Chain Length Effects of Linear Alkanes in Zeolite Ferrierite. 1. Sorption and 13C NMR Experiments

Abstract

Temperature-programmed desorption, heat of adsorption, adsorption isotherm, and 13C NMR measurements are used to study the sorption properties of linear alkanes in ferrierite. Some remarkable chain length effects are found in these properties. While propane, n-butane, and n-pentane fill the ferrierite pore structure completely, n-hexane and n-heptane can only access a part of the pore structure. It is shown by 13C NMR that n-hexane adsorbs only in the 10-ring channels of ferrierite and not in the ferrierite 8-ring cages. Adsorption of n-pentane in this cage is possible but only at relatively high pressures. At low pressures, only the 10-ring channels are filled by n-pentane. This remarkable sorption behavior is caused by the much lower heat of adsorption of n-pentane in the ferrierite 8-ring cage compared to the 10-ring channels and results in a two-stage desorption profile. In contrast to n-pentane, propane and n-butane adsorb easily into the complete pore structure, which is reflected in the normal single-stage desorption profile. The 13C NMR measurements show furthermore that propane is preferentially adsorbed in the ferrierite 8-ring cage, while no clear preference is found for n-butane.