researchers use formaldehyde to protect active hydroxyl groups of the lignin in advance, the protection can realize high proportion reservation of β -O-4 bonds under relatively severe extraction conditions, improve the extraction efficiency of the lignin, the investment of raw materials is increased, meanwhile, the activated hydroxyl groups of the lignin are protected by formaldehyde in advance, the lignin is separated in later stages, the lignin is always subjected to selective catalytic depolymerization, the lignin is degraded by using a catalyst, and the lignin is difficult to be decomposed in situ, and the lignin is degraded by using a catalyst.
Disclosure of Invention
In order to solve the problems in the prior art, the invention aims to provide a lignin separation and extraction method based on FeOCl Fenton reaction, the separation and extraction of lignin in the raw material are promoted by treating the lignocellulose raw material by using a FeOCl Fenton pretreatment system, and the obtained lignin has a large molecular weight, a more complete structure and more uniform components.
In order to achieve the technical purpose, the invention adopts the following technical scheme:
the method for separating and extracting the lignin based on FeOCl Fenton reaction comprises the following steps:
1) placing the lignocellulose raw material in a Fenton-like pretreatment system consisting of FeOCl, and treating for 1-12 h;
the addition amount of the lignocellulose raw material is 1g: 5-30 mL;
2) extracting lignin in the raw materials by an alkaline method or a wood grinding method;
the effective components of the Fenton-like pretreatment system consisting of FeOOL consist of the following components:
0.2-3.5g/L of FeOCl and 0.09-1.55mol/L of hydrogen peroxide; the pH value of the pretreatment system is 3-8.
Preferably, the effective components of the Fenton-like pretreatment system consist of the following components: FeOCl 0.8-1.6g/L and hydrogen peroxide 0.78-1.55 mol/L.
Preferably, the lignocellulose raw material is wood raw material or crop straw; the wood raw material or the crop straw is subjected to degreasing treatment before being subjected to pretreatment.
Preferably, the alkaline extraction method of lignin comprises the following steps:
1) putting the pretreated lignocellulose raw material into 0.025-0.2mol/L strong alkali solution, and reacting for 1-10h at 25-75 ℃;
the solid-liquid ratio of the raw materials to the alkali liquor is 1g: 5-20 mL;
the strong base is any one or a mixture of two of sodium hydroxide, potassium hydroxide and calcium hydroxide in any proportion;
2) filtering to remove the precipitate, and adding the filtrate into 2-15 times of acid solution;
3) and (4) after the precipitation is complete, carrying out solid-liquid separation to obtain the precipitate, namely the lignin.
Further preferably, the acid solution is a hydrochloric acid solution having a pH of 2.
Preferably, the wood grinding method for extracting lignin in the method comprises the following steps:
1) placing the pretreated lignocellulose raw material into a ball mill for ball milling for 1-5 h;
2) extracting for 1-5h at 95-105 ℃ by using dioxane after the ball milling is finished, wherein the load of the dioxane is 1g: 10-30 mL;
3) concentrating the extract to a certain volume, and then adding the concentrated extract into 2-5 times of ethanol;
4) after the precipitation is completed, removing the precipitation, and then concentrating the filtrate again to a certain volume;
5) and adding the concentrated solution into an acid solution with the volume of 5-15 times, and after the precipitation is complete, carrying out solid-liquid separation to obtain a precipitate, namely the lignin.
Based on a general inventive concept, the present invention also includes lignin obtained by the above method, which has a number average molecular weight of 2100-.
