Valorization of Lignin via Oxidative Depolymerization with Hydrogen Peroxide: Towards Carboxyl-Rich Oligomeric Lignin...

The extraction and characterization of defined and carboxyl-rich oligomeric lignin fragments with narrow molecular weight distribution is presented herein. With regard to the well-known pulp bleaching process, oxidative lignin depolymerization was investigated using hydrogen peroxide in an aqueous alkaline solution (i.e., at T = 318 K, t = 1 h) and subsequent selective fractionation with a 10/90 (v/v) acetone/water mixture. While the weight average molecular weight (M_W) of lignin in comparison to the starting material was reduced by 82% after oxidation (T = 318 K, t = 1 h, c_lignin = 40 g L^-1, c_H2O2 = 80 g L^-1, c_NaOH = 2 mol L^-1) and subsequent solvent fractionation (T = 298 K, t = 18 h, c_{cleavage product} = 20 g L^-1), the carboxyl group (-COOH) content increased from 1.29 mmol g^-1 up to 2.66 mmol g^-1. Finally, the successful scale-up of this whole process to 3 L scale led to gram amounts (14% yield) of oligomeric lignin fragments with a M_W of 1607 g mol^-1, a number average molecular weight (M_N) of 646 g mol^-1, a narrow polydispersity index of 3.0, and a high -COOH content of 2.96 mmol g^-1. Application of these oligomeric lignin fragments in epoxy resins or as adsorbents is conceivable without further functionalization.