A carbon nitride / cobalt oxide composite material, a preparation method thereof and application thereof in boron adsorption

Abstract

This invention belongs to the field of materials chemistry and discloses a carbon nitride / cobalt oxide composite material, its preparation method, and its application in boron adsorption. The carbon nitride / cobalt oxide composite material comprises a three-dimensional porous framework composed of cobalt oxide nanoparticles and two-dimensional sheet-like graphitic carbon nitride material. The two-dimensional sheet-like graphitic carbon nitride material is dispersed and physically attached to the outer surface and / or internal channels of the three-dimensional porous framework, forming a physical contact interface. The carbon nitride / cobalt oxide composite material of this invention exhibits excellent boron adsorption capacity and selectivity.

Description

Technical Field

[0001] This invention relates to the field of materials chemistry, and in particular to a carbon nitride / cobalt oxide composite material, its preparation method, and its application in boron adsorption. Background Technology

[0002] Boric acid, with a pKa of 9.2 at room temperature, is a relatively weak Lewis acid. However, its low dissociation constant makes it electrically neutral, hindering its separation from brine. To address this issue, researchers typically employ adsorption methods to separate boron, aiming for a dual improvement in adsorption capacity and selectivity. A common design approach for boron adsorbents utilizes chelation mechanisms to bind boron acid with polyols, aiming to obtain boron adsorbents modified with functional groups rich in vicinal diols. This allows for specific chelation between the adsorbent and boron acid, ultimately achieving effective adsorption. However, NMDG-based materials still suffer from a theoretically much smaller number of grafted -OH groups than the actual number, hindering effective chelation between -OH groups and borate anions, resulting in suboptimal adsorption capacity and poor selectivity. Metal oxides, due to the presence of hydroxyl groups on their hydrated surfaces, can form borate esters. These surface hydroxyl groups are highly sensitive to proton concentration and are therefore affected by changes in the pH of the boron-containing solution. Surface hydroxyl groups are protonated at low pH and deprotonated at high pH.

[0003] Compared to alkaline earth metal oxides such as MgO, transition metal oxides (such as Co3O4) have unfilled d orbitals at their metal centers, thus exhibiting strong Lewis acidity and making them more prone to adsorbing boric acid through coordination reactions. Previous studies have shown that Co3O4 readily dissociates in aqueous solution to form -OH groups, creating an octahedral Co group with a "semi-hydrated layer." 2+ This hydration structure exposes highly unsaturated surface cations, which is beneficial for providing abundant active sites, thereby accelerating the capture of boron and achieving directional adsorption, resulting in a highly selective boron adsorbent. Carbon materials, due to their high specific surface area and porosity, exhibit electrostatic attraction between positively charged carbon atoms and borate anions, promoting effective boron adsorption. However, carbon materials have relatively few active groups on their surface, limiting their boron adsorption. Furthermore, monomeric oxides or carbon materials, due to their fixed surface electronic structures, face the problem of insufficient exposure of active sites. Therefore, research shows that defect engineering combined with the construction of heterojunction interfaces can effectively regulate the electronic structure. According to Lewis acid-base theory, the abundant lone pairs of electrons in boric acid molecules can act as electron donors to react chemically with metal sites. Therefore, if ultrathin carbon nitride with N vacancies is artificially introduced and forms a heterojunction with Co3O4, it is hoped that the interfacial electronic interactions will make the Co sites more electron-deficient, thereby enhancing its Lewis acidity and further facilitating the capture of oxygen lone pairs in boric acid molecules to form chemical bonds. This achieves a dual improvement in the selectivity and adsorption capacity for boron species in salt lake brines.

