Functional group modified MOF (Metal Organic Framework) material as well as preparation method and application thereof

Abstract

The invention provides a functional group modified MOF material as well as a preparation method and application thereof, and relates to the technical field of functional materials and resource recovery. The functional group modified MOF material is prepared by taking 2-aminoterephthalic acid as an organic ligand and chromium ions as a center, forming a three-dimensional network structure through a coordination relationship between the 2-aminoterephthalic acid and the chromium ions to obtain MIL-101 (Cr)-NH2, and then performing post-synthesis modification on the MIL-101 (Cr)-NH2 through ethylene diamine tetraacetic acid dianhydride (EDTA-DA). And carboxyl (-COOH) and amido (-CONH-) functional groups are introduced to the surface of the polymer. According to the method, the defects in the prior art are overcome, the rare earth ions in the wastewater can be effectively and selectively adsorbed, and the recovery effect of the modified rare earth ions is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of functional materials and resource recycling technology, and in particular to a functional group modified MOF material and a preparation method and application thereof. BACKGROUND

[0002] Rare earth elements (REEs) are located in the third subgroup of the periodic table, including actinides, scandium and yttrium elements. According to the differences in the physical and chemical properties of rare earth elements, researchers usually divide them into light, medium and heavy types. Rare earth is an important strategic resource, known as the "seasoning" of modern industry, and is widely used in steel casting, ceramic materials, superconducting materials, glass manufacturing and other fields. China has a decisive position in the world's rare earth market. According to the data released by the United States Geological Survey (2023), the global rare earth reserves are about 130 million tons, and China is the country with the most abundant rare earth resources in the world, accounting for about 35%. For a long time, China has been the world's largest producer of rare earths, and in 2022, China's rare earth mineral production accounted for 69.76% of the world's rare earth mineral production.

[0003] In the process of rare earth resource exploitation and processing, a large amount of rare earth ions are contained in the wastewater, which can be called rare earth secondary resources. If the rare earth elements in this kind of wastewater cannot be properly recovered and utilized, it will not only cause serious environmental pollution, but also cause serious resource waste. Therefore, it has high economic value and environmental value to enrich and recover rare earth elements from solution and realize the sustainable utilization of rare earth resources. Adsorption method can effectively remove and recover rare earth elements from wastewater, and is a commonly used technology for resourceful treatment of rare earth wastewater. However, the adsorbents such as activated carbon, carbon nanotubes and biochar currently used have low adsorption capacity for rare earth metals, no selectivity and are easily affected by coexisting ions, which greatly limits the development and practical application of the technology. Therefore, the development of an adsorbent with high adsorption capacity and strong selectivity will be beneficial to promote the development of adsorption technology in the resourceful treatment of rare earth wastewater.

[0004] MOFs have a series of excellent properties such as high specific surface area, adjustable pore size, and introduction of various functional groups by simple post-synthesis modification, and are considered to be a kind of adsorbent with great potential for separation and recovery of rare earth elements. However, the MOF materials reported in the current research have relatively low adsorption capacity for rare earth elements and slow separation and recovery rate. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a functional group modified MOF material and a preparation method and application thereof, and designs a modified MOF material which can effectively selectively adsorb rare earth ions in wastewater and improve the recovery effect of modified rare earth ions.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions: A functional group modified MOF material, wherein the functional group modified MOF material is MIL-101(Cr)-NH-EDTA, and the chemical formula is C 34 H 31 Cr3N5O 22 , the chemical structure is as follows:

[0007] The preparation method of the functional group modified MOF material comprises the following steps: S1, dissolving 2-aminoterephthalic acid and chromium nitrate nonahydrate in ultrapure water, mixing by magnetic stirring, and subjecting the obtained mixture to a hydrothermal reaction to obtain a reaction solution for later use; S2. The reaction solution was cooled to room temperature and then centrifuged. The precipitate was washed to obtain a dark green solid material for standby use; S3, drying the solid material in a vacuum drying oven, and then grinding and pulverizing to obtain MIL-101(Cr)-NH2; S4, dissolving the above-mentioned MIL-101(Cr)-NH2 and ethylenediaminetetraacetic dianhydride in N,N-dimethylformamide, stirring evenly and heating with stirring to react, to obtain a reaction material for later use; S5. After the reaction material is cooled to room temperature, it is washed with N,N-dimethylformamide and anhydrous ethanol, and the solid powder is collected by high-speed centrifugation. The solid powder is then dried in a vacuum drying oven and ground to obtain MIL-101(Cr)-NH-EDTA.

