Direct Catalytic Hydrogenolysis of Kraft Lignin to Phenols in Choline-Derived Ionic Liquids

A new dual catalyst system composed of choline-derived ionic liquids (ILs) and Pd/C was developed for the selective hydrogenolysis of Kraft lignin to mono-phenols. Among a series of investigated choline-derived ILs, [Ch][MeSO3] displayed a strong acidity, good thermal stability, and excellent lignin solubility. Under the reaction conditions of the mass ratio of [Ch][MeSO3] to Pd/C being 1, the Pd/C loading of 3.5 wt %, H-2 pressure of 2.0 MPa, reaction time of 5 h, and temperature of 200 degrees C, the conversion of Kraft lignin and the selectivity to phenol (PL) and catechol (COL) reached 20.3%, 18.4%, and 18.1%, respectively. In order to rationalize the formation of PL and COL in our [Ch][MeSO3]-Pd/C system, the hydrogenolysis of a suitable lignin model compound (guaiacylglycerol-beta-guaiacyl ether) was studied under the same condition for Kraft lignin. The results suggested that the mechanism involved fragmentation of lignin catalyzed by both acid and Pd/C, followed by acid-catalyzed C-O and C-C cleavage of the fragmented compounds resulting in the formation of PL and COL