Multiscale Mechanistic Insights into Hydrogen Production from Microalgae via Molten Hydroxide-Mediated Thermochemical Conversion

The thermochemical conversion of microalgae in molten hydroxides presents a promising pathway for sustainable hydrogen production, distinguished by high hydrogen yield and minimal tar formation. However, the underlying reaction network governing these advantages remains unclear. To address this, a multiscale analysis was employed to unravel the enhancement mechanisms. This integrated approach combined macro-scale observation using coupled thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS), mesoscale validation via targeted experiments with model aromatics and pyrolytic char, and microscale elucidation through density functional theory (DFT) calculations. Results reveal that molten hydroxides simultaneously lower the biomass decomposition temperature, suppress tar formation, and activate three distinct hydrogen production pathways: organic catalytic cracking (200-450 °C), deep aromatic volatiles reforming (>500 °C), and char alkalization (>550 °C). Aromatics reforming involves both condensation and ring cleavage. Ring cleavage proceeds via OH^- nucleophilic addition and C-H bond heterolysis, with deprotonation as the rate-determining step and the energy barrier governed by the molecular geometry of aromatics. Char alkalization follows fundamental steps with lighter aromatics but encounters higher energy barriers due to its more condensed structure. This work establishes a multiscale mechanistic framework for hydrogen production, paving the way for the rational design of efficient and low-carbon biomass-to-hydrogen technologies based on molten salts.