[0008] Preferably, in step S1, the mass ratio of 2-aminoterephthalic acid to chromium nitrate nonahydrate is 125:58, and the mass ratio of chromium nitrate nonahydrate to ultrapure water is 1:14.

[0009] Preferably, the temperature of the hydrothermal reaction in step S1 is 150° C., and the reaction time is 24 h.

[0010] Preferably, the washing method in step S2 is to repeatedly wash with ultrapure water and anhydrous ethanol until the solution is no longer dark.

[0011] Preferably, in step S4, the mass ratio of MIL-101(Cr)-NH2 to ethylenediaminetetraacetic dianhydride is 363:256, and the mass ratio of MIL-101(Cr)-NH2 to N,N-dimethylformamide is 1:100.

[0012] Preferably, the heating and stirring reaction condition in step S4 is stirring the reaction in an oil bath at 60° C. for 24 hours.

[0013] Preferably, the drying in the vacuum drying oven in steps S3 and S5 is carried out at 60° C. for 12 hours.

[0014] The above functional group-modified MOF materials are used to enrich and recover rare earth elements in solution.

[0015] The present invention provides a functional group-modified MOF material and its preparation method and application, which have the following advantages over the prior art: The functional group-modified MOF material designed in this invention uses 2-aminoterephthalic acid as an organic ligand and a chromium ion as a central ligand, forming a three-dimensional network structure to form MIL-101(Cr)-NH2. This is then post-synthetically modified with ethylenediaminetetraacetic dianhydride (EDTA-DA). While retaining the original MOF octahedral phase, carboxyl (-COOH) and amide (-CONH-) functional groups are introduced onto the surface. This material maintains the octahedral structure of MIL-101(Cr) with a particle size of approximately 100 nm. After post-synthetic modification with the multidentate EDTA ligand, clusters form on the framework surface and within the pores, with slight aggregation, forming an H3-type mesoporous network while maintaining a high specific surface area and transparent pores. Infrared spectroscopy and XPS confirm that the carboxyl-amino synergistic coordination sites are uniformly distributed throughout the framework, effectively enhancing rare earth affinity. Thermogravimetric analysis revealed that the modified framework exhibited enhanced thermal stability, making it adaptable to the complex environments of acidic to neutral aqueous systems. The material maintained its structural integrity and adsorption activity over multiple cycles, enabling the selective capture and regeneration of light and heavy rare earth ions, making it suitable for the green extraction and resource recovery of rare earth-containing wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The SEM comparison images of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA in the examples of the present invention are shown; (a) and (b) are scanning electron microscope (SEM) micrographs of MIL-101(Cr)-NH2; (c) and (d) are scanning electron microscope (SEM) micrographs of MIL-101(Cr)-NH-EDTA; Figure 2 (a) is a comparison of the N2 adsorption-desorption isotherms of the two materials; (b) is a comparison of the X-ray diffraction (XRD) of the two materials; (c) is a comparison of the FTIR spectroscopy identification; (d) is a comparison of the thermogravimetric (TG) curves of the two materials; (e) is a comparison of the differential thermogravimetric (DTG) curves of the two materials; Figure 3(a), (b), (c), (d), and (e) are X-ray photoelectron spectra (XPS) of MIL-101(Cr)-NH2; (f), (g), (h), (i), and (j) are X-ray photoelectron spectra (XPS) of MIL-101(Cr)-NH-EDTA; Figure 4 Langmuir and Freundlich isotherm models for the adsorption of La (a), Ce (b), Gd (c), and Dy (d) by 1 g / L MIL-101(Cr)-NH-EDTA; Figure 5 (a) shows the adsorption kinetics of La, Ce, Gd and Dy by 1 g / L MIL-101(Cr)-NH-EDTA; (b) shows the pseudo-first-order kinetic model for the adsorption of La, Ce, Gd and Dy by 1 g / L MIL-101(Cr)-NH-EDTA; (c) shows the pseudo-second-order kinetic model for the adsorption of La, Ce, Gd and Dy by 1 g / L MIL-101(Cr)-NH-EDTA; (d) shows the thermodynamic adsorption model of La, Ce, Gd and Dy by MIL-101(Cr)-NH-EDTA. Figure 6 (a) is a schematic diagram showing the effect of MIL-101(Cr)-NH-EDTA on the adsorption of La and Gd at different pH values; (b) is a comparison diagram of the Zeta potential of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA in an embodiment of the present invention.

