Reactivity and stability investigation of supported molybdenum oxide catalysts for the hydrodeoxygenation (HDO) of m-cresol

The vapor-phase hydrodeoxygenation (HDO) of m-cresol is investigated at 593 K and H-2 pressures <= 1 bar for supported catalysts comprised of 10 wt% MoO3 dispersed over SiO2, gamma Al2O3, TiO2, ZrO2, and CeO2. Reactivity data show that all catalysts selectively cleave C-O bonds without saturating the aromatic ring, thus effectively transforming m-cresol into toluene at moderate to high conversions. MoO3/ZrO2 and MoO3/TiO2 feature the highest initial site time yields (23.4 and 13.9 h(-1), respectively) and lowest first-order deactivation rate constants (0.013 and 0.006 h(-1), respectively) of all catalysts tested after ca. 100 h on stream. Characterization studies demonstrate that the supports play an important role in stabilizing partially reduced, coordinatively unsaturated (CU) sites in surface oligomeric Mo moieties. Post-reaction X-ray photoelectron spectroscopy shows that the catalysts with higher activity feature larger proportions of intermediate oxidation species (Mo5+ and Mo3+). In contrast, the catalysts with lower reactivity show different oxidation states: bulk MoO3 features mostly Mo4+ and metallic Mo species, while MoO3/CeO2 features a high proportion of Mo6+ species. An inverse correlation is established between the catalyst activity and both the maximum hydrogen consumption temperature obtained during temperature programmed reduction, and the support cation electronegativity (with the exception of MoO3/CeO2). (C) 2015 Elsevier Inc. All rights reserved.