(369c) Effects of Adsorbent Material Properties on the Selective Separation of Supercritical Multicomponent Fluid Mixtures

A great deal of effort has, thus far, been put into screening existing porous materials, including zeolites and metal-organic frameworks (MOFs), for their ability to selectively adsorb desired molecules from a fluid mixture on the basis of molecular size and shape. [1,2] However, when different molecules have similar sizes or lack characteristic structures whose complement can be readily engineered in a porous material, selective separation becomes difficult. Here we use computationally efficient flat-histogram Monte Carlo (MC) simulation methods [3] to study the adsorption of prototypical coarse-grained binary supercritical fluid mixtures in flexible porous media. We demonstrate an extensible methodology for studying multicomponent adsorption which yields the complete free energy surface of the confined mixture. We focus on a canonical binary mixture which is particularly cumbersome to separate via traditional distillation techniques, and thus is a representative candidate for alternative approaches such as adsorbents or other mass-separating agents. Interrogation of the adsorption behavior at different overall bulk compositions reveals regimes in which the material's selectivity for a single component may be a highly non-monotonic function of applied pressure. We illustrate how this may be controlled by systematically tuning the adsorbent's chemical and mechanical properties. By controlling the relative strength and range of interaction each species has with the adsorbent we identify a regime in which the selective capture of dilute components may be enhanced by increasing the bulk pressure, even if the species is less entropically favored inside the material. At high pressure, the adsorbent's flexibility becomes increasingly important and can also lead to local extrema in the selectivity of that material. We elucidate the effects of a material's flexibility on its separation efficiency and discuss its relationship to the resulting heterogeneous fluid structure inside the porous adsorbent. This allows us to propose general heuristics for the design of adsorbent materials to achieve selective separation for certain classes of fluid mixtures.

References

[1] Wilmer et al., Nature Chemistry 4, 83-89 (2012).
[2] Lin et al., Nature Materials 11, 633-641 (2012).
[3] Errington and Shen, Journal of Chemical Physics 123, 164103 (2005).