日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Metal-organic frameworks (MOFs) in which metal ions play a role for linking ligands, have merits of both of organic and inorganic compounds. MOF has fundamental framework units assembled in a different way to give a unique three dimensional (3D) framework structure. Some MOFs that have weak bondings such as hydrogen bonding and π - π interaction exhibit unique adsorption behavior stemming from the structural flexibility. Unusually soft nature of the lattice frame structure of [Cu(bpy)₂(BF₄)₂]_n (bpy=4,4'-bipyridine) crystals is shown. The Cu-complex crystals have no open nanoporosity and thereby this compound is named latent porous crystal (LPC). The CO₂ adsorption isotherm of LPC has a rectangular shape hysteresis loop. The adsorption starts at P/P₀=0.01 at 273 K, giving almost a vertical adsorption branch, which is named gate adsorption. In a similar way, CO₂ is evolved suddenly, leading to the vertical desorption branch (gate desorption) at P/P₀=0.007. The gate adsorption and desorption is accompanied with a remarkable lattice expansion and shrinkage in the direction of c-axis by 26%. The temperature dependence of the gate pressure is expressed by the Clausius-Clapeyron equation, indicating the clathrate formation. The gate behavior for supercritical CH₄ is also observed. The adsorption and desorption isotherm does not vary after 50 cycles, indicating the unusual flexibility of the lattice.