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 (MW) of lignin in comparison to the starting material was reduced by 82% after oxidation (T = 318 K, t = 1 h, clignin = 40 g L-1, cH2O2 = 80 g L-1, cNaOH = 2 mol L-1) and subsequent solvent fractionation (T = 298 K, t = 18 h, ccleavage 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 MW of 1607 g mol-1, a number average molecular weight (MN) 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.

Advanced process for precipitation of lignin from ethanol organosolv spent liquors.

An advanced process for lignin precipitation from organosolv spent liquors based on ethanol evaporation was developed. The process avoids lignin incrustations in the reactor, enhances filterability of the precipitated lignin particles and significantly reduces the liquor mass in downstream processes. Initially, lignin solubility and softening properties were understood, quantified and exploited to design an improved precipitation process. Lignin incrustations were avoided by targeted precipitation of solid lignin at specific conditions (e.g. 100 mbar evaporation pressure, 43°C and 10%wt. of ethanol in lignin dispersion) in fed-batch operation at lab and pilot scale. As result of evaporation the mass of spent liquor was reduced by about 50%wt., thus avoiding large process streams. By controlled droplet coalescence the mean lignin particle size increased from below 10 µm to sizes larger than 10 µm improving the significantly filterability.