Application Of Iron-based MOFs And Its Derivative Composites In Antibiotic Degradation By Catalysis

The excessive use of chemical products and the continuous discharge of organic pollutants in human production activities have caused serious damage to the environment.Among them,the pollution caused by the abuse of antibiotics has seriously affected human health and the ecological environment.Exploring effective technologies and methods to deal with the increasingly serious problem of antibiotic pollution has become a research hotspot.As an environmentally friendly advanced oxidation technology,photocatalysis has attracted much attention because it is driven by clean solar energy to remove pollutants.Semiconductor photocatalysts convert solar energy into chemical energy for redox reactions,and generate active species with water and dissolved oxygen,which can effectively remove pollutants in water or convert pollutants into environmentally friendly substances.Improving the performance of catalysts in photocatalytic technology is the primary goal.Among a variety of new photocatalysts,metal organic framework（MOF）is a crystalline material composed of metal nodes and organic ligands.Because of its porous framework,high surface area,long-range ordered structure,and highly controllability,it has attracted much attention.Among them,iron-based organometallic frameworks（Fe-MOFs）are a class of semiconducting catalysts formed by earth-abundant iron elements with visible light response,low toxicity and hydrothermal stability,and are also excellent catalysts for the Fenton reaction.Fe-MOFs and their composites have been widely used in the research of photocatalytic degradation of organic pollutants and related fields.However,weak visible light absorption,rapid recombination of photogenerated electron-holes,and low degradation efficiency are still challenges for Fe-MOFs photocatalysts.According to the study of the principle of semiconductor photocatalysis,improving light absorption efficiency,increasing the number of catalytic active sites and promoting the separation of photogenerated charges are three effective methods to improve the performance of catalysts.This paper proposes some solutions to the above problems,such as the regulation of the valence state of the metal center,the construction of heterojunctions,and the addition of unsaturated coordination defects,which can effectively improve the performance of Fe-MOFs photocatalysts.Meanwhile,this paper mainly focuses on three Fe-MOFs constructed from terephthalic acid and Fe-O clusters:MIL-53（Fe）,MIL-88B（Fe）,and MIL-101（Fe）,by performing the heating modification of monomeric materials and the method of constructing composite materials can improve the degradation performance of Fe-MOFs on antibiotic pollutants,and combine various characterization analysis methods to explore the mechanism of performance improvement.The processes and mechanisms of degradation were also explored.The specific content and research conclusions are as follows:（1）Mixed valence MIL-53（Fe）photocatalyst and its degradation performance and mechanism for dyes and tetracycline.By adjusting the heating temperature from 120℃to220℃in vacuum to control the reduction of Fe ^IIIin the Fe-O clusters,MIL-53（Fe）with different Fe ^II/Fe ^IIIratios was obtained.The MIL-vac120 prepared under vacuum heating at120℃displayed the best photocatalytic performance,which showed 96.28%and 95.01%removal efficiencies of Rh B and TC in 100 min,respectively.The reaction rate was 1.63times and 2.45 times higher than that of MIL-53（Fe）without vacuum heating treatment.The XPS results show that the vacuum heating method can indeed reduce Fe ^IIIin the Fe-O cluster,and the temperature was proportional to the content of Fe ^II.The Fe ^II/Fe ^IIIratio of the optimal sample MIL-vac120 was 0.2725.The band structure calculated by DRS and other characterizations shows that the appearance of Fe ^IIin the Fe-O cluster reduced the position of the conduction band.The band gap gradually decreases from 2.76 to 2.58 e V,which increased the absorption in the visible region of the photocatalyst.In the photoelectrochemical characterization,MIL-vac120 has the lowest excited fluorescence and impedance,which proved that the introduction of mixed valence one-dimensional Fe-O chain clusters can improve the transfer and separation efficiency of photogenerated charges,and improve the photocatalytic degradation performance.It is verified by experiments that this method is also applicable to MIL-88B（Fe）and MIL-101（Fe）with Fe-O clusters.In addition,MIL-vac120with unsaturated coordination can also effectively activate PDS into·SO₄^-.By coupling PDS catalysis and photocatalysis,the consumption of PDS active site was reduced,and MIL-vac120 can be designed used as a catalyst for all-day catalytic wastewater treatment.