[0004] To enhance activity, existing technologies typically employ complex processes such as in-situ growth, hydrothermal methods, or chemical vapor deposition to "grow" cobalt oxide on the surface of another material (such as carbon materials) in order to form a tight chemically bonded interface. While these "strongly coupled" methods can improve performance to some extent, their processes are demanding, lengthy, and costly, and the reaction conditions are difficult to control, severely hindering the large-scale production and practical application of the materials. Summary of the Invention

[0005] The purpose of this invention is to address the technical deficiencies in the prior art by providing a carbon nitride / cobalt oxide composite material, its preparation method, and its application in boron adsorption.

[0006] The technical solution adopted to achieve the purpose of this invention is: A method for preparing a carbon nitride / cobalt oxide composite material includes the following steps: Step 1: Disperse the cobalt source and the organic ligand 2-methylimidazole in methanol, mix them evenly, age them, centrifuge to separate the solid product, wash and dry to obtain the ZIF-67 precursor; Step 2: The ZIF-67 precursor obtained in Step 1 is immersed in an ethanol solution in which a saturated cobalt source is dispersed. After ultrasonic dispersion and aging, the solid product is separated by centrifugation, washed and dried to obtain Co@ZIF-67. The obtained Co@ZIF-67 is calcined to obtain cobalt oxide material Co3O4. Step 3: Dicyandiamide is calcined in a tube furnace in two stages and cooled to room temperature to obtain bulk carbon nitride-C3N4. Step 4: The bulk g-C3N4 obtained in Step 3 is further calcined in a muffle furnace to obtain carbon nitride g-C3N4 with N vacancies; Step 5: The carbon nitride g-C3N4 with N vacancies from Step 4 and the cobalt oxide material Co3O4 from Step 2 are mechanically stirred in ethanol, centrifuged and dried to obtain the carbon nitride / cobalt oxide composite material g-C3N4@Co3O4.

[0007] In the above technical solution, in step 1, the cobalt source is cobalt nitrate hexahydrate, cobalt chloride, or cobalt sulfate. When the cobalt source is cobalt nitrate hexahydrate, the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is (1.1-1.5):(1.0-1.6). When the cobalt source is cobalt chloride, the mass ratio of cobalt chloride to 2-methylimidazole is (0.25-0.35):(1.2-1.7). When the cobalt source is cobalt sulfate, the mass ratio of cobalt sulfate to 2-methylimidazole is (0.20-0.32):(1.5-1.8).

[0008] In the above technical solution, in step 1, the aging temperature is room temperature and the aging time is 6-24 hours.

[0009] In the above technical solution, in step 1, the drying temperature is 40-80℃ and the drying time is 6-12h.

[0010] In the above technical solution, in step 2, the cobalt source is cobalt nitrate hexahydrate, and the mass ratio of ZIF-67 precursor to cobalt nitrate hexahydrate is (1.0-3.0):(2.0-6.0).

[0011] In the above technical solution, in step 2, the aging temperature is room temperature and the aging time is 6-12 hours.

[0012] In the above technical solution, in step 2, the drying temperature is 40-80℃ and the drying time is 6-12h.

[0013] In the above technical solution, in step 2, the calcination temperature is 320-350℃ and the calcination time is 2-3.5h.

[0014] In the above technical solution, in step 3, the first stage calcination temperature is 300-350℃, the second stage calcination temperature is 600-650℃, the first stage calcination time is 2-2.5h, and the second stage calcination time is 2-2.5h.

[0015] In the above technical solution, in step 4, the calcination temperature is 450-550℃ and the calcination time is 4-6h.

[0016] In the above technical solution, in step 5, the mass ratio of carbon nitride g-C3N4 with N vacancies to cobalt oxide material Co3O4 is (1.2-1.8):(4.0-20.0).

[0017] Another aspect of the present invention includes a carbon nitride / cobalt oxide composite material obtained by the preparation method, wherein the carbon nitride / cobalt oxide composite material comprises a three-dimensional porous main framework composed of cobalt oxide nanoparticles and a two-dimensional sheet-like graphitic carbon nitride material, wherein the two-dimensional sheet-like graphitic carbon nitride material is dispersed and physically attached to the outer surface and / or internal channels of the three-dimensional porous main framework to form a physical contact interface.

