Investigating the CrO^sub x^/Al^sub 2^O^sub 3^ dehydrogenation catalyst model: II. Relative activity of the chromium species on the catalyst surface

In this work a series of the fresh and equilibrated CrOx/Al2O3 catalysts with the Cr loading of 0.58-13.2wt% and unsupported α-Cr2O3 have been investigated to evaluate the relative activity of five chromium species in isobutane dehydrogenation reactions. The study showed that the activity order of five chromium species can be described as: the non redox Cr3+ on α-Cr2O3>the redox Cr3+ generated from the reduction of 'isolated' mono-/poly- aluminum chromates ∼ the non redox Cr3+ on amorphous Cr2O3>the 'isolated' non redox Cr3+ ions and/or very small Cr2O3 clusters>the redox aluminum-chromium-chromate(s). The redox aluminum-chromium-chromate(s) were found also active in the formation of C1-C3 hydrocracking byproducts and coke. It was also found that non-redox Cr3+ on α-Cr2O3 had little contribution to the total dehydrogenation activity in the fresh catalyst, but contributed significantly to that in the equilibrated one. Moreover, a reaction chemistry pathway for the thermal conversion of mono-/poly-aluminum chromate(s) and physically deposited CrO3 into the aluminum-chromium-chromate(s) was proposed.