State of Chromium in CrSiBEA Zeolite Prepared by the Two-Step Postsynthesis Method:  XRD, FTIR, UV−Vis, EPR, TPR, and XAS Studies

CrxSiBEA zeolites (x = 0.2, 0.9, and 3.4 Cr wt %) are prepared by a two-step postsynthesis method that allows controlling the introduction of chromium ions in the zeolite structure. The introduction of Cr ions in vacant T-sites and their reaction with silanol groups are evidenced by XRD and FTIR, respectively. It generates Brønsted acidic sites as shown by FTIR of pyridine adsorbed as a molecular probe. The state of chromium species is characterized by DR UV−vis, EPR, TPR, and XAS. The combined use of these techniques allows one to show that the state of chromium species depend on the treatment condition of CrSiBEA samples. In the as-prepared CrxSiBEA samples chromium species are mainly present as isolated mononuclear octahedral Cr(III) species, as evidenced by DR UV−vis and XAS spectroscopy. In calcined CrxSiBEA samples isolated mononuclear tetrahedral Cr(VI) and octahedral Cr(III) species are present as shown by DR UV−vis. After further outgassing under vacuum, part of the isolated mononuclear tetrahedral Cr(VI) species is reduced to pentavalent chromium species as shown by EPR measurements. After rehydration of calcined CrxSiBEA samples at room temperature less distorted tetrahedral Cr(VI) appear as shown by DR UV−vis. Possible models of mononuclear octahedral Cr(III) and tetrahedral Cr(VI) species in the BEA structure are proposed.