Method for preparing water-soluble lignin chelated calcium fertilizer based on iron-manganese double site catalyst

Abstract

The present application belongs to the technical field of lignin modification and chelate fertilizer preparation, and particularly relates to a method for preparing water-soluble lignin chelated calcium fertilizer based on iron-manganese double-site catalysts. The present application provides a special iron-manganese double-site catalyst for lignin oxidation modification of water-soluble lignin chelated fertilizer preparation. The catalyst can enhance the hydrophilicity of oxidized lignin and increase the effective chelating sites, thereby facilitating the formation of stable chelate structure with calcium source, so that the yield and calcium content of water-soluble lignin chelated fertilizer can be significantly improved. In addition, in the present application, papermaking black liquor is used as raw material, and functionalization and fertilizerization conversion of lignin can be realized under mild temperature range and controllable pressure oxidation conditions, which greatly improves the added value of waste liquid generated in papermaking, and is conducive to the coordinated development of black liquor resource utilization and functional fertilizer preparation industry.

Description

Technical Field

[0001] This invention belongs to the field of lignin modification and chelated fertilizer preparation technology, and particularly relates to an iron-manganese dual-site catalyst for lignin oxidation modification and its application, and more particularly to a method for preparing water-soluble lignin chelated calcium fertilizer based on the iron-manganese dual-site catalyst. Background Technology

[0002] Lignin is the most abundant renewable aromatic polymer resource in nature. Its industrial sources are mainly black liquor from pulp and paper making and by-products from biomass ethanol refining. However, currently more than half of industrial lignin is only used for heat recovery through incineration, which not only wastes resources but also easily causes environmental problems such as air and water pollution.

[0003] In modern agriculture, the effective supply of micronutrients (such as Fe, Zn, Ca, and Mg) is crucial for improving crop yield and quality. However, traditional inorganic micronutrient fertilizers easily combine with phosphates and carbonates in the soil to form insoluble substances, resulting in low nutrient utilization. Chelated fertilizers, through chelating agents, form stable chelates with micronutrients, significantly reducing nutrient fixation and improving nutrient utilization. They are one of the core fertilizer types for green and efficient agricultural production. Lignin molecules naturally contain polar functional groups such as phenolic and alcoholic hydroxyl groups, possessing the potential to act as chelating agents. Furthermore, their renewable and environmentally compatible characteristics align with the development needs of green agriculture. Therefore, lignin can be used as a carrier to develop lignin chelated fertilizers.

[0004] However, on the one hand, protolignin's dense structure, complex molecular structure, wide molecular weight distribution, and strong hydrophobicity result in significantly insufficient solubility and dispersibility in aqueous systems, making it difficult to form a uniform and stable active component; on the other hand, protolignin molecules can react with Ca... 2+ The number of functional groups capable of effective coordination (such as phenolic hydroxyl and carboxyl groups) is limited, and their accessibility and reactivity are low due to the influence of complex three-dimensional spatial structures. These structural characteristics lead to insufficient stability of calcium complexes / chelates formed by lignin and calcium salts, resulting in problems such as poor water solubility or dispersibility of calcium fertilizer products, slow release kinetics of available calcium, low migration efficiency in the soil, and poor absorption and utilization of calcium by crops. These issues make it difficult to meet the requirements of fertilizer application. Therefore, it is necessary to modify and regulate its structure and functional group composition to enhance chelating performance.

[0005] Currently, lignin modification methods focus on increasing its functional group density and improving its reactivity, mainly including three categories: chemical modification, physical modification, and bio-enzyme modification. Among them, physical modification mainly uses methods such as ball milling and ultrasound to refine lignin particle size and increase specific surface area, which can only help improve reaction contact efficiency and is difficult to change the essential properties of functional groups. Bio-enzyme modification relies on lignin-degrading enzymes to gently break molecular chains. Although it is environmentally friendly, the reaction cycle is long, the precision of functional group regulation is low, and the cost of enzyme preparations is high, limiting its industrial applicability. Chemical modification, on the other hand, has become the main technical means of lignin modification because it has advantages such as high reaction efficiency, large range of functional group regulation, strong adjustability of modified product properties, and easy compatibility with existing industrial production equipment.

[0006] Chemical modification includes esterification, amination, and oxidation. Among these, esterification and amination mainly target the active sites such as phenolic and alcoholic hydroxyl groups in lignin molecules, and carry out esterification reactions with carboxylic acid (anhydride) reagents such as maleic anhydride and citric acid, or undergo Mannich grafting reactions with amine compounds such as ethylenediamine, to directionally introduce highly active functional groups such as carboxyl and amino groups, thereby significantly enhancing the reactivity and functional properties of lignin. However, the modification process requires a large amount of organic reagents and has drawbacks such as high reagent costs, high energy consumption, and poor environmental friendliness.

