A catalyst for the preparation of humic acid, its preparation method and application

Abstract

This invention discloses a catalyst for humic acid preparation, its preparation method, and its application, belonging to the field of chemical catalytic humic acid preparation technology. The catalyst for humic acid preparation in this invention exhibits high catalytic activity, effectively transforming macromolecular components in straw into smaller molecular structures, thereby increasing the yield of humic acid prepared from straw. Simultaneously, during the hydrothermal process, the catalyst can catalyze the breakage of some stubborn carbon structures, providing more reaction sites for subsequent oxidation and oxygenation, thus yielding humic acid with a high content of acidic functional groups. This invention overcomes the limitations of existing technologies and realizes high-value-added utilization of straw resources.

Description

Technical Field

[0001] This invention belongs to the field of chemical catalytic preparation of humic acid, specifically relating to a catalyst for humic acid preparation, its preparation method, and its application. Background Technology

[0002] Humic acid, as a natural organic polymer, possesses multiple functions, including improving soil structure, enhancing fertilizer utilization efficiency, and promoting crop growth, and has broad application prospects in green agriculture. Currently, the main extraction method for humic acid utilizes alkaline solutions to extract it from raw materials such as lignite. While this method is simple to operate, it consumes a large amount of chemical reagents and generates alkaline wastewater with high salinity and high color, which is difficult to treat and easily causes secondary pollution.

[0003] Straw is one of the main agricultural wastes, with a huge annual output, but its resource utilization rate remains low. Traditional disposal methods, such as open burning or simple dumping, easily lead to resource waste and environmental pollution. In recent years, the environmentally friendly conversion technology of preparing humic acid from straw using the hydrothermal method has received widespread attention. This method uses water as the reaction medium to decompose components such as lignocellulose in straw under high temperature and high pressure conditions, converting them into humic acid substances. The hydrothermal method has relatively mild reaction conditions, a simple process flow, and good green process characteristics. However, research has found that humic acid prepared by the hydrothermal method alone not only has a limited yield, but also generally has a low content of acidic functional groups such as carboxyl and phenolic hydroxyl groups. These functional groups are the key structural basis for humic acid to perform ion exchange, complex heavy metals, and enhance chemical activity. Insufficient functional group content seriously restricts the application effect of humic acid in fertilizer enhancement, soil improvement, and pollution control, becoming a technical bottleneck in the preparation of high-quality humic acid by the hydrothermal method.

[0004] Based on the above situation, developing a method for hydrothermal conversion of straw that can significantly improve the yield of humic acid and the content of its acidic functional groups has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] In view of the above-mentioned prior art, the present invention provides a catalyst for the preparation of humic acid, its preparation method and application, so as to solve the technical problems of low yield of straw to humic acid and low content of acidic functional groups in the obtained humic acid.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is to provide a method for preparing a catalyst for humic acid preparation, comprising the following steps: S1: Dissolve nickel salt and 2-methylimidazole in methanol to obtain nickel salt solution and 2-methylimidazole solution respectively; then add 2-methylimidazole solution to nickel salt solution under stirring, mix well and let stand at room temperature until the system color turns light green, then centrifuge, collect precipitate, wash, dry and heat to 750~900℃ in inert atmosphere, keep warm and calcine for 1~3h to obtain nitrogen-doped carbon-supported nickel material; S2: Zinc salt, iron salt and nitrogen-doped carbon nickel-supported material are co-dispersed in water, then a mixed alkaline solution is added, mixed well and aged at 60~70℃ for 20~30h, then centrifuged, the precipitate is collected, washed and dried to obtain the catalyst for humic acid preparation; the mixed alkaline solution is a mixture of sodium carbonate solution and sodium hydroxide solution.

[0007] Based on the above technical solution, the present invention can be further improved as follows.

[0008] Furthermore, the nickel salt is Ni(NO3)2·6H2O, and its molar ratio with 2-methylimidazole is 1:5.

[0009] Furthermore, the precipitation washing method in S1 is to rinse three times with methanol; the drying temperature is 60℃ and the drying time is 12h.

[0010] Furthermore, the heating rate in S1 is 1~3℃ / min.

[0011] Furthermore, the zinc salt is Zn(NO3)2·6H2O; the iron salt is Fe(NO3)3·9H2O; and the mass ratio of zinc salt, iron salt and nitrogen-doped carbon-supported nickel material is 0.2~0.3:0.2~0.3:2.

