A pretreatment method for extracting lignin from lignocellulose raw materials

Abstract

The present invention relates to a pretreatment method for extracting lignin from a lignocellulosic raw material, comprising the following steps: (S1) crushing the lignocellulosic raw material and then screening it to obtain a first material; (S2) placing the first material into an impregnation liquid and impregnating it for 1 to 3 hours to obtain a mixed material; the impregnation liquid is a mixture of cyanurate and a polyoxyethylene surfactant with an HLB of 15 to 18 in a mass ratio of 10:0.1 to 0.3, added to water to prepare a mixed solution with a concentration of 0.5 wt% to 2.0 wt%; (S3) steam-exploding the mixed material obtained in step S2; (S4) after the explosion, collecting the exploded material, washing it with water, and drying it to obtain a pretreated material, thereby completing the pretreatment of the lignocellulosic raw material. The pretreatment method provided by the present invention achieves a loose and porous surface structure of the pretreated lignocellulosic raw material, significantly improving the subsequent enzymatic hydrolysis rate.

Description

Technical Field

[0001] The invention belongs to the technical field of extraction of lignin from biomass, and particularly relates to a pretreatment method for extracting lignin from lignocellulose raw materials. Background Art

[0002] As global energy shortages intensify, finding alternatives to fossil fuels has become a crucial path for sustainable development worldwide. Lignocellulosic materials are the world's most abundant and widespread renewable biomass resource, primarily composed of cellulose, hemicellulose, and lignin, with lignin accounting for approximately 20% to 30%. Lignin has broad potential as a substitute for petroleum-based feedstock in the production of renewable chemicals and materials. It can be used not only as an additive in epoxy resins, rubber, and thermoplastics, but also as an animal feed additive and polymer raw material. Therefore, the efficient extraction of lignin from lignocellulosic materials has long been a research hotspot. However, the extremely dense structure of lignocellulosic materials, with the entanglement and constraints between cellulose, hemicellulose, and lignin, makes the clean, efficient, and structurally intact extraction of lignin a daunting challenge. Existing methods for extracting lignin often struggle to achieve both high yield and high quality, either severely damaging the natural structure of lignin or producing lignin of insufficient purity, severely impacting its high-value utilization. Currently, methods for extracting lignin include mechanical, acid-base, enzymatic, and organic solvent methods. Enzymatic hydrolysis is a new method for separating lignin that has been developed in recent years. It uses cellulase to hydrolyze cellulose and hemicellulase in biomass into water-soluble sugars, thereby separating and extracting lignin. Enzymatic hydrolysis has the advantages of mild separation conditions and minimal changes in lignin structure.

[0003] In order to improve the extraction rate of lignin, the lignocellulosic raw materials are generally pretreated before extraction. Common pretreatment methods include acid method, alkali method, organic solvent method, steam explosion method, biological method, etc., as well as a combination of several methods. The treatment effect of acid method, alkali method and organic solvent method is good, but the treatment cost is high and the environmental pollution is serious. Although the biological method is clean, the cycle is long and the treatment efficiency is low, making it difficult to meet the requirements of industrialization. Steam explosion method is pollution-free and has a short cycle. It is a method widely used in the pretreatment technology of lignocellulosic raw materials, but it also has the problem of unsatisfactory treatment effect. Therefore, how to improve the pretreatment efficiency and reduce the pretreatment cost is an urgent problem to be solved.

[0004] CN117025693A discloses a method for pretreating agricultural biomass by steam explosion, which uses spiral extrusion coupled with ultra-low acid spraying to synergize steam explosion and pretreat the agricultural biomass. Prior to steam explosion pretreatment, the agricultural biomass is first subjected to spiral extrusion, then to acid spraying. Steam explosion is performed under the synergistic effect of the acids, effectively improving enzymatic hydrolysis efficiency, lowering steam explosion conditions, reducing energy consumption, and reducing the formation of inhibitors. Furthermore, the method effectively reduces the severity of the steam explosion pretreatment. Furthermore, the method uses a small amount of chemicals, and has the advantages of low production cost, high selectivity, and high environmental friendliness. However, while the addition of acid during the steam explosion process has a synergistic promoting effect, it still causes a certain degree of corrosion to the equipment. Summary of the Invention

