Highly Ion-Permselective Porous Organic Cage Membranes with Hierarchical Channels

Abstract

Membranes of high ion permselectivity are significant for the separation of ion species at the subnanometer scale. Here, we report porous organic cage (i.e., CC3) membranes with hierarchical channels including discrete internal cavities and cage-aligned external cavities connected by subnanometer-sized windows. The windows of CC3 sieve monovalent ions from divalent ones and the dual nanometer-sized cavities provide pathways for fast ion transport with a flux of 1.0 mol m–2 h–1 and a mono-/divalent ion selectivity (e.g., K+/Mg2+) up to 103, several orders of magnitude higher than the permselectivities of reported membranes. Molecular dynamics simulations illustrate the ion transport trajectory from the external to internal cavity via the CC3 window, where ions migrate in diverse hydration states following the energy barrier sequence of K+ < Na+ < Li+ ≪ Mg2+. This work sheds light on ion transport properties in porous organic cage channels of discrete frameworks and offers guidelines for developing membranes with hierarchical channels for efficient ion separation.

Ion Exchange Membranes: Constructing and Tuning Ion Transport Channels

“…Thus, we next developed phase-pure MOF membranes via interfacial polymerization or in situ growth of MOFs, which were readily applicable in ion separation. [184][185][186][187][188] 2D nanosheets such as graphene oxide (GO), MXenes, and graphitic carbon nitride (g-C 3 N 4 ) can be fashioned into lamellar membranes via their multilayer stacking, which has received increasing interest from the membrane community. [189][190][191] Both the through-plane channels and intraplanar gaps between the layers can contribute to mass transfer.…”

“…Thus, we next developed phase‐pure MOF membranes via interfacial polymerization or in situ growth of MOFs, which were readily applicable in ion separation. [ 184–188 ]…”

Recent advances in the applications of porous organic cages

“…Xu et al 71 synthesized CC3 membranes with hierarchical subnanometer windows and nanometer cavities by the contra-diffusion growth method. The novel window-to-window alignment of CC3 molecules provided routes for fast ion transport as the ion fluxes of K + and Na + were both above 1.0 mol m −2 h −1 .…”

Switchable Na+ and K+ selectivity in an amino acid functionalized 2D covalent organic framework membrane

“…15). In general, the competition between cations in a binary-ion system will compromise the ion selectivity for most nanoporous membranes with sub-nanometer channels 19,24 . Instead, the enhanced Na + /K + selectivity of the COF-Cys-60% membrane in the binary-ion system is similar to that observed in the polymeric membranes with specific binding sites, which suggests that a facilitated transport mechanism may dominate the ion transport process in the COF-Cys-60% membranes 16,17,48 .…”

“…Conventional nanoporous membranes made of two-dimensional laminates [6][7][8][9][10] and polymers [11][12][13][14][15][16][17][18] usually exhibit poor mono/monovalent cations selectivity due to their broad channel size distribution. Metalorganic frameworks (MOFs) [19][20][21][22][23] and porous organic cages 24 possess ordered channels, but the weak interactions between monovalent cations and channel walls 18,25,26 compromise the selectivity. Macrocyclic molecules, such as crown ethers 27,28 , cucurbituril 29 , and calixarene 30 , can bind specific monovalent cations (e. g., Na + and K + ), which produce remarkable mono/monovalent cations selectivity 25,27,31,32 .…”