Cage-Confined BF$_4$$^–$ Counteranions Generate Fluorine-Rich Microenvironments for C$_2$H$_2$/CO$_2$ Separation in Isostructural Co/Ni-MOFs

The separation of C$_2$H$_2$ from CO$_2$ 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 (BF$_4$$^–$) counteranions are immobilized within cavities via C–H···F interactions, generating fluorine-rich microenvironments as C$_2$H$_2$ recognition sites. At 298 K and 1 bar, LIFM-801 and LIFM-802 exhibit C$_2$H$_2$ uptakes of 121.4 and 113.5 cm$^3$ g$^{–1}$ with isosteric adsorption enthalpies (Qst) of 30.1 and 27.8 kJ mol$^{–1}$. Ideal adsorbed solution theory (IAST) calculations yield equimolar C$_2$H$_2$/CO$_2$ selectivities of 4.17 and 2.96. Dynamic breakthrough experiments demonstrate efficient C$_2$H$_2$/CO$_2$ separation performance with separation windows of 23 and 22 min g$^{–1}$ at 298 K. The comparable performance indicates that the separation ability is dominated by cage-confined BF$_4$$^–$ anions, while metal node substitution enables tuning of affinity and kinetics. ... mehrGrand canonical Monte Carlo (GCMC) simulations corroborate that cage-confined BF$_4$$^–$ serves as the dominant binding site for C$_2$H$_2$ 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.