Dry distillation of pyrolyzed bio-oils: Exploring the evolution mechanism of heat polymerization in algae-derived bio-oils

Abstract

This study systematically elucidates the thermal polymerization mechanism of dry distillation residues from algal bio-oil, with a focus on the transformation pathways of functional groups. The thermal polymerization process involves two main stages. First, low-boiling small molecules undergo crosslinking polycondensation to form polyaromatic frameworks, while non-aromatic moieties undergo cyclic polycondensation to enhance the aromaticity of residues. Subsequently, deoxygenation reactions occur, particularly dehydration/demethoxylation of aromatic structures, and furanyl compounds accelerate polymerization by thermally activated ring-opening to generate carbonyl intermediates. For nitrogen-containing species (pyridine, pyrrole, indole, nitriles), heat treatment promotes chain cleavage of long-chain nitriles into short-chain analogs, accompanied by cyclization-aromatization cascade reactions to form pyridine derivatives and methyl-indoles. Critically, thermal polymerization enables nitrogen heterocycles to graft nitrile/alkyl side chains, forming uniquely structured polymeric frameworks. Multi-scale analysis methods reveal that algal bio-oil coordinates synergistic chemical pathways of aromatic condensation, deoxygenation, and nitrogen reorganization under temperature to form residues. These findings highlight the thermal polymerization characteristics of algal bio-oil, providing a theoretical basis for optimizing dry distillation separation of algal bio-oil and subsequent utilization of residues.