Modeling Benzene Orientational Randomization In Na-Y Zeolite At Finite Loadings With Kinetic Monte Carlo And Master Equation Methods

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, (C) 2000 American Institute of Physics. [S0021-9606(00)70145-0].