Carbon monoxide adsorption on alkali-metal exchanged BEA zeolite: IR and thermodynamics study

Interaction of carbon monoxide with extra-framework Na+ and K+ cations of the BEA zeolite was investigated by FTIR spectroscopy carried out under various temperatures, which allowed to obtain the corresponding IR spectra and to determine isosteric enthalpies of individual adsorption complexes. Three distinct bands were observed in the spectra: (i) a red-shifted band at 2112 and 2120 cm−1 for Na- and K-BEA, respectively, assigned to O-down (isocarbonylic) complexes (ii) a blue-shifted band at 2180 and 2167 cm−1 for Na- and K-BEA, respectively, corresponding to C-down monocarbonyl complexes, and (iii) a band at the medium frequencies (2161 and 2149 cm−1 for Na- and K-BEA, respectively) ascribing to linearly bridged carbonyl complexes on dual cationic sites, where CO molecule interacts with two nearby cations via both carbon and oxygen atoms. Adsorption enthalpies of CO adsorbed on Na- and K-BEA zeolite were estimated by means of temperature and pressure dependent FTIR spectroscopy. The bridged carbonyl complexes on Na- and K-BEA were found to be about 4.2 and 2.7 kJ/mol, respectively, more stable than the appropriate C-down monocarbonyl complexes.

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► Bridged carbonyl complexes were observed in the Na- and K-BEA zeolite.
► Interaction with secondary cation results in stabilization of carbonyl complex.
► Isosteric heat of adsorption determined by modified multi-dose VTIR spectroscopy.