Preparation and Catalytic Performance of 3D Hierarchically Porous Cobalt-Based Nanomaterials Derived from MOFs

Author:Chen Hui Rong

Supervisor:li ying wei


Degree Year:2019





Metal-organic frameworks(MOFs)are a new class of ordered network materials formed by the self-assembly of inorganic metal cations and bridging organic linkers.Compared with traditional porous materials,MOFs have unique properties like ultra-low densities,high specific surface areas,and tunable pore structures,which have recently attracted increasing research interest and have been used in various fields,such as catalysis.However,due to the limitation of low catalytic activity,insufficient electronic conductivity and chemical stability of most pristine MOFs,converting MOFs into metal compounds,carbonaceous materials,or their composites are of great significance to realized major breakthrough for catalysis-related technologies.In this thesis,we aimed to develop novel and highly efficient MOFs-based catalysts for some industrial important reaction such as photocatalysis,oxidation and hydrogenation by employing MOFs with sophisticated structure as sacrificing template and explore the structure-performance relationships.The main contents of this thesis and experimental results are as follows:A novel ZnO@C-N-Co core-shell nanocomposite towards efficient degradation of organic pollutants was designed and fabricated by directly pyrolyzing a hollow Zn/Co-ZIF matrix consisting of a ZIF-8 shell.The unique core-shell structure can make the intermediate ZnO much more stable during the reaction.And the porous carbon shell can not only provide a high BET specific area and thus high adsorption capability for reactants,but also inhibit the recombination of photogenerated electrons and holes.With many advantages in interms of composition and unique core-shell structure,the as-prepared ZnO@C-N-Co showed enhanced activity and good magnetic recyclability in the photocatalytic degradation of methyl orange,as compared to pure ZnO under the same conditions.Moreover,photocatalytic efficiency of ZnO@C-N-Co showed negligible changes even after five recycles,demonstrating good long-term stability.It was found that the structure of precursors and calcination temperature have important influence on the final structure and morphology of catalysts.Based on systematical optimization of the pyrolysis temperature and the shell-thickness of Zn/Co-ZIFs,a highly efficient and recyclable yolk-shell Co@C-N nanoreactor with controllable properties was fabricated by the direct thermolysis of a hollow Zn/Co-ZIF precursor.The uniform Co nanoparticles(NPs)were completely anchored and stabilized by the Co-ZIF derived N-doped carbon nanosheets,which were further confined by a permeable and robust N-doped carbon(C-N)shell to protect the Co NPs against leaching and also enabled the reaction to take place in the hollow void.Consequently,the optimal yolk-shell Co@C-N nanoreactor showed a significantly enhanced catalytic activity for the aqueous oxidation of alcohols,yielding >99% conversion under atmospheric air and base-free conditions,which was much higher than that of the solid counterparts derived from pure ZIF-67 and solid core-shell ZIF-67@ZIF-8 precursors(with 14% and 59% conversion under the same reaction condition,respectively).The enhanced catalytic activity should be attributed to the yolk-shell structure that could facilitate the transport of reactant/product and the strong interaction between the Co NPs and N-doped carbon nanosheet to aff ord positive synergistic eff ects.Moreover,this catalyst also showed good recyclability,magnetically reusability,and general applicability for a broad substrate scope.Compared with the solid and single shell counterpart,multishell hollow systems have always gained a lot of attention due to their attractive properties such as large surface areas,multiphase heterogeneous interfaces,improved mass diffusion properties,excellent loading capacity and thus more elaborate functionalities.Consequently,a general strategy for the construction of multishell hollow metal/nitrogen/carbon dodecahedrons(metal@NC)with well-defined and precisely controlled architectures was developed.This strategy is based on the pyrolysis of multilayer solid ZIFs prepared by a step-by-step crystal growth approach,which enables the precise controls over the shell number and composition of the resultant hollow metal@NC.Impressively,our strategy can be further extended to the synthesis of yolk@multishell hollow structures or multishell hollow structures that are assembled by carbon nanotubes.The multishell hollow structures can efficiently facilitate the mass diffusion,which together with the high dispersity,are responsible for their significantly enhanced catalytic performances for the selective hydrogenation of biomass-derived furfural to cyclopentanol.Interestingly,the cobalt-based materials could be easily separated from the solution after reaction by using an external magnetic field.Furthermore,the catalyst was reusable and retained high catalytic activity even after recycling for a number of times.