Infrared evidence of a third bronsted site in mordenites

A series of progressively proton-exchanged sodium mordenite was studied by FTIR spectroscopy in the presence of probe molecules to precise the location of protons and sodium ions in the structure of MOR. The atomic ratio between protons in unconstrained and constrained environments in fully exchanged mordenite was 2:1 as shown by the adsorption of both pyridine and CO. On the contrary, this ratio was 1:2 for the Na+ cations in the parent Na-mordenite as deduced from CO adsorption and from the formation of NaNO3 in the structure by NO + O-2 reaction. Therefore, some framework Al atoms lead to unconstrained OH groups in the acidic form but to constrained Na+ cations in the sodium form. This observation, the appearance of a new v(OH) component at ca. 3605 cm(-1), and the quantitative infrared data at intermediate exchange levels allow to conclude that not only two but at least three different OH types can be distinguished by infrared spectroscopy in the MOR structure, in line with recent structural proposals. A model is proposed for the exchange process, in which the unconstrained OH environment classically reported on infrared basis can be split in two different sites.