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

Vanadium oxide supported on calcined hydrotalcite has been investigate d for the oxidehydrogenation of n-butane in the 500-550 degrees C temp erature interval. Hydrotalcite (Mg/Al atomic ratio of 2.77), consistin g of a single phase only, has been employed as a support precursor. Th e vanadium catalysts (0-50, referred as wt.-% V2O5) were prepared by i mpregnation of calcined hydrotalcite (450 degrees C) with ammonium met avanadate (in an aqueous solution) or vanadyl acetylacetonate (in a me thanolic solution), and then calcined at 600 degrees C for 4 h. During the impregnation step, the support is transformed into hydrotalcite i f aqueous solutions are used. However, it is not modified if methanoli c solutions are used. After calcination, diffuse X-ray diffraction pat terns of MgO, in addition of a broad peak at 2 theta=35 degrees which intensity increases with the vanadium loading, were observed. Magnesiu m vanadates, i.e. ortho- and pyro-magnesium vanadates, were observed b y FT-IR, FT-Raman and DR UV-vis. Similar activities for n-butane conve rsion 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 oxydeh ydrogenation products (1-, cis-2-, trans-2-butenes and butadiene) init ially increases with the vanadium loading, showing a maximum on cataly sts with 30 wt.-% V2O5. The nature of active and selective sites is al so discussed.