ELECTRON-SPIN-RESONANCE AND ELECTRON-SPIN ECHO MODULATION STUDIES ON RADIATION-INDUCED SILVER AGGLOMERATION IN A SAPO-42 MOLECULAR-SIEVE - A COMPARISON WITH ISOSTRUCTURAL ZEOLITE-A

This study focuses on paramagnetic silver species produced by gamma-radiolysis of a silver silicoaluminophosphate-42 (Ag-SAPO-42) molecular sieve. Electron spin resonance and electron spin echo modulation spectroscopies have been used to study the structure, location, and coordination of silver adducts and clusters stabilized in dehydrated SAPO-42 and exposed to methanol and ammonia. The results of this study are compared to the results found earlier for isostructural AgNa-A zeolite. It was found that in dehydrated Ag-SAPO-42, small silver clusters are formed with very low yield, whereas, in AgNa-A, Ag32+ or Ag6n+ clusters are efficiently stabilized. The less restricted mobility of Ag0 in SAPO-42 due to its lower cation capacity is postulated to explain the observed differences. In the presence of methanol and ammonia in the molecular sieve cages, the differences are less significant. The major paramagnetic products of radiolysis in both SAPO-42 and type A molecular sieves are silver methoxy radicals (methanol adsorbed) or silver-ammonia adducts (ammonia adsorbed). They are located in alpha-cages, although the electron spin echo modulation results suggest some differences in location, possibly due to a different distribution of negative charge between the aluminophosphate and aluminosilicate frameworks.