Supramolecular assemblies of lignin into nano- and microparticles
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Lignins and their untapped potential Wood fibers are widely utilized in mature industries that take advantage of unique chemical and structural features of cellulose. However, the same cannot be said about lignin, another important component in wood. Lignin is a highly branched and amorphous biomacromolecule that displays different interunit linkages and is derived from complex phenylpropanoid pathways. In plants, lignin provides protection against various biotic/abiotic stresses and support to sap-conducting elements. The existence of lignin has contributed to the capacity of landbased plants to dominate terrestrial environments.1,2 The main source of lignin, readily available for use on a large scale, comprises so-called “black liquors” produced as side streams during fractionation of wood fibers for use in pulp and paper industries.3,4 Lignin extracted from the most typical process, so-called kraft pulping (a method that uses alkaline liquors for lignin solubilization), has a low degree of polymerization, with a weight-averaged molecular weight of 1–5 kDa (1–5 Kg/mol), lower than other commercially available lignins such as lignosulfonates (5–400 kDa [5–400 kg/mol]). Other lignins, such as those extracted with organic solvents (organosolv lignins) are in the range of 0.5–3 kDa ([0.5–3 kDa];
note that the polydispersity of lignosulfonate and kraft lignins is high compared to organosolv lignins). These technical lignins comprise low quantities of oxygen-containing functional groups (such as phenolic hydroxyl, carboxyl, and sulfonate groups, depending on the type), making them a viable feedstock for biomaterials production.5 Water-soluble lignosulfonates are, by far, the most used technical lignin today.5 Lignin represents a suitable source of bioenergy, considering its density and calorific value (similar to that of ethanol, 27 KJ/g).6,7 For this and other practical reasons, most industrial lignins are burned, on-site, for energy cogeneration; only a small fraction are used for materials. Assessments performed on extracted kraft lignin have determined that it may be a preferable precursor compared to synthetic organic counterparts of similar molecular complexity. This is due to the energy efficiency of lignin extraction over organic synthesis. Recent environmental life cycle cradleto-gate analyses of the impact of integrating a commercialscale lignin extraction process and lignin co-product to an existing kraft pulp mill indicated a considerable reduction in global warming.8 Thus, lignin isolated from pulp mills represents an emerging opportunity to create new co-products and
Mariko Ago, Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Finland; mariko.ago@aalto.fi Blaise L. Tardy, Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Finland; blaise.tardy@aalto.fi Ling Wang, Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Finland; ling.wang@aalto.fi Jiaqi Guo, Department of Bioproducts and Biosyste
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