Structural change of molybdenum on silica–alumina in contact with propene studied by ESR and Mo LIII-edge XANES

To obtain structural information of the molybdenum species supported on silica–alumina (SAH-1), as a unique catalyst for propene metathesis, Mo LIII-edge XANES and ESR spectroscopies were applied to characterize. The molybdenum species on SAH-1 are partly reduced to form Mo5+ ions in contact with propene even at room temperature (293 K), and the reduced ions increased with an increase in reaction temperature up to 473 K. From the results of Mo LIII-edge XANES, unique polymolybdate species with tetrahedral and octahedral structures are dominant in MoO3/SAH-1 before the reaction. After the metathesis reaction, Mo ions are partly reduced to form metathesis active species and a ratio of tetrahedral structure decreases. This unique redox behavior of Mo ions is brought about by SAH-1 support. It is suggested that the metathesis active species of MoO3/SAH-1 are not highly dispersed Mo ions in reduced state but are in poorly crystallized polymolybdate species, which can be partly reduced in contact with propene even at room temperature.