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 31 P modulation to determine the distance and number of 31 P 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 D 2 O, ND 3 , CD 3 OH, and CH 3 OD and analyzing the 2 D modulation, the distances between the Cu(II) and the adsorbed molecules are derived. Additionally, the interaction with ND 3 ammonia is supported by ESR results obtained for Cu(II)-15 NH 3 for which the 15 N hyperfine interaction is resolved and reveals the number of interacting 15 N. 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.

Synthesis and Characterization of CuAPO-5 Molecular Sieves: Evidence for the Framework Incorporation of Cu(II) Ions

“…More than one Cu(II)−ammonia complex differing in the number of ammonia ligands has been observed in molecular sieves. A square planar Cu(II)(ND 3 ) 4 complex has been observed in CuH−Y, CuH−SAPO-18, and CuH−SAPO-11. ,, In CuH−SAPO-5, CuH−SAPO-34, and CuH−SAPO-37 three ammonia molecules are found to coordinate with the Cu(II) ions after adsorption of ammonia. , However, Cu(II) species H observed for CuAPO-5 with only two directly coordinated ammonia ligands has not been observed before and is again consistent with a framework site for copper.…”

“…This distance is very long for Cu(II) ions at ion-exchangeable positions. In a recent study on ion-exchanged CuH−SAPO-18, 31 P ESEM results suggest that in dehydrated samples there are three phosphorus nuclei interacting with Cu(II) at a distance of 3.2 Å, consistent with Cu(II) situated near a hexagonal window and coordinating with three framework oxygens . In the CuAPO-5 system, elemental analysis suggests that copper substitutes for phosphorus in the framework.…”

Nature of Cu Species in Cu–SAPO-18 Catalyst for NH3–SCR: Combination of Experiments and DFT Calculations

“…The identical values for all the samples indicate that the coordination environment has no change with the different Cu contents. The values of g ∥ = 2.391 and A = 130.5 G consider Cu 2+ exists in octahedral state for the hydrated samples, resulting from the six coordination to H 2 O molecules and oxygen atoms of the zeolite framework. − One may further consider Cu ions in hydrated SAPO-18 are located near the hexagonal window of D6R, which seems to be similar to the so-called site I in Cu-CHA structure …”

Cupric Ion Location and Adsorbate Interactions in Cu(II) Exchanged Erionite-like SAPO-17 Molecular Sieve

“…In a recent study on CuH-SAPO-18 the single Cu species observed after dehydration has similar ESR parameters to species A1 in CuH-SAPO-17. On the basis of ESEM results, this species was assigned to Cu(II) ions located within the pear-shaped cavity near a six-ring window coordinating with three framework oxygens …”