Lignin is the world’s second most abundant biopolymer and the world’s largest source of biobased aromatics (1). Occurring as an adhesive which binds together the cellulose fibers in the cell walls of woody plants, lignin has been largely cast aside as a waste byproduct of pulping procedures. Since the main industrial pulping procedures focus on the production of paper, the fate of lignin usually involves combustion to produce heat (2). This heat is then used to produce energy to offset the electrical costs of such paper mills, but it has been estimated that lignin can be upgraded in value from $0.18/Kg as a burned fuel to $1.08/Kg as upgraded, useful chemicals (3). The successful chemical upgrading of lignin involves not only the gentle separation of lignin from cellulose and hemicellulose; it involves depolymerization followed by functionalization, defunctionalization, or chemical synthesis to produce value-added compounds that can serve a wide variety of purposes. Functionalized materials can be used as fillers and additives in polymer formulations and concretes (4). De-functionalized materials are converted into bulk chemicals such as benzene and cyclohexane derivatives (1). The products of lignin depolymerization can be further reacted to form monomers of novel biobased polymers that show promise as green alternatives to petrochemically-sourced commodity plastics (5).
Given that the research on the depolymerization of lignin and the subsequent (de)functionalization and polymerization of lignin-based compounds is diverse and growing, there are many topics in these conversations that merit close attention. In this discussion, a narrowed focus on selected recent works is applied with the purpose of informing the audience of how technical lignin is commonly sourced and transformed. The recent works of Dr. Katalin Barta on the mechanistic underpinnings and improvement of acid-catalyzed lignin depolymerization are presented first as an example of how recent developments are seeking to improve yields of lignin-derived materials (6,7,8). Then, some of the recent explorations of lignin functionalization are shown as examples of low lignin itself may be modified and utilized prior to depolymerization as thermostable biobased fillers for conventional polymeric applications (9). Finally, a foray into some of the more recent examples of lignin-based biopolymers is presented, as the upgrading of lignin-based materials may offer strong candidates for renewable biobased alternatives to common polymers (10, 11). Polyesters, poly (ether ferulates), and poly (ether amides) are all presented and discussed. Critical analysis of present works and suggestions of future research trajectories are proposed within the context that the research in this area is still being quickly expanded upon (12).
(1) L.A. Zevallos Torres et al. Lignin as a potential source of high-added value compounds: A review. Journal of Cleaner Production. 2020, 263, 1-18.
(2) C. Chio, M. Sain, W. Qin, Lignin Utilization: A review of lignin depolymerization from various aspects. Renewable and Sustainable Energy Reviews. 2019, 107, 232- 249.
(3) K. Barta et al. Bright Side of Lignin Depolymerization: Toward New Platform Chemicals. Chem. Rev. 2018, 118, 614-678.
(4) A. Duval & L. Averous, Cyclic Carbonates as Safe and Versatile Etherifying Reagents for the Functionalization of Lignins and Tannins. ACS Sustainable Chem. Eng. 2017, 5, 7334-7343.
(5) J.V. Schijndel et al. Repeatable molecularly recyclable semi-aromatic polyesters derived from lignin. J. Polym. Sci. 2020, 58, 1655-1663.
(6) P.J. Deuss, K. Barta et al. Phenolic acetals from lignins of varying compositions via iron (III) triflate catalyzed depolymerization. Green Chem. 2017, 19, 2774-2782.
(7) A. DeSanti et al. Lignin-First Fractionation of Softwood Lignocellulose Using a Mild Dimethyl Carbonate and Ethylene Glycol Organosolv Process. ChemSusChem. 2020, 13, 4468-4477.
(8) A. DeSanti et al. New Mechanistic Insights into the lignin β-O-4 Linkage Acidolysis with Ethylene Glycol Stabilization Aided by Multilevel Computational Chemistry. ACS Sustainable Chem. Eng. 2021, 9, 2388-2399.
(9) L.Y. Liu et al. Uniform Chemical Functionality of Technical Lignin Using Ethylene Carbonate for Hydroxyethylation and Subsequent Greener Esterification. ACS Sustainable Chem. Eng. 2018, 6, 12251-12260.
(10) B.M. Upton & A.M. Kasko Biomass -Derived Poly(ether-amide)s Incorporating Hydroxycinnamates Biomacromolecules 2019, 20, 758-766.
(11) H.T.H. Nguyen et al. Polyethylene ferulate (PEF) and congeners: polystyrene mimics developed from biorenewable aromatics Green Chem. 2017, 17, 4512- 4517.
(12) J. Wenger V. Haas & T. Stern Why Can We Make Anything from Lignin Except Money? Towards a Broader Economic Perspective in Lignin Research Current Forestry Reports 2020, 6, 294-308.