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The trade-off between selectivity and adsorption capacity with porous materials is a major roadblock to reducing the energy footprint of gas separation technologies. To address this matter, we report herein a systematic crystal engineering study of C 2 H 2 removal from CO 2 in a family of hybrid ultramicroporous materials (HUMs). The HUMs are composed of the same organic linker ligand, 4-(3,5-dimethyl-1 H -pyrazol-4-yl)pyridine, pypz, three inorganic pillar ligands, and two metal cations, thereby affording six isostructural pcu topology HUMs. All six HUMs exhibited strong binding sites for C 2 H 2 and weaker affinity for CO 2 . The tuning of pore size and chemistry enabled by crystal engineering resulted in benchmark C 2 H 2 /CO 2 separation performance. Fixed-bed dynamic column breakthrough experiments for an equimolar (v/v = 1:1) C 2 H 2 /CO 2 binary gas mixture revealed that one sorbent, SIFSIX-21-Ni, was the first C 2 H 2 selective sorbent that combines exceptional separation selectivity (27.7) with high adsorption capacity (4 mmol·g -1 )., Competing Interests: The authors declare no competing interests., (© 2021 The Authors.)