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)

Abstract

The metal–organic 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 VIV═O molecular ions are used as local probes to detect the NP to LP transitions. The EPR spectra of VIV═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 st...