David L. Stern

Oxydehydrogenation of n-butane over promoted MgV-oxide based catalysts

Abstract Magnesium vanadate, Mg2V2Ox, and promoted systems of the composition Mg4V2M2Ox, where M = Mo, W, B, Al, Ga, Sb and (SbP), were synthesized, characterized and evaluated for the oxydehydrogenation of n-butane in the temperature range 400–500°C and atmospheric pressure. The selectivities to butene/butadiene of the majority of the promoted catalysts are, at 500°C and constant n-butane conversion (2%), substantially higher than those of the base system: Sb(87), SbP(82), B(82), Mo(68), Al(62), base(55), W(50). The superior selectivities of the antimony and boron containing compositions are probably primarily due to the formation of ternary MgVSb-oxide Mg1−xSbV2/3xO3.5) and MgVB-oxide (Mg1−xBV2/3O2.5) phases, contributing to the structural site isolation of the paraffin activating vanadium sites; the stabilization of the V4+ or lower oxidation states; and their possible interaction with an inferred supra-surface Sb-oxide phase. Highest butene/butadiene per pass yields were obtained with a Mg4V2Ga2Ox catalyst (5.2% at 7.9% conversion, at 500°C, 1 atm, C40/O2/N2 = 5/1/20, and WHSV = 13 h−1). The only composition yielding measurable amounts of furan was the W promoted catalyst.

Mechanistic aspects of propane oxidation over Ni-Co-molybdate catalysts

Publisher Summary The most studied systems for oxidative propane upgrading are vanadium, vanadium-antimony, vanadium-molybdenum, and vanadium-phosphorus based catalysts. Another family of light paraffin oxidation catalysts are molybdenum based systems, for example, nickel-molybdates, cobalt-molybdates, and various metal-molybdates. Ni–Co-molybdates are viable oxidation catalysts for the activation of light paraffins, such as propane and butanes, to produce the respective olefins. Maximum yields are in the range of 16% at about 80% selectivity. The catalysts activate methylene C–H bonds, abstracting the hydrogen of the substrate in the rate limiting step of the reaction. With propane as feed, propylene is the only first formed product and all the higher oxidized products ensue in subsequent steps after the propylene has been formed. Acrolein is formed from the in situ produced propylene and acrolein is the main intermediate, leading to waste products CO and CO 2 . Binary molybdates of the formula AMoO 4 , where A = Ni, Co, Mg, Mn, and/or Zn and some ternary Ni–Co-molybdates promoted with P, Bi, Fe, Cr, V, Ce, K, or Cs, have been recently under study. A specific representative of these systems is the composition Nio.5Co 0.5 MoO 4 that has been recently selected for an in depth kinetic study and whose mechanistic aspects are now further discussed in this chapter.