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 31P modulation to determine the distance and number of 31P from the Cu(II) ion at various stages of hydration of the material. It was found that in freshly ion-exchanged Cu−SAPO-11 at 400 °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, ND...