[0012] Furthermore, the concentration of sodium hydroxide in the mixed alkaline solution is 0.35M, and the concentration of sodium carbonate is 0.15M; the amount of mixed alkaline solution added is based on adjusting the pH of the system to 10.

[0013] Furthermore, the drying temperature in S2 is 60°C.

[0014] The present invention also discloses a catalyst for the preparation of humic acid, which is prepared by the above-described preparation process.

[0015] This invention also discloses the application of the above-mentioned catalyst for humic acid preparation in the preparation of humic acid rich in acidic functional groups. The method for preparing humic acid rich in acidic functional groups includes the following steps: (1) Mix crop straw powder with alkaline solution, then add the catalyst for humic acid preparation according to claim 8, and react at 160~200℃ for 18~30h; then cool to room temperature, centrifuge, collect the supernatant to obtain humic acid solution; (2) Preheat the humic acid solution to 38~42℃, then add hydrogen peroxide solution, keep warm and stir for 1~3h, then adjust the pH of the reaction system to 3.0, then centrifuge and collect the precipitate to obtain humic acid rich in acidic functional groups.

[0016] Furthermore, the crop straw powder is wheat straw powder, the alkaline solution is a 0.5wt% potassium hydroxide solution, the ratio of wheat straw powder to potassium hydroxide solution is 1g:20mL, the mass ratio of the catalyst for humic acid preparation to wheat straw powder is 1:100, the concentration of hydrogen peroxide solution is 4%, and its volume ratio to humic acid solution is 1:2.

[0017] The beneficial effects of this invention are: The catalyst for humic acid preparation in this invention exhibits high catalytic activity, effectively transforming macromolecular components in straw into smaller molecular structures, thereby increasing the yield of humic acid prepared from straw. Simultaneously, during the hydrothermal process, the catalyst can catalyze the breakage of some stubborn carbon structures, providing more reaction sites for subsequent oxidation and oxygenation, thus yielding humic acid with a high content of acidic functional groups. This invention overcomes the limitations of existing technologies and achieves high-value-added utilization of straw resources. Attached Figure Description

[0018] Figure 1 The figure shows the effect of different humic acids on the cumulative amount of urea ammonia volatilization. Detailed Implementation

[0019] The specific embodiments of the present invention will be described in detail below with reference to examples.

[0020] Example 1

[0021] A catalyst for the preparation of humic acid is prepared by the following steps: S1: Dissolve 5 mM Ni(NO3)2·6H2O and 25 mM 2-methylimidazole in 100 mL of methanol to obtain Ni(NO3)2 solution and 2-methylimidazole solution, respectively. Then, under continuous stirring, slowly pour the 2-methylimidazole solution into the Ni(NO3)2 solution and continue stirring for 30 min after the addition is complete. Let the mixture stand at room temperature until the system color turns light green, then centrifuge at 5000 rpm for 10 min, collect the precipitate, wash it three times with methanol, and then dry it at 60 °C for 12 h. The obtained solid is heated to 800 °C at a rate of 2 °C / min in N2 atmosphere and calcined for 2 h to obtain nitrogen-doped carbon-supported nickel material, denoted as NiNC.

[0022] S2: 0.297 g of Zn(NO3)2·6H2O, 0.202 g of Fe(NO3)3·9H2O, and 2 g of NiNC were added to 500 mL of deionized water. The mixture was ultrasonically treated in a water bath for 1 h to ensure thorough dispersion. Subsequently, a mixed alkaline solution containing 0.35 M NaOH and 0.15 M Na2CO3 was slowly added dropwise with stirring, and the pH of the system was maintained at 10.0. The mixture was aged at 65 °C for 24 h. After cooling to room temperature, it was centrifuged at 5000 rpm for 10 min, the precipitate was collected, washed three times with deionized water, and dried at 60 °C to constant weight to obtain the catalyst for humic acid preparation, denoted as ZnFe@NiNC.

[0023] Example 2

[0024] A catalyst for the preparation of humic acid is prepared by the following steps: S1: Dissolve 5 mM Ni(NO3)2·6H2O and 25 mM 2-methylimidazole in 100 mL of methanol to obtain Ni(NO3)2 solution and 2-methylimidazole solution, respectively. Then, under continuous stirring, slowly pour the 2-methylimidazole solution into the Ni(NO3)2 solution and continue stirring for 30 min after the addition is complete. Let the mixture stand at room temperature until the system color turns light green, then centrifuge at 5000 rpm for 10 min, collect the precipitate, wash it three times with methanol, and then dry it at 60 °C for 12 h. The obtained solid is heated to 750 °C at a rate of 1 °C / min in N2 atmosphere and calcined for 3 h to obtain nitrogen-doped carbon-supported nickel material, denoted as NiNC.

