Tuning Pore Size in Square‐Lattice Coordination Networks for Size‐Selective Sieving of CO2

Abstract Porous materials capable of selectively capturing CO 2 from flue‐gases or natural gas are of interest in terms of rising atmospheric CO 2 levels and methane purification. Size‐exclusive sieving of CO 2 over CH 4 and N 2 has rarely been achieved. Herein we show that a crystal engineering approach to tuning of pore‐size in a coordination network, [Cu(quinoline‐5‐carboxyate) 2 ] n ( Qc‐5‐Cu ) ena+bles ultra‐high selectivity for CO 2 over N 2 (S CN ≈40 000) and CH 4 (S CM ≈3300). Qc‐5‐Cu‐sql‐β , a narrow pore polymorph of the square lattice ( sql ) coordination network Qc‐5‐Cu‐sql‐α, adsorbs CO 2 while excluding both CH 4 and N 2 . Experimental measurements and molecular modeling validate and explain the performance. Qc‐5‐Cu‐sql‐β is stable to moisture and its separation performance is unaffected by humidity.