Studies on CrO x /La 2 O 3 /ZrO 2 catalysts modified by Mg

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Catalysis Letters Vol. 112, Nos. 3–4, December 2006 ( 2006) DOI: 10.1007/s10562-006-0199-3

Studies on CrOx/La2O3/ZrO2 catalysts modified by Mg D.L. Hoang,a,* S. Farage,a A. Dittmar,a A. Trunschke,b H. Lieske,a and A. Martina a

Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock, Branch Berlin (former ACA), Richard-Willsta¨tter-Str. 12, D-12489 Berlin, Germany b Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Plack-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

Received 15 September 2006; accepted 4 October 2006

Lanthana–zirconia supported chromium oxide and magnesium chromium mixed oxide catalysts were studied in the dehydrocyclization of n-octane and characterized by temperature-programmed desorption of NH3, temperature-programmed reduction, XPS and DRIFTS. The Mg-free catalyst shows the highest activity, but suffers from rapid deactivation due to coke formation. The addition of Mg decreases the initial activity of the supported chromium oxide and retards its deactivation. The characterization results reveal that the deactivation retarding effect of Mg species not only consists in the deletion of strong acid sites but also in the decoration and/or dilution of Cr3+ oxide cluster, supposedly due to the formation of Mg–Cr surface compounds and, thus, in preventing the formation of coke. KEY WORDS: dehydrocyclization; lanthana–zirconia support; chromium oxide catalysts; chromium-magnesium-lanthanum oxide catalysts; catalyst characterization; coke deposition.

1. Introduction Recently, we have found that CrOx/La2O3/ZrO2 (Cr/ LZ) catalysts are able to catalyze the dehydrocyclization (DHC) of C6+ alkanes, exhibiting high selectivities for alkyl aromatics. At 773-823 K under hydrogen atmosphere the DHC of n-octane on Cr/LZ catalysts mainly produces o-xylene and ethylbenzene [1]. However, the strong deactivation of the catalysts due to coke deposition impedes an industrial application [2–4]. By means of in-situ ESR spectroscopy, the coexistence of catalytically active Cr3+ oxide clusters and isolated Cr3+ ions on the LZ support was revealed under reaction conditions. The isolated Cr species appeared to be less sensitive towards deactivation by coke deposition during the DHC of n-octane [4]. Therefore, our efforts were directed to suppress deactivation by coke formation by specific generation and stabilization of isolated Cr3+ species on the surface of the support. Recently, we have suggested that strong interaction between La and Cr oxides could be responsible for stabilization of highly dispersed Cr species on the surface of Cr/LZ [5–7]. Magnesia is known to be able to interact with chromia or La-Cr mixed oxides resulting in MgCr2O4 with spinel structure or LaCr1-xMgxO3 with a substituted perovskite structure, respectively [8–10]. Moreover, MgO crystals, being simultaneously formed with La–Cr perovskite-type mixed oxide phases in oxidation catalyst systems, were proved to prevent sintering of the latter, keeping Cr containing species in a dispersed state [10]. On the other hand, MgO addi