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

Abstract

X-ray diffraction (XRD) and electron paramagnetic resonance spectroscopy (EPR) were combined to study the structural transformations induced by temperature, pressure and air humidity of the "breathing" metal-organic framework (MOF) MIL-53(Al), doped with paramagnetic V ions, after activation. The correlation between in situ XRD and thermogravimetric analysis measurements showed that upon heating this MOF in air, starting from ambient temperature and pressure, the narrow pore framework first dehydrates and after that makes the transition to a large pore state (lp). The EPR spectra of V[double bond, length as m-dash]O molecular ions, replacing Al-OH in the structure, also allow to distinguish the as synthesized, hydrated (np-h) and dehydrated narrow pore (np-d), and lp states of MIL-53(Al). A careful analysis of EPR spectra recorded at microwave frequencies between 9.5 and 275 GHz demonstrates that all V[double bond, length as m-dash]O in the np-d and lp states are equivalent, whereas in the np-h state (at least two) slightly different V[double bond, length as m-dash]O sites exist. Moreover, the lp MIL-53(Al) framework is accessible to oxygen, leading to a notable broadening of the V[double bond, length as m-dash]O EPR spectrum at pressures of a few mbar, while such effect is absent for the np-h and np-d states for pressures up to 1 bar.

Mixed-metal metal–organic frameworks

“…51 Analysis of the edge position and pre-edge peak intensity allowed us to follow evolutions in the oxidation state of vanadium dopant ions in MIL-53(Al). 57 The as synthesized MM-MOF exhibited hardly any intensity in the pre-edge region, indicating that most of the Vdopant ions were incorporated as (V-OH) 2+ metal nodes which are nearly centrosymmetric. The growth of the pre-edge peak and upshift of the edge position upon solvent extraction and calcination are a clear proof that an important fraction of the dopant is oxidized to (V=O) 2+ by these treatments.…”

“…55 In Section 3.4 the SS-NMR spectra of ligand nuclei, bridging between metal ions will be discussed, which are more sensitive probes for mixing in the SBU's. For Fe-based MOFs, Mössbauer spectroscopy yields similar structural information as EPR and NMR via isomeric shift, quadrupole and magnetic hyperfine effects on the nuclear transition energy for 57 Fe. This gamma-ray technique can in principle also be applied to, e.g.…”

“…Furthermore, the EPR spectra of these paramagnetic probes proved sensitive to transformations of the framework and to the occupation state of the pores. 57,62,64,65 Frameworks where all metal nodes are occupied with paramagnetic ions, e.g. MIL-47(V), MIL-53(Fe) and Cu3(BTC)2, yield broad and featureless EPR spectra at room temperature, without resolved HF or other structure.…”

“…61 Ni, 67 Zn and many lanthanide nuclei, but each of these nuclei requires a dedicated radioactive source. In the practice of MOF research Mössbauer spectroscopy has so far been restricted to studies of 57 Fe. Based on the recoil-free emission and absorption of gamma-rays by nuclides, spectra are very narrow and well-suited for discriminating and quantifying different sites and oxidation states for Fe.…”

“…We made a similar study in the isoreticular series of DUT-5 (an Albiphenyldicarboxylate MOF) and COMOC-2 (the V-variant). 57,144 ( Figure 31). A combination of EPR, XANES, EXAFS and in situ XRD during CO2 sorption revealed that the MM-MOFs, containing both V and Al, had in fact two lp forms, one that is flexible and exists in vacuum and undergoes reversible transformations to the np form, and one lp form that comes exclusively from the np form.…”