IR and29Si NMR investigation of the influence of alkaline modification on the structure of hydrothermally dealuminated Y zeolites

The change in the structure of NH₄NaY zeolite (53 % NH₄ ⁺, Si/Al = 2.37) after hydrothermal treatment at 873 K followed by modification with aqueous KOH solution at 353 K was studied by IR and²⁹Si NMR spectroscopy. It has been shown that hydrolytic cleavage of the Al-O bond of the deammoniated zeolite sites by hydrothermal treatment predominates in the framework groups Si(OAl) _n (OSi)_4–n ,n=2, 3. Molecular water adsorbed on such a sample exists as hydrogen-bonded associates with hydrogen bonds of various strengths reaching that in ice-like structures (the band at 2468 cm^–1). Treatment with an alkali results in partial regeneration of the normal bridge bonds. The exchange of the protons of the terminal silanol groups with the alkaline cation prevents those groups from participating in the regeneration process.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 387–391, March, 1994.