The interaction of porous metal-organic framework (MOF) materials with adsorbed guest molecules can result in reversible structural transformations. Kondo et al. have observed such a transition induced by carbon dioxide adsorption in a [Cu(BF4)2(bpy)2] lattice, where bpy is 4,4′-bipyridine. The authors prepared this MOF, which adopts a two-dimensional (2D) quasi-square grid structure, by crystal-to-crystal transformation of a hydrated 3-D interpenetrating network precursor, which they heated under vacuum for several hours to remove the incorporated water.
Exposure of the MOF to CO2 at 273 K resulted in an abrupt jump in adsorption as the incoming gas pressure approached 35 kPa. To explain this observation, the authors carried out structural modeling of the material, with and without adsorbed CO2, on the basis of extended x-ray absorption fine structure and powder x-ray diffraction data. The results indicated that CO2 adsorption and subsequent clathrate formation increased the interlayer distance in the host lattice by ~50% to 0.68 nm. This process, which though reversible exhibited significant hysteresis, led to a macroscopic volume change of 6.6% at a final pressure of 101 kPa. -- PDS
Nano Lett. 6, 10.1021/nl062032b (2006).
