n-Pentane hopping in zeolite ZK-5 studied with 13C NMR

Using 1D- and 2D-exchange 13C NMR techniques it is shown that n-pentane translation in zeolite ZK-5 occurs by hopping between neighboring and cages. Owing to the structure of the ZK-5 pore network, these techniques give direct information about the translational part of molecular intercage motion, which represents the elementary step of sorbate self-diffusion. Between 247 and 317 K the hopping rates obtained for n-pentane in H- and K-ZK-5 at respective loadings of 8.1 and 9.9 molecules per unit cell are of the order of 1-103 s-1. This corresponds to intra-zeolite self-diffusion constants of 10-18-10-15 m2 s-1. Non-exponential time-correlation functions indicate heterogeneity, possibly reflecting the different loading states of the individual cages at the microscopic level. The temperature dependence of hopping shows that it is an activated process with a barrier of 28 ± 5, kJ mol-1 for both H-ZK-5 and K-ZK-5. The apparent activation entropy determined from a two-site exchange analysis is 130 ± 20 J K-1 mol-1 for both H- and K-ZK-5.