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).

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research

“…The results obtained are shown in the top inset of Figure 11 , where it is seen that the lower Si:Al ratio results in an increased intensity of the IR absorption band at 2355 cm −1 , as compared to that at 2346 cm −1 ; and this provides strong evidence that the former band should come from K + ···OCO···K + adsorption complexes bridging two K + ions (dual sites), while that at 2346 cm −1 should correspond to K + ···OCO species formed on single sites (which are expected to show a definite preponderance in the high silica zeolite). Further confirmation of those assignments was obtained by DFT calculations [ 61 ]; and analogous single and dual cation sites were also reported for the CO 2 /Na-FER system [ 62 ].…”

“…As an example of single and dual cation sites for adsorbates other than CO, the case of carbon dioxide adsorption on a K-FER zeolite will now be briefly considered; full details can be found elsewhere [ 61 ]. Representative VTIR spectra in the ν 3 region (asymmetric stretching) of CO 2 adsorbed on K-FER (Si:Al = 27.5:1) are shown in the main body of Figure 11 .…”