Nandi Shyamapada, Singh Himan Dev, Chakraborty Debanjan, Maity Rahul, Vaidhyanathan Ramanathan
ACS Appl Mater Interfaces. 2021 Jun 2;13(21):24976-24983. doi: 10.1021/acsami.1c05845. Epub 2021 May 20.
Carbon capture from industrial effluents such as flue gas or natural gas mixture (cf. landfill gas), the primary sources of CO emission, greatly aids in balancing the environmental carbon cycle. In this context, the most energy-efficient physisorptive CO separation process can benefit immensely from improved porous sorbents. Metal organic frameworks (MOFs), especially the ultramicroporous MOFs, built from readily available small and rigid ligands, are highly promising because of their high selectivity (CO/N) and easy scalability. Here, we report two new ultramicroporous Co-adeninato isophthalate MOFs. They concomitantly carry basic functional groups (-NH) and Lewis acidic sites (coordinatively unsaturated Co centers). They show good CO capacity (3.3 mmol/g at 303 K and 1 bar) along with high CO/N (∼600 at 313 K and 1 bar and ∼340 at 303 K and 1 bar) selectivity, working capacity, and smooth diffusion kinetics ( = 7.5 × 10 m s). The MOFs exhibit good CO/N kinetic separation under both dry and wet conditions with a smooth breakthrough profile. Despite their well-defined CO adsorption sites, these MOFs exhibit only a moderately strong interaction with CO as evidenced from their HOA values. This counterintuitive observation is ubiquitous among many MOFs adorned with strong CO adsorption sites. To gain insights, we have identified the binding sites for CO using simulation and MD studies. The radial distribution function analysis reveals that despite the amine and bare-metal sites, the pore size and the pore structure determine the positions for the CO molecules. The most favorable sites become the confined spaces lined by aromatic rings. A plausible explanation for the lack of strong adsorption in these MOFs is premised from these collective studies, which could aid in the future design of superior CO sorbents.
从工业废气（如烟道气或天然气混合物，参见垃圾填埋气）中捕获碳，这些废气是二氧化碳排放的主要来源，这对平衡环境碳循环有很大帮助。在这种背景下，最节能的物理吸附二氧化碳分离过程可以从改进的多孔吸附剂中受益匪浅。金属有机框架材料（MOFs），特别是由易于获得的小而刚性的配体构建的超微孔MOFs，因其高选择性（二氧化碳/氮气）和易于规模化而极具前景。在此，我们报道了两种新型的超微孔钴 - 腺嘌呤间苯二甲酸酯MOFs。它们同时带有碱性官能团（-NH）和路易斯酸性位点（配位不饱和的钴中心）。它们表现出良好的二氧化碳吸附容量（在303 K和1 bar下为3.3 mmol/g）以及高二氧化碳/氮气选择性（在313 K和1 bar下约为600，在303 K和1 bar下约为340）、工作容量和平滑的扩散动力学（D = 7.5 × 10⁻¹⁰ m²/s）。这些MOFs在干燥和潮湿条件下均表现出良好的二氧化碳/氮气动力学分离性能，具有平滑的穿透曲线。尽管它们具有明确的二氧化碳吸附位点，但从它们的热重吸附（HOA）值可以看出，这些MOFs与二氧化碳的相互作用仅为中等强度。这种与直觉相悖的观察结果在许多带有强二氧化碳吸附位点的MOFs中普遍存在。为了深入了解，我们通过模拟和分子动力学（MD）研究确定了二氧化碳的结合位点。径向分布函数分析表明，尽管存在胺基和裸金属位点，但孔径和孔结构决定了二氧化碳分子的位置。最有利的位点是由芳香环排列的受限空间。基于这些综合研究，对这些MOFs中缺乏强吸附的一个合理的解释，这可能有助于未来设计出更优异的二氧化碳吸附剂。
