Unusual adsorption-induced phase transitions in a pillared-layered copper ethylenediphosphonate with ultrasmall channels

In this work, we thoroughly assess the CO2 adsorption behaviour of a recently reported, pillared-layered CuII ethylenediphosphonate of formula Cu2(H2O)1.7(O3P–C2H4–PO3)1.5H2O (Cu-EtP), that features narrow channel-like pores (diameter o 5 Å). Once the metal coordinated H2O molecules are removed, Cu-EtP features a very high density of open metal sites (0.0188 sites Å-3) and can adsorb a significant amount of CO2 at saturation (2.9 mmol g-1, 6.9 mmol cm-3). Most interesting, it displays a step-shaped CO2 isotherm,  whereby a preliminary adsorption event takes place, followed by a step occurring at higher pressure, during which adsorption kinetics become very slow. Hysteresis in desorption suggests that structural rearrangements could be responsible for the unusual isotherm shape. Cu-EtP does not adsorb N2, probably due to its very narrow pores, thus it displays virtually infinite CO2/N2 selectivity.
The isosteric heat of adsorption extracted using the Clausius–Clapeyron equation is in the range of 35–40 kJ mol-1, suggesting a strong physisorptive character. This is further confirmed by CO2 adsorption microcalorimetry. In situ synchrotron powder X-ray diffraction analysis, carried out during evacuation and CO2 adsorption, proves that the MOF undergoes phase transitions during both stages, with
shrinking of the interlayer spacing upon removal of water and partial reopening of the structure once CO2 is adsorbed. The phase transitions are slow, in agreement with the slow adsorption kinetics observed. In situ infrared spectroscopy suggests that CO2 interacts with the open metal sites generated after removal of strongly adsorbed water and that CO2 cannot displace H2O if this is not fully removed from the surface. NO was also used as a probe molecule to further demonstrate the existence of open metal sites, finding that it is not able to diffuse within the framework when pure, but that it can displace coordinated water. Optical spectroscopy suggests that changes in the coordination sphere of Cu take place during evacuation and CO2 adsorption. Finally, periodic density functional theory calculations were carried out to unravel the structural evolution of Cu-EtP and the energetics of interaction with H2O and CO2, finding that the change in coordination environment of Cu is mainly responsible for the observed phase transitions.