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 moly bdate significantly increases its performance as a catalyst in the oxi dative dehydrogenation of propane to propene, The most effective compo sition is Mo/Ni = 1.27/1, for which a selectivity of 63 mol% in propen e is obtained at a propane conversion of 22 mol% (500 degrees C, s = 3 .8 s, C-3/O-2/H2O/N-2 = 20/10/30/40). Several methods of preparation h ave 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 t he solid precursor of Mo/Ni > 1 catalysts contains two ammonium salts, NH4(NiMoO4)(2)OH . H2O and (NH4)(4)NiH6Mo6O24. 5H(2)O. During calcina tion these salts give rise to alpha-NiMoO4 and to a mixture of alpha-N iMoO4 and MoO3 (molar ratio NiMoO4/MoO3 = 1/5), respectively DTA/TGA s hows that the relative rates of their decomposition during calcination depend on the method of preparation. These experiments pet-mit the pr ecursors to be classified as type I, II, or III materials. The crystal lization of MoO3 proceeds at a lower temperature for type I than for t ype II material (280 instead of 380 degrees C) and before the crystall ization of alpha-NiMoO4 (ca 450-455 degrees C). No DTA or TGA signal a ccounts for crystallization of MoO3 or alpha-NiMoO4 in type III materi al. In calcined type I material, the polymorphic transition alpha --> beta-NiMoO4 is advanced because of the presence of MoO3, and MoO3 itse lf does not sublime easily, Type I catalysts exhibit better catalytic properties than other types. In differential conditions (500 degrees C , tau = 0.2 s), a synergetic effect is observed with Mo/Ni = 1.27 (typ e I) catalyst, the conversion of propane being maximum, Coherent inter faces between the (010) plane of alpha-NiMoO4 and the (100) plane of M oO3 are shown by transmission electron microscopy. As tentatively expl ained in ?he discussion, these interfaces are formed during calcinatio n of type I precursors, the decomposition of which determines the way the reactive microdomains of NiMoO4 are distributed throughout the cat alyst in the presence of, and/or onto, crystallites of MoO3. In turn, the catalytic properties of NiMoO4/MoO3 (Mo/Ni > 1) are enhanced for t he oxidative dehydrogenation of propane to propene. (C) 1997 Academic Press.