Interaction of NH₃ with Brønsted acid sites in different cages of zeolite Y as studied by ¹H MAS NMR

Ammonia adsorption onto acidic Y zeolites is studied with 1H MAS NMR (at 295-320 K). The influence of the amount of adsorbed ammonia is investigated in the presence of sodium or cesium as exchangeable cations and for different framework compositions (Si/Al = 2.4-12). The Brønsted sites in unloaded or partially loaded zeolites are observed near δ = 3.9 for protons in supercages and near δ = 4.6 for protons in sodalite cages. Ammonium ions under condition of no proton-exchange resonate at δ = 8.1. These species are detected as such only in the sodalite cages. In the supercages, a signal at δ = 6.9 is observed, which is ascribed to an exchange involving NH4+ (δ = 8.1), H+ (δ = 3.9), and NH3 (δ = 0.8). Furthermore, in the sodalite cages, a signal at δ = 6.5 is observed, which is ascribed to an exchange involving NH4+ (δ = 8.1), H+ (δ = 4.6), and NH3 (δ = 0.8), but with different relative residence times. Additionally, ammonium-ammonia signals are found at δ = 5.3-5.9 and at a position determined by the NH3/NH4+ ratio in the zeolite (δ = 7-1). For the ammonium-ammonia species proton-exchange between ammonium and ammonia is fast compared to the time scale of the NMR experiment. For at least two ammonium species proton-exchange with the Brønsted protons is fast, and consequently the lines are shifted to δ = 6.9 and 6.5. This implies that ammonium ions observed at δ = 8.1 have the lowest basicity and the ammonium ions at δ = 6.5 have the highest basicity. However, for the temperature range used, ammonia reacts statistically with the Brønsted sites of different acid strength.