CN115894961B - A green lignin separation method and its products and applications - Google Patents

Abstract

The invention discloses a green lignin separation method, a product and application thereof, and belongs to the technical field of lignocellulose pretreatment. The invention takes the aqueous solution of L-cysteine hydrochloride and organic acid as pretreatment liquid, and the wood fiber raw material is mixed with the pretreatment liquid for pretreatment; extracting and vacuum-filtering the pretreated suspension to obtain filtrate and filter residue, and extracting and purifying the filtrate to obtain the L-cysteine lignin. The solvent used in the invention is green and nontoxic, the lignin beta-O-4 structure is reserved to a certain extent while the efficient separation of lignin is realized, the lignin preferential separation concept is met, and the separated lignin has good application in the fields of ultraviolet resistance, sun protection and the like.

Description

A green lignin separation method and its products and applications

Technical Field

The invention belongs to the technical field of lignocellulose pretreatment, and specifically relates to a green lignin separation method and a product and application thereof.

Background technique

Lignin, as the only renewable aromatic hydrocarbon resource in nature, is widely present in the secondary wall of vascular plant cells and plays an important role in plant growth and development. In the early stage, the research on lignin focused on clarifying its complex chemical structure. The skeleton structure of lignin 4-n-propylphenol was verified by the conversion of raw materials by catalysts. With the increasing demand for energy, the efficient separation and degradation and reduction of lignin into phenolic compounds to produce various chemicals has attracted widespread attention in recent years. However, as a natural physical anti-degradation barrier in the cell wall, lignin is often removed as a "stubborn heterogeneity" substance and thus not fully utilized. Traditional pretreatment methods such as caustic soda method, sulfate method, sulfite method, steam explosion method, etc., due to their violent chemical reaction conditions, lead to irreversible "degradation-condensation" process of lignin macromolecules with highly reactive β-O-4 structure, while realizing the separation and conversion of polysaccharide compounds, ignoring the potential value of lignin. In these pretreatment strategies, the β-O-4-rich aromatic ether bonds of lignin are easily broken, thereby promoting the formation of non-natural lignin-derived polymers. In general, β-aryl ether bonds will form unstable unsaturated C=C free radicals, ketone free radicals or active α-position carbon cation intermediates under severe conditions such as high temperature and acid-base catalysis. These active intermediates easily react with free sites on the aromatic rings of lignin fragments to form irreversible condensed C-C bonds, reducing the application value of lignin. Compared with in-situ lignin, the conditions required for reduction and conversion of industrial lignin with a condensed structure rich in C-C bonds to produce downstream high-value-added products are more stringent. Therefore, burning industrial lignin to generate energy for power generation or as an industrial additive has long been the main way to apply lignin in industry. How to separate lignin from polysaccharides without making it more stubborn and overcome the inherent heterogeneity of lignin are the two major goals of efficient separation and utilization of lignin.

In recent years, biomass refining strategies based on the "lignin priority" strategy have become a research hotspot. The lignin priority strategy focuses on stabilizing the active intermediates of lignin degradation, that is, first dissolving the lignin in natural lignocellulosic biomass, and then separating and extracting it through a series of mild methods, while avoiding the condensation reaction of difficult-to-degrade lignin polymers and the degradation of carbohydrate structures. Based on this strategy, people have developed a series of new pretreatment technologies to separate and extract lignin from wood fiber raw materials with the goal of obtaining good chemical reactivity and high-purity lignin, laying the foundation for the high-value utilization of all components of the cell wall in subsequent biomass refining. In order to stabilize the β-O-4 structure of the separated lignin, mild separation conditions are generally used or solvents and catalysts with pre-protection mechanisms are designed for separation reactions. However, the former has a low lignin separation efficiency and thus cannot obtain a high yield of separated lignin. There are two main methods for combining high lignin separation yield with minimal lignin condensation: (1) catalytic reduction separation (RCF), which uses transition metals as catalysts and, with the assistance of a hydrogen source, nitrogen source or other hydrogen-donating reagent, catalytically reduces lignin into oligomers and monomer derivatives in one step; and (2) while efficiently dissociating lignin under acidic conditions, stabilizing the structure of active intermediates that are easily condensed by adding chemical protecting groups.

