A method for extracting high-purity lignin by adding thiourea dioxide
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SOUTH CHINA UNIV OF TECH
- Filing Date
- 2026-04-03
- Publication Date
- 2026-06-30
Smart Images
Figure CN122302313A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of lignin extraction technology, specifically relating to a method for extracting high-purity lignin by adding thiourea dioxide. Background Technology
[0002] As the second most abundant natural polymer and the richest aromatic organic compound in nature, lignin is a key component of plant cell walls, accounting for 15%-35% of lignocellulose biomass. However, the efficient conversion of lignocellulose resources faces two main challenges: First, cellulose, hemicellulose, and lignin form a complex cross-linked network structure through covalent and non-covalent bonds; second, lignin itself has a complex chemical structure, with monomers randomly forming polymerization sites through C-C and CO bonds, lacking a strictly fixed structure.
[0003] Delignification essentially involves chemically breaking the covalent bonds in the carbohydrate-lignin complex. Due to their strong nucleophilic properties, sulfur-containing chemicals can react with lignin to induce its depolymerization or chemical modification, and are therefore widely used in delignification processes. For example, the sulfate process uses basic sodium sulfide as the main chemical, while the acidic sulfite process relies on sulfite (hydrogen) ions. Similarly, in the research of Juho et al., they treated softwood under mild conditions using an acidic aqueous solution system in the presence of thiourea, achieving an 87% removal rate of the original lignin in a 30-minute treatment time, and also obtaining partially water-soluble cationic lignin (Antti JS, Marja M, Juha A, et al. Highly effective fractionation chemistry to overcome the recalcitrance of softwood lignocellulose [J]. Carbohydrate Polymers, 2023, 312 120815-120815.). The reaction mechanism may involve the nucleophilic addition of thiourea to lignin to form isothiourea-modified lignin, thereby dissolving lignin in an acidic reaction solution. The formation of isothiourea reduces the degradation and condensation of lignin during fractionation, which in turn leads to the production of high molecular weight lignin and prevents the darkening of carbohydrate and lignin fractions. A study on the depolymerization of purified alkaline lignin by TDO showed that selective cleavage of the β-O-4 bond was achieved in TDO / sodium hydroxide treatment, and guaiacol was identified as the major phenolic monomer (64.77%) (Dongpo H, Mengtian Y, Jingyu X, et al. Thiourea dioxide as a green reductant for selective depolymerization of lignin to guaiacol [J]. Industrial Crops & Products, 2023, 194).
[0004] Thiourea dioxide, an oxidation product of thiourea, exhibits higher thermal stability and stronger reducing power under alkaline conditions compared to thiourea. It can react with lignin structural units, reducing the o- and p-benzoquinone structures in lignin to o- and p-phenol structures, and reducing carbonyl and aldehyde groups in lignin to hydroxyl groups. These reactions destroy potential chromophores in lignin. Currently, thiourea dioxide is rarely used directly for the delignification of lignocellulose. Therefore, a lignin extraction technique based on alkaline thiourea dioxide is proposed. Summary of the Invention
[0005] This invention aims to explore a better and cheaper method for extracting water-soluble lignin by adding thiourea dioxide to extract high-purity lignin. Thiourea dioxide is an oxidation product of thiourea, which has higher thermal stability and stronger reducing properties under alkaline conditions compared to thiourea, and has a wider range of applications. While thiourea is stable at room temperature, it may burn at high temperatures or when exposed to open flames. Thiourea dioxide is safer and cheaper, thus resulting in a lower processing cost, simpler process, and greater safety.
[0006] To achieve the above technical objectives, the present invention adopts the following technical solution: This invention provides a method for extracting high-purity lignin by adding thiourea dioxide, comprising the following steps: (1) Add thiourea dioxide and NaOH to water to obtain a reaction solution. Mix the reaction solution with bamboo powder and stir the reaction in an oil bath. (2) After the reaction is completed, the mixture is cooled to room temperature, the bamboo residue is filtered out under reduced pressure, the mixture is rotary evaporated, the pH is adjusted, and the mixture is centrifuged. The precipitate is washed with pure water and dried to obtain lignin.