[0007] Oxidative modification can simultaneously break ether bonds and carbon-carbon crosslinking bonds in lignin molecules, as well as convert hydroxyl groups to carboxyl and aldehyde groups, through oxidants such as hydrogen peroxide and ozone. This helps reduce steric hindrance, significantly increases functional group density, and provides highly efficient and controllable modification effects. Therefore, it has become a major technical means for the chemical modification of lignin, as disclosed in patents such as CN109503221A, CN109503220A, and CN116332681A. However, the degree of catalytic oxidation and the type of catalyst used directly affect the effect of lignin oxidative modification.

[0008] Therefore, it is necessary to develop a lignin oxidation modification catalyst and a new oxidation modification technology that are suitable for the preparation of chelated fertilizers, so as to specifically enhance the chelation ability of modified lignin with metal ions and promote the transformation of lignin resources into high-value green fertilizers. Summary of the Invention

[0009] To address the above technical problems, this invention proposes an iron-manganese dual-site catalyst for catalytic oxidation modification of lignin and its application, and in particular, proposes a method for preparing water-soluble lignin chelated calcium fertilizer based on the modification of lignin using this iron-manganese dual-site catalyst.

[0010] This invention constructs a bimetallic active site catalyst with synergistic interaction between iron single atoms and manganese clusters to catalytically oxidize and modify lignin in papermaking black liquor, thereby enhancing the hydrophilicity of lignin and its chelation ability with calcium ions. Subsequently, a calcium source is introduced to produce water-soluble lignin-chelated calcium fertilizer. After oxidizing and modifying lignin in black liquor using the catalyst prepared in this invention, the calcium fertilizer yield is higher and the calcium content in the calcium fertilizer is increased, thus effectively promoting the coordinated development of high-value utilization of black liquor resources and the preparation of functional fertilizers.

[0011] The technical solution of this invention is: This invention provides an iron-manganese dual-site catalyst for lignin oxidative modification, and the preparation steps of the iron-manganese dual-site catalyst are as follows: (1) Dissolve iron salt and zinc salt in water at a molar ratio of 1:5~10, mix by ultrasonication, stir at 55~65℃ and add dropwise to glucose solution at a uniform rate to obtain glucose-metal salt solution, wherein the molar ratio of glucose to iron salt is 1:20~30, the concentration of glucose solution is 12~20 g / L, the iron salt is selected from ferric chloride and ferric sulfate, and the zinc salt is selected from zinc chloride and zinc sulfate. (2) Add a nitrogen source to the glucose-metal salt solution in (1) and disperse it evenly. Then freeze it at -80°C and vacuum dry it to obtain a nitrogen-doped metal glucose precursor. The nitrogen source is selected from melamine and dicyandiamide, preferably melamine. (3) The precursor in (2) was pyrolyzed under nitrogen protection to obtain a glucose carbon-based iron single-atom catalyst; The pyrolysis conditions were as follows: under a nitrogen protective atmosphere, the temperature was increased in three stages: in the first stage, the temperature was increased to 400~600℃ at a rate of 5℃ / min and held for 0.5~1.5 h; in the second stage, the temperature was increased to 750~950℃ at a rate of 5℃ / min and held for 0.5~1.5 h; and in the third stage, the temperature was increased to 1000~1200℃ at a rate of 5℃ / min and held for 1~5 min. (4) The glucose carbon-based iron single-atom catalyst prepared in (3) is impregnated in a manganese salt solution, stirred at 60~95℃ and a reducing agent is added dropwise. After washing and drying, the iron-manganese dual-site catalyst is obtained. Preferably, the reducing agent is selected from sodium borohydride or formaldehyde, and the manganese salt is preferably manganese chloride. The molar ratio of manganese salt to reducing agent is 1:2500~2800.

[0012] Furthermore, the application of the iron-manganese dual-site catalyst described above is a key technical aspect to be protected in this invention, particularly its application in the preparation of water-soluble lignin-chelated calcium fertilizer.

[0013] This invention also provides a method for preparing water-soluble lignin-chelated calcium fertilizer based on the aforementioned iron-manganese dual-site catalyst, specifically: Under the action of an iron-manganese dual-site catalyst, papermaking black liquor was used as raw material. O2 was introduced into the liquor at pH 13.0, and the reaction was carried out at 100-110℃ for 1-4 h to obtain oxidized black liquor. After cooling to room temperature, the liquor was centrifuged and the supernatant was collected. Calcium chloride was added to the supernatant and stirred at 40-60℃ for 1-2 h. After centrifugation, the supernatant was collected and the supernatant was added to the supernatant and stirred at 40-60℃ for 1-2 h. After centrifugation and filtration, supernatant I and precipitate I were obtained separately. 95% ethanol was added to supernatant I and mixed. After centrifugation and filtration, precipitate II and supernatant II were obtained separately. Precipitate II was then washed 2-3 times with 80% ethanol and freeze-dried to obtain the water-soluble lignin chelated calcium fertilizer.

