Effects of the Ionic Liquid Structure on Porosity of Lignin-Derived Carbon Materials

Converting lignin into advanced porous carbon materials, with desirable surface functionalities, can be challenging. While lignin-derived carbons produced by pyrolysis at >600 °C develop porosity, they also simultaneously lose nearly all their surface functional groups. By contrast, pyrolysis of lignin at lower temperatures (e.g., <400 °C) results in the formation of nonporous char that retains some surface functionalities. However, copyrolysis of lignin with some ionic liquids (ILs) at lower temperatures offers an opportunity to produce porous carbon materials with both large surface areas and an abundance of surface functional groups. This study investigates the effects of IL properties (solubility, thermal, and ionic size) on the specific surface areas of lignin-derived carbons produced by copyrolysis of lignin and ILs at 350-400 °C for 20 min. It was found that ILs that have bulky anions and small cation sizes can induce porosity in lignin-derived carbons with large surface areas. Among 16 ILs that were tested, [C₂MIm][NTF₂] demonstrated the best performance; the inclusion of it in the copyrolysis process resulted in lignin-derived carbons with ∼528 m² g^-1 and 0.48 cm³ g^-1. Lignin-derived carbons produced using no IL, [C₂MIm][NTF₂], and [C₄MIm][OTF] were further characterized for morphology, interfacial chemical, and elemental properties. The copyrolysis of lignin and [C₂MIm][NTF₂], and [C₄MIm][OTF] resulted in doping of heteroatoms (N and S) on the porous carbon materials during pyrolysis reaction. The present findings contribute to a better understanding of the main property of ILs responsible for creating porosity in lignin carbon during pyrolysis.