A high-oxygen-content lignin-based conductive gel for oxygen-alkali pulping black liquor, its preparation method and application

Abstract

This invention discloses a lignin-based conductive gel with high oxygen-functional groups from oxy-alkali pulping black liquor, its preparation method, and its application, belonging to the field of biomass resource recycling. After pretreatment, wheat straw is used as an alkali source to prepare wheat straw oxy-alkali pulp in the presence of oxygen, and black liquor is collected. Crude lignin is obtained by acid precipitation, and then purified by dioxane to obtain lignin. The purified lignin has good molecular uniformity and a high content of oxygen-functional groups. Using the purified lignin as an electroactive material and graphene oxide as a conductive matrix, a conductive hydrogel is prepared by a simple hydrothermal method. Electrochemical testing shows that the mass specific capacitance of the obtained gel is higher than that of a simple graphene gel. This invention provides a new approach for the development of low-cost biomass-based electrode materials, further improving the utilization value of lignin from oxy-alkali pulping black liquor, and is of great significance for promoting resource utilization, environmental protection, and the sustainable development of the pulp and paper industry.

Description

Technical Field

[0001] This invention belongs to the field of biomass resource recycling, specifically relating to a lignin-based conductive gel with high oxygen content functional groups in oxo-alkali pulping black liquor, its preparation method, and its application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Black liquor from pulp and paper production contains a large amount of lignin. Lignin is the second largest biomass resource after cellulose and the only high-molecular-weight substance rich in aromatic rings. Its numerous oxygen-containing functional groups provide abundant active sites. However, it is often burned as waste or low-value fuel, which not only pollutes the environment but also wastes resources. Therefore, the development and high-value utilization of lignin is particularly important.

[0004] However, traditional pulping methods often use high-concentration alkali treatment. During this process, the original structure of lignin in the raw materials is destroyed, the content of oxygen-containing functional groups decreases and the uniformity deteriorates, which seriously limits the development and high-value utilization of lignin. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a lignin-based conductive gel with high oxygen content functional groups from oxy-alkali pulping black liquor, along with its preparation method and application. This invention uses black liquor lignin as a raw material to prepare the conductive gel, realizing the high-value utilization of black liquor lignin, which is of great significance for environmental protection and the sustainable development of the pulp and paper industry.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] The first aspect of this invention provides a method for preparing lignin with high oxygen-functional groups from oxo-alkali pulping black liquor, comprising the following steps:

[0008] (1) Using wheat straw as raw material and sodium carbonate as alkali source, wheat straw oxygen-alkali pulp and its black liquor are obtained after oxygen-alkali pulping under the synergistic effect of oxygen.

[0009] (2) Adjust the pH of the black liquor to 2.0-2.5, centrifuge to collect the precipitate, wash and dry it to obtain crude lignin; under nitrogen protection, purify the crude lignin with dioxane to obtain lignin with high oxygen functional groups in oxygen-alkali pulping black liquor.

[0010] In the method of the present invention, step (1) uses sodium carbonate as an alkali source to ensure that the obtained lignin has a higher oxygen-containing functional group content and good lignin homogeneity. Normally, in an alkaline environment, oxygen molecules react with the active groups of lignin, resulting in side chain shedding, methoxy group shedding, and aromatic nucleus cleavage to generate carboxylic acids. Because sodium carbonate is relatively weakly alkaline, it does not easily cause condensation reactions in lignin during delignification, thus yielding lignin with a high oxygen-containing functional group content and good homogeneity.

[0011] In some embodiments of the present invention, wheat straw is used as raw material after being screened, cut, washed, spirally twisted, and dried.

[0012] Preferably, the wheat straw has a cellulose content of 35-38%, a holocellulose content of 65-69%, and a lignin content of 17-20% by mass. The cellulose, hemicellulose, and lignin contents vary among different raw materials, and the lignin content in the black liquor produced after pulping also varies significantly. The specific use of wheat straw here is to clarify the composition of the raw materials used in this invention and to facilitate differentiation from other fiber raw materials.

[0013] In some embodiments of the present invention, the oxygen-alkali pulping conditions are as follows: sodium carbonate dosage 15%-25%, based on the oven-dry weight of wheat straw; and a material-to-liquid ratio (i.e., the ratio of wheat straw mass (g) to alkali solution volume (mL)).

