SILVER AGGLOMERATION IN SAPO-5 AND SAPO-11 MOLECULAR-SIEVES

Electron spin resonance and electron spin echo modulation (ESEM) spectroscopies have been used to study the location and coordination of silver ionic clusters and silver-alcohol adducts stabilized in gamma-irradiated Ag-SAPO-5 and Ag-SAPO-11 molecular sieves. It was found that in dehydrated samples silver ionic clusters are not stabilized in contrast to zeolites, but after exposure to methanol, ethanol, and propanol before irradiation Ag-2(+) and Ag-3(2+) are efficiently stabilized. Besides silver clusters, the formation of organosilver radicals is observed, i.e., Ag . CH2OH+. By analyzing the ESEM spectra it was shown that organosilver radicals are located in the 10-ring channels of Ag-SAPO-11 and the 12-ring channels of Ag-SAPO-5 and are coordinated by two nonequivalent methanol molecules. Ag-2(+) is located in the 6-ring channels and in SAPO-5 interacts with three equivalent CH3OH groups, whereas in SAPO-11 two methanol molecules are distinctly closer to Ag-2(+) than the third one. The higher yield and stability of Ag-2(+) in SAPO-11 is associated with the preferential location of Ag+ cations in two adjacent 6-ring channels. This increases the probability of encounter between radiation-induced mobile Ag-0 and Ag+.