[0007] Furthermore, thiourea dioxide has the molecular formula CH4N2O2S. It is produced by oxidizing thiourea with hydrogen peroxide. The sulfur atoms are oxidized and combine with two oxygen atoms to form a more complex sulfur oxide structure.
[0008] Furthermore, in step (1), NaOH is added to the water at a mass fraction of 10 g / L to 22 g / L.
[0009] Furthermore, in step (1), thiourea dioxide is added to water at a mass fraction of 6 g / L to 18 g / L.
[0010] Furthermore, in step (1), the solid-liquid ratio of bamboo powder to reaction liquid is 1:10~1:20.
[0011] Furthermore, in step (1), the reaction temperature in the oil bath is 105℃~125℃.
[0012] Furthermore, in step (1), the reaction time is 4h~10h.
[0013] Furthermore, in step (2), 2 / 3 of the liquid volume is removed by rotary evaporation.
[0014] Furthermore, in step (2), the pH is adjusted using a 2 mol / L hydrochloric acid solution to adjust the pH to 1.8-2.2.
[0015] The present invention also provides lignin prepared by the above-described method for extracting high-purity lignin by adding thiourea dioxide.
[0016] Furthermore, the content of lignin C is relatively low, and the molecular weight is also relatively low.
[0017] Furthermore, the lignin extraction rate can reach 16.30%, and the lignin purity is 89.2%.
[0018] Furthermore, lignin exhibits a lighter color compared to direct extraction with NaOH.
[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) Existing technologies mostly use thiourea + alkali to extract lignin. Although the extraction rate is high, thiourea is unstable due to the high temperature environment during the extraction process, which poses a potential fire and explosion hazard. In comparison, thiourea dioxide is safer and cheaper. Therefore, the method provided by this invention has low process cost, and the process is simpler and safer. Moreover, the extraction rate of lignin obtained by the method provided by this invention can reach 16.30%, and the purity of lignin is 89.2%, which is better than conventional extraction processes.
[0020] (2) Most existing extraction processes use alkali + thiourea, which are more complicated and usually require the introduction of an inert atmosphere for protection (nitrogen), multiple acid precipitation, etc. The process is complicated, cumbersome, and costly. The method provided by this invention does not require any inert atmosphere protection, and the process is simple and low-cost. Attached Figure Description
[0021] Figure 1 The effect of alkali concentration on lignin extraction rate.
[0022] Figure 2 The effect of thiourea dioxide concentration on lignin extraction rate.
[0023] Figure 3 The effect of solid-liquid ratio on lignin extraction rate.
[0024] Figure 4 The effect of reaction time on lignin extraction rate.
[0025] Figure 5 The effect of reaction temperature on lignin extraction rate.
[0026] Figure 6 This is a scanning electron microscope (SEM) image of lignin extracted using the optimal process group.
[0027] Figure 7 The image shows the ultraviolet spectrum of lignin extracted using the optimal process group.
[0028] Figure 8 Fourier transform infrared (FT-IR) spectrum of lignin extracted using the optimal process group.
[0029] Figure 9Two-dimensional HSQC nuclear magnetic resonance spectra of lignin extracted by the optimal process group (left: side chain region spectrum: δC / δH 50-90 / 2.5-6.0; right: aromatic ring region spectrum: δC / δH 95-150 / 6.0-8.0).
[0030] Figure 10 The main structures observed in the lignin components extracted by the optimal process group.
[0031] Figure 11 A comparison of lignin extracted from the optimal process group and the lignin extracted from the alkali extraction control group (left: optimal process group, right: alkali extraction control group). Detailed Implementation
[0032] The following embodiments further illustrate the method for extracting high-purity lignin by adding thiourea dioxide provided by the present invention.
