Analysis of bonding nature being operative in the M(Li, Na, K) ion-exchanged zeolites-CO adsorption systems

The nature of the interaction of alkali-metal ion-exchanged ZSM-5 zeolite (MZSM-5; M= Li⁺, Na⁺, and K⁺) with carbon monoxide (CO) molecules was investigated by the adsorption calorimetry and IR spectroscopy, in comparison with the results obtained by the density functional (DF) calculation. For the MZSM-5 - CO systems, a major IR band due to the stretching vibration of CO molecules adsorbed could be resolved into two components. Corresponding to them, two types of adsorbed species with different heats of adsorption were suggested to exist. A linear relationship between the heat of adsorption and the wavenumber of IR band can be established for the MZSM-5 - CO systems, as for the case of copper ion-exchanged ZSM-5 (CuZSM-5) - CO system, though its slope and position are different from the latter case. Taking into consideration that the σ-bonding is operative in the latter system, it is assumed that in the MZSM-5 - CO systems the interaction is of predominantly electrostatic. On the basis of DF theory, the optimized structures for the CO species adsorbed on the two types of alkali-metal ion-exchanged sites were obtained. From these results, it was found that the vibrational frequency and the bonding energy for the CO species adsorbed on the two-coordinated site are higher than those on the three-coordinated site. Such models explained the experimental data satisfactorily.