Schaper Larissa, Keupp Julian, Schmid Rochus
Computational Materials Chemistry Group, Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Bochum, Germany.
Front Chem. 2021 Oct 25;9:757680. doi: 10.3389/fchem.2021.757680. eCollection 2021.
One of the most investigated properties of porous crystalline metal-organic frameworks (MOFs) is their potential flexibility to undergo large changes in unit cell size upon guest adsorption or other stimuli, referred to as "breathing". Computationally, such phase transitions are usually investigated using periodic boundary conditions, where the system's volume can be controlled directly. However, we have recently shown that important aspects like the formation of a moving interface between the open and the closed pore form or the free energy barrier of the first-order phase transition and its size effects can best be investigated using non-periodic nanocrystallite (NC) models [Keupp et al. (Adv. Theory Simul., 2019, 2, 1900117)]. In this case, the application of pressure is not straightforward, and a distance constraint was used to mimic a mechanical strain enforcing the reaction coordinate. In contrast to this prior work, a mediating particle bath is used here to exert an isotropic hydrostatic pressure on the MOF nanocrystallites. The approach is inspired by the mercury nanoporosimetry used to compress flexible MOF powders. For such a mediating medium, parameters are presented that require a reasonable additional numerical effort and avoid unwanted diffusion of bath particles into the MOF pores. As a proof-of-concept, NCs of pillared-layer MOFs with different linkers and sizes are studied concerning their response to external pressure exerted by the bath. By this approach, an isotropic pressure on the NC can be applied in analogy to corresponding periodic simulations, without any bias for a specific mechanism. This allows a more realistic investigation of the breathing phase transformation of a MOF NC and further bridges the gap between experiment and simulation.
多孔晶体金属有机框架材料（MOFs）最受研究的特性之一是其在客体吸附或其他刺激下，晶胞尺寸可能发生巨大变化的潜在灵活性，即所谓的“呼吸”现象。在计算方面，此类相变通常使用周期性边界条件进行研究，在这种条件下系统的体积可直接控制。然而，我们最近发现，诸如开放孔和封闭孔形式之间移动界面的形成、一级相变的自由能垒及其尺寸效应等重要方面，使用非周期性纳米微晶（NC）模型进行研究最为合适[Keupp等人（《理论与模拟进展》，2019年，2卷，1900117）]。在这种情况下，施加压力并不直接，因此使用距离约束来模拟强制反应坐标的机械应变。与之前的工作不同，本文使用中介粒子浴对MOF纳米微晶施加各向同性静水压力。该方法的灵感来源于用于压缩柔性MOF粉末的汞压汞法。对于这种中介介质，给出了相关参数，这些参数需要合理的额外数值计算量，并避免浴粒子意外扩散到MOF孔中。作为概念验证，研究了具有不同连接体和尺寸的柱状层MOF的纳米微晶对浴施加的外部压力的响应。通过这种方法，可以类似于相应的周期性模拟对纳米微晶施加各向同性压力，而不会对特定机制产生任何偏差。这使得对MOF纳米微晶的呼吸相变进行更现实的研究成为可能，并进一步弥合了实验与模拟之间的差距。
