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The upgrading of microalgal biocrude produced via HTL is essential for the production of refinery-compatible drop-in fuels. Despite the high energy density of HTL biocrude, its elevated nitrogen and oxygen content originating from protein and lipid fractions presents significant hurdles for downstream hydroprocessing. This review evaluates the performance and mechanisms of diverse catalysts, ranging from conventional sulfides and oxides to noble metals, TMCs, bifunctional and zeolitic catalysts. While noble metals consistently yield high-quality products with HHVs of 40–45 MJ kg−1, their industrial application is restricted by high costs and vulnerability to nitrogen and sulfur poisoning. Conversely, TMCs, specifically Mo2C, emerge as robust, cost-effective alternatives that resist deactivation while favoring hydrogen-conservative decarboxylation pathways. A critical analysis of catalyst architecture reveals that hierarchical zeolite pore structures are vital for overcoming mass-transfer limitations associated with bulky algal macromolecules. It is observed that achieving industrial-scale viability requires a shift toward feedstock-specific catalyst design, utilizing staged hydroprocessing and applying different catalyst structures to manage the complex heteroatom fingerprint of microalgal feedstocks.
Demir et al. (Tue,) studied this question.