The synthesis and structure of a single-phase, nanocrystalline MoVW mixed-oxide catalyst of the Mo5O14 type

Abstract

The different preparation steps are characterized to the single phase, crystalline, ternary oxide (MoVW) 5O14, which is important for catalytic, mild selective oxidation reactions. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and v anadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed at each preparation step, starting from the precursor to the final product, were studied using scanning and transmission electron microscopy, X -ray powder diffraction, thermal analysis, and Raman spectroscopy. Raman spectroscopy was also applied to shed some light into the aqueous chemistry of the mixed precursor solutions. Raman data indicates that a molecular structure is already formed in solution which seems to be closely related to that of the final crystalline Mo5O14-type oxide. X-ray diffraction revealed that the thermal treatment steps strongly affect the degree of crystallinity of the ternary Mo5O14 oxide. Transmission electron microscopy with energy dispersive microanalysis confirmed the presence of V and W in the molybdenum oxide particles and gave evidence for the (010) plane as the most developed face of the crystals of this phase. Details of the structural transformation of this system at the different preparation and calcination steps are discussed in relation to their performance in the selective partial oxid ation of acrolein to acrylic acid.

In Situ Investigations of Structureactivity correlations of Mixed Molybdenum Oxide Catalysts

“…The Mo 0.68 V 0.23 W 0.09 oxide catalyst was prepared by spray-drying mixed solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate and subsequent calcination at 623 K in air and 713 K in helium [4]. Combined in situ XRD/MS experiments were performed on a STOE Theta/theta diffractometer equipped with a XRK 900 high temperature cell and a mass spectrometer for on-line analysis.…”

Structure stabilizing effect of tungsten in mixed molybdenum oxides with Mo5O14-type structure

“…The precursor of the (MoVW) 5 O 14 material was prepared as described previously [14]. Ammonium heptamolybdate (AHM) (c = 0.963 mol/L) and ammonium metatungstate (AMT) (c = 0.27 mol/L) were dissolved in bidestilled water at 353 K. V 2 O 5 was dissolved in an aqueous solution of 1.93 mol/L oxalic acid at 353 K (vanadyl oxalate aqueous solution c = 0.379 mol/L).…”

“…Hence, (MoVW) dioxide possesses a smaller unit cell volume than (MoV) dioxide. [14]. The corresponding reaction proceeds through a series of polycondensation steps resulting in the three-dimensional Mo 5 O 14 structure.…”

“…Here, (MoVW) 5 O 14 and (MoV) 5 O 14 have been prepared as crystalline single phase materials exhibiting catalytic activity in the selective oxidation of propene [14,23]. In situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) combined with online gas phase analysis are used to explore the structural evolution of these single phase Mo 5 O 14 -type materials under varying reactive atmospheres.…”

Glycerol oxidehydration to acrylic acid on complex mixed-metal oxides

“…When acrolein was fed in analogous conditions (residence time 0.15 s, temperature 360°C), selectivity to acrylic acid was close to 70% at 20% acrolein conversion. This was apparently due to both the low selectivity to acrolein and the specific residence time used (optimal for glycerol dehydration on WVNb-1), which is too low to convert acrolein into acrylic acid on MoV(W) with Mo 5 O 14 -type structure [112]. WVNb-1), the two catalysts showed important differences: the WV-2 sample exhibited both a lower acrolein conversion than WVNb-1 (which may be explained by taking into account both the higher specific surface area of this latter sample) and a lower selectivity into acrylic acid, with higher selectivity to heavy compounds.…”