[0033] Raw materials and reagents Bamboo powder, ethanol, thiourea dioxide, hydrochloric acid, potassium bromide, acetic anhydride, pyridine.
[0034] Process optimization for lignin extraction from thiourea dioxide A reaction solution was prepared by adding 12 g / L of thiourea dioxide and 10 g / L to 22 g / L of NaOH to water. The reaction solution was then mixed with bamboo powder at a certain solid-liquid ratio and stirred in an oil bath.
[0035] After the reaction was completed, the mixture was cooled to room temperature, and the bamboo residue was filtered off under reduced pressure. About 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 1.8-2.2 with 2 mol / L hydrochloric acid solution. Then, the mixture was centrifuged at 8000 r / min for 10 min. The precipitate was washed 2-3 times with pure water and dried at 50℃ for 24 h to obtain lignin.
[0036] Calculate the lignin extraction rate using the following formula: Lignin extraction rate (%)
[0037] Example 1 Effect of alkali concentration Thiourea dioxide with a mass fraction of 12 g / L was added to water, and NaOH was added at mass fractions of 10 g / L, 12 g / L, 14 g / L, 16 g / L, 18 g / L, 20 g / L, and 22 g / L to obtain reaction solutions. The reaction solutions were mixed with bamboo powder at a solid-liquid ratio of 1:10 (g / mL) and reacted at 120℃ for 8 h. The extraction rates of lignin were then compared.
[0038] After the reaction was completed, the mixture was cooled to room temperature, the bamboo residue was filtered off under reduced pressure, and 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 2 with 2 mol / L hydrochloric acid solution, and then centrifuged at 8000 r / min for 10 min. The precipitate was washed three times with pure water and dried at 50℃ for 24 h to obtain lignin.
[0039] The effect of sodium hydroxide concentration on lignin extraction rate was studied under the conditions of thiourea dioxide mass fraction of 12 g / L, solid-liquid ratio of 1:10 (g / mL), reaction time of 8 h, and reaction temperature of 120 °C. The results are as follows: Figure 1 As shown, the overall extraction rate of lignin initially increased and then slightly decreased with increasing sodium hydroxide concentration. The extraction rate reached its maximum of 12.03% when the sodium hydroxide concentration was 18 g / L. Therefore, a sodium hydroxide concentration of 18 g / L was selected as the optimal alkali concentration.
[0040] Example 2 Effect of thiourea dioxide concentration After using the preferred sodium hydroxide concentration, thiourea dioxide was added at mass fractions of 6 g / L, 9 g / L, 12 g / L, 15 g / L, and 18 g / L to obtain reaction solutions. The reaction solutions were mixed with bamboo powder at a solid-liquid ratio of 1:10 (g / mL) and reacted at 120℃ for 8 hours. The extraction rates of lignin were then compared.
[0041] After the reaction was completed, the mixture was cooled to room temperature, the bamboo residue was filtered off under reduced pressure, and 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 2 with 2 mol / L hydrochloric acid solution, and then centrifuged at 8000 r / min for 10 min. The precipitate was washed three times with pure water and dried at 50℃ for 24 h to obtain lignin.
[0042] The effect of sodium hydroxide concentration on lignin extraction rate was studied under the conditions of thiourea dioxide mass fraction of 12 g / L, solid-liquid ratio of 1:10 (g / mL), reaction time of 8 h, and reaction temperature of 120 °C. The results are as follows: Figure 2 As shown, the extraction rate of lignin generally increased and then decreased with increasing sodium hydroxide concentration. The extraction rate reached its maximum of 12.03% when the mass fraction of thiourea dioxide was 12 g / L. Therefore, 12 g / L of thiourea dioxide was selected as the optimal concentration.
