Tuning surface inductive electric field in microporous organic polymers for Xe/Kr separation

Abstract

Understanding how the local polar sites in porous adsorbent surface affect their adsorption performance remains a challenge in molecular related separation process. We present an in-depth study of silver (Ag), cobalt (Co), iron (Fe) and nickel (Ni) ions on hyper cross-linked microporous organic polymer (HCP) in which all metal ions act as polar sites for xenon (Xe) and krypton (Kr) adsorption. Surface electrostatic potential analysis revealed that Xe adsorption strength at metal sites differs from that of Kr. Charge density difference analysis proved that the precise charge transfer between metal ions and Xe. These effects could strengthen the inductive energy caused by surface electric field for Xe adsorption in metal ions decorated HCP (HCP-M). Adsorption kinetics experiments showed that the adsorption rates of Xe on HCP-M (20.45–47.19 cm3/(cm3·s)) are much higher than that of Kr (9.45–17.38 cm3/(cm3·s)). The HCP-Ni can adsorb 90 ml Xe per volume with 16.20 Xe/Kr selectivity in Xe, Kr and N2 ternary gas mixture, which was among the well performing porous organic polymers reported. The Xe/Kr adsorptive separation performance of HCP-M stemmed mainly from the optimized inductive energy and the small-pore confinement effect of HCP-M, which could significantly facilitate preferential selective adsorption of Xe over Kr.