Benchmark C₂H₂/CO₂ and CO₂/C₂H₂ Separation by Two Closely Related Hybrid Ultramicroporous Materials

The removal of CO₂ impurities from C₂H₂-containing gas mixtures is an important step in purifying C₂H₂, a feedstock chemical used in the production of several commodity chemicals. However, that C₂H₂ and CO₂ exhibit similar size and physicochemical properties makes their separation by physisorption extremely difficult. In this work, we detail how two hybrid ultramicroporous materials (HUMs)—known variant SIFSIX-3-Ni and variant TIFSIX-2-Cu-i—exhibit exceptional CO₂/C₂H₂ and C₂H₂/CO₂ selectivity, respectively. SIFSIX-3-Ni sets a benchmark for CO₂/C₂H₂ selectivity at low partial pressures, whereas TIFSIX-2-Cu-i ranks among the best porous materials in the context of C₂H₂/CO₂ selectivity. The performance of these HUMs was confirmed by real-time dynamic breakthrough experiments. To our knowledge, such yin-yang inversion of selectivity in closely related compounds is unprecedented. We attribute this to the distinct sorbate binding sites in SIFSIX-3-Ni and TIFSIX-2-Cu-i, as revealed by modeling studies.