Yield of Phenolic Monomers from Lignin Hydrothermolysis in Subcritical Water System

Lignin is the most noncellulosic based abundant natural organic polymer with the highest number of aromatic units. It is mainly extracted from wood in the Kraft pulping process and used as boiler fuel. However, the number of high-value applications of lignin remains small. The aim of this study is to explore the yield of phenolic monomers from lignin hydrothermal conversion in subcritical water. The hydrothermal experiments were carried out at different subcritical conditions to explore the influence of reaction temperature (200-350 degrees C), time (0-60 min), and lignin to water ratio (1:10-1:80). The results show that the yields of bio-oils (sum of crude-oil and water-soluble organics regarded as light-oil) with a range of 28.9-44.7 wt % (of dry lignin fed) were strongly influenced by these operating parameters. The crude-oil contained a small amount of monomers (0.1-2.8 wt % of lignin), whereas a large amount of monomers was composed in the light-oil (1.2-6.3 wt % of lignin). The most abundant monomers produced from hydrothermal reactions were guaiacol and cate-chol, with the highest yields of 1.18 wt % (11.8 mg/g at 300 degrees C) and 1.88 wt % (18.8 mg/g at 350 degrees C), respectively.