Post-Synthetic Modification Of Metal-Organic Frameworks Withβ-Diketone And The Performance And Mechanism Of Photocatalytic Degradation Of Organic Pollutants

Metal-organic frameworks（MOFs）as novel,porous,adjustable and recyclable hybrid materials bridged by inorganic metal nodes and organic ligands,are considered to be a class of photocatalysts with good application prospects.However,the visible light capture efficiency of MOFs is generally very low,which severely limits the green application of such materials.Currently,two strategies have been developed to improve the visible light responsive photocatalytic activity of MOFs,namely,post-synthetic modification with auxochromic groups or immobilization of photosensitizers.Amino functionalization is the most widely used strategy.However,the absorption edges of amino substituted MOFs were still confined in a short wavelength range（generally less than 450 nm）.The second strategy is to immobilize photosensitizers into the structure of MOFs.MOFs obtained through the second strategy tend to produce reactive oxygen species（ROS）through energy transfer,especially singlet oxygen.However,this strategy requires complex synthesis routes or precious metals,and has disadvantages such as high cost and poor availability of precious metals.In order to solve the above problems,the universality ofβ-diketone post-synthetic modification strategy and its influence on the physicochemical properties of MOFs were studied,and the performance and mechanism of the modified MOFs materials（AA-MOFs）for photocatalytic degradation of organic pollutants were discussed.The main conclusions of this research are as follows:（1）The universality ofβ-diketone post-synthetic modification strategy was confirmed.The amino substituted MOFs with metal nodes of Ti⁴⁺,Cr³⁺and Al³⁺were employed as precursors,the amino group was azidated,and then the condensation reaction between acetylacetone and azide was used to make the diketone structure loaded into MOFs.Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy and hydrogen nuclear magnetic resonance were employed to prove thatβ-diketone was successfully integrated into the side chain of MOFs ligand.Through scanning electron microscopy,transmission electron microscopy,specific surface area calculation,particle size analysis,pore size analysis,and adsorption performance evaluation,it was proved thatβ-diketone post-synthetic modification strategy had no significant effect on the morphology,particle size distribution,specific surface area,pore size distribution and adsorption performance of MOFs.The band structure of MOFs before and after modification was determined by ultraviolet-visible diffuse reflectance spectroscopy and Mott-Schottky curve analysis,which proved thatβ-diketone post-synthetic modification could effectively improve the visible light capture ability of MOFs and reduce the band gap.The above results indicate thatβ-diketone post-synthetic modification strategy is universal,and the modification can enhance the visible light capture efficiency of MOFs and reduce the band gap width.（2）The photocatalytic performance of AA-MOFs was investigated.The photocatalytic activity of MIL-125-Xs,MIL-101（Cr）-Xs,and MIL-101（Al）-Xs was tested with decolorization of dyes and activation of molecular oxygen,which provedβ-diketone post-synthetic modification could significantly improve the performance of MOFs for photocatalytic activation of molecular oxygen and degradation of various organic pollutants.The above results indicate thatβ-diketone post-synthetic modification could enhance the visible light catalytic activity of MOFs.（3）The main ROS in the AA-MOFs photocatalytic system were analyzed,and the mechanism of singlet oxygen generation in the system was clarified.Through ROS quenching experiments,electron paramagnetic resonance spectroscopy and singlet oxygen phosphorescence detection,it was proved that in the AA-MOFs photocatalytic system,the main ROS were not conventional hydroxyl radicals and superoxide anions,but singlet oxygen.The addition of disodium ethylenediaminetetraacetate completely inhibited the photocatalytic decolorization of Acid Orange 7,indicating that holes played a key role in the formation of the triplet state.Through triplet quenching experiments,fluorescence and phosphorescence spectroscopy,electrochemical impedance spectroscopy and transient photocurrent analysis,it was proved that the generation of singlet oxygen depends on triplet energy transfer.Combining electron paramagnetic resonance spectroscopy,sputtering X-ray photoelectron spectroscopy and the analysis of the role of holes in photocatalysis,the path of generation of singlet oxygen was clarified:Acetylacetone modification induced the formation of oxygen vacancies in MOFs.The oxygen vacancies acted as trap states for photogenerated electrons and holes,and promoted the formation of singlet excitons.Singlet excitons became triplet excitons through intersystem crossing.The excitons activated the ground state oxygen to singlet oxygen through energy transfer.This research provides a new strategy for the design of MOFs-based photocatalysts,and provides a reference for understanding the exciton behavior and photocatalytic mechanism of MOFs.