Design And Synthesis Of Metal-organic Frameworks And Their Heterojunction For Water Pruification

Water pollution caused by the overuse of dyes and antibiotics is part of the most severe global environmental concerns and threatens the sustainability of life on the earth,which arises from the combination of population growth,industrialization,and urbanization.Persistent organic pollutants(POPs)including dyes and antibiotics,as an important class of toxic and recalcitrant organic pollutants,have been one of the main sources of water pollution.As a highly efficient,economical and environmentally friendly“green”technology,photocatalysis has become one of the prospective treatment methods for organic pollutants elimination.Metal-organic frameworks(MOF)are an important class of crystalline functional materials with a well-defined porous structure composed of metal centers(nodes)and bridging organic ligands(linkers).MOFs,featuring semiconductor-like behavior,have recently captured broad interest toward adsorption and photocatalytic purification because of their unique merits,such as uniform and tunable porous structure,high-density coordination-unsaturated metal sites and high surface areas,etc.However,most of MOF present in the form of particles or powder and the poor processability significantly hinders their extensive applications.At the same time,the direct application of nanoparticles will cause difficulties in recycle with extra costs and safety concerns.In addition,nanoparticles tend to aggregate in practical applications,which dramatically decreases the catalytic efficiency.These problems can be overcome by immobilizing the nanoparticles onto porous supports or desired substrates,such as carbon cloth and functionalized fibrous membranes.However,these single-component MOF are difficult to achieve rapid separation of charge carriers because of the poor electrical conductivity,resulting in unsatisfactory photocatalytic efficiency.Construction of porous photoactive MOF-based composite systems is regarded as one of the most effective strategies to reduce the recombination efficiency of photogenetated electron-holes.In addition,introducing oxidants to produce free radicals like persulfate(PS)or hydrogen peroxide(H2O2)as electron acceptors is an advisable way to accelerate the separation and migration of photo-generated electron-hole pairs resulting in the production of during photocatalytic process.Through the synergistic effect of the catalyst and the oxidant,the degradation efficiency of the target pollutant can be further improved.Based on these,this study combines a summary of the most recent research on water-stable MOF-based photocatalysts,the novel functional materials based on functionalized MOF and MOF-semiconductor heterojunction were developed and applied to remove the organics from wastewater.Photocatalytic performance evaluation was evaluated by a series of physicochemical characterizations and the analysis of catalytic mechanism was also discussed.The validity and reliability of the simple approaches were confirmed by above analysis.The results of the research not only supplied new ideas and methods for designing and preparing functional materials,but also provided a scientific basis for the degradation of organic contaminants.The main contents and achievements of the paper were summarized as follows:(1)The visible-light responsive Ti-based MOF/polymer fibrous hybrid membranes were successfully prepared via electrospinning technique followed by a solvothermal method in this study.MIL-125-NH₂ particles were firmly adhered to the surface of the fibers and the resulting MOF hybrid nanofibers showed abundant pores,large specific area(452 m² g^-1)and easy recovery and regeneration.The membranes exhibited great adsorption capacity to methylene blue and sodium fluorescein,and the adsorption was dominated by steric hindrance of dye molecule,not the?–?interactions and the zeta potential.The rapid adsorption of dye occurred in the early time,more importantly,the MOF membranes displayed an efficient photoactivity for the degradation of organic dyes under visible light and could be easily separated for reuse.The enhanced photocatalytic performance of the composites could be attributed to the introduced H2O2.It not only formed reactive·OH(H2O2+e^-?·OH+OH^-)but also captured the free electron to reduce recombination rate of the electron and hole.The performance of the MOF membranes remained unchanged after three reaction runs.This work demonstrated a direction for development of practical photocatalytic materials for the application in wastewater treatment.(2)Designing recyclable photocatalysts with high activity and stability has drawn considerable attention in the fields of sewage treatment.A series of heterojunctions constructed by zirconium-based MOF(Ui O-66-NH2)and tungsten trioxide(WO3)was immobilized on carbon cloth via a facile solvothermal method,resulting in highly recyclable photocatalysts.The surface of the carbon fiber was uniformly covered by pine-needle-like WO3 with high density.A dense layer of globular Ui O-66-NH2 with 40-90 nm in diameter was in intimate contact with pine-needle-like WO3 to form heterojunction,which was beneficial for quick charge transfer.Results showed that the optimal heterojunction photocatalyst UWC-1.2 exhibited excellent photocatalytic degradation efficiency for the removal of tetracycline(TC)from water,for which nearly 100%of TC was degraded within60 min under visible light.Trapping experiments and electron spin resonance(ESR)spectra analyses demonstrate that the superoxide radicals(·O2^-)and photogenerated hole(h⁺)played a dominant role in the degradation process.Excellent photocatalytic activity was dominantly attributed to the effective separation of photoinduced carriers in this type-?heterostructure system.Moreover,the possible photocatalytic oxidation degradation pathway was confirmed by analyzing intermediates using liquid chromatography mass spectrometry(LC-MS).This study offerred a highly efficient strategy to design recyclable heterojunction photocatalysts for the degradation of refractory antibiotics in sewage.(3)The immobilization of powdery photocatalysts has always been a key technology for the practical application in waste-water purification.Photocatalysis technology has great potential to become one of the most promising solutions for environmental pollution,and a prerequisite for the photocatalytic application was to obtain efficient,easily recyclable and large-area photocatalysts with nanostructures.Meanwhile,the composites evidenced great specific surface area and excellent photoactivity.To solve these problems,with carbon cloth(CC)as the fixing substrate,a series of MIL-101(Fe)/WO3 heterojunction were constructed as a model on CC by a solvothermal method.The growth of MIL-101(Fe)nanodots with 5-10nm as a porous co-photocatalyst to decorate WO3 nanorods.In addition,because the existence of coordinatively unsaturated Fe sites in the abundant surface of MIL-101(Fe),by introducing oxidants like persulfate(PS)as electron acceptors was an advisable way to significantly improve the PS activation efficiency in the generation of·OH and·SO₄^-radicals under visible light.Among all composite materials with different ratios,the optimum heterojunction photocatalyst MWC-1.5 exhibited the best photocatalytic degradation efficiency in the presence of persulfate under visible light.MWC-1.5 could remove 100%TC in 20 min,which was three times compared to that by WO3/CC.The excellent photocatalytic activity of the heterojunction could be attributed to two reasons.One was the suppressing recombination of photogenerated electron-hole pairs by the Z-scheme heterojunction scheme,the other was the formation of·SO4^-radicals originated from the catalysts activated persulfate.In addition,the degradation rate of TC after five cycles was still above 95%,indicating the good recyclability and reusability of MWC-1.5.This research revealed that the synergetic catalytic effects of MIL-101(Fe)/WO3 and persulfate in the photocatalysis possessed great potential in wastewater remediation.