Cellulose is one of the most abundant bio-renewable materials on the earth and its conversion to biofuels provides an appealing way to satisfy the increasing global energy demand. However, before carrying out the process of enzymolysis to glucose or polysaccharides, cellulose needs to be pretreated to overcome its recalcitrance. In recent years, a variety of ionic liquids (ILs) have been found to be effective solvents for cellulose, providing a new, feasible pretreatment strategy. A lot of experimental and computational studies have been carried out to investigate the dissolution mechanism. However, many details are not fully understood, which highlights the necessity to overview the current knowledge of cellulose dissolution and identify the research trend in the future. This perspective summarizes the mechanistic studies and microscopic insights of cellulose dissolution in ILs. Recent investigations of the synergistic effect of cations/anions and the distinctive structural changes of cellulose microfibril in ILs are also reviewed. Besides, understanding the factors controlling the dissolution process, such as the structure of anions/cations, viscosity of ILs, pretreatment temperature, heating rate, etc., has been discussed from a structural and physicochemical viewpoint. At the end, the existing problems are discussed and future prospects are given. We hope this article would be helpful for deeper understanding of the cellulose dissolution process in ILs and the rational design of more efficient and recyclable ILs.

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Towards a molecular understanding of cellulose dissolution in ionic liquids: anion/cation effect, synergistic mechanism and physicochemical aspects

Cellulose, in addition to hemicellulose and lignin, makes the major fraction of lignocellulosic biomass – the only sustainable feedstock to meet the long-term sustainable energy need of the world. Cellulose is soluble in a number of solvents, e.g., concentrated phosphoric acid (CPA), N-methylmorpholine-N-oxide (NMMO), and ionic liquids (ILs), and can be regenerated by an anti-solvent without major derivatization for various applications. For one, the regenerated and much less crystalline cellulose is highly reactive for its biological conversion to sugars, fuels, and chemicals mediated by enzymes and/or microbes. This ability can be used as a core pretreatment step for improved bioprocessing of lignocelluloses. In this comprehensive review, cellulose solvent-based lignocellulosic fractionation technologies for enhanced enzymatic hydrolysis to improve biofuel and renewable chemical production are reviewed. The first part is focused on the background information of lignocellulosic biomass, lignocellulosic derived biogas, biohydrogen, and ethanol as well as acetone, butanol, and ethanol (ABE) production, and enzymatic hydrolysis. In the second part, the conditions for pretreatments applying CPA, NMMO, and IL solvents, improvements in enzymatic hydrolysis rates and yields for solids resulting from application of these pretreatments, and the features of the lignocellulosic structure affecting the improved bioprocessing have been thoroughly reviewed.

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Cellulose solvent-based pretreatment for enhanced second-generation biofuel production: a review

Recent progress in theoretical and computational studies on the utilization of lignocellulosic materials

Transforming lignocellulosic biomass into biofuels enabled by ionic liquid pretreatment.

The highly crystalline nature of cellulose results in poor processability and solubility, necessitating the search for solvents that can efficiently dissolve this material. Thus, ionic liquids (ILs) have recently been shown to be well suited for this purpose, although the corresponding dissolution mechanism has not been studied in detail. Herein, we adopt a molecular dynamics (MD) approach to study the dissolution of model cellulose crystal structures in imidazolium-based ILs and gain deep mechanistic insights, demonstrating that dissolution involves IL penetration-induced cleavage of hydrogen bonds between cellulose molecular chains. Moreover, we reveal that in ILs with high cellulose dissolving power (powerful solvents, such as 1-allyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium chloride), the above molecular chains are peeled from the crystal phase and subsequently dispersed in the solvent, whereas no significant structural changes are observed in poor-dissolving-power solvents. Finall...

Cellulose Crystal Dissolution in Imidazolium-Based Ionic Liquids: A Theoretical Study.

In recent years, ionic liquids (ILs) have become a promising solvent for cellulose pretreatment in biorefinery. However, almost all the ILs that can dissolve cellulose have an unsaturated heterocyclic cationic structure, while the ILs with cations of a saturated ring can hardly dissolve cellulose. To reveal the underlying mechanism, four kinds of ILs composed of unsaturated and saturated cations (1-butyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-1-methylpyrrolidinium, and 1-butyl-1-methylpiperidinium) and the acetate anion were explored as the solvents for a cellulose bunch by molecular dynamics simulation. It was shown that the cellulose bunch only dissolved in the ILs containing cations with an unsaturated heterocyclic ring. The reason lies in two aspects. One is the structure factor: the π electron delocalization of the unsaturated heterocyclic ring makes the cation more active to interact with cellulose and provides more space for acetate anions to form hydrogen bonds with cellulose. The other i...

Why Only Ionic Liquids with Unsaturated Heterocyclic Cations Can Dissolve Cellulose: A Simulation Study

Conversion of carbon dioxide (CO2) to valuable materials is important to academia and the chemical industry. Fixation of carbon dioxide catalyzed by ionic liquids (ILs) is an attractive option. Much work has been done in the fixation of carbon dioxide catalyzed by ILs, but most of it focused on reactions catalyzed by one molecule. In this work, we studied the existence of IL clusters in a series of systems containing hydroxyl-functionalized ILs (HFILs) through a series of theoretical calculations. The intermolecular interactions and microstructures of IL clusters were studied by molecular dynamics (MD) simulations. The results indicated that three-ion clusters to six-ion clusters exist in systems, while the number of clusters decreased as the cluster size increased. Instead of studying the single ion or ionic pair catalytic processes, we gained more insight into the catalytic effects of ionic liquid clusters on the fixation of CO2 by using density functional theory (DFT) calculations. Results suggested th...

Yaqin Zhang

Papers

Why Only Ionic Liquids with Unsaturated Heterocyclic Cations Can Dissolve Cellulose: A Simulation Study

Theoretical Study of Ionic Liquid Clusters Catalytic Effect on the Fixation of CO2