The invention applies the Fenton-like catalytic reaction composed of FeOCl and hydrogen peroxide to the separation and extraction of lignin in the lignocellulose raw material: the method comprises the steps of treating a lignocellulose raw material in a pretreatment system for a period of time, and extracting lignin in the lignocellulose raw material under mild conditions (including an alkaline method and a wood grinding method), wherein the results show that under different extraction conditions, the extraction rate of the lignin in the pretreated raw material is higher than that under the conditions without pretreatment; the lignin obtained after pretreatment was determined to have a number average molecular weight of about 2136, a weight average molecular weight of about 2905, and a dispersibility of 1.36, whereas the lignin obtained without pretreatment under the same extraction conditions had a number average molecular weight of about 1627, a weight average molecular weight of about 2263, and a dispersibility of 1.39; the lignin separation and extraction method based on FeOCl Fenton reaction can improve the extraction rate of lignin and obtain lignin with large molecular weight and concentrated distribution;
by carrying out infrared characterization analysis on lignin structures obtained under different conditions, we find that after a pretreatment system based on FeOCl is obtained, the lignin chemical structure is not influenced, and the lignin separation is promoted, so that lignin with more complete molecular structure and functional groups is obtained; and proton nuclear magnetic resonance spectrum of the obtained lignin structure1H-NMR analysis results show that the lignin obtained after pretreatment has H, G, S characteristic peaks belonging to a typical herbaceous lignin structure, and the lignin is pretreated and subjected toThe β -O-4 connecting bond of the lignin is not damaged, the structure of the original lignin is basically reserved, and the influence on the skeleton structure of the lignin is small.
The implementation result of the invention fully proves that the lignin separation and extraction method based on FeOCl Fenton reaction can keep β -O-4 bonds with high proportion in lignin, is beneficial to high-value conversion in the later period, has high lignin extraction efficiency, overcomes the technical problem of low high-quality lignin extraction rate in the prior art, has low FeOCl cost and easy preparation, and effectively reduces the use cost of the catalyst, thereby providing the high-quality lignin separation and extraction method which is economic and efficient.
Drawings
FIG. 1 is an XRD analysis pattern of FeOCl prepared;
FIG. 2 is an XPS analysis of FeOOL;
FIG. 3 is an SEM electron micrograph of FeOOL prepared in FIG. 3;
FIG. 4 shows the effect of pretreatment system on the enzymatic hydrolysis of lignocellulose at different pH values;
FIG. 5 shows the effect of different FeOOCl additions on the pretreatment effect;
FIG. 6 shows a modification H2O2Influence of the addition amount on the pretreatment effect;
FIG. 7 the effect of different pretreatment times on the pretreatment effect;
FIG. 8 is a Fourier infrared spectrum analysis spectrum of lignin obtained under different extraction conditions;
FIG. 9 NMR spectra of lignin obtained before and after pretreatment1H-NMR spectrum.
Detailed Description
The present invention will be described in further detail with reference to specific examples.
Relevant experiments of the inventor show that the pretreatment system can obviously improve the degradation efficiency of lignocellulose in the wood raw materials after pretreatment is carried out on the wood raw materials of eucalyptus, zelkova, pine, birch and poplar, and crop straws of corn, wheat, sorghum, cotton and bagasse; 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;
cellulose complex enzyme (CTec) used in the invention2) Purchased from novicent (china) biotechnology limited; 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.
Example 1
The FeOCl used in the invention is prepared by the following steps:
putting a proper amount of ferric trichloride hexahydrate into a mortar, fully grinding, and then transferring into a crucible; covering a crucible cover, putting the crucible cover into a muffle furnace, and heating for 60min at 220 ℃ in an air atmosphere; and taking out the sample, cleaning the sample by using anhydrous acetone, and removing unreacted ferric trichloride to obtain mauve FeOCl.
In order to verify the purity of the FeOCl, the FeOCl is characterized and analyzed by X-ray diffraction, X-ray photoelectron spectroscopy and a Scanning Electron Microscope (SEM);
FIG. 1 is an XRD (X-ray diffraction) spectrum of prepared FeOOL, and it can be seen that the sample prepared by the invention completely corresponds to the standard peak position of FeOOL, and the diffraction peak shape is sharp, which indicates that the prepared FeOOL has high purity and complete crystal form;
FIG. 2 is an XPS spectrum of FeOCl, which shows that the valence structure of each element in the obtained FeOCl conforms to the valence and energy level distribution of the element in the FeOCl;
FIG. 3 is an SEM electron micrograph of the prepared FeOCl sample, and it can be seen from the SEM images under different magnifications that the microstructure of the prepared FeOCl is in an obvious layered structure and conforms to the characteristics of the typical layered structure of FeOCl.
In summary, the FeOCl prepared in this embodiment has a complete structure, and the valence, energy level distribution and microstructure of each element in the compound all conform to the characteristics of typical FeOCl, and the FeOCl is used for subsequent tests.
