Propane dehydrogenation on chromia/zirconia catalysts

Abstract

The activity of CrO x /ZrO 2 (ZC) catalysts (0.05 to 0.8% chromium by weight) in the dehydrogenation of propane at 723–823 K has been investigated in a flow apparatus. The apparent activation energy is 70 ± 10 kJ mol −1. Samples were activated by heating with oxygen at 773 K followed, in most cases, by reduction to a controlled extent with carbon monoxide or hydrogen. At T ⩾ 773 K the catalyst prereduction is not significant in determining both the initial rate and its trend with time-on-stream, while at 723 K the catalyst prereduction affects these features. The formation of coke leads to deactivation, which is slowed down by adding hydrogen or water to the reactants, while it is made faster by adding propene. Activity and selectivity are completely restored by treating the catalyst with oxygen in flow at 773 K for 0.5 h. Zirconia displays a dehydrogenation activity at T ⩾ 773 K, poisoned by small amounts of water. The average turnover frequency (per total surface chromium atom) is nearly independent of the chromium content, indicating that the active centre is mononuclear. After reduction with carbon monoxide, samples contain Cr II and Cr III in nearly equal amounts. A subsequent selective oxidation of Cr II to Cr III with water at 773 K hardly affects the activity, therefore suggesting that Cr III species are active. The detrimental effect of the potassium addition on the activity suggests that O 2− ions adjacent to Cr III active species are involved in the dehydrogenation site.