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

Abstract

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 AlO 4 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.