[0005] In view of the above-mentioned problems in the prior art, the present invention provides a pretreatment method for extracting lignin from lignocellulosic raw materials, that is, the lignocellulosic raw materials are first impregnated and then steam exploded, which can effectively improve the enzymatic hydrolysis efficiency and lignin extraction rate without any pollution or corrosion problems.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A pretreatment method for extracting lignin from lignocellulose raw materials comprises the following steps:

[0008] (S1) crushing the lignocellulosic raw material and then sieving it to obtain a first material;

[0009] (S2) placing the first material obtained in step S1 into an impregnation solution and impregnating it for 1 to 3 hours to obtain a mixed material; the impregnation solution is a mixture of cyanurate and a nonionic surfactant added to water in a mass ratio of 10:0.1 to 0.3 to prepare a concentration of 0.5 wt% to 2.0 wt%; the nonionic surfactant is a polyoxyethylene surfactant with an HLB of 15 to 18;

[0010] (S3) placing the mixed material obtained in step (S2) into a reaction tank of a steam explosion device, and then introducing high-pressure saturated steam into the reaction tank to perform steam explosion;

[0011] (S4) After the blasting is completed, the blasted materials are collected, washed, and dried to obtain pretreated materials, thereby completing the pretreatment of the wood fiber raw materials.

[0012] Furthermore, the lignocellulosic raw material in (S1) is at least one of corn straw, wheat straw, reed and rice straw.

[0013] Furthermore, (S1) the pulverization is to grind the lignocellulose raw material into 3-10 mm using a pulverizer; and the screening is to sieve with a mesh size of 40-60 meshes.

[0014] Furthermore, (S2) the solid-liquid ratio of the first material to the impregnation liquid is 1 kg: 2 to 4 L.

[0015] Furthermore, (S2) the cyanurate is at least one of sodium cyanurate and potassium cyanurate; and the polyoxyethylene surfactant is polyoxyethylene fatty acid ester.

[0016] Furthermore, the polyoxyethylene fatty acid ester is at least one of S-49 and S-52.

[0017] Furthermore, the immersion in (S2) is at 50-60°C and is supplemented by ultrasonic treatment, with an ultrasonic treatment power of 200-300W and a frequency of 5-10kHz.

[0018] Furthermore, (S3) the steam explosion is performed by controlling the explosion pressure to be 1.2-1.5 MPa and maintaining the pressure for 6-10 minutes.

[0019] Furthermore, (S4) the water washing is to press the blasted material dry and then add it into water at a mass ratio of 1:1 to 3, stir for 5-20 minutes, and wash it with water 2-4 times; the drying is to press the washed material dry and then dry it in a drying oven at 80 to 90°C for 8 to 12 hours.

[0020] Pre-treating lignocellulosic raw materials using steam explosion technology can disrupt the dense three-dimensional network structure of the lignocellulosic raw materials, promote component separation, and facilitate subsequent fiber enzymatic hydrolysis. The present invention impregnates the lignocellulosic raw materials with the combined action of cyanurate and surfactant before steam explosion, softening the fibers and promoting the destruction of the dense three-dimensional network structure of the lignocellulosic raw materials. This facilitates the separation of the fibers without mechanical damage during steam explosion. Simultaneously, the fibers undergo a certain degree of swelling, which facilitates increased water vapor penetration, better promotes the rupture of the raw material cell walls, and makes the raw material structure loose and porous, thereby providing more enzyme contact sites and significantly improving the efficiency of subsequent enzymatic hydrolysis and lignin extraction.

