Preparation of magnetic silica supported Brønsted acidic ionic liquids for the depolymerization of lignin to aromatic monomers

Abstract

Lignin, the most abundant renewable resource of aromatics in nature, is recognized as an alternative for fossil-based fuels and chemicals. Herein, we propose an efficient method to obtain aromatic monomers by lignin depolymerization using magnetic silica supported Brønsted acidic ionic liquid (BAIL–Fe₃O₄@SiO₂), which has higher catalytic activity and easier separate property compared with the traditional lignin depolymerization in ionic liquids. The prepared BAIL (30 wt%)–Fe₃O₄@SiO₂ catalyst is magnetic, mesoporous, thermostable, and core–shell structured, with a diameter of 500–600 nm, BET surface area of 185.7 m² g⁻¹, and pore size of 4.362 nm. The results showed that in the presence of 40 wt% of BAIL (30 wt%)–Fe₃O₄@SiO₂ at 160 °C for 3 h, the conversion of lignin increased markedly to 54.0% and 33.0% yield of aromatic monomers were acquired. Among the ethanol-soluble liquid products from lignin depolymerization over BAIL (30 wt%)–Fe₃O₄@SiO₂, the aryl aldehyde is predominant and has a yield of 19.2%. Furthermore, other products were also detected, for which the molecular mass was less than 250 Da, the double bond equivalent value varied from 4 to 7, and the carbon number was within the range of 6–15. The reusability experiment showed that the conversion of lignin and the yield of aromatic monomers can still achieve 90.4% and 91.2% of that catalyzed by fresh catalyst after the 3rd cycle.