Method for separating lignin from corn cob residues and preparing 5-HMF

Abstract

The invention relates to a method for separating lignin from corn cob residues and preparing 5-hydroxymethylfurfural (5-HMF). The method comprises the following steps: adding corn cob residues serving as a main raw material into an alkaline hydrogen peroxide solution to form high-solid slurry, and storing the slurry at room temperature for pretreatment; performing solid-liquid separation on the pretreated slurry to obtain filter residues and pretreatment liquid, adding sulfuric acid into the pretreatment liquid to separate out lignin, and performing solid-liquid separation to obtain lignin and acid precipitation filtrate; further taking the filter residue as a raw material, adding the filter residue into a mixed solution composed of the acid precipitation filtrate and an organic solvent, and purifying after high-temperature hydrolysis to obtain the 5-HMF. The method is simple in process, mild in process and less in pollution, and high-valued graded utilization of the corn cob residues can be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of high-value utilization of biomass, and in particular relates to a method for separating lignin from xylose residue and preparing 5-HMF. Background Art

[0002] Corncobs are a common biomass feedstock for the industrial production of xylose. During the acid hydrolysis of corncobs to produce xylose, the hemicellulose in the cobs is converted into xylose, simultaneously producing a large amount of xylose residue, which is rich in lignin and cellulose. Currently, manufacturers often burn xylose residue as fuel or sell it directly as a raw material at a low price, significantly reducing its value. In recent years, efforts have been made to develop various ways to increase the value of xylose residue. For example, direct separation and extraction of xylose residue components is one approach. Patents CN119102135A and CN119350653A disclose methods for extracting high-purity cellulose and lignin from xylose residue, respectively. Xylose residue can also be used to produce bio-based materials, such as board materials (CN106313258B), wood-plastic materials (CN107602970A), cellulose films (CN117924761A), nanocellulose (CN111499768B), and polyurethane materials (CN116854883A). In addition, the graded utilization of lignin and cellulose components of xylose residue is also an effective strategy. Patent CN118994266A discloses a method for co-producing lignin and glucose from xylose residue.

[0003] 5-Hydroxymethylfurfural (5-HMF) is an important bio-based platform chemical with widespread applications in pharmaceuticals, new materials, fuels, and other fields. However, the main challenge with 5-HMF is its high production cost. Xylose residue, rich in cellulose, is a potential raw material for 5-HMF production. Using xylose residue as a raw material for 5-HMF production would help reduce the raw material cost of 5-HMF. Furthermore, the catalytic system is another key factor affecting the cost of 5-HMF production. Therefore, developing a green, efficient, and economically viable catalytic system for 5-HMF remains a current challenge. Summary of the Invention

[0004] The present invention aims to provide an environmentally friendly process for separating lignin from xylose residue and preparing 5-HMF by employing a xylose residue component fractionation strategy and utilizing the acid-precipitated wastewater generated by xylose residue pretreatment to construct an inexpensive catalytic reaction system. First, xylose residue, as the primary raw material, is added to an alkaline hydrogen peroxide solution to form a high-solidity slurry. The slurry is then stored at room temperature for pretreatment. The pretreated slurry undergoes solid-liquid separation to yield a filter residue and a pretreatment liquid. Sulfuric acid is added to the pretreatment liquid to precipitate lignin, and solid-liquid separation is performed to yield lignin and an acid-precipitated filtrate. The acid-precipitated filtrate contains NaSO produced by acid-base neutralization and unreacted sulfuric acid, which together constitute an acid / metal sulfate binary catalyst, providing the catalytic system with suitable Brønsted and Lewis acid sites, thereby promoting the synthesis of 5-HMF. Furthermore, the filter residue is added to a mixture of the acid-precipitated filtrate and an organic solvent, and 5-HMF is purified after high-temperature hydrolysis. The present invention has the characteristics of simple process, mild process and little pollution, and can not only realize the graded utilization of xylose residues, but also help to reduce the cost of 5-HMF.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: A method for separating lignin from xylose residue and preparing 5-HMF comprises the following steps: using xylose residue as a raw material, adding the xylose residue to an alkaline hydrogen peroxide solution to form a high-solidity slurry, and pre-treating the slurry at room temperature; subjecting the pre-treated slurry to solid-liquid separation to obtain a filter residue and a pre-treatment liquid, adding sulfuric acid to the pre-treatment liquid to adjust the pH value to 1-3 to precipitate lignin, and subjecting the pre-treatment liquid to solid-liquid separation to obtain lignin and an acid precipitation filtrate; and adding the filter residue as a raw material to a mixture of the acid precipitation filtrate and an organic solvent, and purifying the mixture after high-temperature hydrolysis to obtain 5-HMF.