[0043] Example 3 Effect of solid-liquid ratio The reaction solution was prepared using preferred concentrations of sodium hydroxide and thiourea dioxide. Bamboo powder was mixed with the reaction solution at solid-liquid ratios of 1:10, 1:12, 1:15, 1:18, and 1:20 (g / mL), and reacted at 120°C for 8 hours. The lignin extraction rates were compared.
[0044] After the reaction was completed, the mixture was cooled to room temperature, the bamboo residue was filtered off under reduced pressure, and 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 2 with 2 mol / L hydrochloric acid solution, and then centrifuged at 8000 r / min for 10 min. The precipitate was washed three times with pure water and dried at 50℃ for 24 h to obtain lignin.
[0045] The effect of the solid-liquid ratio on the lignin extraction rate was studied under the conditions of thiourea dioxide concentration of 12 g / L, sodium hydroxide concentration of 18 g / L, reaction time of 8 h, and reaction temperature of 120 °C. The results are as follows: Figure 3 As shown, when the solid-liquid ratio increased from 1:10 (g / mL) to 1:15 (g / mL), the extraction rate also increased from 12.03% to 16.30%. This is because an increase in the solid-liquid ratio leads to an increase in the solvent medium content, which is beneficial for the contact between bamboo powder and the solvent medium. However, when the solid-liquid ratio is further increased, a small amount of lignin cannot be effectively precipitated, thus affecting the extraction rate. Therefore, 1:15 (g / mL) was chosen as the optimal solid-liquid ratio.
[0046] Example 4 Effect of reaction time After using the preferred sodium hydroxide concentration, thiourea dioxide concentration, and solid-liquid ratio, the reaction was carried out at 120℃ for 4h, 6h, 8h, and 10h, respectively, and the lignin extraction rate was compared.
[0047] After the reaction was completed, the mixture was cooled to room temperature, the bamboo residue was filtered off under reduced pressure, and 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 2 with 2 mol / L hydrochloric acid solution, and then centrifuged at 8000 r / min for 10 min. The precipitate was washed three times with pure water and dried at 50℃ for 24 h to obtain lignin.
[0048] The effect of reaction time on lignin extraction rate was studied under the conditions of thiourea dioxide concentration of 12 g / L, sodium hydroxide concentration of 18 g / L, solid-liquid ratio of 1:15 (g / mL), and reaction temperature of 120℃. The results are as follows: Figure 4 As shown, the extraction rate was highest at 16.30% when the reaction time was 8 hours. Further increases in reaction time did not significantly change the lignin extraction rate. Therefore, 8 hours was chosen as the reaction time.
[0049] Example 5 Effect of reaction temperature After using the preferred sodium hydroxide concentration, thiourea dioxide concentration, solid-liquid ratio, and reaction time, the reaction was carried out at 105℃, 110℃, 115℃, 120℃, and 125℃, respectively, and the lignin extraction rate was compared.
[0050] After the reaction was completed, the mixture was cooled to room temperature, the bamboo residue was filtered off under reduced pressure, and 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 2 with 2 mol / L hydrochloric acid solution, and then centrifuged at 8000 r / min for 10 min. The precipitate was washed three times with pure water and dried at 50℃ for 24 h to obtain lignin.
[0051] The effect of reaction temperature on lignin extraction rate was studied under the conditions of thiourea dioxide concentration of 12 g / L, sodium hydroxide concentration of 18 g / L, solid-liquid ratio of 1:15 (g / mL), and reaction time of 8 h. The results are as follows: Figure 5 As shown, the lignin extraction rate gradually increased as the reaction temperature increased from 105℃ to 120℃. When the reaction temperature was further increased, the lignin extraction rate did not change significantly. Therefore, 120℃ was chosen as the reaction time, at which point the lignin extraction rate was 16.30%.
[0052] Comparative example (equivalent to conventional alkaline extraction process, without the addition of thiourea dioxide) Under optimized conditions, a control group without the addition of thiourea dioxide was used for extraction, i.e., sodium hydroxide concentration of 18 g / L, reaction temperature of 120℃, and reaction time of 8 h. The extraction rates of lignin were compared.
