Chemistry and Spectroscopy of Chromium in Zeolites

The diffuse reflectance spectra (DRS) and electron spin resonance spectra (ESR) of chromium containing zeolites prepared by ion exchange and impregnation (Y, Ga-Y, X and Mordenite) and hydrothermal synthesis (CrAPO-5 and Cr-silicalite) have been investigated before and after drying, after calcination, after CO-reduction and recalcination. In the Cr exchanged zeolites, Cr3+is octahedrally coordinated before and after drying and almost quantitatively oxidized to Cr5+and Cr6+upon calcination at 550°C. This Cr6+is a chromate-like species with two lattice and two extra-lattice oxygens in the coordination sphere. The coordination and location of Cr5+and the Cr-dispersion depends on the zeolite type and preparation method. A method of quantifying Cr6+and Cr5+-ions was developed. The reducibility of Cr6+and Cr5+to Cr3+and Cr2+follows the order : Cr-Mordenite < Cr-Y (impregnated) < Cr-Y (ion exchange) < Cr-X < Cr-Ga-Y < Cr-Y (solid state exchange). Series of CrAPO-5 and Cr-silicalite molecular sieves were synthesized hydrothermally starting with different Cr-precursors and Cr- and template-contents. In the gels of CrAPO-5, Cr3+and Cr6+are present as respectively octahedral Cr(H2O)63+and dichromate, both in the liquid phase. In the as-synthesized CrAPO-5's and Cr-silicalites, only extra-lattice octahedral Cr3+is present. Its redox chemistry is similar to that of the ion exchanged materials and Cr supported catalysts.