[0018] Another aspect of the present invention includes the application of the carbon nitride / cobalt oxide composite material as an adsorbent for the adsorption and separation of boric acid.

[0019] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention utilizes a three-dimensional porous cobalt oxide as the main body and two-dimensional graphitic carbon nitride as a small amount of auxiliary agent to form a composite adsorbent through physical compounding. Its core structural feature is that two-dimensional sheet-like materials are dispersed and physically attached to the surface and pores of a three-dimensional framework. The preparation method is simple (a two-step process): first, a porous cobalt oxide main body is obtained by treating the MOF precursor; second, the main body is compounded with a small amount of g-C3N4 through mechanical mixing. The core of this method lies in eliminating all complex chemical bonding processes. It possesses both low cost and large-scale production potential. This invention creatively employs a mechanical mixing method to achieve the preparation of high-performance composite materials. This method eliminates the complex in-situ growth, hydrothermal, or solvothermal processes in traditional composite material preparation, and has significant advantages such as short process flow, no waste discharge, low energy consumption, and universal equipment, clearing cost and process barriers for the industrial application of this high-performance adsorbent.

[0020] 2. This invention aims to fundamentally enhance the Lewis acidity of cobalt oxide active sites through electron injection. We have discovered that two-dimensional graphitic carbon nitride (g-C3N4) can serve as an excellent electron donor. By introducing it into the cobalt oxide system, a directional transfer of electrons from g-C3N4 to the Co3O4 host can be induced. This electron injection reduces the electron cloud density around the Co sites, thereby greatly enhancing its ability to capture borate ions as a Lewis acid. This fundamentally solves the problem of insufficient selectivity and achieves a non-obvious synergistic effect of "1+1>2".

[0021] 3. Traditionally, to achieve synergistic effects such as electron transfer, it is necessary to construct robust chemical bonds or tight heterojunctions at the lattice level. This invention overturns the traditional understanding that performance improvement requires "strong coupling," aiming to demonstrate and utilize the highly efficient synergistic effects in "weakly coupled" systems. Even through simple mechanical-physical mixing, as long as the band structures of the two materials match, significant and effective electron transfer can still occur at their physical interface. Therefore, this invention provides an extremely simple and low-cost preparation method that allows for precise control of the activity of the host material simply by mixing two powder materials. It successfully transforms the complex problem of electronic structure control into a simple, scalable physical mixing process problem, opening up a new shortcut for the industrialization of high-performance adsorbent materials.

[0022] 4. A significant leap forward in adsorption performance, solving application challenges in complex water bodies. The composite material prepared by this invention exhibits excellent boron adsorption capacity and selectivity. In real salt lake brines with extremely complex compositions (such as Laguocuo and Qarhan Salt Lake), its saturated adsorption capacity reaches as high as 81 mg / g and 79 mg / g, respectively. This performance not only far exceeds that of single-component cobalt oxide or carbon nitride materials, but also surpasses most boron adsorbents reported in existing literature, proving that this invention effectively solves the technical problem of the rapid performance degradation of traditional materials in high salinity and multi-ion competition environments. Attached Figure Description

[0023] Figure 1 (a) XRD pattern, (b) FTIR spectrum, (d) electron paramagnetic resonance (EPR) spectrum, and (c), (e) and (f) XPS spectra of CN and composite adsorbent before and after adsorption for g-C3N4(CN), Co3O4 and composite adsorbents with different CN introduction amounts. Figure 2 (a) Zeta potential, (b) adsorption capacity plot, (c) Gibbs free energy step plot of the composite adsorbent for adsorbing boric acid in the pH range of 2-12, (d) and (e) differential charge plot, and (f) Bader Charge plot. Figure 3 The diagram shows the selectivity of the composite adsorbent (a), the adsorption capacity of the actual salt lake brine (b), and the cycle stability (c). Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0025] Example 1 A method for preparing a carbon nitride / cobalt oxide heterojunction adsorbent material includes the following steps: Step 1: 1.092 g of 2-methylimidazole was used as the organic ligand, and 1.232 g of cobalt nitrate hexahydrate was used as the cobalt source. These solutions were dispersed separately in 15 ml of methanol solution, denoted as solutions A and B. The solutions were ultrasonically dispersed and stirred at room temperature for 30 min. Solution A, after complete dispersion, was added to solution B and stirred for 30 min, then aged for 12 h. Finally, the samples were centrifuged and washed with methanol at least three times, then dried at 50 °C for 6 h to obtain the ZIF-67 precursor powder.