[0021] During the research process, the inventors also found that not all organic salts and surfactants can synergistically enhance the pretreatment effect of lignocellulosic raw materials with steam explosion. The applicant found in the study that cyanurate solutions containing six-membered large π bonds can interact with the large π bonds of the benzene rings of lignin to form π-π interactions. The possible reason is that this promotes the destruction of the dense three-dimensional network structure of the lignocellulosic raw materials. At the same time, polyoxyethylene surfactants (HLB between 15-18) have good solubilization and wetting effects. After comparative studies, the present invention uses an impregnation solution composed of cyanurate and polyoxyethylene surfactants mixed in a certain mass ratio to impregnate the lignocellulosic raw materials. Through the π-π interaction, solubilization, wetting, swelling and dispersion of the cyanurate-polyoxyethylene surfactants, combined with the effect of steam explosion, efficient pretreatment of the lignocellulosic raw materials is achieved, the separation of the components in the lignocellulosic raw materials is promoted, and the subsequent enzymatic hydrolysis efficiency is improved. In practice, if the concentration of the cyanurate-polyoxyethylene surfactant in the impregnation solution is too low, it will not effectively reduce the surface tension of the interfacial liquid of the lignocellulosic raw material. Consequently, the cyanurate-polyoxyethylene surfactant cannot evenly penetrate the fiber interior to achieve the corresponding π-π interaction, solubilization, and swelling effects. If the concentration of the cyanurate-polyoxyethylene lipid surfactant in the impregnation solution is too high, exceeding the critical micelle concentration, material waste will result. Experimental results show that the optimal synergistic effect is achieved when the cyanurate-polyoxyethylene surfactant concentration in the impregnation solution is 0.5wt% to 2.0wt%.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The pretreatment method of lignocellulose raw materials provided by the present invention performs steam explosion under the synergistic action of a cyanurate-polyoxyethylene surfactant, that is, before steam explosion, the lignocellulose raw materials are impregnated with an aqueous solution of the cyanurate-polyoxyethylene surfactant, which is beneficial for separating the fibers without mechanical damage during steam explosion. At the same time, the fibers are caused to swell to a certain extent, which is beneficial for increasing the penetration of water vapor, thereby improving the effect of steam explosion, and further improving the subsequent enzymatic hydrolysis efficiency and lignin extraction rate, while avoiding pollution and corrosion problems.

[0024] The pretreatment method of lignocellulosic raw materials provided by the present invention can reduce the steam explosion conditions due to the impregnation before steam explosion. Compared with the explosion pressure of 2 to 4 MPa in the prior art, a better explosion effect can be achieved at an explosion pressure of 1.2 to 1.5 MPa. The reduction of steam explosion conditions can also reduce energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1This is the SEM image of the corn straw before pretreatment in Example 1.

[0026] Figure 2 SEM image of corn straw after pretreatment in Example 1.

[0027] Figure 3 This is the SEM image of the corn straw after pretreatment in Comparative Example 1. DETAILED DESCRIPTION

[0028] To make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. The following examples are provided to facilitate a better understanding of the present invention, but are not intended to limit the present invention.

[0029] Unless otherwise specified, the "parts" in the examples of the present invention are parts by mass, and the "%" in the examples of the present invention are percentages by mass unless otherwise specified.

[0030] The examples and comparative examples all used corn stalks from the same batch.

[0031] Mai Ze 52 is a polyoxyethylene fatty acid ester surfactant, a non-ionic surfactant with an HLB of 16.9.

[0032] Tween 80 is a polyoxyethylene sorbitan ester surfactant, a nonionic surfactant with an HLB of 16.7.

[0033] Span 20 is sorbitan monolaurate, a nonionic surfactant with an HLB of 8.6.

[0034] Example 1

[0035] A pretreatment method for extracting lignin from lignocellulose raw materials comprises the following steps:

[0036] (S1) crushing corn stalks into about 5 mm using a crusher, and then sieving through a 40-mesh sieve to obtain a first material;

[0037] (S2) sodium cyanurate and surfactant (Mei Ze 52) were added to water in a mass ratio of 10:0.1 to prepare an impregnation solution with a concentration of 0.5 wt%, 1 kg of the first material obtained in step (S1) was added to 2 L of the impregnation solution, mixed evenly, and impregnated for 2 h to obtain a mixed material;

[0038] (S3) placing the mixture obtained in step (S2) into a reaction tank of a steam blaster, and introducing high-pressure saturated steam into the reaction tank for steam explosion, with a blasting pressure of 1.5 MPa and a pressure maintenance time of 6 min.

[0039] (S4) After the blasting is completed, the blasted material is collected, pressed dry, and then added to water at a mass ratio of 1:2 and stirred for 10 minutes for water washing, and washed twice; after the water washing is completed, the material is pressed dry and then dried in a drying oven at 80° C. for 8 hours to obtain the pretreated material, thereby completing the pretreatment of the corn straw.

[0040] Example 2

[0041] The rest is the same as Example 1, except that the concentration of the impregnation solution in step (S2) is 1.0 wt%.

