Synthesis and Characterization of Iso-Reticular Metal-Organic Frameworks and Their Applications for Gas Separations
Nanoporous metal-organic frameworks (MOFs) have attracted tremendous interest due to their potential applications in gas-storage, gas separation, gas sensing, and catalysis. MOFs consist of metal-oxygen polyhedera interconnected with a variety of organic linker molecules, resulting in tailored nanoporous materials. With a judicious choice of organic linker groups, it is possible to fine-tune size, shape, and chemical functionality of the cavities and the internal surfaces. This unique structural feature offers unprecedented opportunities in small-molecule separations as well as chiral separations and catalysis. Prototypical iso-reticular metal-organic frameworks (IRMOFs) have been extensively studied among MOFs due to the simplicity of their synthesis and the variety of their potential applications. IRMOFs are a specific series of metal-organic frameworks developed by Yaghi and his coworkers. All IRMOFs are composed of oxygen-centered Zn4O tetrahedra interconnected with dicarboxylate linkers, forming a cubic type three dimensional (3D) porous network with high surface area. Despite a great deal of research in the synthesis and characterization of MOFs, there have been relatively few reports on the development of their applications, such as the fabrication of MOF thin films and membranes for gas separations. This is mainly due to the challenges associated with relatively difficult heterogeneous nucleation (seeding) and growth of MOFs on supports, and crack formation compared to their counterparts. Thin films and membranes of MOFs have great potentials for applications in membranebased gas separations, reactors, chemical sensors, and nonlinear optical devices. In this dissertation, the fabrication of IRMOF-1 membrane using a novel seeding method and its gas diffusion properties has been demonstrated. Introduction of the new seeding method for MOFs using microwaves resulted in well inter-grown IRMOF membranes showing Knudsen type transport of small gases through its pore. The heteroepitaxial growth of one IRMOF on another produced multi-layered IRMOF membranes. In addition, postsynthetic modification (PSM) of IRMOFs created functionalized membranes with enhanced stability against water as well as reduced crack formation during membrane fabrication. Lastly, hierarchical IRMOFs with improved CO2 adsorption properties were synthesized via PSM with cyanuric chloride.