SiO2-supported vanadium magnesium mixed oxides as selective catalysts for the oxydehydrogenation of short chain alkanes

Unsupported and SiO2-supported VMgO mixed oxides catalysts have been prepar ed. characterized and tested in the oxidation of propane, n-butane and prop ylene. Mg-vanadates are observed by XRD, IR and Raman spectroscopy in both pure and supported VMgO catalysts, although the effective Mg/V ratio, in th e VMgO phases formed, depends on the silica content. Since SiO2 reacts part ially with MgO during the calcination step. forming Mg2SiO4, the effective Mg/V ratio depends on the silica content. In this sense, pyro-Mg2V2O7 is ma inly formed on supported catalysts with Mg/V atomic ratios lower than 4, wh ile ortho-Mg3V2O8 is observed on supported catalysts with Mg/V atomic ratio s 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 t han those observed on unsupported ones. The catalytic tests for the oxidati on of propane and n-butane indicate that the catalytic behavior of supporte d catalysts depends on both the SiO2 content and the Mg/V atomic ratio, alt hough a different behavior during the oxidation of n-butane or propane is o bserved: the most selective catalysts for the oxydehydrogenation (OXDH) of propane were those with ortho-Mg3V2O7/MgO or ortho-Mg3V2O8/MgO mixed phases while the most selective catalysts for the OXDH of n-butane are formed by ortho-Mg3V2O8/MgO mixed phases. On the other hand, V-containing compounds w ith V=O double bonds are required in order to obtain selective catalysts fo r the oxidation of propylene to acrolein. In the selective oxidation of pro pylene, acrolein is not formed on catalysts in which ortho-Mg3V2O8/MgO mixe d phases are mainly detected. However, acrolein was obtained on catalysts i n which V=O double bonds are present. (C) 2001 Elsevier Science B.V. All ri ghts reserved.