Synthesis of lignin-based epoxy resins: optimization of reaction parameters using response surface methodology

Abstract

Owing to the presence of phenolic groups in the lignin structure, this provides the potential to substitute bisphenol-A in synthesis of epoxy resin. In this work, organosolv lignin (OL) was first depolymerized by reductive depolymerization in supercritical acetone at 350 °C in the presence of Ru/C catalyst and 10 MPa H₂. The obtained depolymerized organosolv lignin (DOL), with a low average molecular weight (M_w) and high hydroxyl number, was used to synthesize lignin-based epoxy pre-polymers. A set of experiments was designed by utilizing the central composite design (CCD) to synthesize lignin-based epoxy resin. Three synthesis variables including reaction temperature, reaction time and NaOH/DOL molar ratio were investigated and the synthesized epoxy pre-polymers were characterized by FTIR and potentiometric titration. The mathematical model derived from the CCD was found to be accurate to predict the optimum conditions. At the optimal synthesis conditions, i.e., 8 h at 55 °C with NaOH/DOL molar ratio of 6.3, a high product yield (99%) and high epoxy content of ∼8 were achieved.