Journal of Physical Chemistry B, Vol.103, No.11, 1841-1853, 1999
Adsorption of linear alkanes in the alpha-cages and gamma-cages of H-ZK-5 and K-ZK-5
The adsorption properties of linear alkanes in H-ZK-5 and K-ZK-5 were studied at room temperature by means of sorption and C-13 NMR measurements and by molecular simulations. Alkane molecules up to n-heptane are able to adsorb in the alpha- and gamma-cages of ZK-5, but adsorption of longer alkane molecules is limited to the large alpha-cages as a result of size exclusion from the small gamma-cages. These gamma-cages are the preferred adsorption sites for propane and n-butane because of the more favorable heats of adsorption. In contrast, n-hexane and n-heptane adsorb preferentially in the large alpha-cages because of the larger entropy in these cages. For n-pentane, the gamma-cages are the preferred adsorption sites in H-ZK-5, whereas in K-ZK-5 the alpha-cages are the preferred adsorption sites. This difference is caused by the presence of the potassium cations in the gamma-cages of K-ZK-5 which constrains the adsorption in these cages. The difference in siting of molecules with different chain lengths is the result of a steep decrease of the entropy of the adsorbed molecules in the gamma-cages with increasing chain length. The C-13 NMR measurements and molecular simulations show that the molecules in the gamma-cages are in general more coiled than those in the alpha-cages. Although the conformational equilibria of the molecules in the alpha-cages of H-ZK-5 and K-ZK-5 are comparable, the molecules in the gamma-cages of K-ZK-5 are considerably more coiled than those in the gamma-cages of H-ZK-5. The simulation parameters describing the zeolite-alkane interaction were tested by a comparison of the simulation results with the experimental findings.