Cluster-Aggregation-Triggered Isomeric Ultra-Microporous Fe-Isonicotinate Frameworks for Efficient C₂H₂/CO₂ Adsorption and Separation

Because of the similar physical properties of acetylene (C2H2) and carbon dioxide (CO2), their efficient separation remains challenging in industry. In this work, two isomeric ultra-microporous Fe-isonicotinate frameworks (SNNU-131 and SNNU-132) were prepared by a cluster-aggregation strategy, and both show prominent acetylene and carbon dioxide adsorption and separation ability. The linear trinuclear cluster [FeIII2FeII(μ2-O)2(COO)4] and its dimer, hexanuclear cluster [FeIII4FeII2(μ3-O)2(μ2-O)2(COO)8] were extended by isonicotinic acid linkers to generate two 8-connected supramolecular isomers with typical bcu and hex topology, respectively. Thanks to the existence of 1D quadrilateral channels with crossing size of about 7 Å and open metal sites from linear trinuclear clusters, SNNU-131 has better acetylene and carbon dioxide adsorption capacities of 88.5 and 49.9 cm3 g–1 at 298 K and 1 bar. On the other hand, SNNU-132 with smaller triangular channels shows higher C2H2/CO2 ideal adsorbed solution theory (IAST) selectivity values of 3.54–4.44, as well as a longer breakthrough interval time of 30 min g–1 at 298 K and 1 bar. The preferred adsorption sites and density distributions of C2H2 and CO2 molecules in these two metal–organic frameworks (MOFs) were further calculated by the Grand Canonical Monte Carlo (GCMC) simulations to understand their adsorption and separation performance.