Study On The Surface Modification Of Metal Organic Frameworks And Its Adsorption/Photocatalytic Properties

Metal organic framework materials（MOFs）are assembled by coordination between metal ions or clusters and organic ligands.MOFs have adjustable metal nodes,diverse ligand structures,rich and uniform channel structures,high specific surface area.Therefore,they not only provide large amounts of coordination sites,but also have excellent mass transfer performance.In addition,metal nodes and organic ligands in some series of MOFs have electrochemical and photochemical activities.As a result,the construction of MOF-based materials provides an effective path for adsorption and photocatalytic treatment on pollutants.In this work,a series of adsorption and photocatalytic materials were prepared based on MOFs by surface modification and element doping.The main results are as follows:（1）ZIF-67 was synthesized by a simple method.Methylene blue（MB）and tetracycline（TC）were selected as model substrates of dyes and antibiotics to study the selective adsorption of TC by ZIF-67.In the mixed solution（p H=6.5）with the concentration of MB and TC of 40 and 60 mg·L^-1respectively,the adsorption capacity is 192.3 mg·g^-1 and the adsorption selectivity is 92.6%.The maximum theoretical adsorption capacity of TC by ZIF-67 is 735.3 mg·g^-1.One reason of the selective adsorption of TC is that the large specific surface area(1085.6 cm²·g^-1)and rich pores of ZIF-67 provide a large number of adsorption sites for TC removal.Another reason is that the electropositive surface potential of ZIF-67 in p H=3.0-9.0 has electrostatic repulsion to cationic dye MB,while there is no adsorption barrier with TC component.The adsorption in the mixed system is a monolayer chemical adsorption process,and the electrostatic interaction,π-πinteraction and formation of hydrogen bond promote the adsorption of TC on ZIF-67.（2）Based on the surface electrical properties of ZIF-67,a strategy of adding highly electronegative phosphotungstic acid（HPW）to regulate the surface charge potential of ZIF-67 in the crystallization process was proposed.ZIF-67-n HPW composites were prepared by controlling the added mass（0.050,0.200,0.400 g）during synthesis.The successful loading of HPW on ZIF-67 was confirmed by various characterization methods.The cage structure in ZIF-67 was confirmed to has restrictions on HPW.Thanks to the loaded HPW,the composite shows strong electronegativity in the range of p H=3.0-11.0 and has good adsorption performance for MB.The maximum theoretical adsorption capacity of ZIF-67-0.2 HPW is 446.4 mg·g^-1.At the same time,p H values,temperature and coexisting ions have little impact on the performance of the adsorbent.The removal rate of MB solution(50 mg·L^-1)by ZIF-67-0.2 HPW can still reach 87.2%after 8cycles.（3）In order to realize the degradation treatment of organic pollutants,photocatalytic degradation was considered.A strategy of preparing MOF-based photocatalytic materials by element doping was proposed.MOF-199 was synthesized by solvothermal method,and different mass（0.002,0.020,0.030 g）of thiourea were added in the synthesis process to synthesize S-doped MOF-199-n S in situ for the photocatalytic degradation of TC.Compared with MOF-199,the band gap E_g of MOF-199-0.02S is reduced from 3.27 e V to 3.15 e V,and the charge transfer resistance of the material is also reduced.The degradation rate of TC by MOF-199-0.02S was increased from37.0%to 42.6%,and the photocatalytic degradation constant was 1.61 times that of MOF-199.MOF-199-n S requires less energy to generate electrons excited by light and has higher utilization efficiency of light energy.The smaller charge transfer resistance is conducive to the transfer of photogenerated electrons to the material surface and promotes the separation of photogenerated electrons and holes.After four cycles,the photocatalytic degradation rate of TC by MOF-199-0.02S decreased by only 4.8%.（4）Based on the work of MOF-199 element doping modification,a more environment-friendly photocatalytic material is developed.A strategy of in-situ S element doping with thiourea in the solvothermal synthesis of CAU-17（Bi-based MOF）is proposed.The characterization and experimental results show that the doped S elements promote the generation of oxygen vacancies in CAU-17,which can act as adsorption and catalytic sites,and promote the separation of photogenerated electrons and holes.Compared with CAU-17,the degradation rate of TC by CAU-17-0.02S was increased from 54.9%to 62.9%,and the photocatalytic degradation constant was1.44 times that of CAU-17.The E_g of CAU-17-0.02S is reduced from 3.57 e V of CAU-17 to 3.37e V,and the material is more likely to produce photogenerated electrons when exposed to light.At the same time,its charge transfer resistance is also smaller than that of CAU-17,which is conducive to the transfer of photogenerated electrons and the improvement of TC degradation performance.The photocatalytic degradation rate of TC by CAU-17-0.02S decreased by 10.1%after four cycles.