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种类金刚石（diamondoid，简称dia）金属有机材料（metal–organic materials，简称MOMs）家族的合成、结构与吸附性能，该系列材料由钴(II)或锌(II)阳离子与三种刚性配体之一配位构建而成，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dia网状结构系列材料的孔径调控可通过两种策略实现：一是改变桥联配体的链长（从配体1到配体3）；二是以溶剂为模板，调控基于配体1和配体3的网状结构的互穿程度。 所得到的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及dia-4i-1，它们均拥有一维孔道，其孔径极限直径（pore limiting diameters，简称PLD）分别为1.64、2.90、5.06、5.28、8.57、8.83、11.86与18.25 Å。 本研究选用dia网状结构的原因如下：其一，这类材料的一维孔道便于在孔道化学环境相似（吡啶基苯甲酸型桥联配体）或完全一致（多晶型情况）的条件下，研究孔径对气体吸附参数的影响；其二，其金属中心处于饱和配位状态，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容热（isosteric heat of adsorption，简称Qst）以及低压区吸附等温线的陡升程度予以验证。 在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Qst值均为所有材料中最高。其中，dia-7i-1-Co对CH₄的Qst可达26.7 kJ/mol，这一结果通过CH₄吸附的巨正则蒙特卡洛模拟研究得到了验证。 该Qst值远高于共价有机框架以及其他带有不饱和金属中心的MOMs的对应数值。综上，本研究结果进一步证实了PLD在多孔MOMs气体吸附过程中所发挥的关键作用。