Regeneration and Secondary Building Unit Transformation of Metal-organic Frameworks via Post-synthetic Treatment

Abstract

Ever since the first metal-organic framework (MOF) was synthesized, there have been a lot of different MOFs reported along with new approach of post-synthetic modification of MOFs, utilizing their diverse metal nodes and organic struts. A myriad of applications was available from various types of MOFs exhibiting the ease of tuning their structures. In this dissertation, I proposed simple protocols, the post acid-base treatment to iconic MOFs (MOF-5 and HKUST-1) to regenerate them from deteriorated form to pristine form. In addition, I altered the metal cluster of a MOF by post-synthetic metal exchange from [Zn4O(COO)4] secondary building unit (SBU) to [Fe3O(COO)6] SBU via a single-crystal-to-single-crystal transformation. One of the weaknesses of metal-organic frameworks are their coordination bonds between metals and organic linkers are vulnerable to hydrothermal conditions. This becomes one of the obstacles that hinder metal-organic frameworks used as a practical application. In first part, I proposed a simple regeneration method by treating acid-base to the degraded MOF-5 and HKUST-1. By adjusting the solution pH, degraded MOF materials are easily recovered either by in situ base formation via solvothermal reaction or ex situ base addition to regenerate HKUST-1 in a short period of time. Post-synthetic metal exchange (PME) has been proven to be a straightforward strategy to generate new MOFs with desirable metal nodes from their parent MOFs. While most SBUs of MOFs are substituted in an isostructural manner, changing the local symmetry of SBU of MOFs by PME is hardly achievable owing to the increase of framework strain caused by symmetry-mismatch of the new SBU. In last part, a two-fold interpenetrated Zn-based MOF with [Zn4O(COO)6] cluster as an octahedral symmetric SBU has been transformed into a Fe-based MOF with [Fe3O(COO)6] cluster as a trigonal prismatic symmetric SBU. The transformation occurred in a single-crystal-to-single-crystal fashion from surface to core while maintaining its net topology. Zn-based MOFs with neutral [Zn4O(COO)6] SBU, Fe-based MOFs with cationic [Fe3O(COO)6]+ SBU, and their intermediate hybrid core-shell structures with both SBUs were further characterized by guest molecule adsorption to show different dye selectivity based on their size and charge of dyes.