Fast diffusion and high C2H2 capture in a 2D MOF with oxygen-riched wide channels for efficient C2H2/CO2 separation

Abstract

The separation of C₂H₂/CO₂ presents an arduous challenge due to their similar physicochemical properties. In this study, we propose SUM-1(Zr), a two-dimensional layered MOF that effectively captures C₂H₂ molecules by utilizing electronegative oxygen as hydrogen bond donors and separates C₂H₂/CO₂ mixtures by competitive adsorption between C₂H₂ and CO₂ molecules. The adsorption capacity of SUM-1(Zr) for C₂H₂ was measured to be 3.07 mmol g⁻¹ at 298 K and 1 bar, with an IAST selectivity for C₂H₂/CO₂ reaching 3.33. Kinetic studies demonstrated faster diffusion rates of C₂H₂ and CO₂ molecules in hexagonal channels with larger pore sizes. The electronegative oxygen atoms and –NH molecules in SUM-1(Zr) create a favorable adsorption environment for the guest molecules, while the –NH moiety in SUM-1(Zr) is oriented towards the narrow triangular channels, and the wide hexagonal channels contains numerous electronegative oxygen atoms that act as hydrogen bond donors, selectively trapping C₂H₂ molecules. Theoretical calculations indicate that C₂H₂ prefers to adsorb near the oxygen atoms in the wide hexagonal channels, forming multiple hydrogen bonds with the oxygen atoms in the two adjacent parallel layers. It is worth noting that the binding energies of these two types of channels for C₂H₂ are significantly higher than those for CO₂, resulting in competitive adsorption between C₂H₂ and CO₂. This study highlights the potential of utilizing the unique pore surface environment and competitive adsorption among diverse gas molecules for efficient separation of gas mixtures in MOFs.