Synthesis and Catalytic Applications of Metal-Organic Framework Compositions

Author:Chen Xiao Dong

Supervisor:li ying wei

Database:Doctor

Degree Year:2019

Download:165

Pages:156

Size:10484K

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Metal-organic frameworks(MOFs)are a new class of porous materials that are constructed by organic bridging ligands and inorganic secondary building units.Due to their excellent physical and chemical properties,such as large specific surface areas and high porosities,MOFs have shown great potential and special advantages in catalytic applications over the past two decades.Involving abundant coordination unsaturated metal sites and active sites on the functional organic ligands,pristine MOFs can serve as acid or base catalysts.However,due to the low catalytic activity and stability,their applications in catalysis are limited.Design and preparation of high-performance MOFs-based catalysts are of great significance for advancing the practical application of MOFs in the field of catalysis.In this thesis,MOFs are employed as support to incorporate catalytic active sites or as precursors to derive porous carbon composites.A series of new MOFs-based catalysts with high catalytic activity and high stability have been designed and developed for important catalytic applications,such as industrial hydrogenation,pollutant catalytic conversion and energy conversion.The relationships between the structural properties of the catalyst and the catalytic performance are also investigated in detail.The main contents and findings of the thesis are as follows:Due to its high porosity,high structural stability,and abundant Lewis acid sites,MIL-101 is selected as support to load highly active Pd nanoparticles through a colloid precipitation method.The prepared Pd/MIL-101 presents the characteristics of a bifunctional catalyst.Due to the synergistic effect of Lewis acidity and Pd,Pd/MIL-101 can simultaneously catalyze the reduction of nitro group,hydrogenation of benzene ring and cross-coupling reaction of amino groups.In consequence,the highly efficient conversion from nitrobenzene to dicyclohexylamine can be achieved.In addition,we find that the polarity of the solvents play an important role in the hydrogenation of nitrobenzene,and the aniline derivatives or the dicyclohexylamine derivatives can be obtained at will by adjusting the polarity of the solvent.A series of PS@Zn1-xCox-ZIF with various molar ratio of Zn2+/Co2+and a core-shell structure are synthesized using polystyrene beads(PS)as template.By carbonizing the above materials at high temperature,a series of Co/N co-doped porous carbon materials(Co-HNCS-x)with hollow structure are prepared.Suitable Zn/Co molar ratio in the precursor material can endow the corresponding derived porous materials with large specific surface area and pore volume,high N content,ultrafine Co nanoparticles and high degree of graphitization.Moreover,the unique hollow structure can not only reduce the diffusion resistance but also increase the exposure of the catalytic active sites.When employed as ORR catalyst,Co-HNCS-0.2 exhibits significantly improved catalytic activity,whose half-wave potential reaches 0.82 V,which is far superior to that of single-metal ZIFs-derived Co-HNCS and HNCS hollow materials,and also superior to bimetallic ZIFs-derived Co-NC-0.2 solid material.Moreover,the catalyst shows 4 electron mechanism and good stability.A simple and efficient melamine and MOF co-calcination strategy is developed to prepare Fe@NMC-x catalysts with both highly dispersed Fe atoms and 3D interconnected mesoporous structure.An appropriate amount of melamine in the precursor allows the generation of rich mesopores and increased density of Fe-Nx moieties in the Fe-ZIF-8 derived materials.Excess amount of melamine in the precursor could convert the Fe-ZIF-8 derivative into carbon nanotubes,however,its specific surface area and the amount of active sites decrease.Among the Fe@NMC-x catalysts,Fe@NMC-1 possessing the advantages both in structure and composition,can efficiently catalyze the ORR reaction in both acid and alkaline media through the 4 electron pathway.In addition,it also exhibits much better stability and methanol tolerance than commercial Pt/C catalyst.These results would provide a viable approach for the preparation of highly efficient non-Pt-based ORR catalysts.It has been reported that doping heteroatoms in porous carbons can significantly change their surface chemistry and catalytic properties.On the other hand,constructing unique hollow mesoporous structure in catalysts can promote the rapid diffusion of the reactants and imrove the utilization of the internal active sites.To combine the above two merits in one catalyst,we have developed a facile strategy to prepared B,N co-doped hollow mesoporous carbons by directly calcining the composites of ZIF-8,melamine and phenylboronic acid.The materials are subsequently applied to the reaction of 4-nitrophenol degradation.The results show that both B and N atoms doping in porous carbon can promote the catalytic reaction,and B and N co-doping leads to much better catalytic performance than single atom(B or N)doping.Among them,BN/HMC-0.05 exhibits the best catalytic performance,which can convert 4-nitrophenol to 4-aminophenol completely within 3.5 min,surpassing other porous carbon composites.In addition,the catalyst also shows good stability.