Due to the complexity and variability of native biomass, early studies on possible mechanisms have focused on purified cellulose/lignin substrates [3, 14, 28, 67, 86, 87], oligomers of glucose and lignin models [26, 28, 75, 76]. Indeed, biomass is a complex heterogeneous substrate constituted of cellulose, hemicellulose, and lig­nin at varying ratios depending on the biomass feedstock. Cellulose can have several different crystalline structures [71]. Lignin is a branched polymer com­posed of different types of aryl-ether units and bonds that ionic liquids can cleave (aryl-ethers and aryl-alkyl linkages) [75]. The composition of lignin can affect its structure. Hardwood lignins have usually a higher ratio of syringyl/guaicyl units, giving them a more linear structure. In contrast, softwoods contain mostly guaiacyl phenolic units, giving them a branched structure [32].

The dissolution of Avicel cellulose was studied in different 1-alkyl-3-methyl — imidazolium chloride ILs prepared with alkyl chains of various lengths (2-10 atoms). It was found that Avicel cellulose was more soluble with alkyl chains with an even number of carbon atoms [61]. The depolymerization of cellulose was studied also in [BMIM][Cl] using an acid resin as a catalyst [88, 89]. It was proposed that the hydrolysis of cellulose is initiated with the protonation of the oxygen atom in the glycosidic bond. The glycosidic bond then breaks to form a cyclic carbocation, followed by a nucleophilic attack of water to add a hydroxyl group [89].

The cleavage of a particular type of linkage was studied on specifically designed lignin models with the desired linkage. For example, the IL 1-H-3- methylimidazolium chloride was effective in the cleavage of the b-O-4 bond in guaiacylglycerol-b-guaiacyl ether and veratrylglycerol-b-guaiacyl ether [75]. The cleavage of the same lignin models in [BMIM][Cl] required the presence of metal chloride catalysts, such as FeCl3, CuCl2, and AlCl3 [76]. The reactivity of 2-methoxy-4-(2-propenyl)phenol (similar to guaiacyl unit), 4-ethyl-2-methoxy — phenol (alkyl substitution), and 2-phenylethyl phenyl ether (with b-aryl ethers linkage) was studied in 1-ethyl-3-methylimidazolium triflate and [EMIM][Cl] with metal chlorides and acid catalysts [28].

Another study focused on the dissolution of pine kraft lignin. It was found soluble at temperatures above 50°C in 1,3-dimethylimidazolium methylsulfate ([MMIM][MeSO4]), 1 — hexyl-3-methylimidazolium trifluoromethanesulfonate

([HMIM][CF3SO3]), [BMIM][MeSO4]. However, it was insoluble in 1-butyl-3- methylimidazolium hexafluorophosphate ([BMIM][PF6]) even at 120°C. The anion in imidazolium-based ILs affected the solubility dramatically: the methyl — sulfate anion was more effective than the chloride and bromide anions at dis­solving lignin [3].