Characterization of bismuth–cerium-molybdate selective propylene ammoxidation catalysts

Abstract

a b s t r a c tAcrylonitrile is a major chemical intermediate used in the production of a wide range of chemical and polymer products. Central to the commercial process is a proprietary catalyst consisting of a complex mixture of metal oxides containing a bismuth-containing molybdate phase that is active and selective for propylene ammoxidation to acrylonitrile. Among the most active and selective is a solid solution of bismuth and cerium molybdate. Solid state structural studies were undertaken to characterize this active phase. The results show that the mixed bismuth-cerium molybdate consists of a solid solution phase having the scheelite-related structure of cerium molybdate with a monoclinic unit cell. Analysis of the cation site occupancy using synchrotron X-ray diffraction indicates that bismuth preferentially occupies the Ce(3) site of the monoclinic cerium molybdate structure. It is therefore possible to singularly identify the structure of the active site for propylene ammoxidation given that bismuth is a necessary constituent of a site for selective propylene (amm)oxidation. The proposed active site consists of bismuth located next to a cation vacancy in the structure, presumably in order to accommodate its lone pair of electrons. Bismuth serves as the site for the rate determining ␣-hydrogen abstraction from propylene to form an allyl intermediate and subsequent nitrogen insertion and loss of lattice oxygen. The bismuth site is surrounded by two cerium cations in this active site configuration. Thus the model that emerges from this study is Bi 3+ and Ce 3+ in a molybdate structural framework with cerium readily able to undergo Ce 3+ ↔ Ce 4+ redox that facilitates lattice oxygen transfer to the active site as required by the operative Mars-van Krevelen mechanism for selective propylene ammoxidation. The presence of two cerium cations adjacent to bismuth as a key component of the active site is expected to promote the rapid re-oxidation of the catalytic site effecting enhanced catalytic performance with respect to selective product yields and productivity.

Synthesis and Characterization of Cerium Molybdate Semiconductor Nanoparticles

“…The pattern of the powders calcined at 800 ºC shows the diffraction peaks of the monoclinic phase, but it was also observed the appearance of Ce 11 O 20 and Ce 7 O 12 phases due to cerium oxidation. This result was also related by Brazdil et al (2015) 19 that demonstrated the appearance of CeO 2 phases in the synthesis of cerium molybdate at temperatures above 600 ºC. The diffractograms of the cerium molybdate powders were indexed with lanthanum molybdate cards, as they are isostructural and due to the unavailability of tetragonal and monoclinic cerium molybdate cards in the literature.…”

“…(28 º -34 º and 46 º -58 º), plus the weaker low-angle (10 º -25 º) superstructure diffraction peaks which are the fingerprints of the defect monoclinic phase 19 . The pattern of the powders calcined at 800 ºC shows the diffraction peaks of the monoclinic phase, but it was also observed the appearance of Ce 11 O 20 and Ce 7 O 12 phases due to cerium oxidation.…”

The effects of Bi2O3 on the selective catalytic reduction of NO by propylene over Co3O4 nanoplates

“…Bi 2 O 3 , a common oxide semiconductor, is widely used in the elds of chemical engineering and electronics such as NO detection 23 and the oxidation or ammoxidation of propylene. 24,25 In the oxidation/ammoxidation of propene over bismuth/ molybdate catalyst, bismuth was thought to involve the ratedetermining hydrogen abstraction from propylene, 26 exhibiting its mild oxidizing characteristics. This property might be also benecial for the partial oxidation of propylene in C 3 H 6 -SCR for NO reduction.…”

{CeO2/Bi2Mo1−xRuxO6} and {Au/Bi2Mo1−xRuxO6} Catalysts for Low-Temperature CO Oxidation

“…Compounds with a Bi/Mo ratio of 3:1 was found to produce a α-Bi 2 Mo 3 O 12 compound exhibiting acrolein selectivity in the range of 36%-56%, providing a 60%-63% propylene conversion. Brazdil et al [14] reported on the synthesis of bismuth-cerium molybdate compounds with a Bi x Ce 2−x Mo 3 O 12 formulation. They studied the selectivity as a function of cerium content, to prove its effectiveness in reactions for catalytic propylene ammoxidation.…”