[0026] Step 2: Disperse 1g of ZIF-67 precursor and 2g of Co3(NO3)2·6H2O in 80mL of ethanol solution and stir for 1h. After aging for 10h, collect by centrifugation and vacuum dry at 80℃ for 6h. Take 80mg of the obtained sample and place it in a covered boat, transfer it to a muffle furnace, and maintain it at 320℃ for 2h. After the muffle furnace cools to room temperature, a black powder is obtained, which is the Co3O4 material.

[0027] Step 3: Place 4g of dicyandiamide in a covered, semi-enclosed crucible. Transfer the crucible to a tube furnace, raise the temperature to 330°C and maintain it for 2 hours, then raise it to 550°C and maintain it for 2.5 hours. After the thermal polycondensation reaction is complete, allow the tube furnace to cool naturally to room temperature, and collect the resulting powder, which is the bulk-g-C3N4 material.

[0028] Step 4: Weigh an appropriate amount of bulk-g-C3N4 and place it in a covered boat. Keep it in a muffle furnace at 500℃ for 5 hours. After cooling to room temperature, a light yellow powder is obtained, which is denoted as g-C3N4.

[0029] Step 5: Using mechanical stirring, composite adsorbents with different mass ratios are obtained. g-C3N4(CN) and Co3O4 with a mass ratio of 1:5 are dispersed in 20 ml of ethanol solution. The solution is stirred for 60 min under the heating condition of a water bath at 80 °C. The stirred solution is then collected and dried in an oven at 50 °C for 8 h to obtain the composite adsorbent.

[0030] The composite adsorbent obtained in Example 1 was characterized, such as... Figure 1 The diffraction peaks of Co3O4 shown in (a) are (111), (220), (311), (400), (422), (511), and (440) crystal planes, which perfectly match the standard card of Co3O4 (JCPDS NO.43-1003). The diffraction peaks at 2θ = 12.87° and 27.7° correspond to the (100) and (002) crystal planes of CN, respectively. By magnifying the diffraction peaks located at 2θ = 12.87°-27.71°, it can be found that the composite material exhibits characteristic peaks belonging to CN, indicating that CN was successfully introduced into C3O4 and the composite material was successfully prepared. Figure 1 The FT-IR spectrum in (b) shows that the composite adsorbent exhibits characteristic peaks attributed to CN and Co3O4, respectively. The above spectral results preliminarily prove the successful binding of CN and Co3O4.

[0031] Example 2 A method for preparing a carbon nitride / cobalt oxide heterojunction adsorbent material includes the following steps: Step 1: Take 1.15g of 2-methylimidazole as the organic ligand and 1.25g of cobalt nitrate hexahydrate as the cobalt source, and disperse them separately in 15ml of methanol solution, denoted as solutions A and B. Sonicate the solutions until homogeneous and stir at room temperature for 30min. Add the completely dispersed solution A to solution B, stir for 30min, and age for 18h. Finally, centrifuge the sample and wash it three times with methanol, then dry it at 50℃ for 10h to obtain the powder, which is the ZIF-67 precursor.

