Catalytic depolymerization of alkali lignin in ionic liquids on Pt-supported La2O3–SO42−/ZrO2 catalysts

Abstract

A series of PtLaχ/SO₄²⁻/ZrO₂ catalysts, where χ is the atomic ratio of La to Pt of 1, 3 and 6, were prepared and used for the catalytic depolymerization of alkali lignin with ionic liquids in a continuous flow fixed-bed reaction system. Compared to the SO₄²⁻/ZrO₂ catalyst, the introduction of Pt and La₂O₃ significantly enhanced the catalytic activity in alkali lignin depolymerization. According to the characterization results of the catalysts obtained from N₂-physisorption, FT-IR spectra of CO or pyridine adsorption, and XPS and TEM analysis, the introduction of Pt and La₂O₃ resulted in the highly covalent character of sulfate species on the catalyst surface and strong interactions between Pt and La₂O₃, which led to an increase of the number of Lewis acid sites for PtLaχ/SZ catalysts. The PtLa3/SO₄²⁻/ZrO₂ catalyst showed the highest yield of phenolic compounds (28.7%) at 210 °C. Furthermore, based on the results of two model compounds of lignin in pretreatment and reaction units, the synergistic effect of ionic liquids and catalysts was also elucidated and a hypothetical reaction pathway of lignin depolymerization was elucidated.