Propane oxydehydrogenation over molybdate-based catalysts

Single and binary metal molybdates, supported on silica (80 wt% active phase/20 wt% SiO2), having the formula AMoO(4), where A=Ni, Co, Mg, Mn, and/or Zn, and some ternary molybdates having the formula Ni0.45Co0.45X0.066MoO4, where X=P, Bi, Fe, Cr, V, and Ce, were investigated for the oxydehydrogenation of propane to propylene. The reaction is catalytic and is first order in propane disappearance, consistent with the abstraction of a methylene hydrogen being the rate limiting step. Propane conversion and yields of propylene produced vary greatly with the choice of the A metal of the molybdate and the surface area of the catalyst. At 560 degrees C and atmospheric pressure, the highest propane conversion and highest propylene yields are obtained with NiMoO4/SiO2 (16% at 27% conversion), closely followed by Ni0.5Co0.5MoO4/SiO2. The molybdenum content of the compositions greatly influences the catalytic activity and useful product yields. The effect is largest for the Ni-molybdate system whose optimum lies at stoichiometry. The optimum activity of Co-molybdate lies at molybdenum lean compositions. Binary Ni-Co-molybdates are less sensitive to molybdenum level, offering a conveniently stable catalytic system for further mechanistic and technological optimization studies. Addition of redox elements V, Fe, Ce, and Cr enhances the activity of the Ni-Co-molybdates, with Cr addition holding most promise by retaining high selectivity at enhanced conversions and hence a possibility for desired lower process temperature operation. Our studies suggest, that the Ni-Co molybdate system provides an alternate to V-based catalysts for the activation of light paraffins. Futuristically, it holds promise as the paraffin activating component of a two component catalyst system for the direct conversion, in a single reactor, of propane to acrylic acid or acrylonitrile, with the second component being composed of one of the well known olefin converting, multicomponent mixed metal molybdates. (C) 1997 Academic Press.