Active Sites Modified On The Metal-Organic Framework MOF-808 And Its Catalytic Performance

Using single ion catalysts as precursors,ultra-fine and controllable noble metal nanoparticles were synthesized by directional reduction method,which maintained the highly active catalytic centers of single ions,and the active sites uniformly dispersed.Thus,the stability and activity of the catalyst are guaranteed,and the conversion strategy from single ion to nanoparticle is realized.We used a simple grafting method to disperse and anchor the ligand in porous MOF-808.Palladium ions and the ligand formed stable coordination bonds and fixed on the ligand to form a new type of highly active and dispersed organo-metallic skeleton material(Pd@MOF-808-Pza).The effective grafting of the ligands was confirmed by infrared spectroscopy,nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy.DFT density functional theory calculation that it has lower energy and tends to form five-membered ring coordination.Then Pd-nano@MOF-808-Pza catalyst was prepared by reduction method.The HAADF-STEM and EDS characterization revealed that the sub 3 ～ 6 nm intermetallic Pd nanoparticles were uniformly dispersed in MOF-808.Pd-nano@MOF-808-Pza shows excellent catalytic activity for hydrogenation of nitrogen-containing unsaturated compounds.More importantly,we have tested and expanded substrates for reactions such as Knoevenagel-Hydrogenation cascade and Suzuki-Hydrogenation cascade,and obtained catalytic yields of more than 99%.The porous structure of MOF-808 enhances the interaction between Pd ions and ligands,thus stabilizing and maintaining the dispersion of the nano-catalyst,preventing the aggregation of Pd nanoparticles and improving its catalytic activity in the reaction system.We also prepared Pd-nano@MOF-808 for direct reduction of palladium without ligands.The ultra-small metal particles have more advantages than large particle clusters in terms of catalytic performance and reutilization in hydrogenation of nitrogen-containing unsaturated compounds.This is because the ligand-dispersed Pd nanoparticles can not only form a uniform and stable structure,but also better adsorb on the surface of MOF-808,which is not easy to remove.This strategy is conducted from the assembly method of a single atomic catalyst,which enables the preparation of ultrafine nanoparticles that remain active and avoid the polymerization of precious metals,Providing a new idea in synthesis.Based on this idea,Pza was grafted onto MOF-808 in the same way,and then loaded with cobalt acetate as the catalytic active center to prepare the catalyst Co@MOF-808-Pza.The ligand was linked to MOF-808 by infrared spectroscopy,nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy.Co@MOF-808-Pza has superior catalytic activity for Biginelli reaction compared with the reported catalysts,more moderate reaction conditions,less time,the expansion of the substrate and the catalytic cycle stability is also within the expectations of the experiment,it provides a better experimental conditions for the synthesis of pharmaceutical intermediates,drug synthesis for reasonable development provides a new way of synthesis.