Enhanced Acid-Catalyzed Lignin Depolymerization in a Continuous Reactor with Stable Activity

A continuous process to depolymerize maple wood lignin dissolved in γ-valerolactone (GVL) to monomeric phenols is demonstrated in a stirred reactor using a mesoporous Zr-KIT-5 catalyst. The yield of identified aromatic monomers is stable for 18 h of continuous operation and reaches 7 wt % when the lignin feed is subjected to thermo-solvolytic pretreatment. Without the thermal pretreatment, the monomer yield is only 2.3 wt %. The thermal pretreatment causes partial lignin depolymerization (0.8 wt % monomer yield) improving catalyst pore accessibility of the lignin fragments. The 7 wt % monomer yield observed in the two-step lignin depolymerization route surpasses monomer yields observed in batch processes using identical feeds: ∼2.6 wt % during hydrogenolysis in methanol over Pd/C catalyst and ∼1.6 wt % with a microporous H-ZSM-5 catalyst. While the major monomers observed in the various techniques are guaiacol, syringol, and phenolic derivatives, their relative distribution varies depending on the technique. The demonstration of continuous acid-catalyzed lignin depolymerization solution with stable activity and high monomer yields, eliminating the need for additional reagents and issues related to handling solid lignin feed, represents a significant advance in lignin depolymerization science and technology.