Design,Preparation And Catalytic Performance Of Macroporous/Mesoporous MOFs And Their Derived Carbon Materials

Metal-organic frameworks(MOFs)are porous materials with periodic pores assembled by metal ions/clusters and organic ligands.MOFs have unique physicochemical properties such as controllable pore size and shape,modifiable surface chemical group etc.In recent years,MOFs have attracted extensive attention in the field of catalysis.However,the metal sites of MOFs are mostly saturated and most of MOFs only contain micropores,which greatly limit the accessibility of catalytically active sites.In this thesis,aiming at improving the accessibility of active sites of MOFs-based catalysts,two-dimensional ultrathin MOFs and three-dimensional hierarchically porous MOFs-based materials are fabricated.The relationships between the structural properties of the materials and their catalytic properties in the conversion of biomass based platform molecules are investigated.Furthermore,the catalytic reaction mechanisms of the designed MOFs and MOFs-based materials are explored.The main contents and results of this thesis are as follows.An ultrasound-assisted method is developed to fabricate ultrathin nickel imidazole-framed nanosheets self-assembled porous spheres(Ni-ZIF-UNS).The unique structure can effectively inhibit the stacking of two-dimensional ultrathin nanosheets and expose a large number of unsaturated-coordination metal sites,thus improving the accessibility of active sites.The as-prepared Ni-ZIF-UNS material shows high catalytic efficiencies in the Meerwein-Ponndorf-Verley reduction of ketones to alcohols,even for the organic molecules with large sizes,e.g.,4-(diphenylamino)benzaldehyde and 4-(di-p-tolylamino)benzaldehyde.In addition,Ni-ZIF-UNS also affords high catalytic activities and selectivities for the conversion ofα,β-unsaturated aldehydes.The reaction and characterization results indicate that the unsaturated Ni-N in the Ni-ZIF-UNS material is the main active site of the hydrogen transfer reaction.The ultrathin two-dimensional structure Ni-ZIF-UNS synthesized with the assistance of ultrasound can greatly improve the accessibility of the unsaturated site,thus furnishing excellent hydrogenation activity.The conversion rate is 5.2 times faster than that of Co-ZIF-UNS and 1.2times of Ni-ZIF-NS.ZnCo-ZIF@PS material is first prepared by using polystyrene spheres(PS)as templates.Then,nitrogen-doped carbon loaded with Co single atoms(3DOM-Co SAs/NC)with three dimensional ordered macropores are synthesized by one-step pyrolysis,employing ZnCo-ZIF@PS as precursor.The three-dimensional ordered macroporous structure of 3DOM-Co SAs/NC can promote the long-range mass transfer and expose more active site,and the monodisperse active Co sites may maximize the utilization of metal elements.3DOM-Co SAs/NC achieves 99%yield of methyl pyromucate in the oxidation esterification of furfural,much superior to the catalysts without three-dimensional ordered macroporous structures(C-Co SAs/NC and C-Co NPs/NC),and Co nanoparticles(3DOM-Co NPs/NC).Experimental and characterization results suggest that Co N_x might be the main active site for the oxidative esterification of furfural.Cobalt/nitrogen-doped carbon composites with three-dimensional(3D)ordered macropores and hollow walls(H-3DOM-Co/NC)are prepared,using a single-crystal ordered macroporous(SOM)ZIF-8@ZIF-67 as precursor.During the pyrolysis,the interconnected microporous structure of SOM-ZIF-8@ZIF-67 is mostly preserved,whereas the pore wall achieves a solid-to-hollow transformation with Co nanoparticles formed in the hollow walls.The 3D-ordered macroporous carbon skeleton may effectively promote long range mass transfer and the hollow wall can facilitate local accessibility of active sites.This unique structure greatly boosts its catalytic activity in the selective hydrogenation of biomass-derived furfural to cyclopentanol,much superior to its counterparts without this hierarchically porous structure.A facile tannic acid(TA)-etching strategy for constructing mesoporous and hollow structures in the walls of ordered macroporous ZnCo-ZIF single crystals is developed.The prepared H-3DOM-ZnCo ZIF/ZnCoTA material after TA etching not only inherits the 3DOM structure,but also has attractive mesoporous hollow-wall architecture.Furthermore,cobalt/nitrogen-doped carbon composites with 3DOM and mesoporous hollow-wall structures(H-3DOM-Co/ONC)are successfully synthesized by pyrolysis of H-3DOM-ZnCo ZIF/ZnCoTA.The ordered macropores can facilitate kinetic accessibility of the active sites,while mesoporous hollow-wall structure may further improve the electrochemically active surface areas.Moreover,the H-3DOM-Co/ONC material with abundant oxygen-containing functional groups can enhance the hydrophilcity and facilitate the wetting of catalyst by the electrolyte.The H-3DOM-Co/ONC catalyst exhibits outstanding electrocatalytic oxygen reduction activity in alkaline media,which is even comparable to that of the benchmark Pt/C catalyst.