IR spectroscopic study of alkane interaction with the Bronsted acid sites of hydrogen-exchanged zeolites

Diffuse-reflectance IR spectroscopy is used to study the interaction of C-3 and C-6 alkanes (propane, n-hexane, 3-methylpentane, and cyclohexane) with the Bronsted acid sites of hydrogen-exchanged mordenite, ferrierite, ZSM-5, and faujasite. It is found that a shift of the absorption band of the stretching vibrations of acidic Si(OH)Al groups toward lower frequencies (DeltanuOH) due to the formation of a hydrogen bond with adsorbed alkanes increases in the following series: DeltanuOH(propane) < AnuOH(n-hexane) = DeltanuOH(3-methylpentane). The accessibility of Si(OH)AI groups to alkane molecules is determined by the dimension of rings through which molecules enter zeolite channels and cavities. It follows from the measured DeltanuOH values that the strength of Bronsted acid sites decreases in the following series: HZSM-5 > H-mordenite approximate to H-ferrierite much greater than HY. The difference between the three high-silica zeolites is not great. The results obtained are compared with the published IR data on Si(OH)Al groups of zeolites with adsorbed alkanes and other weak bases.