Ammoxidation of ethylene to acetonitrile over chromium or cobalt alumina catalysts prepared by sol–gel method
Résumé
A series of Cr/Al2O3 and Co/Al2O3 catalysts were tested in the selective ammoxidation of ethylene to acetonitrile. Catalysts were prepared either by sol–gel method or by impregnation with chromium or cobalt acetylacetonate salts. Physicochemical properties of catalysts were accomplished by several techniques such as chemical analysis, physisorption of N2, X-ray diffraction (XRD), 27Al MAS NMR, UV–Visible diffuse reflectance (DRS) and Raman spectroscopy and temperature programmed reduction of H2 (H2–TPR). Textural analysis reveals that mesoporous materials with pronounced surface areas were obtained using sol–gel procedure while impregnation of the support produces a moderate decrease of its surface area and pore volume. XRD analysis confirms the presence of highly dispersed metal species which reside essentially on the surface and measure less than 4 nm. Furthermore, 27Al MAS NMR shows that for xerogels, part of metal species occupies sites on/in A12O3 in close vicinity of octahedral 27Al. This, apparently, is not the case for aerogels. For Cr/Al2O3 catalysts, isolated Cr6+, mono and polychromate species were identified using DRS, Raman Spectroscopy and H2–TPR which seem to play a key role in the ammoxidation of ethylene. Furthermore, for cobalt doped catalysts, CoAl2O4 was identified as active phase on the basis of DRS and H2–TPR results. From the supercritical drying, it results generally better catalysts than catalysts calcined by ordinary procedure which leads to inactive agglomerated Co3O4 and CoO–Al2O3 phase.