[0014] Preferably, the lignin content in the papermaking black liquor is 8% to 10% by mass, and the mass-to-volume ratio of calcium chloride to black liquor is 1 g: 0.02 to 0.1 L. More preferably, the mass-to-volume ratio of calcium chloride to black liquor is 1 g: 0.04 to 0.06 L. Most preferably, the mass-to-volume ratio of calcium chloride to black liquor is 1 g: 0.05 L.

[0015] Preferably, the mass ratio of the black liquor to the iron-manganese dual-site catalyst is 100-150:1, more preferably 100-120:1, and most preferably 100:1.

[0016] As a preferred method, precipitate II is washed 2-3 times with 80% ethanol.

[0017] Furthermore, the present invention also provides a water-soluble lignin-chelated calcium fertilizer, wherein the calcium fertilizer is obtained by chelating calcium salt and oxidized lignin at a mass ratio of 0.1~0.3 g:1 g, and the oxidized lignin is prepared by catalytic oxidation reaction of the iron-manganese dual-site catalyst provided by the present invention with oxygen at 100~110℃ for 1~4 h.

[0018] Preferably, the "water" mentioned in this invention includes not only water in the traditional sense, but more preferably, the water can also be "deionized water", "distilled water" or other commonly used laboratory water.

[0019] The present invention has the following advantages and effects compared with the prior art: (1) A special iron-manganese dual-site catalyst for lignin oxidation modification is provided for the preparation of water-soluble lignin chelate fertilizer. Using this catalyst to oxidize and modify lignin can significantly enhance the hydrophilicity of lignin and increase the effective chelation sites, which is more conducive to the subsequent formation of a stable chelate structure with calcium source. Compared with black liquor lignin calcium fertilizer (calcium fertilizer yield of 13.26% and calcium content of 3.87%) prepared without catalytic oxidation treatment, after black liquor lignin is catalytically oxidized with this catalyst, the yield of the calcium fertilizer is 27.95%~42.35% and the calcium content is 6.37%~11.85% based on the mass of lignin in black liquor. The yield and calcium content of calcium fertilizer are significantly improved. (2) A highly efficient water-soluble lignin chelated calcium fertilizer was provided. Experimental results showed that after applying the chelated calcium fertilizer to corn seedlings, the corn plants grew more vigorously, and the plant height, root length, chlorophyll and other traits were significantly improved compared with water-insoluble lignin calcium fertilizer and unmodified lignin calcium fertilizer. (3) In this invention, papermaking black liquor is used as raw material. Under relatively mild temperature range and controllable pressure oxidation conditions, lignin functionalization and fertilizer conversion can be realized, which greatly improves the added value of waste liquid generated in the papermaking process and has good industrial adaptability. (4) The chelated fertilizer preparation process provided by the present invention is highly operable, easy to scale up, and can be stably and repeatedly realized. In addition, the entire fertilizer preparation process is green and environmentally friendly, does not use any organic solvents, and the catalyst is easy to recycle and reuse, which helps to reduce operating costs and environmental burden, and further improves the greenness and industrialization of the process. Attached Figure Description

[0020] Figure 1 This is a diagram showing the growth status of corn plants in each fertilization group in Application Example 1 of this invention; Figure 2 This is a bar graph comparing the corn plant height of each fertilization group in Application Example 1 of this invention; Figure 3 This is a bar graph comparing the root length of corn in each fertilization group in Application Example 1 of the present invention; Figure 4 This is a bar chart comparing the chlorophyll content of maize in each fertilization group in Application Example 1 of the present invention. Detailed Implementation

[0021] To enable those skilled in the art to better understand the present invention, the present invention will now be further described in conjunction with specific embodiments.

[0022] Example 1 The method for preparing water-soluble lignin-chelated calcium fertilizer based on an iron-manganese dual-site catalyst comprises the following steps: 30 mL of black liquor from papermaking pulping, with a lignin content of 10% (w / w), was adjusted to pH 13.0 with NaOH and placed in a reaction vessel. Then, an iron-manganese dual-site catalyst was added to the vessel at a mass ratio of 100:1. After thorough mixing, O2 gas at a pressure of 1 MPa was introduced into the vessel, and the reaction was carried out at 100°C for 2 h to obtain oxidized black liquor. After the reaction, the reaction vessel was cooled to room temperature, the oxidized black liquor was removed, centrifuged, and the supernatant was collected. 0.6 g of calcium chloride was added to this supernatant, and the mixture was stirred uniformly at 50°C for 1 hour. h, centrifuge again to obtain supernatant I and precipitate I respectively. Add 3 times the volume of ethanol solution (volume fraction of 95%) to supernatant I, mix well, centrifuge, and separate light brown precipitate II and supernatant II. Wash precipitate II three times with 80% ethanol solution, freeze dry, and it is water-soluble lignin chelated calcium fertilizer. The calcium fertilizer yield is 42.35% based on the mass of lignin in black liquor. Further analysis of its calcium content shows that the calcium content is 11.85%.