[0053] After the reaction was completed, the mixture was cooled to room temperature, the bamboo residue was filtered off under reduced pressure, and 2 / 3 of the liquid volume was removed by rotary evaporation. The pH was adjusted to 2 with 2 mol / L hydrochloric acid solution, and then centrifuged at 8000 r / min for 10 min. The precipitate was washed three times with pure water and dried at 50℃ for 24 h to obtain caustic soda lignin.
[0054] Measurement of lignin-related components The contents of Klason lignin, acid-soluble lignin, and ash were determined according to the National Renewable Energy Laboratory (NREL) standard method NREL / TP-510-42618. The contents of Klason lignin, acid-soluble lignin, and ash were measured separately in bamboo powder raw materials after treatment with the thiourea dioxide-alkali process, in wood powder, thiourea dioxide-alkali process lignin, and in alkali process lignin.
[0055] Characterization methods for lignin Scanning electron microscopy analysis The prepared lignin suspension was diluted to near colorless and then slowly dropped onto a smooth silicon wafer, which was then allowed to air dry at room temperature for 12 hours. The surface morphology and size of the lignin were observed using a China National Quantum-SEM500X scanning electron microscope (SEM).
[0056] UV-Vis spectrophotometer detection Aromatic compounds have the property of absorbing ultraviolet light. The type and number of structural units of lignin will affect the shape and absorption coefficient of its absorption spectrum. The absorption spectrum of lignin was analyzed in the wavelength range of 250nm to 400nm by dissolving lignin at a concentration of 200μg / ml in a 1% NaOH solution.
[0057] Infrared spectroscopy detection Fourier transform infrared spectroscopy (FTIR) is an important method for analyzing functional groups in organic compounds. The types of functional groups are determined based on the selective absorption of lignin and its synthetic products within different wavelength ranges. This experiment analyzed the functional groups of alkali lignin and experimental products using the KBr pellet method. A suitable amount (1-2 mg) of the crushed sample was mixed with a certain amount of KBr (sample:potassium bromide = 1:150 (w / w)), ground, and manually pelleted at a wavelength of 400 cm⁻¹. -1 ~4000cm -1 .
[0058] Gel permeation chromatography analysis The molecular weight distribution of lignin was determined using an Agilent 1260 Infinity gel electrophoresis system. First, lignin was acetylated in an acetic anhydride / pyridine (1 / 1, v / v) solution for 4 h at room temperature to derive 50 mg of acetylated lignin. Then, 2 mg of the acetylated lignin was completely dissolved in tetrahydrofuran and filtered through a 0.22 μm filter. The lignin in the eluent was detected using a 245 nm variable wavelength detector.
[0059] Elemental analysis Elemental analysis is a testing method for the quantitative analysis of elements in a sample. This experiment used a German Elementar UNICUBE elemental analyzer, heating the sample to 1150℃ with He as the carrier gas to decompose it. The contents of carbon, hydrogen, nitrogen, oxygen, and sulfur in the products were analyzed.
[0060] HSQC nuclear magnetic resonance spectrum analysis Nuclear magnetic resonance (NMR) spectra were obtained using a Bruker-AVANCE III HD 600 spectrometer (Germany). The lignin obtained in the experiment dissolved in deuterated DMSO. 1 H NMR frequency 600MHz and 13 C NMR at 100 MHz, chemical shift expressed as δ (ppm), using deuterated solvent DMSO, with solvent shift values δC / δH 39.52 / 2.50 used as reference standards.
[0061] Results and Analysis Comparison of extraction processes and extraction rates Initial process (before optimization): thiourea dioxide mass fraction 12 g / L, sodium hydroxide concentration 10 g / L, solid-liquid ratio of bamboo powder to reaction solution 1:10 (g / mL), reaction temperature 120℃, reaction time 8 h. Under these conditions, the lignin extraction rate was 4.11%. Due to its low extraction rate, it had no practical application significance and was therefore not analyzed in the determination of lignin content.
