Hydrocracking of Organosolv Lignin in Subcritical Water to Useful Phenols Employing Various Raney Nickel Catalysts

Hydrothermal conversion of organosolv lignin has been conducted under a reductive hydrogen atmosphere near the critical point of water. Several Raney nickel catalysts were tested with respect to degree of liquefaction, yields of phenols, and influence on the product distribution. The application of Raney nickel catalysts is crucial for the hydrothermal liquefaction of lignin since the liquefaction rate increased approximately 2.5 times compared to hydrothermal conversion without catalysts. In addition, Raney nickel caused complete demethoxylation of guaiacyl and syringyl moieties resulting in an oil composed mainly of six highly reactive simple phenols (phenol and monoalkylated derivatives). Depending on the catalyst type, yields of these phenols were 1.3-8.0 wt % (approximate to 2.6%-16% theoretical yield). Flash pressure release with subsequent condensation of the hot vapors proved to be a fast and easy approach to obtain a liquid fraction with a high content (25%) of phenol and para- or ortho-alkylated derivatives, which might be used in resin formulations.