[0023] The iron-manganese dual-site catalyst used in this embodiment was prepared using the following method: (1) Dissolve 0.20 mmol ferric chloride and 1.30 mmol zinc chloride in 100 mL of water, sonicate for 30 min, stir at 55 °C and add dropwise to 0.1 L of 15 g / L glucose solution, stir at 600 rpm for 3 h to obtain glucose-metal salt solution; (2) Add 10 g of melamine to the glucose-metal salt solution in (1), mix well, freeze at -80℃, and then freeze-dry to obtain nitrogen-doped metal glucose precursor. (3) The precursor in (2) was pyrolyzed under nitrogen protection. The specific conditions were as follows: N2 (40 mL / min) was introduced, the temperature was raised from room temperature to 550℃ at a rate of 5℃ / min and held for 1 h, then raised to 850℃ at a rate of 5℃ / min and held for 1 h, then raised to 1000℃ at a rate of 5℃ / min and held for 1 min, and finally cooled to room temperature to obtain glucose carbon-based iron single-atom catalyst. (4) Take 0.1 g of glucose carbon-based iron single-atom catalyst and 0.02 mmol of manganese chloride obtained in (3), add them to 30 mL of water, heat to 80 °C, add 2.23 mL of 37% formaldehyde solution (so that the molar ratio of manganese chloride to formaldehyde is 1:2600), stir for 3 h, wash with water 3 times, and dry in a vacuum oven at 60 °C for 12 h to obtain the iron-manganese dual-site catalyst.

[0024] Example 2 The method for preparing water-soluble lignin-chelated calcium fertilizer based on an iron-manganese dual-site catalyst differs from Example 1 in that the catalytic oxidation modification temperature of black liquor is lowered. The specific steps are as follows: 30 mL of black liquor from papermaking pulping, with a lignin content of 10% (w / w), was adjusted to pH 13.0 with NaOH and then placed in a reaction vessel. Next, an iron-manganese dual-site catalyst (same as in Example 1) was added to the reaction vessel at a mass ratio of 100:1 (black liquor to catalyst). After stirring thoroughly, O2 gas at a pressure of 1 MPa was introduced into the reaction vessel, and the reaction was carried out at 90 °C for 2 h to obtain oxidized black liquor. After the reaction, the reaction vessel was cooled to room temperature, the oxidized black liquor was removed, centrifuged, and the supernatant was collected. 0.6 g of calcium chloride was added to this supernatant, and the mixture was stirred uniformly at 50 °C for 1 h. h, centrifuge again to obtain supernatant I and precipitate I respectively. Add 3 times the volume of ethanol solution (volume fraction of 95%) to supernatant I, mix well, centrifuge, and separate light brown precipitate II and supernatant II. Wash precipitate II three times with 80% ethanol solution, freeze dry, and it is water-soluble lignin chelated calcium fertilizer. The yield of calcium fertilizer is 27.95% based on the mass of lignin in black liquor. Further analysis of its calcium content shows that the calcium content is 8.07%.

[0025] Example 3 The method for preparing water-soluble lignin-chelated calcium fertilizer based on an iron-manganese dual-site catalyst differs from Example 1 in that the catalytic oxidation modification pressure of black liquor is increased. The specific steps are as follows: 30 mL of black liquor from papermaking pulping, with a lignin content of 10% (w / w), was adjusted to pH 13.0 with NaOH and then placed in a reaction vessel. Next, an iron-manganese dual-site catalyst (same as in Example 1) was added to the reaction vessel at a mass ratio of 100:1 (black liquor to catalyst). After stirring thoroughly, O2 gas at a pressure of 2 MPa was introduced into the reaction vessel, and the reaction was carried out at 100 °C for 2 h to obtain oxidized black liquor. After the reaction, the reaction vessel was cooled to room temperature, the oxidized black liquor was removed, centrifuged, and the supernatant was collected. 0.6 g of calcium chloride was added to this supernatant, and the mixture was stirred uniformly at 50 °C for 1 hour. h, centrifuge again to obtain supernatant I and precipitate I respectively. Add 3 times the volume of ethanol solution (volume fraction of 95%) to supernatant I, mix well, centrifuge, and separate light brown precipitate II and supernatant II. Wash precipitate II three times with 80% ethanol solution, freeze dry, and it is water-soluble lignin chelated calcium fertilizer. The calcium fertilizer yield is 28.44% based on the mass of lignin in black liquor. Further analysis of its calcium content shows that the calcium content is 6.37%.

