Heterogeneity of OH Groups in Faujasites Studied by IR Spectroscopy

Abstract

Heterogeneity of OH groups in faujasites was studied by analysis of the IR absorption band of free hydroxyls and hydroxyls engaged in hydrogen-bonding with chlorobenzene. The Si-O 1H-Al groups were found to be homogeneous in NaHX zeolite (Si/Al = 1.06) and heterogeneous in NaHY zeolite (Si/Al = 2.56). Experiments of pyridine sorption and desorption have shown that the more acidic hydroxyls desorb pyridine at higher temperatures than the less acidic hydroxyls. The IR spectroscopic results agree well with previously published 29Si MAS NMR results. Only one Si(4A1) signal is present in the NMR spectrum of X zeolite (Si/Al ≅ 1), but there are four signals (Si(3Al), Si(2Al), Si(1Al), and Si(OAl)) in Y zeolite. As Si(OAl) cannot create bridging hydroxyls, this implies three kinds of hydroxyls of various acid strengths in NaHY zeolite and one kind in NaHX, as actually observed in the present IR study. The hydroxyl groups in NaHX zeolite can be assigned to (AlO) 3Si-OH-Al(SiO) 3, and the three kinds of hydroxyls in NaHY to (AlO) 2(SiO)Si-OH-Al(SiO) 3, (AlO)(SiO) 2Si-OH-Al(SiO) 3, and (SiO) 3Si-OH-Al(SiO) 3, respectively. The elimination of a small amount of the most acidic S-O 1H-Al by pyridine significantly reduces the catalytic activity of NaHY zeolite in butene-1 isomerization.