[0032] Step 2: 1.5g of ZIF-67 precursor and 3g of Co3(NO3)2·6H2O were dispersed in 80mL of ethanol solution and stirred for 1h. After aging for 12h, the mixture was collected by centrifugation and vacuum dried at 80℃ for 4h. 80mg of the obtained sample was placed in a covered boat, transferred to a muffle furnace, and heated to 330℃ for 2 hours. After the muffle furnace cooled to room temperature, a black powder was obtained, which is the Co3O4 material.

[0033] Step 3: Place 2g of dicyandiamide in a covered, semi-enclosed crucible. Transfer the crucible to a tube furnace, raise the temperature to 320°C and maintain it for 2.5 hours, then raise it to 580°C and maintain it for 2 hours. After the thermal polycondensation reaction is complete, allow the tube furnace to cool naturally to room temperature, and collect the resulting powder, which is the bulk-g-C3N4 material.

[0034] Step 4: Weigh an appropriate amount of bulk-g-C3N4 and place it in a covered boat. Keep it in a muffle furnace at 520℃ for 4 hours. After cooling to room temperature, a light yellow powder is obtained, which is denoted as g-C3N4.

[0035] Step 5: Using mechanical stirring, composite adsorbents with different mass ratios are obtained. g-C3N4(CN) and Co3O4 with a mass ratio of 1:4 are dispersed in 20 ml of ethanol solution. The solution is stirred for 30 min under the heating condition of a water bath at 80 °C. The stirred solution is then collected and dried in an oven at 60 °C for 10 h to obtain the composite adsorbent, denoted as COCN-5.

[0036] The composite adsorbent obtained in Example 2 was characterized, such as... Figure 1In (c), (e), and (f), a significant blue shift in N 1s was observed during the two processes from the construction of the composite material to adsorption, indicating a change in the chemical environment. The integral ratio of the CN=C to C-(N)3 peaks in the CN material was 1.15, which decreased to 0.68 after the composite adsorbent was constructed, indicating that N vacancies were constructed and originated from CN=C sites. The high-resolution energy dispersive spectroscopy (EDS) spectra of COCN-5 before and after adsorption showed a significant blue shift in O 1s after adsorption, and a new characteristic peak belonging to Co-OB appeared at 530.02 eV. Furthermore, the quantitative integral of Co-OH decreased from 60.7% to 29.5%, indicating that the adsorption process led to a change in the chemical environment and consumed Co-OH. The typical B 1s peak at 191.86 eV directly proved that boric acid was successfully adsorbed on the COCN-5 adsorbent. Figure 1 The middle (d) electron paramagnetic resonance (EPR) spectrum further confirmed that the typical peak at g=2.005 is the signal peak of the N vacancy, confirming that the N vacancy has been successfully constructed.

[0037] Example 3 A method for preparing a carbon nitride / cobalt oxide heterojunction adsorbent material includes the following steps: Step 1: Take 1.23 g of 2-methylimidazole as the organic ligand and 1.48 g of cobalt nitrate hexahydrate as the cobalt source, and disperse them separately in 15 ml of methanol solution, denoted as solutions A and B. Sonicate the solutions until homogeneous and stir at room temperature for 30 min. Add the completely dispersed solution A to solution B, stir for 30 min, and age for 20 h. Finally, centrifuge the sample and wash it with methanol at least three times, then dry it at 60℃ for 12 h to obtain the powder, which is the ZIF-67 precursor.

[0038] Step 2: 1.8 g of ZIF-67 precursor and 3.6 g of Co3(NO3)2·6H2O were dispersed in 80 mL of ethanol solution and stirred for 1 h. After aging for 11 h, the mixture was collected by centrifugation and vacuum dried at 80 °C for 5 h. 80 mg of the obtained sample was placed in a covered boat, transferred to a muffle furnace, and heated to 340 °C for 2.2 h. After the muffle furnace cooled to room temperature, a black powder was obtained, which is the Co3O4 material.

