Heterogeneity of OH groups in mordenites

Abstract

Hydroxyl groups in H-mordenites are localized both in broad channels and in side pockets. Heterogeneity of OH groups in large channels was studied by IR experiments of benzene and CO sorption. It has been found that the band of hydroxyls interacting with both probe molecules was asymmetric and split. The submaxima of lower Δν (206 cm−1 for CO and 240 cm−1 for benzene) were assigned to less acidic (SiO)2(AlO)Si–OH–Al(OSi)3 groups, and submaxima of higher Δν (300 cm−1 for CO and 360 cm−1 for benzene) were assigned to more acidic (SiO)3Si–OH–Al(OSi)3. The presence of Si(1Al) and Si(2Al), which are indispensable for the formation of the above proposed hydroxyls, was confirmed by our 29Si MAS NMR experiments. The values of deprotonation energies (PA) of both kinds of hydroxyls were estimated from Δν of hydroxyls interacting with CO. Analysis of the second derivative diagram of IR bands of more acidic (SiO)3Si–OH–Al(OAl)3 hydroxyls interacting both with benzene and CO suggested that these bands were also split and that these hydroxyls were also heterogenous.