Example 2
In order to investigate the pretreatment effect of a Fenton-like catalytic reaction system in which FeOOL participates on a lignocellulose raw material, the invention utilizes the FeOOL and hydrogen peroxide to form the Fenton-like reaction system to pretreat corn straws, and then inspects the degradation effect of cellulose in the pretreated corn straws, wherein the specific test process is as follows:
1. test method
(1) Fenton-like pretreatment
Weighing 5g of crushed corn straws, adding the crushed corn straws into a 100mL pretreatment system, processing the crushed corn straws for a period of time at room temperature, and washing and drying a sample for subsequent enzymolysis;
(2) enzymolysis treatment
Weighing 0.5g of the treated corn straw sample, placing the corn straw sample into 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 cellulose complex enzyme 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 48 h; adding an amount of antibiotic (ampicillin) to prevent the growth of microorganisms; after enzymolysis, a proper amount of saccharification liquid is diluted by 10 times with water to measure the sugar concentration.
2. Measurement method
Determination of cellulose, hemicellulose and lignin the determination was carried out according to the biomass trix determination procedure established by the us renewable energy laboratory (NREL);
the sugar concentration is measured by a Dionex P680 high performance liquid chromatograph, and the detector is an RI101 refractive index detector; hydrogen ion exchange chromatography column A Minex HPX-87H, column temperature 55 deg.C, mobile phase 5mmol/L H2SO4(pH2.0) and a flow rate of 0.6 mL/min.
3. Formula for calculation
The following formula for calculating the retention of cellulose, hemicellulose and lignin is shown in formula 1:
4. Results and analysis
4.1 Effect of pretreatment on the major constituents in corn stover
In order to investigate the influence of a pretreatment system consisting of FeOOCl on the components of the corn stalks, 5g of the corn stalks are placed in a 100mL pretreatment system for treatment for 12 hours, wherein the addition amount of FeOOCl is 0.16g, and H is added2O20.78 mol/L; after treatment, determining the content of lignin, cellulose and hemicellulose in the corn straws, and calculating the retention rate;
through determination, the retention rate of lignin in the pretreated corn straws is 94.34%, the retention rate of cellulose is 93.56%, and the retention rate of hemicellulose is 93.79%; the pretreatment process can cause the loss of partial components in the raw materials, which is inevitable, but after the pretreatment system is used for pretreating the corn straws, the retention rate of the three elements in the sample exceeds 93 percent, which indicates that the pretreatment system is a mild pretreatment system and has little influence on the content of the three elements in the corn straws.
4.2 Effect of the pH value of the pretreatment System on the treatment Effect
Next, we investigated the degradation effect of pretreatment systems on cellulose in corn stalks under different pH conditions: respectively adjusting the pH value of the pretreatment system to 3, 4, 5, 6, 7 and 8, treating for 24 hours, and then washing and drying a sample; adding the pretreated corn straws into a biological enzymolysis reaction system for enzymolysis, determining the sugar concentration in an enzymolysis solution, and converting the sugar concentration and the amount of the added corn straws into sugar content (the unit is g/100g, and the sugar content is expressed by the amount of sugar generated by 100g of straws); three groups of parallels are arranged in the test, and the test results are averaged;
the sugar content of the enzymolysis liquid obtained after 48h enzymolysis of corn stalks under different pH values is shown in figure 4;
it can be seen that after the corn straws are treated by the pretreatment systems under different pH values, the sugar content in the enzymolysis liquid has no obvious difference; the Fenton-like catalytic pretreatment system with participation of FeOCl can perform Fenton-like catalytic reaction within the pH value range of 3-8, and overcomes the defect that the traditional Fenton catalytic reaction can only be performed under acidic conditions.
4.3 Effect of FeOCl content in pretreatment System on treatment Effect
In order to investigate the influence of different component contents of a pretreatment system on the enzymolysis efficiency of the corn straws, the corn straws are respectively placed in the pretreatment systems with the addition amounts of FeOCl of 0.02g, 0.04g, 0.08g, 0.16g and 0.32g for treatment for 12 hours; then carrying out enzymolysis treatment on the pretreated corn straws, and determining the sugar content of an enzymolysis solution; taking corn straws which are not pretreated as a blank control group (CK); three groups of parallel tests are set in the tests, and the test results are averaged; the sugar content of the corn stalk enzymatic hydrolysate is shown in figure 5;
it can be seen that the sugar content in the pretreated corn stalk enzymatic hydrolysate is higher than that in the blank control group which is not pretreated; with the increase of the addition of FeOCl, the sugar content in the pretreated corn straw enzymatic hydrolysate is gradually increased, and when the addition is 0.16g, the sugar content in the corn straw enzymatic hydrolysate reaches 19.18g/100g at most; then, with further increase of the addition amount, the sugar content in the enzymolysis liquid is not greatly increased, and the enzymolysis liquid basically presents a steady state.
