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 inv estigated in the presence of sodium or cesium as exchangeable cations and for different framework compositions (Si/Al = 2.4-12). The Bronste d sites in unloaded or partially loaded zeolites are observed near del ta = 3.9 for protons in supercages and near delta = 4.6 for protons in sodalite cages. Ammonium ions under condition of no proton-exchange r esonate at delta = 8. 1. These species are detected as such only in th e sodalite cages. In the supercages, a signal at delta = 6.9 is observ ed, which is ascribed to an exchange involving NH4+ (delta = 8.1), H(delta = 3.9), and NH3 (delta = 0.8). Furthermore, in the sodalite cag es, a signal at delta = 6.5 is observed, which is ascribed to an excha nge involving NH4+ (delta = 8. 1), H+ (delta = 4.6), and NH3 (delta = 0.8), but with different relative residence times. Additionally, ammon ium-ammonia signals are found at delta = 5.3-5.9 and at a position det ermined by the NH3/NH4+ ratio in the zeolite (delta = 7-1). For the am monium-ammonia species proton-exchange between ammonium and ammonia is fast compared to the time scale of the NMR experiment. For at least t wo ammonium species proton-exchange with the Bronsted protons is fast, and consequently the lines are shifted to delta = 6.9 and 6.5. This i mplies that ammonium ions observed at delta = 8.1 have the lowest basi city and the ammonium ions at delta = 6.5 have the highest basicity. H owever, for the temperature range used, ammonia reacts statistically w ith the Bronsted sites of different acid strength.