Multi-frequency (S, X, Q and W-band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal-Organic Framework MIL-53

Identification of vanadium dopant sites in the metal-organic framework DUT-5(Al)

Studying the structural environment of the V^IV ions doped in the metal-organic framework (MOF) DUT-5(Al) ((Al^IIIOH)BPDC) with electron paramagnetic resonance (EPR) reveals four different vanadium-related spectral components. The spin-Hamiltonian parameters are derived by analysis of X-, Q- and W-band powder EPR spectra. Complementary Q-band Electron Nuclear DOuble Resonance (ENDOR) experiments, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction (XRD) and Fourier Transform InfraRed (FTIR) measurements are performed to investigate the origin of these spectral components. Two spectral components with well resolved ⁵¹V hyperfine structure are visible, one corresponding to V^IV[double bond, length as m-dash]O substitution in a large (or open) pore and one to a narrow (or closed) pore variant of this MOF. Furthermore, a broad structureless Lorentzian line assigned to interacting vanadyl centers in each other's close neighborhood grows with increasing V-concentration. The last spectral component is best visible at low V-concentrations. We tentatively attribute it to (V^IV[double bond, length as m-dash]O)²⁺ linked with DMF or dimethylamine in the pores of the MOF. Simulations using these four spectral components convincingly reproduce the experimental spectra and allow to estimate the contribution of each vanadyl species as a function of V-concentration.

Mixed-metal metal–organic frameworks

Mixed-metal MOFs contain at least 2 different metal ions presenting promising potential in heterogeneous catalysis, gas sorption/separation, luminescence and sensing.

Sensing the framework state and guest molecules in MIL-53(Al) via the electron paramagnetic resonance spectrum of VIV dopant ions

The EPR spectrum of V^IV dopant ions was used to discriminate different states of MIL-53(Al) and to detect oxygen gas.

Microwave induced “egg yolk” structure in Cr/V-MIL-53

Using a one pot microwave procedure, mixed-metal “egg yolk” MOFs are created, with a core of (Cr/V)-MIL-53 and a shell of Cr-MIL-53.

Mixed-metal or mixed-linker metal organic frameworks as heterogeneous catalysts

This review illustrates the recent developments in heterogeneous catalysis using mixed metal or mixed linker MOFs.