A QUANTITATIVE DIFFUSE-REFLECTANCE SPECTROSCOPY STUDY OF CHROMIUM-CONTAINING ZEOLITES

The diffuse reflectance spectra of chromium-containing zeolites (Y, GaY, X, and mordenite) have been investigated before and after drying, after calcination, after successive CO reduction, and after recalcination. Different methods of Cr zeolites preparation (ion exchange, solid-state ion exchange, and impregnation) were investigated. A method for quantifying Cr(VI) ions was developed. Before and after drying, Cr(III) is octahedrally coordinated, which is almost quantitatively oxidized upon calcination at 550-degrees-C to Cr(VI). Cr(VI) is a chromatelike species with two lattice and two extralattice oxygens in the coordination sphere. The Cr dispersion depends on the zeolite type and preparation method. The reducibility of Cr(VI) to Cr(III) and Cr(II) follows the order Cr mordenite > CrY (impregnated) > CrY (ion exchange) > CrX > CrGaY > CrY (solid state). This order can be qualitatively explained in terms of zeolite properties and Cr dispersion.