Heterogeneously Catalyzed Hydroprocessing of Organosolv Lignin in Sub- and Supercritical Solvents

A highly effective method for catalytic hydroprocessing of lignin to liquid products has been developed in supercritical ethanol (scEtOH) medium at moderate reaction conditions. The problems associated with the low lignin conversion (<20%) and char formation at higher temperatures in hot compressed water (HCW) were overcome in scEtOH. A significant increase in the calculated “higher heating value” (HHV) of the formed liquid was observed in a reaction using a heterogeneous catalyst compared to a reaction without a catalyst. The liquid product contains mainly substituted phenols, such as guaiacols and syringols, as major components. On the basis of the product distribution, a possible reaction pathway for the formation of the phenolic products is described. A maximum HHV up to 36.2 MJ/kg was calculated for a product obtained over 5% Ru/γ-Al2O3 at 300 °C after 20 h of reaction time. Lignin conversion of ca. 98% was observed. The yield for the liquid product reached values up to 92%.