Molecular insights into the role of O2 in reversed C2H6/C2H4 separation on metal–organic frameworks

Abstract

Previous experimental studies have shown that C2H4-selective metal–organic frameworks (MOFs) can be reversed to C2H6-selective MOFs by introducing an oxygen molecule (O2) into the MOF. The O2 functionalization strategy provides a new insight into one-step C2H4 purification; however, the molecular-level understanding of the reversed separation mechanism remains theoretically and experimentally unclear, which limits further material refinement. In this study, we explored the influence of O2 on the C2H6/C2H4 selectivity of a Cr-BTC MOF via density functional theory (DFT) calculations and grand canonical Monte Carlo simulations within the newly DFT-derived force fields. Results suggest that O2 was antiferromagnetically coupled to the open Cr site, with an end-on pattern forming the superoxo site. The presence of superoxide adducts decreased the dipole moment of the open-metal site by 5.95 debye. Furthermore, the altered gas selectivity could be attributed to the superoxo site blocking the donation and back-donation interactions between Cr and C2H4. This removes the highly polarized C2H4-selective site, while enhancing the pore channel environmental inertness for preferential C2H6 adsorption. To assess the performance of the reversed separation strategy under humid air conditions, ab initio molecular dynamics with metadynamics were used to determine the water stabilities of Cr-BTC and oxidized conjugates. We revealed that O2 introduction weakened the metal-linker bond and reduced the free-energy barrier for the initial framework decomposition process, which may hinder the application of gas separation in further practical environments. These findings provide theoretical support for oxidized MOFs with reversed separation performance and facilitate intentional post-synthetic MOF modifications for paraffin-selective adsorbent design, a critical need for energy applications and the petroleum industry.