Highly selective gas separation by two isostructural boron cluster pillared MOFs

Two isostructural closo-dodecaborate [B₁₂H₁₂]^2- pillared metal organic frameworks (MOFs), BSF-4 and BSF-9, were synthesized and compared in the performance of selective gas adsorption and separation. BSF-9 with symmetrical interpenetration and two contracted 1D channel exhibited efficient uptake of C₂H₂ (85.1/76.3 mLg⁻¹, 278/298 K) with high separation selectivity over C₂H₄ (41.4, 298 K), outperforming BSF-4 (7.3, 298 K) and many benchmark MOFs. The selectivity of C₂H₆/CH₄, CO₂/CH₄, CO₂/N₂ on BSF-9 (25.2, 9.9, 56.6) and BSF-4 (19.0, 8.9, 41.7) was all relatively high and comparable. The practical gas separation ability for C₂H₂/C₂H₄ mixtures on BSF-9 was confirmed by mixed gas breakthrough experiments with negligible capacity loss after 4 cycles, indicating its potential for purification of ethylene from acetylene-containing industrial gas mixtures. The C₂H₂ and C₂H₄ bonding sites and bonding energies within the framework of BSF-9 were further compared by density functional theory (DFT) study, indicating that C₂H₂ can be trapped tightly between two anionic [B₁₂H₁₂]^2- by four synergistic dihydrogen bonds while C₂H₄ only interacts with single [B₁₂H₁₂]^2- with weaker bonding energy. BSF-4 with asymmetrical interpenetration and single extended 1D channel showed higher selectivity of C₃H₈/CH₄ (138.5, 298 K) than that of BSF-9 (64.0, 298 K). Inverse ambient adsorption capacity of C₃H₈ < C₂H₆ was observed in BSF-9 due to the small pore window and 1D channel.