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

摘要

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.