[0006] Furthermore, the xylose residue is the solid residue after xylose is hydrolyzed from corn cobs.

[0007] Furthermore, the solid-liquid mass-to-volume ratio of the xylose residue to the alkaline hydrogen peroxide solution is 1:1-3 (g:mL).

[0008] Furthermore, the alkali in the alkaline hydrogen peroxide solution is sodium hydroxide, the mass concentration of NaOH is 1.0-5.0%, and the mass concentration of hydrogen peroxide is 2.0-5.0%.

[0009] Furthermore, the pretreatment is to store the slurry at room temperature for 5 to 7 days, in order to ensure a certain pretreatment time so as to better dissolve the lignin and facilitate the subsequent separation and extraction of the lignin.

[0010] Furthermore, the organic solvent is tetrahydrofuran, the volume ratio of the acid precipitation filtrate to the organic solvent is 1:1-10, and the solid-liquid mass volume ratio of the filter residue to the mixed liquid is 1:10-15; the reaction temperature of the high-temperature hydrolysis is 170-210° C., and the reaction time is 60-90 min.

[0011] Furthermore, the purification operation is to subject the hydrolyzate to rotary evaporation and water washing to obtain a crude extract, then add 3 times the volume of MIBK for extraction, collect the organic phase and perform rotary evaporation, and after rotary evaporation to remove MIBK, obtain 5-HMF.

[0012] The advantages of the present invention are: 1. The present invention can achieve mild pretreatment of high-solidity xylose residue system and lignin separation; 2. The present invention can utilize lignin acidification liquid to construct a catalytic system, thereby reducing the catalytic cost of 5-HMF; 3. The present invention provides a green process for separating lignin from xylose residue and preparing 5-HMF, which can achieve the graded and high-value utilization of xylose residue. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the process of the present invention; Figure 2 The effect of different sulfate types in the acid solution on the yield of 5-HMF; Figure 3 is the effect of sulfate dosage on 5-HMF yield; Figure 4 SEM images of xylose residue before and after pretreatment; Figure 5 FTIR images of xylose residue before and after pretreatment; Figure 6 FTIR graph of separated lignin; Figure 7 This is the hydrogen nuclear magnetic resonance spectrum of 5-HMF. DETAILED DESCRIPTION

[0014] The process flow chart of separating lignin from xylose residue and preparing 5-HMF of the present invention is as follows: Figure 1 As shown in the figure, xylose residue is used as the main raw material. It is added to an alkaline hydrogen peroxide solution to form a high-solidity slurry, which is then stored at room temperature for pretreatment. The pretreated slurry undergoes solid-liquid separation to obtain a filter residue and a pretreatment liquid. Sulfuric acid is added to the pretreatment liquid to precipitate lignin, and solid-liquid separation is performed to obtain lignin and an acid precipitation filtrate. The filter residue is further added to a mixture of the acid precipitation filtrate and THF, and 5-HMF is purified after high-temperature hydrolysis.

[0015] The detection method used in the embodiment of the present invention is: (1) 5-HMF was detected using a high performance liquid chromatograph (Agilent 1260). The column model was ZorbaxSB-C18; the column temperature was 30°C; the mobile phase was methanol and water (Vmethanol / Vwater = 1 / 4) at a flow rate of 0.6 mL / min; the detector was an ultraviolet detector at a wavelength of 284 nm. The yield of 5-HMF was calculated as follows: Where: x1 is the mass of 5-HMF obtained after the reaction, g; x2 is the mass of xylose residue before reaction, g; M1 is the molar mass of 5-HMF, 126.1 g / mol; M2 is the molar mass of cellulose monomer, 162.1 g / mol; ω is the cellulose content in xylose residue.