[0014] Cooking is carried out at a ratio of 1:4 to 1:5, with an initial oxygen pressure of 0.8 MPa to 1.7 MPa and a temperature of 130 to 145°C for 2 to 4 hours. The alkali used in this invention is sodium carbonate, which has the following advantages compared to other alkalis: using sodium carbonate as the alkali solution allows for the direct use of sodium carbonate generated during the alkali recovery process, avoiding the causticization stage and calcination section required in traditional pulping waste liquor alkali recovery processes to convert sodium carbonate into sodium hydroxide for reuse. This greatly simplifies the alkali recovery process and saves enterprises alkali recovery costs.

[0015] In some embodiments of the present invention, a high-temperature and high-pressure resistant reactor may be selected for oxygen-alkali pulping.

[0016] In some embodiments of the present invention, concentrated hydrochloric acid can be added to adjust the pH of black liquor, or other acids can be added.

[0017] In some embodiments of the present invention, the centrifugation may be performed in a high-speed centrifuge at a speed of 10,000 r / min for 5-15 min.

[0018] In some embodiments of the present invention, the washing can be carried out using acidic water with pH=2, and the precipitate can be washed 3-5 times to remove the black liquid on the surface of the precipitate.

[0019] In some embodiments of the present invention, the drying may be freeze-drying, and crude lignin is obtained after freeze-drying.

[0020] In some embodiments of the present invention, the conditions for purifying lignin with dioxane are: reaction at 80-90°C for 1-5 hours.

[0021] In a second aspect, the present invention provides a lignin with high oxygen-containing functional groups in oxo-alkali pulping black liquor, which is prepared by the above-described preparation method.

[0022] In some embodiments of the present invention, the lignin with high oxygen-containing functional groups in the oxygen-alkali pulping black liquor has a molecular weight of 3000-3500, a polydispersity index of 2.5-3.0, a phenolic hydroxyl content of 0.3-0.4 mmol / g, and a carboxyl content of 1.0-1.5 mmol / g.

[0023] Numerous researchers have prepared lignin-based carbon materials using methods such as direct high-temperature carbonization of lignin, preliminary pyrolysis and carbonization of lignin followed by physical activation with CO2, and chemical activation with activators such as KOH. However, most of these carbon materials are predominantly microporous, limiting the improvement of capacitance performance. Building upon this, a three-dimensional lignin gel structure with abundant pores has been pre-prepared through chemical cross-linking, followed by high-temperature carbonization and activation to induce conductivity and generate micropores. This effectively constructs an interconnected hierarchical porous structure, but the potential for increasing energy density is limited. In fact, the phenolic hydroxyl and carboxyl groups in black liquor lignin can be effectively converted into quinone structures with redox activity, which can be used in electrode materials to effectively increase energy storage properties. However, lignin has electrical insulation properties, making it difficult to use directly as an electrode material to construct high-performance lignin-based electrode materials. Graphene possesses excellent conductivity, self-assembly properties, and flexibility, making it an ideal structural assembly unit. Chemically converted graphene can construct composite structures with various functional molecules through non-covalent bonds. More importantly, both lignin and chemically converted graphene have a six-membered carbon ring structure, which can be combined through π-π conjugation. Furthermore, both possess abundant oxygen-containing functional groups, which can form hydrogen bonding sites. All of the above demonstrates that chemically converted graphene oxide and lignin and its derivatives have the advantage of constructing composite materials, which can be used to build high-performance lignin-based electrode materials.

[0024] Therefore, a third aspect of the present invention provides a method for preparing a lignin-based conductive gel material with high oxygen content functional groups from oxygen-alkali pulping black liquor, comprising the following steps:

[0025] Using lignin with high oxygen-containing functional groups from the oxygen-alkali pulping black liquor as an electroactive material and graphene oxide as a conductive matrix, a conductive gel material was prepared hydrothermally.

[0026] In some embodiments of the present invention, the mass ratio of graphene oxide to lignin with high oxygen functional groups in alkali pulping black liquor is 1-2:1-2, preferably 1:1, and the concentration of graphene oxide in the reaction system is 2 mg / mL.

[0027] In some embodiments of the present invention, the hydrothermal process involves ultrasonically mixing a mixture of lignin and graphene with high oxygen functional groups from oxygen-alkali pulping black liquor, and then reacting it at 150–200°C for 10–12 hours.

[0028] Preferably, the ultrasound treatment lasts for 10-15 minutes.

[0029] In a fourth aspect, the present invention provides a lignin-based conductive gel material with high oxygen content functional groups for oxo-alkali pulping black liquor, which is prepared by the preparation method described in the third aspect.

