Tuning flexibility to control selectivity in soft porous crystals.

We use flat-histogram Monte Carlo simulations to study how changing the flexibility of soft porous crystals (SPCs) affects their selective adsorption of a binary, size-asymmetric supercritical fluid. Specifically, we consider mesoporous SPCs which have multiple minima in their free energy profiles as a function of pore size such that they are capable of exhibiting polymorphism between a narrow and large pore phase. While specific fluid-pore interactions determine the shape of both pores' selectivity curve as a function of adsorbate pressure, an individual pore tends to selectively adsorb a species based on the size of the adsorbate molecule relative to itself, thereby shifting the pore's selectivity curve relative to its polymorph. By controlling the flexibility of a SPC, the relative thermodynamic stability of the two pore phases may be varied, thereby changing the overall selectivity of the SPC during adsorbate loading. We investigate this for two classes of SPCs: one representative of "gate-opening" materials and another of "breathing" materials. For gate-opening materials, this control is much more salient than in breathing ones. However, for the latter, we illustrate how to tune the free energy profile to create materials which breathe multiple times during adsorption/desorption.