Microwave-Assisted Catalytic Solvolysis of Lignin to Phenols: Kinetics and Product Characterization

Lignin, a major component of lignocellulosic biomass, is a valuable source of phenolic and aromatic compounds. It is, therefore, vital to develop strategies to selectively deconstruct lignin to valuable chemicals. This study focuses on the kinetics of depolymerization of lignin and the production of phenols via a microwave-assisted catalytic process at mild conditions of 80 °C in dimethyl sulfoxide/water medium. Four different catalysts used in this study, viz., Fe2O3, LaFeO3, ZrO2, and zeolite-Y hydrogen (ZYH), were characterized for structure, specific surface area, and surface morphology. The molecular weight reduction of lignin and the evolution of phenolic monomers and oligomers were monitored using various techniques, and the rate constants of lignin degradation in the presence of different catalysts were determined using a continuous distribution kinetics model, assuming scission of the lignin macromolecule at any random position. The rate constants (min–1) followed the trend: ZYH (26 × 10–4) ≈ LaFeO3 (25 × 10–4) > ZrO2 (22 × 10–4) > Fe2O3 ≈ no catalyst (16 × 10–4). Vanillic acid (15 mg g–1) and methyl phenol (17 mg g–1) were the major phenolics obtained with LaFeO3, whereas coniferaldehyde (13 mg g–1) was the major phenolic compound with Fe2O3. Vanillin was produced at ca. 11 mg g–1 with both Fe2O3 and ZYH. LaFeO3 is shown to be a promising catalyst for both molecular weight reduction of lignin and the production of monomeric phenols, whereas the use of Fe2O3 results in the formation of only phenols, possibly via specific end-chain depolymerization. The selectivities of the monomeric phenols were higher with these two catalysts, whereas with ZYH and ZrO2, the selectivities of the oligomers were better. The reusability of the catalysts and the effect of catalyst loading on kinetics of lignin depolymerization were also evaluated