Variable‐Temperature IR Spectroscopic and Theoretical Studies on CO 2 Adsorbed in Zeolite K‐FER

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 .