(303f) Exploring the Link Between the Properties of Flexible, MOF-like Adsorbent Materials and Adsorption Behavior Via Flat-Histogram Monte Carlo

Authors: 
Siderius, D. W., National Institute of Standards and Technology
Mahynski, N. A., 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. Materials that deform due to gas pressure may prove to be extremely useful in practical engineering applications. Beneficial properties of deformable sorbents include enhanced gas selectivity due to expansion or contraction of pore space and relatively large operating capacity due to large changes in pore volume [1]. Simulation modeling of such materials is advancing, but it is typically difficult to separate the individual effects of sorbent flexibility and the gas-adsorbent interaction from each other; only the net result is available. Recently, Shen and Siderius introduced a technique for simulating flexible porous materials based on flat-histogram Monte Carlo methods for studying adsorption in deformable adsorbents [2]. The method is able to extract adsorption isotherms, free energies, and limits of stability. In particular, this method allows for independent specification of the adsorbent deformation characteristics and the gas-adsorbent interaction properties. Furthermore, for a given gas-adsorption interaction, it allows for rapid evaluation the effect of different deformation characteristics on the net adsorption characteristics of the adsorbent-adsorbate system. In the present work, we apply this simulation method to study flexible adsorbents that resemble metal-organic frameworks (i.e., materials composed of coordinated metal centers and connecting organic ligands) and construct the adsorption isotherms and free-energy diagrams for adsorption of a single-component gas. We then identify certain adsorption characteristics that follow from specific material properties, including the relative strength of adsorbing regions, the nature of our model's â??ligandsâ?, and the material's flexibility, that together give rise to the observed behavior. One can envision the use of this method for designing new sorbent materials to achieve certain performance objectives, such as high operating capacity via a pore-opening response.

[1] J. A. Mason et al., Nature, 527:357 (2015).
[2] V. K. Shen and D. W. Siderius, J. Chem. Phys., 140:244106, (2014).