[0062] The optimal process group was as follows: with a thiourea dioxide mass fraction of 12 g / L, a sodium hydroxide mass fraction of 18 g / L, a solid-liquid ratio of bamboo powder to reaction solution of 1:15 (g / mL), a reaction time of 8 hours, and a reaction temperature of 120℃, the extraction rate was 16.3%.
[0063] Alkali extraction control group (comparative example): Sodium hydroxide mass fraction was 18 g / L, solid-liquid ratio of bamboo powder to reaction solution was 1:15 (g / mL), reaction time was 8 hours, reaction temperature was 120℃, and extraction rate was 14.16%.
[0064] As shown above, the extraction rate of this process is 2.14% higher than that of the alkali extraction control group.
[0065] The contents of bamboo powder and lignin-related components before and after the reaction are shown in Table 1.
[0066] Table 1. Content of bamboo powder and lignin-related components before and after the reaction.
[0067] Comparing the lignin content in bamboo powder before and after the optimized extraction process, the total lignin content in the bamboo powder decreased from 26.77% (the sum of 24.73% Klason lignin (%) + 2.04% acid-soluble lignin (%)) in the original bamboo powder to 8.12% (the sum of the two types of lignin listed in the residue after optimal extraction, i.e., 6.28% + 1.84%), showing a significant decrease in lignin content. The purity of lignin extracted using the optimal process was 89.20%. In contrast, the purity of lignin extracted in the alkali extraction control group (comparative example) was only 66.00%.
[0068] Scanning electron microscopy analysis results SEM images of lignin extracted using the optimal process group are shown below. Figure 6 As shown, it presents a three-dimensional porous framework with a large number of irregular pores (the pore size range is estimated to be about 1-10μm) and interconnected channels, and the overall appearance is "sponge-like" or "honeycomb-like".
[0069] UV-Vis spectrophotometer test results Lignin is a substance containing an aromatic ring structure, and substances containing aromatic ring structures have strong absorption of ultraviolet light, while carbohydrates have almost no absorption in the ultraviolet region. Ultraviolet absorption spectroscopy was performed on lignin extracted using the optimal process group, and the results are as follows: Figure 7 As shown, there is a relatively large absorption peak at 280 nm, indicating that the extracted lignin has a relatively complete structure.
[0070] Infrared spectroscopy detection results To observe the functional groups of extracted lignin, Fourier transform infrared spectroscopy (FT-IR) was performed on the lignin extracted using the optimal process group. The results are as follows: Figure 8 As shown, at 1600cm -1 1510cm -1 and 1425cm -1 The absorption peak at 1460 cm⁻¹ is a characteristic absorption peak of the aromatic ring in lignin. -1 The absorption peak at 1326 cm⁻¹ is related to -CH₃. -1 The absorption peak at 1220 cm⁻¹ is related to the benzene ring in the lilac core. -1 and 1121cm -1 The absorption peak at 1031 cm⁻¹ is related to the absorption peak of the CH stretching vibration in the lilac nucleus. -1 The absorption peak at 830 cm⁻¹ is related to CH on the guaiac matrix. -1 The absorption peaks at the lignin are related to the CH functional group on the aromatic core. The assignments of the infrared absorption bands of lignin are shown in Table 2.
[0071] Table 2. Assignment of infrared absorption bands in lignin.
[0072] Gel permeation chromatography analysis results Table 3 shows the molecular weight data of lignin extracted using the optimal process group after acetylation treatment. The number-average molecular weight and weight-average molecular weight of the obtained lignin were 756 g / mol and 3505 g / mol, respectively. The low molecular weight and high PDI value of the obtained lignin indicate that the molecular weight distribution of the lignin extracted using this process is uneven.
