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

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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 Bphys 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.

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