[0017] Figure 7 (a) is a comparison of X-ray diffraction (XRD) patterns of MIL-101(Cr)-NH-EDTA before and after La adsorption; (b) is a comparison of FTIR spectroscopic identification patterns of MIL-101(Cr)-NH-EDTA before and after La adsorption. Figure 8 This is the X-ray photoelectron spectroscopy (XPS) diagram of MIL-101(Cr)-NH-EDTA after adsorption of La; Figure 9 (a) is a schematic diagram of the optimized structure of MIL-101(Cr)-NH-EDTA; (b) is a schematic diagram of the electrostatic potential distribution of MIL-101(Cr)-NH-EDTA before adsorption; (c) is a schematic diagram of the adsorption configuration of MIL-101(Cr)-NH-EDTA (adsorption of La); (d) is a schematic diagram of the electrostatic potential distribution of MIL-101(Cr)-NH-EDTA after adsorption of La; Figure 10HOMO (a) and LUMO (b) molecular orbital surfaces and energy levels of MIL-101(Cr)-NH-EDTA before adsorption and HOMO (c) and LUMO (d) molecular orbital surfaces and energy levels after La adsorption; Figure 11 Schematic diagram of the adsorption mechanism of rare earth elements (Ln=La, Ce, Gd or Dy) by MIL-101(Cr)-NH-EDTA.

[0018] Figure 12 Schematic diagram of the recovery characteristics of La adsorbed by MIL-101(Cr)-NH-EDTA. DETAILED DESCRIPTION

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0020] Example 1: Preparation of functional group modified MOF material MIL-101(Cr)-NH-EDTA: (1) Weigh chromium nitrate nonahydrate and 2-aminoterephthalic acid in a mass ratio of 125:58, add them to ultrapure water, control the mass ratio of chromium nitrate nonahydrate to ultrapure water to be 1:14, and magnetically stir the mixture for 3 hours; (2) The mixed solution was transferred to a 100 mL polytetrafluoroethylene high-pressure tank and subjected to a hydrothermal reaction at 150°C in a forced air drying oven for 24 h to obtain a reaction solution; (3) After the reaction solution is cooled to room temperature, the material is separated by centrifuge to obtain a precipitate, and the precipitate is repeatedly washed with ultrapure water and anhydrous ethanol until the washed solution is no longer dark, obtaining a black-green solid; (4) Dry the dark green solid in a vacuum drying oven at 60 °C for 12 h to obtain a dark green block solid, which was then ground into a green powder using an agate mortar, namely MIL-101(Cr)-NH2; (5) MIL-101(Cr)-NH2 and ethylenediaminetetraacetic acid dianhydride were mixed in a mass ratio of 1:100 and dissolved in N,N-dimethylformamide. The mass ratio of MIL-101(Cr)-NH2 to N,N-dimethylformamide was controlled to be 1:100, and the mixture was stirred evenly in a round-bottom flask (with a condenser placed on the top). The mixture was stirred and reacted in an oil bath at 60°C for 24 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylformamide and anhydrous ethanol and the precipitated solid was collected by high-speed centrifugation. The solid was vacuum dried at 60°C for 12 hours and ground into a green powder using an agate mortar to obtain MIL-101(Cr)-NH-EDTA. The chemical formula of the reaction is:

[0021] Detection: 1. Detect the differences in microstructural evolution between MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA: Specific results such as Figure 1 As shown, Figure 1 (a) and (b) show the microstructure of MIL-101(Cr)-NH2; MIL-101(Cr)-NH2 consists of irregularly shaped particles with a diameter of approximately 100 nm. Figure 1 (c) and (d) show the microstructure of MIL-101(Cr)-NH2-EDTA. After MIL-101(Cr)-NH2 was modified with EDTA, the material showed obvious agglomeration, indicating that EDTA was not only fixed on the surface but also diffused into the pore network of the framework.

