Mechanism of [3-13C]propene ammoxidation on bismuth molybdate catalyst

Abstract

To study the mechanism of acrylonitrile formation, the ammoxidation of [3-13C]propene was studied in the presence of a bismuth molybdate catalyst on a silica support. The reaction was carried out at 723 K with a conversion of about 40 vol % of the propene to acrylonitrile and a small amount of acetonitrile. The 13C distribution among the reaction products was examined quantitatively with a high-resolution mass spectrometer. Half of the I3C in the acrylonitrile was found to be in the CN-group and the other half in the C2H3-group. This result points to a symmetrical intermediate, such as an allylic species, in the ammoxidation reaction. A similar intermediate was observed in the catalytic oxidation of propene to acrolein. The total 13C content in the acetonitrile amounted to only half that of the acrylonitrile, which indicates carbon-carbon bond fissure of the allylic intermediate in the mechanism of acetonitrile formation. Ammoxidation of propene does not seem to involve acrolein as an intermediate. As a matter of fact, the addition of ammonia to a mixture of propene + oxygen + helium completely suppresses acrolein formation at 623 K.

Selective C–H Bond Activation via NOx-Mediated Generation of Strong H-Abstractors

“…In contrast, the allylic C–H bonds in C 3 H 6 are weaker than the bonds in C 3 H 8 (371 kJ mol –1 , Table ), which indicates that the C 3 H 6 molecule can undergo facile C–H activations at the allylic position and is more reactive for C–H activations than the reactants. Such allylic C–H activations are often rate-limiting steps in oxidation of C 3 H 6 . − C 3 H 6 is the desired product in C 3 H 8 oxidative dehydrogenation, and therefore, these facile C–H activations represent the most significant limitations to the selectivity to this product on typical transition metal oxide catalysts such as V 2 O 5 because they lead to more facile activation of weaker C–H bonds.…”

Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations

“…Oxidation and ammoxidation reactions have the same overall activation energy, E a = 19−21 kcal/mol for bismuth molybdates. , The rate-determining step is the α-methyl hydrogen abstraction (step 2), as established from the isotope effect ( k H / k D = 1.82) and the isotopic distributions of oxygen and nitrogen insertion products from either allyl or vinyl-D-labeled propenes. − According to the observed 64:36 ratio of acrylonitrile-[2,2- d 2 ]:[d 0 ] produced from propene-[3,3,3- d 3 ] or -[1,1- d 2 ] and 70:30 ratio of acrylonitrile-[2,2- d 2 ]:[ d 0 ] produced from allyl alcohol-[1,1- d 2 ] or -[3,3- d 2 ], the initial rate-determining step results in formation of π-allyl intermediate, where allyl is π bonded to a coordinately unsaturated Mo (step 3). − , This π-allyl species is rapidly and reversibly converted to σ- N -allyl intermediate through the formation of a C−N bond (step 4a), which is followed by the subsequent hydrogen abstractions (steps 4a, 4b, 4c) to give acrylonitrile. Subsequent steps in the conversion of σ- N -allyl intermediate to acrylonitrile have been studied using molecular probes, such as allyl alcohol, allylamine, and selected D- and 18 O-labeled derivatives over molybdates. ,− The second hydrogen abstraction (step 4a) is the rate-determining step in the conversion of pregenerated σ- N -allyl species to acrylonitrile.…”

Mechanism of Selective Oxidation of Propene to Acrolein on Bismuth Molybdates from Quantum Mechanical Calculations

“…Experimental studies also suggest that the rate-determining step is a α-methyl hydrogen abstraction to form allylic radical intermediates. This was established from both the kinetic isotope effect (KIE) ( k H / k D = 1.82) and the isotopic distributions of oxygen insertion products from either allyl- or vinyl-D-labeled propenes. − Martir and Lunsford reported an activation energy of 14 kcal/mol for allyl radical formation over pure Bi 2 O 3 , over the temperature range of 523−723 K. However, White and Hightower reported an activation energy of 22 kcal/mol for the catalytic reaction of propene and oxygen to form 1,5-hexadiene over Bi 2 O 3 , whereas Swift et al reported the value of 27.5 kcal/mol for cyclic reduction of Bi 2 O 3 by propene . It is not clear why these reported values are so distinctly different, although it should be noted that the studies employ a wide range of experimental conditions.…”