[0016] (2) Scanning electron microscopy characterization of the samples before and after the reaction was performed using a JEOL JSM-7500F scanning electron microscope. The samples were crushed to 80 mesh before testing, dried, and gold-sprayed. The analysis parameters were: resolution 1.0 nm (15 kV) / 1.4 nm (1 kV); acceleration voltage 0.1~30 kV; magnification 250,000~1 million times; beam intensity 10 -13 ~2×10 -9 .

[0017] (3) The samples before and after the reaction were characterized by infrared spectroscopy using a Perkin Elmer Spetrum-2 infrared analyzer. The KBr pellet method was used and the scanning wavenumber range was 400–4000 cm -1 , the measurement resolution is 2 cm -1 .

[0018] (4) Take about 10 mg of purified 5-HMF and dissolve it in 0.6 mL of deuterated DMSO solution. After the sample is completely dissolved, filter it with a 0.2 μm syringe filter. Place the sample in a nuclear magnetic resonance tube and use a 400 MHz nuclear magnetic resonance spectrometer produced by Bruker, Switzerland to detect the nuclear magnetic H spectrum.

[0019] Example 1 Xylose residue was mixed with an alkaline hydrogen peroxide solution at a solid-liquid mass-volume ratio of 1:3 (g:mL) to form a high-solidity slurry. The alkaline hydrogen peroxide solution contained 1.0% NaOH and 2.0% H₂O₂. The solid slurry was pretreated by storing it at room temperature for 5 days. The pretreated slurry was then subjected to solid-liquid separation to obtain a filter residue and a pretreatment solution. Sulfuric acid was added to the pretreatment solution to adjust the pH to 1.0. Lignin precipitated and was then filtered or centrifuged to obtain lignin and an acid precipitate. The lignin extraction yield was 71.8%. The filter residue from the pretreatment solid-liquid separation was added to a mixture of the acid precipitate and THF. The volume ratio of the acid precipitate to the organic solvent was 1:10, and the solid-liquid mass-volume ratio of the filter residue to the mixture was 1:15. The above mixture was reacted in an autoclave at 210°C for 60 min. After the reaction, the hydrolyzed liquid was subjected to rotary evaporation and washed with water to obtain a crude extract, which was then extracted by adding MIBK (3 times the volume of the liquid). The organic phase was collected and rotary evaporated to remove MIBK, resulting in 5-HMF with a yield of 44.5%.

[0020] Example 2 Xylose residue was mixed with an alkaline hydrogen peroxide solution at a solid-liquid mass-volume ratio of 1:3 (g:mL) to form a high-solidity slurry. The alkaline hydrogen peroxide solution contained 2.0% NaOH and 3.0% H₂O₂. The solid slurry was pretreated by storing it at room temperature for 6 days. The pretreated slurry was then subjected to solid-liquid separation to obtain a filter residue and a pretreatment solution. Sulfuric acid was added to the pretreatment solution to adjust the pH to 2.0. Lignin precipitated and was then filtered or centrifuged to obtain lignin and an acid precipitate. The lignin extraction yield was 70.3%. The filter residue from the pretreatment solid-liquid separation was added to a mixture of the acid precipitate and THF. The volume ratio of the acid precipitate to the organic solvent was 1:7, and the solid-liquid mass-volume ratio of the filter residue to the mixture was 1:12. The above mixture was reacted in an autoclave at 190°C for 70 min. After the reaction, the hydrolyzed liquid was subjected to rotary evaporation and washed with water to obtain a crude extract, which was then extracted by adding MIBK (3 times the volume of the liquid). The organic phase was collected and rotary evaporated to remove MIBK, resulting in 5-HMF with a yield of 42.3%.