[0073] Table 3. Molecular weight analysis of lignin
[0074] Elemental analysis results Table 4 lists the elemental composition of lignin extracted using the optimal process. The small amount of nitrogen (N) may be due to minor impurities in the lignin, and the resulting lignin has a low carbon (C) content. This is likely because thiourea dioxide undergoes a reduction reaction with residual lignin, disrupting quinone structures and conjugated double bonds.
[0075] Table 4 Elemental Analysis Content
[0076] HSQC two-dimensional NMR spectrum analysis results The lignin extracted using the optimal process group was subjected to NMR analysis. Figure 9 The side-chain region spectrum (δC / δH 50-90 / 2.5-6.0) (left) and aromatic region spectrum (δC / δH 95-150 / 6.0-8.0) (right) of the two-dimensional HSQC NMR spectrum of lignin extracted by the optimal process group are shown. The main peak assignments are shown in the table. In the aliphatic side-chain region and aromatic region, relative integration of each signal region corresponding to the HSQC spectrum was performed to semi-quantitatively determine the content of each fragment group in the lignin treatment; the aryl region mainly includes S (eugenyl), G (guaiacol), and H (p-hydroxyphenyl) structural units. In the side-chain region, corresponding integration of position signals was performed to roughly calculate the connection and content of different units, mainly including β-O-4, β-5, and β-β connections (the 13C-1H assignments in the spectrum are shown in Table 5). Combining the integrated area of the sample in the NMR, the proportion of S was calculated to be 42.22%, the proportion of G to be 46.51%, and the proportion of H to be 11.27%.
[0077] Table 5. Lignin 2D-HSQC spectra 13 C- 1 H belongs to
[0078] Figure 10 The two-dimensional HSQC NMR spectrum of lignin extracted using the optimal process is shown, along with the main structures observed in the lignin fraction extracted using the optimal process. This indicates that the structure of the lignin extracted by this process is consistent with the theoretical lignin structure, further confirming the high purity and low impurity content of the extracted lignin at the microstructural level.
[0079] Table 6 lists the L content of alkali-extracted lignin from the control group and the light-colored lignin extracted from the optimal process group. a b Values. Each sample was measured three times, and the average value was recorded in a table.
[0080] Where L The value represents the lightness or darkness of the color, ranging from 0 (pure black) to 100 (pure white). L The higher the value, the stronger the ability of the object's surface to reflect light, and the closer the color is to white; conversely, the lower the value, the darker the object.
[0081] Where a The value indicates the color's bias on the red-green axis; positive values bias towards red, and negative values bias towards green.
[0082] Where b The value indicates the color's bias on the yellow-blue axis; positive values bias towards yellow, and negative values bias towards blue.
[0083] Table 6. L of lignin extracted by different extraction processes a b Value Comparison Table
[0084] From Table 6, L of lignin a b Analysis of the data results shows that, using the method provided by this invention for extracting high-purity lignin by adding thiourea dioxide, the obtained lignin has a high L... The L value was significantly better than that of lignin extracted from the alkali-extracted control group (comparative example). Value, and close to a white background L The value was increased by nearly 1.78 times. Therefore, the method provided by this invention is very suitable for the extraction of light-colored lignin. The process is simple, low-cost, and safe. When used as a raw material for plant-based sunscreens, it can be applied directly without additional decolorization processes, resulting in low cost and a simple and convenient process. Figure 11 A comparison diagram of lignin extracted using the optimal process group and the lignin extracted using the alkali extraction control group is shown (left: optimal process group, right: alkali extraction control group); Reference Figure 11 It is known that the lignin extracted by the optimal process group of the present invention has a lighter color compared with the lignin extracted by the alkali extraction control group, which increases the possibility of its application as a raw material for sunscreen (one of the biggest obstacles to lignin as a raw material for sunscreen is that the extracted lignin is too dark in color to be widely used).
