Biochar-coated catalyst turns wet microalgae into cleaner fuel-building chemicals

Microalgae have long been viewed as a promising source of renewable fuel. They grow quickly, capture carbon dioxide efficiently, and do not compete with food crops for farmland. Yet turning algae into usable liquid fuels remains difficult because algae-derived bio-oil often contains high levels of oxygen and nitrogen compounds, which can lower fuel quality, reduce stability, and create pollution concerns during combustion.

A new study published in Biochar reports a promising strategy to address this challenge. Researchers developed a HZSM-5 coated biochar catalyst, known as HZSM-5@biochar, and paired it with wet torrefaction, a water-based pretreatment process, to convert Chlorella microalgae into valuable aromatic hydrocarbons. These compounds, especially benzene, toluene, and xylene, commonly known as BTX, are important chemical building blocks for fuels, plastics, and other industrial products.

“Our goal was not only to improve the quality of algae-derived pyrolysis products, but also to understand why the catalyst works,” said corresponding author Liangliang Fan. “By combining wet torrefaction with a biochar-supported zeolite catalyst, we were able to promote deoxygenation and denitrogenation while suppressing catalyst deactivation.”

In the study, the team first treated Chlorella through wet torrefaction at different temperatures. This step helped remove part of the oxygen and nitrogen from the biomass before pyrolysis. The pretreated microalgae were then rapidly heated in the presence of the HZSM-5@biochar catalyst. Under optimized conditions, including wet torrefaction at 200 °C, pyrolysis at 500 °C, and a catalyst-to-feedstock ratio of 20:1, the process achieved up to 96.06% aromatic selectivity. Among these products, BTX selectivity reached 83.24%, with a BTX yield of 94.64 mg per gram of feedstock.

The catalyst also sharply reduced unwanted compounds. Under non-catalytic conditions, oxygen-containing and nitrogen-containing products accounted for 82.14% of the pyrolysis products. With HZSM-5@biochar, that share fell to just 3.26%, showing strong removal of heteroatoms that normally limit the use of algae-based bio-oil.

To explain the mechanism, the researchers studied model compounds representing the main components of microalgae, including proteins, lipids, and carbohydrates. They also examined typical pyrolysis products such as long-chain alkenes, amides, fatty acids, aldehydes, and nitrogen-containing heterocycles. These experiments suggested that biochar and HZSM-5 play complementary roles. Biochar provides mesopores and surface sites that help adsorb and pre-crack larger molecules, while HZSM-5 provides strong acidic sites and micropores that drive aromatization.

This two-step catalytic effect appears to reduce one of the major problems in catalytic pyrolysis: coking. Coke deposits can block catalyst pores and cause rapid deactivation. After one use, HZSM-5@biochar produced only 0.33% coke, compared with 1.87% coke on conventional HZSM-5. The composite catalyst also maintained excellent performance over six reuse cycles after regeneration.

“The biochar support acts like a protective front line,” Fan said. “It helps break down unstable large molecules before they can clog the zeolite pores, allowing the catalyst to keep producing aromatics more efficiently.”

The findings provide new insight into how biochar-based composite catalysts can upgrade nitrogen-rich biomass such as microalgae. By improving fuel quality, increasing BTX production, and enhancing catalyst stability, the work could help advance cleaner and more efficient pathways for producing renewable fuels and industrial chemicals from algae.

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Journal Reference: Hu, J., Wang, Y., Jiang, H. et al. In-depth into the mechanism of aromatic production from catalytic pyrolysis of wet-torrefied microalgae with HZSM-5 coated biochar. Biochar 8, 91 (2026).   

https://doi.org/10.1007/s42773-026-00612-0   

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About Biochar

Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field. 

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