[0021] Example 3 Xylose residue was mixed with an alkaline hydrogen peroxide solution at a solid-liquid mass-volume ratio of 1:2.5 (g:mL) to form a high-solidity slurry. The alkaline hydrogen peroxide solution contained 3.0% NaOH and 4.0% H₂O₂. The solid slurry was pretreated by storing it at room temperature for 7 days. The pretreated slurry was then subjected to solid-liquid separation to obtain a filter residue and a pretreatment solution. Sulfuric acid was added to the pretreatment solution to adjust the pH to 2.0. Lignin precipitated and was then filtered or centrifuged to obtain lignin and an acid precipitate. The lignin extraction yield was 71.2%. The filter residue from the pretreatment solid-liquid separation was added to a mixture of the acid precipitate and THF. The volume ratio of the acid precipitate to the organic solvent was 1:5, and the solid-liquid mass-volume ratio of the filter residue to the mixture was 1:10. The above mixture was reacted in an autoclave at 180°C for 80 min. After the reaction, the hydrolyzed liquid was subjected to rotary evaporation and washed with water to obtain a crude extract, which was then extracted by adding MIBK (3 times the volume of the liquid). The organic phase was collected and rotary evaporated to remove MIBK, resulting in 5-HMF with a yield of 43.9%.

[0022] Example 4 Xylose residue was mixed with an alkaline hydrogen peroxide solution at a solid-liquid mass-volume ratio of 1:2 (g:mL) to form a high-solidity slurry. The alkaline hydrogen peroxide solution contained 4.5% NaOH and 3.5% H₂O₂. The solid slurry was pretreated by storing it at room temperature for 7 days. The pretreated slurry was then subjected to solid-liquid separation to obtain a filter residue and a pretreatment solution. Sulfuric acid was added to the pretreatment solution to adjust the pH to 1.5. Lignin precipitated and was then filtered or centrifuged to obtain lignin and an acid precipitate. The lignin extraction yield was 69.0%. The filter residue from the pretreatment solid-liquid separation was added to a mixture of the acid precipitate and THF. The volume ratio of the acid precipitate to the organic solvent was 1:3, and the solid-liquid mass-volume ratio of the filter residue to the mixture was 1:10. The above mixture was reacted in an autoclave at 170°C for 90 min. After the reaction, the hydrolyzed liquid was subjected to rotary evaporation and washed with water to obtain a crude extract, which was then extracted by adding MIBK (3 times the volume of the liquid). The organic phase was collected and subjected to rotary evaporation. After removing MIBK by rotary evaporation, 5-HMF was obtained with a yield of 42.1%.

[0023] Example 5 Xylose residue was mixed with an alkaline hydrogen peroxide solution at a solid-liquid mass-volume ratio of 1:1 (g:mL) to form a high-solidity slurry. The alkaline hydrogen peroxide solution contained 5.0% NaOH and 5.0% H₂O₂. The solid slurry was stored at room temperature for 7 days for pretreatment. The pretreated slurry was then subjected to solid-liquid separation to obtain a filter residue and a pretreatment solution. Sulfuric acid was added to the pretreatment solution to adjust the pH to 1.0. Lignin precipitated and was filtered or centrifuged to obtain lignin and an acid precipitate. The lignin extraction yield was 69.9%. The filter residue from the pretreatment solid-liquid separation was added to a mixture of the acid precipitate and THF. The volume ratio of the acid precipitate to the organic solvent was 1:1, and the solid-liquid mass-volume ratio of the filter residue to the mixture was 1:15. The above mixture was reacted in an autoclave at 170°C for 90 min. After the reaction, the hydrolyzed liquid was subjected to rotary evaporation and washed with water to obtain a crude extract, which was then extracted by adding MIBK (3 times the volume of the liquid). The organic phase was collected and rotary evaporated to remove MIBK, resulting in 5-HMF with a yield of 40.6%.

[0024] Control trial 1 Under the same experimental conditions, the components of the samples of high-solidity xylose residue pretreated for different days were analyzed, and the results of the relationship between the number of days and the cellulose retention rate and hemicellulose removal rate were compared, as shown in Table 1.

[0025] Table 1 Number of days Cellulose retention rate (%) Lignin removal rate (%) Day 1 99.6 40.6 Day 2 98.7 60.8 Day 3 98.5 80.5 Day 4 97.2 82.3 Day 5 96.5 89.0 Day 6 95.4 89.2 Day 7 93.6 89.2 Day 8 93.6 89.0 Day 9 93.4 88.9 Day 10 93.1 89.7 The results of pretreatment of high-solidity xylose residues show that cellulose retention and lignin removal rates increase and decrease, respectively, with increasing storage pretreatment time. This is because lignin dissolves in the liquid phase during pretreatment, increasing the cellulose content and decreasing the lignin content in the pretreated xylose residue. Furthermore, the lignin removal rate remains stable after the fifth day of pretreatment. Considering both removal effectiveness and pretreatment efficiency, a 5-7 day pretreatment period is optimal.

