Solid-state ion exchange of CoCl2 center dot 6H(2)O into NH4+-Beta zeolite: Pathway analysis

摘要

Solid-state ion exchange of CoCl center dot 6H(2)O into NH4+-Beta zeolite (Co/Al = 1, Si/Al = 12.5) was studied throughout the temperature range of 30-500 degrees C, and isothermally at 500 degrees C for 12 h. In the presence of zeolite, the thermal analysis coupled to mass spectrometry revealed that the hydration/dehydration of the salt is a reversible process. Therefore, the coordination of Co2+ central ion was elucidated by the evaluation of Racah and octahedral ligand-field-splitting parameters, determined from visible diffuse reflectance (DR) spectra recorded between 250 and 350 degrees C. Following the non-isothermal exchange of Co2+ into Beta zeolite between 400 and 500 degrees C, the partially hydrated Co(II) complex occupied, temporarily, alpha, beta, and gamma-sites, while epsilon-site is less-accessible to Co2+ ions on the basis of the quantitative study performed by visible DR spectroscopy. Nevertheless, following the isothermal exchange at 500 degrees C, Co2+ ions occupied, definitively, alpha, beta, and gamma-sites through a slow migration, extended up to 12 h. Using Infrared spectroscopy upon desorption of C O at -196 degrees C, we were able to reveal the presence of [Co(III)O](+) species. Nonetheless, by coupling both Infrared and DR results, we were able to identify, on one hand, the specific bands (in v(C O) region) relative to each Co2+ ion type and to reveal, on the other hand, that the rate of Co2+ ions mobility during isothermal exchange increases in the following order: gamma-type beta-type alpha-type.