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 [Fe^III₂Fe^II(μ₂-O)₂(COO)₄] and its dimer, hexanuclear cluster [Fe^III₄Fe^II₂(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.