Facile synthesis of bimetallic zeolitic imidazolate frameworks with tailored structures

• Main Author: Yao, Bingqing
• Other Authors: Dong Zhili
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Nanostructured materials
• Online Access: https://hdl.handle.net/10356/149620
• Institution: Nanyang Technological University

Recently, metal-organic frameworks (MOFs) have become a promising candidate for many applications such as catalysis, gas storage, sensors and drug delivery. They are composed of self-assembled metal ions or clusters with organic ligands, leading to their ultrahigh surface area, diversity of pore structure and multifunctionality features. Among MOFs, zeolitic-imidazolate frameworks (ZIFs) are becoming a shining star owing to their uniform pore size and structural stability. In the past decades, MOFs with tailored and sophisticated structures have attracted great attention for their capability of integrating different functional materials into one particle and further enhancing functionalities. However, the synthesis of MOFs with well-defined morphology, composition as well as tailored structure via a mild and efficient method remains a big challenge. In this thesis, firstly, an optimized solvothermal method was developed to synthesize bimetallic ZIFs at room temperature. The as-prepared bimetallic ZIFs microcrystals show rhombic dodecahedron shape with uniform particle size at about 1 μm and high surface area of over 1400 m2/g. Influence of cobalt doping on the crystal structure, morphology, surface area and pore volume was investigated in detail. Among them, Zn1Co2-ZIF has the highest specific surface area, thus, H2 and CO2 uptake capabilities of Zn1Co2-ZIF were further investigated. Moreover, a hollow structure inside ZIF particles was successfully obtained via a facile chemical etching process, while the cavity is controllable by tuning the etching time. Then, in order to realize the effective and efficient cobalt doping in ZIF-8 framework, a rapid and facile strategy was reported to synthesize ZIF-8, Co/ZIF-8 and ZIF-67 nanoparticles via ultrasound-assisted method. Interestingly, a monotonic increase in particle size was obtained when gradually doping cobalt in ZIF-8. In addition, the nucleation and growth mechanism of ZIF nanoparticles was theoretically described. This can provide a novel strategy for rational design of bimetallic MOFs based on different nucleation and growth kinetics of metal salts. Moreover, hierarchical porous structure of bimetallic ZIFs with high surface area was introduced with ultrasound irradiation, making them highly suitable for heterogeneous catalysis. Finally, the Co/ZIF-8 exhibits excellent catalytic activity and stability towards the decolorization of organic dye in water. Therefore, this synthetic strategy will open up opportunities to the predictable design and controlled synthesis of bimetallic MOF nanostructures with desired structures and compositions for catalytic applications. In addition, based on the benefit of ultrasound irradiation on nucleation and growth process during bimetallic ZIFs synthesis, a facile and rapid seed-mediated method was developed to fabricate ZIF-67@ZIF-8 via epitaxial growth of ZIF-8 shell on ZIF-67 seed under ultrasound irradiation at room temperature. The core-shelled nanoparticles exhibit uniform particle size with rhombic dodecahedron shape and high surface area of over 2000 m2/g. Impressively, it was demonstrated to be a single-crystalline material, which is highly desirable for heterogeneous photocatalysis. Then, hollow structure was introduced into ZIF-67@ZIF-8 via chemical etching process accompanied with surface modification. Moreover, the H2 and CO2 uptake capabilities of the core-shelled ZIF-67@ZIF-8 and hollow ZIF-67@ZIF-8 with acid etching treatment as well as their ability about removal of cationic dye were further investigated.