Key Takeaways & Executive Findings
- •• • The 10%Ni-Mo/h-BN catalyst with Ni:Mo = 3:1 achieved a maximum H2 yield of 38.48 mmol at 280 °C and formic acid-to-water molar ratio of 1:10, demonstrating optimal in situ hydrogen production for lignin hydroprocessing. • • The highest heavy bio-oil yield of 18.93% was obtained at 280 °C for 1 h, indicating that moderate temperatures maximize bio-oil yield from alkali lignin under hydrothermal conditions. • • At 280 °C, the relative content of aromatic hydrocarbons peaked at 13.81%, while H-type phenolics reached 34.98%, showing temperature-dependent selectivity toward valuable aromatic compounds. • • Extending reaction time beyond 1 h decreased heavy bio-oil yield and aromatic hydrocarbon content while increasing furan derivatives, highlighting the need for precise residence time control to optimize product quality.
Abstract
Alkali lignin, a high-volume byproduct from pulp and paper manufacturing and biomass refining, is a promising feedstock for aromatic hydrocarbon production in liquid fuels due to its high energy density and abundant aromatic moieties. However, its highly cross-linked polymeric structure hinders efficient valorization. This work investigates catalytic conversion of alkali lignin into bio-oil under in situ H2 supply from formic acid. A series of Ni-Mo/h-BN bimetallic catalysts with varied metal ratios were synthesized by impregnation and characterized by XPS, XRD, and other techniques. The effects of reaction parameters on H2 production via aqueous-phase reforming (APR) of formic acid were evaluated. Optimal H2 yield was achieved at a formic acid-to-water molar ratio of 1:10 and a Ni/Mo atomic ratio of 3:1. H2 yield increased monotonically with temperature from 220 to 280 °C, reaching a maximum of 38.48 mmol. Subsequently, influences of reaction temperature and residence time on bio-oil production were examined. The highest heavy bio-oil yield (18.93%) and maximum relative content of aromatic hydrocarbons (13.81%) were both achieved at 280 °C. Prolonged reaction time reduced heavy bio-oil yield and aromatic hydrocarbon abundance while favoring furan derivatives. This work demonstrates good synergy between in situ hydrogen generation from formic acid and lignin hydrogenation in the temperature range 240–280 °C.
1. Introduction
The valorization of alkali lignin, a major byproduct of pulp and paper manufacturing and emerging biorefineries, remains a critical bottleneck in biomass conversion. Its highly cross-linked polymeric structure resists conventional depolymerization, leading to low yields of valuable aromatic hydrocarbons and excessive char formation. Traditional hydrogenation routes require external high-pressure H2, which poses safety and economic challenges. This study addresses these limitations by employing formic acid as a liquid hydrogen donor, enabling in situ H2 generation under hydrothermal conditions, thereby eliminating the need for external hydrogen infrastructure.
The novelty lies in the synergistic coupling of aqueous-phase reforming of formic acid with lignin hydrogenation over a Ni-Mo/h-BN bimetallic catalyst. The catalyst's tailored metal ratio and support interactions facilitate both H2 production and subsequent hydrodeoxygenation of lignin fragments. By systematically varying reaction temperature and residence time, the study identifies optimal conditions that maximize heavy bio-oil yield and aromatic hydrocarbon content, offering a feasible and efficient strategy for converting alkali lignin into high-quality bio-oil.
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LI Bingshuo, LIU Longfei, ZHANG Bowen, WANG Zhicai, YANG Tianhua (2026). Hydrothermal Liquefaction of Alkaline Lignin with In Situ Hydrogen Supply from Formic Acid. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(26)60717-7
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Frequently Asked Questions
What is the optimal catalyst composition and reaction temperature for maximizing H2 yield from formic acid APR, and how does this correlate with bio-oil production?
The optimal catalyst is 10%Ni-Mo/h-BN with Ni:Mo atomic ratio of 3:1. The maximum H2 yield of 38.48 mmol was achieved at 280 °C with a formic acid-to-water molar ratio of 1:10. This temperature also corresponds to the highest heavy bio-oil yield (18.93%) and aromatic hydrocarbon content (13.81%), indicating a strong synergy between H2 generation and lignin hydrogenation in the 240–280 °C range.
How does reaction time affect the yield and composition of bio-oil, and what is the recommended residence time for industrial application?
Prolonged reaction time (beyond 1 h) reduces heavy bio-oil yield and aromatic hydrocarbon content while increasing furan derivatives. The maximum bio-oil yield of 18.93% was obtained at 280 °C for 1 h. For industrial processes, a residence time of approximately 1 h is recommended to balance yield and product quality, as longer times lead to secondary reactions that degrade valuable aromatic compounds.
What are the main challenges in scaling up this process, particularly regarding catalyst stability and formic acid cost?
Catalyst stability under hydrothermal conditions is a key concern; Ni-Mo/h-BN must resist sintering and leaching. The study does not report long-term stability tests, but the catalyst's performance at 280 °C suggests moderate stability. Formic acid is relatively inexpensive and can be derived from biomass, but its cost and handling at scale must be evaluated. The process operates at moderate temperatures (240–280 °C) and pressures typical of HTL, which may reduce energy costs compared to high-pressure H2 systems.
How does the use of formic acid as an in situ hydrogen donor compare to conventional external H2 supply in terms of process economics and safety?
Using formic acid eliminates the need for high-pressure H2 storage and compression, reducing capital and safety costs. Formic acid is a liquid, easy to handle, and can be produced from biomass, aligning with renewable goals. However, the molar ratio of formic acid to water (1:10) implies significant water usage, and the APR reaction produces CO2, which must be managed. Overall, the in situ approach offers a safer, more integrated process, though the cost of formic acid and its decomposition byproducts must be considered.
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