Thermodynamic Study of CO2 Sorption by Polymorphic Microporous MOFs with Open Zn(II) Coordination Sites

Two Zn-based metal organic frameworks have been prepared solvothermally, and their selectivity for CO2 adsorption was investigated. In both frameworks, the inorganic structural building unit is composed of Zn­(II) bridged by the 2-carboxylate or 5-carboxylate pendants of 2,5-pyridine dicarboxylate (pydc) to form a 1D zigzag chain. The zigzag chains are linked by the bridging 2,5-carboxylates across the Zn ions to form 3D networks with formulas of Zn4(pydc)4­(DMF)2·3DMF (1) and Zn2(pydc)2­(DEF) (2). The framework (1) contains coordinated DMF as well as DMF solvates (DMF = N,N-dimethylformamide), while (2) contains coordinated DEF (DEF = N,N-diethylformamide). (1) displays a reversible type-I sorption isotherm for CO2 and N2 with BET surface areas of 196 and 319 m2/g, respectively. At low pressures, CO2 and N2 isotherms for (2) were not able to reach saturation, indicative of pore sizes too small for the gas molecules to penetrate. A solvent exchange to give (2)-MeOH allowed for increased CO2 and N2 adsorption onto the MOF surface with BET surface areas of 41 and 39 m2/g, respectively. The binding of CO2 into the framework of (1) was found to be exothermic with a zero coverage heat of adsorption, Qst0, of −27.7 kJ/mol. The Qst0 of (2) and (2)-MeOH were found to be −3 and −41 kJ/mol, respectively. The CO2/N2 selectivity for (1), calculated from the estimated KH at 296 K, was found to be 42. At pressures relevant to postcombustion capture, the selectivity was 14. The thermodynamic data are consistent with a mechanism of adsorption that involves CO2 binding to the unsaturated Zn­(II) metal centers present in the crystal structures.本研究通过溶剂热法合成了两种锌基金属有机框架（metal organic frameworks, MOF），并对其二氧化碳（CO₂）吸附选择性展开了考察。两种框架的无机结构基元均由被2,5-吡啶二羧酸（2,5-pyridine dicarboxylate, pydc）的2-位或5-位羧基桥联的Zn(II)构成，形成一维锯齿状链结构。该锯齿链通过桥联的2,5-位羧基跨接Zn离子，进而构筑出三维网状骨架，其分子式分别为Zn₄(pydc)₄(DMF)₂·3DMF（记为化合物1）与Zn₂(pydc)₂(DEF)（记为化合物2），其中DMF为N,N-二甲基甲酰胺。化合物1同时包含配位DMF与溶剂化DMF，而化合物2则含有配位的DEF（即N,N-二乙基甲酰胺）。化合物1对CO₂与N₂均表现出可逆的I型吸附等温线，其布鲁诺尔-埃米特-特勒（Brunauer-Emmett-Teller, BET）比表面积分别为196 m²/g与319 m²/g。在低压条件下，化合物2的CO₂与N₂吸附等温线未达到饱和，说明其孔径过小，无法容纳气体分子渗透进入孔道。通过溶剂交换得到的2-MeOH材料，其MOF表面对CO₂与N₂的吸附量显著提升，对应的BET比表面积分别为41 m²/g与39 m²/g。研究表明，CO₂在化合物1骨架中的结合过程为放热反应，其零覆盖吸附热Qst0（zero coverage heat of adsorption）为-27.7 kJ/mol。化合物2与2-MeOH的零覆盖吸附热Qst0分别为-3 kJ/mol与-41 kJ/mol。基于296 K下估算的亨利常数（KH）计算可得，化合物1的CO₂/N₂吸附选择性为42；在与燃烧后碳捕获相关的压力区间内，该选择性为14。上述热力学数据与吸附机制一致，该机制涉及CO₂与晶体结构中暴露的不饱和Zn(II)金属中心的配位结合作用。