(582c) Temperature Extrapolation of Henry Law Constants: Expanding the Thermodynamic Space of Adsorbent Screening

With the continual increase in computational resources and improvement in molecular simulations and force fields, computational screening of adsorbent materials for gas adsorption applications has become widespread [1,2]. Various thermodynamic properties are used for screening, but frequently the first property used for screening is the Henry Law Constant, i.e., the proportionality constant between pressure and adsorption at low pressure or, equivalently, infinitely dilute conditions for a particular adsorbent/adsorptive combination. The Henry Law Constant is especially attractive as an initial screening metric, as it can be efficiently computed by Monte Carlo integration as opposed to longer Metropolis Monte Carlo molecular simulation. This quantity is especially useful for screening materials for gas separations [3] as the ratio of the Henry Law Constants for two species is equivalent to the low-pressure selectivity of those same species. This approach to materials screening can inadvertently limit the thermodynamic search space, however, as the Henry Law Constant is necessarily an isothermal quantity. The simplest remedy is, of course, computing the property at many temperatures, though at the cost of extra computation, especially for lower temperatures. In the present work, we present a method for thermodynamically ‘extrapolating’ the Henry Law Constant based on manipulation [4] of the rigorous statistical mechanical definitions of the constant, with minimal added calculations. Thus, a single Monte Carlo prediction of the Henry Law Constant can simultaneously be reapplied to compute the same property at other temperatures. Owing to the manner of extrapolation, the extrapolation range is not narrow, but extends of tens or hundreds of degrees Kelvin, drastically expanding the space for screening combinations of adsorbent materials and adsorptive gases. Furthermore, this extrapolation scheme can also be applied to computation of the low pressure isosteric heat of adsorption from the same set of Monte Carlo integration data. We present results showing the high accuracy of the extrapolation scheme as well as demonstration screening experiments for a handful of adsorbent materials and adsorptive gases. Lastly, we demonstrate how this extrapolation scheme can be simply implemented in the FEASST Monte Carlo toolkit [5], providing modularity and ease of use for many materials and gases.

[1] Wilmer et al, Nat Chem, 4:83 (2011)
[2] Colón and Snurr, Chem Soc Rev, 43:5375 (2014)
[3] Sikora et al, Chem Sci, 3:2217 (2012)
[4] Sarkisov, J Phys Chem C, 116:3025 (2012)
[4] Mahynski, Errington, and Shen, J Chem Phys, 147:054105 (2017)
[5] Hatch, Mahynski, and Shen, J Res NIST, 123:123004 (2018)