School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, USA.
J Am Chem Soc. 2013 May 15;135(19):7172-80. doi: 10.1021/ja310770c. Epub 2013 May 1.
Fundamental insight into how low pressure adsorption properties are affected by chemical functionalization is critical to the development of next-generation porous materials for postcombustion CO2 capture. In this work, we present a systematic approach to understanding low pressure CO2 affinity in isostructural metal-organic frameworks (MOFs) using molecular simulations and apply it to obtain quantitative, molecular-level insight into interesting experimental low pressure adsorption trends in a series of pillared MOFs. Our experimental results show that increasing the number of nonpolar functional groups on the benzene dicarboxylate (BDC) linker in the pillared DMOF-1 [Zn2(BDC)2(DABCO)] structure is an effective way to tune the CO2 Henry's coefficient in this isostructural series. These findings are contrary to the common scenario where polar functional groups induce the greatest increase in low pressure affinity through polarization of the CO2 molecule. Instead, MOFs in this isostructural series containing nitro, hydroxyl, fluorine, chlorine, and bromine functional groups result in little increase to the low pressure CO2 affinity. Strong agreement between simulated and experimental Henry's coefficient values is obtained from simulations on representative structures, and a powerful yet simple approach involving the analysis of the simulated heats of adsorption, adsorbate density distributions, and minimum energy 0 K binding sites is presented to elucidate the intermolecular interactions governing these interesting trends. Through a combined experimental and simulation approach, we demonstrate how subtle, structure-specific differences in CO2 affinity induced by functionalization can be understood at the molecular-level through classical simulations. This work also illustrates how structure-property relationships resulting from chemical functionalization can be very specific to the topology and electrostatic environment in the structure of interest. Given the excellent agreement between experiments and simulation, predicted CO2 selectivities over N2, CH4, and CO are also investigated to demonstrate that methyl groups also provide the greatest increase in CO2 selectivity relative to the other functional groups. These results indicate that methyl ligand functionalization may be a promising approach for creating both water stable and CO2 selective variations of other MOFs for various industrial applications.
深入了解低压吸附性能如何受到化学功能化的影响，对于开发下一代用于后燃烧 CO2 捕获的多孔材料至关重要。在这项工作中，我们使用分子模拟方法提出了一种系统的方法来理解同构金属-有机骨架（MOF）中的低压 CO2 亲和力，并应用该方法从分子水平上定量了解一系列支柱 MOF 中有趣的实验低压吸附趋势。我们的实验结果表明，在支柱 DMOF-1 [Zn2(BDC)2(DABCO)]结构的苯二甲酸二羧酸酯（BDC）连接体上增加非极性官能团的数量是调节同构系列中 CO2亨利系数的有效方法。这些发现与常见的情况相反，常见情况是极性官能团通过极化 CO2 分子诱导低压亲和力的最大增加。相反，同构系列中的 MOF 包含硝基、羟基、氟、氯和溴官能团，对低压 CO2 亲和力的增加很小。从代表性结构的模拟中获得了模拟和实验亨利系数值之间的强烈一致，并且提出了一种强大而简单的方法，涉及分析模拟吸附热、吸附物密度分布和最低能量 0 K 结合位点，以阐明控制这些有趣趋势的分子间相互作用。通过实验和模拟相结合的方法，我们展示了如何通过经典模拟理解功能化引起的 CO2 亲和力的细微、结构特异性差异。这项工作还说明了化学功能化导致的结构-性能关系如何非常特定于感兴趣结构的拓扑和静电环境。鉴于实验和模拟之间的良好一致性，还研究了预测的 CO2 对 N2、CH4 和 CO 的选择性，以证明甲基基团相对于其他官能团提供了对 CO2 选择性的最大增加。这些结果表明，甲基配体功能化可能是为各种工业应用创建其他 MOF 的既耐水又具有 CO2 选择性的变体的有前途的方法。
