(606d) Predicting Molecular Adsorption Entropies in Confined Environments

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 (Figure
1).

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.

Figure 1: Entropy
Loss Upon Adsorption on Surfaces and Confined Environments.