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

Akihiko MIYAKOSHI*, Noriyoshi KAKUTAõ, Akifumi UENOõõ and Masaru ICHIKAWAõõõ

Department of Materials Chemistry, Asahikawa National College of Technology; 2-2 Shunkoudai, Asahikawa-shi 071-8142 Japan
õ Department of Materials Science, Toyohashi University of Technology; 1-1 Hibarigaoka, Tempaku, Toyohashi-shi 441-8580 Japan
õõ Department of Materials Science, Shizuoka University; Johoku Hamamatsu-shi 432-8561 Japan
õõõ Catalysis Research Center, Hokkaido University; N11, W10, Kita-ku, Sapporo-shi 060-0811 Japan

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 ZnII and CrIII 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 ZnII and CrIII 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 CrIII 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 CrIII and CrVI. The results suggest that CrVI 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.
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