Selective oxidation and oxidative dehydrogenation of hydrocarbons on bismuth vanadium molybdenum oxide

Abstract

a b s t r a c tA systematic investigation of the oxidative dehydrogenation of propane to propene and 1-and 2-butene to 1,3-butadiene, and the selective oxidation of isobutene to methacrolein was carried out over Bi 1Àx/3 V 1Àx Mo x O 4 (x = 0-1) with the aim of defining the effects of catalyst and reactant composition on the reaction kinetics. This work has revealed that the reaction kinetics can differ significantly depending on the state of catalyst oxidation, which in turn depends on the catalyst composition and the reaction conditions. Under conditions where the catalyst is fully oxidized, the kinetics for the oxidation of propene to acrolein and isobutene to methacrolein, and the oxidative dehydrogenation of propane to propene, 1-butene and trans-2-butene to butadiene are very similar-first order in the partial pressure of the alkane or alkene and zero order in the partial pressure of oxygen. These observations, together with XANES and UV-Vis data, suggest that all these reactions proceed via a Mars van Krevelen mechanism involving oxygen atoms in the catalysts and that the rate-limiting step involves cleavage of the weakest CAH bond in the reactant. Consistent with these findings, the apparent activation energy and pre-exponential factor for both oxidative dehydrogenation and selective oxidation correlate with the dissociation energy of the weakest CAH bond in the reactant. As the reaction temperature is lowered, catalyst reoxidation can become rate-limiting, the transition to this regime depending on ease of catalyst reduction and effectiveness of the reacting hydrocarbons as a reducing agent. A third regime is observed for isobutene oxidation at lower temperatures, in which the catalyst is more severely reduced and oxidation now proceeds via reaction of molecular oxygen, rather than catalyst lattice oxygen, with the reactant.

Bismuth Molybdate Catalysts Prepared by Mild Hydrothermal Synthesis: Influence of pH on the Selective Oxidation of Propylene

“…According to the group of Keulks [8,20], re-oxidation of the catalyst is the rate-determining step in propylene oxidation at temperatures below 400 °C, whereas abstraction of an α-hydrogen atom to form an allylic intermediate is the rate determining step at higher temperatures (>400 °C), in agreement with theory [14]. Recently, it was further reported that the reaction order in oxygen was zero at 340 °C and 400 °C for Bi2Mo3O12 catalysts, i.e., the reaction rate for acrolein formation, was independent of the partial pressure of oxygen, indicating that the transition temperature where re-oxidation of the catalyst becomes the rate determining step was lower than 340 °C [61,62]. The hydrothermally synthesized samples at pH = 4-8 exhibited high propylene conversion and also relatively high acrolein selectivities already at 360 °C.…”

Isotope Studies in Oxidation of Propane over Vanadium Oxide

“…It might be, therefore, possible that different mechanisms contribute to acrolein formation over 6 V/SBA‐15, such as a low‐temperature (200–300 °C) oxidation pathway catalyzed by activated oxygen species at the surface, such as peroxovanadate, via propylene oxide as intermediate followed by oxidative dehydrogenation (Scheme a), and the allylic oxidation of propylene at higher temperatures (>300 °C) (Scheme b). In the low‐temperature range acetone and propionaldehyde may be formed as isomerization products of propylene oxide . Indication of the participation of peroxide species in oxidative dehydrogenation of propane over supported vanadium oxide catalysts has been found recently by experiment, and theory .…”

Effect of O2, CO2 and N2O on Ni–Mo/Al2O3 catalyst oxygen mobility in n-butane activation and conversion to 1,3-butadiene

“…Low amounts of butane and butadiene were detected at low CO 2 -to-butane molar ratios despite the fact that C-H bond dissociation can occur on the molybdenum metal. 38 In the presence of CO 2 , significant amounts of hydrogen and CO are formed. Aside from a dehydrogenation reaction, a CO 2 reforming reaction can also take place.…”

“…The oxidative dehydrogenation of n-butane to butenes and 1,3-butadiene follows the Mars-van Krevelen mechanism, as also reported by several researchers. 2,7,8,38 In this mechanism, chemisorption of n-butane, activation of C-H bonds, and abstraction of hydrogen from n-butane to form an alkyl radical-metal cation intermediate are important steps in the oxidative dehydrogenation of n-butane. 27,49,50 The rates of ODH are enhanced if the cation present in the catalyst is a weak acid and can be reduced by two electrons transferred from C-H bonds.…”