[0026] Control trial 2 The acid precipitation liquid contains sulfuric acid and metal salts, which are the key to the acid precipitation liquid's catalytic effect. In order to compare the catalytic effects of sulfuric acid and metal salts, different metal salt acid precipitation liquid systems were configured and hydrolysis catalysis comparison tests were carried out. The results are as follows: Figure 2As shown. As can be seen from the figure, the five sulfuric acids Al2(SO4)3, Fe2(SO4)3, ZnSO4, CuSO4 and Na2SO4 have significantly different effects on the yield of 5-HMF. Na2SO4 has the highest yield. This is because the combination of sulfuric acid and Na2SO4 in the acid solution provides more suitable Brønsted acid and Lewis acid sites, which promotes the effective conversion of xylose residue to 5-HMF. Therefore, when designing the alkaline hydrogen peroxide solvent used for the pretreatment of xylose residue, the present invention only considers the use of NaOH as the base in the solution. The purpose is that NaOH can produce Na2SO4 after subsequent neutralization, which can be used for the catalysis of subsequent raw materials, while also avoiding the additional use of other catalysts.

[0027] Control experiment 3: The comparative effect of the amount of Na2SO4 in the acid precipitation filtrate on the preparation of 5-HMF is as follows: Figure 3 As shown in the figure, it can be seen that the 5-HMF yield increases with the increase in the amount of Na2SO4 used. When the amount of Na2SO4 is 5.0%, the 5-HMF yield is the highest. Further increasing the amount of Na2SO4, the 5-HMF yield decreases. This is because excessive Na2SO4 promotes further conversion of 5-HMF, resulting in a decrease in the 5-HMF yield. Therefore, if a catalytic system with acid precipitation filtrate as the main body is constructed, an appropriate amount of sulfuric acid can be added to the xylose residue pretreatment liquid to neutralize the NaOH in the pretreatment liquid, and the Na2SO4 content can be adjusted by regulating the pH value of the acid precipitation liquid. At the same time, after the xylose residue is pretreated with alkaline hydrogen peroxide, most of the lignin dissolves in the alkaline solution, and the lignin can be precipitated and separated by acid precipitation. Therefore, regulating the pH value of the acid precipitation liquid is also the key to precipitating and separating lignin. Based on this, through experimental screening, the present invention finally determined that when the pH value of the pretreatment liquid is controlled at 1-3, it can meet the dual goals of both having an appropriate Na2SO4 content and precipitating most of the lignin.

[0028] To further illustrate the effect of the present invention, the present invention takes Example 1 as an example to perform SEM comparative characterization of xylose residue before and after pretreatment ( Figure 4 ) and FTIR comparative characterization ( Figure 5 ), and the acid-precipitated lignin was characterized by FTIR ( Figure 6 ), 5-HMF was labeled with nuclear magnetic resonance ( Figure 7 ).Depend on Figure 4 It can be seen that the original xylose residue has a dense structure, a rough surface, and some small particles. After pretreatment, the overall structure becomes looser, and a large number of holes and cracks appear on the surface. This is because the pretreatment significantly improves the structural characteristics of the raw material, and the dissolution of lignin components destroys the anti-degradation barrier of the biomass. Figure 5 It can be seen that at 3413 cm -1The position corresponds to the stretching vibration absorption peak of -OH, which is the characteristic band of cellulose fiber, and the 2909 cm -1 and 1427 cm -1 The absorption near 1024 cm is the characteristic peak of -CH2- and -CH functional groups in cellulose. -1 is the cellulose CC and CO stretching vibration absorption peak, while 897 cm -1 It is the characteristic peak of cellulose β-1,4-glycosidic bond. After pretreatment, these characteristic peaks are enhanced, indicating that the relative content of cellulose in the pretreated xylose residue is increased. -1 and 1458 cm -1 The absorption peaks at these two locations are mainly attributed to the vibration absorption of the aromatic nucleus of the benzene ring. The absorption peaks of the pretreated xylose residue sample are weakened, indicating that the pretreatment effectively removes lignin.

