Rational Construction Of Functional Metal-organic Frameworks(MOFs) And Their Applications In Adsorption

Metal-organic frameworks(MOFs)have been widely used in adsorption and separation due to their advantages of high specific surface area,adjustable pore size,and ease of modification.The design and preparation of high-efficiency,high-selectivity and high-stability MOFs are the focus of the research.The use of MOFs for the adsorption of hexavalent uranium U(VI)has been reported,but reported MOFs either have low stability or have low adsorption efficiency.Based on the high stability MOFs reported in the literature,MOFs generate a large number of unsaturated coordination sites by the modulation of ligand defects and metal nodes to improve the adsorption of U(VI)on MOFs.By studying the structure-activity relationship of MOFs in adsorption applications,the development of highly efficient and highly stable U(VI)adsorbents based on MOFs was developed to provide potential materials for spent fuel reprocessing and uranium-containing wastewater treatment.The main findings and innovations in this article are as follows:(1)A series porous metal-organic frameworks MIL-100(M)(M=Al,Fe,Cr)with the same MTN topology was prepared by a hydrothermal method.The structure and surface properties of the prepared materials were characterized by SEM,nitrogen adsorption/desorption and Zeta potential measurements.The adsorption behavior of U(VI)on the MOFs by the static batch method was systematically studied.The results show that MIL-100(Al,Fe)has good adsorption performance to U(VI),respectively.Under the condition of p H=5.0,the adsorption capacity reaches 210 mg/g and 120 mg/g,respectively,while MIL-100(Cr)under the same conditions hardly adsorbs U(VI).The adsorption kinetics of U(VI)on MIL-100(Al,Fe)accorded with the pseudo-second-order kinetics model,indicating that was a chemical adsorption process;the adsorption isotherm conformed to the Langmuir adsorption model,indicating that the U(VI)was uniformly adsorbed on a single layer.Combined with Zeta potential and solid-state NMR(27Al NMR),it was found that MIL-100(Al)mainly adsorbs U(VI)through terminal hydroxyl group(Al-OH)coordination and ion exchange,while MIL-100(Cr)surface end Hydroxy groups have low cation-binding capacity and shown inertness to U(VI)adsorptions.This study shows that the metal nodes change the surface properties of the MOFs themselves,which in turn affect their U(VI)adsorption performance.(2)A series of Ui O-66-type MOFs were prepared by introducing modulater in the reaction mixture.(Ui O-66-n D,n represents the proportion of molecular modifiers,D represents defects.).The characterization of the defected MOFs was performed using TGA,N2adsorption/desorption,X-ray diffraction(XRD)and positron annihilation spectroscopy(PALS).The adsorption behavior of U(VI)with different defect Ui O-66 s was studied.At p H 5.0,Ui O-66-20 D has a saturated adsorption capacity of 350 mg/g,which reaches the adsorption equilibrium within 30 min.Under the same conditions,Ui O-66 with perfect crystal form has almost no adsorption to U(VI).By molecular dynamics simulation,it was found that the diffusion free energy of uranyl hydrate [UO2(H2O)5]2+ in the perfect crystal form Ui O-66 was 31kcal/mol,and that in the defective Ui O-66-20 D was 17.4 Kcal/mol,thus indicating that U(VI)is more easily diffused in the defected MOFs.For the first time,the highly efficient and selective adsorption of U(VI)by deficient MOFs has been reported.The formation of defect enhances theporosity of the MOFs itself and introduces a large number of active sites such as hydroxyl groups,thereby significantly improving the U(VI)adsorption performance of the MOFs.At the same time,the excellent hydrothermal stability and acid resistance of Ui O-66 are maintained in the defect process.This work provides new stratigies for the development of lanthanide solid-phase adsorbents based on MOFs.