Single and Dual Cation Sites in Zeolites: Theoretical Calculations and FTIR Spectroscopic Studies on CO Adsorption on K-FER

Interaction of CO with K-FER zeolite was investigated by a combination of variable-temperature IR spectroscopy and computational study. Calculations were performed using ωCO/rCO correlation method in combination with a periodic density functional theory model. On the basis of agreement between experimental and calculated results, the following carbonyl complexes were identified: (i) mono- and dicarbonyl C-down complexes on single K+ sites characterized by IR absorption bands at 2163 and 2161 cm-1, respectively; (ii) complexes formed by CO bridging two K+ ions separated by about 7−8 Å (dual sites) characterized by a band at 2148 cm-1; and (iii) isocarbonyl (O-down) complexes characterized by a band at 2116 cm-1. The bridged carbonyl complexes on dual K+ sites are about 5 kJ/mol more stable than monodentate (monocarbonyl) CO complexes. The C−O stretching frequency of monocarbonyl species in K-FER depends on K+ location in the zeolite, and not on K+ coordination to the framework. A combination of theoretical calculations using a periodic density functional model and experimental results showed formation of two types of monocarbonyls. The most abundant type appears at 2163 cm-1, and the less abundant one at 2172 cm-1. These experimentally determined wavenumber values coincide, within ±2 cm-1, with those derived from theoretical calculations.