One-pot depolymerization, demethylation and phenolation of lignin catalyzed by HBr under microwave irradiation for phenolic foam preparation

High-value utilization of lignin is critical to improve the economic viability of integrated biorefineries. In this study, HBr was employed as a trifunctional catalyst to perform depolymerization, phenolation and demethylation of lignin in phenol to improve the reactivity of lignin and the obtained product was further directly used to prepare phenolic foams without purification. With the aid of microwave irradiation, a reduced reaction time and mild reaction conditions were achieved. The results showed that the phenolic hydroxyl content increased from 2.89 mmol/g to 5.90 mmol/g (increased by 104%) while the methoxyl group content dropped from the 4.75 mmol/g to 3.37 mmol/g (decreased by 30%) under the optimized conditions (10% HBr, 90 °C and 2 h). The structure changes of lignin were investigated by FT-IR, GPC, 1H, 31P, and 2D HSQC NMR analyses. Due to the enhanced activity of lignin, the activation energy and characteristic curing temperature of the modified lignin-based phenolic resin (103.27 kJ/mol, 130.1 °C) was markedly lower than that of the unmodified lignin-based phenolic resin (113.48 kJ/mol, 139.4 °C) according to differential scanning calorimetry (DSC) analysis. Further characterization of the modified lignin-based phenolic foam showed it has better thermal insulation and mechanical properties than that of unmodified lignin-based phenolic foam, which provides a value-added application of lignin as insulating foam material.