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, M n, 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 catal ytic and is first order in propane disappearance, consistent with the abstraction of a methylene hydrogen being the rate limiting step. Prop ane conversion and yields of propylene produced vary greatly with the choice of the A metal of the molybdate and the surface area of the cat alyst. 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 catalyti c activity and useful product yields. The effect is largest for the Ni -molybdate system whose optimum lies at stoichiometry. The optimum act ivity of Co-molybdate lies at molybdenum lean compositions. Binary Ni- Co-molybdates are less sensitive to molybdenum level, offering a conve niently stable catalytic system for further mechanistic and technologi cal optimization studies. Addition of redox elements V, Fe, Ce, and Cr enhances the activity of the Ni-Co-molybdates, with Cr addition holdi ng most promise by retaining high selectivity at enhanced conversions and hence a possibility for desired lower process temperature operatio n. Our studies suggest, that the Ni-Co molybdate system provides an al ternate to V-based catalysts for the activation of light paraffins. Fu turistically, it holds promise as the paraffin activating component of a two component catalyst system for the direct conversion, in a singl e reactor, of propane to acrylic acid or acrylonitrile, with the secon d component being composed of one of the well known olefin converting, multicomponent mixed metal molybdates. (C) 1997 Academic Press.