Residual biomass shows potential to be used as a feedstock for fast pyrolysis bio-oil production for energetic and chemical use. Although environmentally advantageous, further catalytic upgrading is required in order to increase the bio-oil stability, by reducing reactive compounds, functional oxygen-containing groups and water content. However, bio-oils may separate in fractions either spontaneously after ageing or by fractionated condensation. Therefore the effects of upgrading on different fast pyrolysis bio-oil (FPBO) fractions obtained from a commercially available FPBO were studied by elemental analysis, GC-MS and ¹H-NMR. Not only the FPBO was upgraded by catalytic hydrotreatment, but also the heavy phase fraction formed after intentional aging and phase separation. The reactions were conducted between 175 and 325 °C and 80–100 bar by using a nickel–chromium catalyst in batch experiments. The influence of the hydrotreatment conditions correlated with the composition of the upgraded products. Higher oxygen removal was obtained at higher temperatures, whereas higher pressures resulted in higher hydrogen consumption with no significant influence on deoxygenation. At 325 °C and 80 bar 42% of the oxygen content was removed from the FPBO. Compounds attributed to pyrolysis oil instability, such as ketones and furfural were completely converted while the number of alcohols detected in the upgraded products increased. Coke formation was observed after all reactions, especially for the reaction with the fraction rich in lignin derivatives, likely formed by polymerization of phenolic compounds mainly concentrated in this phase. Independently of the feedstock used, the upgraded bio-oils were very similar in composition, with reduced oxygen and water content, higher energy density and higher carbon content.

Four nickel-based catalysts are synthesized by wet impregnation and evaluated for the hydrotreatment/hydrodeoxygenation of beech wood fast-pyrolysis bio-oil. Parameters such as elemental analysis, pH value, and water content, as well as the heating value of the upgraded bio-oils are considered for the evaluation of the catalysts’ activity and catalyst reuse in cycles of hydrodeoxygenation after regeneration. The reduction temperature, selectivity and hydrogen consumption are distinct among them, although all catalysts tested produce upgraded bio-oils with reduced oxygen concentration, lower water content and higher energy density. Ni/SiO₂, in particular, can remove more than 50% of the oxygen content and reduce the water content by more than 80%, with low coke and gas formation. The evaluation over four consecutive hydrotreatment reactions and catalyst regeneration shows a slightly reduced hydrodeoxygenation activity of Ni/SiO₂, mainly due to deactivation caused by sintering and adsorption of poisoning substances, such as sulfur. Following the fourth catalyst reuse, the upgraded bio-oil shows 43% less oxygen in comparison to the feedstock and properties comparable to the upgraded bio-oil obtained with the fresh catalyst. Hence, nickel-based catalysts are promising for improving hardwood fast-pyrolysis bio-oil properties, especially monometallic nickel catalysts supported on silica.

The catalytic activity of high-loaded Ni-based catalysts for beech wood fast-pyrolysis bio-oil hydrotreatment is compared to Ru/C. The influence of promoter, temperature, reaction time, and consecutive upgrading is investigated. The catalytic activity is addressed in terms of elemental composition, pH value, H₂ consumption, and water content, while the selectivity is based on the GC-MS/FID results. The catalysts showed similar deoxygenation activity, while the highest hydrogenation activity and the highest upgraded oil yields were obtained with Ni-based catalysts. The elemental composition of upgraded oils was comparable for 2 and 4 h of reaction, and the temperature showed a positive effect for reactions with Ni–Cr and Ru/C. Ni–Cr showed superior activity for the conversion of organic acids, sugars and ketones, being selected for the 2-step upgrading reaction. The highest activity correlates to the strength of the acid sites promoted by Cr₂O₃. Consecutive upgrading reduced the content of oxygen by 64.8% and the water content by 90%, whereas the higher heating value increased by 90.1%. While more than 96% of the organic acid content was converted, the discrepancy of aromatic compounds quantified by ¹H-NMR and GC-MS/FID may indicate polymerization of aromatics taking place during the second upgrading step.

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, Nb₂O₅ was evaluated as a support of Pd-based catalysts for HDO of fast pyrolysis oil. A Pd/SiO₂ 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 H₂ uptake and composition of the upgraded products (gas phase, upgraded oil and aqueous phase) through elemental analysis, Karl Fischer titration, GC-MS/FID and ¹H-NMR. In comparison to SiO₂, due to its acid sites, Nb₂O₅ 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 Nb₂O₅ had a lower average molecular weight and was therefore less viscous than the oil obtained with SiO₂. 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).

In the present work, we compared the chemical profile of the organic compounds produced in non-catalytic pyrolysis of sugarcane bagasse at 500 °C with those obtained by the in-line catalytic upgrading of the vapor phase at 350 °C. The influence over the chemical profile was evaluated by testing two Ni-based catalysts employing an inert atmosphere (N₂) and a reactive atmosphere (H₂) under atmospheric pressure with yields of the liquid phase varying from 55 to 62%. Major changes in the chemical profile were evidenced in the process under the H₂ atmosphere, wherein a higher degree of deoxygenation was identified due to the effect of synergistic action between the catalyst and H₂. The organic fraction of the liquid phase, called bio-oil, showed an increase in the relative content of alcohols and phenolic compounds in the GC/MS fingerprint after the upgrading process, corroborating with the action of the catalytic process upon the compounds derived from sugar and carboxylic acids. Thus, the thermal conversion of sugarcane bagasse, in a process under an H₂ atmosphere and the presence of Ni-based catalysts, promoted higher deoxygenation performance of the pyrolytic vapors, acting mainly through sugar dehydration reactions. Therefore, the adoption of this process can potentialize the use of this waste biomass to produce a bio-oil with higher content of phenolic species, which have a wide range of applications in the energy and industrial sectors.

The social, economic, and environmental impacts of climate change have been shown to affect poorer populations throughout the world disproportionally, and the COVID-19 pandemic of 2020–2021 has only exacerbated the use of less sustainable energy, fuel, and chemical sources. The period of economic and social recovery following the pandemic presents an unprecedented opportunity to invest in biorefineries based on the pyrolysis of agricultural residues. These produce a plethora of sustainable resources while also contributing to the economic valorization of first-sector local economies. However, biomass-derived pyrolysis liquid is highly oxygenated, which hinders its long-term stability and usability. Catalytic hydrogenation is a proposed upgrading method to reduce this hindrance, while recent studies on the use of nickel and niobium as low-cost catalysts, both abundant in Brazil, reinforce the potential synergy between different economic sectors within the country. This review gathers state-of-the-art applications of these technologies with the intent to guide the scientific community and lawmakers alike on yet another alternative for energy and commodities production within an environmentally sustainable paradigm.