HETEROGENEITY OF SI-OH-AL GROUPS IN HNAY ZEOLITES

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 c learly 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 d esorption 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 des orption energy distribution of 90-120 kJ mol(-1). This assignment is c onfirmed by evaluation of the OH stretching region [high frequency (HF ) and low frequency (LF) stretching vibrations]. At both NH4+ adsorpti on 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 therefor e assigned to isolated Bronsted acid sites. The acidity distribution s eems 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.