[0022] 2. Testing the gas adsorption and related structural properties of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH2-EDTA: N2 adsorption-desorption isotherms were used to analyze the specific surface area and pore size distribution of the materials. The test conditions were: pure N2 atmosphere, degassing temperature of 100°C, and degassing time of 6 h. The sample mass ranged from 0.01 g to 0.025 g. Liquid nitrogen adsorption-desorption experiments were performed at relative pressures P0 / P = 0.011-0.9943 and 0.9943-0.1401.

[0023] Specific results Figure 2 、 Figure 3 As shown in Table 1 below: Table 1

[0024] Figure 2(a) shows that both MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA show type IV physical adsorption curves with H3 hysteresis, confirming their mesoporous structure; and MIL-101(Cr)-NH2 exhibits higher nitrogen absorption rate and pore volume retention rate. This obvious reduction of MIL-101(Cr)-NH-EDTA compared with MIL-101(Cr)-NH2 is related to the pore narrowing observed by SEM. The pore narrowing is attributed to the pore blocking effect caused by EDTA that restricts gas diffusion, but MIL-101(Cr)-NH-EDTA maintains sufficient mesoporosity (average pore width of 2.98 nm) to accommodate rare earth ions.

[0025] Figure 2 Panel (b) shows the XRD patterns of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA. The diffraction peak positions and intensities of MIL-101(Cr)-NH2 are consistent with those reported in the literature, and no additional peaks are present, indicating successful sample preparation. MIL-101(Cr)-NH-EDTA exhibits no significant structural changes, indicating that the grafting of EDTA groups onto MIL-101(Cr)-NH2 does not result in significant loss of crystallinity.

[0026] Figure 2 (c) is a schematic diagram of the infrared characteristic peaks of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA, which are between 3250 and 3500 cm -1 Both asymmetric and symmetric NH stretching vibrations of -NH2 were observed, indicating the presence of amino groups. -1 The stretching vibration peak of CN (aromatic) can be observed at ∼1400 cm -1 The split peak at 1740~1650cm indicates that there is an intermolecular hydrogen bond between the grafted amino and carboxyl groups in the MOF structure; MIL-101(Cr)-NH-EDTA has a peak at 1740~1650cm -1 and 1680~1630cm -1 The vibration peaks are attributed to the stretching modes of the carboxyl group (-COOH) and the carbonyl group (-C=O) in the amide bond, respectively.

[0027] Figure 2TG curves of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA and DTG curve of MIL-101(Cr)-NH-EDTA, compared with MIL-101(Cr)-NH2, the thermal stability of MIL-101(Cr)-NH-EDTA is improved, the weight loss process of the sample is divided into three stages. The first stage, 30-120℃ temperature range, due to the loss of physically adsorbed water and solvent molecules, 10% weight loss; the second stage, 120-310℃ temperature range, 30%-40% weight loss, which is due to the loss of their internal bound water, the surface of MIL-101(Cr)-NH-EDTA also decomposes. The third stage, 310-370℃ temperature range, is the fastest stage of thermal degradation rate.

[0028] Figure 3 XPS spectra of MIL-101(Cr)-NH2 and MIL-101(Cr)-NH-EDTA, which shows that after introducing EDTA, the position and area of Cr 2p absorption peak of the material do not change significantly, indicating that the modification has little effect on the main structure of the material; the introduction of DTPA increases the C-O peak ratio in C 1s and O1s spectra and the C-N peak ratio in N 1s spectrum, which is caused by the increase of -CONH- after modification reaction.

