Cluster-Aggregation-Triggered\nIsomeric Ultra-Microporous\nFe-Isonicotinate Frameworks for Efficient C<sub>2</sub>H<sub>2</sub>/CO<sub>2</sub> Adsorption and Separation

Because of the similar physical properties\nof acetylene (C<sub>2</sub>H<sub>2</sub>) and carbon dioxide (CO<sub>2</sub>), their\nefficient separation remains challenging in industry. In this work,\ntwo isomeric ultra-microporous Fe-isonicotinate frameworks (SNNU-131\nand SNNU-132) were prepared by a cluster-aggregation strategy, and\nboth show prominent acetylene and carbon dioxide adsorption and separation\nability. The linear trinuclear cluster [Fe<sup>III</sup><sub>2</sub>Fe<sup>II</sup>(μ<sub>2</sub>-O)<sub>2</sub>(COO)<sub>4</sub>] and its dimer, hexanuclear cluster [Fe<sup>III</sup><sub>4</sub>Fe<sup>II</sup><sub>2</sub>(μ<sub>3</sub>-O)<sub>2</sub>(μ<sub>2</sub>-O)<sub>2</sub>(COO)<sub>8</sub>] were extended by isonicotinic\nacid linkers to generate two 8-connected supramolecular isomers with\ntypical <i>bcu</i> and <i>hex</i> topology, respectively.\nThanks to the existence of 1D quadrilateral channels with crossing\nsize of about 7 Å and open metal sites from linear trinuclear\nclusters, SNNU-131 has better acetylene and carbon dioxide adsorption\ncapacities of 88.5 and 49.9 cm<sup>3</sup> g<sup>–1</sup> at\n298 K and 1 bar. On the other hand, SNNU-132 with smaller triangular\nchannels shows higher C<sub>2</sub>H<sub>2</sub>/CO<sub>2</sub> ideal\nadsorbed solution theory (IAST) selectivity values of 3.54–4.44,\nas well as a longer breakthrough interval time of 30 min g<sup>–1</sup> at 298 K and 1 bar. The preferred adsorption sites and density distributions\nof C<sub>2</sub>H<sub>2</sub> and CO<sub>2</sub> molecules in these\ntwo metal–organic frameworks (MOFs) were further calculated\nby the Grand Canonical Monte Carlo (GCMC) simulations to understand\ntheir adsorption and separation performance.