Elucidation of puzzling questions regarding the CrOx/Al2O3 catalyst I. X-ray absorption spectroscopy aided identification of the nature of the chromium oxide species in the CrOx/Al2O3 dehydrogenation catalyst system

Abstract

The CATOFIN© process produces propylene from propane using a CrOx /Al2O3 catalyst. Despite its use, there are still uncertainties surrounding the nature of Cr species formed during synthesis, regeneration, and aging. It has been observed that ppm levels of Cr2O3 can be oxidized to Cr(VI) in air; however, when Cr2O3 is in direct contact with Al2O3, Cr(VI) formation increased more than 100-fold. To confirm physical contact between Cr2O3 and Al2O3 results in increased Cr(VI) formation, Cr2O3 and Cr2O3/Al2O3 samples were synthesized, calcined, and characterized. Catalytic activity measurements confirm propylene selectivity inversely tracks with Cr(VI). Wet chemical titration and X-ray absorption spectroscopy (XAS) quantified and determined the structure of the Cr(VI) species. FEFF X-ray absorption near edge structure simulations determined the potential sensitivity of the XAS to the presence of Al3+/Cr3+ neighboring atoms. XAS correlated to FEFF simulations suggested that Al3+xCr3+2−xCr6+3O12 comprises a significant fraction of the Cr(VI) species formed.