Deep Eutectic Solvent Assisted Facile Synthesis of Lignin-Based Cryogel Kuan-Ting Lin, ⊥,† Ruoshui Ma, ⊥,†,‡ Peipei Wang, † Junna Xin, § Jinwen Zhang, § Michael P. Wolcott, § and Xiao Zhang* ,†,‡ † Voiland School of Chemical Engineering & Bioengineering Bioproducts, Science & Engineering Laboratory, Washington State University, 2710 Crimson Way, Richland, Washington 99354, United States ‡ Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Composite Materials & Engineering Center, Washington State University, Pullman, Washington 99164, United States * S Supporting Information ABSTRACT: In this study, we reported a method to prepare a lignin based cryogel by cross-linking deep eutectic solvent (DES) extracted lignin and formaldehyde as an alternative to replace resorcinol-formaldehyde (RF) based aerogel. The resulting lignin-formaldehyde cryogel has a highly porous structure (172.8 m 2 /g surface area) with a high dimension stability. We found that the hydroxyl group and carbonyl group of DES lignin provide reactive sites to cross-link with formaldehyde through electrophilic addition and aldol con- densation reaction. We also found that using choline chloride- lactic acid (ChCl-Lac) DES as a solvent during the lignin-formaldehyde cryogel formation is critical to prevent the shrinkage of the final cyrogel. The ensuing lignin-formaldehyde cryogel has promising properties such as high thermostability, low thermal conductivity, and good fire retardancy. ■ INTRODUCTION Resorcinol-formaldehyde (RF) based aerogel was first demonstrated in 1989. 1 Since then it has gained increasing interest because of its unique physical properties such as high specific surface area and low thermal conductivity. 2-4 RF- based gels have found many potential applications as adsorbents, electrodes, energy storages, and thermal insu- lates. 5-8 However, the cost of RF aerogel manufacturing has significantly limited its applications. Two major cost barriers are associated with the expensive precursors (i.e., resorcinol) and drying process to maintain the porosity structure. Resorcinol is an expensive chemical, and supercritical dry is an expensive process, which is difficult for large scale commercial production. 9 Thus, identifying alternative pre- cursors and economically viable drying methods can bring breakthrough toward expanding RF gel commercialization. Lignin is the largest renewable resource with an aromatic skeleton. 10 Its chemical structure as well as monomeric constituents resembles many types of phenolic polymers (e.g., acrylonitrile butadiene styrene (ABS), rosin, etc.) and their precursors (e.g., polyurethane, epoxy resin, polyethylene terephthalate, etc.) respectively. 11-14 There has been a significant amount of effort toward modifying lignin macro- molecular structure/chemistry for material applications 15-17 or depolymerizing lignin to low molecular weight and monomeric phenolic compounds for polymer synthesis. 18-20 The phenyl- propane units in lignin are connected through a variety of chemical linkages as well as intricate electronic interac- tions, 21,22 which makes lignin a compact and amorphous macromolecule. Lignin separated from the plant by various means and processes typically show a low surface area and porosity (Table S1). Recently, Jiang and Lubineau have demonstrated the preparation of a lignin-resorcinol-formaldehyde aerogel in alkaline solution, incorporating alkaline lignin into bacterial cellulose. 23 The materials have shown promising application in energy storage. However, the protocol requires the presence of 70 wt % of bacterial cellulose gel as a support, and the aerogel can only form with a low lignin content up to 7.5 wt %. A few other studies also investigated the incorporation of lignin into polymer composites to facilitate the formation and support the pore structure formed by these composites. 24-28 However, blending polymeric materials with lignin loses the feasibility of replacing petroleum precursors with lignin since the higher cost of polymer become the obstacle to prepare porous materials with reasonable price. The low surface area is a major shortcoming hinders the use of lignin macromolecules for many materials applications. Identifying ways to convert lignin to highly porous material can expand the potential for lignin Received: October 23, 2018 Revised: December 7, 2018 Article pubs.acs.org/Macromolecules Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b02279 Macromolecules XXXX, XXX, XXX-XXX Macromolecules Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/22/18. For personal use only.