Computational Investigation of Correlations in Adsorbate Entropy for Pure-Silica Zeolite Adsorbents

Vast
numbers of unstudied hypothetical porous frameworks
continue to spark interest in optimizing adsorption and catalytic
processes. Evaluating the use of such materials depends on the accessibility
of thermodynamic metrics such as the free energy, which, in turn,
depend on the satisfactory estimation or calculation of the adsorption
entropy, which often remains elusive. Previous works using simulations
and experimental data have demonstrated relationships between the
entropy and system descriptors, allowing for sensible predictions
based on more-easily obtained physical parameters. However, the resultant
conclusions were either based on experimental data for industrially
relevant alkanes or lacked a significant sample size. In this paper,
we evaluate correlations between gas-phase and adsorbed-phase entropies
for a larger and more chemically diverse set of adsorbate molecules
by using force fields and statistical mechanical expressions to calculate
those entropies. In total, we perform calculations for 37 molecules
across 10 chemical categories available in the TraPPE force field
set, as adsorbed in five siliceous zeolites. Our results show that
linear correlations between the gas- and adsorbed-phase entropies
persist for the larger and diverse set of adsorbate molecules studied
here, proving a broader applicability and justifying the use of simple
correlations for many adsorbates and, presumably, adsorbent materials.