Xenon/Krypton Separation on a Bromine Functionalized Benzimidazole-Linked Porous Covalent Organic Polymer

A bromine-functionalized benzimidazole-linked porous covalent organic polymer (COP) was designed and synthesized to explore its potential for xenon/krypton (Xe/Kr) separation and systematically compared with its nonbrominated analogue. The incorporation of bromine atoms protruding into the pore channels modulates the framework polarity and creates specific host-guest interactions. Gas adsorption measurements combined with isosteric heat of adsorption analyses demonstrate that bromine functionalization significantly enhances Xe uptake and Xe/Kr selectivity relative to the nonhalogenated counterpart. ¹²⁹Xe nuclear magnetic resonance (NMR) spectroscopy provides direct insight into the adsorption behavior, revealing a bimodal pore environment in the brominated polymer during Xe adsorption, indicative of heterogeneous binding sites. Complementary computational modeling elucidates the synergistic interplay between stronger confinement effects in the smaller pores of the brominated framework and energetically favorable adsorption sites in close proximity to the bromine functionalities. This study introduces a distinct separation mechanism for Xe/Kr based on polarity-induced specific interactions rather than conventional size-shape exclusion, offering a rare and effective example of organic porous materials for noble gas separation. The findings highlight halogen functionalization as a powerful strategy for tuning noble gas adsorption energetics and selectivity in covalent organic polymers.