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 fastpyrolysis 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.

Phase Equilibrium Calculation of Bio-Oil-Related Molecules Using Predictive Thermodynamic Models

“…The condensation of these vapors ultimately gives rise to a liquid product commonly termed bio oil or pyrolysis oil. This bio oil is a complex mixture featuring a multitude of oxygenated species such as water, alcohols, sugars, acids, aldehydes, ketones, pyrans, low-molecular-weight (LMM) lignin, and high-molecular-weight (HMM) lignin derivatives. , To improve the quality of the bio oil in view of fuel applications, e.g., by reducing oxygen content and reactivity, catalytic upgrading needs to be employed. This upgrading process can take place either in situ within the fast pyrolysis reactor or ex situ in a separate reactor downstream of the fast pyrolysis reactor. , …”