O—H stretching frequencies in NaHX and NaHY zeolites: IR spectroscopic studies and quantum chemical calculations

Abstract

Four types of bridging hydroxy groups in H-faujasites may be defined depending on the number of aluminium atoms in the vicinity of the bridge. Such hydroxy groups have been investigated by quantum chemical calculations for cluster models of the type (AlO)_{n– 1}(SiO)_{4 –n}Si—OH—Al(OSi)₃, where n= 1–4. Hydroxy groups in the series show decreasing acidity with increasing n which may be correlated with increasing deprotonation energies and OH stretching frequencies. In this work, hydroxy stretching frequencies evaluated from both IR spectroscopic studies and cluster model quantum chemical calculations are presented. Experimental and theoretical results show the same trends. Good correlation between experiment and calculation for explicit cluster models, including the correct number of aluminium atoms, confirms the dependence of acidic properties of bridging hydroxy groups on the chemical environment of the zeolites studied.