DSC was done under nitrogen, whereas VT-XRD was carried out under argon). The change in the crystal structure returns to the initial (low-temperature) state upon cooling (Figure ). Figure indicates selected lattice planes, in particular (400), to which the most pronounced variations are assigned. These lattice planes are crossing the largest pores in CuBTC, where most of the IL molecules are likely located. In addition to the transition at 220 °C, another change occurs at ~ 100 °C. This low-temperature conversion only occurs during the first heating, and is not reversed upon cooling; most probably, it is related to the release of water and the uptake of IL within the pores of HKUST-1. Furthermore, it is much more gradual, particularly observable in the lattice planes (220), (400), and (440) (see