Computational and Fourier transform infrared spectroscopic studies on carbon monoxide adsorption on the zeolites Na-ZSM-5 and K-ZSM-5:: Evidence of dual-cation sites

A combination of variable-temperature Fourier transform infrared (FTIR) spectroscopy with calculations performed at the periodic density functional theory (DFT) level was used in the investigation of carbon monoxide adsorption on zeolites Na-ZSM-5 and K-ZSM-5. On the basis of a very good agreement between experimental and calculated frequencies and adsorption enthalpies, it is shown that the IR absorption band appearing in the intermediate frequency range for adsorbed CO (2155 and 2150 cm(-1) for Na-ZSM-5 and K-ZSM-5, respectively) is due to the formation of linearly bridged CO adsorption complexes on dual-cation sites (M+center dot center dot center dot CO center dot center dot center dot M+, M = Na, K). The population of such adsorption complexes increases with increasing cation radius and with decreasing Si/Al ratio. Bridged adsorption complexes are slightly more stable than carbonyl complexes formed on isolated extraframework metal cations. Adsorption enthalpies and CO stretching frequencies of carbonyl complexes formed on isolated extraframework metal cations were found to depend on the metal cation coordination with the zeolite framework. This dependence is particularly apparent for Na-ZSM-5, where cations located on the intersection sites are coordinated to only two framework oxygen atoms and CO adsorption on these sites is up to 8 kJ/mol more stable than adsorption on the channel wall sites; CO stretching frequencies of carbonyls formed on intersection sites are up to 7 cm(-1) higher than frequencies of carbonyls formed on channel wall sites.