Mechanism of cellulose regeneration from its ionic liquid solution as revealed by infrared spectroscopy

Dissolution followed by regeneration is a typical processing route to cellulose, which triggers much attention in the research of regeneration mechanism, especially for the new solvent ionic liquids (ILs). It has been known that H-bond (HB) interaction between IL and anti-solvent through double diffusion triggers the cellulose regeneration. The structural change including H-bonds transformation mainly during regeneration process is still unclear, however. In the present work, a device for in situ characterization of infrared spectroscopy was set up to monitor the cellulose regeneration from microcrystalline cellulose (MCC)/1-ethyl-3-methylimidazolium acetate ([Emim]Ac) solution. With the help of time-dependent infrared spectroscopy combined with two-dimensional spectral analysis, that is perturbation-correlation moving-window two-dimensional (PCMW2D) correlation spectroscopy and generalized two-dimensional correlation spectroscopy (2DCOS), the mechanism of cellulose regeneration from MCC/[Emim]Ac solution as water diffusion was revealed. We proposed a two-stage model. The diffusion of the free [Emim]Ac and water contributes to the structural changes occurred at the former stage. Water interacts with free IL to form HBW-IL in one hand, and with non-H bonded hydroxyl groups on cellulose to form HBC-W in another hand. HBC-IL is destroyed as water diffusion further at the later stage. Continuous HBC-W is constructed, accompanied by more HBW-IL formation between released anion of [Emim]Ac and water. No crystallization or aggregation of cellulose other than improved regularity of the chain arrangement is revealed by infrared spectroscopy. This study provides a new testing technique for the characterization of H-bonds structures of cellulose on the one hand. On the other hand, it is conductive to the in-depth understanding of the regeneration mechanism of cellulose in ionic liquid systems.