Role of iron in multicomponent molybdate catalysts for selective oxidation of propylene

In order to investigate the role of iron in multicomponent molybdate (MCM) catalysts, a series of MCM catalysts of the composition Mg<SUB>11-x</SUB>Fe<SUB>x</SUB>BiMo<SUB>12</SUB>O<SUB>n</SUB>(0&#8804;x&#8804;8) have been prepared and characterized by X-ray diffraction, infrared, thermal analysis, Mössbauer spectroscopy, scanning electron microscopy and electron spectroscopy for chemical analysis (ESCA). In samples with a value of x up to 2.0, the formation of a ternary compound, Bi<SUB>3</SUB>(FeO<SUB>4</SUB>)(MoO<SUB>4</SUB>)<SUB>2</SUB> is observed. At x=2.5 the ternary compound and ferric molybdate coexist and on increasing the value of x, ferric molybdate is formed. At x=8, ferric molybdate exists as a major phase with &#946;-MgMoO<SUB>4</SUB>, Bi<SUB>2</SUB>(MoO<SUB>4</SUB>)<SUB>3</SUB> and small amounts of &#945;-Fe<SUB>2</SUB>O<SUB>3</SUB>. ESCA data have shown that the surface concentrations of Bi and Mo increase with increasing addition of Fe, though their bulk composition remains unaltered throughout the series. The highest Bi/Mo ratio is observed at x=2.5. Activity and selectivity for oxidation of propylene at 430&#176;C over the series of catalysts also increase with the value of x, exhibiting maxima again at x=2.5. An attempt has been made to rationalize the observed activity pattern on the basis of the physiochemical characteristics. Formation of ternary compound and enrichment of the surface with key components, i.e., Bi (for rate-determining &#945;-H abstraction step) and Mo (for selective oxygen insertion) are responsible for the enhancement observed up to x=2.5. Maximum surface Bi/Mo ratio corresponds to maximum activity and selectivity. At higher values of x, absence of ternary compound and formation of higher amounts of Fe<SUB>2</SUB>(MoO<SUB>4</SUB>)<SUB>3</SUB> and some free &#945;-Fe<SUB>2</SUB>O<SUB>3</SUB> results in poor performance