Direct Propane Ammoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase

The kinetics of the direct synthesis of acrylonitrile from propane on V-Sb-Al-(W) mixed oxides indicate that acrylonitrile (ACN) forms by two parallel pathways, one directly from propane and the second, which is the prevalent path, through the intermediate formation of propylene (C3=). The limiting factor in the formation of ACN is the relative slowness of the step of allylic oxidation to ACN of the intermediate C3=, and the higher rate of C3= oxidation to carbon oxides as compared to that of ACN to COx. The step of C3= oxidation to ACN is controlled by the surface availability of NH3 which, in turn, depends considerably on the side reaction of NH3 oxidation to N2. The catalytic behavior of different modified V-Sb-(Al)-O systems and their characterization by X-ray diffraction analysis and Raman, Infrared and X-ray Photoelectron spectroscopies indicate that i) a reduction of both V and Sb occurs during the catalytic reaction, ii) the presence of vanadium not stabilized in the rutile-like phase is responsible for the side reaction of NH3 oxidation and lowering of the selectivity, iii) alumina reacts with antimony forming an AlSbO4 rutile phase which could be epitaxially intergrown or in solid solution with the VSbO4/Sb2O4 system, which, in turn, limits the presence of not stabilized (unselective) vanadium species, and iv) antimony oxide supported on alumina is also selective in propane ammoxidation, but forming acetonitrile as the main product. The doping with vanadium of this sample increases slightly the activity, but especially gives rise to the formation of acrylonitrile instead of acetonitrile.