CHROMIA ZIRCONIA CATALYSTS WITH CR CONTENT EXCEEDING THE MONOLAYER - A COMPARISON WITH CHROMIA/ALUMINA AND CHROMIA/SILICA FOR ISOBUTANE DEHYDROGENATION/

CrOx/ZrO2, CrOx/gamma-Al2O3 and CrOx/SiO2 catalysts were prepared by w et impregnation in the loading range 0.05-14 atoms Cr nm(-2), that is below and above the monolayer capacity of support. The samples were ch aracterized by chemical analysis, including carbon monoxide reduction and Cr-VI extraction with sodium hydroxide or water, and by XRPD, UV-V is DRS and ESR. Several Cr species were identified, namely Cr-VI, Cr-V (ESR gamma-signal), Cr-III (ESR delta-signal), small Cr-III oxide clu sters (ESR beta-signal) and alpha-chromia. The relative abundance of t hese Cr species was found to be mainly controlled by the specific supp ort employed and the Cr concentration. The catalytic activity in the i sobutane dehydrogenation was studied at 723 and 773 K in a flow micror eactor. It was found that the activity per total Cr atom was markedly higher for the zirconia-based catalysts compared with the other two sy stems in all the explored Cr concentration range. That is, the CrOx/Zr O2 interaction maintains its positive effect well beyond the monolayer capacity of this support. Moreover, the catalytic activity correlated with the amount of high-oxidation state chromium (Cr-VI and Cr-V), ev aluated by the quantity of extractable Cr or by the consumption of car bon monoxide in the catalyst reduction. The larger concentration of mo nonuclear Cr-V species on zirconia, confirmed here, is thought to be r esponsible for the higher activity of the zirconia-based catalysts, th rough reduction to active mononuclear Cr-III species under reaction co nditions. (C) 1998 Elsevier Science B.V.