[0025] S2: 0.2 g of Zn(NO3)2·6H2O, 0.2 g of Fe(NO3)3·9H2O, and 2 g of NiNC were added to 500 mL of deionized water. The mixture was ultrasonically treated in a water bath for 1 h to ensure thorough dispersion. Subsequently, a mixed alkaline solution containing 0.35 M NaOH and 0.15 M Na2CO3 was slowly added dropwise under stirring, and the pH of the system was maintained at 10.0. The mixture was aged at 60 °C for 30 h. After cooling to room temperature, it was centrifuged at 5000 rpm for 10 min, the precipitate was collected, washed three times with deionized water, and dried at 60 °C to constant weight to obtain the catalyst for humic acid preparation, denoted as ZnFe@NiNC.

[0026] Example 3

[0027] A catalyst for the preparation of humic acid is prepared by the following steps: S1: Dissolve 5 mM Ni(NO3)2·6H2O and 25 mM 2-methylimidazole in 100 mL of methanol to obtain Ni(NO3)2 solution and 2-methylimidazole solution, respectively. Then, under continuous stirring, slowly pour the 2-methylimidazole solution into the Ni(NO3)2 solution and continue stirring for 30 min after the addition is complete. Let the mixture stand at room temperature until the system color turns light green, then centrifuge at 5000 rpm for 10 min, collect the precipitate, wash it three times with methanol, and then dry it at 60 °C for 12 h. The obtained solid is heated to 900 °C at a rate of 3 °C / min in N2 atmosphere and calcined for 1 h to obtain nitrogen-doped carbon-supported nickel material, denoted as NiNC.

[0028] S2: 0.3 g of Zn(NO3)2·6H2O, 0.3 g of Fe(NO3)3·9H2O, and 2 g of NiNC were added to 500 mL of deionized water. The mixture was ultrasonically treated in a water bath for 1 h to ensure thorough dispersion. Subsequently, a mixed alkaline solution containing 0.35 M NaOH and 0.15 M Na2CO3 was slowly added dropwise under stirring, and the pH of the system was maintained at 10.0. The mixture was aged at 70 °C for 20 h. After cooling to room temperature, it was centrifuged at 5000 rpm for 10 min, the precipitate was collected, washed three times with deionized water, and dried at 60 °C to constant weight to obtain the catalyst for humic acid preparation, denoted as ZnFe@NiNC.

[0029] Experimental Example The catalysts for humic acid preparation prepared in Examples 1-3 have similar performance. Taking the catalyst for humic acid preparation prepared in Example 1 as an example, the performance of the catalyst will be described in detail.

[0030] The preparation of humic acid rich in acidic functional groups using the catalyst for humic acid preparation prepared in Example 1 specifically includes the following steps: (1) Add 100 mL of 0.5 wt% potassium hydroxide solution to the inner liner of a 250 mL reactor containing 5 g of wheat straw powder, then add 0.05 g of ZnFe@NiNC catalyst for humic acid preparation, then place the reactor in an oven and heat it to 180 °C, keep it at that temperature for 24 h, and then let it cool naturally to room temperature; centrifuge at 5000 rpm for 10 min, separate and collect the supernatant to obtain humic acid solution.

[0031] (2) Take 50 mL of humic acid solution and add it to a 500 mL beaker, and place a clean magnetic stir bar in it; place the beaker on the heating plate of the temperature-controlled magnetic stirrer, turn on the magnetic stirring (1000 rpm) and heating function, preheat the humic acid solution and maintain it at 40℃ (±0.5℃); quickly add 25 mL of 4% w / v H2O2 solution to the preheated humic acid solution, keep it warm and stir for 2 h; after the reaction is completed, immediately add 50 mL of catalase solution (3000 U / L) to remove excess hydrogen peroxide in the solution; then adjust the pH of the reaction system to 3.0, and centrifuge at 5000 rpm for 10 min, collect the precipitate, and obtain humic acid rich in acidic functional groups, denoted as ZnFe-OHA.

