Structural Properties and Reactivity Trends of Molybdenum Oxide Catalysts Supported on Zirconia for the Hydrodeoxygenation of Anisole

Abstract

Vapor-phase hydrodeoxygenation (HDO) of anisole was investigated at 593 K and H2 pressures of ≤1 bar over supported MoO3/ZrO2 catalysts with MoO3 loadings ranging from 1 to 36 wt % (i.e., 0.5–23.8 Mo/nm2). Reactivity studies showed that HDO activity increased proportionally with MoO3 coverage up to a monolayer coverage (∼15 wt %) over the ZrO2 surface. Specific rates declined for catalysts with high loadings exceeding the monolayer coverage, because of a decreasing amount of redox-active species, as confirmed by oxygen chemisorption experiments. For low catalyst loadings (1 and 5 wt %), the selectivities toward fully deoxygenated aromatics were 13 and 24% on a C-mol basis, respectively, while at intermediate and high loadings (10–36 wt %), the selectivity was ∼40%. Post-reaction characterization of the spent catalysts using X-ray diffraction and X-ray photoelectron spectroscopy showed that the catalysts with 25 and 36 wt % MoO3 loadings were over-reduced, as evidenced by the prevalence of Mo4+ and Mo3+ oxidation states summing to 54 and 67%, respectively. In contrast, catalysts with low and intermediate Mo loadings exhibited a prevalence of Mo6+ species (∼60%). We hypothesize that Mo5+ species are more easily stabilized in oligomeric and isolated forms over the zirconia support. The catalysts with intermediate loadings feature HDO and alkylation rates higher than those of catalysts with low loadings because the latter feature a higher proportion of isolated species. Once the monolayer coverage is exceeded, MoO3 crystallites are formed, which can undergo facile reduction to less reactive MoO2.