(297b) X-Ray Absorption Spectroscopic Investigation Of Mo-V Based Mixed Oxide Catalysts For The Selective Ammoxidation Of Propane
Direct ammoxidation of propane is being presently explored as a potential new route for the production of acrylonitrile (1). The current process for making acrylonitrile use propylene as the feed which is more costly and less abundant than propane. Several studies have examined a number of catalyst systems for the direct ammoxidation of propane and mixed oxides based on Mo and V are among the most promising catalyst candidates reported so far (2-17). Mo-V-Te-Nb-O system, the most active and selective for this reaction, shows the presence of orthorhombic (so called M1) and hexagonal (M2) phases, the former being more active and selective for this reaction (18-21). The structural and compositional features that make this catalyst system unique in selective propane ammoxidation are still being investigated requiring characterization by a variety of spectroscopic techniques. X-ray absorption spectroscopy (XAS) is a highly useful technique to investigate the local structure and oxidation states of constituent metal ions of the Mo-V based mixed oxide catalysts. In this study, we employed XAS to probe the local structures and oxidation states of the above-mentioned catalyst system and report the results obtained.
The catalysts were synthesized by hydrothermal treatment of the slurry obtained by mixing appropriate amounts of source compounds, such as ammonium molybdate, vanadyl sulfate, niobium oxalate, and telluric acid or tellurium oxide. After synthesis, the materials obtained were calcined in a flow of ultra-pure N2 at 773-873 K to yield crystalline, phase pure samples which was confirmed by X-ray diffraction. The XAS experiments were conducted on beamline X18-B at the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory. The data reduction was carried out with Athena software (22). XANES spectra of transition metal oxides are characterized by the pre-edge peak caused by the s(d transition. The intensity depends on the coordination symmetry of oxygen atoms around the central metal atom. The peak is intense for a tetrahedral coordination and low for an octahedral coordination. The fine structure near the edges of the X-ray absorption spectrum is determined by the electron structure and the geometric arrangement of the neighboring atoms of an absorbing atom. Mo complexes with tetrahedral geometry show pre-edge peaks of relatively high intensity, attributed to the 1s(Mo) → 4d(Mo)+2p(O) transition, which is dipole-allowed for tetrahedral symmetry. For a regular octahedron (e. g. in MoO2), this transition is very weak and nearly invisible at the Mo K-edge having large natural broadening. When the coordination is distorted octahedral, this transition is visible as a weak pre-edge or shoulder, as in α-MoO3. The absorption edges of the catalytic M1 phases are shifted to higher energies compared to that of MoCl5 and exhibit clear similarity to that of MoO3. The occurrence of pre-peak and the energy values indicate that Mo exists in a distorted octahedral coordination in the 6+ oxidation state in all M1 phases. In the case of Nb, the K edge spectrum of the Nb2O5 reference was clearly different from that of NbO2. The spectrum is shifted to higher energy for Nb2O5 due to the higher oxidation state and the distinct pre-edge structure observed for Nb2O5 does not occur clearly for NbO2. The absorption edge of M1 phases coincides with the absorption edge of the pentavalent niobium oxide, Nb2O5. The local structures of V and Te ions were investigated similarly by conducting a comparison with corresponding model compounds. Further details will be presented and discussed at the conference.
The authors acknowledge the financial support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG02-04ER15604. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. We acknowledge the help received from Nebojsa Marinkovic, Syed Khalid and Viviane Schwartz. We also thank the Geology Department, University of Cincinnati, for access to the X-ray diffraction equipment.
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