[0026] Example 4 The method for preparing water-soluble lignin-chelated calcium fertilizer based on an iron-manganese dual-site catalyst differs from Example 1 in that the catalytic oxidation modification time of black liquor is reduced. The specific steps are as follows: 30 mL of black liquor from papermaking pulping, with a lignin content of 10% (w / w), was adjusted to pH 13.0 with NaOH and then placed in a reaction vessel. Next, an iron-manganese dual-site catalyst (same as in Example 1) was added to the reaction vessel at a mass ratio of 100:1 (black liquor to catalyst). After stirring thoroughly, O2 gas at a pressure of 1 MPa was introduced into the reaction vessel, and the reaction was carried out at 100°C for 1 h to obtain oxidized black liquor. After the reaction, the reaction vessel was cooled to room temperature, the oxidized black liquor was removed, centrifuged, and the supernatant was collected. 0.6 g of calcium chloride was added to the supernatant, and the mixture was stirred uniformly at 50°C for 1 h. h, centrifuge again to obtain supernatant I and precipitate I respectively. Add 3 times the volume of ethanol solution (volume fraction of 95%) to supernatant I, mix well, centrifuge, and separate light brown precipitate II and supernatant II. Wash precipitate II three times with 80% ethanol solution, freeze dry, and it is water-soluble lignin chelated calcium fertilizer. The yield of calcium fertilizer is 28.48% based on the mass of lignin in black liquor. Further analysis of its calcium content shows that the calcium content is 7.76%.

[0027] Comparative Example 1 The only difference between this comparative example and Example 1 is that the preparation method of the iron-manganese dual-site catalyst is different, while the other operations and parameters are exactly the same as in Example 1.

[0028] In this comparative example, the iron-manganese dual-site catalyst was prepared by only the first stage of pyrolysis in (3), without the latter two stages of pyrolysis (heating to 850℃ at 5℃ / min and holding for 1 h, then continuing to heat to 1000℃ at 5℃ / min and holding for 1 min). Specifically: Introduce N2 (40 mL / min) and heat from room temperature to 550℃ at a rate of 5℃ / min, then maintain the temperature for 2 h.

[0029] Experimental results show that after the iron-manganese dual-site catalyst prepared by this comparative method is used to catalytically oxidize and modify black liquor, and then to produce chelated calcium fertilizer, the calcium fertilizer yield is 22.38% based on the mass of lignin in the black liquor. Further analysis of its calcium content shows that the calcium content is 7.19%. This indicates that the latter two stages of pyrolysis were not carried out, and the pore-forming agent zinc was not fully volatilized.

[0030] Comparative Example 2 The only difference between this comparative example and Example 1 is that the preparation method of the iron-manganese dual-site catalyst is different, while the other operations and parameters are exactly the same as in Example 1.

[0031] In this comparative example, during the preparation of the iron-manganese dual-site catalyst, the holding time in the third stage of the pyrolysis process (heating to 1000℃ at 5℃ / min) was increased to 10 min, while other conditions remained unchanged. Experimental results showed that after the iron-manganese dual-site catalyst prepared by this comparative example was used to catalytically oxidize and modify black liquor, it was then used to make chelated calcium fertilizer. Based on the mass of lignin in the black liquor, the calcium fertilizer yield was 23.16%. Further analysis of its calcium content showed that the calcium content was 6.98%. This indicates that the excessively long holding time during the pyrolysis process caused thermal agglomeration of the metal, forming an Fe / Mn dual-cluster catalyst.

[0032] Comparative Example 3 The only difference between this comparative example and Example 1 is that a single glucose carbon-based iron single-atom catalyst is used as the catalyst for black liquor oxidation to modify the black liquor, i.e., without manganese sub-nano clusters.

[0033] In this comparative example, a glucose-based iron single-atom catalyst was prepared using the same methods (1) to (3) as in Example 1, omitting step (4). Experimental results showed that, based on the mass of lignin in the black liquor, the calcium fertilizer yield was 20.47%. Further analysis of the calcium content revealed a calcium content of 8.11%. It is evident that both the calcium fertilizer yield and the calcium chelation rate in the calcium fertilizer are significantly lower than in Example 1 (calcium fertilizer yield 42.35%, calcium content 11.85%). This sufficiently demonstrates that the introduction of manganese sub-nano clusters into the catalyst provided by this invention significantly enhances the catalyst's catalytic performance.

