Formation of active phases in the Sb-V-, Al-Sb-V-, and Al-Sb-V-W-oxide systems for propane ammoxidation

The Sb-V-O, Al-Sb-V-O and Al-Sb-V-W-O systems were investigated for the ammoxidation of propane to acrylonitrile. Compared with pure approximate to SbVO4, the approximate to SbVO4 that was obtained by co-calcination of pure approximate to SbVO4 with alpha-Sb2O4 at 800 degrees C and subsequent sieving was found to be much more selective to acrylonitrile. XRD data showed no variation of the rutile unit cell for synthesis ratios Sb:V greater than or equal to 1, indicating that the active phase is approximate to SbVO4 with a surface enriched with Sb. Characterisation of syntheses in the Al-Sb-V-O system allowed the identification of a trirutile-like phase with the composition Al1-xSbVxO4 (0 < x < 0.5). The synthesis of this phase, which is active and selective to acrylonitrile, requires excess of aluminium. Over a fresh preparation with Al, Sb, V and W the activity and the selectivity to acrylonitrile increased considerably with time-on-stream. This behaviour shows that the active rutile phase is formed in situ, and EDX analyses gave the average composition Al0.1Sb0.8V0.7W0.4O4. The highest yield to acrylonitrile that was observed for the three systems was 37 % and was obtained over the Al-Sb-V-W-oxide.