IR Spectroscopic Study of Alkane Interaction with the Bronsted Acid Sites of Hydrogen-Exchanged Zeolites

摘要

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 (△vOH) due to the formation of a hydrogen bond with adsorbed alkanes increases in the following series: △vOH(propane) < △vOH(n-hexane) = △vOH(3-methylpen-tane). The accessibility of Si(OH)Al groups to alkane molecules is determined by the dimension of rings through which molecules enter zeolite channels and cavities. It follows from the measured △vOH values that the strength of Bronsted acid sites decreases in the following series; HZSM-5>H-mordenite ≈ H-ferrierite 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.