Variable-Temperature IR Spectroscopic and Theoretical Studies on CO2 Adsorbed in Zeolite K-FER

Adsorption of CO2 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 CO2 adsorption complexes on single extra-framework K+ sites and on dual-cation sites where CO2 interacts simultaneously with two extra-framework K+ cations (-40 and -44 kJmol(-1), respectively) are in excellent agreement with experimental values. The analysis of effects on the frequency of the asymmetric CO2 stretching mode nu(3) shows that polarization of CO2 by the K+ cation leads to an increase in nu(3), while the interaction of CO2 with the zeolite framework leads to a decrease in nu(3). In the case of K-FER, the latter effect is slightly larger than the former, and thus a small redshift in nu(3) results (-3 cm(-1) with respect to free CO2). For adsorption complexes on dual K+ sites, where CO2 interacts with one K+ cation on each end of the molecule, the polarization of CO2 molecules on both sides results in a blueshift of nu(3). The origin of the redshift in nu(3) when CO2 is adsorbed in purely siliceous FER is also investigated computationally. Calculations show that the dispersion interaction does not affect the vibrational frequency of adsorbed CO2.