Dual Function Lewis Acid Catalyzed Depolymerization of Industrial Corn Stover Lignin into Stable Monomeric Phenols

Depolymerization of corn stover lignin from an industrial source was demonstrated over a predominantly Lewis acidic Zr-KIT-5 catalyst with stable activity and reusability. Among the solvents screened (1,4-dioxane, acetic acid, and gamma-valerolactone), only acetic acid, the solvent used for separating the lignin from the cellulose fraction, showed a combination of adequate solubility and thermal stability at depolymerization temperatures (similar to 250 degrees C). Thermo-solvolytic pretreatment of the lignin followed by catalytic depolymerization enhances the yield of low molecular weight products significantly. This is attributed to better accessibility of the mesopores by the smaller lignin fragments formed during the thermal pretreatment. The yield of ethyl acetate-extractable components is similar to 43 wt % (based on the initial lignin), made up of low molecular weight compounds (<1.5 kDa) as confirmed by gel permeation chromatography (GPC) analysis. Twelve phenolic monomers, with a cumulative yield of 28 wt % (based on the initial lignin) were detected in this extracted product. The Lewis acid sites in the Zr-KIT-5 catalyst also facilitate acetylation of the resulting monomers, protecting them from further condensation/repolymerization. Furthermore, the catalyst displays stable activity and product quality for up to five catalytic cycles tested.