Catalytic Depolymerization of Alkaline Lignin into Phenolic-Based Compounds over Metal-Free Carbon-Based Catalysts

Low-cost alkaline lignin, a major waste from pulp manufacturers, is currently a promising renewable feedstock for converting pulp to high-value phenolic compounds via a catalytic depolymerization reaction. Among potential catalysts for this reaction, metal-free carbon-based catalysts have been widely developing due to several favorable characteristics such as high catalytic activity and selectivity, long catalyst life, and ease in recovery. In this study, five types of synthesized metal-free carbon-based catalysts, e.g., graphene oxide (GO), nitrogen-doped GO (N-GO), solvothermal carbon (STC), functionalized STC with H2SO4 (SO3-STC), and nitrogen-doped STC (N-STC), were tested for alkaline lignin depolymerization. It was found that, in the presence of catalysts, a significantly higher phenolic monomers yield and a lesser amount of char formation were observed from the reaction. Under the optimized reaction conditions (at 250 degrees C for 3 h), N-STC enhanced the highest phenolic monomers yield (7.52%). Various techniques, such as gas chromatography-mass spectrometry (GC-MS), gas, chromatography-flame ionization detector (GC-FID), pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), Fourier transform-infrared spectroscopy (FT-IR), proton nuclear magnetic resonance with respect to hydrogen-1 nuclei (H-1 NMR), and gel permeation chromatography (GPC), were used to analyze for complete identification and quantification of raw lignin, lignin product, and residual lignin.