Diffusion of n-pentane in the zeolite ZK5 studied by high-temperature configuration-space exploration

The extremely slow diffusion of the molecule n-pentane caused by the hopping from cage-to-cage in zeolite ZK5 has been investigated by transition state theory (TST). Such slow diffusion cannot be accessed by usual molecular dynamics simulation techniques. The calculation of the partition function ratio needed for TST was enabled by a recently developed method, the so-called high-temperature configuration-space exploration (HTCE). Dynamical corrections for recrossing events have also been taken into account. The obtained intra-zeolite self-diffusion constant between 247 and 317 K of 10−16-10−15 m2 s−1 falls in the range of 10−18-10−15 m2 s−1 observed experimentally. The calculated energetic barrier between two neighboring cages of 29 kJ mol−1 is in good agreement with that of 28 ± 5 kJ mol−1 obtained from NMR measurement.