Influence of the Content and Environment of Chromium in CrSiBEA Zeolites on the Oxidative Dehydrogenation of Propane

Abstract

CrxSiBEA catalysts (x = 0.2, 1.0, and 3.4 Cr wt %) are prepared by a two-step postsynthesis method. The introduction of Cr ions in the framework of dealuminated SiBEA zeolite evidenced by XRD generates Brønsted acidic sites as shown by FTIR of pyridine used as molecular probe. In CrxSiBEA as prepared, chromium is mainly present as octahedral Cr(III) and in calcined CrxSiBEA (773 K, 3 h, flowing air) as distorted tetrahedral Cr(VI), as evidenced by DR UV−vis, TPR, and XPS. Upon outgassing of calcined CrxSiBEA at 773 K for 1 h (10−3 Pa), part of Cr(VI) is reduced to distorted tetrahedral Cr(V), as shown by EPR. Isolated distorted tetrahedral Cr sites present in activated CrxSiBEA are responsible for their catalytic activity in the oxidative dehydrogenation (ODH) of propane, performed in the presence of air or N2O. The propane conversion increases with Cr content in the presence of air or N2O, while the selectivity toward propene depends on the reaction temperature and oxidant. In the presence of air, the selectivity toward propene is highest for 1.0 Cr wt %. The lower selectivity for the content of 3.4 Cr wt % may be the result of the presence of some amount of extra-framework octahedral Cr sites. The lower propane conversion and the higher selectivity toward propene in the presence of N2O probably depend on the oxidation state of Cr under reaction conditions and on the nature of surface oxygen. The presence of isolated tetrahedral chromium sites with oxidation state lower than (VI) on activated CrxSiBEA is essential for maintaining a high selectivity.