Design And Synthesis Of Zr-MOF-Based Photocatalysts And Their Performance Towards Selective Oxidation Of Aromatic Alcohols

As a novel crystalline porous coordination polymer,metal-organic framework（MOF）,featuring functionality,high BET surface area,structural adjustability and abundant active sites,has been studied in the fields of gas adsorption,separation and storage,catalysis,drug delivery,supercapacitors,etc.Because of its metal-oxo clusters,MOF shows the potential of photocatalysis and has raised great attention.Therefore,rational design and synthesis of MOF-based photocatalysts will be of great significance for understanding the photocatalytic behavior of MOF and developing new porous photocatalysts.In this paper,the wide-band gap UiO-66 was modified by metal nanoparticles loading and multi-step post-synthetic ligand exchange.The morphology,structure,optical and electrochemical properties of MOF-based materials were analyzed.The relationship between structure and activity of the catalysts was investigated.Further more,the transfer route of photo-generated electrons in photocatalytic process was explored,and the possible mechanism of the reaction was proposed.For the first time,using advanced double-solvent approach,Cu（II）precursor was introduced both on the outer surface and in the cavities of UiO-66.After H₂ reduction,Cu/Cu@UiO-66 was obtained.Using only one kind of guest metal,Cu/Cu@UiO-66achieves the synergies of surface plasmon resonance and Schottky junction,and its photocatalytic activity is significantly improved.The characterization results indicate that Cu in Cu/Cu@UiO-66 can be divided into two parts:Cu on the surface of UiO-66has surface plasmon resonance effect,which can be excited by visible light to generate hot electrons,which broadens the light absorption range of MOF.Cu encapsulated in UiO-66 acts as electron acceptors and forms Cu-MOF Schottky junctions,which steers the photo-generated electrons to Cu to promote the separation of the photogenerated charge carriers.Based on the synergies,we explored the photo-generated electron transfer route in Cu/Cu@UiO-66 and the mechanism of photocatalytic selective oxidation of benzyl alcohol.The guest ligands such as citric acid（C）and 2-aminoterephthalic acid（N）were introduced into UiO-66 by multi-step post-synthetic ligand exchange to obtain N-C-U（UiO-66 which was exchanged with ligand C and ligand N sequentially）.Owing to the synergies of MOF defects and enhanced ligand-to-metal-cluster charge transfer,N-C-U exhibits the highest photocatalytic activity while maintaining good stability:After citric acid exchange,MOF defects are formed on the surface of UiO-66,which enriches photo-generated electrons and promotes photo-generated electron-hole separation;and the exchange of 2-aminoterephthalic acid partially heals MOF defects,improving the stability of the defect-containing materials.Simultaneously,the photoactive ligands introduced on the outer layer of N-C-U enhance the charge transfer of MOF ligand to metal-oxo and broaden the light absorption of MOF.We quantitatively analyzed the defect concentration by thermogravimetric curves,discussed the photo-generated charge transfer in N-C-U,and proposed the possible reaction mechanism.