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.

中文翻译：

---

ZIF-7 中合成后配体交换对开门压力和 CO2/CH4 混合物分离的影响


针对特定分离任务量身定制的吸附剂的有针对性开发为优化选择性、能源使用和循环提供了机会。沼气就是这种分离挑战的一个例子，沼气是一种主要由甲烷和二氧化碳组成的可再生能源。柔性金属有机框架 （MOF） 领域因其在气体储存和捕获应用中的潜力而受到研究人员的极大关注。这些 MOF 中相变的吸附物依赖性阈值压力表明存在选择性气体分离的可能性。在这些系统中掺入功能化配体会产生两个显着效果：在内表面引入特定的吸附位点以及改变灵活的开门行为。在这里，我们对掺入柔性 MOF ZIF-7 中的不同含氮配体 （2-氨基苯并咪唑、苯并三唑和 5-氮杂苯并咪唑） 进行了比较。这些接头包括杂环中的氮原子或作为连接的官能团。目的是评估它们在模拟沼气场景中的二氧化碳-甲烷分离性能。我们进一步合成了性能最佳材料的其他衍生物，其中具有不同数量的 5-氮杂苯并咪唑配体。在实际混合物条件下进行的突破性实验表明，存在一个最佳配体交换点 （10%） 促进相变，而不会显着增强甲烷吸附。 密度泛函理论模拟证实，由于额外的氢键提高了开相的稳定性，再加上减弱的坍缩相，功能化程度的增加导致更容易的相变。为了评估这种最有前途的材料的有效性，使用变压吸附 （PSA） 模拟将其性能与基线 ZIF-7 吸附剂进行了比较。高压进料步骤的优化机会表明，由于能量输入减少，表面对二氧化碳的亲和力增强，以及更尖锐的等温线步骤和更窄的滞后性，转化为更高的选择性、更快的循环和更低的工艺成本。