Chain length effects of linear alkanes in zeolite ferrierite. 1. Sorption and ¹³C NMR experiments

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.