Electron spin resonance and X-ray diffraction studies of copper(II)-ion-doped Zr4-pillared montmorillonite clay

Abstract

Zr₄-pillared montmorillonites have been synthesized by treating montmorillonite with Zr₄-polymer solutions prepared either by stirred (S) or refluxed (R) methods. These stirred or refluxed samples were either oven-dried (OD) or calcined (CAL) to give a set of pillared montmorillonites, (a) stirred and oven-dried (S/OD), (b) stirred and calcined (S/CAL), (c) refluxed and oven-dried (R/OD) and (d) refluxed and calcined (R/CAL). These different sample preparations were doped with Cu²⁺ and different Cu²⁺ species (A, B and E) were found by electron spin resonance dependent on (a) the method of Zr₄-polymer preparation (S or R) and (b) the heat treatment (OD or CAL). Cu²⁺ species A is observed in an air-dried S/OD sample and is assigned to an octahedral Cu²⁺ coordinated to water and located in the interlayer space. In contrast, species B_phys is observed in an air-dried R/OD sample. However, only one type of Cu²⁺ ion, species B, is observed in an air-dried Cu²⁺-doped calcined samples (S/CAL and R/CAL). This species is of considerable interest since it appears to be coordinated to the pillar. Cu²⁺ species B is assigned to square planar Cu²⁺ within the Zr₄-polymer pillar coordinated to hydroxyl groups. Cu²⁺ species E is formed when any of the different sample preparations is heated to 360 °C. Cu²⁺ species E is assigned to square planar Cu²⁺ within the Zr₄-polymer pillar coordinated to oxygens.