[0029] FTIR of acid-precipitated lignin Figure 6 As shown. Located at 1601 cm -1 The C=C stretching vibration peak of 1510 cm-1 is derived from the aromatic ring skeleton of syringyl and guaiacyl. -1 and 1451 cm -1 The absorption peaks at 1221 cm-1 correspond to the stretching vibration and CH bending vibration of the aromatic ring, respectively. -1 The guaiacyl CO stretching vibration peak at 1126 cm -1 The strong characteristic peaks of phenolic bond / ether bond at 1126 cm -1 The peak is the characteristic band of lignin, 782 cm -1 The peaks at are the CH stretching peaks of guaiacyl and syringyl.

[0030] Figure 7 The H NMR spectrum of the purified 5-HMF product shows the following: the peak near 2.5 ppm is the DMSO solvent peak, and the peak at 3.47 ppm is the water peak. The chemical shifts of the aldehyde peaks range from 9.0 to 10.0 ppm. The doublet at a is the aldehyde hydrogen, and the doublets at b and c are aromatic hydrogens. The hydrogens in the -CH2 group of the hydroxymethyl group have chemical shifts between 4.4 and 4.7 ppm, the peak at d is the hydrogen in the -CH2 group, and the hydrogens in the -OH group of the hydroxymethyl group have chemical shifts between 4.0 and 6.0 ppm. The multiplet at e is the hydroxyl hydrogen. The peak characteristics of the spectrum are consistent with theoretical expectations for hydrogen atoms in the 5-HMF molecular structure, confirming that the sample is 5-HMF. The absence of other impurity peaks in the spectrum indicates high purity.

Claims

1. A method for separating lignin from xylose residue and preparing 5-HMF, characterized in that: The following steps are involved: Xylose residue is used as raw material, and is added into an alkaline hydrogen peroxide solution to form a high solid slurry, which is then stored at room temperature for pretreatment; The pretreated slurry is subjected to solid-liquid separation to obtain a filter residue and a pretreatment liquid. Sulfuric acid is added to the pretreatment liquid to adjust the pH value to 1-3, lignin is precipitated, and lignin and an acid precipitation filtrate are obtained through solid-liquid separation. The filter residue is then used as a raw material and added to a mixture of the acid precipitation filtrate and an organic solvent, and 5-HMF is purified after high-temperature hydrolysis.

2. The method for separating lignin from xylose residue and preparing 5-HMF according to claim 1, wherein: The xylose residue is the solid residue after the xylose in corn cobs is hydrolyzed.

3. The method for separating lignin from xylose residue and preparing 5-HMF according to claim 1, wherein: The solid-liquid mass-to-volume ratio of the xylose residue to the alkaline hydrogen peroxide solution is 1:1-3 g / mL.

4. The method for separating lignin from xylose residue and preparing 5-HMF according to claim 3, wherein: The alkali in the alkaline hydrogen peroxide solution is sodium hydroxide, the mass concentration of NaOH is 1.0-5.0%, and the mass concentration of hydrogen peroxide is 2.0-5.0%.

5. The method for separating lignin from xylose residue and preparing 5-HMF according to claim 1, wherein: The pretreatment is to store the slurry at room temperature for 5 to 7 days.

6. The method for separating lignin from xylose residue and preparing 5-HMF according to claim 1, wherein: The organic solvent is tetrahydrofuran, the volume ratio of the acid precipitation filtrate to the organic solvent is 1:1-10, and the solid-liquid mass volume ratio of the filter residue to the mixed liquid is 1:10-15; the reaction temperature of the high-temperature hydrolysis is 170-210° C., and the reaction time is 60-90 min.

7. The method for separating lignin from xylose residue and preparing 5-HMF according to claim 1, wherein: The purification operation comprises the following steps: subjecting the hydrolyzed liquid to rotary evaporation and washing with water to obtain a crude extract, then adding 3 times the volume of MIBK for extraction, collecting the organic phase and subjecting it to rotary evaporation, and removing MIBK by rotary evaporation to obtain 5-HMF.

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