Characterization of thermally treated Co2+-exchanged zeolite X

Abstract

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.