[0042] Example 3

[0043] The rest is the same as Example 1, except that the concentration of the impregnation solution in step (S2) is 2.0 wt%.

[0044] Example 4

[0045] The rest is the same as Example 1, except that in step (S2), sodium cyanurate and cyanurate 52 are added to water at a mass ratio of 10:0.3 to prepare an impregnation solution with a concentration of 0.5wt%.

[0046] Example 5

[0047] The rest is the same as Example 1, except that in step (S2), the immersion is heated to 50° C. and supplemented with ultrasonic treatment, with a power of 200 W and a frequency of 6 kHz.

[0048] Example 6

[0049] The rest is the same as Example 1, except that wheat straw is used instead of corn straw, the blasting pressure in step (S3) is 1.2 MPa, and the pressure maintenance time is 8 minutes.

[0050] Example 7

[0051] The rest is the same as Example 1, except that rice straw is used instead of corn straw, the blasting pressure in step (S3) is 1.3 MPa, and the pressure maintenance time is 8 minutes.

[0052] Example 8

[0053] The rest is the same as Example 1, except that reed straw is used instead of corn straw, the blasting pressure in step (S3) is 1.4 MPa, and the pressure maintenance time is 8 min.

[0054] Example 9

[0055] The rest is the same as Example 1, except that Tween-80 is used instead of Tween 52, and the HLB of Tween 80 is 16.7.

[0056] Comparative Example 1

[0057] Without performing step (S2), the first material obtained in step (S1) is directly subjected to steam explosion in step (S3).

[0058] Comparative Example 2

[0059] The rest is the same as Example 1, except that sodium cyanurate is not used and only surfactant 52 is used.

[0060] Comparative Example 3

[0061] The rest is the same as Example 1, except that the disk 20 is used instead of the valve 52.

[0062] Test Case

[0063] The pretreated materials of the embodiment and comparative example were subjected to enzymatic hydrolysis according to the following steps:

[0064] Enzymatic hydrolysis: The pretreated lignocellulosic raw material was enzymatically hydrolyzed using the company's homemade cellulase (prepared by the preparation method disclosed by the applicant in the previous patent 201810076624.X, with an enzyme activity of 20 FPU / ml). The concentration of the hydrolyzate was 70%, the pH of the hydrolyzate was 4.8, and the enzymatic hydrolysis conditions were 15% substrate amount (the volume ratio of the mass of the pretreated lignocellulosic raw material to the hydrolyzate, g / 100 mL) and enzymatic hydrolysis at 180 rpm at 50°C for 96 h. After the enzymatic hydrolysis was completed, the hydrolyzate was centrifuged, and the solid after enzymatic hydrolysis was collected and dried to obtain crude lignin.

[0065] The enzymatic hydrolysis rates of the pretreated materials obtained in the Examples and Comparative Examples were measured by differential weight method, i.e., the enzymatic hydrolysis rates were calculated according to the following formula 1, as shown in Table 1. Since the lignocellulose components are mainly cellulose and hemicellulose that can be enzymatically hydrolyzed into sugars, and lignin that cannot be enzymatically hydrolyzed, the higher the enzymatic hydrolysis rate, the higher the purity of the lignin in the enzymatic hydrolysis residue, that is, the higher the lignin extraction rate.

[0066] Enzymatic hydrolysis rate = (m1-m2) / m1*100% (Formula 1)

[0067] in,

[0068] m1 = mass of solid before enzymatic hydrolysis, g;

[0069] m2 = mass of solid after enzymatic hydrolysis, g;

[0070] Results and Analysis

[0071] Figure 1 The SEM images of corn straw before and after pretreatment in Example 1 are shown. Figure 2 The SEM images of corn stalks before and after pretreatment of Example 1 are shown. Figure 1It can be seen that before pretreatment, the surface of corn stalks was smooth and the structure was dense and flat. Figure 2 It can be seen that after pretreatment with cyanurate-polyoxyethylene surfactant aqueous solution and steam explosion, the structure of corn stalks becomes loose and porous, and the explosion effect is obvious.

[0072] Figure 3 The following are SEM images of corn stalks before and after pretreatment in Comparative Example 1. Figure 2 and Figure 3 It can be seen that the surface cracks of the corn stalks pretreated in Comparative Example 1 are relatively few, a small amount of debris is attached to the fiber surface, and the blasting effect is not obvious.

