Quantitative analysis of IR spectra of carbonylic species in alkali–metal exchanged ZSM-5 and FER zeolites

• Absorption coefficients determination of carbonyls in alkali–metal FER and ZSM-5.
• Quantification of bridged complexes in Na- and K-form of ZSM-5 and FER zeolites.
• No general validity of molar absorption coefficients of M–CO complexes.

The molar absorption coefficients of IR bands previously assigned to monocarbonyls on isolated cations, bridged carbonyls interacting with two nearby cations and isocarbonyls formed on potassium and sodium cations in the FER and ZSM-5 zeolites were determined by analyzing data obtained under specific adsorption conditions by using IR spectroscopy coupled with volumetry. In accordance with the prediction of the electrostatic model of CO interaction with potassium and sodium cations, molar absorption coefficient of isocarbonyl species is higher than molar absorption coefficient of C-down monocarbonyl complex. Behavior of bridged carbonyl complexes does not obey general rule according to which, for localized vibrations, the band intensity increases as their frequency decreases. Calculated molar absorption coefficients of given type of carbonyl complex in different zeolite structures are not the same and cannot be generalized. Thanks to these results, the quantity of different surface carbonyl species was estimated. Especially, amounts of bridged carbonyl complexes in FER and ZSM-5 zeolites were determined for the first time.

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