[0034] Comparative Example 4 The only difference between this comparative example and Example 1 is that a single glucose carbon-based manganese sub-nano cluster catalyst is used as the catalyst for black liquor oxidation to modify the black liquor, that is, no complexation treatment of metallic iron and glucose is performed during the preparation process.

[0035] The specific steps for catalyst preparation in this comparative example are as follows: (1) Dissolve 1.5 mmol zinc chloride in 100 mL of water, sonicate for 30 min, stir at 55 °C and add dropwise to 0.1 L of 15 g / L glucose solution, stir at 600 rpm for 3 h to obtain zinc-glucose solution; The operation and reaction conditions of (2) to (4) are exactly the same as those in Example 1.

[0036] The results showed that, based on the mass of lignin in black liquor, the calcium fertilizer yield was 19.88%. Further analysis of the calcium content revealed that it was 6.04%. Compared to Comparative Example 3, the catalytic effect of the glucose carbon-based manganese sub-nano cluster catalyst in this comparative example was worse than that of the single glucose carbon-based iron single-atom catalyst. Only under the catalysis of iron and manganese bimetallic sites was the catalyst most effective for the catalytic oxidation of black liquor. The introduction of single atoms such as iron and manganese can significantly improve the catalytic performance and facilitate the catalytic oxidation reaction of lignin.

[0037] Comparative Example 5 The only difference between this comparative example and Example 1 is that a catalyst prepared by loading Fe single atoms and Mn sub-nano clusters onto conventional carbon materials (activated carbon) (the preparation method is the same as in Example 1, but activated carbon is used instead of glucose-derived carbon-based materials) is used for the black liquor catalytic oxidation reaction, and the reaction conditions are the same as in Example 1.

[0038] The catalyst prepared using the method described in this comparative example was used to catalytically oxidize black liquor, resulting in chelated calcium fertilizer. The calcium fertilizer yield, based on the mass of lignin in the black liquor, was 21.64%. Further analysis of the calcium content revealed a content of 7.66%. This result further demonstrates that the glucose-derived carbon-based material used in this invention, compared to traditional activated carbon-based materials, possesses unique structural advantages that play a crucial role in enhancing catalyst performance.

[0039] Comparative Example 6 The only difference between this comparative example and Example 1 is that the catalyst is prepared differently.

[0040] The iron-manganese dual-site catalyst in this comparative example was formed by mixing the catalysts prepared in Comparative Example 1 and Comparative Example 2 at a mass ratio of 1:1 and mechanically grinding them for 30 min, resulting in a physical mixture labeled as Fe / Mn physical mixture.

[0041] The results showed that, based on the mass of lignin in the black liquor, the calcium fertilizer yield was 28.74%. Further analysis of the calcium content revealed that the calcium content was 7.36%. This result indicates that, compared to the catalyst in Example 1, which was chemically combined with loaded iron single atoms and manganese sub-nano clusters, the performance of the simple physically mixed two-site catalyst was significantly reduced, and it could not effectively improve the catalytic oxidation performance of the black liquor.

[0042] Comparative Example 7 The difference between this comparative example and Example 1 is that manganese acetate was used as the catalyst for the catalytic oxidation reaction of black liquor (the amount used was 50% of the mass of lignin in the black liquor). The reaction was carried out in an ethanol / water mixed solvent (mass ratio 15:1), with an oxygen pressure of 1.0 MPa and a temperature of 180°C for 6 h to oxidize the lignin in the black liquor. Subsequently, chelated calcium fertilizer was prepared according to the method of Example 1. The manganese acetate catalyst used in this comparative example was prepared according to the method disclosed in patent CN108947784B. The experimental results of this comparative example showed that the calcium fertilizer yield was 21.5% based on the mass of lignin in the black liquor. Further analysis of the calcium content showed that the calcium content was 4.8%, and the obtained lignin chelated calcium fertilizer easily agglomerated and precipitated in water. This indicates that under these conditions and with this catalyst dosage, the functional transformation of lignin into a stable water-soluble calcium chelate cannot be achieved.

