Electron spin resonance and electron spin echo modulation studies of Cu(II)-exchanged silicoaluminophosphate type 18 molecular sieve

Abstract

The silicoaluminophosphate type 18 (SAPO-18) is a medium-pore molecular sieve and is of interest as heterogeneous catalysts. A SAPO-18 material was synthesized with an Al:P:Si ratio of 10:9:1 with the silica substituting selectively for the phosphorus sites, thus giving a negatively charged framework. Divalent copper ion exchanged into SAPO-18 from an aqueous solution was used as a spin probe to examine its location in SAPO-18 and its interaction with small molecular adsorbates. To monior the Cu(II) interactions with the local environment, electron spin resonance (ESR) and three-pulse electron spin echo modulation (ESEM) were used. ESEM has been used to detect P-31 modulation to determine the distance and number of P-31 from the Cu(II) ion at various stages of hydration of the material. It was found that in freshly ion-exchanged Cu-SAPO-II at 400 degrees C the Cu(II) is directly coordinated to a hexagonal window inside of the pear-shaped large cavity in SAPO-18. Using deuterium-labeled adsorbates including D2O, ND3, CD3OH, and CH3OD and analyzing the D-2 modulation, the distances between the Cu(II) and the adsorbed molecules are derived. Additionally, the interaction with ND3 ammonia is supported by ESR results obtained fur Cu(II)-(NH3)-N-15 for which the N-15 hyperfine interaction is resolved and reveals the number of interacting N-15. The results show that rehydration involves coordination by three water molecules in a six-membered ring of the framework. For methanol, steric hindrances limit the coordination of only two molecules to Cu(II). Results for ammonia show that the Cu(II) forms a complex with four ammonia molecules most probably in square-planar arrangement.