Influence of Postsynthetic Ligand Exchange in ZIF‑7\non Gate-Opening Pressure and CO<sub>2</sub>/CH<sub>4</sub> Mixture\nSeparation

The targeted development\nof adsorbents tailored for specific separation\ntasks offers an opportunity to optimize selectivity, energy usage,\nand cycling. One instance of such a separation challenge is presented\nby biogas, a renewable energy source primarily composed of methane\nand carbon dioxide. The field of flexible metal–organic frameworks\n(MOFs) has garnered significant attention from researchers due to\ntheir potential for gas storage and capture applications. The adsorbate-dependent\nthreshold pressure for the phase transition in these MOFs indicates\npotential for selective gas separation. The incorporation of functionalized\nligands in these systems results in two notable effects: the introduction\nof specific adsorption sites on the internal surface as well as a\nmodification of the flexible gate-opening behavior. Here, we conducted\na comparison among different nitrogen-containing ligands (2-aminobenzimidazole,\nbenzotriazole, and 5-azabenzimidazole) incorporated into the flexible\nMOF ZIF-7. These linkers include a nitrogen atom either in a heterocycle\nor as an attached functional group. The aim was to evaluate their\nperformance for carbon dioxide–methane separation in a simulated\nbiogas scenario. We further synthesized additional derivatives of\nthe best-performing material, featuring the 5-azabenzimidazole ligand\nin varying quantities. Breakthrough experiments conducted under actual\nmixture conditions reveal that there is an optimal ligand exchange\npoint (10%) facilitating the phase transition without significantly\nenhancing methane adsorption. Density functional theory simulations\nconfirm that increased functionalization led to an easier phase transition\ndue to heightened stability of the open phase resulting from additional\nhydrogen bonding, coupled with a weakened collapsed phase. To assess\nthe effectiveness of this most-promising material, its performance\nwas compared to the baseline ZIF-7 adsorbent using pressure swing\nadsorption (PSA) simulations. The enhanced surface affinity toward\ncarbon dioxide, along with a sharper isotherm step and narrower hysteresis,\ntranslated to increased selectivity, faster cycling, and reduced process\ncosts due to decreased energy input as shown by optimization opportunities\nof the high-pressure feed step.