Flexibility of metal-organic frameworks for separations: utilization, suppression and regulation

Framework flexibility and dynamic behavior are unique characteristics of metal-organic frameworks (MOFs). Gate-opening, as one of flexibility modes of MOFs, is mainly triggered by guest adsorption. The gate-opening feature of MOFs is influenced by the types of guest molecules, pressure, temperature and crystal size of MOFs. Gate-opening of MOFs led to an unexpected selectivity when used as adsorbents, as a result of guest-dependent gate-opening pressure. However, when MOFs were adapted for membrane separations, the framework flexibility sometimes can be disadvantage, resulting in the absence of sharp molecular sieving. This evokes considerable research efforts in recent years to suppress the negative impacts of framework flexibility, for example, hindering the gate-opening effects, on the gas separation performance of MOF membranes. Very recently, accurate regulation of gate-opening of MOF membranes by applying an external electric field was demonstrated, opening new scenario to develop MOFs with excellent performances for molecular separations.