Oxidative dehydrogenation of propane over chromium-loaded calcium-hydroxyapatite

Chromium-loaded hydroxyapatite catalysts Cr(x)/CaHAp (0.1 <= x <= 3.7 wt.% Cr) were prepared by ion-exchange and characterized by several techniques including FTIR, Raman, XPS, UV-vis-NIR spectroscopies, EPR, DRX and TPR. After calcination in air, several isolated chromium species were identified: (i) surface Cr3+ ions in distorted octahedral symmetry, (ii) bulk octahedral Cr3+ ions, (iii) octahedral Cr5+ ions in low concentration and (iv) Cr6+ ions. Cr6+ ions present as monochromates are predominant only at very low loadings (x <= 0.1 wt.% Cr) whereas, at higher Cr amounts (up to 3.7 wt.% Cr), Cr3+ species are preponderant. The majority of Cr3+ ions are located on the apatite surface; they do not form Cr2O3 crystallites but isolated Cr3+-O-Ca2+ or Cr3+-O-Cr3+ entities at the highest Cr amounts. The Cr(x)/CaHAp catalysts were tested in propane oxidative dehydrogenation in the 300-550 C temperature range. The Cr6+ centres initially present on the catalysts may initiate the cracking of propane because of their acidity and improve the conversion: however, as they are reduced by the reaction mixture, the propane conversion decreases upon running at 550 C.Cr2+ ions are also formed upon running. Isolated Cr3+ species are believed to be responsible for the propylene formation (propylene yield around 7% at 550 C). This limitation of performance is ascribed to the decrease of the basicity induced by the fixation of Cr3+ which counterbalances the positive effect of chromium on oxygen reactivity. The proposed mechanism involves the contribution of oxygen vacancies or Cr2+ species. (C) 2009 Elsevier B.V. All rights reserved.