Are Polycyclic Aromatic Hydrocarbons in the Hydro-Liquefaction Products of Lignin Derived from the Lignin or Hydrogen Donor Solvent?

Identifying the source of products in lignin hydro-liquefaction oil is crucial for assessing the lignin conversion efficiency. The utilization of hydrogen donor solvents (HDSs) complicates the determination of whether polycyclic aromatic hydrocarbons (PAHs) derive from lignin or HDS. Thus, identifying the source of PAHs in the oil can improve our understanding of lignin structures and enhance its utilization. To address this challenge, 1,2,3,4-tetrahydroquinoline (THQ) was used as an HDS to trace products within the oil. The results showed that (1) lignin could be completely liquefied in THQ under optimal conditions: a THQ/lignin mass ratio of 0.5, a reaction temperature of 450 °C, an initial H2 pressure of 8 MPa, a reaction time of 4 h, and a molybdenum sulfide/lignin mass ratio of 1; (2) PAHs with 2–6 rings were identified in the oil and confirmed to derive solely from lignin through qualitative and quantitative analyses via GC–MS rather than THQ.