[0043] Comparative Example 8 The difference between this comparative example and Example 1 is that a lanthanum-based porous catalyst was used as the catalyst for the catalytic oxidation reaction of black liquor. The catalyst reacted with black liquor (pH 13.0) at 100°C and an oxygen pressure of 1.0 MPa for 2 hours to obtain catalytically modified black liquor. Chelated calcium fertilizer was then prepared using the same method as in Example 1. In this comparative example, the preparation of the lanthanum-based porous catalyst was carried out according to the method of patent CN120679540A, specifically as follows: Lanthanum nitrate, nickel nitrate, and citric acid were mixed in a solvent, and polymethyl methacrylate was used as a template. The mixture was calcined to obtain a porous material with a microporous-mesoporous-macroporous structure, i.e., the lanthanum-based porous catalyst. The results showed that the calcium fertilizer yield was 18.6% based on the mass of lignin in the black liquor. Further analysis of the calcium content showed that the calcium content was 6.2%, and the calcium fertilizer had poor dispersibility in water, exhibiting significant precipitation after standing. This indicates that the catalyst failed to significantly improve the solubility and stability of lignin in the aqueous phase, resulting in a significantly lower yield, calcium content, and water solubility of the final chelated calcium fertilizer compared to the method of this invention.

[0044] Comparative Example 9 The difference between this comparative example and Example 1 is that Mn / α-Fe2O3-D is used as the catalyst for lignin oxidation modification in black liquor. The Mn / α-Fe2O3-D catalyst is prepared according to the method in patent CN120920004A, specifically using highly defective α-Fe2O3 as a support and prepared via atomic layer deposition. The preparation method of the lignin-chelated calcium fertilizer is exactly the same as in Example 1.

[0045] Experimental results showed that, based on the mass of lignin in black liquor, the calcium fertilizer yield was 19.4%. Further analysis of the calcium content revealed that the calcium fertilizer contained 7.9% calcium, and the water solubility of the chelated calcium fertilizer product was generally poor, with precipitation occurring after standing. This indicates that the catalyst caused excessive degradation of lignin, resulting in a significantly lower yield, calcium content, and water solubility of the final chelated calcium fertilizer compared to the method of this invention.

[0046] Comparative Example 10 The only difference between this comparative example and Example 1 is that a catalyst (Cu / CA) with copper nanoparticles loaded on a cellulose carbon aerogel as a carrier is used to catalytically oxidize black liquor. The catalyst is prepared according to the method disclosed in patent CN120515413A.

[0047] The catalyst prepared using the method described in this comparative example was used to catalytically oxidize and modify black liquor before being used to produce chelated calcium fertilizer. Based on the mass of lignin in the black liquor, the calcium fertilizer yield was 23.7%, with a calcium content of 5.8%. Furthermore, the solubility and stability of the calcium fertilizer in water were significantly reduced, making it prone to aggregation and precipitation. This catalyst is similar to those in Comparative Examples 8-9. This indicates that the catalyst failed to achieve the targeted regulation of lignin hydrophilicity and chelation site enhancement required for the calcium fertilizer preparation system of this invention.

[0048] Comparative Example 11 The difference from Example 1 is that no catalyst was used for catalytic oxidation of the black liquor. Instead, 30 mL of black liquor with a lignin mass fraction of 10% was directly chelated with 0.6 g of calcium chloride to prepare chelated calcium fertilizer. The calcium fertilizer yield was 13.26% and the calcium content was 3.87%.

[0049] Comparative Example 12 The difference from Example 1 is that no catalyst was added, and the black liquor was directly oxidized. The oxidized black liquor was then chelated with calcium chloride to prepare chelated calcium fertilizer. The calcium fertilizer yield was 16.84%, and the calcium content was 4.62%.

[0050] Comparative Example 13 Unlike Example 1, light brown precipitate I was collected to obtain water-insoluble lignin chelated calcium fertilizer.

[0051] Application Example 1 The fertilizer efficacy verification of the water-soluble lignin chelated calcium fertilizer prepared in Example 1 was carried out as follows: The water-soluble lignin-chelated calcium fertilizer prepared in Example 1 was applied to corn planting. Specifically, corn seeds were germinated in water and grew to the two-leaf-one-heart stage. They were then transferred to a Hoagland nutrient solution for cultivation. At this time, water-soluble lignin-chelated calcium fertilizer was added to the nutrient solution at a concentration of 1 g / L. The hydroponic device was placed in a plant cultivation box, with a cold light lamp as the plant light source. The temperature was 25℃, the relative humidity was 70±5%, and the continuous light exposure was 16 h followed by 8 h of darkness. When the corn seedlings were 15 days old, basic physiological indicators (plant height, root length, chlorophyll) were randomly selected from the hydroponic box and photographed for recording.

[0052] Meanwhile, CK (calcium deficiency treatment) and comparative examples 11 to 13 were used as controls.

