Cellulose can be regenerated from the IL/biomass solution with an anti-solvent, such as acetone, water, dichloromethane, or acetonitrile, in excess [4, 7, 36].

Lignin and the IL can be washed away with NaOH [48]. Lignin can be precipitated with HCl [48] or H2SO4 [27]. Cellulose and lignin can also be extracted separately: cellulose was precipitated with ethanol and lignin with water after dissolution of wheat straw and pine wood with [EMIM][Cl] [47]. After regeneration, cellulose usually becomes amorphous or changes into its cellulose II structure [7, 36].

Changes in crystallinity have been characterized in purified cellulose substrates and native biomass by X-ray diffraction [8, 10, 25, 38, 71, 91]. The intensity of the (002), (110), and (1-10) reflections from cellulose usually decrease in intensity after the IL pretreatment and regeneration, indicating a loss in crystallinity [2, 14, 17, 67, 78, 149]. This was the case for cellulose in maple flour dissolved in [BMIM][OAc] and [EMIM][OAc] [38], and in spruce sawdust dissolved in [AMIM][Cl] [7]. The lower crystallinity led to improved access for hydrolytic enzymes and an improved glucose yield after hydrolysis [7].

However, it is possible for cellulose to retain its native cellulose I structure. Upon application of [EMIM][OAc] on poplar microtome section, the (002), (110), and (1-10) reflections of cellulose I disappeared and the diffraction pattern was dominated by a diffuse ring from the [EMIM][OAc]. When the pretreated sample was exposed to water, the IL was expelled from the wood and the diffraction pattern of cellulose was gradually recovered (Fig. 4.3). This recovery of cellulose I is in contrast to studies on biomass dissolution at high temperatures, where the regenerated cellulose is either amorphous or in the cellulose II phase. This is explained by the partial solubilization of cellulose microfibrils. Those microfibrils that retained their cellulose I structure acted as nucleation sites for cellulose I recrystallization after expulsion of [EMIM][OAc] [71].