Large-Scale Metal-Organic Framework Nanoparticle Monolayers with Controlled Orientation for Selective Transport of Rare-Earth Elements.

Long-range ordered membranes comprised of porous nanoparticles have been pursued in precise separations for a long time. Yet most of the fabrication methods suffer from limited substrates or lack of precise control over crystal orientation. Herein, large-scale metal-organic framework (MOF) monolayer membranes with controlled orientations are prepared through an interfacial self-assembly process confined by superlyophilic substrates. The superspreading of reactant microdroplets results in an ultrathin liquid layer under an immiscible oil as a confined reactor. The concomitant MOF (ZIF-8) particles spontaneously assemble into monolayers with controlled orientations, determined by the particles' contact angles at the liquid/liquid interface, which can be regulated by solvent compositions. Therein both gas-adsorption and ion-transport tests prove that the ⟨111⟩-oriented membrane exhibits a minimized mass-transfer resistance. The as-prepared membrane can selectively transport rare-earth elements (REEs), and a La3+/K+ selectivity of 14.3 is achieved. Molecular dynamics simulations reveal that the REEs-selectivity is associated with the distinct difference in ion-membrane binding energies, demonstrating the potential of ZIF-8 membranes for use in high-efficiency recovery of REEs from industrial wastes.