FTIR study of CO and N2 adsorption on [Ge]FAU zeolites in their Na- and H-forms

Abstract

A [Ge]FAU zeolite sample (2.36 wt. % Ge) was synthesized by a new approach comprising utilization of mother liquids instead of zeolite seeds and characterized by FTIR spectroscopy of adsorbed CO and N2 as probe molecules. A Ge-free sample with similar Al concentration was also prepared and studied for comparison. The background spectra of the H-[Ge]FAU do not give indication of the presence of Ge(OH)Al bridging hydroxyls. However, the incorporation of Ge affects the properties of the bridging hydroxyls. Thus, the CO-induced shifts of the bands due to the HF bridging hydroxyls are larger with the Ge-free sample indicating a higher protonic acidity of this material. The same results are obtained with N2. Adsorption of N2 on NaFAU zeolite shows that the accessible Na+ sites are uniform and low coordinated: At high coverage Na+(N2)2 geminal species are observed and are converted into Na+–N2 adducts at lower coverage. Similar results are obtained by CO adsorption, i.e. conversion between Na+(CO)2 and Na+–CO species. The same phenomena were also detected with the Na-[Ge]FAU sample. However, in this case, two different Na+ sites are clearly detected. Because the accessible for adsorption Na+ ions are believed to be located in SII positions, the results suggest that some of the Na+ sites are affected by the presence of Ge in the second coordination sphere. This is consistent with the fact that, contrary to the proton (bonded to one oxygen), Na+ cations are coordinated to three framework oxygens.