Dynamics of benzene in zeolite KL

Zeolites can be viewed as 'solid solvents', wherein an adsorbed molecule is confined within narrow pores and interacts with the zeolite framework and the exchangeable cations. However, it is not clear what part entropy plays in this confinement effect. It is often assumed, especially in cation-Containing zeolites, that the entropy effects are negligible, so that all conclusions about adsorption and diffusion are drawn from energetic considerations alone, such as adsorption energies and energy barriers to the diffusion. In order to test this hypothesis, we have undertaken the study of adsorption and dynamics of benzene in zeolite L, with (KL) and without (LTL) compensating K cations. We have constructed and validated against experimental data a model of zeolite KL with Si/Al = 3. Adsorption and dynamics of benzene in this zeolite were studied using static simulation methods, i.e. molecular docking and constrained energy minimization, as well as molecular dynamics (MD) simulations at various temperatures and loadings. All simulations used standard techniques available within MSI'S Cerius and InsightII environments. Our simulations show a large discrepancy between the two types of methods, revealing the importance of entropy, even in KL: energy tends to localize adsorption on the cation, entropy to delocalize it over the cage; energy tends to favor adsorption at a 12-Membered ring (12T) window between two cages, entropy to destabilize this site; energy tends to cluster up to three benzene molecules within the same cage, entropy to spread the molecules over all cages. From a linear fit with temperature of the free-Energy barrier due to the potential of mean-Force (MF) along the channel axis, we can estimate an order of magnitude of the entropic barrier to the diffusion through the 12T window as 0.02 kJ mol K between 200 and 600 K at infinite dilution, both in LTL and KL.