Understanding the Crystallization of Nanosized Cobalt Aluminate Spinel from Ion-Exchanged Zeolites Using Combined in Situ QEXAFS/XRD

Abstract

By using combined QEXAFS/XRD (supported by measurements obtained using DSC, XRD, and IR spectroscopy), we have studied, in situ, the evolution of nanosized cobalt aluminate spinel (CoAl2O4) particles from cobalt-ion-exchanged zeolite precursors. All of the cobalt-ion-exchanged zeolites were observed to first undergo dehydration, followed by amorphization before the CoAl2O4 phase was observed to form. For two of the zeolites studied, this phase was observed to form at temperatures as low as 450 °C. However, with the exception of zeolite Y, a minor amount of a secondary phase (Co2SiO4) was also observed to form from amorphized zeolites when the Si/Al ratio was >1.85 and the calcination temperature exceeded 850 °C. This unusual ability of zeolite Y to yield phase-pure CoAl2O4, even though it has a Si/Al ratio of ∼2.52 can be attributed to the tendency for the cobalt ion to predominantly locate within the double six rings (SI site) of the zeolite Y structure in close proximity to the Al3+ ions (as charge-compensating cations), thereby creating nucleation sites for the spinel material. In contrast, other zeolites that lose their framework structure allow random distribution of cobalt ions, away from Al3+, allowing the formation of other phases such as Co2SiO4.