[0073] The cellulose and hemicellulose contents of the pretreated materials in the examples and comparative examples were measured according to the National Renewable Energy Laboratory (NREL) method, and the relative enzymatic hydrolysis rates were calculated according to Formula 2, as shown in Table 1.

[0074] Relative enzymatic hydrolysis rate = (enzymatic hydrolysis rate / total content of cellulose and hemicellulose) * 100% (Formula 2)

[0075] Table 1 Component content and enzymatic hydrolysis rate of pretreated materials

[0076] Implementation Group Enzymatic hydrolysis rate Relative enzymatic hydrolysis rate Example 1 56.8％ 89.6％ Example 2 58.9％ 93.3％ Example 3 59.9％ 94.7％ Example 4 58.8％ 93.3％ Example 5 61.6％ 95.6％ Example 6 58.4％ 89.5％ Example 7 59.2％ 89.6％ Example 8 56.1％ 89.7％ Example 9 54.9％ 86.2％ Comparative Example 1 43.0％ 67.1％ Comparative Example 2 45.8％ 71.8％ Comparative Example 3 50.9％ 80.2％

[0077] As can be seen from Table 1, compared with the comparative example, the enzymatic hydrolysis rate of the lignocellulosic raw material after pretreatment by the pretreatment method provided by the present invention is significantly improved, especially the relative enzymatic hydrolysis rate of Preferred Example 5 reaches 95.6%. At the same time, the present invention uses an aqueous solution of a cyanurate-polyoxyethylene surfactant to impregnate the lignocellulosic raw material before steam explosion, which can reduce the conditions for steam explosion and thus reduce energy consumption.

Claims

1. A pretreatment method for extracting lignin from lignocellulose raw materials, characterized in that: The following steps are involved: (S1) crushing the lignocellulosic raw material and then screening it to obtain a first material; (S2) placing the first material obtained in step S1 into an impregnation solution and impregnating for 1 to 3 hours to obtain a mixed material; the impregnation solution is a mixture of cyanurate and a nonionic surfactant added to water in a mass ratio of 10:0.1 to 0.3 to prepare a concentration of 0.5 wt% to 2.0 wt%; the nonionic surfactant is a polyoxyethylene surfactant with an HLB of 15 to 18; the cyanurate is at least one of sodium cyanurate and potassium cyanurate; (S3) placing the mixed material obtained in step (S2) into a reaction tank of a steam explosion device, and then introducing high-pressure saturated water vapor into the reaction tank to perform steam explosion; (S4) After the blasting is completed, the blasted materials are collected, washed, and dried to obtain pretreated materials, thereby completing the pretreatment of the wood fiber raw materials.

2. The pretreatment method according to claim 1, characterized in that (S1) The lignocellulosic raw material is at least one of corn straw, wheat straw, reed and rice straw.

3. The pretreatment method according to claim 1, characterized in that (S1) The pulverization is to grind the lignocellulose raw material into 3-10 mm using a pulverizer; and the screening is to sieve with a mesh size of 40-60 meshes.

4. The pretreatment method according to claim 1, wherein (S2) The solid-liquid ratio of the first material to the impregnation liquid is 1 kg: 2-4 L.

5. The pretreatment method according to claim 1, characterized in that The polyoxyethylene surfactant described in (S2) is polyoxyethylene fatty acid ester.

6. The pretreatment method according to claim 5, characterized in that The polyoxyethylene fatty acid ester is at least one of Maize 49 or Maize 52.

7. The pretreatment method according to claim 1, wherein (S2) The immersion is carried out at a temperature of 50-60° C. and is supplemented by ultrasonic treatment, wherein the ultrasonic treatment power is 200-300 W and the frequency is 5-10 kHz.

8. The pretreatment method according to claim 1, characterized in that (S3) The steam explosion is performed by controlling the explosion pressure to be 1.2-1.5 MPa and maintaining the pressure for 6-10 minutes.

9. The pretreatment method according to claim 1, characterized in that (S4) The water washing is to press the blasted material dry, add it into water at a mass ratio of 1:1-3, stir for 5-20 minutes, and wash it with water 2-4 times.

10. The pretreatment method according to claim 1, characterized in that (S4) The drying step is to press the washed material dry and then dry it in a drying oven at 80-90°C for 8-12 hours.

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