Understanding the relationship between the structure and depolymerization behavior of lignin

Various lignin depolymerization methods have been proposed. Nevertheless, the relationship between the structure of lignin and its depolymerization behavior has not been widely investigated. Herein, six types of lignin samples were produced from oakwood (OW, hardwood) and pinewood (PW, softwood) using three different delignification techniques (ethanolsolv, formasolv, and Klason). The content of ether linkages in the OW-derived lignins was approximately three times higher than that in the PW-derived lignins because of the presence of the sinapyl alcohol unit in the former. The contents of ether linkages in the lignin isolated via the different methods followed the order: formasolv > ethanolsolv > Klason. The lignin samples were depolymerized in a mixture of supercritical ethanol (scEtOH) and formic acid at temperatures of 250–350 °C. At 350 °C, regardless of the lignin type, high conversion (>95%) and a high bio-oil yield (>81 wt%) could be achieved, demonstrating that the combined use of scEtOH-HCOOH was very effective for the depolymerization of various types of lignin. At the low temperatures of 250–300 °C, the lignin conversion and bio-oil yield were highly dependent on the amount of ether linkages; for example, at 300 °C, the use of OW-derived formasolv lignin resulted in a high bio-oil yield (86.2 wt%), whereas the use of OW-derived Klason lignin resulted in a very low bio-oil yield (27.9 wt%). The properties of the bio-oils produced from the different types of lignin were discussed.