[0039] Step 3: Place 3g of dicyandiamide in a covered, semi-enclosed crucible. Transfer the crucible to a tube furnace, raise the temperature to 340°C and maintain it for 2.2 hours, then raise it to 560°C and maintain it for 2.5 hours. After the thermal polycondensation reaction is complete, allow the tube furnace to cool naturally to room temperature, and collect the resulting powder, which is the bulk-g-C3N4 material.

[0040] Step 4: Weigh an appropriate amount of bulk-g-C3N4 and place it in a covered boat. Keep it in a muffle furnace at 540℃ for 5 hours. After cooling to room temperature, a light yellow powder is obtained, which is denoted as g-C3N4.

[0041] Step 5: Using mechanical stirring, composite adsorbents with different mass ratios are obtained. g-C3N4(CN) and Co3O4 with a mass ratio of 1:10 are dispersed in 20 ml of ethanol solution. The solution is stirred for 45 min under the heating condition of a water bath at 80 °C. The stirred solution is then collected and dried in an oven at 50 °C for 8 h to obtain the composite adsorbent.

[0042] The composite adsorbent obtained in Example 3 was characterized, such as... Figure 2 The composite adsorbents shown in (a) and (b) have an isoelectric point (pHPZC) of approximately 7.5. The material exhibits a negative charge and the highest adsorption capacity at pH 8, where H3BO3 molecules dominate the solution. Therefore, the excellent adsorption capacity is attributed to ligand exchange-dominated chemical complexation. Figure 2 Figure (c) shows that in the absence of defects, the adsorption free energy is as high as +1.14 eV, indicating that the reaction cannot proceed spontaneously. When N defects are introduced, the adsorption Gibbs free energy (ΔG) drops to -4.28 eV. This indicates that the chemical chelation process has a very strong thermodynamic driving force, and this coordination process can form a stable inner spherical surface complex (Co-OB).

[0043] Example 4 A method for preparing a carbon nitride / cobalt oxide heterojunction adsorbent material includes the following steps: Step 1: Take 1.52 g of 2-methylimidazole as the organic ligand and 1.71 g of cobalt nitrate hexahydrate as the cobalt source, and disperse them separately in 15 ml of methanol solution, denoted as solutions A and B. Sonicate the solutions until homogeneous and stir at room temperature for 30 min. Add the completely dispersed solution A to solution B, stir for 30 min, and age for 22 h. Finally, centrifuge the sample and wash it three times or more with methanol, then dry it at 60℃ for 9 h to obtain the powder, which is the ZIF-67 precursor.

[0044] Step 2: Disperse 2.6g of ZIF-67 precursor and 5.2g of Co3(NO3)2·6H2O in 80mL of ethanol solution and stir for 1h. After aging for 9h, collect by centrifugation and vacuum dry at 70℃ for 10h. Take 80mg of the obtained sample and place it in a covered boat, transfer it to a muffle furnace, and maintain it at 350℃ for 2.5h. After the muffle furnace cools to room temperature, a black powder is obtained, which is the Co3O4 material.

[0045] Step 3: Place 3g of dicyandiamide in a covered, semi-enclosed crucible. Transfer the crucible to a tube furnace, raise the temperature to 350°C and maintain it for 2.2 hours, then raise it to 550°C and maintain it for 2.5 hours. After the thermal polycondensation reaction is complete, allow the tube furnace to cool naturally to room temperature, and collect the resulting powder, which is the bulk-g-C3N4 material.

[0046] Step 4: Weigh an appropriate amount of bulk-g-C3N4 and place it in a covered boat. Keep it in a muffle furnace at 580℃ for 4.5 hours. After cooling to room temperature, a light yellow powder is obtained, which is denoted as g-C3N4.

[0047] Step 5: Using mechanical stirring, composite adsorbents with different mass ratios are obtained. g-C3N4(CN) and Co3O4 with a mass ratio of 1:10 are dispersed in 20 ml of ethanol solution. The solution is stirred for 70 min under the heating condition of a water bath at 80 °C. The stirred solution is then collected and dried in an oven at 70 °C for 12 h to obtain the composite adsorbent.

