Cage-Confined BF4- Counteranions Generate Fluorine-Rich Microenvironments for C2H2/CO2 Separation in Isostructural Co/Ni-MOFs.

The separation of C2H2 from CO2 remains a critical challenge because of their similar molecular dimensions and boiling points. Herein, we report two isostructural cage-based metal-organic frameworks (MOFs), LIFM-801 and LIFM-802, with Co and Ni metal nodes. Built from linear trinuclear metal nodes and tritopic pyridyl ligands, both adopt a nested cage architecture. Cage-confined tetrafluoroborate (BF4-) counteranions are immobilized within cavities via C-H···F interactions, generating fluorine-rich microenvironments as C2H2 recognition sites. At 298 K and 1 bar, LIFM-801 and LIFM-802 exhibit C2H2 uptakes of 121.4 and 113.5 cm3 g-1 with isosteric adsorption enthalpies (Qst) of 30.1 and 27.8 kJ mol-1. Ideal adsorbed solution theory (IAST) calculations yield equimolar C2H2/CO2 selectivities of 4.17 and 2.96. Dynamic breakthrough experiments demonstrate efficient C2H2/CO2 separation performance with separation windows of 23 and 22 min g-1 at 298 K. The comparable performance indicates that the separation ability is dominated by cage-confined BF4- anions, while metal node substitution enables tuning of affinity and kinetics. Grand canonical Monte Carlo (GCMC) simulations corroborate that cage-confined BF4- serves as the dominant binding site for C2H2 via C-H···F interactions. This work highlights a guest-anion confinement strategy for fluorine-rich adsorption microenvironments toward gas separations, with metal node modulation for performance optimization.