Chromium oxide supported on Zr modified alumina for stable and selective propane dehydrogenation in oxygen free moving bed process

Abstract Propane dehydrogenation in oxygen free environment over alumina-supported chromium oxide catalysts with different chromium loading was studied within short time-on-stream range for the fluidized moving bed applications. For this purpose, a series of CrO x /Al 2 O 3 and CrO x /Zr-Al 2 O 3 catalysts were prepared and examined. Increase in the chromium loading resulted in reduction of monochromate species, possibly leading to formation of polychromate species. Further increase of chromium loading led to formation of Cr 2 O 3 species. Accordingly, specific activity of the catalyst decreased with increasing chromium loading. Propane conversion significantly decreased and propylene selectivity increased during an induction period due to the rapid consumption of limited lattice oxygen in the catalyst. Equilibrated and artificially-aged catalysts were characterized in order to investigate the factors affecting propylene selectivity and catalyst durability by pore analysis, temperature-programmed reduction (H 2 -TPR), temperature-programmed desorption (NH 3 -TPD), X-ray diffraction patterns (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). After an artificial catalyst aging treatment, all catalysts showed reduction in catalytic activity and the most drastic activity decrease was found in the catalyst with the highest loading of 20 wt% Cr/γ-Al 2 O 3 sample. The activity loss was mainly due to agglomeration of the chromium species, reduction of the surface area, and transformation of Cr 6+ species into stable Cr 2 O 3 species. Cr species were agglomerated on the surface of alumina support during the aging treatment and the agglomeration was more pronounced at the high Cr loading. Moreover, Raman spectroscopy and XRD analyses revealed the formation of chromia/alumina solid solution in the catalyst after the aging treatment which also significantly contributed to the activity loss. Contrarily, Zr modified alumina support successfully suppressed the formation of chromia/alumina solid solution phase and substantially improved the catalytic activity after the aging treatment by increasing the dispersion of chromium oxide. Zr-Al 2 O 3 support showed lower catalytic activity toward propylene than Al 2 O 3 support, which also contributed to the higher propylene yield of Cr/Zr-Al 2 O 3 .