Influence of Postsynthetic Ligand Exchange in ZIF-7 on Gate-Opening Pressure and CO₂/CH₄ Mixture Separation

The targeted development of adsorbents tailored for specific separation tasks offers an opportunity to optimize selectivity, energy usage, and cycling. One instance of such a separation challenge is presented by biogas, a renewable energy source primarily composed of methane and carbon dioxide. The field of flexible metal–organic frameworks (MOFs) has garnered significant attention from researchers due to their potential for gas storage and capture applications. The adsorbate-dependent threshold pressure for the phase transition in these MOFs indicates potential for selective gas separation. The incorporation of functionalized ligands in these systems results in two notable effects: the introduction of specific adsorption sites on the internal surface as well as a modification of the flexible gate-opening behavior. Here, we conducted a comparison among different nitrogen-containing ligands (2-aminobenzimidazole, benzotriazole, and 5-azabenzimidazole) incorporated into the flexible MOF ZIF-7. These linkers include a nitrogen atom either in a heterocycle or as an attached functional group. The aim was to evaluate their performance for carbon dioxide–methane separation in a simulated biogas scenario. We further synthesized additional derivatives of the best-performing material, featuring the 5-azabenzimidazole ligand in varying quantities. Breakthrough experiments conducted under actual mixture conditions reveal that there is an optimal ligand exchange point (10%) facilitating the phase transition without significantly enhancing methane adsorption. Density functional theory simulations confirm that increased functionalization led to an easier phase transition due to heightened stability of the open phase resulting from additional hydrogen bonding, coupled with a weakened collapsed phase. To assess the effectiveness of this most-promising material, its performance was compared to the baseline ZIF-7 adsorbent using pressure swing adsorption (PSA) simulations. The enhanced surface affinity toward carbon dioxide, along with a sharper isotherm step and narrower hysteresis, translated to increased selectivity, faster cycling, and reduced process costs due to decreased energy input as shown by optimization opportunities of the high-pressure feed step.