Structure and Tunable Properties of Hybrid Zeolitic Imidazolate Frameworks

Abstract

Metal Organic Frameworks (MOFs) are a relatively novel class of nanoporous materials that have shown tremendous promise for application in energy-efficient separations. Thousands of structures of MOFs have been synthesized and studies for facilitating several different separation processes that are vital to the chemical engineering industry. However, much work remains to be done in understanding and exploiting their structure-property relationships. We examine unique structural and functional aspects of MOFs versus other nanoporous materials. The pore size and available adsorption sites in MOFs can be tuned via incorporation of two or more organic linkers in the MOF framework, resulting in a ‘mixed-linker’ material. We have chosen an important family of MOFs known as Zeolitic Imidazolate Frameworks (ZIFs) to highlight different facets of mixed-linker synthesis. Importantly, this work develops different methodologies to study the structure of mixed-linker materials at multiple length-scales, which allows a reliable connection to their molecular adsorption and transport properties. Tunability of properties such as adsorption and diffusion of guest molecules such as water and alcohols in mixed-linker ZIF-8-90 is demonstrated. Microscopic structure elucidation of mixed-linker ZIF-8-90 materials is accomplished using advanced NMR spectroscopy combined with computational modeling of structures and NMR spectra. The effect of synthesis route on the structure and properties of mixed-linker ZIFs is explored by comparing and contrasting solvent assisted linker exchange and de novo synthesis. Mechanistic insight of linker exchange process is studied in detail. A modified linker exchange method is used to repair ZIF crystals that have undergone acid gas damage and restore crystallinity and porosity, thereby increasing their useful lifetime.