Comparative analysis of selective gas adsorption on metal-organic frameworks: Cu–(abtc) vs. Cu–(hbtc)

Abstract

In this work, the adsorption characteristics of di-isophthalate-based metal-organic frameworks (MOFs) were evaluated by measuring the adsorption behavior of industrially relevant gases—including CO₂, CO, CH₄, N₂, C₂H₆, C₃H₈, and O₂—with varying polarity and polarizability. The functional group was modified by replacing the double bond (N = N) in a MOF derived from abtc with an NH–NH group (named hbtc). This modification highlights the enhanced affinity of the NH–NH group compared to N = N. Both frameworks feature open metal sites, which contribute significantly to their adsorption behavior. For all measured gases, Type–I isotherms were observed. The isotherms were modeled using the modified Virial equation for CO₂ and CO gases and the Langmuir model for nonpolar gases (O₂, N₂, CH₄, C₂H₆, and C₃H₈). Adsorption capacities increased with carbon chain length at low pressures, attributed to stronger dispersion interactions with longer hydrocarbons. Model parameters were used to calculate the enthalpies of adsorption, and the Ideal Adsorbed Solution Theory was employed to predict the selectivity of binary mixtures. CO₂ selectivity over N₂ increased significantly with pressure, with higher CO₂ selectivity observed for Cu–hbtc compared to Cu–abtc due to the stronger affinity of the functional group and framework–adsorbate interactions. The findings of this work indicate the potential of these MOFs for sustainable applications, including carbon capture for climate change mitigation, biogas upgrading, and industrial gas separations, contributing to energy-efficient and environmentally friendly solutions.