SiO 2-supported vanadium magnesium mixed oxides as selective catalysts for the oxydehydrogenation of short chain alkanes

Abstract

Unsupported and SiO 2-supported VMgO mixed oxides catalysts have been prepared, characterized and tested in the oxidation of propane, n-butane and propylene. Mg-vanadates are observed by XRD, IR and Raman spectroscopy in both pure and supported VMgO catalysts, although the effective Mg/V ratio, in the VMgO phases formed, depends on the silica content. Since SiO 2 reacts partially with MgO during the calcination step, forming Mg 2SiO 4, the effective Mg/V ratio depends on the silica content. In this sense, pyro-Mg 2V 2O 7 is mainly formed on supported catalysts with Mg/V atomic ratios lower than 4, while ortho-Mg 3V 2O 8 is observed on supported catalysts with Mg/V atomic ratios higher than 4. For this reason, it can be concluded that the Mg/V atomic ratio of crystalline VMgO phases observed on supported catalysts is lower than those observed on unsupported ones. The catalytic tests for the oxidation of propane and n-butane indicate that the catalytic behavior of supported catalysts depends on both the SiO 2 content and the Mg/V atomic ratio, although a different behavior during the oxidation of n-butane or propane is observed: the most selective catalysts for the oxydehydrogenation (OXDH) of propane were those with ortho-Mg 3V 2O 8/ pyro-Mg 2V 2O 7/MgO or ortho-Mg 3V 2O 8/MgO mixed phases while the most selective catalysts for the OXDH of n-butane are formed by ortho-Mg 3V 2O 8/MgO mixed phases. On the other hand, V-containing compounds with VO double bonds are required in order to obtain selective catalysts for the oxidation of propylene to acrolein. In the selective oxidation of propylene, acrolein is not formed on catalysts in which ortho-Mg 3V 2O 8/MgO mixed phases are mainly detected. However, acrolein was obtained on catalysts in which VO double bonds are present.