[0048] The composite adsorbent obtained in Example 4 was characterized, such as... Figure 2 As shown in (d) and (e), in the defect-free system, charge transfer between Co and borate is not significant, and the electron overlap region at the interface is small, corresponding to weak physical interactions. Conversely, in the N-defect system, we observed significant charge rearrangement. A distinct blue (electron loss) and yellow (electron gain) overlapping region appeared between the Co atom and the O atom of the borate ion. This strong electron accumulation demonstrates strong orbital hybridization between the Co -3d orbitals and the O -2p orbitals, forming a stable Co-OB bond. Figure 2 As shown in (f), the charge of the defect-free active Co site is only 2.93|e|. However, after introducing N vacancies, the corresponding Co site charge increases to 2.99-3.00|e|. This increase in positive charge density significantly enhances the Lewis acidity of the Co site. As a stronger Lewis acid, Co at the defect site can more strongly attract electron-rich boron species.

[0049] Example 5 A method for preparing a carbon nitride / cobalt oxide heterojunction adsorbent material includes the following steps: Step 1: Take 1.42 g of 2-methylimidazole as the organic ligand and 1.68 g of cobalt nitrate hexahydrate as the cobalt source, and disperse them separately in 15 ml of methanol solution, denoted as solutions A and B. Sonicate the solutions until homogeneous and stir at room temperature for 30 min. Add the completely dispersed solution A to solution B, stir for 30 min, and age for 24 h. Finally, centrifuge the samples and wash them with methanol at least three times, then dry them at 60℃ for 9 h to obtain the powder, which is the ZIF-67 precursor.

[0050] Step 2: 2.8 g of ZIF-67 precursor and 5.6 g of Co3(NO3)2·6H2O were dispersed in 80 mL of ethanol solution and stirred for 1 h. After aging for 10 h, the mixture was collected by centrifugation and vacuum dried at 50 °C for 12 h. 80 mg of the obtained sample was placed in a covered boat, transferred to a muffle furnace, and heated to 350 °C for 3 h. After the muffle furnace cooled to room temperature, a black powder was obtained, which is the Co3O4 material.

[0051] Step 3: Place 3g of dicyandiamide in a covered, semi-enclosed crucible. Transfer the crucible to a tube furnace, raise the temperature to 350°C and maintain it for 3 hours, then raise it to 550°C and maintain it for 2 hours. After the thermal polycondensation reaction is complete, allow the tube furnace to cool naturally to room temperature, and collect the resulting powder, which is the bulk-g-C3N4 material.

[0052] Step 4: Weigh an appropriate amount of bulk-g-C3N4 and place it in a covered boat. Keep it in a muffle furnace at 600℃ for 4 hours. After cooling to room temperature, a light yellow powder is obtained, which is denoted as g-C3N4.

[0053] Step 5: Using mechanical stirring, composite adsorbents with different mass ratios are obtained. g-C3N4(CN) and Co3O4 with a mass ratio of 1:2 are dispersed in 20 ml of ethanol solution. The solution is stirred for 70 min under the heating condition of a water bath at 80 °C. The stirred solution is then collected and dried in an oven at 50 °C for 12 h to obtain the composite adsorbent, denoted as COCN-5.

[0054] The composite adsorbent obtained in Example 5 was characterized, such as... Figure 3 (a) indicates NO3 - SO4 2- K + It has almost no effect on the adsorption of borate ions, Ca 2+ Mg 2+ and Cl - The impact is relatively low, and the adsorption capacity remains above 90%. Figure 3 The boron adsorption capacities of the two salt lakes in (b) were 81 and 79 mg g, respectively. -1 This confirms that COCN-5 maintains a high boron adsorption capacity in actual samples. Figure 3 In (c), the cycling experiment results showed that COCN-5 has high cycling stability, which is due to the complete desorption rate, resulting in a decrease in cycling adsorption capacity of 1.5%, 2%, 5% and 14%, respectively.

