INTERACTION OF NH(3) WITH BRONSTED ACID SITES IN DIFFERENT CAGES OF ZEOLITE-Y AS STUDIED BY H-1 MAS NMR

Ammonia adsorption onto acidic Y zeolites is studied with H-1 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 Bronsted sites in unloaded or partially loaded zeolites are observed near delta = 3.9 for protons in supercages and near delta = 4.6 for protons in sodalite cages. Ammonium ions under condition of no proton-exchange resonate at delta = 8. 1. These species are detected as such only in the sodalite cages. In the supercages, a signal at delta = 6.9 is observed, which is ascribed to an exchange involving NH4+ (delta = 8.1), H+ (delta = 3.9), and NH3 (delta = 0.8). Furthermore, in the sodalite cages, a signal at delta = 6.5 is observed, which is ascribed to an exchange involving NH4+ (delta = 8. 1), H+ (delta = 4.6), and NH3 (delta = 0.8), but with different relative residence times. Additionally, ammonium-ammonia signals are found at delta = 5.3-5.9 and at a position determined by the NH3/NH4+ ratio in the zeolite (delta = 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 Bronsted protons is fast, and consequently the lines are shifted to delta = 6.9 and 6.5. This implies that ammonium ions observed at delta = 8.1 have the lowest basicity and the ammonium ions at delta = 6.5 have the highest basicity. However, for the temperature range used, ammonia reacts statistically with the Bronsted sites of different acid strength.