Oxidative Depolymerization of Lignin in Ionic Liquids

Beech lignin was oxidatively cleaved in ionic liquids to give phenols, unsaturated propylaromatics, and aromatic aldehydes. A multiparallel batch reactor system was used to screen different ionic liquids and metal catalysts. Mn(NO3)(2) in 1-ethy1-3-methylimidazolium trifluoromethanesulfonate [EMIM][CF3SO3] proved to be the most effective reaction system. A larger scale batch reaction with this system in a 300 mL autoclave (11 g lignin starting material) resulted in a maximum conversion of 66.3% (24 h at 100 degrees C, 84 x 10(5) Pa air). By adjusting the reaction conditions and catalyst loading, the selectivity of the process could be shifted from syringaldehyde as the predominant product to 2,6-dimethoxy-1,4-benzoquinone (DMBQ). Surprisingly, the latter could be isolated as a pure substance in 11.5 wt% overall yield by a simple extraction/crystallization process.