[0029] 3. Prepare 1 L of La, Ce, Gd and Dy stock solution with a concentration of 1000 mg / L, and dilute to different concentrations during the experiment for adsorption experiment. Specifically, dilute 1000 mg / L of La, Ce, Gd and Dy ion stock solution into a series of concentration gradients (10, 20, 50, 100, 200, 500 mg / L), take 20 mL of each concentration gradient solution in a centrifuge tube, adjust the pH of the solution to 5.0 with 0.1 M HNO3 and NaOH, then add 20 mg of adsorbent material, the solid-liquid ratio is 1.0 g / L, then place the above mixed solution in a constant temperature water bath shaker, set the temperature to 25℃, the rotation speed is 200 rpm, take sample (24 h) after adsorption is completed, filter the reaction solution with 0.22 μm water filter head for measurement.

[0030] (1) The equilibrium adsorption capacity and adsorption rate of the adsorbent material are obtained by the following method: ; ; Where q e is the equilibrium adsorption capacity (mg / g), η b (%) is the removal rate, C0and Ce are the initial concentration of a metal ion solution and the residual concentration of a certain ion in the solution after the reaction (mg / L), V (mL) and m (g) are the volume of the metal ion solution and the mass of the added material, respectively.

[0031] Based on the above studies, the adsorption isotherms of La, Ce, Gd, and Dy on the adsorbent materials were determined; Fit the adsorption equilibrium data to the Langmuir or Freundlich model to understand the interaction between rare earth ions and the adsorption sites of the adsorbent material and estimate its adsorption capacity. The two isotherm equations are: ; where q e is the adsorption capacity at adsorption equilibrium; C e is the concentration of the substance at adsorption equilibrium; K L is the Langmuir adsorption constant; K F and n is the Freundlich adsorption constant. f The larger the value of and n, the better the adsorption performance. F It will decrease with increasing temperature; the range of 1 / n is 0~1, and its size indicates the influence of concentration on adsorption capacity: 1 / n=0.1~0.5 indicates easy adsorption, and 1 / n>2 indicates difficult adsorption.

[0032] The adsorbent materials were MIL-101(Cr)-NH-EDTA, MIL-101(Cr)-CH2Cl-ED, MIL-101(Cr)-ED, poly-γ-glutamic acid @ polyvinyl alcohol, EDTA-β-CD, and CNCs / 5%GO-NIIPs. By using the Langmuir and Freundlich isotherm theories to establish an equilibrium model, the interfacial adsorption mechanism between rare earth ions and functionalized MOFs was elucidated. Figure 4 As shown in Table 2 and Table 3: Table 2

[0033] Table 3

[0034] like Figure 4 As shown, among all the rare earth ions tested, the Langmuir model showed a higher correlation coefficient (R 2= 0.908-0.991), indicating that monolayer adsorption is dominant on uniform binding sites. Langmuir analysis (Table 2) shows that La 3+ 、Ce 3+ 、Gd 3+ and Dy 3+ The maximum adsorption capacity (q m ), the calculated results were 70.72, 35.86, 89.18 and 59.84 mg / g respectively.

[0035] The maximum adsorption capacity of MIL-101(Cr)-NH-EDTA for REEs in Table 3 is significantly higher than that of other related adsorbents. This indicates that MIL-101(Cr)-NH-EDTA can be used for efficient recovery of REEs from aqueous solutions.

[0036] (2) Detect the adsorption kinetics of La, Ce, Gd, and Dy on MIL-101(Cr)-NH-EDTA.

[0037] Adsorption kinetics experiments were performed at specified time intervals (5 min, 10 min, 20 min, 30 min, 60 min, 90 min, 120 min, 240 min, 360 min, and 720 min). To better understand the adsorption process, pseudo-first-order and pseudo-second-order kinetics were used for fitting, as shown below: Pseudo-first-order kinetics: ; Pseudo-second-order kinetics: ; where q e and q t They represent the adsorption capacity of the adsorbent material for metal ions at adsorption equilibrium (mg / g) and the adsorption capacity of the adsorbent material for metal ions at adsorption time t (mg / g), k1 is the apparent rate constant of pseudo-first-order kinetics, and k2 is the rate constant of pseudo-second-order kinetics.

[0038] Specific results can be found in Figure 5 and Table 4: Table 4

[0039] Figure 5(a) shows the time-dependent adsorption curve of MIL-101(Cr)-NH-EDTA, which exhibits two-stage behavior: a rapid adsorption phase occurs within the first 30 minutes, likely due to the migration of rare earth ions to the outer surface of MIL-101(Cr)-NH-EDTA, where there are abundant active sites. This is followed by a slower adsorption phase, with the system gradually approaching equilibrium after approximately 360 minutes, indicating that chemical adsorption is the primary process in the latter stage.