[0030] A fifth aspect of the present invention provides the application of the above-mentioned oxygen-containing lignin-based conductive gel material from oxygen-alkali pulping black liquor in supercapacitors.

[0031] In some embodiments of the present invention, the supercapacitor is a symmetrical supercapacitor.

[0032] Preferably, the symmetrical supercapacitor uses a lignin-based conductive gel material with high oxygen content functional groups from alkali pulping black liquor as the electrode material, a cellulose film as the separator, a platinum sheet as the current collector, and 0.1 mol / L HClO4 as the electrolyte.

[0033] In a further preferred embodiment, during the preparation of the symmetrical supercapacitor, the oxygen-alkali pulping black liquor high-oxygen functional group lignin-based conductive gel material needs to be cut into thin slices of 1-2 mm and soaked together with the separator in the electrolyte for 8-10 hours.

[0034] The beneficial effects of this invention are as follows:

[0035] This invention discloses a high-oxygen-functional-group lignin from oxy-alkali pulping black liquor. Using wheat straw as raw material and sodium carbonate as the alkali source, oxy-alkali pulping is performed under the synergistic effect of oxygen to obtain wheat straw oxy-alkali pulp and its black liquor. The pH of the black liquor is adjusted to 2.0-2.5, the precipitate is collected by centrifugation, washed, and dried to obtain crude lignin. Under nitrogen protection, the crude lignin is purified with dioxane to obtain high-oxygen-functional-group lignin from oxy-alkali pulping black liquor. This invention uses more environmentally friendly and equipment-friendly sodium carbonate as the cooking liquor, eliminating the need for causticization and calcination stages, greatly simplifying the alkali recovery process. The oxy-alkali pulping method effectively reduces the silicon content in wheat straw pulp black liquor, alleviating silicon interference problems. Furthermore, based on the good uniformity and high oxygen-functional-group content of lignin molecules obtained from sodium carbonate oxy-alkali pulping, this invention develops it as an electrode material and assembles it into a supercapacitor, exhibiting excellent electrochemical performance, providing technical support for the high-value utilization of lignin.

[0036] This invention also discloses a lignin-based conductive gel material with high oxygen-containing functional groups from oxy-alkali pulping black liquor. Using lignin from oxy-alkali pulping black liquor as the electroactive material and graphene oxide as the conductive matrix, a self-supporting composite hydrogel is prepared via a simple hydrothermal method, and this hydrogel is used as the electrode material to directly assemble supercapacitors. The research results can provide new ideas for the development of low-cost biomass-based electrode materials, further improve the utilization value of black liquor lignin, and promote the full utilization of resources. Attached Figure Description

[0037] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0038] Figure 1 The images show the cyclic voltammetry curves of the lignin gel-based supercapacitor prepared in Example 1 and the pure graphene gel-based supercapacitor prepared in Comparative Example 3 at a scan rate of 5 mV / s.

[0039] Figure 2 The graphs show the charge-discharge curves of the lignin gel-based supercapacitor prepared in Example 1 and the pure graphene gel-based supercapacitor prepared in Comparative Example 3 at a current density of 1 A / g.

[0040] Figure 3 This is a cyclic voltammetry curve of the lignin gel-based supercapacitor prepared in Example 1 at different scan rates;

[0041] Figure 4 This is a charge-discharge curve of the lignin gel-based supercapacitor prepared in Example 1 at different current densities. Detailed Implementation

[0042] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0043] Example 1

[0044] After screening, washing, spiral extrusion, and drying, wheat straw was mixed with 18% sodium carbonate alkali solution at a material-to-liquid ratio of 1:4. The mixture was kneaded thoroughly and then transferred to a high-temperature, high-pressure reactor, maintaining an initial oxygen pressure of 1.4 MPa. The stirring speed of the reactor was adjusted to 15 rpm, the temperature was set to 135℃, and the mixture was kept at this temperature for 2 hours before collecting the black liquor. The pH of the black liquor was adjusted to 2.0 using concentrated hydrochloric acid. After centrifugation at 10000 rpm for 10 minutes, the precipitate was washed 3-5 times with acidic water at pH 2. The lignin was then purified using dioxane under a nitrogen atmosphere. The weight-average molecular weight, polydispersity index, and oxygen-containing functional group content of the obtained lignin are shown in Table 1.

