IR studies of the formation of ammonia dimers in zeolites TON

Ammonia may be a very convenient probe molecule to study the acid properties of zeolites, the pores of which are so narrow that they are inaccessible to pyridine. Ammonia molecules interact with acidic hydroxyls forming ammonium ions, however upon the neutralization of acidic hydroxyls next introduced ammonia molecules can react with ammonium ions (which also act as Brønsted acid sites), forming dimers NH4+·NH3. If ammonia is adsorbed at low temperatures (room temperature or less), ammonia can react with ammonium ions even before the neutralization of all acidic hydroxyls. Based on the results of our IR studies of formation and decomposition of dimers in zeolite TON optimal experimental conditions of IR quantitative studies of Brønsted acid concentration with ammonia were performed. The concentration of Brønsted sites determined by this method in zeolites TON of various Si/Al was practically the same as the values calculated from chemical analysis.