[0040] Comparative analysis of linear regression results is as follows Figure 5 (b), (c) and Table 4; the pseudo-second-order kinetic model has higher prediction accuracy and higher correlation coefficient (R 2 >0.993). This confirms that the coordination-driven adsorption mechanism, rather than simple physical forces, dominates the electron transfer between lanthanide ions and MIL-101(Cr)-NH-EDTA.

[0041] (3) Verification of the thermodynamic performance of MIL-101(Cr)-NH-EDTA adsorption of La, Ce, Gd and Dy: The interaction mechanism between rare earth ions and MIL-101(Cr)-NH-EDTA was revealed by thermodynamic adsorption models at 298.15K, 308.15K, and 318.15K: ; ; ; where ΔG is the standard Gibbs free energy (kJ·mol −1 ), ΔH is the enthalpy change (kJ·mol −1 ), ΔS is the entropy change (kJ·mol −1 ·K −1 ); T (K) is the absolute temperature, R is the ideal gas constant (8.314 J·mol −1 ·K −1 ), K is the dimensionless equilibrium coefficient; K T is derived from the Langmuir constant (K L ) formula to calculate the dimensionless equilibrium constant, C w is the water concentration (5.56×10 4 mmol·L -1 ).

[0042] The specific results are shown in Table 5: Table 5

[0043] As shown in the table above, the positive enthalpy change (ΔH > 0) confirms the endothermic nature of adsorption, indicating that chemical interactions predominate. The concurrent positive entropy change (ΔS > 0) indicates increased interfacial disorder, likely due to the displacement of water molecules during REE complexation and reorganization of the adsorbent framework. The gradual shift in the Gibbs free energy (ΔG > 0) indicates increased spontaneity with increasing temperature, confirming the thermodynamically favorable nature of this process. This indicates that the adsorption of rare earth elements by MIL-101(Cr)-NH-EDTA is a spontaneous, endothermic, and entropically driven process.

[0044] (4) Detection of the effect of different pH on the adsorption of rare earth ions (La, Gd) by MIL-101(Cr)-NH-EDTA: To avoid the formation of hydroxide precipitation of rare earth ions at higher pH values, the pH range studied was limited to 2–6.

[0045] Specific results such as Figure 6 As shown, Figure 6 (a) shows the effect of MIL-101(Cr)-NH-EDTA on La 3+ and Gd 3+ The adsorption amount reached the maximum at pH 5.0 (22.64 mg / g, 23.41 mg / g); Figure 6 (b) shows the zeta potential analysis of MIL-101(Cr)-NH2-EDTA. The introduction of EDTA effectively alters the surface charge distribution of MIL-101(Cr)-NH2, increasing its electronegativity. The unmodified MIL-101(Cr)-NH2 material possesses a positive charge, which is unfavorable for rare earth ion binding. The zero point of the modified MIL-101(Cr)-NH2-EDTA appears at pH 5.0, due to the addition of -COOH groups on the EDTA. With increasing pH, the carboxyl groups in MIL-101(Cr)-NH2-EDTA undergo deprotonation, causing the zeta potential of MIL-101(Cr)-NH2-EDTA to shift from positive to negative. This enhances REE removal performance at high pH through electrostatic attraction.

[0046] (5) Verify the interaction mechanism between rare earth ions and MIL-101(Cr)-NH-EDTA, and characterize the adsorption of La on MIL-101(Cr)-NH-EDTA by FT-IR, XRD and XPS. 3+ The functional group changes of MIL-101(Cr)-NH-EDTA were investigated using density functional theory (DFT). These calculations help to elucidate electrostatic interactions, electron redistribution, and binding energy at the molecular level. Specific results such as Figure 7-11 As shown, Figure 7 (a) shows that the characteristic peak of MIL-101(Cr)-NH2 in the diffraction angle range of 8°-9° did not change significantly before and after the adsorption of rare earth metals, indicating that the adsorbent has good stability; Figure 7 (b) shows the stretching vibration of -COOH in the Fourier transform infrared spectrum (1620-1550 cm -1 The weakening of ) can be attributed to the complexation between rare earth ions and the carboxyl groups on the EDTA ligand. The amine-related features (including NH stretching vibration (3250-3500 cm) -1 ) and aromatic CN modes (~1260 cm -1 )) in the adsorption of La 3+ The attenuation of the adsorbent decreases with time, indicating that there is an interaction between the rare earth ions and the amino groups of the adsorbent.