4.4 pretreatment of System H2O2Influence of the content on the treatment Effect
Next, consider the difference H2O2Influence of the addition amount on the enzymolysis efficiency of the corn straws; wherein H2O2The addition amounts are respectively 0.09mol/L, 0.19mol/L, 0.38mol/L, 0.78mol/L and 1.55 mol/L; after 12h of treatment, the determination result of the sugar content in the corn straw enzymatic hydrolysate is shown in figure 6;
it can be seen that when H is2O2When the addition amount is 0.09-0.38mol/L, the sugar content in the pretreated corn straw enzymatic hydrolysate is slowly increased from 7.56g/100g to 10.21g/100 g; when H is present2O2When the addition amount is 0.78mol/L, the sugar content in the pretreated corn straw enzymatic hydrolysate is rapidly increased to about 2 times under the condition of 0.38mol/L, and the sugar content reaches 21.13g/100 g; when H is present2O2The addition amount is continuously increased to 1.55mol/L, the sugar content in the corn straw enzymolysis liquid finally reaches 21.73g/100g, and is only increased by 2.84 percent compared with the 21.13g/100g when the addition amount is 0.78 mol/L.
As can be seen from the above test results, FeOCl is associated with H2O2The formed Fenton-like pretreatment system can improve the yield of the corn strawsThe enzymolysis efficiency of the cellulose is high, but the more the FeOOCl is added in the system, the better, so 0.08-0.16g is the better addition range of FeOOCl, and 0.78-1.55mol/L is H2O2The preferred addition range of (3).
4.5 Effect of pretreatment time on treatment Effect
To examine the influence of pretreatment time on the enzymolysis efficiency of corn stalks, 5g of stalks were added to a mixture containing 0.16g of FeOCl and 0.78mol/L H2O2Setting pretreatment time gradients of 1h, 2h, 3h, 6h, 9h and 12h in a 100mL pretreatment system, carrying out enzymolysis treatment on the pretreated corn straw sample, and determining the sugar content in an enzymolysis solution; 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 change curve of sugar content in the corn straw enzymatic hydrolysate pretreated at different times is shown in figure 7;
as can be seen from the figure, when the pretreatment time is 6 hours, the sugar content in the corn straw enzymatic hydrolysate reaches the maximum value of 21.68g/100 g; then, the sugar content in the enzymolysis liquid of the treated corn straws is not increased continuously but is reduced to a certain extent along with the continuous increase of the pretreatment time to 9h and 12h, but the sugar content in the enzymolysis liquid of the pretreated corn straws is closer to and higher than the sugar content in the enzymolysis liquid of the pretreated corn straws after the pretreatment time of 9h and 12 h; the optimum treatment effect can be achieved after the Fenton-like catalytic system composed of FeOOL is treated for 6 hours, and compared with the treatment time of 120 hours of the existing Fenton catalytic system in the background art, the Fenton-like catalytic system composed of FeOOL can obviously shorten the pretreatment time of the corn straws, so that the degradation efficiency of cellulose in the corn straws is effectively improved.
