Cellulose is the most abundant and renewable organic compound on Earth, it is however not soluble in common organic solvents and aqueous solutions. Cellulose dissolution is a key aspect to promote its value-added applications. Ionic liquids (ILs) have been shown to solubilize cellulose under relatively mild conditions. The easy processability of cellulose with ILs and their environmental-friendly nature prompted research in various fields such as biomass pretreatment and conversion, cellulose fiber and composite production, and chemical conversion of cellulose in ILs. Progress has been made on understanding the mechanism of cellulose dissolution in ILs, including the structural characteristics of ILs that are cellulose solvents, however many details remain unknown. In light of rapid development and importance of cellulose dissolution in the field of IL-based cellulose and biomass processing, it is necessary to provide an overview of current understanding of cellulose dissolution in ILs and outline possible future research trends. Recent literature studies suggest that synergistic effects between the anions and the cations of ILs need to be revealed, which requires refining the structure of cellulose elementary fibrils, simulation of more realistic cellulose fibrils and detailed studies on the solution structure of cellulose in ILs. After analyzing literature studies, three interacting modules are identified, which are crucial to understand the process of cellulose dissolution in ILs: (1) the structure of elementary fibrils; (2) solvation of cellulose in ILs; and (3) solution structure of cellulose solubilized in ILs. A coherent analysis of these modules will aid in better design of more efficient ILs and processes.

A distillation apparatus was used in the pretreatment of white poplar samples with aqueous 1-ethyl-3-methylimidazolium acetate ([C2C1Im]­[OAc]) solutions. During biomass pretreatment, the concentration of [C2C1Im]­[OAc] was increasing due to removal of water via distillation. This allowed utilization of aqueous IL solutions with initially low IL concentrations that were not considered as effective pretreatment media previously. Aqueous [C2C1Im]­[OAc] solutions with initial concentrations of 7 and 15 wt % were tested to pretreat white poplar samples at 130 °C for 3 h. The biomass loading in [C2C1Im]­[OAc] was 20 wt %. After pretreatment, sugar conversion was found to account for 72–77% of the sample pretreated in neat [C2C1Im]­[OAc]. The encouraging results of this operation prompted a new approach to recycle and reuse [C2C1Im]­[OAc]: use the liquor collected after biomass pretreatment in neat [C2C1Im]­[OAc] without first separating water from it. In a similar fashion, biomass pretreated with the recycled liquor was exposed to dynamically increasing [C2C1Im]­[OAc] concentration, combining biomass pretreatment and water evaporation into one step. Three cycles of recycling and reuse were performed in this work, and the sugar conversion was found to decrease with the number of recycling cycles. It was believed that process optimization could improve sugar conversion further. The potential of this approach of biomass pretreatment, IL recycling, and reuse may stimulate the design of new IL pretreatment technologies.

A systematic study was performed to understand interactions among biomass loading during ionic liquid (IL) pretreatment, biomass type and biomass structures. White poplar and eucalyptus samples were pretreated using 1-ethyl-3-methylimidazolium acetate (EmimOAc) at 110°C for 3h at biomass loadings of 5, 10, 15, 20 and 25wt%. All of the samples were chemically characterized and tested for enzymatic hydrolysis. Physical structures including biomass crystallinity and porosity were measured by X-ray diffraction (XRD) and small angle neutron scattering (SANS), respectively. SANS detected pores of radii ranging from ∼25 to 625Å, enabling assessment of contributions of pores with different sizes to increased porosity after pretreatment. Contrasting dependences of sugar conversion on white poplar and eucalyptus as a function of biomass loading were observed and cellulose crystalline structure was found to play an important role.

Small-angle neutron scattering (SANS) was used to study the porosity of pine samples before and after ionic liquid (IL) pretreatment. Pine samples were pretreated using 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) at 110 °C for 3 h at biomass concentrations of 5, 10, 15, 20, and 25 wt % and at 130 °C for 3 h at biomass concentrations of 5 and 25 wt %. For the first time, relative changes in porosity of pretreated pine samples derived from SANS were compared with those obtained from the nitrogen adsorption analysis. Biomass crystalline structures were measured by X-ray diffraction (XRD). Both porosity and XRD data suggest that [C2C1Im][OAc] interacted with pine samples more efficiently at higher biomass concentrations during pretreatment. This was attributed to the presence of resin acid in the pine samples.