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Free keywords:
OXYGEN BOND DISTANCES, X-RAY-ABSORPTION, AQUEOUS-SOLUTION, POTENTIOMETRIC DATA, MOLYBDENUM, ISOPOLYMOLYBDATES, MOO3, EQUILIBRIUM, SCATTERING, MOLYBDATE
8 Mixed V/W/Mo Oxides for selective oxidation
Abstract:
In situ Raman spectroscopy was used to characterize the synthesis and activation steps of mixed metal oxide catalysts starting from the mixed solutions, through subsequent drying and activation procedures, to the catalytic propene partial oxidation reaction. Comparison with Raman spectra recorded for defined, well-crystallized reference oxides allowed the assignment of the spectra obtained during the catalyst preparation to certain oxides, such as Mo5O14. Especially the latter oxide phase is relevant for selective partial oxidation catalysis. The selectivity for partial oxidation products could be improved between two- and three-fold when the amount Of Mo5O14-type oxide increased. This phase forms from mixtures of molybdenum and tungsten and from mixtures of molybdenum, tungsten and vanadium, but not from binary mixtures of molybdenum and vanadium with high vanadium concentrations. Tungsten and vanadium play an important role as structural promoters in the formation and stabilization of this oxide and for high catalytic activity. A resonance Raman effect was proved for reduced molybdenum oxides due to resonant coupling of the exciting laser to the IVCT transitions between fivefold- coordinated Mo5+ and sixfold-coordinated Mo6+. This resonance enhancement renders possible the in situ characterization of operating, reduced molybdenum oxide catalysts. In contrast to the Mo5O14-type phase, MoO3-x, exhibits a high selectivity to total oxidation, and it even becomes re-oxidized during propene partial oxidation. These different catalytic properties Of MoO3-x, and the Mo5O14-type oxide led to the development of a structure-activity relationship which explains the behavior of industrial catalysts. A model is proposed on the basis of a bond order-Raman wavenumber relationship, which explains the different selectivities of these two oxides in terms of metal- oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. Copyright (C) 2002 John Wiley Sons, Ltd.
