Effect of titanium substitution in approximate to SbVO4 used for propane ammoxidation

Catalysts of the nominal composition Sb0.9V0.9-xTixOy, 0.0 less than or equ al to x less than or equal to 0.9, were prepared and characterized with X-r ay diffraction, Fourier transform-Raman spectra, diffuse reflectance infrar ed Fourier transform spectra, transmission electron microscopy, electron di ffraction, and X-ray microanalysis. The catalysts were used for the ammoxid ation of propane to give acrylonitrile. Compared with the pure approximate to SbVO4 (Sb0.92V0.92O4) it is observed that the activity decreases while t he selectivity to acrylonitrile formation is improved when the Ti : V ratio of the preparation is increased. The characterization of the catalysts sho ws the formation of a rutile-type phase in all preparations with vanadium, Additionally, alpha-Sb2O4 is formed in an amount that increases with the am ount of titanium in the sample. X-ray microanalysis data confirm that two s ubstitution mechanisms occur in parallel, namely, one Ti4+ substitutes for one V4+ and two Ti4+ substitute for one V3+ and one Sb5+, forming the solid solution series Sb-(0.92-z/2)(5+) V-(0.28-z/2)(3+) V-(0.64-u)(4+) Ti-(z+u) (4+) square(0.16)O(4) (square is a cation vacancy). Quantitative model calc ulations considering data obtained by the characterization methods give the content of cations in the unit cell of the rutile phase. It is demonstrate d that the activity can be correlated with the content of V3+ in the unit c ell, indicating that the activation of propane occurs on a V3+ center. More over, the selectivity to acrylonitrile can be correlated to the Sb5+/V3+ ra tio, which indicates that the ammonia is activated on a Sb5+ moiety. Consid ering both activity and selectivity, the best performance for propane ammox idation is obtained for a sample with the nominal composition Sb0.9V0.3Ti0. 6Oy. The selectivity to acrylonitrile is 53% at 25% propane conversion to b e compared with 12% at 15% propane conversion for the nonsubstituted approx imate to SbVO4, The improved selectivity can be explained by isolation of t he vanadium centers in the active phase, which gives fewer bridging V-O-V m oieties that are active for degradation and combustion of propane and the s ubsequently formed propylene. (C) 2000 Academic Press.