Modeling benzene orientational randomization in Na–Y zeolite at finite loadings with kinetic Monte Carlo and master equation methods

Abstract

We have modeled the orientational dynamics of benzene in Na–Y zeolite, motivated by the NMR study of Isfort et al. at loadings of five benzenes per cage [Chem. Phys. Lett. 288, 71 (1998)]. We consider guest-guest interactions in two stages: first, we include only site blocking; next, we consider both site blocking and nearest-neighbor attractions. We calculated orientational correlation functions using kinetic Monte Carlo and also with a mean field master equation (MFME). Both methods produce correlation functions exhibiting biexponential decay in time. Analytically solving the MFME shows that long-time decay is controlled by a composite of intracage and cage-to-cage jumps. The apparent activation energy is greater than the fundamental cage-to-cage barrier when considering only site blocking, but is less than the same fundamental barrier when also including guest-guest attractions. This suggests that the actual cage-to-cage barrier is greater than the 40 kJ mol−1 reported by Isfort et al., which lends credence to previous simulations of benzene in Na–Y.