Probing the Structural Stability of and Enhanced CO2 Storage in MOF MIL-68(In) under High Pressures by FTIR Spectroscopy

The unique structural topology of metal-organic framework (MOF) MIL-68, featuring two types of channels with distinct pore sizes, makes it a promising candidate for application in gas storage and separation. In this study, the behavior of as-made and activated MIL-68(In) was investigated in a diamond-anvil cell under high pressure by in situ IR spectroscopy. The framework exhibits high stability under compression up to 9 GPa, whereas the bridging OH groups appear to be very sensitive to compression. Pressure-induced structural modifications were found to be completely reversible for as-made MIL-68(In) but irreversible for the activated framework. Moreover, the addition of Nujol as pressure-transmitting medium makes the framework more resilient to pressure. Finally, when loaded with CO2, the framework exhibited interesting differential binding affinities with CO2 in the hexagonal and triangular pores at different pressures. The pressure-enhanced CO2 storage behavior and the guest-host interaction mechanism between CO2 and the MOF framework were explored with the aid of Monte Carlo simulations. These studies demonstrated great potential for MIL-68(In) in gas-storage applications that require extreme loading pressures.