(66f) Valorization of Lignin: Development Towards a Continuous Fractionation by Precipitation Process

The constantly growing global energy demand, concerns about the negative effects of increasing greenhouse gas emissions, and the finite nature of fossil raw materials call for alternative sources, which are low cost, renewable, and environmentally friendly. Lignocellulosic biomass (LCB) is a suitable raw material for carbon-neutral materials and chemical value chains [1]. LCB, can be pretreated by (acetone) organosolv pulping for fractionation into cellulose, hemicellulose and lignin. The latter is an amorphous polymer widely available from various sources [2]. Efficient processes for lignin precipitation from organosolv pulping liquors (“LigniSep”) have been developed earlier [3]. However, current technical lignins are hardly industrially utilized due to their complex heterogeneous nature. Tailored narrow lignin fractions could e.g. be used as functional additives or blends in polymer formulations. Possible functionalities of lignin fractions include antioxidant properties, antimicrobial activities and thermal stabilization for materials. Lignin classes of different molecular weight distribution can be precipitated by a stepwise decrease of the acetone concentration of lignin-acetone-water solution [1]. Therefore, the major goal of this work is to design a process for the continuous production of lignin fractions with targeted molecular weight for high-value applications.

Initially stepwise batch fractionation of lignin from different acetone-water compositions was conducted. The obtained fractions showed narrower molecular weight distributions and differ in weight averages and various physicochemical parameters. In this regard, a new High Performance Size Exclusion Chromatography based on acetone as an eluent was developed for determining the molecular weight distribution (MWD) relatively and absolutely (HPSEC-MALS). Through the polarity change in the stepwise fractionation, lignin solubility decreases with reducing the acetone concentration. By enhancing the polarity with increasing water concentration, larger lignin molecules, which indicate non-polar behavior, tend to precipitate. Also in the work of Sameni et al. low molecular weight lignin showed the highest solubility, whereas lignin with the highest aliphatic hydroxyl number had the lowest solubility in an organic solvent [4]. All lignin fractions show antioxidant capacity, whereby a trend towards higher antioxidant activity can be found towards smaller molecular weight lignin fractions. Espinoza–Acosta et al. have reported a similar tendency, where the antioxidant capacity of lignin was higher in fractions with an higher content of phenolic hydroxyl groups [5]. The identification and amount of functional groups was revealed by ³¹P NMR analysis.

A first concept for an evaporation-based fed-batch process has been derived from the solubility data of fractions obtained by batch processing. Controlling and adjusting the acetone concentration and temperature (by distillation pressure) are essential for the precipitation process. An ATR-FTIR probe is placed into the reactor medium to monitor in-line the acetone concentration and a FBRM probe is used to detect the phase separation of the lignin-rich phase from the solution. The new suggested (semi-)continuous lignin fractionation process, characteristics of the obtained lignin fractions (e.g. MWD-analysis, functional groups, glass temperatures, antioxidant assay etc.) will be presented in the conference contribution along with prospective product applications.

[1] A. Ponnudurai, P. Schulze, A. Seidel-Morgenstern, H. Lorenz (2021): Separation strategies for valorization of lignin by targeted molecular weight fractionation, Poster presented at ISIC 21 – 21^st International Symposium on Industrial Crystallization., Virtually

[2] A. Smit, W. Huijgen (2017): Effective fractionation of lignocellulose in herbaceous biomass and hardwood using a mild acetone organosolv process. Green Chem. 19 (22), 5505-5514

[3] P. Schulze, M. Leschinsky, A. Seidel-Morgenstern and H. Lorenz (2019): Continuous Separation of Lignin from Organosolv Pulping Liquors: Combined Lignin Particle Formation and Solvent Recovery, Ind. Eng. Chem. Res. 58, 9, 3797-3810

[4] J Sameni, S. Krigstin, M.Sain (2017): Solubility of lignin and acetylated lignin in organic solvent, BioRes. 12(1), 1548-1565

[5] J. Espinoza-Acosta, P. Torrws-Chávez, B. Ramirez-Wong, C. López-Saiz, B. Montano-Leyva (2016): Antioxidant, antimicrobial and antimutagenic properties of technical lignins and their applications, BioRes. 11(2), 5452-5381