Controlled Preparation And Catalytic Properties Of Pd Supported On MOFs

Metal organic frameworks (MOFs) are a new class of crystal porous organic-inorganic hybrid material self-assembled by organic ligands and metal ions. They possess extremely high surface area and porosity, regular pore canal and tunable structure, for which they not only show extraordinary talent in the fields of gas storage and sorption, but also could be a promising candidate material for applications in heterogeneous catalysis. Based on the unique structure and property features of MOFs, simple and efficient methods under mild conditions have been developed for preparing novel and effective catalysts supported on MOFs applied in Suzuki-Miyaura coupling and selective hydrogenation reactions. The main research contents and results are listed as follow:(1) MOF-5synthesized via room temperature dynamic crystallization method was loaded with metal-organic compound Pd(C3H5)(C5H5) by metal-organic chemical vapor deposition (MOCVD) method. The solid-state nuclear resonance spectroscopy results revealed the chemisorption happened between the Pd precursor and MOF-5. The obtained Pd@MOF-5catalyst showed a good activity in the Suzuki coupling reaction of bromobenzene and phenylboronic acid with DMF as solvent and K2CO3as base in the first catalytic run. However, the crystal structure of MOF-5was completely destroyed during the following reaction runs, which caused a big loss of activity. MOFs with higher stability was needed to solve this problem.(2) The highly porous metal-organic framework ZIF-8synthesized via solvothermal method was loaded with Pd particles by MOCVD method under mild conditions. Pd nano-particles of about1.5-3.5nm in size were homogeneously dispersed on ZIF-8support. Pd@ZIF-8catalysts showed good activity in Suzuki-Miyaura coupling reactions for different kinds of aryl halides at70℃with MeOH as solvent and K2CO3as base. Besides, the catalyst can be used at least6times without dramatic loss of activity and obvious decomposition of the structure of ZIF-8as confirmed by PXRD and N2adsorption results, which can be attributed to high chemical stability of ZIF-8compared with MOF-5.(3) A basic anion exchange resin, Amberlite IRA-900, with the similar matrix structure with MOFs, was simply grounded with the precursor of Pd(C3H5)(C5H5). The final heterogeneous catalyst Pd@IRA-900was obtained through reduction at room temperature. The Pd@IRA-900catalysts showed a high activity for different kinds of aryl halides in Suzuki-Miyaura cross coupling reaction under mild and aerobic conditions in eco-friendly solvent. An excellent recyclability was also observed in the Suzuki-Miyaura coupling reaction of iodobenzene and phenylboronic acid. A bifunctional catalyst was also synthesized by pretreating the anion exchange resin support with the solution of sodium hydroxide. The modified catalyst referred as Pd@IRA-900(OH) successfully catalyzed the Suzuki-Miyaura reaction of iodobenzene and phenylboronic acid without base.(4) Catalysts with good dispersion can be gained by MOCVD method for its simpleness and the mild conditions. But, it is still very difficult to control the nano particles to site only inside the MOFs crystal, which caused a big waste of the porous property of MOFs. PVP protected cubic Pd nano-sol was added during the synthesis of ZIF-8. The affinity between PVP and ZIF-8made the encapsulation of ZIF-8around Pd nano-sol. The obtained PVP-Pd@ZIF-8showed excellent activity and almost100%selectivity for BED (1,4-butenediol) in the hydrogenation of BYD (1,4-butynediol). The outstanding catalytic performance can be attributed to the special morphology of Pd and the structure and composition feature of ZIF-8. Another acetate protected Pt nano-sol was supported on ZIF-8also showed very excellent activity and BED selectivity in the hydrogenation of BYD.