[0045] The obtained lignin and graphene were mixed uniformly at a mass ratio of 1:1 and hydrothermally reacted at 180℃ for 12 h to prepare a gel electrode material, abbreviated as LGH. Finally, a symmetrical supercapacitor was assembled using a 1-2 mm gel sheet as the electrode material, a cellulose film as the separator, a platinum sheet as the current collector, and 0.1 mol / L HClO4 as the electrolyte, and its electrochemical performance was tested. The specific capacitance of the gel at a current density of 1 A / g was measured to be 144 F / g. Figure 1 As shown, it can still maintain a good rectangular shape even at a scanning rate as high as 100mV / s.

[0046] Example 2

[0047] After screening, washing, spiral extrusion, and drying, wheat straw was mixed with 15% sodium carbonate alkali solution at a material-to-liquid ratio of 1:4. The mixture was kneaded thoroughly and then transferred to a high-temperature, high-pressure reactor, maintaining an initial oxygen pressure of 1.0 MPa. The reactor stirring speed was adjusted to 15 rpm, the temperature was set to 135℃, and the mixture was kept at this temperature for 2 hours before collecting the black liquor. The pH of the black liquor was adjusted to 2.0 using concentrated hydrochloric acid. After centrifugation at 10000 r / min for 10 min in a high-speed centrifuge, the precipitate was washed 3-5 times with acidic water at pH=2. The lignin was then purified using dioxane under a nitrogen atmosphere. The weight-average molecular weight, polydispersity index, and oxygen-containing functional group content of the obtained lignin are shown in Table 1.

[0048] Example 3

[0049] After screening, washing, spiral extrusion, and drying, wheat straw was mixed with 25% sodium carbonate alkali solution at a material-to-liquid ratio of 1:4. The mixture was kneaded thoroughly and then transferred to a high-temperature, high-pressure reactor, maintaining an initial oxygen pressure of 1.7 MPa. The stirring speed of the reactor was adjusted to 15 rpm, the temperature was set to 135℃, and the mixture was kept at this temperature for 2 hours before collecting the black liquor. The pH of the black liquor was adjusted to 2.0 using concentrated hydrochloric acid. After centrifugation at 10000 rpm for 10 minutes, the precipitate was washed 3-5 times with acidic water at pH 2. The lignin was then purified using dioxane under a nitrogen atmosphere. The weight-average molecular weight, polydispersity index, and oxygen-containing functional group content of the obtained lignin are shown in Table 1.

[0050] Comparative Example 1

[0051] After screening, washing, spiral extrusion, and drying, wheat straw was mixed with 13.5% sodium hydroxide alkaline solution at a material-to-liquid ratio of 1:5. The mixture was kneaded thoroughly and then transferred to a high-temperature, high-pressure reactor, maintaining an initial oxygen pressure of 1.4 MPa. The stirring speed of the reactor was adjusted to 15 rpm, the temperature was set to 135℃, and the mixture was kept at this temperature for 2 hours before collecting the black liquor. The pH of the black liquor was adjusted to 2.0 using concentrated hydrochloric acid. After centrifugation at 10000 rpm for 10 minutes, the precipitate was washed 3-5 times with acidic water at pH 2. The lignin was then purified using dioxane under a nitrogen atmosphere. The weight-average molecular weight, polydispersity index, and oxygen-containing functional group content of the obtained lignin are shown in Table 1.

[0052] Comparative Example 2

[0053] Take an appropriate amount of 3-5cm wheat straw and grind it into a powder of 40-60 mesh in a grinder. Extract with benzene alcohol for 12 hours to remove resin, fat, and other substances. Then, grind the wheat straw powder without extract in a planetary ball mill under the following conditions: grind for 5 minutes, rest for 5 minutes, for a total of 24 hours, at a speed of 200 rpm. Mix the ball-milled wheat flour with a mixed enzyme solution of cellulase and xylanase, transfer it to a buffer solution of pH=5, and run it in a shaker at 150 rpm and 50℃. After 72 hours, centrifuge the mixture, wash the residue several times with acidic water of pH=2, and freeze-dry to obtain a crude enzymatically hydrolyzed and mildly acid-hydrolyzed crude lignin sample. Adjust the pH of the black liquor to 2.0 with concentrated hydrochloric acid, centrifuge at 10000 rpm for 10 minutes in a high-speed centrifuge, wash the precipitate 3-5 times with acidic water of pH=2, and then purify the lignin with dioxane under a nitrogen atmosphere. The weight-average molecular weight, polydispersity index, and oxygen-containing functional group content of the obtained lignin are shown in Table 1.