（2）MIL-53（Fe）/PDI（PM）heterojunction photocatalyst and its degradation performance and mechanism for tetracycline.The PDI nanofibers were ultrasonically dispersed and added to the DMF solution for the synthesis of MIL-53（Fe）precursors.The PM composites prepared by a simple solvothermal synthesis method had good photocatalytic degradation performance for TC.The degradation and removal rate of TC in 5PM was close to 94.08%within 30 min,and its photodegradation kinetics was 0.07647 min^-1,which were 4times that of PDI and 33 times that of MIL-53（Fe）,respectively.Through characterization analysis,the 5PM has a unique hair-like structure,and the PDI nanofibers were uniformly dispersed and fixed in MIL-53（Fe）,and were connected by valence bonds,which enhances the photogenerated charge transfer between the interfaces and the stability of the photocatalyst.According to the photoelectrochemical characterization,the band structures of PDI and MIL-53（Fe）were calculated,combined with the electron transfer direction of the PM heterojunction interface characterized by XPS,it was proved that the PM conforms to the Z-scheme heterojunction mechanism,and this heterojunction accelerated the separation and transfer of photogenerated charges,and provided favorable redox capabilities for degradation.The radical trap experiments and ESR analysis showed that hydroxyl radicals（·OH）,superoxide radicals（·O₂^-）and photogenerated holes（h⁺）were the active species in the degradation reaction,and the LC-MS analyzed the possible degradation pathways of TC and its intermediate products.（3）The photothermal catalytic degradation performance and mechanism of MIL-101（Fe）and its annealed derivatives for organic pollutants.Based on TG curves,the annealed derivatives of MIL-101（Fe）prepared at 200℃,300℃,400℃and 500℃under nitrogen protection were studied,and the relationship between the change of material composition and catalytic performance was discussed.The changes of crystal and chemical structure were systematically analyzed by XRD and other characterization methods.It was verified that M-300 belongs to annealing defect state.The thermal removal of ligands made MIL-101（Fe）have defect crystal form and Fe-O center with enriched oxygen vacancy.According to the photoelectrochemical characterization,the band gap of annealed defective M-300 was reduced by about 0.1 e V due to the frame distortion,while over-pyrolyzed M-400and M-500 lose the characteristics of semiconductors and form Fe₃O₄-like derivatives.Among them,the M-300 can remove 65%and 91%of TC and Rh B within 60 min of illumination,and its photodegradation kinetics were 2.18 and 3.09 times that of MIL-101（Fe）,respectively.In addition,the photo-thermal catalytic mechanism of M-300 in the TC degradation were analyzed through experiments and ESR characterization.M-300 increased the temperature through photothermal conversion,accelerated the photocatalytic reaction and activated the thermal catalytic degradation at the same time.As an active site,the Fe-O clusters with oxygen defect were excited by photo-thermal to generate oxygen active species such as·O₂^-and·OH,which acted on TC degradation.（4）Thermal catalytic degradation performance and mechanism of Fe-based MILs（Fe-MILs）and their annealed derivatives on organic pollutants.Three kinds of Fe-MILs（MIL-53（Fe）,MIL-88B（Fe）,and MIL-101（Fe））and their annealed derivatives at 300 to400°C were prepared to further study their thermocatalytic degradation performance and mechanism on TC and dyes.The annealed Fe-MILs in this temperature range has excellent thermocatalytic degradation performance for TC.The optimal sample was MIL-101-350.Its degradation and removal rate of TC was 94.22%at 70℃within 60 min,and the removal rate of TC can reach 90.6%at room temperature 25℃within 10 h.By comparison,it is found that the improved thermocatalytic degradation efficiency was attributed to the exposure of Fe-O clusters and the formation of oxygen vacancies.XRD proves that the derivatives transform into the similar annealing-deficient Fe-MILs.Radical trap experiments and ESR prove that the main active substance in the degradation of TC was OH·.TC can also promoted the formation of·O₂^-and ¹O₂,and participate in the decolorization and degradation of dye pollutants.The Mars-van Krevelen mechanism and the surface electron transfer mechanism between the catalyst and pollutants could explain the thermocatalytic degradation of TC and Rh B,and the possible thermocatalytic degradation products were analyzed by LC-MS.