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Challenges and Opportunities of Molecular Simulations for Negative Gas Adsorption
Accounts of Materials Research ( IF 14.0 ) Pub Date : 2024-04-25 , DOI: 10.1021/accountsmr.4c00071 Jack D. Evans 1 , François-Xavier Coudert 2
Accounts of Materials Research ( IF 14.0 ) Pub Date : 2024-04-25 , DOI: 10.1021/accountsmr.4c00071 Jack D. Evans 1 , François-Xavier Coudert 2
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Negative gas adsorption (NGA) is a particularly eye-catching phenomenon, involving the spontaneous desorption of gas upon pressure increase during adsorption in a flexible nanoporous material. The material undergoes a structural transition from an “open-pore” phase to a contracted “closed-pore” phase upon gas adsorption, leading to macroscopic gas desorption visible to the naked eye. It was initially evidenced experimentally in 2016 for the adsorption of methane and n-butane in the DUT-49 metal–organic framework (DUT = Dresden University of Technology) and later demonstrated to be a general phenomenon, occurring for different gases and in a variety of materials with the same topology. NGA materials belong to the category of metamaterials, displaying behavior that is not found (or rarely observed) in “natural” or simple materials. The negative adsorption transition takes place outside of thermodynamic equilibrium, and its characterization requires the use of many complementary experimental techniques (adsorption measurements, in situ X-ray diffraction, EXAFS, NMR, etc.), as well as molecular simulation techniques. In order to obtain a full and consistent picture of the NGA phenomenon, it is indeed necessary to combine computational modeling with a variety of methods, at different scales, in order to understand the microscopic behavior of the host framework and guest molecules to the macroscopic experimental results. At the smallest scale, density functional theory calculations have been used to understand the energetics and structure of the NGA materials, as well as the micromechanical properties of their organic linkers: the buckling of these linkers explains the large metastability of the open-pore phase and gives rise to the NGA transition. At a larger scale, classical grand canonical Monte Carlo simulations in the “rigid host” structures can predict the adsorption capacity of different phases, elucidating the driving force behind the structural transition. To explicitly couple the flexibility of the framework and the adsorption of guest molecules, molecular dynamics simulations (relying on a classical force field for the flexible metal–organic framework) can be coupled with free energy methods to investigate the thermodynamics of NGA, obtaining free energy profiles that determine the relative stability of different phases with varying amounts of adsorbed gas. Finally, mesoscopic-scale modeling methods are required in order to understand the phenomenon at a scale larger than one unit cell and explain experimental findings about the influence of crystal size effects on the NGA transition. This Account summarizes the computational approaches that have been used so far to better understand negative gas adsorption and highlights open questions and perspectives in this field of research.
中文翻译：
负气体吸附分子模拟的挑战和机遇
负气体吸附（NGA）是一种特别引人注目的现象，涉及柔性纳米多孔材料吸附过程中压力增加时气体的自发解吸。该材料在气体吸附时经历从“开孔”相到收缩“闭孔”相的结构转变，导致肉眼可见的宏观气体解吸。 2016 年，DUT-49 金属有机框架（DUT = 德累斯顿工业大学）中甲烷和正丁烷的吸附最初被实验证明，后来被证明是一种普遍现象，发生在不同的气体和各种环境中。具有相同拓扑结构的材料。 NGA 材料属于超材料类别，表现出“天然”或简单材料中未发现（或很少观察到）的行为。负吸附转变发生在热力学平衡之外，其表征需要使用许多补充实验技术（吸附测量、原位 X 射线衍射、EXAFS、NMR 等）以及分子模拟技术。为了获得 NGA 现象的完整一致的图像，确实有必要在不同尺度上将计算建模与多种方法相结合，以了解主框架和客体分子的微观行为到宏观实验结果。在最小尺度上，密度泛函理论计算已用于了解 NGA 材料的能量学和结构，以及其有机连接体的微观机械性能：这些连接体的屈曲解释了开孔相的大亚稳定性和引起 NGA 转变。 在更大的尺度上，“刚性主体”结构中的经典正则蒙特卡罗模拟可以预测不同相的吸附能力，阐明结构转变背后的驱动力。为了明确耦合框架的柔性和客体分子的吸附，分子动力学模拟（依赖于柔性金属有机框架的经典力场）可以与自由能方法相结合来研究 NGA 的热力学，获得自由能确定具有不同吸附气体量的不同相的相对稳定性的曲线。最后，需要介观尺度建模方法来理解大于一个晶胞的尺度上的现象，并解释有关晶体尺寸效应对 NGA 转变影响的实验结果。本报告总结了迄今为止用于更好地理解负气体吸附的计算方法，并强调了该研究领域中的悬而未决的问题和观点。
更新日期：2024-04-25
中文翻译：
负气体吸附分子模拟的挑战和机遇
负气体吸附（NGA）是一种特别引人注目的现象，涉及柔性纳米多孔材料吸附过程中压力增加时气体的自发解吸。该材料在气体吸附时经历从“开孔”相到收缩“闭孔”相的结构转变，导致肉眼可见的宏观气体解吸。 2016 年，DUT-49 金属有机框架（DUT = 德累斯顿工业大学）中甲烷和正丁烷的吸附最初被实验证明，后来被证明是一种普遍现象，发生在不同的气体和各种环境中。具有相同拓扑结构的材料。 NGA 材料属于超材料类别，表现出“天然”或简单材料中未发现（或很少观察到）的行为。负吸附转变发生在热力学平衡之外，其表征需要使用许多补充实验技术（吸附测量、原位 X 射线衍射、EXAFS、NMR 等）以及分子模拟技术。为了获得 NGA 现象的完整一致的图像，确实有必要在不同尺度上将计算建模与多种方法相结合，以了解主框架和客体分子的微观行为到宏观实验结果。在最小尺度上，密度泛函理论计算已用于了解 NGA 材料的能量学和结构，以及其有机连接体的微观机械性能：这些连接体的屈曲解释了开孔相的大亚稳定性和引起 NGA 转变。 在更大的尺度上，“刚性主体”结构中的经典正则蒙特卡罗模拟可以预测不同相的吸附能力，阐明结构转变背后的驱动力。为了明确耦合框架的柔性和客体分子的吸附，分子动力学模拟（依赖于柔性金属有机框架的经典力场）可以与自由能方法相结合来研究 NGA 的热力学，获得自由能确定具有不同吸附气体量的不同相的相对稳定性的曲线。最后，需要介观尺度建模方法来理解大于一个晶胞的尺度上的现象，并解释有关晶体尺寸效应对 NGA 转变影响的实验结果。本报告总结了迄今为止用于更好地理解负气体吸附的计算方法，并强调了该研究领域中的悬而未决的问题和观点。
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