Effects of various reaction parameters on solvolytical depolymerization of lignin in sub- and supercritical ethanol

Organosolv lignin was treated with ethanol at sub/supercritical temperatures (200, 275, and 350 °C) for conversion to low molecular phenols under different reaction times (20, 40, and 60 min), solvent-to-lignin ratios (50, 100, and 150 mL g(-1)), and initial hydrogen gas pressures (2 and 3 MPa). Essential lignin-degraded products, oil (liquid), char (solid), and gas were obtained, and their yields were directly influenced by reaction conditions. In particular, concurrent reactions involving depolymerization and recondensation as well as further (secondary) decomposition were significantly accelerated with increasing temperature, leading to both lignin-derived phenols in the oil fraction and undesirable products (char and gas). As the main components in the oil fraction, oxygenated phenols, guaiacol, and syringol as well as their alkylated forms were detected. The yield of alkylated phenols showed a drastic increase at 350 °C in the presence of initial hydrogen gas due to prevailing hydrodeoxygenation and hydrogenation reactions of the vinyl/allyl/oxygenated phenols. These reactions were also demonstrated indirectly from the results of atomic H/C and O/C of the oils. The highest amount of monomeric phenols released from lignin (1.0 g) was measured as ca. 96.7 mg at 350 °C, 40 min, 100 mL g(-1), and 3 MPa of H2. In addition, GPC analysis suggested a possibility of condensation between lignin-degraded fragments during the solvolysis reaction.