Chemical Titration and Transient Kinetic Studies of Site Requirements in Mo₂C-Catalyzed Vapor Phase Anisole Hydrodeoxygenation

The turnover frequency (TOF) of benzene synthesis from vapor phase anisole hydrodeoxygenation (HDO), estimated via in situ CO titration, was found to be invariant (1.1 ± 0.3 × 10^-3 s^-1) over molybdenum carbide (Mo₂C) catalysts with varying CO chemisorption uptakes (∼70 to ∼260 μmol g^-1, measured ex situ at 323 K). Accumulation of oxygen (∼0.29 monolayer) over Mo₂C catalysts was determined by an oxygen mass balance during the transient of anisole HDO at 423 K under ambient pressure (H₂/anisole molar ratio ∼ 110). Similar product selectivity, apparent activation energy, and TOF of benzene synthesis for an oxygen treated (with oxygen incorporation: O/Mo_bulk (molar ratio) = 0.075) and freshly prepared Mo₂C catalysts (no exposure to air prior to kinetic measurements) demonstrate that the effect of oxygen at these low concentrations is solely to reduce the number of active sites for anisole HDO, resulting in a lower (∼3 times) benzene synthesis rate per gram of catalyst for the oxygen-modified material. The observed benzene synthesis rates per CO chemisorption site for bulk molybdenum oxide (MoO_x) catalysts were found to be ∼10 times lower than those for Mo₂C catalysts, suggesting that bulk molybdenum oxide phases are not associated with the dominant active sites for anisole HDO at 423 K under ambient pressure.