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

✍ Scribed by P. C. M. M. Magusin; D. Schuring; E. M. van Oers; J. W. de Haan; R. A. van Santen

Publisher
John Wiley and Sons
Year
1999
Tongue
English
Weight
143 KB
Volume
37
Category
Article
ISSN
0749-1581

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✦ Synopsis

Using 1D-and 2D-exchange 13 C 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 1-10 3 s 1 . This corresponds to intra-zeolite self-diffusion constants of 10 18 -10 15 m 2 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 š 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 š 20 J K 1 mol 1 for both H-and K-ZK-5.

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