The Effect of Sterically Active Ligand Substituents on Gas Adsorption within a Family of 3D Zn-Based Coordination Polymers

An investigation of the adsorption properties of two structurally related, 3D coordination polymers of composition Zn­(2-Mehba) and Zn­(2,6-Me2hba) (2-Mehba = the dianion of 2-methyl-4-hydroxybenzoic acid and 2,6-Me2hba = the dianion of 2,6-dimethyl-4-hydroxybenzoic acid) is presented. A common feature of these structures are parallel channels that are able to accommodate appropriately sized guest molecules. The structures differ with respect to the steric congestion within the channels arising from methyl groups appended to the bridging ligands of the network. The host network, Zn­(2-Mehba), is able to take up appreciable quantities of H2 (77 K) and CO2 and CH4 (298 K) in a reversible manner. In regard to the adsorption of N2 by Zn­(2-Mehba), there appears to be an unusual temperature dependence for the uptake of the gas such that when the temperature is increased from 77 to 298 K the uptake of N2 increases. The relatively narrow channels of Zn­(2,6-Me2hba) are too small to allow the uptake of N2 and CH4, but H2 molecules can be adsorbed. A pronounced step at elevated pressures in CO2 and N2O isotherms for Zn­(2,6-Me2hba) is noted. Calculations indicate that rotation of phenolate rings leads to a change in the available intraframework space during CO2 dosing.本研究报道了两种结构相近、组成为Zn(2-Mehba)与Zn(2,6-Me2hba)的三维配位聚合物（3D coordination polymers）的吸附性能，其中2-Mehba为2-甲基-4-羟基苯甲酸的二价阴离子，2,6-Me2hba为2,6-二甲基-4-羟基苯甲酸的二价阴离子。两类聚合物的结构共性在于均拥有可容纳尺寸匹配客体分子的平行孔道；二者的结构差异源于骨架桥联配体上修饰的甲基所引发的孔道内空间位阻差异。主体骨架Zn(2-Mehba)可在77 K下吸附足量氢气（H2），并在298 K下可逆吸附二氧化碳（CO2）与甲烷（CH4）。针对Zn(2-Mehba)对氮气（N2）的吸附行为，其吸附量呈现反常的温度依赖性：当温度从77 K升高至298 K时，N2的吸附量反而上升。Zn(2,6-Me2hba)的孔道相对狭窄，尺寸不足以容纳N2与CH4，但可吸附H2分子。该材料在CO2与一氧化二氮（N2O）的吸附等温线中，于高压区域出现显著的吸附台阶。计算结果表明，在CO2吸附过程中，酚氧环（phenolate rings）的旋转会改变骨架内的可用空间。