L-cysteine is the only amino acid with a reducing thiol group among the more than 20 amino acids that make up protein. It has antioxidant effects, prevents food from browning, and prevents stains. Shi Lanlan and others first discovered that L-cysteine can effectively retain the β-O-4 structure of separated lignin without reducing the separation efficiency of wood fiber raw material components, and to a certain extent improve the yield of separated lignin to produce phenolic chemicals (Study on the Separation of L-cysteine Lignin and Its Hydrocracking Performance). However, its relatively complex pretreatment steps and low lignin separation efficiency have limited its industrial application to a certain extent.

Summary of the invention

In view of the shortcomings of the prior art, the present invention provides a green lignin separation method and its products and applications, a wood fiber pretreatment technology based on the lignin priority separation strategy, and a green DES separation system of L-cysteine hydrochloride/organic acid (lactic acid, etc.) is designed to efficiently separate lignin from plant cell walls while retaining the β-O-4 structure in the separated lignin as much as possible, laying a foundation for the development and application of L-cysteine lignin.

In order to achieve the above object, the present invention adopts the following technical solutions:

A green lignin separation method comprises the following steps:

The aqueous solution of L-cysteine hydrochloride and organic acid is used as a pretreatment liquid, and the wood fiber raw material is mixed with the pretreatment liquid for pretreatment; the suspension obtained by the pretreatment is extracted and vacuum filtered to obtain a filtrate and a filter residue, and the filtrate is extracted and purified to obtain L-cysteine lignin.

Preferably, the pretreatment solution is prepared from L-cysteine hydrochloride, an organic acid solution and water; the mass volume ratio of L-cysteine hydrochloride, the organic acid solution and water is 1g:(5-15)mL:(2-5)mL; the concentration of the organic acid solution is 80-90wt%. The water content in the pretreatment solution is 25-40Φ%.

Further preferably, the pretreatment solution is synthesized into a uniform and transparent solution by heating and stirring L-cysteine hydrochloride, an organic acid solution and water, and the synthesis conditions are: a temperature of 60-90° C. and a stirring time of 0-4 h.

Preferably, the pretreatment conditions are: pretreatment temperature is 80-140° C., and pretreatment time is 4-6 h; further preferably, the pretreatment time is 6 h.

Preferably, the organic acid includes at least one of lactic acid, acetic acid, formic acid and oxalic acid.

Preferably, the solid-liquid ratio of the lignocellulosic raw material to the pretreatment liquid is 1:12-1:15.

Preferably, the extraction is extraction by adding an organic solvent;

Further preferably, the organic solvent is a mixed solvent of acetone and water; the ratio of the volume of the organic solvent to the absolute dry mass of the wood fiber raw material is 20:1-30:1. More preferably, the volume ratio of acetone to water in the mixed solvent of acetone and water is 1-3:3-1. More preferably, the volume ratio of acetone to water in the mixed solvent of acetone and water is 1:1.

Preferably, the stirring time of the extraction is 1-3 hours. Further preferably, the stirring time of the extraction is 2 hours.

Preferably, the extraction and purification steps are: subjecting the obtained filtrate to rotary evaporation and adjusting the pH to obtain a lignin suspension; refrigerating the lignin suspension, allowing it to stand, diluting, centrifuging, washing, and freeze-drying it to obtain L-cysteine lignin.

More preferably, the temperature of the rotary evaporation is 40-60°C; more preferably, the pH is adjusted to 5-7 using NaOH with a concentration of 2-6 mol/L.

More preferably, the refrigerated storage temperature is 3-6°C, and the refrigerated storage time is 8-16 hours. More preferably, the refrigerated storage time is 12 hours.

Preferably, the wood fiber raw material is crushed to 40-60 mesh by a plant crusher; then extracted with a mixed solvent of ethanol and water for 8-12 hours, repeated at least three times, and finally extracted with dichloromethane for 8-12 hours once, and the dewaxed sample is dried at 50-60°C for 12-24 hours for use.

