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Vanadium-antimony oxide selective oxidation catalysts are prepared using a novel peroxide-based sol-gel synthesis procedure. The catalysts are prepared by reaction of soluble peroxovanadium complexes with antimony trioxide under refluxing conditions. The procedure produces a stable colloidal sol of microcrystalline rutile vanadium antimonate having an average particle size of 6.7 nm. Drying the sol gives a gel consisting of pure rutile vanadium antimonate having the vanadium/antimony ratio of about 1/1, consistent with the stoichiometry of the well-characterized crystalline phase V 8/9 Sb 8/9 0 2/9 O 4 . The driving force for the formation of this phase is the redox reaction between a V 5+ peroxo complex and Sb 3+ oxide to form the V 4+ ,Sb 5+ -containing vanadium antimonate. The peroxide-based solgel preparation procedure allows formation of vanadium antimonate under milder conditions than previous synthesis methods that rely on heating of mixtures of vanadium and antimony oxides.

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.

The crystal structure of Sb͑C 2 O 4 ͒OH has been solved by charge flipping in combination with difference Fourier techniques using laboratory X-ray powder data exhibiting significant preferred orientation and refined by the Rietveld method. The compound crystallizes in Pnma with a = 5.827 13͑3͒, b = 11.294 48͑10͒, c = 6.313 77͑3͒ Å, V = 415.537͑5͒ Å 3 , and Z = 4. The crystal structure contains pentagonal pyramidal Sb 3+ cations, which are bridged by hydroxyl groups to form zigzag chains along the a axis. Each oxalate anion chelates to two Sb in approximately the ab plane, linking the chains into a three-dimensional framework. The H of the hydroxyl group is probably disordered in order to form stronger more-linear hydrogen bonds. The highest energy occupied molecular orbitals are the Sb 3+ lone pairs. The structure is chemically reasonable compared to other antimony oxalates and to Bi͑C 2 O 4 ͒OH.