Variable-temperature IR spectroscopic and theoretical studies on CO2 adsorbed in zeolite K-FER

Abstract

Adsorption of CO(2) in K-FER zeolite is investigated by a combination of variable-temperature IR spectroscopy and periodic DFT calculations augmented for description of dispersion interactions. Calculated adsorption enthalpies for CO(2) adsorption complexes on single extra-framework K(+) sites and on dual-cation sites where CO(2) interacts simultaneously with two extra-framework K(+) cations (-40 and -44 kJ mol(-1), respectively) are in excellent agreement with experimental values. The analysis of effects on the frequency of the asymmetric CO(2) stretching mode ν(3) shows that polarization of CO(2) by the K(+) cation leads to an increase in ν(3), while the interaction of CO(2) with the zeolite framework leads to a decrease in ν(3). In the case of K-FER, the latter effect is slightly larger than the former, and thus a small redshift in ν(3) results (-3 cm(-1) with respect to free CO(2)). For adsorption complexes on dual K(+) sites, where CO(2) interacts with one K(+) cation on each end of the molecule, the polarization of CO(2) molecules on both sides results in a blueshift of ν(3). The origin of the redshift in ν(3) when CO(2) is adsorbed in purely siliceous FER is also investigated computationally. Calculations show that the dispersion interaction does not affect the vibrational frequency of adsorbed CO(2).