SYNTHESES, STRUCTURES AND PROPERTIES OF NANOPOROUS METAL-ORGANIC FRAMEWORK MATERIALS

Abstract

This dissertation describes the synthesis and characterization of a number of extended framework materials. Firstly, a series of transition metal and lanthanide metal-organic framework materials (MOFs) possessing both two and three-dimensional (2-D/ 3-D) structures as a result of linkage with different rigid carboxylate ligands have been synthesized. The crystal structures have been determined by single crystal X-ray diffraction, and their thermal/ structural stability evaluated by thermogravimetric analysis, powder neutron diffraction, in-situ and combined synchrotron powder X-ray and Raman spectroscopy. The structural stability strongly relates to the dimensionality and chemical bonding within the frameworks. In this dissertation, a geometric strategy has been proposed to elucidate these complex structures. A nanoporous structure stable up to 500oC can be obtained by dehydrating holmium MOF with 1, 3, 5-Benzene tricarboxylic acid (H3BTC) ligand. Its porosity is shown to be 879.1m2/g specific surface area by gas isotherm experiments and absorbing 1.46wt% excess of hydrogen at 77K. High resolution neutron powder diffraction has been performed on the dehydrated Ho-MOF-BTC system with P4122 chiral space group under different deuterium gas pressures and a new mechanism for gas adsorption in framework materials. A unique "helical" deuterium adsorption phenomenon results from direct interaction between the guest molecules and the non-centrosymmetric (chiral) framework architecture. Reconstruction of the nuclear scattering density using the charge flipping method has been utilized as a novel strategy for the qualitative analysis of the adsorbed molecules, in combination with maximum entropy method (MEM) density maps that provide quantitative information of the exact distribution of the diffusion pathways. In addition, the structure, magnetic and photo-luminescent properties of three lanthanide- and transition metal MOFs with shorter formate ligand are investigated by single crystal X-ray and neutron diffraction as well as solid-state fluorescence spectroscopy techniques, demonstrating unusual magnetic behaviors of Co-formate systems and strongly characteristic emission spectra from europium (Eu(III)) and terbium (Tb(III)) compounds.