[0032] In addition, the preparation of humic acid by direct hydrothermal extraction specifically includes the following steps: 100 mL of 0.5 wt% potassium hydroxide solution was added to the inner liner of a 250 mL reaction vessel containing 5 g of wheat straw powder. The reaction vessel was then placed in an oven and heated to 180 °C, and the reaction was maintained at this temperature for 24 h. Afterward, it was allowed to cool naturally to room temperature. The supernatant was collected by centrifugation at 5000 rpm for 10 min. Subsequently, a 6 mol·L⁻¹ potassium hydroxide solution was used. -1 The pH of the supernatant was adjusted to 1.0 with HCl solution, and centrifuged (5000 rpm for 10 min). The insoluble component obtained was humic acid extracted by direct hydrothermal method, denoted as HA.

[0033] Secondly, modified humic acid is prepared by direct oxidation after hydrothermal treatment, specifically including the following steps: (1) Add 100 mL of 0.5 wt% potassium hydroxide solution to the inner liner of a 250 mL reactor containing 5 g of wheat straw powder. Then place the reactor in an oven and heat it to 180 °C. Keep it warm for 24 h and then let it cool naturally to room temperature. Centrifuge at 5000 rpm for 10 min and separate and collect the supernatant to obtain humic acid solution.

[0034] (2) Take 50 mL of humic acid solution and add it to a 500 mL beaker, and place a clean magnetic stir bar in it; place the beaker on the heating plate of the temperature-controlled magnetic stirrer, turn on the magnetic stirring (1000 rpm) and heating function, preheat the humic acid solution and maintain it at 40℃ (±0.5℃); quickly add 25 mL of 4% w / v H2O2 solution to the preheated humic acid solution, keep it warm and stir for 2 h; after the reaction is completed, immediately add 50 mL of catalase solution (3000 U / L) to remove excess hydrogen peroxide in the solution; then adjust the pH of the reaction system to 3.0, and centrifuge at 5000 rpm for 10 min, collect the precipitate, and obtain humic acid rich in acidic functional groups, which is denoted as OHA.

[0035] Results Analysis I. Structural characteristics of humic acid rich in acidic functional groups from straw Table 1 lists the yields and elemental compositions of HA, OHA, and ZnFe-OHA. As shown in Table 1, the yield of humic acid (HA) obtained through conventional hydrothermal extraction is 12.30%. Compared to HA, the yield of OHA decreased by 15.28%, while the yield of ZnFe-OHA increased by 73.66%. This indicates that without catalyst addition, direct oxidation after conventional hydrothermal extraction leads to excessive oxidation of some carbon structures in humic acid, generating CO2 gas. However, the combined use of hydrothermal catalysis and H2O2 oxidation can improve the yield of humic acid extracted from straw, suggesting that adding the catalyst ZnFe@NiNC during the hydrothermal process helps promote the conversion of straw into humic acid. In terms of elemental composition, ZnFe-OHA has a lower C content and a higher O content than HA and OHA. This is because ZnFe@NiNC catalyzes the breakage of some stubborn carbon structures during the hydrothermal process, providing more reaction sites for subsequent oxidation and oxygenation.

[0036] Table 1. Yield and elemental composition of humic acid extracted from straw using different processes. Table 2 lists the acidic functional group content and distribution results of HA, OHA, and ZnFe-OHA. As shown in Table 2, compared with HA, OHA showed an increase in total acidity and carboxyl content of 6.34% and 14.29%, respectively, while the phenolic hydroxyl content decreased slightly. In contrast, ZnFe-OHA showed an increase in total acidity, carboxyl content, and phenolic hydroxyl content of 22.68%, 41.33%, and 5.61%, respectively. These results indicate that the addition of ZnFe@NiNC during the hydrothermal process promotes the conversion of straw into humic acid and forms more carboxyl precursor structures, which is beneficial for carboxyl group formation during oxidation.

[0037] Table 2. Content and distribution of acidic functional groups in humic acid extracted from straw using different processes. II. The Influence of Humic Acid Rich in Acidic Functional Groups from Straw on Ammonia Volatilization Three types of humic acid (HA, OHA, and ZnFe-OHA) were added to molten urea at 0.5% (mass of humic acid divided by the total mass of humic acid and urea) at 130℃. The mixture was stirred for 30 seconds, cooled, pulverized, and passed through a 100-mesh sieve to obtain three types of humic acid urea: HAU (corresponding to HA), OHAU (corresponding to OHA), and ZnFe-OHAU (corresponding to ZnFe-OHA). Urea (U) obtained by melting without the addition of humic acid served as a control.

