Vanadium antimonate

Partial oxidation of toluene to benzaldehyde over vanadium antimonate catalysts doped with titanium. The influence of the antimony content over the deactivation process. 

In a previous work [7] we have studied the changes produced by the replacing of antimony sites by titanium on the structure of a vanadium antimonate catalyst and on its activity and selectivity levels during the vapour phase oxidation of toluene. 

Hie catalytic behavior of vanadium-antimonate samples in propane ammoxidation and Ae... 

The heat treatment in the absence of oxygen avoids the oxidation of vanadium, but even so the formation of the vanadium-antimonate phase is not complete in the Sb V=1.0 sample prepared by redox reaction in solution and some a-Sb204 is present... 

X-Sb204. Vanadium, therefore, catalyzes the leoxidadon of antimony, reasonably that antimony situated in interface sites between Sb-oxide and vanadium-antimonate phases. 

These results clearly indicate that Sb -oxide, stabilized at the surface of vanadium-antimonate, is responsible for the selective synthesis of acrylonitrile from the intermediate propylene in agreement with previous suggestions [16], and that vanadium catalyzes the leoxidation of reduced antimony, as well as plays other roles in the mechanism of oxidative dehydrogmia-tion of propane to propylene and in the side reaction of NH3 oxidation to N2 (see above). 

The segregation phenomena in V-Sb-0 catalyst for selective propylene oxidation to acrolein were studied by means of X-ray photoelectron spectroscopy, scanning electron microscopy with EDS, as well as electron and X-ray diffraction. The vanadium antimonate crystals were found to be a main component of the catalyst. It was stated that epitaxial layers of antimony tetroxide on the base faces of vanadium antimonate crystals, exposing (010) plane, were responsible for high catalyst selectivity. 

In Fig. 5 X-ray diffraction patterns (XRDPs) of the used antimony trioxide (a), of antimony trioxide slowly heated from room temperature to 973 K for 5 hours (b), and of the freshly prepared catalyst are presented. As it is seen, slow heating of of antimony trioxide from room temperature to 973 K causes oxidative transformation of the antimony trloxide into a-antimony tetroxide containing traces of /9-antimony tetroxide. While, the similar heating of the mixture of vanadia and antimony trioxide followed by its annealing at 973 K results in formation of the mixture of crystals of vanadium antimonate, a—antimony tetroxide, and /3-antimony tetroxide. Vanadium antimonate is the base phase of this mixture. There are not lines of the starting oxides in XRDP of the catalyst. 

The closest packed (110) planes of vanadium antimonate lattice form the best developed faces of vanadium antimonate crystals. The selected area diffraction pattern (SADP) with [110] zone axis taken from vanadium antimonate crystal is presented in Fig. 6. The high catalyst selectivity of the oxidized catalyst should be mainly connected with evolution of (110) surface of the vanadium antimonate crystals occurring in oxidizing atmosphere. 

Typical propane ammoxidation catalysts are essentially constituted by a combination of metallic mixed oxides. To date, there are two catalytic systems i) vanadium-antimonates with a rutile-type structure, represented by the VSbxMyOz formula, where M are elements used as the promoter such as W, Al, Te, Nb, Sn, Bi, Cu, or andii) vanadium-molybdates with a bronze structure, rep-... 

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