Brønsted acid site and pore controlled siting of alkane sorption in acidic molecular sieves

The sorption of alkanes in zeolites was investigated using calorimetry, gravimetry, and in situ infrared spectroscopy. At temperatures below 373 K alkanes are found to sorb localized on the strong Brønsted acid sites (bridging Si-OH-Al groups) of H-MFI, H-MOR, and H-FAU via hydrogen bonding of the dipole induced in the alkane and the hydroxyl group. Two n-alkanes, but only one isoalkane, simultaneously interact with the Brønsted acid sites of H-MFI. With the other zeolites only one alkane molecule is sorbed locally per strong Brønsted acid site. About one-third of these sites are accessible to alkanes in H-MOR and about 70% in H-FAU. The heat of sorption decreases in the sequence H-MFI > H-MOR > H-FAU. With increasing size of the hydrocarbon the heat of adsorption increases primarily due to an increase in the dispersion forces. The linear relationships between the enthalpies and the entropies of sorption suggest that the interaction is of similar nature for all hydrocarbons and characteristic for a particular zeolite.