Further preferably, the volume ratio of ethanol to water in the ethanol and water mixed solvent is 7-9:3-1. More preferably, the volume ratio of ethanol to water in the ethanol and water mixed solvent is 8:2.

The L-cysteine lignin separated by the above-mentioned green pretreatment method of wood fiber based on the lignin priority separation strategy.

The application of the above-mentioned L-cysteine lignin in anti-ultraviolet sunscreen.

The fiber residue yield of the invention is between 39.72% and 67%, the lignin removal rate is between 70.86% and 97.22%, and the L-cysteine lignin yield is between 15.71% and 54.79%.

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

(1) For the first time, L-cysteine, which is widely available and green and non-toxic, was used as a hydrogen bond acceptor and organic acids such as lactic acid were used as hydrogen bond donors to synthesize binary green DES for use in wood fiber pretreatment. While achieving efficient separation of lignin, the β-O-4 structure of lignin was retained to a certain extent, which is in line with the concept of lignin priority separation.

(2) Compared with blank commercial sunscreen, the lignin-based sunscreen prepared by using the L-cysteine lignin separated by the present invention can significantly improve the anti-ultraviolet radiation performance of commercial sunscreen, and the SPF increase of some can exceed 100%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a 2D-HSQC NMR spectrum of L-cysteine lignin obtained by separation of bagasse in Example 3.

FIG. 2 is a 2D-HSQC NMR spectrum of L-cysteine lignin obtained by separation of bagasse in Example 4.

FIG. 3 is a 2D-HSQC NMR spectrum of L-cysteine lignin obtained by separation of bagasse in Example 5.

FIG. 4 is a 2D-HSQC NMR spectrum of L-cysteine lignin obtained by separation of bagasse in Example 6.

FIG. 5 is a 2D-HSQC NMR spectrum of L-cysteine lignin isolated from Pinus koraiensis in Example 7.

FIG. 6 is a 2D-HSQC NMR spectrum of L-cysteine lignin isolated from Eucalyptus yunnanensis in Example 8.

Detailed ways

In order to better understand the technical features of the present invention, the present invention is further described in detail below based on examples, but the scope of protection claimed by the present invention is not limited thereto.

The calculation formulas for the fiber residue yield, L-cysteine lignin yield, and lignin removal rate in the present invention are shown in formulas (1-1)(1-2)(1-3), wherein the lignin content is determined using the U.S. Renewable Energy Laboratory biomass analysis method (NREL method).

All NMR spectra of L-cysteine lignin samples were collected using a Bruker Ascend-500M NMR instrument at 25°C, and dimethyl sulfoxide (DMSO-d ₆ ) was used as the solvent. 50-70 mg of lignin sample was weighed into an NMR tube, and 0.6 mL of dimethyl sulfoxide (DMSO-d ₆ ) was added to dissolve it. The data was processed using Bruker Topspin 4.1.4.

The β-O-4 content was calculated by integration using the nuclear magnetic resonance relative quantitative method, and the calculation formulas are shown in (1-4) and (1-5).

Among them, formula (1-4) is applicable to grass materials, and formula (1-5) is applicable to coniferous and broad-leaved materials. They represent the two-dimensional NMR signals of lignin G unit at C ₂ , S unit at C _2,6 , and H unit at C _2,6 respectively.

In order to evaluate the application prospects of the separated L-cysteine lignin in the field of sun protection, the present invention prepared a lignin-based sunscreen and measured its sun protection factor (SPF value). The measurement steps are as follows:

Grind and sieve the separated and extracted lignin, and collect the lignin that passes 200 mesh. Separately mix 20 mg of lignin with 980 mg of SPF15 sunscreen (control the lignin content to be 2%, mass ratio), and stir at 500 rpm for more than 24 hours under light-proof conditions. Take 25 mg of lignin-based sunscreen sample, evenly apply it on a 1 mm thick glass sheet with 3M tape, air-dry for 20 minutes under light-proof conditions, and then take 7 different points to test the transmittance of the sample at 290-400 nm, and calculate the SPF value of the sample. The calculation formula is shown in formula (1-6).

