Dehydrogenation of ethylbenzene over double oxide catalysts of ZnO-Cr2O3 -Catalyst preparation and their catalytic performances

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 a s 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 nitrate s. It was of interest to find that the calcined BSP catalyst exhibited much higher yields and selectivities for the catalytic dehydrogenation of ethyl benzene(EB) than the ASP catalysts. This was suggested to be associated wit h the different crystal structures and surface compositions between the res ulting BSP and ASP catalysts. The XRD and XPS characterization data reveale d that the BSP catalysts were preferentially consisted with the spinel ZnCr 2O4, 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-VI. Th e results suggest that Cr-VI is derived from potassium-chromium oxalate and acts as a catalytic poisoning substance to decrease its structural stabili zation and suppress the generation of the active sites for dehydrogenation of EB on the resulting mixed oxide catalysts.