GAS ADSORPTION PROPERTIES OF VARIOUS METAL-ORGANIC FRAMEWORKS WITH COORDINATIVELY UNSATURATED METAL CENTERS
Wendy L. Queen, Craig M. Brown, Matthew R. Hudson, Kenji Sumida, Eric D. Bloch, Leslie J. Murray, Jeffrey R. Long, David K. Britt, Omar M. Yaghi
Metal organic frameworks (MOFs) are crystalline materials that contain metal ions or metal-ion clusters as nodes and organic ligands as linkers to form 1-, 2-, and 3-D structures. In addition to their convenient modular synthesis and chemical tunability, this class of materials has been under intense investigation for gas storage and separation applications due to the discovery of many 3-D frameworks with high internal surface areas. Through careful selection of the ligand and metal, which control pore size / shape and MOF-adsorbate interactions, their uptake properties, such as gas selectivity, can be tuned. In the past, MOFs have been hampered by low binding energies due to weak dispersive type interactions between the framework and adsorbed gas. An effective strategy to improve binding energy is through the generation of MOFs that, upon solvent removal, contain high concentrations of coordinatively unsaturated metal centers (UMCs) on the framework surface. Due to their highly reactive, electron deficient nature, UMCs can drastically enhance gas uptake / selectivity, increase adsorbate surface packing density, and serve as a source for catalytic activity. In order to obtain a better understanding of how structure dictates function during adsorption / desorption processes we have performed detailed neutron diffraction experiments on several new porous frameworks with UMCs, allowing us to probe structural changes in the framework as well as the exact positions, occupancy, and site affinity of the gas as it is loaded.