Encapsulation Of Metal Nanostructures In MOFs And Their Catalytic Applications

Metal–organic frameworks(MOFs)are constructed by bridging organic linkers and inorganic secondary building units.Compared with traditional porous materials,MOFs possess the advantages of controllable pore size,shape,and functionality,which make them ideal platforms for the encapsulation of metal catalysts.Selectively encapsulating metal nanostructures in the pores of MOFs and further tailoring the size,composition and structure of the embedded nanostructures are very important for the design of advanced heterogeneous catalysts with high stability,activity and selectivity.In this thesis,we developed novel strategies for the encapsulation of metal ions and nanoparticles(NPs)to achieve advanced MOFs-based catalysts by employing bipyridine-based MOFs(e.g.,UiO-67 and MOF-253)as supports.Moreover,the structure-performance relationships were explored.The main contents of his thesis are as follows:Ligand grafting and mixed ligand direct incorporation strategies were developed for the synthesis of isolated Pd single sites uniformly distributed in the MOF network,enabling the as-prepared catalysts to show high activity and stability in the carbon-carbon coupling reactions of a wide range of aryl halides.The prepared MOF-253·0.05PdCl2 was shown to be highly active and selective for the Suzuki–Miyaura cross-coupling and Ullmann homocoupling of a wide range of aryl halides under mild reaction conditions,showing very good substrate tolerance.Pd(II)doped UiO-67 was highly efficient in Heck and Suzuki–Miyaura coupling reactions of chloroarenes bearing a wide range of substituents,giving the corresponding cross-coupling products in high yields(> 80%).Studies indicated that the 2,2'-bipyridine moieties in MOFs ligands showed strong coordinating ability to PdII,which could seperate and disperse the active metal sites,as well as provide electron-rich environment,thus allowing the as-synthesized catalysts to combine the advantages of both homo-and heterogeneous systems.In contrast to previous reports,we developed a novel synthesis strategy through introducing metal precursors by being anchored to the linker prior to the assembly of the MOF,achieving uniformly distributed metal NPs inside the cavities of MOFs,which lead to enhanced stability for the as-synthesized metal catalysts.This strategy involved the first immobilization of metal ions on t he organic ligands through anchoring sites,followed by assembling the functional ligands into MOFs.This methodology can avoid the different diffusion resistance between external and internal surfaces,and thus allow metal precursors to be easily deposited into the pores and evenly distributed within the MOF networks.Studies indicated that the superior catalytic activity and stability came from the synergetic effects of electron-donation and nano-confinement offered by the MOF framework,which could not achieved by using a traditional impregnation method.We further developed effective strategies such as “in situ incorporation of metal precursors”,“one-step encapsulation via a temperature control program”,and “one-pot synthesis” to simplify the preparation procedure with facile handling for easy up-scaling.A seed-mediated growth strategy was developed for the encapsulation of bimetallic core-shell and crown-jewel NPs into the MOF pores,achieving the structure and composition control of the embedded bimetallic NPs in the MOF pores.The reducing agents employed by traditional methods were dispersed in the solvent,thus could not avoid the individual nucleation and growth of the secondary metal as individual particles.As hydrogen could be dissociated into atomic hydrogen and spillover onto the surface of embedded Pd NPs,which possessed a strong reducing ability at room temperature thus could selectively direct the deposition of the second metal(e.g.,Ag,Ni,Cu,and Fe)onto Pd.By controlling the amount of transition metal on t he surface of Pd,we could achieve the encapsulation of bimetallic core-shell and crown-jewel NPs in the MOF pores.Studies indicated that the encapsulation of such unique structure in the MOF pores could allow the tuning of the surface electronic environment by intermetallic charge transfer and the structural strain effect,thus allowing the as-synthesized catalysts to show enhanced activity and selectivity than their monometallic counterparts.We demonstrated a new concept where covalent organic frameworks(COFs)doped with metal cations can be readily used as novel precursors for the in situ encapsulation of metal NPs into N doped hollow carbon spheres(NHCS).The eclipsed nitrogen atoms in adjacent layers of COFs can serve as the coordination sites for Pd,which could stabilize the as-formed Pd NPs during the annealing process and serve as both the carbon and nitrogen sources for the formation of NHCS.This proposed protocol simplified the synthesis process with the elimination of hard templates,which provided a new route for the preparation of metal NPs@NHCS composites.The composites exhibited excellent catalytic activity and selectivity in the hydrogenation of nitrobenzene and oxidation of alcohol,which may be attributed to the synergism of the porous hollow spheric structure,highly dispersed Pd NPs,and uniform distribution of N dopants on the materials.