Preparation Of Metal-Organic Frameworks And The Conposites Based On Waste Plastics

Plastic waste and water pollution brought about severe energy crises and environmental pollution.Photo-Fenton and photocatalysis,as emerging advanced oxidation technologies,were cost-effective green methods for removing pollutants from the aqueous phase.Fe-MOFs were widely used in photo-Fenton and photocatalysis because of their high chemical stability,excellent photo responsiveness and abundant iron-unsaturated active sites.However,Fe-MOFs were usually synthesized by solvothermal methods with expensive organic ligands,requiring harsh synthetic conditions and energy-consuming processes.Waste polyethylene terephthalate（PET）,one of the most commonly used plastics today,had a structural unit of terephthalic acid（TPA）which was one of the most widely used organic ligands for the synthesis of MOFs.If TPA could be extracted from waste PET plastic and converted into high-value-added Fe-MOFs to remove pollutants from water,it would have a vital resource and environmental significance.Dielectric barrier discharge（DBD）low-temperature plasma technology provided an effective means for rapidly preparing Fe-MOFs.In this paper,two kinds of Fe-MOFs composites were prepared by two-step method and one-step method,respectively,using DBD low-temperature plasma technology with waste PET plastic as the organic ligand source of MOFs.Fe-MOFs composites were used to degrade pollutants in water,aiming to achieve high-value recycling of waste plastics,reduce the preparation cost and preparation time of Fe-MOFs,as well as obtain materials with high catalytic performance.TPA was extracted from PET using water and ethylene glycol as solvents by a solvothermal method.Then Fe-MOFs and g-C₃N₄/Fe-MOFs composites were prepared with the assistance of DBD plasma.XRD,TEM characterization and photo-Fenton degradation of doxycycline experiments revealed that the Fe-MOFs formed from PET-derived TPA were relatively similar to those formed from commercially purchased TPA in terms of structure,morphology and properties.The g-C₃N₄/Fe-MOFs-10 showed better catalytic properties,which was synthesized under the conditions that Fe（NO₃）₃·9H₂O as the iron source,the discharge residence time of22.5 min,the discharge voltage of 7.3 k V,the metal-to-ligand ratio of 1:1,and the addition of g-C₃N₄10%of the raw material mass of Fe-MOFs.The degradation rate of doxycycline reached 90.6%with the addition of g-C₃N₄/Fe-MOFs-10 at 15 mg,doxycycline concentration of 50 mg/L,H₂O₂addition of 60μL,and p H 7 for 40 min of the photo-Fenton reaction.g-C₃N₄/Fe-MOFs composites exhibited good catalytic performance due to the formation of favourable heterojunctions,and the photo-Fenton degradation efficiency of doxycycline was 4.42 and 3.30 times that of Fe-MOFs and g-C₃N₄alone.The radical capture experiments revealed that·OH,·O₂^-and h⁺were the main active species,with h⁺occupying a key position.Under visible light irradiation,h⁺was concentrated on the surface of g-C₃N₄to react directly with doxycycline,and e^-was focused on the surface of Fe-MOFs to promote Fe³⁺/Fe²⁺conversion to enhance the Fenton reaction and react with H₂O₂and O₂to generate strong oxidative·OH and·O₂^-,which were involved in the degradation of doxycycline reaction.To improve the preparation efficiency and reduce energy consumption,Fe-MOFs/Fe₂O₃composites derived from waste PET were prepared in a one-step process by DBD plasma.PET depolymerization into TPA and Fe-MOFs composites formation was carried out simultaneously with DBD plasma.Compared with Fe₂O₃alone,Fe-MOFs/Fe₂O₃showed a larger specific surface area,which could expose more active sites.Also,Fe-MOFs/Fe₂O₃had a wider band gap,which could effectively suppress photogenerated electron-hole complexation and significantly enhance photocatalytic performance.The Fe-MOFs/Fe₂O₃-4 showed better photocatalytic performance,which were prepared under the conditions of Fe（NO₃）₃·9H₂O as the iron source,10 mmol PET addition,ethanol/water mixed solution as the solvent,9.5 k V discharge voltage and 20 min discharge residence time.Under visible light irradiation,the degradation rate of malachite green reached 99.3%after 30 min,when the catalyst added of 0.3 g,the concentration of malachite green of 15 mg/L.In the photocatalytic process,h⁺and·OH played the leading role for degradation.Fe-MOFs composites with high catalytic performance were successfully synthesized with DBD plasma derived from waste plastics,which could effectively protect the environment and save energy,providing an idea for the large-scale,low-cost and high-efficiency preparation of MOFs materials and green treatment of wastewater.