[0047] Figure 8 MIL-101(Cr)-NH-EDTA adsorbs La 3+ XPS spectra before and after; Figure 8 (a) The appearance of La 3d photoelectron signal confirms that La 3+ Successful adsorption of Figure 8 (c) shows the high-resolution O 1s photoelectron spectrum. A positive shift of 0.15 eV in the CO bonding energy (531.97 eV → 532.12 eV) is observed, indicating oxygen-mediated coordination via redistribution of electron density at the carboxyl site. In addition, compared with the N 1s XPS spectrum before adsorption, the N–H peak intensity decreases, and the La 3+ After adsorption, a new peak appears near 400 eV ( Figure 8 (d) This is likely due to the formation of chelate bonds between the rare earth ions and the –NH2 groups, resulting in an N–La structure. This evidence confirms that the amino groups in MIL-101(Cr)-NH-EDTA play an active role in the rare earth element binding process. Figure 8 The Cd 2p and La 3d spectra in (e) and (f) further show that the Cd 2p signal remains relatively stable in both peak position and intensity, indicating that Cr maintains a constant valence during the adsorption process. Figure 8 (f) shows the La 3d characteristic double peaks, located at 835.83 / 852.60 eV (3d 5 / 2 ) and 839.07 / 855.81 eV (3d 3 / 2 ), only corresponds to La 3+oxidation state, confirming that the trivalent coordination was stable and no redox transformation occurred during the adsorption process.

[0048] The adsorption energy results confirm the feasibility and strength of adsorption. Specifically, the calculated La 3+ The adsorption energy on MIL-101(Cr)-NH-EDTA is -339.37 kJ·mol per ion. −1 , which indicates that the adsorption process is spontaneous and thermodynamically favorable. Figure 9 As shown, upon adsorption of La 3+ Before and after adsorption, the optimized molecular structure and corresponding electrostatic potential (ESP) distribution of MIL-101(Cr)-NH-EDTA were obtained. In these visualization images, the red area represents the electron-rich (negatively charged) area, and the blue area represents the electron-deficient (positively charged) area. Figure 9 In (b), the strongest electron-rich regions are mainly located around the carboxyl oxygen atoms of the EDTA ligands, highlighting that they are potential coordination sites for La3+. Figure 9 In (d), the electron density in these regions decreases significantly, indicating that the electrons have obviously transferred to La 3+ ions. This observation highlights the role of carboxyl groups in promoting La 3+ Important role in coordination.

[0049] like Figure 10 As shown, the spatial distribution of HOMO and LUMO orbitals shifts significantly towards La during the adsorption process. 3+ Coordination site migration. Before adsorption ( Figure 10 In (a) and (b), HOMO and LUMO are mainly localized on the aromatic ring and donor atoms (O, N), indicating potential active sites. 3+ After combining ( Figure 10 In (c) and (d)), both the HOMO and LUMO electron densities move closer to the metal center, suggesting increased electron delocalization and stronger orbital interactions. Quantitative evaluation of the HOMO–LUMO energy gap (Δε) supports these observations (Table 6). Δε reflects the trend of charge transfer: lower Δε indicates greater electron delocalization during binding, leading to a more stable complex. After adsorption, Δε decreases from 0.238 eV to 0.217 eV, indicating enhanced chemical activity and a more stable adsorbed complex.

[0050] Table 6

[0051] Mulliken charge analysis showed that the carboxyl oxygen atoms in EDTA undergo more significant charge changes than the nitrogen atoms in the amine groups. One carboxyl oxygen atom exhibited a Δq of -0.064, while the change in nitrogen atoms generally remained below -0.01. This finding highlights the role of carboxyl groups as La 3+ Incorporate the key roles of major contributors.

