Putting the Squeeze on CH₄ and CO₂ through Control over Interpenetration in Diamondoid Nets

We report the synthesis, structure, and sorption properties of a family of eight diamondoid (dia) metal–organic materials (MOMs) that are sustained by Co­(II) or Zn­(II) cations linked by one of three rigid ligands: 4-(2-(4-pyridyl)­ethenyl)­benzoate (1), 4-(pyridin-4-yl)­benzoate (2), and 4-(pyridin-4-yl)­acrylate (3). Pore size control in this family of dia nets was exerted by two approaches: changing the length of the linker ligand from 1 to 3, and using solvent as a template to control the level of interpenetration in nets based upon 1 and 3. The resulting MOMs, dia-8i-1, dia-5i-3, dia-7i-1-Zn, dia-7i-1-Co, dia-4i-3-a, dia-4i-3-b, dia-4i-2, and dia-4i-1, exhibit 1D channels with pore limiting diameters (PLDs) of 1.64, 2.90, 5.06, 5.28, 8.57, 8.83, 11.86, and 18.25 Å, respectively. We selected dia nets for this study for the following reasons: their 1D channels facilitate study of the impact of pore size on gas sorption parameters in situations where pore chemistry is similar (pyridyl benzoate-type linkers) or identical (in the case of polymorphs), and their saturated metal centers eliminate open metal sites from dominating sorbent–solvate interactions and possibly masking the effect of pore size. Our data reveal that smaller pore sizes offer stronger interactions, as determined by the isosteric heat of adsorption (Qst) and the steepness of the adsorption isotherm in the low-pressure region. The porous MOM with the smallest PLD suitable for physisorption, dia-7i-1-Co, was thereby found to exhibit the highest Qst values for CO2 and CH4. Indeed, dia-7i-1-Co exhibits a Qst for CH4 of 26.7 kJ/mol, which was validated through grand canonical Monte Carlo simulation studies of CH4 adsorption. This Qst value is considerably higher than those found in covalent organic frameworks and other MOMs with unsaturated metal centers. These results therefore further validate the critical role that PLD plays in gas adsorption by porous MOMs.本研究报道了8种金刚石型（diamondoid，缩写dia）金属有机材料（metal–organic materials，MOMs）的合成、结构与吸附性能。该系列材料由Co(II)或Zn(II)阳离子与三种刚性配体之一配位构建，三种配体分别为4-(2-(4-吡啶基)乙烯基)苯甲酸根（1）、4-(吡啶-4-基)苯甲酸根（2）以及4-(吡啶-4-基)丙烯酸根（3）。该系列dia拓扑网络的孔径调控可通过两种策略实现：一是改变连接配体的链长（从配体1至配体3）；二是以溶剂为模板，调控基于配体1和3构建的网络的互穿程度。所得到的8种MOMs分别为dia-8i-1、dia-5i-3、dia-7i-1-Zn、dia-7i-1-Co、dia-4i-3-a、dia-4i-3-b、dia-4i-2以及dia-4i-1，它们均拥有一维孔道，其孔径极限直径（pore limiting diameters，PLDs）分别为1.64、2.90、5.06、5.28、8.57、8.83、11.86和18.25 Å。 本研究选择dia拓扑网络的原因如下：其一，这类网络的一维孔道可在孔道化学环境相似（如吡啶苯甲酸酯类连接配体）或完全一致（如多晶型物）的条件下，便于探究孔径对气体吸附参数的影响；其二，其饱和金属中心可避免开放金属位点主导吸附剂-溶剂相互作用，进而掩盖孔径本身的调控效应。本研究数据表明，孔径越小，吸附相互作用越强，这一结论可通过吸附等量热（isosteric heat of adsorption，Qst）以及低压区吸附等温线的陡峭程度得以验证。 在所有适用于物理吸附的材料中，孔径极限直径最小的多孔MOM为dia-7i-1-Co，其对CO₂和CH₄的吸附等量热数值最高。具体而言，dia-7i-1-Co对CH₄的吸附等量热可达26.7 kJ/mol，该结果通过CH₄吸附的巨正则蒙特卡洛（grand canonical Monte Carlo）模拟研究得到了验证。该吸附等量热数值远高于共价有机框架以及其他带有不饱和金属位点的MOMs所报道的结果。上述研究结果进一步证实了孔径极限直径在多孔MOMs气体吸附过程中所发挥的关键作用。