Dehydrogenation of Ethylbenzene over Double Oxide Catalysts of ZnO-Cr₂O₃—Catalyst Preparation and Their Catalytic Performances—

Abstract

The double oxide catalysts of ZnO/Cr₂O₃ were prepared by the two different methods; (a) the precipitates were formed by adding a mixed acidic solution of Zn^II and Cr^III nitrates into an alkaline solution of K₂CO₃ which are denoted as BSP(base-side precipitated) catalysts, and (b) those reffered as ASP(acid-side precipitated) catalysts were obtained by the reverse order, that is, the K₂CO₃ solution was added to that of Zn^II and Cr^III nitrates. It was of interest to find that the calcined BSP catalyst exhibited much higher yields and selectivities for the catalytic dehydrogenation of ethylbenzene(EB) than the ASP catalysts. This was suggested to be associated with the different crystal structures and surface compositions between the resulting BSP and ASP catalysts. The XRD and XPS characterization data revealed that the BSP catalysts were preferentially consisted with the spinel ZnCr₂O₄ having Cr^III as the catalytically active and stable structure for the EB dehydrogenation reaction, while the ASP catalysts contained the spinel phase in the minor part and had mixed contributions of Cr^III and Cr^IV. The results suggest that Cr^IV is derived from potassium-chromium oxalate and acts as a catalytic poisoning substance to decrease its structural stabilization and suppress the generation of the active sites for dehydrogenation of EB on the resulting mixed oxide catalysts.