[0055] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon nitride / cobalt oxide composite material, characterized in that, Includes the following steps: Step 1: Disperse the cobalt source and the organic ligand 2-methylimidazole in methanol, mix them evenly, age them, centrifuge to separate the solid product, wash and dry to obtain the ZIF-67 precursor; Step 2: The ZIF-67 precursor obtained in Step 1 is immersed in an ethanol solution in which a saturated cobalt source is dispersed. After ultrasonic dispersion and aging, the solid product is separated by centrifugation, washed and dried to obtain Co@ZIF-67. The obtained Co@ZIF-67 is calcined to obtain cobalt oxide material Co3O4. Step 3: Dicyandiamide is calcined in a tube furnace in two stages and cooled to room temperature to obtain bulk carbon nitride-C3N4. Step 4: The bulk g-C3N4 obtained in Step 3 is further calcined in a muffle furnace to obtain carbon nitride g-C3N4 with N vacancies; Step 5: The carbon nitride g-C3N4 with N vacancies from Step 4 and the cobalt oxide material Co3O4 from Step 2 are mechanically stirred in ethanol, centrifuged and dried to obtain the carbon nitride / cobalt oxide composite material g-C3N4@Co3O4.

2. The preparation method according to claim 1, characterized in that, In step 1, the cobalt source is cobalt nitrate hexahydrate, cobalt chloride, or cobalt sulfate. When the cobalt source is cobalt nitrate hexahydrate, the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is (1.1-1.5):(1.0-1.6). When the cobalt source is cobalt chloride, the mass ratio of cobalt chloride to 2-methylimidazole is (0.25-0.35):(1.2-1.7). When the cobalt source is cobalt sulfate, the mass ratio of cobalt sulfate to 2-methylimidazole is (0.20-0.32):(1.5-1.8).

3. The preparation method according to claim 1, characterized in that, In step 1, the aging temperature is room temperature and the aging time is 6-24 hours; the drying temperature is 40-80℃ and the drying time is 6-12 hours.

4. The preparation method according to claim 1, characterized in that, In step 2, the cobalt source is cobalt nitrate hexahydrate, and the mass ratio of ZIF-67 precursor to cobalt nitrate hexahydrate is (1.0-3.0):(2.0-6.0).

5. The preparation method according to claim 1, characterized in that, In step 2, the aging temperature is room temperature and the aging time is 6-12 hours; the drying temperature is 40-80℃ and the drying time is 6-12 hours; the calcination temperature is 320-350℃ and the calcination time is 2-3.5 hours.

6. The preparation method according to claim 1, characterized in that, In step 3, the calcination temperature in the first stage is 300-350℃, the calcination temperature in the second stage is 600-650℃, the calcination time in the first stage is 2-2.5h, and the calcination time in the second stage is 2-2.5h.

7. The preparation method according to claim 1, characterized in that, In step 4, the calcination temperature is 450-550℃ and the calcination time is 4-6h.

8. The preparation method according to claim 1, characterized in that, In step 5, the mass ratio of carbon nitride g-C3N4 with N vacancies to cobalt oxide material Co3O4 is (1.2-1.8):(4.0-20.0).

9. The carbon nitride / cobalt oxide composite material obtained by the preparation method according to any one of claims 1 to 8, characterized in that, The carbon nitride / cobalt oxide composite material comprises a three-dimensional porous main framework composed of cobalt oxide nanoparticles and a two-dimensional sheet-like graphitic carbon nitride material. The two-dimensional sheet-like graphitic carbon nitride material is dispersed and physically attached to the outer surface and / or internal channels of the three-dimensional porous main framework, forming a physical contact interface.

10. The application of the carbon nitride / cobalt oxide composite material as described in claim 9 as an adsorbent for the adsorption and separation of boric acid.

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