Synergistic basic-acid catalytic upgrading of lignin pyrolysis volatiles over CaO-HZSM-5

As a major component of lignocellulosic biomass, lignin supports sustainable biorefineries and the circular economy as a renewable feedstock for biofuels and high-value chemicals. Catalytic upgrading of lignin pyrolysis volatiles offers a promising pathway. This study investigated lignin pyrolysis with in-situ upgrading using CaO (basic), HZSM-5 (acid), and CaO-HZSM-5 relay catalysts at 550 °C with a lignin-to-catalyst weight ratio of 1:1. Raw tar contained H-type (phenol, alkylphenols, naphthalenols), G-type (methoxyphenols), and S-type (dimethoxyphenols) phenolics, esters, alkanes, alkenes and aromatics. CaO increased the total relative content of phenolics by 17.96%, favoring H- and G-type phenolics while reducing S-type phenolics and esters. HZSM-5 significantly promoted aromatics formation, raising the relative content of mono- and bicyclic aromatics by 11.21% and 23.86%, respectively. The relay catalyst simultaneously increased the relative content of phenolics and aromatics, achieving 69.96% selectivity for H-type phenolics among total phenolics and 80.23% combined selectivity for benzenes and naphthalenes within the aromatic fraction, with the lowest coke yield (4.02%). A synergistic catalytic mechanism is proposed, where primary tar undergoes decarbonylation and demethoxylation over CaO, followed by aromatization over HZSM-5. This integrated basic-acid strategy promotes production of light phenolics and aromatics while suppressing coke deposition, advancing efficient lignin conversion into value-added chemicals. • An integrated pyrolysis-catalytic upgrading system was constructed for lignin conversion. • Basic CaO catalytic upgrading led to demethoxylation and decarbonylation, boosting H- and G-type phenolics. • Acidic HZSM-5 showed high aromatization activity, substantially promoting aromatic production. • Synergistic basic-acid catalysis efficiently converted lignin to light phenols and aromatics. • Basic-acid tandem catalysis suppressed coking more effectively than individual catalysts.