Zirconium-Incorporated Mesoporous Silicates Show Remarkable Lignin Depolymerization Activity

Abstract

The depolymerization of dealkaline lignin in CH3OH + H2O mixtures was investigated over various solid acid catalysts, including commercial zeolites (H-FER, H-MOR, H-Beta, H-USY, and H-ZSM-5) with predominantly Brønsted acid sites and zirconium-incorporated mesoporous silicates (Zr-KIT-5 and Zr-KIT-6 with different Si/Zr ratios) with predominantly Lewis acid sites. Gel permeation chromatography (GPC) analyses of the product mixture, obtained from half hour batch reactions on the various catalysts at 250 °C and under a nitrogen pressure of 0.7 MPa, revealed a clear decrease in the molecular weight distribution, compared to the parent lignin. The yields of aromatic monomers (7.5 wt % of the initial lignin) and overall soluble aromatic species (∼77%) observed over Zr-KIT-5 is higher, compared to those observed with H-ZSM-5 (4.1% and 67%, respectively) under identical conditions. Furthermore, the yield of catalyst deposits is lower in Zr-KIT-5 (14%), compared to H-ZSM-5 (27%). The predominant Lewis acidity of Zr-KIT-5 appears to reduce aldehyde condensation reactions promoted by the Brønsted acid sites present in zeolites. However, a lack of hydrothermal stability of the mesoporous silicates is a drawback that must be overcome for practical viability.