Co-production and upgrading of multiple products from hydrothermal carbonization of microalgae with organic solvent assistance

Hydrothermal carbonization facilitates microalgae utilization but suffers from low product quality, with no effective solutions. This study investigated the product characteristics and quality enhancement methods of hydrochar, bio-oil, and carbon dots prepared through hydrothermal carbonization of microalgae assisted by non-polar organic solvents (petroleum ether and n-hexane) and polar organic solvents (ethyl acetate, isopropanol, and ethanol). The results showed that non-polar organic solvents were more effective than polar organic solvents in extracting fatty acids hydrolyzed from lipids into the organic phase, reducing amidation with amino acids and lowering bio-oil nitrogen content by 47.0%, with n-hexane exhibiting the best effect. They also promoted Maillard reactions, increasing hydrochar and carbon dots yields by 41.2% and 63.3% while boosting nitrogen content by 48.9% and 15.7%, respectively. This study explored the co-production and upgrading of multiple products in microalgae hydrothermal carbonization with organic solvent assistance, offering insights into process optimization for efficient utilization of microalgae.