文章精读 - Paper Reading 1：Elucidating the Competitive Adsorption of H2O and CO2 in CALF-20

链接 - Link：https://pubs.acs.org/doi/10.1021/acsami.3c11092?ref=pdf

作者 - Author：Ching-Hwa Ho* and Francesco Paesani*

年份 - Year：2023

期刊 - Journal：ACS Applied Materials & Interfaces (8.3)

标题 - Title：Elucidating the Competitive Adsorption of H2O and CO2 in CALF-20: New Insights for Enhanced Carbon Capture Metal–Organic Frameworks

关键词 - keywords：metal−organic frameworks; carbon capture; molecular dynamics; adsorption

摘要 - Abstract：

鉴于对高效碳捕获解决方案的迫切需求，我们的研究调查了 CALF-20 中水 (H2O) 和二氧化碳 (CO2) 的同时吸附与相对湿度的函数关系，CALF-20 是一种高度可扩展且稳定的金属有机材料。框架（MOF）。先进的计算机模拟表明，由于它们与框架的相互作用相似，H2O 和 CO2 分子竞争相同的结合位点，占据 CALF-20 孔内相似的空隙区域。这种竞争导致 H2O 和 CO2 分子产生不同的热力学和动力学行为，具体取决于是否存在一种或两种客体物种。值得注意的是，CO2分子的存在迫使H2O分子在较小的区域内形成更多连接的氢键网络，减缓水的重新取向动力学并降低水的熵。相反，水的存在会加速 CO2 分子的重新定向，降低 CO2 熵，并由于水介导的相互作用更强而增加 CO 被吸附在框架内的倾向。由于对相同空隙空间的竞争，当存在其他客体物种的分子时，H2O 和 CO2 分子都表现出较慢的扩散速度。这些发现为增强专为碳捕获应用而设计的 MOF 中 H2O 和 CO2 的差异亲和力提供了有价值的策略和见解。

In light of the pressing need for efficient carbon capture solutions, our study investigates the simultaneous adsorption of water (H2O) and carbon dioxide (CO2) as a function of relative humidity in CALF-20, a highly scalable and stable metal–organic framework (MOF). Advanced computer simulations reveal that due to their similar interactions with the framework, H2O and CO2 molecules compete for the same binding sites, occupying similar void regions within the CALF-20 pores. This competition results in distinct thermodynamic and dynamical behaviors of H2O and CO2 molecules, depending on whether one or both guest species are present. Notably, the presence of CO2 molecules forces the H2O molecules to form more connected hydrogen-bond networks within smaller regions, slowing water reorientation dynamics and decreasing water entropy. Conversely, the presence of water speeds up the reorientation of CO2 molecules, decreases the CO2 entropy, and increases the propensity for CO2 to be adsorbed within the framework due to stronger water-mediated interactions. Due to the competition for the same void spaces, both H2O and CO2 molecules exhibit slower diffusion when molecules of the other guest species are present. These findings offer valuable strategies and insights into enhancing the differential affinity of H2O and CO2 for MOFs specifically designed for carbon capture applications.

研究方法 - Research Methods:

使用MD模拟，研究了CALF-20中H2O和CO2的竞争吸附。 - 灵活力场 - Zn：UFF；其他原子：GAFF - 使用MB-pol 和 MB-nrg 力场（他们实验室自己开发的）模拟水和二氧化碳的多体势能 - 竞争吸附模拟（competitive adsorption simulation） - 随机生成H2O 和 CO 分子的初始位置 Packmol - 平衡 100 ps。 - 熵和所有动力学特性通过对 200 多个独立的 50 ps 轨迹进行平均来计算 - 使用两相热力学 (2PT) 模型将熵计算为相对湿度 (RH) 的函数，该模型将 H2O 和 CO2 分子的态密度分解为类气体和类固体成分，并能够计算热力学性质。 - 计算扩散系数，作为 RH 函数。

Using molecular dynamics (MD) simulations, the competitive adsorption of H₂O and CO₂ in CALF-20 was investigated. - Flexible Force Field: - Zn atoms: UFF; other atoms: GAFF. - The many-body potentials of H₂O and CO₂ were modeled using MB-pol and MB-nrg force fields (developed in-house). - Competitive Adsorption Simulation: - Initial positions of H₂O and CO₂ molecules were randomly generated with Packmol. - Equilibrium phase lasted 100 ps. - Entropy and all dynamic properties were averaged over more than 200 independent 50 ps trajectories. - Entropy was calculated as a function of relative humidity (RH) using the two-phase thermodynamics (2PT) model, which decomposes the density of states of H₂O and CO₂ into gas-like and solid-like components, enabling the calculation of thermodynamic properties. - Diffusion coefficients were also calculated as a function of RH.

结论 - Conclusions：

CALF-20框架内H2O和CO分子之间的竞争可归因于两个因素： - H2O和CO分子与框架之间的相互作用强度相当 - H2O和CO分子偏好占据框架内相似的空隙空间

CALF-20框架发挥限制效应，导致H2O和CO分子的空间排列类似于高压下液态H2O和CO分子的空间排列。

The competition between H₂O and CO molecules within the CALF-20 framework can be attributed to two main factors: - The interaction strengths between the framework and each of the H₂O and CO molecules are comparable. - H₂O and CO molecules both prefer to occupy similar void spaces within the framework.

The CALF-20 framework imposes a confinement effect, resulting in a spatial arrangement of H₂O and CO molecules similar to that found in high-pressure liquid phases of H₂O and CO.

金句 - Quotes:

与 H2O 相比，极化电荷分布较少的框架可能对 CO2 表现出更强的亲和力。

Frameworks with a lower degree of polarized charge distribution compared to H₂O may exhibit a stronger affinity for CO₂.

一方面，水的存在增加了 CO2 分子对 CALF-20 的能量亲和力，而不会显着影响其在低相对湿度值下的熵，另一方面，由于体积排阻效应，它会导致CALF-20 的 CO2 吸附能力降低。

On one hand, the presence of water increases the energetic affinity of CO₂ molecules for CALF-20 without significantly affecting their entropy at low relative humidity. On the other hand, due to steric hindrance, it reduces the CO₂ adsorption capacity of CALF-20.