Investigation of Nb2O5 and Its Polymorphs as Catalyst Supports for Pyrolysis Oil Upgrading through Hydrodeoxygenation

Mild catalytic hydrogenation is an interesting process to upgrade and stabilize raw fast-pyrolysis oil, allowing higher ratios of bio-oil to be coprocessed in conventional refineries. In the search for hydrodeoxygenation (HDO) catalysts with high activity, high selectivity, and long-term stability, Nb2O5-supported catalysts are stressed owing to their water-resistant acid sites. Due to the Nb2O5 polymorphism, its properties such as acidity, morphology, and crystalline structure are adjustable. This study evaluated the suitability of Nb2O5 as a catalyst support and the impact of its different polymorphs on the upgrading of pyrolysis oil. Four different Nb2O5 polymorphs were prepared by thermal treatment of niobic acid, and nanostructured Nb2O5 was prepared by hydrothermal synthesis with niobium oxalate. The supports were wet impregnated with Pd (1 wt %) and applied for the upgrading of the light phase of beech wood fast-pyrolysis oil (FPO-LP) in a batch reactor, pressurized at 80 bar H2 (room temperature), and heated to 250 °C for 2 hours. The activity and selectivity of catalysts and the dominant reaction pathways were addressed in terms of upgraded product properties (elemental composition, water content, functional groups via 1H-NMR, and chemical composition via GC–MS/FID), H2 consumption, generated gases, deoxygenation degree, and catalyst deactivation via coke formation. Nb2O5 showed outstanding potential as a catalyst support for FPO-LP HDO, taking into account the improved properties of the upgraded oil, such as higher carbon content and lower water and oxygen contents. The polymorphs without thermal treatment (Pd/TT&A-Nb2O5) or with a low-temperature thermal treatment (Pd/TT&A-Nb2O5) presented higher activity than the one obtained at high temperature (Pd/M&T-Nb2O5). The high stability of the Pd/A-Nb2O5 was demonstrated by regeneration and re-use. Finally, the nanostructured TT-Nb2O5 presented a remarkable activity toward hydrogenation and hydrogenolysis, owing to its higher acidity and surface area, resulting in the most improved oil.