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

Abstract

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 30min results in a high monomeric yield of 21.5wt%. Under identical reaction conditions, a low amount of CSAHL (17.9wt%) is converted to aromatic monomers; the low yield results from the resistant nature of CSAHL. In scMeOH, the monomeric yields are much lower (3.3–7.8wt%), and the solid residue yields are much higher (about 26–31wt%) than those obtained from subH2O (monomeric yields, 6.2–17.9wt%; solid-residue yields, 16–17wt%). 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.