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

Abstract

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, Nb 2 O 5 -supported catalysts are stressed owing to their water-resistant acid sites. Due to the Nb 2 O 5 polymorphism, its properties such as acidity, morphology, and crystalline structure are adjustable. This study evaluated the suitability of Nb 2 O 5 as a catalyst support and the impact of its different polymorphs on the upgrading of pyrolysis oil. Four different Nb 2 O 5 polymorphs were prepared by thermal treatment of niobic acid, and nanostructured Nb 2 O 5 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 H 2 (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 1 H-NMR, and chemical composition via GC–MS/FID), H 2 consumption, generated gases, deoxygenation degree, and catalyst deactivation via coke formation. Nb 2 O 5 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-Nb 2 O 5 ) or with a low-temperature thermal treatment (Pd/TT&A-Nb 2 O 5 ) presented higher activity than the one obtained at high temperature (Pd/M&T-Nb 2 O 5 ). The high stability of the Pd/A-Nb 2 O 5 was demonstrated by regeneration and re-use. Finally, the nanostructured TT-Nb 2 O 5 presented a remarkable activity toward hydrogenation and hydrogenolysis, owing to its higher acidity and surface area, resulting in the most improved oil.