Depolymerization of concentrated sulfuric acid hydrolysis lignin to high-yield aromatic monomers in basic sub- and supercritical fluids

Conversion of lignin into high-yield aromatic monomers is investigated using base-catalyzed depolymerization (BCD) in sub- and supercritical fluids. The effects of the base catalysts (NaOH, KOH, and Na₂CO₃), reaction media (subcritical water (subH₂O) and supercritical methanol (scMeOH)), and type of lignin (concentrated sulfuric acid hydrolysis lignin (CSAHL) and Kraft lignin (KL)) on the lignin conversion and product yields (gas, bio-oil, solid residue, and aromatic monomers) are explored to understand dominant factors determining the aromatic monomer yield. Treatment of KL lignin in subH₂O-NaOH at 330 °C for 30 min results in a high monomeric yield of 21.5 wt%. Under identical reaction conditions, a low amount of CSAHL (17.9 wt%) is converted to aromatic monomers; the low yield results from the resistant nature of CSAHL. In scMeOH, the monomeric yields are much lower (3.3–7.8 wt%), and the solid residue yields are much higher (about 26–31 wt%) than those obtained from subH₂O (monomeric yields, 6.2–17.9 wt%; solid-residue yields, 16–17 wt%). The most abundant monomers produced in the subH₂O-basic media are catechols and their alkylated and alkoxylated derivatives, while those produced in the scMeOH-basic media are methoxylated benzene and toluene species because of the unique O-alkylation ability of scMeOH.