Stabilization of Scandium Terephthalate MOFs against Reversible Amorphization and Structural Phase Transition by Guest Uptake at Extreme Pressure

Previous
high-pressure experiments have shown that pressure-transmitting
fluids composed of small molecules can be forced inside the pores
of metal organic framework materials, where they can cause phase transitions
and amorphization and can even induce porosity in conventionally nonporous
materials. Here we report a combined high-pressure diffraction and
computational study of the structural response to methanol uptake
at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized
derivative (Sc2(NO2–BDC)3)
and compare it to direct compression behavior in a nonpenetrative
hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible
upon pressure release, whereas Sc2(NO2–BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically
identical polymorph. In the presence of methanol, the reversible amorphization
of Sc2BDC3 and the displacive phase transition
of the nitro-form are completely inhibited (at least up to 3 GPa).
Upon uptake of methanol on Sc2BDC3, the methanol
molecules are found by diffraction to occupy two sites, with preferential
relative filling of one site compared to the other: grand canonical
Monte Carlo simulations support these experimental observations, and
molecular dynamics simulations reveal the likely orientations of the
methanol molecules, which are controlled at least in part by H-bonding
interactions between guests. As well as revealing the atomistic origin
of the stabilization of these MOFs against nonpenetrative hydrostatic
fluids at high pressure, this study demonstrates a novel high-pressure
approach to study adsorption within a porous framework as a function
of increasing guest content, and so to determine the most energetically
favorable adsorption sites.