(509av) Investigation Of The Mechanism Of Propane Ammoxidation Over Mo-V-Te-Nb-O Mixed Oxide Catalyst System

Production of acrylonitrile by direct ammoxidation of propane is being intensely investigated worldwide as an alternative to current industrial process of propene ammoxidation over the Bi-Mo-O catalysts (1). A process based on the direct ammoxidation of propane is highly attractive, since propane is much cheaper and more abundant than propene (2-17). The so-called M1 phase of Mo?V?Nb?Te?O mixed metal oxide system is the most active and selective catalyst reported to date for direct ammoxidation of propane (18-21). However, the mechanism of this reaction over the M1 phase has not been studied in sufficient detail. The dominant hypothetical pathway for propane (amm)oxidation over mixed oxide catalysts involves oxidative dehydrogenation of propane with subsequent stepwise transformation of the propene intermediate to acrolein and acrylic acid (or acrylonitrile) (4). A similar hypothetical pathway proceeding via a butadiene intermediate was proposed for butane oxidation to maleic anhydride over VPO catalysts. According to these views, the alkane backbones do not undergo any skeletal rearrangements during these transformations. However, a recent study of a related reaction of propane oxidation over compositionally similar MoVNbPO heteropolyacids, found evidence supporting dimerization of the propene intermediate followed by its oxidation to final products, acrylic acid and maleic anhydride (22-24). Similarly, 13C-labeled butane was observed to undergo C-C bond scission between C2 and C3 and form a new C-C bond between C1 and C4 during its oxidation to maleic anhydride over a VPO catalyst (25). Therefore, these recent observations challenge the accepted, although largely hypothetical mechanistic views. In this study we probed the reaction mechanism using selective 13C isotopic labeling and examined the reaction products by 13C NMR spectroscopy under realistic, steady-state conditions in order to elucidate propane ammoxidation pathways over the M1 phase catalysts.

The Mo-V-Te-Nb-O catalysts were synthesized by hydrothermal treatment and were characterized by X-ray diffraction which confirmed the phase purity and crystallinity of the M1 phase. The SEM images also confirmed the typical crystal morphology of the M1 phase. 1-13C labeled propane was used as the oxidation substrate and the reaction was carried out in a fixed bed tubular flow reactor at atmospheric pressure. The products and unreacted reactants were collected in deuterated chloroform using liquid N₂ trap and analyzed using Bruker 400 MHz NMR. 1H NMR of the product collected after ammoxidation of unlabeled propane at 693 K show the presence of propylene, acetonitrile, acetic acid and acrylonitrile and unreacted propane. 13C-NMR of this product shows signals due to ?CH₃ group of propane and =CH₂, =CH- and ?CN groups of acrylonitrile, the main product. The intensities of groups in acrylonitrile are in the order =CH₂ ~ =CH- > -CN. In the case of 13C NMR of the product collected after ammoxidation using labeled propane, the signal of CH₂- group of acrylonitrile is most intense followed by that of -CN and =CH-. The absence of 13C label at middle =CH- group rules out the formation of C6 intermediates during the reaction or other types of skeletal rearrangements. The results will be presented and discussed at the conference in detail.

Acknowledgement

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. We acknowledge Dr. Suri Iyer and Dr. Ramesh Kale (Chemistry Department, University of Cincinnati) for NMR studies. We also thank the Geology Department, University of Cincinnati, for access to the X-ray diffraction equipment.

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