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(452e) Exploration of the Relationship Between Bare Sorbent Flexibility and Resulting Adsorption Behavior Via Flat-Histogram Monte Carlo

Authors: 
Siderius, D. W., National Institute of Standards and Technology
Shen, V. K., National Institute of Standards and Technology

Recent years have seen the discovery and development of adsorbent materials that deform or flex under stress, including gas pressure due to exposure to an adsorbate and external mechanical stress. Such materials are interesting from a fundamental physical point of view, but also for practical engineering applications. In particular, deformable sorbent materials might prove to be superior materials for gas separation applications since their pore space or apertures can expand or contract in a manner that favors the adsorption of one gas in a mixture over the others. Molecular simulation of such materials is advancing, but progress has been limited by the inability of conventional simulation methods to account for framework flexibility in a computationally tractable manner. In this work, we discuss a method for simulating adsorption in flexible porous materials based on flat-histogram Monte Carlo methods. The method, which is applicable in both the “flexible” grand canonical ensemble and the osmotic ensemble, computes a multi-dimensional probability distribution, from which we are able to extract adsorption isotherms, free energies, and limits of stability. We use this method with for a variety of models of adsorbent/adsorbate systems to explore the link between a bare sorbent's flexibility and the range of adsorption isotherms that can result, while keeping the gas-adsorbent interaction fixed. Our results show that a particular mode of flexibility results in qualitatively similar adsorption isotherms for a wide variety of gas-adsorbent interactions. Consequently, one can envision the use of this method for designing new sorbent materials or specifying the material characteristics that an adsorbent must possess to yield desired or specified behavior.