[0053] Table 1. Growth status of maize plants in each fertilization group CK Example 1 Comparative Example 11 Comparative Example 12 Comparative Example 13 Plant height (cm) 22±2.0 33±1.3 31.5±1.1 31±1.4 30±2.2 Root length (cm) 12±1.1 20±1.7 18.5±1.4 18±0.9 17.5±1.1 Chlorophyll (SPAD) 50±2.0 62±2.3 60±1.3 59±3.1 58±1.3 Figure 1 The effects of calcium fertilizer in Example 1 and other comparative examples on the growth of maize plants are shown. Figures 2-4 The figures show the plant height, root length, and chlorophyll content of maize plants in each fertilization group. Figures 1-4 As can be seen from the specific data in Table 1, compared with the calcium deficiency treatment group (CK), the corn plant height, root length, chlorophyll content and other parameters of the other groups have increased. Moreover, the overall growth level of the Example 1 group is better than that of the other groups, which further verifies that the chelated calcium fertilizer prepared by catalytic oxidation modification of lignin in this invention has higher biofertilizer efficiency.

[0054] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. All equivalent changes and modifications made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. An iron-manganese dual-site catalyst for lignin oxidation modification, characterized in that, The preparation steps of the iron-manganese dual-site catalyst are as follows: (1) Dissolve iron salt and zinc salt in water at a molar ratio of 1:5~10, mix by ultrasonication, stir at 55~65℃ and add dropwise to glucose solution at a uniform rate to obtain glucose-metal salt solution, wherein the molar ratio of glucose to iron salt is 1:20~30 and the concentration of glucose solution is 12~20 g / L. (2) Add a nitrogen source to the glucose-metal salt solution in (1) and disperse it evenly. Then freeze it at -80°C and vacuum dry it to obtain nitrogen-doped metal glucose precursor. (3) The precursor in (2) was pyrolyzed under nitrogen protection to obtain a glucose carbon-based iron single-atom catalyst; The pyrolysis conditions were as follows: under a nitrogen protective atmosphere, the temperature was increased in three stages: in the first stage, the temperature was increased to 400~600℃ at a rate of 5℃ / min and held for 0.5~1.5 h; in the second stage, the temperature was increased to 750~950℃ at a rate of 5℃ / min and held for 0.5~1.5 h; and in the third stage, the temperature was increased to 1000~1200℃ at a rate of 5℃ / min and held for 1~5 min. (4) The glucose carbon-based iron single-atom catalyst prepared in (3) is impregnated in a manganese salt solution, stirred at 60~95℃ and a reducing agent is added dropwise. After washing and drying, the iron-manganese dual-site catalyst is obtained.

2. The iron-manganese dual-site catalyst as described in claim 1, characterized in that, (1) The iron salt is selected from either ferric chloride or ferric sulfate, and the zinc salt is selected from either zinc chloride or zinc sulfate; (2) The nitrogen source is selected from either melamine or dicyandiamide, and the mass ratio of the nitrogen source to glucose is 5~8:

1.

3. The iron-manganese dual-site catalyst as described in claim 1, characterized in that, (4) The reducing agent is selected from sodium borohydride and formaldehyde, the manganese salt is manganese chloride, and the molar ratio of the manganese salt to the reducing agent is 1:2500~2800.

4. The application of the iron-manganese dual-site catalyst as described in claim 1 in the preparation of water-soluble lignin chelated calcium fertilizer.

5. A method for preparing water-soluble lignin-chelated calcium fertilizer based on the iron-manganese dual-site catalyst according to any one of claims 1 to 3, characterized in that, Under the action of an iron-manganese dual-site catalyst, papermaking black liquor was used as raw material. O2 was introduced into the liquor at pH 13.0 and the reaction was carried out at 100-110℃ for 1-4 h to obtain oxidized black liquor. After cooling to room temperature, the liquor was centrifuged and the supernatant was collected. Calcium chloride was added to the supernatant and stirred at 40-60℃ for 1-2 h. After centrifugation and filtration, supernatant I and precipitate I were obtained. 95% ethanol was added to supernatant I and mixed thoroughly. After centrifugation and filtration, precipitate II and supernatant II were obtained. Precipitate II was then washed with ethanol and freeze-dried to obtain the water-soluble lignin chelated calcium fertilizer.

6. The method as described in claim 5, characterized in that, The lignin content in the papermaking black liquor is 8%~10% by mass, and the mass-volume ratio of calcium chloride to black liquor is 1 g: 0.02~0.1 L.

7. The method as described in claim 5, characterized in that, The mass ratio of the black liquor to the iron-manganese dual-site catalyst is 100~150:

1.

8. The method as described in claim 5, characterized in that, Wash precipitate II with 80% ethanol 2-3 times.

9. A water-soluble lignin-chelated calcium fertilizer, characterized in that, The calcium fertilizer is obtained by chelating calcium salt and oxidized lignin at a mass ratio of 0.1~0.3 g:1 g. The oxidized lignin is prepared by catalytic oxidation of iron-manganese dual-site catalyst and oxygen at 100~110℃ for 1~4 h according to any one of claims 2~3.

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