Studies of catalytic activity and coke deactivation of spinel oxides during ethylbenzene dehydrogenation

Catalytic dehydrogenation of ethylbenzene with CO₂ to produce styrene was carried out on MAl₂O₄ (where M = Cu²⁺, Ni^2+, or both), and also with pure NiO and CuO catalysts. The synthesized materials were characterized by various physico-chemical techniques and catalytic activity, coke deactivation, and recycling of spent catalysts were studied. The NiAl₂O₄ phase showed the highest activity under moderate conditions (activity = 160 × 10^-3 mol g^-1 h^-1; selectivity to styrene = 90.7%). Pure NiO and CuO were virtually inactive in the reaction due to the easy reduction of the phases in these oxides. NiAl₂O₄, an active phase for the reaction, was cooperatively associated with Cu in a highly stable ternary system (Cu-Ni-Al). However, this association did not result in high conversion or selectivity. Analyses of the coked catalysts revealed the presence of carbon on the solid surface that was in the form of graphite and polyaromatics. Deactivation studies on NiAl₂O₄ proved that Ni²⁺ sites were covered with a moderate amount of crystalline graphite that grew progressively and was subsequently eliminated by using higher CO₂/EB ratios. These results suggest that a reverse water shift reaction may be occurring when this solid is used. On the other hand, polyaromatic carbons were not eliminated and were the leading cause of the catalyst deactivation, rather than the physical degradation (reduction) of the solid. Recycling studies have demonstrated the high tendency of NiAl₂O₄ to regain the oxidation state of nickel, and thus, the styrene selectivity was maintained for over 40 h. The conditions leading to the highest conversion were a CO₂/EB ratio of 30 and a T = 550 °C at ambient pressure. Under these conditions, activities were comparable to those of Fe-K commercial catalysts for dehydrogenation, leading over the steam.