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% Ni₃CO₅Fe₃BiPK_0.1Mo₁₂O_52.5-50% SiO₂was 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 Fe₂(MoO₄)₃, β-CoMoO₄, NiMoO₄, and Bi, as Bi₂O₃·MoO₃and Bi₂O₃·3MoO₃, respectively, in the fresh catalyst (I), while Fe₂MoO₄)₃ is reduced to FeMoO₄ in the used catalyst (II). Mössbauer and ESR spectra show that Fe exists in the Fe³⁺ state in I and in the Fe²⁺ 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.