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

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.