In-situ X-ray diffraction study of phase crystallization from an amorphous MoVTeNb oxide catalyst precursor

The formation of the crystalline phases M2, M1, orthorhombic MoO3, and a tetragonal M5O14 (M=Mo,V,Nb) type phase during heat treatment of an amorphous Mo-V-Te-Nb oxide precursor in helium is investigated applying in-situ X-ray diffraction. The precursor was synthesized by precipitation, spray-drying and subsequent calcination of the dried orange powder in air at 275°C. Crystallization of M2 starts at 450°C, and it occurs simultaneously with the initial liberation of tellurium. Concurrent formation of M1, M5O14, and MoO3 at temperatures higher than 550°C suggests that structural building blocks consumed during crystallization are competitively supplied from a common amorphous precursor. The results indicate that the availability of tellurium could play a decisive role with respect to phase distribution in the final catalyst providing one possible explanation for the susceptible response of the phase composition of Mo-V-Te-Nb oxide catalysts for selective oxidation of propane to acrylic acid on the experimental conditions of thermal activation.