Depolymerization of organosolv lignin in sub/supercritical ethanol and evaluation of chemical properties of depolymerized products

Abstract

Subcritical and supercritical treatments were performed to depolymerize organosolv lignin produced from poplar wood. The chemical and structural features of organosolv lignin were studied before the subcritical and supercritical treatments. The results showed that organosolv lignin has a smaller and more condensed structure as well as lower functional group (methoxyl group and phenolic hydroxyl group) contents compared with the structure and functional group contents of milled wood lignin from poplar. In subcritical and supercritical treatments, the reaction temperature ranged from 200ºC (subcritical state) to 350ºC (supercritical state), and the reaction time was controlled from 20 min to 60 min. The solvent ratio was set from 50 (v of ethanol/w of lignin) to 150 (v/w). Furthermore, hydrogen gas was injected into the reactor before the subcritical and the supercritical treatment as a way to determine the effect of hydrogen gas in lignin depolymerization, with the initial pressure derived from hydrogen gas ranging from 10 bar to 30 bar. Other variables such as heating rate, cooling rate, and revolutions per minute (RPM) were fixed to 10°C/min, -20°C/min, and 200, respectively. The results revealed that the distribution of lignin depolymerized products (oil, char, and gas) was heavily influenced by all three variables (temperature, reaction time, and solvent ratio). More specifically, with increasing temperature, reaction time, and solvent ratio the yield of oil decreased whereas the yields of char and gas increased. Moreover, it was also found that higher initial hydrogen pressure resulted in higher oil yield and lower char yield. Elemental analysis was performed to observe the atomic ratios of hydrogen to carbon (H:C) and oxygen to carbon (O:C), respectively; the results showed that these atomic ratios (H:C and O:C) were significantly affected by temperature and reaction time but not by solvent ratio. Using gas chromatography/mass spectrometry (GC/MS) analysis, 28 chemical components were observed. Only 19 phenolic compounds were identified from the oil. These compounds were classified into 4 main groups: H-unit, G-unit, S-unit, and fatty acid ester. Among them, phenolic compounds (H-, G-, S-unit) were quantified with response factor between each phenolic compound and an internal standard (fluoranthene) using an external standard method. From the GC/MS results, no compounds were detected at the temperature of 200°C, and the amount of phenolic monomer increased with increasing reaction time in 275°C and 350°C. However, the amount of phenolic monomer decreased at 275°C, whereas it increased at 350°C with increasing solvent ratio.