Phase transitions of V-Mo-W mixed oxides during reduction/re-oxidation cycles

Tungsten-doped vanadium molybdenum oxide catalysts for the selective oxidation of acrolein to acrylic acid prepared from evaporated or spray-dried precursors form different phases. Treatments under reducing and oxidizing mostly acrolein-containing atmospheres between 653 and 773 K uncover steps of active phase formation and catalyst deactivation depending on tungsten content and preparation. The results are related to industrial acrylic acid production. V-Mo-W mixed oxides are prepared by a hydrochemical preparation route with a stoichiometry of V 2Mo 8W x O y with 0 ≤ x ≤ 5 via evaporation/crystallization and by spray-drying of ammonium polyoxometallate solutions combined with a subsequent calcination procedure. The phase composition has been examined by electron and X-ray powder diffraction. Significant changes are observed after reduction/re-oxidation and after/during reactions in an air–acrolein atmosphere for all crystalline samples. Samples prepared from spray-dried precursors with tungsten contents x > 1 are amorphous/nanocrystalline. Independent of the preparation and reaction procedure, the formation of the thermodynamically metastable, tetragonal Mo 5O 14 phase is detected. Deactivation starts at the latest at 723 K in air with the formation of thermodynamically stable phases like orthorhombic MoO 3. Smaller particles exhibit higher stability against deactivation or reduction. Reaction networks are established to illustrate the effect of various chemical reactions on crystalline and amorphous/nanocrystalline structures. The temperatures of the industrial processes are interpreted on the basis of the presented results.