Ethylbenzene dehydrogenation over Mg 3Fe 0.5− xCo xAl 0.5 catalysts derived from hydrotalcites: Comparison with Mg 3Fe 0.5− yNi yAl 0.5 catalysts

Abstract

A series of Mg 3Fe 0.5− x Co x Al 0.5 ( x = 0–0.5) catalysts were prepared from hydrotalcite precursors and their activities in the dehydrogenation of ethylbenzene were compared with those of a series of Mg 3Fe 0.5− y Ni y Al 0.5 ( y = 0–0.5) catalysts also derived from hydrotalcite. The hydrotalcites prepared by co-precipitation were calcined at 550 °C to the mixed oxides with a high surface area of 150 – 240 m 2 g cat − 1 ; they were composed of Mg(Fe,Me,Al)O periclase and Mg(Me)(Fe,Al) 2O 4 spinel (Me = Co or Ni). Bimetallic Fe 3+–Co 2+ system showed a synergy, i.e., an increase in the activity, whereas Fe 3+–Ni 2+ bimetallic system showed no synergy. The high styrene yield was obtained on Mg 3Fe 0.1Co 0.4Al 0.5; however, a large substitution of Fe 3+ with Co 2+ caused a decrease in styrene selectivity along with coking on the catalysts, due to an isolation of CoO x on the catalyst surface. The highest yield as well as the highest selectivity for styrene production was obtained at x = 0.25 at time on stream of 30 min. The coprecipitation at pH = 10.0 and the composition of Mg 3Fe 0.25Co 0.25Al 0.5 were the best for preparing the active catalyst. This is partly due to the formation of a good hydrotalcite structure. On this catalyst, the active Fe 3+ species was reduced at a low temperature by the Fe 3+–Co 2+ bimetal formation, leading to a high activity. Simultaneously, the amount of reducible Fe 3+ was the smallest, resulting in a high stability of the active Fe 3+ species. It is likely that the dehydrogenation was catalyzed by the reduction–oxidation between Fe 3+ and Fe 2+ and that Co 2+ assisted the reduction–oxidation by forming Fe 3+–Co 2+ (1/1) bimetallic active species.