FTIR Study of CO Interactions with Li⁺Ions in Micro- and Mesoporous Matrices:  Coordination and Localization of Li⁺Ions

Abstract

The interaction of CO with extraframework Li+ ions coordinated in ZSM-5 and FER zeolites with different compositions is investigated by FTIR spectroscopy, and the correlation between Li+ coordination, stability of CO adsorption complexes, and νCO frequencies is discussed in terms of the theoretical investigation of the stability and vibrational dynamics of CO adsorption complexes. Differences in IR spectra of adsorbed CO due to the following effects were considered: (i) various Si/Al ratios, (ii) differences in zeolite topology, (iii) temperature, and (iv) the effect of a co-cation on relative stability and metal cation site preferences. The correlation between Li+ coordination and νCO frequencies of CO adsorption complexes discussed for Li zeolites is extrapolated to mesoporous Li−(Al)MCM-41 materials. The features in the IR spectra of CO/Li−(Al)MCM-41 are assigned to CO adsorption complexes formed on top of five- and six-membered rings on the surface of a channel wall. The spectra of CO/Li−ZSM-5 with different Si/Al ratios and various ion exchange levels bring evidence on the nonstatistical distribution of aluminum in the framework and on the preference of Li+ ions for channel-wall sites.

Microcalorimetric and FTIR Study of the Adsorption of Carbon Dioxide on Alkali-Metal Exchanged FER Zeolites

“…4. In contrast to bridged adsorption complexes of carbon monoxide described on these samples previously [29][30][31]37], where second cation lowers stretching frequency of CO, presence of second cation caused positive shift of asymmetric stretching mode (m 3 ) in the CO 2 bridged adsorption complex. These complexes are characterized by IR bands centered at 2370 and 2357 cm -1 for Na-and K-FER, respectively.…”

“…9) and can be ascribed to CO 2 bridged adsorption complexes. In contrast to bridged adsorption complexes of carbon monoxide described on these samples previously [29][30][31]37], where the interaction of CO molecule with second cation lowers stretching frequency of C-O bond, presence of second cation caused positive shift of asymmetric stretching mode (m 3 ) in the CO 2 bridged adsorption complex. The CO 2 is slightly distorted in the adsorbed form on the isolated cations, the length of the C-O bond adjacent to the cation increases slightly, whereas the length of the second C-O bond slightly decrease.…”

“…Spectra of CO 2 adsorbed on Na-FER with Si/Al 27. but in this case we cannot assign the band to this complex because its intensity is relatively high and correspond neither with results of chemical elemental analysis (Na/Al atomic ratio is 0.98) nor experimental results of FTIR study of low-temperature adsorption of CO molecules on the same samples [27,30,31,37]. The population of individual bands depends on the content of aluminum in the zeolite framework (cf.…”

“…In parallel with our previous investigation of CO adsorption on the same alkali-metal exchanged FER zeolites [29][30][31]37], we believed that differences in the adsorption heats of CO 2 on the investigated samples are caused by existence of bridged M ?… O=C=O … M ? complexes on dual cationic sites, which are more populated in the zeolites with higher concentration of extraframework cations, thus with lower Si/Al ratio.…”

“…7, high-frequency band of m 3 stretching vibration spectra of Na-and K-FER system is more populated for sample with higher aluminum content. Based on our previous studies on CO interaction with the same adsorbents [29,30,37] and general model for vibration of small molecules inside zeolite channels derived from these data [31] we believe that the band at 2370 and 2357 cm -1 for Na-FER and K-FER, respectively, can be assigned to bridged adsorption complexes, where CO 2 molecule interact at the same time with two cations forming so-called dual cationic site. The existence of dual cationic sites, and thus bridged adsorption complexes, in the Li-FER zeolites cannot be ruled out, but our IR spectra did not bring clear evidence of this type of complex.…”