Where _Ser (λ) is the erythema effect weighting, _Ss (λ) is the spectral irradiance of the UV source, and T(λ) is the transmittance of the sample.

The wood fiber raw materials (bagasse, Simiao pine, Yunnan eucalyptus, etc.) used in the embodiment were crushed to 40-60 mesh by a plant crusher; then extracted with a mixed solvent of ethanol and water (8:2, V/V) for 12 hours, repeated three times, and finally extracted with dichloromethane for 12 hours once, and the dewaxed sample was dried at 50°C for 24 hours for use.

Example 1 (Comparative Example - L-cysteine hydrochloride - water system - 90°C - 6h - bagasse, L-cysteine hydrochloride: water = 1:12)

The pretreatment liquid was prepared by mixing L-cysteine hydrochloride and deionized water at a mass volume ratio of 1:12 (g:mL) at room temperature. The pretreatment liquid and the bagasse raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask, and the mass volume ratio of bagasse to pretreatment liquid was 1:12 (g:mL). The bagasse was heated and magnetically stirred at 90°C for 6h at a speed of 250rpm/min. After the reaction was completed, an acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreatment solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5 to 7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 66.52% and the lignin removal rate was 43.79%.

The obtained lignin is L-cysteine lignin, and the separation yield is 24.33%.

Example 2 (Comparative Example - L-cysteine hydrochloride - hydrochloric acid system - 90°C - 6h - bagasse, L-cysteine hydrochloride: hydrochloric acid = 0.6554:12)

The pretreatment liquid was prepared by using L-cysteine hydrochloride and 0.2M hydrochloric acid solution at a mass volume ratio of 0.6554:12 (g:mL) at room temperature. The pretreatment liquid and the bagasse raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask. The mass volume ratio of bagasse to pretreatment liquid was 1:12 (g:mL). The bagasse was heated and magnetically stirred at 90°C for 6h at a speed of 250rpm/min. After the reaction was completed, an acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreatment solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5 to 7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 79.62% and the lignin removal rate was 24.51%.

The obtained lignin is L-cysteine lignin, and the separation yield is 16.73%.

Example 3 (Comparative Example - 90% lactic acid-water system - 90°C - 6h - bagasse, lactic acid solution: water = 5:1)

A 90% lactic acid solution-water (5:1, V/V) system was used as the pretreatment liquid. The pretreatment liquid and the bagasse raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask. The mass volume ratio of bagasse to pretreatment liquid was 1:12 (g:mL). The bagasse was heated and magnetically stirred at 90°C for 6h at a speed of 250rpm/min. After the reaction was completed, an acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreatment solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5 to 7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 84.05% and the lignin removal rate was 28.81%.

The obtained lignin is lactic acid lignin, and the separation yield is 13.99%. Its 2D HSQC NMR spectrum is shown in Figure 1, and the β-O-4 content is 67.22%.

The SPF value of the blank sunscreen was 16.25±0.48, and after mixing with the obtained lactic acid lignin, its SPF value increased to 22.69±1.45.

Example 4 (DES-water system-90°C-6h-bagasse, L-cysteine hydrochloride: lactic acid solution: water=1:5:2)

DES solution was prepared by mixing L-cysteine hydrochloride, lactic acid solution (90 wt%) and deionized water at a mass volume ratio of 1:5:2 (g:mL:mL) at 80°C. DES pretreatment solution and bagasse raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask, and the mass volume ratio of bagasse to pretreatment solution was 1:12 (g:mL). The bagasse was heated and magnetically stirred at 90°C for 6h at a speed of 250rpm/min. After the reaction was completed, acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreatment solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5 to 7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 49.4% and the lignin removal rate was 84.77%.

The obtained lignin is L-cysteine lignin, and the separation yield is 28.72%. Its 2D HSQC NMR spectrum is shown in FIG2 , and the β-O-4 content is 45.27%.

The SPF value of the blank sunscreen was 16.25±0.48, and after mixing with the obtained L-cysteine lignin, its SPF value increased to 25.65±2.99.

