A study of metal phosphides for the hydrodeoxygenation of phenols and pyrolysis oil (2013)
This dissertation addresses the hydrodeoxygenation (HDO) of the model compound 4-methylphenol and pyrolysis oil, over alternative, non-sulfided catalysts. The HDO of 4-methylphenol was studied over unsupported, low surface area MoS₂, MoO₂, MoO₃, and MoP catalysts. The initial turn over frequency (TOF) for the HDO of 4-methylphenol decreased in the order MoP > MoS₂ > MoO₂ > MoO₃. Among the catalysts examined, MoP had the highest hydrogenating selectivity, lowest activation energy, and per site activity (TOF) for the HDO of 4-methylphenol. However, the observed conversion over MoP was limited by its low surface area and CO uptake. Addition of citric acid (CA) improved the properties of unsupported MoP. CA acted as a structural promoter and formed a metal citrate during the catalyst preparation, which increased the surface area and CO uptake of the MoP. High surface area Ni₂P catalysts were prepared similarly and based on initial TOFs, Ni₂P was 6 times more active than MoP for the HDO of 4-methylphenol. The HDO of 4-methylphenol was found to be structure insensitive over both MoP and Ni₂P. However, the Ni₂P catalysts deactivated due to C deposition on the catalyst surface. A kinetic model of the direct deoxygenation and hydrogenation reaction pathways for the HDO reaction over MoP showed the former to have a higher barrier energy (Ea = 106 kJ/mol) than the latter (Ea = 85 kJ/mol).Finally, to validate the use of model compounds to screen catalysts, the HDO of 4-methylphenol was compared to the HDO of pyrolysis oil over sulfide, oxide, and phosphide catalysts. MoP was found to have the highest yield of O-free liquid and the lowest coke yield, followed by Ni₂P, NiMoS/Al₂O₃, MoS₂, and MoO₃ for the HDO of pyrolysis oil. Those catalysts displaying high hydrogenating abilities had a high degree of O free liquid and a low yield of coke (MoP), while those catalysts displaying high isomerization abilities (MoO₃) had a high coke yield. Overall, this thesis identified phosphide catalysts as a new class of catalysts for HDO reactions with strong hydrogenating abilities, and their activity was superior to commercial NiMoS/Al₂O₃ for the HDO of pyrolysis oil.