Electron paramagnetic resonance study of the active vanadium sites in MIL-47

Metal Organic Frameworks (MOF) interesting for catalysis are highly porous, stable up to high temperatures (T) and can withstand a broad range of solvents and reagents. In addition, the metal ions need to be accessible for reaction. Recently we reported the catalytic activity of the V-containing porous MIL-47 [1]. In the pristine material the V-ions are coordinately saturated and certain ligands fold away to allow V-ions to coordinate with peroxide in the selective oxidation of alkenes with hydroperoxide. Electron paramagnetic resonance (EPR) investigation of the coordination and oxidation state of the active metal ion during catalysis may help to elucidate the catalytic reaction mechanism, which may allow to tune the structure of the MOF for customized catalytic activity. Monitoring the room T V(+IV)-related EPR spectrum of MIL-47, we found a fraction of the V-ions oxidizes during the first hours of catalysis [2]. In this contribution, this EPR spectrum is investigated in detail. At RT its anisotropy is partially resolved at microwave frequencies higher than X-band (9.5 GHz). Substructure becomes apparent at lower T. Below 20 K the spectrum exhibits a very strong T-dependence in which certain components strongly broaden. In search for the origin of these phenomena a comparison with other V-MOFs is made. A spin calibration sample is used for quantification of the observable V-sites. [1] K. Leus et al., Chem. Commun. 46, 5085 (2010) [2] K. Leus et al., J. Catal. 285, 196 (2012