The ZnCl2 Induced Catalytic Upgrading of Biomass-Derived Lignin to Aromatics

Selective cleavage of the C-O-C bonds in lignin without disrupting the C-C linkages can result in releasing aromatic monomers and dimers that can be subsequently converted into chemicals and fuels. In this study, we found that both biomass-derived lignin and lignin model compounds were depolymerized in high concentrated ZnCl₂ solution under relatively mild conditions (160 °C~200 °C, 6~14 h). Negligible conversion of lignin was found in low concentration ZnCl₂ (30 wt.%), while nearly complete conversion of lignin was observed in ZnCl₂ solution above 63 wt.%. As expected, aromatic monomers and dimers were detected as major products. High selectivity to the aromatic products in high concentration ZnCl₂ indicated that the incompletely coordinated Zn²⁺ ions could play a very important role in catalytic degradation of lignin since they could further coordinate with oxygen atoms in lignin, selectively weakening and finally breaking the C-O-C bonds.