Example 3
The test result of the embodiment 2 shows that the Fenton-like pretreatment system composed of FeOCl can improve the enzymolysis efficiency of the cellulose in the corn straw within the range of pH value of 3-8, that is, the Fenton-like pretreatment system can promote the degradation of the cellulose in the corn straw, and effectively shorten the pretreatment time. In the natural lignocellulose raw material, cellulose, hemicellulose and lignin exist in a certain covalent bonding mode, and the degradation of the cellulose is theoretically accompanied with the degradation of the lignin and the hemicellulose; therefore, the inventor next inspects the influence of the pretreatment system on the degradation and extraction of lignin in the corn straws, and the specific test process is as follows:
1. test method
(1) Degreasing treatment
When the corn straws are used for extracting lignin, firstly carrying out hydroalcoholic treatment to remove pigments and lipids of the straws, and specifically comprising the following steps: taking a certain amount of corn straw raw material, and mixing the raw material according to a solid-liquid ratio of 1g: adding 10mL of distilled water, and treating for 2h at 60 ℃; filtering to remove filtrate by vacuum pump, drying the raw materials, wrapping with filter paper, and performing Soxhlet extraction with ethanol; putting the degreased raw materials in a fume hood for airing, and then drying for later use;
(2) fenton-like pretreatment
5g of corn stover feedstock treated with hydroalcoholic solvent was placed in 100ml of FEOCl and H2O2The pretreatment system is used for 6 hours, the addition amount of FeOCl in the pretreatment system is 1.6g/L, and H is added2O20.78 mol/L; after pretreatment, cleaning and drying the raw materials for subsequent use;
(3) separating and extracting lignin
Taking 20g of corn straws before and after Fenton-like 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 lignin;
(4) acetylation of lignin
50mg of lignin is dissolved in 3 ml of DMSO NMS (2:1, v/v) mixed solution and reacted for 24 hours at room temperature in the dark; adding 1 mL 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 the acetylated lignin for molecular weight determination.
2. 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 4 mg 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 254nm, 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 500M HZ based on DMSO-d6 (2.5 ppm).
3. Results and discussion
3.1 Effect of Fenton-like pretreatment on extraction yield of Lignin from corn stover
In order to investigate the influence of Fenton-like pretreatment on the extraction rate of lignin in the corn straws, the corn straws subjected to Fenton-like pretreatment are respectively placed in 0.025mol/L, 0.05mol/L and 0.1mol/L NaOH solutions, and are respectively reacted for 2 hours at the conditions of 25 ℃, 50 ℃ and 75 ℃, and then the lignin in the corn straws is extracted; the ratio of the total content of the lignin in the obtained lignin and the corn straw is the extraction rate of the lignin; in the test, untreated corn straws are used as a control group, three groups of parallel tests are set in each group, and the test results are averaged; the extraction rate of lignin in the corn stalks under different pretreatment time and extraction conditions is shown in tables 1 to 3;
TABLE 125 deg.C lignin amount extracted at different NaOH concentrations
TABLE 250 ℃ quantity of lignin extracted at different NaOH concentrations
TABLE 375 deg.C amount of lignin extracted at different NaOH concentrations
From the aspect of NaOH extraction concentration, the extraction rate of lignin obtained from the corn straws is gradually increased before and after pretreatment along with the increase of the concentration of the used NaOH; under the conditions of different reaction temperatures, the extraction rate of lignin in the corn straws subjected to Fenton-like pretreatment is higher than that under the conditions without pretreatment; the lignin extraction rate of the pretreatment for 6 hours is higher than that of the pretreatment for 3 hours;
when the lignin is extracted by an alkaline method, the connecting bonds of the lignin, cellulose and hemicellulose in the corn straws are dissociated under the action of strong alkali, so that the higher the concentration of the alkali used is, the higher the extraction rate of the lignin is; the results show that the Fenton-like catalytic reaction system can promote the dissociation of lignin in the pretreatment stage, so that the extraction rate of the lignin extracted by a subsequent alkaline method is improved.