[0054] Table 1 Physicochemical properties of different black liquor lignins

[0055]

[0056] As shown in Table 1, the lignin obtained by the method of this invention retains a high content of oxygen-containing functional groups in the black liquor. In Comparative Example 2, the lignin was prepared from wheat straw through enzymatic hydrolysis and mild acid hydrolysis. Because this lignin was extracted directly from wheat straw without undergoing the alkali-oxygen synergistic treatment in the oxo-alkali pulping process, it retains the characteristics of wheat straw lignin to the greatest extent, resulting in the highest phenolic hydroxyl content but low carboxyl and oxygen-containing functional group content. Comparing Example 1 with this example demonstrates that even after the oxo-alkali pulping process, the lignin obtained by this invention still retains a high content of oxygen-containing functional groups.

[0057] Comparative Example 3

[0058] A gel electrode material, abbreviated as GH, was prepared by hydrothermal treatment of graphene at 180℃ for 12 h with a concentration of 2 mg / mL. Finally, a symmetrical supercapacitor was assembled using a 1-2 mm thick gel sheet as the electrode material, a cellulose film as the separator, a platinum sheet as the current collector, and 0.1 mol / L HClO4 as the electrolyte. The electrochemical performance was then tested. The specific capacitance of the gel at a current density of 1 A / g was measured to be 104.6 F / g. Figure 2 As shown, at a scan rate of 5 mV / s, the rectangular area is much smaller than the rectangular area of ​​the gel after adding black liquor lignin.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing lignin with high oxygen-containing functional groups from oxygen-alkali pulping black liquor, characterized in that, Includes the following steps: (1) Using wheat straw as raw material and sodium carbonate as alkali source, wheat straw oxygen-alkali pulp and its black liquor are obtained after oxygen-alkali pulping under the synergistic effect of oxygen; the oxygen-alkali pulping conditions are: sodium carbonate dosage 15%-25%, based on the oven-dry weight of wheat straw; material-liquid ratio 1:4-1:5, initial oxygen pressure 0.8 MPa-1.7 MPa, and cooking at 130-145℃ for 2-4 h; (2) Adjust the pH of the black liquor to 2.0-2.5, centrifuge to collect the precipitate, wash and dry it to obtain crude lignin; Under nitrogen protection, crude lignin was purified with dioxane to obtain lignin with high oxygen functional groups in alkali pulping black liquor; the conditions for lignin purification with dioxane were: reaction at 80-90℃ for 1-5 h. The oxygen-alkali pulping black liquor has a molecular weight of 3000-3500 for the high oxygen-functionalized lignin, a polydispersity index of 2.5-3.0, a phenolic hydroxyl content of 0.3-0.4 mmol / g, and a carboxyl content of 1.0-1.5 mmol / g.

2. The preparation method according to claim 1, characterized in that, Wheat straw is selected, cut, washed, spirally twisted, and dried before being used as raw material.

3. A lignin with high oxygen-containing functional groups in oxygen-alkali pulping black liquor, characterized in that, It is prepared by any of the preparation methods described in claims 1-2.

4. A method for preparing a lignin-based conductive gel material with high oxygen content functional groups from oxygen-alkali pulping black liquor, characterized in that, Includes the following steps: Using the oxygen-containing functional group lignin of the oxoalkali pulping black liquor described in claim 3 as the electroactive material and graphene oxide as the conductive matrix, a conductive gel material is prepared hydrothermally.

5. The preparation method according to claim 4, characterized in that, The mass ratio of graphene oxide to lignin with high oxygen-containing functional groups in alkali pulping black liquor is 1-2:1-2, and the concentration of graphene oxide in the reaction system is 2 mg / mL. Alternatively, the hydrothermal process involves ultrasonically mixing a mixture of lignin with high oxygen functional groups and graphene oxide from oxygen-alkali pulping black liquor, and then reacting it at 150-200℃ for 10-12 hours.

6. A lignin-based conductive gel material with high oxygen content functional groups for oxygen-alkali pulping black liquor, characterized in that, It is prepared by the preparation method described in claim 4 or 5.

7. The application of the oxygen-alkali pulping black liquor high oxygen-containing functional group lignin-based conductive gel material as described in claim 6 in supercapacitors.

8. The application as described in claim 7, characterized in that, The supercapacitor is a symmetrical supercapacitor.

9. The application as described in claim 8, characterized in that, The symmetrical supercapacitor uses lignin-based conductive gel material with high oxygen content functional groups from alkali pulping black liquor as electrode material, cellulose film as separator, platinum sheet as current collector, and 0.1 mol / L HClO4 as electrolyte.

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