Interaction of benzene with hydroxyl groups in zeolites: A FTIR study of C 6H 6 and C 6D 6 adsorption on H-ZSM-5 and D-ZSM-5

Abstract

Adsorption of benzene on H-ZSM-5 leads, at low coverages, mainly to formation of 1:1 adducts with the zeolite bridging hydroxyl groups (3612 cm −1). As a result, the O–H modes are shifted to two discrete bands with maxima at ca. 3350 and 3250 cm −1. The appearance of two bands is attributed to Fermi resonance of the shifted O–H stretchings with the 3δ(OH) modes. Small amounts of benzene molecules interacting simultaneously with two hydroxyls are also observed. At high benzene coverages ordered structures are formed. As a result, the shifted band appears at 3175 cm −1, a value misleading to overestimated acidity. In addition, a part of benzene with time turns bonded to bridging OH groups via hydrogen atoms thus producing another shifted band at ca. 3500 cm −1. Small amount of Al–OH species were also present (band at 3663 cm −1). It was concluded that the O–H stretchings of these groups were shifted by ca. 225 cm −1 upon benzene adsorption. Silanol groups (3745–3700 cm -1) are heterogeneous and produce two shifted bands at 3625 and 3585 cm −1. Adsorption of C 6D 6 leads to similar results. The shifted O–H bands coincide in position with those produced by adsorption of C 6H 6. In this case, however, D–H exchange of the bridging OH groups easily occurs. The C 6D 6-induced shift of the O–D modes of the bridging deuteroxyls is lower than expected taking into account the H–D isotopic shift factor. In addition, the ratio between the intensities of the two bands and the split value also differ. These results confirm the occurrence of Fermi resonance.