Gallium and self-nitrogen co-doped algal biochar catalysts for selective production of monoaromatic hydrocarbons from microalgae pyrolysis

Catalytic pyrolysis of microalgae to produce monocyclic aromatic hydrocarbons (MAHs) represents a promising route for sustainable aromatic chemical production. In this study, a gallium and self-nitrogen co-doped algae-derived activated carbon (Ga/N-SAC) catalyst was synthesized and applied to Spirulina pyrolysis. Catalytic pyrolysis experiments showed that the catalyst containing 5   wt.% Ga and used at a catalyst-to-biomass mass ratio of two exhibited the optimal performance. Ga/N-SAC increased the relative content of benzene, toluene, and xylene (BTX) to 51.33%, with selectivities of 10.51% for benzene, 50.27% for toluene, and 12.15% for xylene. The pyridinic-N and pyrrolic-N functionalities facilitated the deoxygenation and deamination of heteroatom-rich intermediates, while Ga-derived Lewis acid sites promoted dehydrogenation, cyclization, and aromatization pathways. The hierarchical pore structure of Ga/N-SAC further enhanced mass transfer and strengthened vapor–catalyst interactions, enabling efficient upgrading of pyrolysis vapors. This strong synergy between nitrogen functionalities and Ga species significantly enhanced MAHs formation while suppressing oxygenated and nitrogen-containing by-products. Ga/N-SAC serves as an efficient and cost-effective catalyst for microalgae-derived BTX and provides a useful framework for developing metal–nitrogen co-doped carbon catalysts.