Mixed iron and molybdenum oxide catalysts supported on nanostructured silica, SBA-15, were synthesized with various Mo/Fe atomic ratios ranging from 0.07/1.0 to 0.57/1.0. Structural characterization of as-prepared MoxOy_FexOy/SBA-15 samples was performed by nitrogen physisorption, X-ray diffraction, and DR-UV-Vis spectroscopy. Adding molybdenum resulted in a pronounced dispersion effect on supported iron oxidic species. Increasing atomic ratio up to 0.21Mo/1.0Fe was accompanied by decreasing species sizes. Strong interactions between iron and molybdenum during the synthesis resulted in the formation of Fe-O-Mo structure units, possibly Fe2(MoO4)3-like species. Reducibility of MoxOy_FexOy/SBA-15 catalysts was investigated by temperature-programmed reduction experiments with hydrogen as reducing agent. The lower reducibility obtained when adding molybdenum was ascribed to both dispersion and electronic effect of molybdenum. Catalytic performance of MoxOy_FexOy/SBA-15 samples was studied in selective gas-phase oxidation of propene with O2 as oxidant. Adding molybdenum resulted in an increased acrolein selectivity and a decreased selectivity towards total oxidation products.
Keywords: dispersion effect; electronic effect; green chemistry; heterogeneous catalysis; iron oxides.