Low temperature CO adsorption on Na-ZSM-5 zeolites: an FTIR investigation

CO was adsorbed at 77 K on two Na-ZSM-5 zeolites which had Si/Al = 14 and Si/Al = 35, respectively. IR spectra in the OH-stretching region showed absorption bands at 3747 and 3673 cm−1 assigned, respectively, to terminal silanols and to hydroxyls located on extraframework aluminium species. These OH groups were more strongly perturbed than silanols by adsorbed CO, thus suggesting a more pronounced Brønsted acidity. The major features of the spectra in the CO-stretching region are two intense absorptions centered, respectively, at 2178 and 2138 cm−1. Neither of them, however, is a single-peak band. The 2178-cm−1 band was assigned to CO molecules polarized by Na+ ions. It could be resolved into two components, revealing coordination of Na+ ions by one molecule (high-frequency component) or by more than one (low-frequency component). The high-frequency component showed an asymmetric profile which was assigned to the presence of Na+ ions in slightly different environments. The 2138-cm−1 band corresponds to liquid-like CO inside the zeolite channels where the molecule behaves as a hindered rotor, thus explaining the fine structure observed. A weak IR absorption was also observed at 2112 cm−1 and assigned to CO simultaneously interacting through both ends with two Lewis acid centres. Finally, a combination mode observed at 2305–2315 cm−1 allowed assignation of a value of ca. 140 cm−1 to the Na+-CO stretching vibration.