Structures and catalytic properties of magnesium molybdate in the oxidative dehydrogenation of alkanes

Magnesium molybdates have been prepared from an aqueous solution of magnesi um nitrate and ammonium paramolybdate under various pH conditions and used for the selective oxidation of propane and isobutane with gaseous oxygen un der atmospheric pressure in the temperature range of 360-520(degrees)C. The structure analyses by XRD and FT-IR showed the formation of three phases, alpha-MgMoO4, beta-MgMoO4 and Mg2Mo3O11, in the catalysts calcined below 55 0(degrees)C. The catalyst prepared at pH=5.7 showed the highest activity fo r the oxidative dehydrogenation of the alkanes as well as the strongest aci dity. By XPS measurements, an excess amount of Mo compared to Mg was observ ed over the active catalysts. It is likely that the excess molybdenum speci es is present as molybdate and creates acidic sites over the catalyst surfa ce.