Phenol-rich bio-oil production from Choerospondias axillaris endocarps via microwave pyrolysis coupled with ex-situ catalytic upgrading

As a promising lignocellulosic biomass, Choerospondias axillaris endocarps hold great potential for the production of phenol-rich bio-oil. However, the catalytic differences among single metal oxides in the ex-situ upgrading of pyrolysis vapors, alongside the formation pathways of phenolic compounds, lack systematic comparison under identical conditions, and their underlying mechanisms remain insufficiently elucidated. In this study, Choerospondias axillaris endocarps were converted by microwave pyrolysis coupled with ex-situ metal oxide catalysis. Seven representative single metal oxides were systematically compared under identical reaction conditions to investigate their effects on product distribution and phenolic selectivity. Furthermore, by combining pyrolysis of the real feedstock with fast pyrolysis of lignin, cellulose, and hemicellulose model compounds, together with in-situ characterization, the formation of phenolic compounds and the evolution of deoxygenation reactions were clarified. The results showed that MgO exhibited the best catalytic performance. Under the optimal conditions (550 °C pyrolysis, 450 °C catalysis, and 1 g catalyst), the liquid yield and relative phenolic content reached 53.75 wt% and 57.66%, respectively, with the latter being 17.03% higher than that of the non-catalytic system. MgO promoted bond cleavage, decarboxylation, decarbonylation, and aromatization of oxygenated intermediates, thereby enriching low-oxygen phenols. Lignin was identified as the dominant direct precursor of phenols, whereas cellulose and hemicellulose contributed only indirectly and produced no G- or S-type phenols. This study provides theoretical support and technical guidance for the targeted production of value-added phenolic chemicals from biomass.