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Acidic anions of ionic liquids have been demonstrated as efficient catalysts for the cleavage of the β-O-4 ether linkage prevalent in the lignin superstructure. Through the use of lignin model compounds with varying functionality and by monitoring reaction kinetics, a full mechanistic investigation into the hydrolysis of the β-O-4 linkage in acidic ionic liquid solutions is reported. Hammett acidities are reported for different 1-butyl-3-methylimidazolium hydrogen sulfate [C4C1im][HSO4] ionic liquid systems with varying acid and water concentrations and were correlated to substrate reactivity. Results show that the rate of ether cleavage increases with an increase in acidity and the initial dehydration of the model compound is the rate-determining step of the reaction. The Eyring activation parameters of the reaction in hydrogen sulfate ionic liquids with a variety of cations are reported, indicating a consistent E1 dehydration mechanism. Hydrogen bonding in protic ionic liquids was shown to significantly influence anion–cation interactions, consequently altering the solvation of the protonated starting material and therefore the overall rate of reaction. Comparison of reaction rates in these ionic liquids with results within aqueous or aqueous/organic media indicate that the ionic liquids facilitate more rapid cleavage of the β-O-4 ether linkage even under less acidic conditions. All the reported results give a complete overview of both the mechanistic and solvation effects of acidic ionic liquids on lignin model compounds and provide scope for the appropriate selection and design of ionic liquids for lignin processing. [ABSTRACT FROM AUTHOR]

A number of ionic liquids have been shown to be excellent solvents for lignocellulosic biomass processing, and some of these are particularly effective in the production of the versatile chemical building block 5-hydroxymethylfurfural (HMF). In this study, the production of HMF from the simple sugar glucose in ionic liquid media is discussed. Several aspects of the selective catalytic formation of HMF from glucose have been elucidated using metal halide salts in two distinct ionic liquids, 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium hydrogen sulfate as well as mixtures of these, revealing key features for accelerating the desired reaction and suppressing byproduct formation. The choice of ionic liquid anion is revealed to be of particular importance, with low HMF yields in the case of hydrogen sulfate-based salts, which are reported to be effective for HMF production from fructose. The most successful system investigated in this study led to almost quantitative conversion of glucose to HMF (90% in only 30 minutes using 7 mol% catalyst loading at 120°C) in a system which is selective for the desired product, has low energy intensity and is environmentally benign. [ABSTRACT FROM AUTHOR]

Recently, acidic ionic liquid water mixtures based on the hydrogen sulfate anion have been shown to effectively extract lignin from lignocellulosic biomass. This study analyses Miscanthus giganteus lignin isolated after extraction with the protic ionic liquid 1-butylimidazolium hydrogen sulfate ([HC4im][HSO4]) followed by precipitation with the antisolvent water. Several analytical techniques were employed, such as quantitative 13C-NMR, 1H–13C HSQC NMR, 31P-NMR, Py-GC-MS, GPC and elemental analysis. The analysis shows that the ionic liquid pretreatment breaks lignin-hemicellulose linkages and depolymerizes the lignin through the cleavage of glycosidic, ester and β-O-4 ether bonds. This is accompanied by solubilization of the newly generated lignin fragments. At longer pretreatment times, repolymerization of lignin fragments through condensation reactions occurs. The isolated lignins were carbohydrate-free, had low sulfur contents, low molecular weights. Early stage lignins were structurally similar to ball-milled lignin, while more treated lignins were enriched in p-hydroxyphenyl and guaiacyl units and had a high phenolic hydroxyl group content. We conclude that, depending on the treatment conditions, lignins with a variety of characteristics can be isolated using this type of ionic liquid solution. [ABSTRACT FROM AUTHOR]

We report on the pretreatment of poplar wood with three different 1-ethyl-3-methylimidazolium ionic liquids, [EMim][OAc], [EMim][MeSO3], and [EMim][HSO4], at varying water contents from 0–40 wt% at 100 °C. The performance was evaluated by observing the lignin and hemicellulose removal, as well as enzymatic saccharification and lignin yield. The mechanism of pretreatment varied between the ionic liquids studied, with the hydrogen sulfate ionic liquid performing delignification and hemicellulose hydrolysis more effectively than the other solvents across the investigated water content range. The acetate ionic liquid produced superior glucose yield at low water contents, while the hydrogen sulfate ionic liquid performed better at higher water contents and produced a recoverable lignin. The methanesulfonate ionic liquid did not introduce significant fractionation or enhancement of saccharification yield under the conditions used. These findings help distinguish the roles of anion hydrogen bonding, solvent acidity, and water content on ionic liquid pretreatment and can aid with anion and water content selections for different applications. [ABSTRACT FROM AUTHOR]