Catalyst Property Effects on Product Distribution during the Hydrodeoxygenation of Lignin Pyrolysis Vapors over MoO₃/γ-Al₂O₃

Abstract

Hydrodeoxygenation (HDO) of three model compounds (i.e., anisole, 4-propylphenol, and 2-butanone) and real lignin pyrolysis vapors was investigated at 673 K and ∼1.7 bar of H2 over a series of MoO3/γ-Al2O3 catalysts with MoO3 loadings ranging from 0 to 19 wt % as well as bulk MoO3. Extensive characterization revealed catalyst acidity (strength) and the degree of MoOx oligomerization as the two main parameters in determining product distribution. Strong Lewis acid sites of γ-Al2O3 were found to catalyze transalkylation, dealkylation, dehydration, and condensation reactions, the latter of which also led to high coke yields (up to 50 C%). The addition of MoO3 progressively reduced the amount of strong Lewis acid sites and generated weaker Lewis and Brønsted acid sites with lower selectivity to condensation reactions resulting in lower coke yields. The growth of MoOx domains depended on MoO3 loading over the γ-Al2O3 support. At MoO3 loadings higher than 8 wt %, crystalline orthorhombic MoO3 phases were found, which behaved similar to bulk MoO3 in catalyzing hydrogenation and HDO reactions. The integration of MoOx species and acidity from the γ-Al2O3 support enabled the modulation of product selectivity. This work provides information for enabling the rational design of supported MoO3 catalysts to allow for maximizing the production of valuable chemicals (i.e., alkenes and aromatics) from HDO of lignin (or biomass) pyrolysis vapors.