Hydrotropy and Cosolvency in Lignin Solubilization with Deep Eutectic Solvents Belinda Soares, † Armando J. D. Silvestre, † Paula C. Rodrigues Pinto, ‡ Carmen S. R. Freire, † and Joã o A. P. Coutinho* ,† † CICECO, Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal ‡ RAIZ, Research Institute of Forest and Paper (The Navigator Company), Rua José Estevã o, 3800-783 Eixo, Aveiro, Portugal * S Supporting Information ABSTRACT: The mechanisms responsible for the good solubility of lignin in aqueous solutions of deep eutectic solvents are here investigated using both monomer model compounds and technical lignins (kraft and organosolv). The results show the ability of deep eutectic solvents to act either as hydrotropes or cosolvents, enhancing the solubility of poorly soluble solutes (technical lignins or their monomers). Hydrotropy was shown to lead to a remarkable enhancement of organosolv lignin solubility of 474.7 ± 2.7 times using an aqueous solution of propionic acid:urea (2:1), while for the systems where the solubilization mechanism was cosolvency, the best solubility enhancement was 194.2 ± 4.1 times using ethylene glycol:tetrabutylphosphonium chloride (2:1). The solubility of kraft lignin was also enhanced using an aqueous solution of propionic acid:urea (2:1) (228.3 ± 8.2 times) and ethylene glycol:tetraethylammonium chloride (2:1) (163.0 ± 16.1 times) by the same mechanisms. KEYWORDS: Deep eutectic solvents, Solubility, Hydrotropy, Cosolvency, Lignin monomer model compounds, Technical lignins, Biorefinery ■ INTRODUCTION Lignocellulosic biomass is the most important feedstock in the biorefinery framework. 1 However, its complexity and structural heterogeneity represent a major challenge to the development of efficient fractionation processes. 1 A pulp and paper mill is an example of a primary lignocellulosic biorefinery, which targets cellulose fibers. 2 Lignin is the main byproduct, which is mainly used for energetic valorization. 2 For a more sustainable and competitive pulp and paper industry, the valorization of lignin is fundamental. 3 High-value products from isolated lignin include low-cost carbon fiber, engineering plastics and thermoplastic elastomers, polymeric foams and membranes, and a variety of fuels and chemicals. 4 Kraft and sulfite pulping processes are performed at high temperatures in strong alkaline and acidic media, respectively. 2 These harsh processing conditions induce the lignin depolymerization. These technical lignins are characterized by the incorporation of sulfur into its chemical structure, which further complicates its downstream valorization. 2 Organosolv uses a mixture of water and organic solvents in the presence or absence of catalyst 5 and has a smaller environmental footprint and simpler solvent recovery and operates at milder conditions, 5 producing a high-quality, sulfur-free lignin fraction. Recently, novel solvents such as ionic liquids attracted great attention. 6 IonoSolv is a process using low-cost ionic liquids in aqueous solution to dissolve lignin and hemi- celluloses, preserving the cellulose properties. Its main advantages over organosolv are higher lignin removal yield, lower solvent cost, and lower process pressure. 6 However, this process induces significant structural changes in the lignin that may compromise its valorization. The development of fractionation processes able to selectively extract high-purity lignin under mild conditions and to induce minimal structural degradation while achieving high lignin yield and preserving cellulose properties is imperative to achieve a complete and sustainable valorization of lignocellulosic biomass. Recently, deep eutectic solvents (DESs) have emerged as promising alternatives for biomass fractionation. 7,8 DESs are mixtures of hydrogen-bond donors (HBDs) and acceptors (HBAs) with low melting points. Francisco et al. 9 were the first to study the solubility of alkali lignin, cellulose, and starch, in DESs combining carboxylic acids with choline chloride ([Ch]Cl) and amino acids. They concluded that most of the selected combinations show high lignin solubility and very poor or negligible cellulose solubility. However, the best DES to solubilize lignin reported was lactic acid:[Ch]Cl (9:1). 9 Other DESs were also investigated for wood fractionation [of Received: April 15, 2019 Revised: June 15, 2019 Published: June 16, 2019 Research Article pubs.acs.org/journal/ascecg Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12485-12493 © 2019 American Chemical Society 12485 DOI: 10.1021/acssuschemeng.9b02109 ACS Sustainable Chem. Eng. 2019, 7, 12485-12493 Downloaded via UNIV ESTADUAL PAULISTA on September 19, 2019 at 15:12:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.