Physicochemical studies on silica-supported multicomponent molybdate catalyst before and after use in ammoxidation of propylene

Abstract

A multicomponent molybdate catalyst of the composition 50% Ni 3CO 5Fe 3BiPK 0.1Mo 12O 52.5-50% SiO 2 was examined for structural and surface changes before and after its use in ammoxidation of propylene at 480 °C for 10 hr. The techniques employed were TG, DTA, X-ray diffraction, ir spectra, ESR, Mössbauer spectra, and ESCA. TG and DTA show that the catalyst is thermally stable up to 1000 °C and it does not contain free metal oxides. X-Ray data and ir spectra indicate that Fe, Co, and Ni are present as Fe 2(MoO 4) 3,β-CoMoO 4, NiMoO 4, and Bi, as Bi 2O 3 · MoO 3 and Bi 2O 3 · 3MoO 3, respectively, in the fresh catalyst (I), while Fe 2(MoO 4) 3 is reduced to FeMoO 4 in the used catalyst (II). Mössbauer and ESR spectra show that Fe exists in the Fe 3+ state in I and in the Fe 2+ state in II, the latter also being in two different chemical environments. ESCA reveals the existence of Co, Mo, Ni, Fe, Bi, Si, O I, and O II in the surface layer of I; the surface concentrations in II differ markedly: Mo, Bi, Fe, and O II increase and Co, Ni, Si, and O I decrease upon exposure of the catalyst to ammoxidation conditions.