| Summary: | Due to the high acid, oxygen and water contents of fast pyrolysis oil, it requires the improvement of its fuel properties by further upgrading, such as catalytic hydrodeoxygenation (HDO). In this study, Nb<sub>2</sub>O<sub>5</sub> was evaluated as a support of Pd-based catalysts for HDO of fast pyrolysis oil. A Pd/SiO<sub>2</sub> catalyst was used as a reference. Additionally, the impact of iron as a promoter in two different loadings was investigated. The activity of the synthesized catalysts was evaluated in terms of H<sub>2</sub> uptake and composition of the upgraded products (gas phase, upgraded oil and aqueous phase) through elemental analysis, Karl Fischer titration, GC-MS/FID and <sup>1</sup>H-NMR. In comparison to SiO<sub>2</sub>, due to its acid sites, Nb<sub>2</sub>O<sub>5</sub> enhanced the catalyst activity towards hydrogenolysis and hydrogenation, confirmed by the increased water formation during HDO and a higher content of hydrogen and aliphatic protons in the upgraded oil. Consequently, the upgraded oil with Nb<sub>2</sub>O<sub>5</sub> had a lower average molecular weight and was therefore less viscous than the oil obtained with SiO<sub>2</sub>. When applied as a promoter, Fe enhanced hydrogenation and hydrogenolysis, although it slightly decreased the acidity of the support, owing to its oxophilic nature, leading to the highest deoxygenation degree (42.5 wt.%) and the highest product HHV (28.2 MJ/kg).
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