Fluorine-mediated single-step ethylene purification in face-transitive metal-organic frameworks from binary to...

Ethylene is a pivotal feedstock for the chemical industry. Obtaining polymer-grade ethylene in a single step from either binary ethane/ethylene or ternary acetylene/ethane/ethylene mixtures via porous adsorbents is highly energy-efficient yet remains a formidable challenge. Face-transitive topologies, a particular class of nets in reticular chemistry, possess only one window type and thus hold exceptional promise for discriminating between closely related C₂ hydrocarbons. Guided by the nia-d topology, we synthesized two isoreticular, trinuclear-manganese-cluster-based, ternary metal-organic frameworks (MOFs), namely nia-d-TZB and nia-d-FTZB, under solvothermal conditions using MnCl₂, the tritopic linker 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT), and the heterofunctional linear linkers 4-(1H-tetrazol-5-yl)benzoic acid (H₂TZB) or 2-fluoro-4-(1H-tetrazol-5-yl)benzoic acid (H₂FTZB). Although the resultant trigonal-bipyramidal cages remain dimensionally invariant, the introduction of fluorine in the latter linker subtly reduces the size of the antiprismatic cages and the sole triangular window in nia-d-FTZB. Single-component adsorption isotherms reveal that nia-d-TZB preferentially adsorbs ethane, whereas nia-d-FTZB preferentially adsorbs both acetylene and ethane. Consequently, nia-d-TZB enables one-step purification of ethylene from an ethane/ethylene mixture, while nia-d-FTZB achieves simultaneous removal of acetylene and ethane from an acetylene/ethane/ethylene ternary stream, again delivering polymer-grade ethylene in a single pass. These findings are corroborated by ideal adsorbed solution theory (IAST), breakthrough experiments with both binary and ternary gas mixtures, and detailed theoretical simulations. This study furnishes compelling evidence for the rational design of face-transitive MOFs to tackle complex gas-separation tasks.