FTIR studies of unusual OH groups in steamed HNaY zeolites: Influence of Na insertion

Starting from a prepared series of NaHY zeolites with different number of sodium ion per unit cell (u.c.), the corresponding series of steamed HNaY(S) zeolites were obtained by a modified method of steaming in order to obtain less acidic but nevertheless thermally stable zeolites. All prepared solids exhibit large amounts of unusual OH groups with stretch frequency at 3700 cm−1 which are thermally stable and characterized by their soft acid strength measured by CO adsorption at low temperature: Δν(OH)=ca. 145 cm−1, i.e. H0=−5. All the solids are highly crystalline (80%). Chemical and FTIR analysis of the two series denoted HNaY and HNaY(S) allowed the determination of their molar formula after calcination at 650 K. The concentration of the unusual (3700)OH groups depends of the level of Na exchange and reaches a maximum for 34 Na per u.c. as deduced from the intensity of the corresponding IR absorption band. This study gives precise details about the role of Na in forming these unusual (3700)OH groups.