In situ electron paramagnetic resonance and X-ray diffraction monitoring of temperature-induced breathing and related structural transformations in activated V-doped MIL-53(Al)

The metalorganic framework MIL-53(Al) is characterized by a distinct reversible structural transition between a narrow pore (NP) and a large pore (LP) state, resulting in expansion or contraction of this three-dimensional porous framework also called breathing. This transition is studied for vanadium-doped MIL-53(Al), induced by temperature (T) using in situ electron paramagnetic resonance (EPR) and X-ray diffraction (XRD) in air and in vacuum. The EPR active V-IV=O molecular ions are used as local probes to detect the NP to LP transitions. The EPR spectra of V-IV=O embedded in the NP and LP MIL-53(Al) states are clearly distinguishable. The temperature-dependent EPR and XRD data can consistently be interpreted in terms of T-ranges in the experiments where one of the states is predominantly present and a narrow T-range in which the two states coexist. In addition the XRD data indicate that the NP state undergoes a transition to a metastable state characterized by different lattice parameters than the NP state at room temperature, before the transition to the LP state occurs. The EPR spectra, however, show that only in the LP state the V-IV=O ions can exhibit an interaction with paramagnetic O-2 molecules from air