Structural Regulation And Catalytic Properties Of Metal-organic Framework Nanomaterials

Metal-organic frameworks (MOFs) possess tunable porous structures and large surface areas, as well as structural and functional tunability, which are promising for a variety of applications, including gas separation, drug delivery,and catalysis. Particularlly, utilizing the atomic-scale dispersed metal nodes,functional linkers, and/or encapsulated functional materials to achieve size-selective catalytic and tandem catalysis has been rapidly growing in the field of catalysis. However, for most of the research, solid MOFs were used,which will lead to the poor reaction efficiency due to the low utilization rate of the active sites in its interior and the low diffusion rate of molecules in the narrow channel of the MOF shell. In this dissertation, the composition and structure of MOF nanomaterials were regulated through the construction of multi-shelled hollow structure, hierarchical porous structure and composite materials, by the rational design of the nucleation and crystallization of MOF.The structure-activity relationship was studied by using the important industrial reactions, such as styrene oxidation and olefin hydrogenation, as the probes, and finally the efficient MOF-based catalysts were obtained. The main contents and conclusions are as follows:1. The construction of multi-shelled hollow structure can improve the efficiency of the active site within the material, and can concentrate the substrate in the hollow interiors between each layer, so as to improve its catalytic efficiency. We demonstrated a rational strategy to fabricate multi-shelled hollow MIL-101 (MOF with high thermal and chemical stability and large cavities) with single-crystalline shells via step-by-step crystal growth and subsequent etching processes. The formation mechanism of inhomogeneous MOF crystal and multi-shelled hollow MOF were studied.The cavity size and shell thickness of multi-shelled hollow MOF were well controlled by tuning the etching time, and the thickness of each shell was independently manipulated by tuning the growth processes. The properties of multi-shelled hollow MOF were studied by means of drug adsorption and catalytic oxidation of styrene. It was found that the construction of the multi-shelled hollow structure can effectively improve its catalytic activity and drug adsorption capacity. Prove the structural advantages of the multi-shell hollow structure in the above applications.2. The small pore size of MOF nanomaterials inherently limits the diffusion of catalytic substrate and their interaction with active sites in MOF,thereby limiting its application. Fabrication of MOF nanomaterials with hierarchical porous to enhance the molecular diffusion is an efficient strategy to solve this problem. Herein, chrome aluminum bimetallic MOF was synthesized under hydrothermal condition. Due to the stability difference of the coordination of metal ions and ligands, Al nodes in the framework is easier to be etched away in hot acetic acid solution, resulting in hierarchical porous MOFs. Furthermore, hierarchical porous MIL-101 shows higher catalytic activity than solid MIL-101 in benzyl alcohol oxidation reaction, due to its structural advantage. In addition, after loading Pd nanoparticles, the obtained Pd/hierarchical porous MIL-101 exhibited excellent catalytic performance.3. By localizing the encapsulated nanoparticles closer to the surface of MOFs, the resultant composites not only exhibit effective selectivity derived from MOF cavities, but also much more enhanced catalytic activity due to the improved mass transport of the molecules to and from the active sites within MOF. For this purpose, we developed a facile strategy to regulate the spatial localization of encapsulated nanoparticles in MOF, through in situ conversion of metal oxides template to MOFs and simultaneous encapsulation of the nanoparticles. The spatial localization of nanoparticles can be regulated at the interface between the MOF crystal and the metal oxide, or far away from the metal oxide template (close to the MOF surface) by optimization of the crystallization behavior of the MOF under different concentrations of organic ligands. In addition, the catalytic efficiency of the as-prepared NPs@MOF composites was significantly enhanced by control of the NPs as close as possible to the MOF surface, mainly due to the shortened distance of the reactants from the surface of the MOF to the active sites of NPs encapsulated in MOF. Meanwhile, the size- and shape-selective behavior that originates from the microporous nature of the MOF component was maintained.4. In the practical application of the catalysts, the powder catalyst has the problems of difficult recovery, high bed pressure drop and poor mechanical properties. We constructed supported nanoarray structured catalyst to solve these problems. Thin bimetallic nanosheet arrays were firstly synthesized by a two-step hydrothermal method, and then used as supports to deposit and stabilize gold nanoparticles. The obtained structured catalyst shows excellent catalytic activity and stability in the reduction of p-nitrophenol, which was contributed to the evenly arranged nanosheet arrays and the uniformly deposited Au nanoparticles. In addition, the structured catalyst could be easily recycled owing to the unique architecture of nanoarrays.