Synergetic Effects in the Ni-Mo-O System: Influence of Preparation on Catalytic Performance in the Oxidative Dehydrogenation of Propane

In the Ni-Mo-O system, the addition of molybdenum oxide to nickel molybdate significantly increases its performance as a catalyst in the oxidative dehydrogenation of propane to propene. The most effective composition is Mo/Ni = 1.27/1, for which a selectivity of 63 mol% in propene is obtained at a propane conversion of 22 mol% (500≥C,τ = 3.8 s, C 3 /O 2 /H 2 O/N 2 = 20/10/30/40). Several methods of preparation have been used and Mo/Ni ratios were varied from 0.90 to 2.15. Chemical analyses, X-ray diffraction patterns and infrared spectra show that the solid precursor of Mo/Ni > 1 catalysts contains two ammonium salts, NH 4 (NiMoO 4 ) 2 OH · H 2 O and (NH 4 ) 4 NiH 6 Mo 6 O 24 · 5H 2 O. During calcination, these salts give rise toα-NiMoO 4 and to a mixture of α-NiMoO 4 and MoO 3 (molar ratio NiMoO 4 /MoO 3 = 1/5), respectively. DTA/TGA shows that the relative rates of their decomposition during calcination depend on the method of preparation. These experiments permit the precursors to be classified as type I, II, or III materials. The crystallization of MoO3 proceeds at a lower temperature for type I than for type II material (280 instead of 380≥ C) and before the crystallization of α-NiMoO4 (ca 450–455≥C). No DTA or TGA signal accounts for crystallization of MoO 3 or α-NiMoO 4 in type III material. In calcined type I material, the polymorphic transition α → β -NiMoO 4 is advanced because of the presence of MoO3, and MoO3 itself does not sublime easily. Type I catalysts exhibit better catalytic properties than other types. In differential conditions (500≥C, t = 0.2 s), a synergetic effect is observed with Mo/Ni = 1.27 (type I) catalyst, the conversion of propane being maximum. Coherent interfaces between the (010) plane of α-NiMoO 4 and the (100) plane of MoO 3 are shown by transmission electron microscopy. As tentatively explained in the discussion, these interfaces are formed during calcination of type I precursors, the decomposition of which determines the way the reactive microdomains of NiMoO 4 are distributed throughout the catalyst in the presence of, and/or onto, crystallites of MoO 3 . In turn, the catalytic properties of NiMoO 4 /MoO 3 (Mo/Ni>1) are enhanced for the oxidative dehydrogenation of propane to propene.