(364f) Carbon Dioxide and Organic Base Additives for Polar Aprotic Organosolv Pulping of Lignocellulosic Biomass

Lignocellulose is the most abundant terrestrial biomass on earth, with over 181.5 billion tons produced annually.¹ Lignocellulosic biomass is comprised of three major macromolecular components, which include cellulose (40-60 wt%), hemicellulose (10-40 wt%), and lignin (15-30 wt%).² Collectively the constituent macromolecules form a lignin-carbohydrate complex that facilitates the structural integrity and chemical recalcitrant of plant tissues. The chemical recalcitrance of lignocellulose can be overcome by thermochemical pretreatments which fractionate lignocellulose into its constituent components. Chemical pretreatments cleave lignin-carbohydrate linkages, solubilize and fractionate lignins, and produce cellulose-rich pulps. Historically, chemical pulping of lignocellulose (e.g., kraft pulping) has focused on solely isolating the cellulose fraction for producing paper pulp. Lignin fractions are removed as black liquor, a mixture consisting of condensed lignins and pulping chemicals, which is subsequently incinerated in order to generate low-grade heat and recoup the pulping reagents.² Alternatively, as lignins are the largest biogenic source for aromatic compounds, “lignin-first” pulping methods, such as organosolv pulping, focus on the isolation of lignin fractions in order to enhance the economic viability of lignocellulose valorization processes.³ However, organosolv pulping finds limited commercialization due to the volatility of the organic solvents (e.g., methanol, ethanol, acetone) which require elevated pressures (15-35 bar) and specialized equipment to keep the solvent species in a condensed liquid phase.^4,5

In the present study, polar aprotic solvents and organic bases with low vapor pressures are evaluated for the organosolv pulping of lignocellulosic biomass. Carbon dioxide (CO₂) atmospheres are also evaluated as additives to facilitate delignification. Under mild thermal and pressure conditions (T < 210 °C, < 10 bar), high degrees of delignification (80-90 wt%) of softwood species are observed with 2 hours. The influence of process conditions (e.g., temperature, pressure, time) and solvent composition on the organosolv pulping process is evaluated through characterization of the isolated cellulose and lignin fractions via spectroscopic, microscopic, and wet chemistry analytical methods. The outcomes of the present research advance the development of mild and scalable chemical pretreatments for fractionating lignocellulose into its constituent polymers, which can serve as feedstocks for value-added, biobased materials.

References

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(2) Schutyser, W.; Renders, T.; Van den Bosch, S.; Koelewijn, S. F.; Beckham, G. T.; Sels, B. F. Chemicals from Lignin: An Interplay of Lignocellulose Fractionation, Depolymerisation, and Upgrading. Chem. Soc. Rev. 2018, 47, 852–908. https://doi.org/10.1039/c7cs00566k.

(3) Abu-Omar, M. M.; Barta, K.; Beckham, G. T.; Luterbacher, J. S.; Ralph, J.; Rinaldi, R.; Roman-Leshkov, Y.; Samec, J. S.; Sels, B. F.; Wang, F. Guidelines for Performing Lignin-First Biorefining. Energy Environ. Sci. 2021, 14, 262–292. https://doi.org/10.1039/d0ee02870c.

(4) Galbe, M.; Wallberg, O. Pretreatment for Biorefineries: A Review of Common Methods for Efficient Utilisation of Lignocellulosic Materials. Biotechnol. Biofuels 2019, 12 (1), 1–26. https://doi.org/10.1186/s13068-019-1634-1.

(5) Brosse, N.; Hussin, M. H.; Rahim, A. A. Organosolv Processes. Adv. Biochem. Eng. Biotechnol. 2019, 166 (March 2017), 153–176. https://doi.org/10.1007/10_2016_61.