A Rational Design of Microporous Nitrogen-Rich Lanthanide Metal–Organic Frameworks for CO₂/CH₄ Separation

Three new lanthanide metal–organic frameworks IRHs-(1–3) supported by cyamelurate linkers have been synthesized and structurally characterized. The incorporation of numerous heteroatoms (N and O) into the pore walls and the relatively small microchannels of these porous solids enhance bonding force of the host–guest interactions, thus promoting the adsorption of carbon dioxide (CO2) over methane (CH4). The nonpolar covalent bonds in methane also favor the less uptake due to the hydrophilic walls of these frameworks. Grand canonical Monte Carlo simulations were performed to determine the origin of the adsorption. The density isocontour surfaces show that CO2 is mainly adsorbed on the walls composed of organic linkers and around the metal sites, whereas no specific adsorption site is observed for CH4, which indicates weak interactions between the framework and the adsorbed gas. As expected, the simulations show that CH4 is not observed around the metal center due to the presence of H2O molecules. The excellent selectivity of CO2/CH4 binary mixture was predicted by the ideal adsorbed solution theory (IAST) via correlating pure component adsorption isotherms with the Toth model. At 25 °C and 1 bar, the CO2 and CH4 uptakes for IRH-3 were 2.7 and 0.07 mol/kg, respectively, and the IAST predicated selectivity for CO2/CH4 (1:1) reached 27, which is among the best value for MOF materials.本研究合成了三种以氰尿酸酯连接基（cyamelurate linkers）为桥连配体的新型镧系金属有机框架（lanthanide metal–organic frameworks）IRHs-1~3，并完成了结构表征与解析。这些多孔固体的孔道壁中引入了大量杂原子（氮、氧），且其微通道尺寸相对较小，这强化了主客体相互作用的结合强度，进而提升了二氧化碳（CO₂）相较于甲烷（CH₄）的吸附选择性。甲烷分子所含的非极性共价键，加之该类框架的亲水孔道壁，使其吸附量更低。为阐明该吸附行为的起源，本研究开展了巨正则蒙特卡洛（Grand canonical Monte Carlo）模拟。密度等值面分析结果显示，CO₂主要吸附于有机连接基构成的孔道壁及金属位点周围，而CH₄未表现出特定吸附位点，这表明框架与吸附气体之间的相互作用较弱。正如预期，模拟结果显示，由于水分子的存在，CH₄未在金属中心附近出现吸附。通过将纯组分吸附等温线与Toth模型进行关联拟合，本研究借助理想吸附溶液理论（ideal adsorbed solution theory, IAST）预测了CO₂/CH₄二元混合体系的优异吸附选择性。在25 ℃、1 bar条件下，IRH-3的CO₂与CH₄吸附量分别为2.7 mol/kg与0.07 mol/kg，其对1:1型CO₂/CH₄混合体系的IAST预测选择性可达27，该数值位列金属有机框架（metal-organic framework, MOF）材料的最优性能之列。