Mechanistic study of propane selective oxidation with H2 and O2 on Au/TS-1

The selective oxidation of propane to acetone and 2-propanol with H-2 and O-2 was studied on Au/TS-1 by kinetic and spectroscopic analysis. A kinetic study using a factorial design at conditions where the catalyst was stable and gave propane conversions of < 6% and combined oxygenates selectivity of > 90%, resulted in a power-rate law expression of the form roxyg(center dot) = kroxyg(center dot)(H-2)(0.74)(O-2)(0.36)(C3H8)(0.29). In situ Au L-3-edge X-ray absorption near-edge spectroscopy (XANES) measurements showed activation of 02 on Au, whereas in situ ultraviolet-visible (UV-vis) spectroscopy evidenced the presence of Ti-hydroperoxo species. The role of the Ti-hydroperoxo species was probed by a transient technique in which changes in Ti K-edge XANES spectra were used to determine the evolution of coverage with time (d theta/dt). It was shown that the rate of reaction by XANES (6.7 x 10(-4) s(-1)) was close to the turnover rate measured in a catalytic flow reactor (5.6 x 10(-4) s(-1)), indicating that the hydroperoxci species were true intermediates in the reaction. A proposed reaction sequence in which H2O2 forms on Au sites and propane is partially oxidized on Ti centers accounts for the spectroscopic results and the reaction orders obtained experimentally for the power-rate law expression. (c) 2008 Elsevier Inc. All rights reserved.