[0038] 0.068 g of U, HAU, OHAU, and ZnFe-OHAU were each mixed with 100 g of soil and added to culture bottles. A control group (CK) without added fertilizer was used. Soil moisture content was adjusted to 20%, and the bottles were cultured in a 25°C climate chamber in the dark. During cultivation, soil moisture content was maintained at 20% by weighing. Each treatment was repeated three times. NH3 was absorbed using a sponge method on days 1, 2, 3, 4, 5, 6, 7, 14, 21, and 28 after cultivation.

[0039] The effects of different humic acids on the cumulative volatilization of urea ammonia are as follows: Figure 1 As shown. From Figure 1 As can be seen, compared with U, the three humic acid urea products can reduce the cumulative ammonia volatilization by 4.16%~15.20%, and compared with HAU, OHAU reduces the cumulative ammonia volatilization by 4.79%, indicating that increasing the content of acidic functional groups in humic acid through oxidation can reduce ammonia volatilization. Compared with HAU and OHAU, ZnFe-OHAU further reduces the cumulative ammonia volatilization by 7.07%~11.53%, indicating that the high-yield humic acid rich in acidic functional groups obtained by the combined hydrothermal catalysis and H2O2 oxidation has a stronger potential to reduce ammonia volatilization losses.

[0040] Although specific embodiments of the present invention have been described in detail with reference to examples, this should not be construed as limiting the scope of protection of this patent. Various modifications and variations that can be made by those skilled in the art without inventive effort within the scope described in the claims still fall within the scope of protection of this patent.

Claims

1. A method for preparing a catalyst for humic acid preparation, characterized in that, Includes the following steps: S1: Dissolve nickel salt and 2-methylimidazole in methanol to obtain nickel salt solution and 2-methylimidazole solution respectively; then add 2-methylimidazole solution to nickel salt solution under stirring, mix well and let stand at room temperature until the system color turns light green, then centrifuge, collect precipitate, wash, dry and heat to 750~900℃ in inert atmosphere, keep warm and calcine for 1~3h to obtain nitrogen-doped carbon-supported nickel material; S2: Zinc salt, iron salt and nitrogen-doped carbon nickel-supported material are co-dispersed in water, then mixed with alkaline solution, and aged at 60~70℃ for 20~30h. After centrifugation, the precipitate is collected, washed and dried to obtain the catalyst for humic acid preparation. The mixed alkaline solution is composed of a sodium carbonate solution and a sodium hydroxide solution.

2. The preparation method according to claim 1, characterized in that: The nickel salt is Ni(NO3)2·6H2O, and its molar ratio with 2-methylimidazole is 1:

5.

3. The preparation method according to claim 1, characterized in that: The precipitation in S1 was washed three times with methanol; the drying temperature was 60℃ and the drying time was 12h.

4. The preparation method according to claim 1, characterized in that: The heating rate in S1 is 1~3℃ / min.

5. The preparation method according to claim 1, characterized in that: The zinc salt is Zn(NO3)2·6H2O; the iron salt is Fe(NO3)3·9H2O; the mass ratio of zinc salt, iron salt and nitrogen-doped carbon-supported nickel material is 0.2~0.3:0.2~0.3:

2.

6. The preparation method according to claim 1, characterized in that: The concentration of sodium hydroxide in the mixed alkaline solution is 0.35M, and the concentration of sodium carbonate is 0.15M; the amount of mixed alkaline solution added is based on adjusting the pH of the system to 10.

7. The preparation method according to claim 1, characterized in that: The drying temperature in S2 is 60℃.

8. A catalyst for the preparation of humic acid, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the catalyst for humic acid preparation according to claim 8 in the preparation of humic acid rich in acidic functional groups, characterized in that, Includes the following steps: (1) Mix crop straw powder with alkaline solution, then add the catalyst for humic acid preparation as described in claim 8, and react at 160~200℃ for 18~30h; then cool to room temperature, centrifuge, collect the supernatant to obtain humic acid solution; (2) Preheat the humic acid solution to 38~42℃, then add hydrogen peroxide solution, keep warm and stir for 1~3h, then adjust the pH of the reaction system to 3.0, then centrifuge and collect the precipitate to obtain humic acid rich in acidic functional groups.

10. The application according to claim 9, characterized in that: The crop straw powder is wheat straw powder, the alkaline solution is a 0.5wt% potassium hydroxide solution, the ratio of wheat straw powder to potassium hydroxide solution is 1g:20mL, the mass ratio of the catalyst for humic acid preparation to wheat straw powder is 1:100, the concentration of hydrogen peroxide solution is 4%, and its volume ratio to humic acid solution is 1:2.

Images