PREPARATION, CHARACTERIZATION AND CATALYTIC PROPERTIES OF VANADIUM-OXIDES SUPPORTED ON CALCINED MG/AL-HYDROTALCITE

Vanadium oxide supported on calcined hydrotalcite has been investigated for the oxidehydrogenation of n-butane in the 500-550 degrees C temperature interval. Hydrotalcite (Mg/Al atomic ratio of 2.77), consisting of a single phase only, has been employed as a support precursor. The vanadium catalysts (0-50, referred as wt.-% V2O5) were prepared by impregnation of calcined hydrotalcite (450 degrees C) with ammonium metavanadate (in an aqueous solution) or vanadyl acetylacetonate (in a methanolic solution), and then calcined at 600 degrees C for 4 h. During the impregnation step, the support is transformed into hydrotalcite if aqueous solutions are used. However, it is not modified if methanolic solutions are used. After calcination, diffuse X-ray diffraction patterns of MgO, in addition of a broad peak at 2 theta=35 degrees which intensity increases with the vanadium loading, were observed. Magnesium vanadates, i.e. ortho- and pyro-magnesium vanadates, were observed by FT-IR, FT-Raman and DR UV-vis. Similar activities for n-butane conversion are observed on all the catalysts studied, although the specific activity increases with the vanadium loading. However, independently of the catalysts preparation procedure, the selectivity to each oxydehydrogenation products (1-, cis-2-, trans-2-butenes and butadiene) initially increases with the vanadium loading, showing a maximum on catalysts with 30 wt.-% V2O5. The nature of active and selective sites is also discussed.