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

Abstract

In this work, we thoroughly assess the CO₂ adsorption behaviour of a recently reported, pillared-layered Cu^II ethylenediphosphonate of formula Cu₂(H₂O)_1.7(O₃P–C₂H₄–PO₃)·1.5H₂O (Cu-EtP), that features narrow channel-like pores (diameter < 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.