Molecular modelling of copper sites in ZSM-5: DFT and IR studies on the properties of Cu~(2+) and Cu~+ centres and their interaction with NO

摘要

In this paper we present quantum chemical calculations for extended cluster model of ZSM-5 framework based on seven T sites forming two fused 5T rings, which mimics the a site in MFI structure. This model is large enough to be a good host for two forms of exchanged copper cation (Cu~(2+) or Cu~+) and their complexes with NO molecule. Position and coordination of each copper system suggested by model calculations are related to IR measurements of T-O-T skeletal vibration. Good agreement between experimental and theoretical changes in the strength of the bonding between the cation and framework oxygens caused by copper migration after oxidation/reduction and NO adsorption is found. The effect of the interaction between Cu~(2+) or Cu~+ site and NO seen from the shift in NO stretching frequency is strikingly different. Both IR measurements and DFT vibrational analysis show that NO molecule becomes significantly activated only after sorption on Cu~+. The effect may be explained by electron-donating ability of monovalent copper site in ZSM-5 framework.