Tuning lignin fast pyrolysis with CO2: PCA-guided synchronized fabrication of phenol-rich bio-oil and porous biochar

Decoding the decisive role of CO2-mediated pyrolysis mechanism is essential for achieving directional biomass conversion and precise product regulation. This study strategically employs CO2 as a dynamic reaction medium to drive fast pyrolysis of lignin, simultaneously producing phenol-enriched bio-oil and functionalized biochar. CO2's multifunctional mechanism during lignin pyrolysis was investigated through multimodal characterization combined with response surface methodology (RSM), principal component analysis (PCA) and t-distributed Stochastic Neighbor Embedding (t-SNE). Results demonstrate that CO2 participates in radical reactions, catalyses the key demethoxylation step, and triggers the Boudouard reaction, simultaneously achieving enrichment of phenolic compounds in bio-oil (reaching 62.07% phenolic content under optimal conditions) and significant development of biochar pore structure (with specific surface area increasing up to 3.6-fold). Under conditions of 537 °C, 97 s, and 40% CO2, bio-oil yield reached 12.8 wt%, oxygen content decreased by 38%, and the yield of the high-value-added product 4-vinylphenol exceeded 18%. This study quantitatively revealed for the first time through PCA the non-linear regulatory effect of CO2 concentration on pyrolysis reaction pathways, constructing a reaction network involving CO2. This provides theoretical basis and process references for directed pyrolysis of lignin and high-value utilisation of CO2. Furthermore, self-derived biochar was engineered into a bifunctional catalyst, further expanding the valorization strategy.