[0052] The above results confirm that rare earth elements (La 3+ There is a strong stabilizing coordination effect between the carboxyl sites of ethylenediaminetetraacetic acid (EDTA) in the MIL-101(Cr)-NH-EDTA framework. The proposed mechanism of rare earth element adsorption on MIL-101(Cr)-NH-EDTA is as follows: Figure 11 shown.

[0053] (6) Detection of the cumulative adsorption performance of MIL-101(Cr)-NH-EDTA: The adsorption performance of MIL-101(Cr)-NH-EDTA for rare earth ions (La 3+ ) adsorption efficiency. In this test, 10 mg of MIL-101(Cr)-NH-EDTA was mixed with 10 mL of a 100 mg / L rare earth ion solution, shaken in 25°C water for 6 hours, and then centrifuged to collect the adsorbed MIL-101(Cr)-NH-EDTA. This was then reintroduced into the 100 mg / L rare earth ion solution for adsorption experiments, and this cycle was repeated five times.

[0054] Specific results can be found in Figure 12 After five regeneration cycles, MIL-101(Cr)-NH-EDTA 3+ The adsorption capacity was only 30 mg / g below the initial adsorption capacity. This gradual decrease in efficiency stems from the partial irreversible occupancy of coordination sites—a common phenomenon in chelating adsorbents. Specifically, EDTA-derived carboxyl and amino functional groups exhibited low protonation efficiency during acidic regeneration, leading to the continued formation of metal-ligand complexes at these sites.

[0055] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A functional group modified MOF material, characterized in that: The functional group modified MOF material is MIL-101(Cr)-NH-EDTA, and the chemical formula is C 34 H 31 Cr3N5O 22 , the chemical structure is as follows: 。 2. A method for preparing a functional group-modified MOF material according to claim 1, characterized in that: The preparation method comprises the following steps: S1, dissolving 2-aminoterephthalic acid and chromium nitrate nonahydrate in ultrapure water, mixing by magnetic stirring, and subjecting the obtained mixture to a hydrothermal reaction to obtain a reaction solution for later use; S2. The reaction solution was cooled to room temperature and then centrifuged. The precipitate was washed to obtain a dark green solid material for standby use; S3, drying the solid material in a vacuum drying oven, and then grinding and pulverizing to obtain MIL-101(Cr)-NH2; S4, dissolving the above-mentioned MIL-101(Cr)-NH2 and ethylenediaminetetraacetic dianhydride in N,N-dimethylformamide, stirring evenly and heating with stirring to react, to obtain a reaction material for later use; S5. After the reaction material is cooled to room temperature, it is washed with N,N-dimethylformamide and anhydrous ethanol, and the solid powder is collected by high-speed centrifugation. The solid powder is then dried in a vacuum drying oven and ground to obtain MIL-101(Cr)-NH-EDTA.

3. The preparation method according to claim 2, wherein: In step S1, the mass ratio of 2-aminoterephthalic acid to chromium nitrate nonahydrate is 125:58, and the mass ratio of chromium nitrate nonahydrate to ultrapure water is 1:

14.

4. The preparation method according to claim 2, wherein: The temperature of the hydrothermal reaction in step S1 is 150° C., and the reaction time is 24 hours.

5. The preparation method according to claim 2, wherein: The washing method in step S2 is to repeatedly wash with ultrapure water and anhydrous ethanol until the solution is no longer dark.

6. The preparation method according to claim 2, wherein: In step S4, the mass ratio of MIL-101(Cr)-NH2 to ethylenediaminetetraacetic dianhydride is 363:256, and the mass ratio of MIL-101(Cr)-NH2 to N,N-dimethylformamide is 1:

100.

7. The preparation method according to claim 2, characterized in that: The heating and stirring reaction conditions in step S4 are stirring the reaction in an oil bath at 60° C. for 24 hours.

8. The preparation method according to claim 2, wherein: The drying method in the vacuum drying oven in steps S3 and S5 is drying at 60° C. for 12 hours.

9. Use of the functional group-modified MOF material according to claim 1 in enriching and recovering rare earth elements in a solution.

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