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+···CO···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.

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Adsorption of CO(2) in K-FER zeolite is investigated by a combination of variable-temperature IR spectroscopy and periodic DFT calculations augmented for description of dispersion interactions. Calculated adsorption enthalpies for CO(2) adsorption complexes on single extra-framework K(+) sites and on dual-cation sites where CO(2) interacts simultaneously with two extra-framework K(+) cations (-40 and -44 kJ mol(-1), respectively) are in excellent agreement with experimental values. The analysis of effects on the frequency of the asymmetric CO(2) stretching mode ν(3) shows that polarization of CO(2) by the K(+) cation leads to an increase in ν(3), while the interaction of CO(2) with the zeolite framework leads to a decrease in ν(3). In the case of K-FER, the latter effect is slightly larger than the former, and thus a small redshift in ν(3) results (-3 cm(-1) with respect to free CO(2)). For adsorption complexes on dual K(+) sites, where CO(2) interacts with one K(+) cation on each end of the molecule, the polarization of CO(2) molecules on both sides results in a blueshift of ν(3). The origin of the redshift in ν(3) when CO(2) is adsorbed in purely siliceous FER is also investigated computationally. Calculations show that the dispersion interaction does not affect the vibrational frequency of adsorbed CO(2).