In situ investigations of the bulk structural evolution of vanadium containing heteropolyoxomolybdate catalysts during thermal activation

The bulk structural evolution of a vanadium containing heteropolyoxomolybdate (HPOM), H4[PVMo11O40] * 13 H2O, with vanadium substituting for Mo in the Keggin ion was studied under reducing (propene) and partial oxidation reaction conditions (propene and oxygen) by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) combined with mass spectrometry. During treatment in propene, the loss of crystal water in the temperature range from 373 K to 573 K is followed by a partial decomposition, reduction of the average Mo valence, and formation of a characteristic cubic HPOM at 573 K. This behavior is similar to the structural evolution of H3[PMo12O40] * 13 H2O during treatment in propene. The formation of cubic Mox[PVMo11-xO40] with Mo centers on extra Keggin framework positions and V centers remaining in the lacunary Keggin ion coincides with the onset of catalytic activity at ~573 K. The detailed investigations of the local structure around the vanadium centers in Mox[PVMo11-xO40] permit to propose a model for the geometric structure of the active site in Mo and V containing metal oxide catalysts. The cubic Mox[PVMo11-xO40] phase prepared from H4[PVMo11O40] * 13 H2O is stable in propene and oxygen up to ~ 620 K and exhibits an onset of activity at ~ 573 K. This onset of activity is correlated to characteristic changes in the average local Mo structure indicating a reversible transition from the reduced state of the active site in Mox[PVMo11-xO40] to an oxidized state under propene oxidation reaction conditions.