Theoretical investigation of site-specific characteristics of CO adsorption complexes in the Li+-FER zeolite

The coordination of extraframework Li+ cations in FER and the formation of mono- and dicarbonyl complexes in the Li+-FER zeolite were investigated using a periodic density function theory (DFT) model. The Li+ cations strongly prefer the coordination on top of five- or six-member rings on the channel wall. The Li+ cation is always coordinated to at least three framework oxygen atoms. Calculated CO stretching frequencies are in excellent agreement with experimental results [Microporous Mesoporous Mater. 34 (2000) 67]; thus, the interpretation of experimental data at the atomic scale level is proposed. The dicarbonyl complexes can be readily formed on Li+ cations coordinated in the eight-member ring entrance window of perpendicular channel. The differences in IR spectra of CO adsorbed on Li+-FER and Li+-MFI zeolites can be understood based on the theoretical investigation. The IR band at 2193 cm−1 observed for CO/Li+-MFI system but missing in the IR spectra of CO/Li+-FER is due to the Li+ sites on the channel intersection where Li+ is coordinated only to two oxygen atoms of AlO4 tetrahedron. While such sites are populated in Li+-MFI they are not significantly populated in Li+-FER. Therefore, the differences in vibrational dynamics of CO adsorbed on Li+ in FER and MFI are due to the differences in Li+ coordination in these frameworks.