Incorporating Flexibility Effects into Metal-Organic Frameworks Adsorption Simulations

Computationally predicting the adsorption properties of metal-organic frameworks (MOFs) in a high throughput way has become a useful means of identifying promising materials for chemical separations. In almost all cases, high throughput simulations use a rigid framework assumption for computational convenience. However, framework flexibility triggered by thermal vibration or external stimuli exists in nearly all MOFs. Thorough flexibility studies have been conducted previously for some well-known MOFs, including MIL-53 and ZIF-8, etc. These studies illustrated that flexibility could have a strong influence on MOF adsorption properties. In this work, we sought to examine the impact of framework flexibility on adsorption in MOFs more generally. To this end, we performed extensive MC/MD simulations that include all degrees of freedom of MOFs during molecular adsorption, using the UFF4MOF force field to describe MOF flexibility. We first examined a variety of materials for which multiple replicate experimental isotherms are available. These comparisons show the flexibility effects are important in making quantitative predictions for several well-known materials. We also performed similar calculations for a wide range of MOFs selected from the CoRE MOF database to explore the characteristics of materials for which flexibility effects are most important. Our results point towards the ability to perform high throughput simulations of adsorption in MOFs (and similar porous materials) that incorporate the important role of flexibility in the adsorbing materials.