Flexible MOFs as Pressure-Tunable Filters for Hydrocarbon Separation

Metal–organic frameworks (MOFs) hold significant promise for separating gas mixtures, especially hydrocarbons. While the main focus in the field is the development of new adsorbents for specific separations of binary mixtures, a more preferable situation is the use of a single framework for the separation of various species. This has been achieved in some flexible MOFs, where ternary mixtures can be separated at different temperatures. Here, we propose a simple yet fascinating way of utilizing the flexibility of MOFs to create tunable filters as a function of external pressure. As a proof of concept, we select the flexible CaMOF framework for C6 hydrocarbon separation; however, our results are applicable to other flexible frameworks and gas molecules. We find that the pressure-tunable separation in this material can function via both thermodynamic and kinetics principles. While the pressures required for a thermodynamic separation may be impractically high (in the GPa range), kinetic-based separation may occur at noticeably lower pressures (in the 100 MPa range). Our findings provide valuable mechanistic and fundamental insight into engineering separation filters by designing flexible pores with critical sizes that can be manipulated by external pressure.