Predicting Molecular Adsorption Entropies in Confined Environments

Omar Abdelrahman

Department of Chemical Engineering
University of Massachusetts Amherst, MA, Il, USA

The adsorption of a molecule on a solid surface is at the heart of all heterogeneously catalyzed processes, ultimately influencing the rate at which surface reactions will proceed. While a molecule is typically stabilized through enthalpic contributions when adsorbing on a surface, the more restricted motion of an adsorbed molecule leads to a significant loss in entropy. While our understanding of adsorption has historically focused on enthalpic effects, significant strides have been recently made in providing quantitative descriptions of adsorption entropies for molecular adsorbates on single crystal surfaces. It however remains unclear whether such descriptions can be readily applied to more realistic systems, such as those of porous materials, where other effects may become relevant. One such significant effect is that of confinement, where an adsorbed molecule loses more entropy upon adsorption as it “feels” more the presence of its host adsorbent.

To this end, we examine the entropy of molecular adsorption in confined environments, using the adsorption of alkanes and permanent gases in zeolites as a model system. Here we consider nine different zeolite frameworks including: MFI, TON, FER, CHA, BEA, MOR, LTL, KFI and FAU. Using only experimentally measured adsorption entropies, characteristic zeolite framework dimensions and statistical mechanics, we propose a simple predictive tool to for the entropy of molecular adsorption in any confined environment. Predictive capabilities of this tool can also be extended to systems where no confinement is experienced. Implications of the developed correlation and its broader applicability, beyond zeotype materials, is discussed.