Study On The Construction Of Defective Metal-organic Framework And Degradation Mechanism Of Typical Antibiotic In Aqueous

When the potential danger of emerging contaminants in the water resources environment expands,there is an increasing demand for creating emerging contaminants treatment technology.Advanced oxidation technology may catalyze the formation of reactive radicals with considerable redox capacity,resulting in the structural resolution as well as quick destruction of large molecules of resistant emerging contaminants.Metal organic frameworks（MOFs）are innovative porous materials that were developed in recent years.Because of their assemblable architectures,diversified synthesis,and coordinated unsaturated active sites,they have been intensively researched as heterogeneous catalysts in advanced oxidation systems.Fe-based MOFs are environmentally friendly and devoid of secondary pollution,affordable to produce,have Lewis acid sites in the center of the Fe metal,and the coordination structure is easier to create coordinatively unsaturated metal sites（CUS）,resulting in improved catalytic performance.However,present Fe-based MOFs have a low number of coordinated unsaturated active sites,and poor catalytic activity.MOFs can be modified to generate defects utilizing the modulator approach,which has garnered widespread interest because it can change the pore properties of the material and improve adsorption catalytic performance by competitive substitution replacement of ligands.However,research on defect construction for Fe-based MOFs is still in the initial stages,and there are still issues such as difficult to regulate defect synthesis and limited exposure of defects.In this paper,MOFs with varying degrees of defect were generated by using metal ions of different elements and ions of different valence states of the same metal as metal centers,with the aim of improving catalytic performance by increasing the exposure of unsaturated active sites.As well as further investigating the coordination structure and form of metal centers in the defects and analyzing the mechanism of the synergistic effect of defects on the oxidative degradation process.The specific study content and main conclusions are listed as follows:（1）Employing the modulator approach with Fe²⁺as the metal center,Fe-MOFs with varying defect degrees were created by adding differed quantities of formic acid（FA）.The modulator produces lattice distortion in Fe-MOFs,resulting in longitudinal extension of the material structure.Formic acid increases the pore capacity and specific surface area of Fe-MOFs,allowing more metal-centered Fe species to be exposed.Fe-MOFs-2 synthesized with5 mmol formic acid had the highest Fe II/Fe III,I_D/I_G（The ratios of Intensity D bond/Intensity G bond）,Fe-O/COOH,and specific surface area values,indicating that Fe-MOFs-2 possessed the largest defect concentration with a high density of unsaturated active sites.The optimum reaction conditions for the degradation of Sulfamethoxazole（SMX）by Fe-MOFs-2 activated Persulfate（PS）were:p H=5.0,[SMX]=10 mg/L,[PS]:[SMX]=100:1,catalyst concentration=1 g/L,SMX concentration=10 mg/L,and the removal rate for SMX was 95.1%in 120 min.Its improved degradation performance was attributed to the ligand competition between formic acid and the original ligand 2-aminoterephthalic acid,which led to the exposure of the ligand-unsaturated active site,thus enhancing the generation of active species during the reaction.The active species present in the system included SO₄^-·,·OH and ¹O₂,with SO₄^-·and·OH playing a dominant role in the reaction process and ¹O₂ playing an auxiliary function.87.6%SMX degradation rate of Fe-MOFs-2 can be preserved after five cycles,and the morphological characteristics of Fe-MOFs-2 do not change significantly before and after the reaction,indicating that Fe-MOFs-2 has strong stability and recyclability,and the mechanism of Fe-MOFs-2/PS degradation of SMX was further proposed.Following the addition of PS,Fe（II）CUS will participate in the reaction and activate PS to generate more reactive radicals and singlet oxygen,resulting in the effective degradation of SMX.（2）The NH₂-MIL-101 with different defect characteristics were prepared by combining the hydrothermal method with the modifier method,using acetic acid as the modifier and Fe³⁺as the metal center.With the introduction of the acetic acid modulator,the crystal structure was gradually detached to different degrees.the changes of Fe-O characteristic peaks indicated that the acetic acid modulator changed the coordination environment of the ligand and affected the generation of metal cluster defects.Defect evaluation revealed a 1.96 times higher Fe（II）/Fe（III）ratio compared to that without the addition of modulator,indicating a higher concentration of defects in the catalyst.the proportion of Fe-O clusters was significantly increased,demonstrating the exposure of more Fe metal centers,which increases their opportunity to coordinate with oxygen.Solid-state EPR and Musburger spectroscopy results demonstrate that the absence of ligand vacancies leads to the conversion of NH₂-MIL-101 from its original saturated Fe-O hexa-ligand structure[Fe O₆]to an unsaturated tetra-ligand structure[Fe O₄].With a total active site density of 7.93 umol/g for NH₂-MIL-101@AA1,the density of strong Lewis acid active sites reached 4.12 umol/g,which is a 10 times increase compared to 0.41umol/g for NH₂-MIL-101 without the addition of the modulator.This suggests that the acetic acid modifier does more than increase the total concentration of defects within the material,but also substantially increases the density of strong Lewis acid sites and exposes more active centers with stronger catalytic properties.In the mechanistic study,the introduction of acetic acid led to the formation of Fe-O coordination defects in the structure,which progressively transformed from saturated hexa-ligand to unsaturated tetra-ligand,while the tetra-ligand structure Fe O₄ was more favorable forπ-electron transfer during the reaction.The accelerated electron transfer prompted the formation of defective electric field,which strengthened the charged transfer effect between NH₂-MIL-101@AA1 and pollutants,shortened the transfer distance with pollutants,and realized the efficient degradation of pollutants by SO₄^-·and·OH.（3）Construction of bimetallic defective MIL-101（Fe,Cu）by the combination of Fe,Cu doping and modulator approach,which was used as PS activator to degrade emerging contaminants in aqueous water.With Cu doping and acetic acid modulator construction,the material morphology gradually changed from ortho-octahedral structure to ellipsoidal shape,which still maintained a relatively complete structure after the reaction.Both the doping of Cu and the introduction of acetic acid cause competition between the metal center and the organic ligand,increasing the pore values and specific surface area of the material as well as exposing more active sites.The mineralization rate of MIL-101（Fe,Cu）-1@AA1 for SMX degradation reached 74.5%and showed good degradation performance for a variety of typical emerging contaminants,proving its broad application potential.Multiple active species such as·OH,SO₄^-·,·O₂^-and ¹O₂ were present in the system,which synergistically degraded the pollutants under the dominant effect of SO₄^-·.Fe（II）and Cu（I）are the major active centers in the reaction process.Except for the direct activation of PS in the reaction,Cu（I）can also accelerate the conversion of Fe（III）to Fe（II）,promoting the cyclic regeneration of Fe（II）and enhancing the catalytic and cyclic performance of MIL-101（Fe,Cu）-1@AA1.Eventually,various reactive species degrade the pollutants by attacking the S-N bond and benzene ring in the SMX molecular structure combined with the hydroxylation reaction process mineralization.In summary,this study constructs defects in iron-based MOFs by the modulator method,and provides a theoretical basis for the development and application of advanced oxidation technology for heterogeneous defective MOFs materials in the field of water treatment.