In
this work, mid-infrared (mid-IR), far-IR, and Raman spectra
are presented for the distinct (meta)stable phases of the flexible
metal–organic framework MIL-53(Al). Static density functional
theory (DFT) simulations are performed, allowing for the identification
of all IR-active modes, which is unprecedented in the low-frequency
region. A unique vibrational fingerprint is revealed, resulting from
aluminum-oxide backbone stretching modes, which can be used to clearly
distinguish the IR spectra of the closed- and large-pore phases. Furthermore,
molecular dynamics simulations based on a DFT description of the potential
energy surface enable determination of the theoretical Raman spectrum
of the closed- and large-pore phases for the first time. An excellent
correspondence between theory and experiment is observed. Both the
low-frequency IR and Raman spectra show major differences in vibrational
modes between the closed- and large-pore phases, indicating changes
in lattice dynamics between the two structures. In addition, several
collective modes related to the breathing mechanism in MIL-53(Al)
are identified. In particular, we rationalize the importance of the
trampoline-like motion of the linker for the phase transition.
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 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.
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.
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. In contrast, the solvothermal method produces homogeneous mixed-metal MOFs. The influence of Cr and V on the flexibility and breathing was studied via T-XRPD and CO adsorption measurements.
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