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

Abstract

The bulk structural evolution of a vanadium-containing heteropolyoxomolybdate (HPOM), H 4[PVMo 11O 40] × 13H 2O, 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 to 573 K was 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 H 3[PMo 12O 40] ×13H 2O during treatment in propene. The formation of cubic Mo x [PVMo 11− x O 40] 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. Detailed investigations of the local structure around the vanadium centers in Mo x [PVMo 11− x O 40] have made it possible to propose a model for the geometric structure of the active site in Mo- and V-containing metal oxide catalysts. The cubic Mo x [PVMo 11− x O 40] phase prepared from H 4[PVMo 11O 40] × 13H 2O 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 Mo x [PVMo 11− x O 40] to an oxidized state under propene oxidation reaction conditions.