3.2 Infrared characterization of Lignin
In order to investigate the influence of pretreatment and different extraction conditions on the chemical structure and functional groups of the obtained lignin, infrared spectrum analysis is carried out on the structure of the lignin by using a Fourier infrared spectrum analyzer (figure 8); the extraction temperature of the obtained lignin is 75 ℃; a, B, C, D in the figure are infrared spectra of lignin obtained under the conditions of NaOH concentration of 0.025mol/L, 0.05mol/L, 0.1mol/L and 0.2mol/L respectively;
the characteristic absorption of each functional group of lignin is mainly concentrated at 800-1800cm-1Of the fingerprint area, wherein, 1707cm-1C = O stretching vibration absorption peak of ketone and carbonyl representing non-conjugated carbonyl, 1656cm-1Represents a C = O stretching shock absorption peak of carbonyl conjugated aromatic ketone, 1513cm-1Representing the absorption peak of stretching vibration of benzene ring skeleton, 1241cm-1Represents the C = O condensation absorption peak of the aromatic nucleus associated with the syringyl nucleus, 1126cm-1Representing the C-C, C-O stretching shock absorption peak of guaiacyl and syringyl, 835cm-1Represents a C-H telescopic shock absorption peak of the purple lower fragrant group;
as can be seen from the figure, the lignin sample after Fenton-like pretreatment has 1513cm of lignin when NaOH is 0.025M-1And 1126cm-1The absorption peak-to-average vibration amplitude is larger than that of the lignin which is not pretreated, and is 835cm-1The absorption peak is obvious, and 835cm in the lignin sample without pretreatment-1The absorption peak at (a) almost disappeared; the fact shows that the Fenton-like 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;
with the increase of the concentration of NaOH used for extracting lignin, the influence of pretreatment on the lignin structure is gradually reduced, for example, under the condition of 0.05-0.1M, the absorption peaks of two lignin samples are relatively close; 1513cm in lignin sample obtained without pretreatment under 0.2M condition-1、1126cm-1And 835cm-1The absorption peak is larger than the absorption peak amplitude of lignin obtained by pretreatment; however, the infrared spectra of the lignin obtained under the conditions of different NaOH concentrations are compared together, so that the peak patterns of absorption peaks in the lignin are widened along with the increase of the NaOH concentration, and the discrimination between the absorption peaks is reduced; the reason for this may be that the high concentration of alkali can open the connection between lignin and hemicellulose and cellulose, thereby promoting the separation and extraction of lignin, but too high concentration of alkali can also cause the degradation of lignin, destroy the main structure and functional groups of lignin molecules, and the destruction increases with the increase of alkali concentration, thereby counteracting the promotion of the fenton pretreatment on the complete separation of lignin.
3.3 determination of the molecular weight of Lignin
The results show that the Fenton-like pretreatment system can promote the separation of lignin in the corn straws, so that the extraction rate of the lignin is improved, and the molecular structure (including a benzene ring framework and functional groups) of the obtained lignin is more complete; however, the alkali concentration in the extraction process is too high (such as 0.2mol/L NaOH), so that the structure of the separated lignin is damaged, and the promotion effect of the pretreatment effect is weakened; in order to further reveal the promotion effect of the pretreatment system on lignin separation, the method extracts lignin from the corn straws before and after pretreatment by using a milder wood grinding method, and measures the molecular weight of the obtained lignin;
the wood grinding method comprises the following specific steps:
(1) 30g of corn straws subjected to 6h Fenton-like pretreatment or non-pretreated corn straws are put into a ball mill for ball milling for 2h at 500 revolutions per minute;
(2) extracting for 2 hours at 100 ℃ by using 96% dioxane after the ball milling is finished, wherein the load of the dioxane is 1g and is 20 mL;
(3) concentrating the extracting solution to 60mL, and then dropwise adding the extracting solution into 95% ethanol with 3 times of volume; removing the precipitate, and then concentrating the filtrate to 60mL again;
(4) slowly adding the concentrated solution into 10 times of hydrochloric acid solution with pH value of about 2, and filtering or centrifuging to obtain precipitate, i.e. lignin;
purifying the obtained lignin by a Beckmann lignin purification method, and measuring the molecular weight of the lignin by acetylation; the number average molecular weight M of the obtained ligninnWeight average molecular weight MwAnd polydispersity index (M)w/Mn) Are listed in Table 4;
TABLE 4 comparison of the molecular weights of the lignins obtained under different conditions
The Fenton-like pretreatment is carried out for 6 hours, the number average molecular weight and the weight average molecular weight of the obtained lignin are both larger than those of lignin obtained without pretreatment, and the dispersibility of the lignin is more concentrated, so that the Fenton-like pretreatment system provided by the invention can promote the separation of lignin in the corn straws, and the obtained lignin has a more complete molecular structure and is more concentrated in distribution.
