On the role of the pore size and tortuosity for sorption of alkanes in molecular sieves

Adsorption of light alkanes on the acidic zeolites theta-1 (II-TON), ferrierite (H-FER), and ZK-5 (H-KFI) was studied by gravimetry, calorimetry, and infrared spectroscopy. Sorption of alkanes on the Bronsted acid sites is energetically favored, and the uptake at low alkane partial pressures is directly correlated to the concentration of acid sites. With H-FER, the accessibility of the two intersecting channel systems depends on the chain, length of the alkanes. Although propane and n-butane sorb in both channels, the sorption of n-pentane is hindered in the 8-ring channels and sorption of n-hexane only occurs in the 10-ring channels. In the presence of cavities (such as found in H-KFI) pronounced intermolecular interactions between sorbed alkanes were monitored. In general, the heat of adsorption of alkanes increases with decreasing pore size. At pore sizes smaller than 0.5 nm repulsion forces start to be important, their impact being most prominent for larger isoalkanes.