Li Zhao, Turner Jake, Snurr Randall Q
Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208, USA.
Commun Chem. 2023 May 8;6(1):90. doi: 10.1038/s42004-023-00889-3.
Adsorption hysteresis is a phenomenon related to phase transitions that can impact applications such as gas storage and separations in porous materials. Computational approaches can greatly facilitate the understanding of phase transitions and phase equilibria in porous materials. In this work, adsorption isotherms for methane, ethane, propane, and n-hexane were calculated from atomistic grand canonical Monte Carlo (GCMC) simulations in a metal-organic framework having both micropores and mesopores to better understand hysteresis and phase equilibria between connected pores of different size and the external bulk fluid. At low temperatures, the calculated isotherms exhibit sharp steps accompanied by hysteresis. As a complementary simulation method, canonical (NVT) ensemble simulations with Widom test particle insertions are demonstrated to provide additional information about these systems. The NVT+Widom simulations provide the full van der Waals loop associated with the sharp steps and hysteresis, including the locations of the spinodal points and points within the metastable and unstable regions that are inaccessible to GCMC simulations. The simulations provide molecular-level insight into pore filling and equilibria between high- and low-density states within individual pores. The effect of framework flexibility on adsorption hysteresis is also investigated for methane in IRMOF-1.
吸附滞后是一种与相变相关的现象，它会影响诸如多孔材料中的气体存储和分离等应用。计算方法能够极大地促进对多孔材料中相变和相平衡的理解。在这项工作中，通过在具有微孔和介孔的金属有机框架中进行原子级巨正则蒙特卡罗（GCMC）模拟，计算了甲烷、乙烷、丙烷和正己烷的吸附等温线，以便更好地理解不同尺寸连通孔与外部主体流体之间的滞后现象和相平衡。在低温下，计算得到的等温线呈现出伴有滞后现象的尖锐台阶。作为一种补充模拟方法，证明了采用维道姆测试粒子插入的正则（NVT）系综模拟能够提供有关这些系统的额外信息。NVT + 维道姆模拟提供了与尖锐台阶和滞后现象相关的完整范德华回路，包括旋节线点以及GCMC模拟无法触及的亚稳和不稳定区域内的点的位置。这些模拟提供了分子水平上对单个孔内的孔填充以及高密度和低密度状态之间平衡的见解。还研究了IRMOF - 1中骨架柔性对甲烷吸附滞后的影响。