Example 5 (DES-water system-90°C-6h-bagasse, L-cysteine hydrochloride: lactic acid solution: water=1:10:2)

DES solution was prepared by mixing L-cysteine hydrochloride, lactic acid solution (90 wt%) and deionized water at a mass volume ratio of 1:10:2 (g:mL:mL) at 80°C. DES pretreatment solution and bagasse raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask, and the mass volume ratio of bagasse to pretreatment solution was 1:12 (g:mL). The bagasse was heated and magnetically stirred at 90°C for 6h at a speed of 250rpm/min. After the reaction was completed, acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreatment solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5 to 7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 58.65%, and the lignin removal rate was 86.04%.

The obtained lignin is L-cysteine lignin, and the separation yield is 52.97%. Its 2D HSQC NMR spectrum is shown in FIG3 , and the β-O-4 content is 37.57%.

The SPF value of the blank sunscreen was 16.25±0.48, and its SPF value was increased to 31.88±3.37 after mixing with the obtained L-cysteine lignin.

Example 6 (DES-water system-90°C-6h-bagasse, L-cysteine hydrochloride: lactic acid solution: water=1:15:3)

DES solution was prepared by mixing L-cysteine hydrochloride and lactic acid solution (90 wt%) and deionized water at a mass volume ratio of 1:15:3 (g:mL:mL) at 80°C. DES pretreatment solution and bagasse raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask, and the mass volume ratio of bagasse to pretreatment solution was 1:12 (g:mL). The bagasse was heated and magnetically stirred at 90°C for 6h at a speed of 250rpm/min. After the reaction was completed, acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreatment solid-liquid mixture, and the rotation speed was 350rpm/min, and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5-7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 52.1% and the lignin removal rate was 83.92%.

The obtained lignin is L-cysteine lignin, and the separation yield is 40.22%. Its 2D HSQC NMR spectrum is shown in FIG1 , and the β-O-4 content is 30.46%.

The SPF value of the blank sunscreen was 16.25±0.48, and after mixing with the obtained L-cysteine lignin, its SPF value increased to 34.54±1.76.

Example 7 (DES-water system-120°C-6h-Simao pine, L-cysteine hydrochloride: lactic acid solution: water=1:10:2)

DES solution was prepared at 80°C with L-cysteine hydrochloride, lactic acid solution (90wt%) and water in a mass volume ratio of 1:10:2 (g:mL:mL). DES pretreatment solution and 40-60 mesh sieved and extracted Simiao pine raw material were added to a round-bottom flask, and the mass volume ratio of Simiao pine to pretreatment solution was 1:12 (g:mL). Bagasse was heated and magnetically stirred at 120°C for 6h at a speed of 250rpm/min. After the reaction was completed, acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreated solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5-7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 46.03% and the lignin removal rate was 95.01%.

The obtained lignin is L-cysteine lignin, and the separation yield is 48.06%. Its 2D HSQC NMR spectrum is shown in FIG5 , and the β-O-4 content is 16.51%.

The SPF value of the blank sunscreen was 16.25±0.48, and after mixing with the obtained L-cysteine lignin, its SPF value increased to 28.39±2.04.

Example 8 (DES-water system-110°C-6h-Yunnan eucalyptus, L-cysteine hydrochloride: lactic acid solution: water=1:10:5)

DES solution was prepared by mixing L-cysteine hydrochloride, lactic acid solution (90wt%) and deionized water at a mass volume ratio of 1:10:5 (g:mL:mL) at 80°C. DES pretreatment solution and Yunnan eucalyptus raw material that had been sieved through 40-60 mesh and extracted were added to a round-bottom flask, and the mass volume ratio of Yunnan eucalyptus to pretreatment solution was 1:15 (g:mL). Yunnan eucalyptus was heated and magnetically stirred at 110°C for 6h at a speed of 250rpm/min. After the reaction was completed, acetone/water solution (1:1, V/V) with a volume mass ratio of 30:1 (mL:g) relative to the absolute dry raw material was added to the pretreated solid-liquid mixture at a speed of 350rpm/min and stirred at room temperature for 2h.

