Understanding pretreatment efficacy of four cholinium and imidazolium ionic liquids by chemistry and computation

Certain ionic liquids (ILs) offer a potentially more sustainable and environmentally responsible alternative to organic solvents for many industrial applications, including biorefineries, where they are used to pretreat lignocellulose. To gain a more robust understanding of the roles of cations and anions in the process, we monitored the impact of the respective ILs on Panicum virgatum (switchgrass) in terms of lignin content, cellulose crystallinity, and enzymatic digestibility. The behaviors of four ILs, based on one of two cations, 1-ethyl-3-methylimidazolium ([C₂mim]⁺) and cholinium ([Ch]⁺), and one of two anions, acetate ([OAc]^-) and lysinate ([Lys]^-), were compared. While all four ILs were effective in pretreating switchgrass, ILs containing [Lys]^- anions provided greater delignification (70-80% vs. 16-50%) after addition of water as an anti-solvent and higher glucose yields (78-96% vs. 56-90%) compared to those obtained by the use of ILs containing [OAc]^- anions. Measurements of the Kamlet-Taft parameters using a series of dyes indicated a greater hydrogen bond basicity for the ILs with [Lys]^- anions as compared to acetate ILs. To understand the effective delignification ability of lysinate-based ILs, interaction energies of individual ions and ion pairs with a model dilignol substrate were determined by quantum chemical calculations. The results show that the addition of antisolvent significantly influenced the interaction energies governing lignin removal during the process.