[0085] Table 7 lists the solubility of lignin extracted from the alkali extraction control group and the lignin extracted from the optimal process group in common solvents. Table 7. Solubility of lignin extracted by different extraction processes in common solvents
[0086] As can be seen from the test results in the table above, the method for extracting high-purity lignin by adding thiourea dioxide provided by this invention not only produces lignin with a lighter color, higher purity, and higher yield, but also exhibits excellent solubility in common solvents in the skincare industry, almost comparable to the currently common alkaline extraction process. In fact, its solubility in propylene glycol, glycerin, and isopropanol is even better than that of the lignin extracted by the alkaline extraction control group.
[0087] In summary, compared with the prior art, the present invention has the following beneficial effects: (1) Existing technologies mostly use the caustic soda method or the DES method to extract lignin. However, the purity of the lignin extracted by both methods is relatively low, and the extraction cost of the DES system is too high. Moreover, the solvents commonly used in the DES system include polyols, which makes it difficult to effectively and completely remove the residual polyols from the lignin obtained, which may pose certain risks to the application of the extracted lignin as a natural plant sunscreen. In contrast, this invention creatively adds thiourea dioxide, which has no risk of solvent residue. This process can effectively improve the purity of lignin, and the extraction cost is low and the process is simple.
[0088] (2) Thiourea dioxide is an oxidation product of thiourea, which has higher thermal stability and strong reducing properties under alkaline conditions, and has a wider range of applications. Thiourea dioxide is stable in acidic solutions, but decomposes in alkaline aqueous solutions to produce urea and hyposulfuric acid, releasing nascent hydrogen. Its reduction potential (-1220mV) is significantly higher than that of sodium hydrosulfite (-1080mV), making it a stronger reducing agent. This can help make the extracted lignin exhibit a lighter color, thus making it more suitable for use as a raw material for sunscreen.
Claims
1. A method for extracting high purity lignin by adding sulfurous anhydride, characterized by, Includes the following steps: (1) Add thiourea dioxide and NaOH to water to obtain a reaction solution. Mix the reaction solution with bamboo powder and stir the reaction in an oil bath. (2) After the reaction is completed, the mixture is cooled to room temperature, the bamboo residue is filtered out under reduced pressure, the mixture is rotary evaporated, the pH is adjusted, and the mixture is centrifuged. The precipitate is washed with pure water and dried to obtain lignin.
2. The method for extracting high-purity lignin by adding thiourea dioxide according to claim 1, characterized in that, In step (1), the NaOH is added to the water at a mass concentration of 10 g / L to 22 g / L.
3. The method for extracting high-purity lignin by adding thiourea dioxide according to claim 1, characterized in that, In step (1), the thiourea dioxide is added to water at a mass concentration of 6 g / L to 18 g / L.
4. The method for extracting high-purity lignin by adding thiourea dioxide according to claim 1, characterized in that, In step (1), the solid-liquid ratio of the bamboo powder to the reaction solution is 1:10 to 1:20 in g / mL.
5. The method for extracting high-purity lignin by adding thiourea dioxide according to claim 1, characterized in that, In step (1), the reaction temperature in the oil bath is 105℃~125℃.
6. The method for extracting high-purity lignin by adding thiourea dioxide according to claim 1, characterized in that, In step (1), the reaction time is 4h to 10h.
7. The method for extracting high-purity lignin by adding thiourea dioxide according to claim 1, characterized in that, In step (2), 2 / 3 of the volume of liquid is removed by rotary evaporation, and the pH is adjusted to 1.8-2.2 using a 2 mol / L hydrochloric acid solution.
8. Lignin prepared by the method for extracting high-purity lignin by adding thiourea dioxide as described in any one of claims 1 to 7.
9. The lignin according to claim 8, characterized in that, The content of lignin C is low, and the molecular weight is low.
10. The lignin according to claim 8, characterized in that, The lignin extraction rate reached 16.30%, and the lignin purity reached 89.2%.