3.4 Lignin proton Nuclear magnetic resonance Spectroscopy1H-NMR analysis
1H-NMR is a means for analyzing the hydrogen-containing structure of lignin, and the structure of lignin can be analyzed by the appearance of resonance peaks due to different chemical shifts in a map; in order to reveal the mechanism of the promotion action of the pretreatment system on the separation and extraction of lignin, the inventor carries out proton nuclear magnetic resonance spectroscopy on the lignin extracted by the wood grinding method1The H-NMR analysis and the specific analysis result are shown in figure 9;
as can be seen from the figure, the sample extracted from the corn straws before and after the pretreatment has characteristic peaks of lignin H, G, S belonging to a typical herbaceous lignin structure, wherein chemical shifts of 6.7-6.79ppm are generated by protons on aromatic rings on syringyl phenylpropane (S) and guaiacyl (G) structural units, indicating that the contents of the S unit and the G unit are equivalent, 7.5ppm is hydrogen in p-hydroxyphenyl, 4.17ppm is generated by H β and H gamma connected with carbon in aromatic ether bonds, indicating that β -O-4 bonds exist in lignin, and the absorption intensity of the lignin sample after the Fenton treatment is consistent with that of the lignin sample without damaging β -O-4 connecting bonds of the lignin basically, 3.72ppm is a strong lignin methoxyl signal peak, 1.51ppm is a signal peak of methyl and methylene in carbonyl, and the aliphatic proton signal peak of lignin side chains is between 1.24-0.86ppm, and the graph shows that the lignin skeleton structure after the pretreatment has no influence on the lignin structure.
In summary, the pretreatment system of the invention can promote the separation of lignin in the corn straw, improve the extraction rate of the lignin, and after the pretreatment system is used for treatment, the lignin framework structure is not affected, and the original structure of the lignin is basically reserved, so that the high-quality lignin with larger molecular weight, more concentrated distribution and more complete structure is obtained, and the foundation is laid for the further development and utilization of the subsequent lignin.
Claims (7)
1. The method for separating and extracting the lignin based on FeOCl Fenton reaction is characterized by comprising the following steps of:
placing the lignocellulose raw material in a Fenton-like pretreatment system consisting of FeOCl, and treating for 1-12 h;
the addition amount of the lignocellulose raw material is 1g: 5-30 mL;
extracting lignin in the raw material obtained in the step (1) by using an alkaline method or a wood grinding method;
the effective components of the Fenton-like pretreatment system consisting of FeOOL consist of the following components:
0.2-3.5g/L of FeOCl and 0.09-1.55mol/L of hydrogen peroxide;
the pH value of the pretreatment system is 3-8.
2. The lignin separation and extraction method according to claim 1, wherein the effective components of the Fenton-like pretreatment system comprise: FeOCl 0.8-1.6g/L and hydrogen peroxide 0.78-1.55 mol/L.
3. The lignin separation and extraction method according to claim 1, wherein: the lignocellulose raw material is a wood raw material or crop straws; the wood raw material or the crop straw is subjected to degreasing treatment before being subjected to pretreatment.
4. The lignin separation and extraction method according to claim 1, wherein the alkaline extraction step comprises: 1) putting the pretreated lignocellulose raw material into 0.025-0.2mol/L strong alkali solution, and reacting for 1-10h at 25-75 ℃;
the solid-liquid ratio of the raw materials to the alkali liquor is 1g: 5-20 mL;
the strong base is any one or a mixture of two of sodium hydroxide, potassium hydroxide and calcium hydroxide in any proportion;
2) filtering to remove the precipitate, and adding the filtrate into 2-15 times of acid solution;
3) and (4) after the precipitation is complete, carrying out solid-liquid separation to obtain the precipitate, namely the lignin.
5. The method for separating and extracting lignin according to claim 4, wherein: the acid solution is hydrochloric acid solution with the pH value of 2.
6. The lignin separation and extraction method according to claim 1, wherein the step of extracting lignin by a grinding method comprises the following steps:
placing the pretreated lignocellulose raw material into a ball mill for ball milling for 1-5 h;
extracting for 1-5h at 95-105 ℃ by using dioxane after the ball milling is finished, wherein the load of the dioxane is 1g: 10-30 mL;
concentrating the extract to a certain volume, and then adding the concentrated extract into 2-5 times of ethanol;
after the precipitation is completed, removing the precipitation, and then concentrating the filtrate again to a certain volume;
and adding the concentrated solution into an acid solution with the volume of 5-15 times, and after the precipitation is complete, carrying out solid-liquid separation to obtain a precipitate, namely the lignin.
7. Lignin obtainable by the process according to any one of claims 1 to 6, characterized in that: the number average molecular weight is 2100-.
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