Heterogeneity of Si–OH–Al groups in HNaY zeolites

Abstract

Using time-resolved FTIR spectroscopy it has been shown for NH₃ TPD on an HNaY zeolite that ammonia is bound to the Si–OH–Al groups in two clearly distinguishable adsorption states. Adsorbed ammonia causing the band at 1450 cm^–1 in the NH bending region desorbs at temperatures above 473 K. The course of desorption can be described by a unimodal desorption energy distribution function in the energy range of 100–135 kJ mol^–1. More weakly adsorbed ammonia, characterised by NH bands at 1680, 1500 and partly 1390 cm^–1, is desorbed below 473 K with a desorption energy distribution of 90–120 kJ mol^–1. This assignment is confirmed by evaluation of the OH stretching region [high frequency (HF) and low frequency (LF) stretching vibrations]. At both NH₄⁺ adsorption complexes there are contributions from both the HF and the LF bands. The stronger acid sites are characterised by OH sub-bands at 3635 cm^–1(HF) and 3565 cm^–1(LF), i.e. at the same wavenumbers as found for the OH groups in highly dealuminated faujasites. They are therefore assigned to isolated Brønsted acid sites. The acidity distribution seems to be mainly determined by the number of Al atoms in the vicinity of the Si–OH–Al groups, rather than by the different position of the corresponding framework oxygen.