Selective production of monocyclic aromatics from lignin catalytic fast pyrolysis: The role of modified Beta zeolite

As the only renewable aromatic polymer in lignocellulose, lignin accounts for 40% of biomass energy density. However, its complex three-dimensional structure and high thermal stability lead to dominant coke formation and low selectivity for monocyclic aromatic hydrocarbons (MAHs) in conventional pyrolysis. This study aims to elucidate the influence of pore characteristics and acid distribution on MAHs selectivity during lignin catalytic fast pyrolysis (CFP) by modifying Beta zeolites through alkali treatment and metal loading, which provide a novel strategy for lignin valorization. Beta zeolites were modified using different alkali solutions (TEAOH, TPAOH, Na₂CO₃, NaOH) and concentrations (0.1, 0.3, and 0.5 M), as well as various metal species (Co, Ga, Zn, Cu) and loadings (0.5, 1, 2, 3 wt%). Results showed that hierarchical Beta treated with 0.3 M Na₂CO₃ achieved a bio-oil yield of 9.5 wt% and increased MAHs selectivity from 17.8 area% (for parent Beta) to 28.9 area%. 1 wt% Cu-loaded Beta exhibited 23.7 area% MAHs selectivity, where altered acid distribution suppressed polycyclic aromatic hydrocarbon formation. Then, an β-O-4 model compound (α-OH-PPE) was employed to analyze bond cleavage pathways in lignin CFP, and correlation (R2) analysis was conducted to bridge the gap between zeolite structural parameters and MAHs production. It was revealed that mesopores facilitated C_β-O bond cleavage, while Cu-modification accelerated decarbonylation of acetophenone to generate toluene. Correlation analysis demonstrated that synergistic improvement in bio-oil yield and quality relied on dual “acid-pore” regulation, requiring tailored catalyst design for target products.