The solid-liquid mixture was vacuum filtered through a G4 sand core funnel to separate the filtrate and the residue. The residue was washed 2 to 3 times with an acetone/water solution (1:1, V/V) with a volume mass ratio of 20:1 (mL: g) relative to the absolute dry raw material, and finally washed with deionized water for 2 to 3 times. After washing, the filtrate was evaporated under reduced pressure at 50°C to remove acetone. Subsequently, 6 mol/L NaOH was added to adjust the pH of the filtrate to 5 to 7, and refrigerated at 4°C for 12 hours.

After the refrigeration, deionized water was added to the filtrate at a volume mass ratio of 100:1 (mL:g) relative to the absolute dry raw material, and the lignin was separated by centrifugation at 9000 rpm/min for 20 min, and the lignin was washed by centrifugation with deionized water 2 to 3 times. The obtained lignin and residue were freeze-dried for later use.

The residue yield was 61.13% and the lignin removal rate was 70.86%.

The obtained lignin is L-cysteine lignin, and the separation yield is 15.71%. Its 2D HSQC NMR spectrum is shown in FIG6 , and the β-O-4 content is 28.72%.

The SPF value of the blank sunscreen was 16.25±0.48, and after mixing with the obtained L-cysteine lignin, its SPF value increased to 28.58±2.04.

The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Adjusting the ratio of L-cysteine hydrochloride, lactic acid, and water and the order of addition are all within the protection scope of the present invention.

Claims (7)

1. A green lignin separation method comprising the steps of:

Taking an aqueous solution of L-cysteine hydrochloride and organic acid as a pretreatment liquid, and mixing a wood fiber raw material with the pretreatment liquid for pretreatment; extracting and vacuum filtering the pretreated suspension to obtain filtrate and filter residue, and extracting and purifying the filtrate to obtain L-cysteine lignin;

the organic acid is lactic acid;

the pretreatment liquid is prepared from L-cysteine hydrochloride, an organic acid solution and water; the mass volume ratio of the L-cysteine hydrochloride, the organic acid solution and the water is 1g (5-15 mL: (2-5) mL; the concentration of the organic acid solution is 80-90 wt%;

The pretreatment conditions are as follows: the pretreatment temperature is 80-140 ℃, and the pretreatment time is 4-6h;

the extraction is extraction by adding an organic solvent; the organic solvent is a mixed solvent of acetone and water; the volume ratio of the acetone to the water in the mixed solvent of the acetone and the water is 1-3:3-1;

The wood fiber raw material is bagasse, pinus massoniana or eucalyptus yunnanensis.

2. The green lignin separation method according to claim 1 wherein the pretreatment solution is heated and stirred by L-cysteine hydrochloride, organic acid solution and water to a uniform transparent solution under synthesis conditions: the temperature is 60-90 ℃, and the stirring time is 0-4 h.

3. The green lignin separation method according to claim 1 wherein the lignocellulosic feedstock to pretreatment liquor solids to liquid ratio is 1:12 to 1:15;

the ratio of the volume of the organic solvent to the absolute dry mass of the wood fiber raw material is 20:1-30:1.

4. The green lignin separation method according to claim 1 wherein the extraction and purification steps are: the obtained filtrate is subjected to rotary evaporation and pH adjustment to obtain lignin suspension; refrigerating and standing lignin suspension, diluting, centrifuging, washing and freeze-drying to obtain L-cysteine lignin.

5. The green lignin separation method according to claim 4 wherein the temperature of rotary steaming is 40-60 ℃;

The pH is adjusted to 5-7 by NaOH with the concentration of 2-6 mol/L.

6. The green lignin separation method according to claim 1 wherein the lignocellulosic feedstock is crushed to 40-60 mesh by a plant crusher; extracting with mixed solvent of ethanol and water for 8-12 hr, repeating for at least three times, extracting with dichloromethane, and drying the dewaxed sample at 50-60deg.C for 12-24 h.

7. The use of L-cysteine lignans prepared by the green lignin separation process of any of claims 1-6 in uv sunscreening.