Characterization of thermally treated Co2+-exchanged zeolite X

This study investigated thermal stabilization of Co2+-exchanged zeolite X (CoX) using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and leaching tests. From SEM-EDX analysis, cobalt was dispersed randomly at ≤600 °C, suggesting its presence as an extraframework cation in exchange sites. At ≥800 °C, cobalt was locally concentrated with Al on the vitreous surface. Consistent with such observations, XRD data indicated that CoX maintained the zeolite framework at ≤600 °C, and that it became vitrified and transformed to nepheline (NaAlSiO4(s)) and cobalt aluminate (CoAl2O4(s)) at ≥800 °C. Cobalt-K edge XAS was subjected to both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analyses. In XANES spectra, the pre-edge peaks and edge-shoulders, characteristic of 4-fold coordinated cobalt (e.g., CoAl2O4(s)), were not evident at ≤600 °C, but such features were strong at ≥800 °C. The EXAFS spectra of CoX at ≤600 °C lacked in the coordination shells beyond the first CoO shell. In contrast, CoX at ≥800 °C showed the EXAFS spectra similar to CoAl2O4(s). Taken together, cobalt was likely present as 6-fold coordinated Co2+ in exchange sites at ≤600 °C and mainly incorporated into a non-exchangeable CoAl2O4-like phase in both vitreous and crystalline forms at ≥800 °C. In agreement with this proposition, leaching tests with concentrated CaCl2 solutions supported the greater stability of cobalt at ≥800 °C.

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► At ≤600 °C, Co2+-exchanged zeolite X (CoX) maintains the zeolite framework.
► At ≤600 °C, cobalt is present as 6-fold coordinated Co2+ in exchange sites.
► At ≥800 °C, CoX transforms to glassy material and secondary crystalline phases.
► At ≥800 °C, cobalt is incorporated into a CoAl2O4(s)-like phase.