Formation and Substructure of Cellulose/Ionic Liquid Complex Crystals: Results from Wide-Angle X-ray Scattering and Solid-State 13C NMR Spectroscopy

Complex crystal formation is a representative approach for controlling the crystal structure of cellulose. In a previous study (Endo et al. Cryst. Growth Des. 2020, 20, 6267- 6271), it was discovered that cellulose forms complex crystals with two ionic liquids (ILs) such as 1-methyl-3-propylimidazolium paired with chloride ([C3mim]Cl) and dimethyl phosphate ([C3mim][DMP]). In this study, we investigated the formation of complex cellulose crystals with 12 ILs that are analogous to the previous ones, that is, 1,3-dimethylimidazolium ([C1mim]+), 1ethyl-3-methylimidazolium ([C2mim]+), 1-allyl-3-methylimidazolium ([Amim]+), and 1-butyl-3-methylimidazolium ([C4mim]+) paired with Cl-, acetate ([OAc]-), and [DMP]-, using wide-angle X-ray scattering. It was established that four ILs ([C1mim][OAc], [C1mim][DMP], [C2mim][DMP], and [Amim][DMP]) formed a complex crystal with cellulose, in addition to the two previously reported ILs. Based on the chemical structure of the ILs that formed the complex crystal, it was implied that the complex crystal formation tends to occur when the ion sizes of the cation and the anion are close. The substructures of these complex crystals were elucidated through solid-state 13C NMR spectroscopy. Almost the same crystal structure was observed, independent of the starting cellulose structure (cellulose I or II). The measurements of the regenerated cellulose strongly suggested that the cellulose chains aligned in an anti-parallel manner for all complex crystals. The conformations of the exocyclic group at the C6 position in the cellulose residue are also discussed.