Renewable and green energy resources have been rapidly developed to face the global challenge of energy crisis and the environment in the past few years. Cellulose is one of the richest biopolymers in the world with an estimated annual yield of more than 1012 tons(Klemm D et al., 2005). As a raw material, cellulose can be broadly used in paper, paints, textiles, and pharmaceutical compounds(Emam H E, 2019; Nongbe M C et al., 2018). Furthermore, it can be used as an appropriate feedstock for biofuel and bioproducts(Satari B et al., 2019; Sharma H K et al., 2019). The utilization of renewable cellulose has shown great promise for the future.
Cellulose is a linear condensation polymer that connects the D-glucose units through the β-1,4-glycosidic bonds, with degrees of polymerization (DP) from 100 to 20,000 depending on the sources(O'Sullivan A C, 1997; Samayam I P et al., 2011). Adjacent glucose molecules are directly 180 degrees apart, connected by hydrogen bonds (H-bonds) and van der Waals forces, resulting in parallel structure and crystal structure(Demain A L et al., 2005; Notley S M et al., 2004). H-bonds formed with the hydroxyl groups of the chains lead to its structural robustness with strong mechanical strength. Therefore, cellulose cannot be dissolved in water and general organic solvents, such as ethanol, ether, acetone, etc. Ionic liquids (ILs), which are organic molten salts with melting points below 100°C, are considered green sustainable solvents for the dissolution of cellulose(Lei Z et al., 2017; Zhu S et al., 2006). There are some special physicochemical characteristics for ILs, such as negligible vapor pressure, nonflammability, wide liquid range, and strong thermal stability. These have led ILs to being widely used in many fields, including catalysis, extraction, electrochemistry, organic synthesis etc(Freudenmann D et al., 2011; Zakrzewska M E et al., 2010). In 2002, Rogers and co-workers reported the use of ILs as cellulose solvents for both physical cellulose dissolution and regeneration, opening up a new class of solvents to the cellulose research community(Swatloski R P et al., 2002). Uto et al. studied the dissolution of cellulose in imidazolium-based ILs by molecular dynamics (MD) simulation, and they proposed the solubility of cellulose is closely related to the number of intermolecular H-bonds in cellulose crystals, both anions and cations in ILs can promote the breakage of H-bonds(Uto T et al., 2018). So far, approximately 300 kinds of ILs have been tested experimentally for dissolving lignocellulosic biomass(Badgujar K C and B M Bhanage, 2015; Wang H et al., 2012; Zhang J et al., 2017). After the cellulose dissolves in ILs, it can be regenerated from the cellulose/ILs/anti-solvent for further treatment. However, the study of cellulose regeneration is far less than the dissolution.
The regenerated cellulose by coagulation with anti-solvent is an important pathway for the industrialization of cellulose materials. Regenerated cellulose has high crystallinity and smoother surface morphology, not easy to generate static electricity, making it more ideal in clothes. The stretching property can greatly improve the crystallinity and orientation of fibers, no flutter, high thermal stability(Li J et al., 2021; Medronho B and B Lindman, 2015). Zhu et al. used sol-gel technology to prepare regenerated cellulose membranes, and the mechanical properties of regenerated cellulose membranes are significantly improved due to the formation of H-bonds(Zhu Q et al., 2013). Hauru et al. measured the threshold of cellulose regeneration by water using nephelometry and rheometry and found that regeneration from wet ILs was asymmetric compared to dissolution into wet ILs(Hauru L K J et al., 2012). At present, most of the research on cellulose is focused on the yield and properties of cellulose regeneration. However, the microcosmic mechanism of cellulose regeneration, the interactions between ILs and anti-solvents, and the roles of anti-solvent in cellulose regeneration have not been revealed yet.
With the rapid development of computer technology, computational modeling methods such as MD, ab-initio, and density functional theory (DFT) calculations have been successfully applied to the transformation mechanism of lignocellulose in ionic liquids(Gupta K M and J Jiang, 2015; Ju Z et al., 2020a; Zhang Y et al., 2019). Payal et al investigated the structures and dissolution mechanism for cellobiose and xylan in ILs by DFT calculations in the gas phase, implicit and explicit solvent, and proposed inter/intramolecular H-bonds play an important role in the dissolution(Payal R S et al., 2012). Zhao et al. reported the effect of cosolvents on the solubility of cellulose in imidazolium-based ILs systems by influencing H-bond interactions using MD and quantum chemistry calculations(Zhao Y et al., 2013). Liu et al. reported binary and ternary mixtures of 1-ethyl-3-methylimidazolium acetate with water and a cellulose oligomer by MD simulations and they proposed the introduction of water changes the structural organization of ILs and disrupts the interactions between ILs and cellulose(Liu H et al., 2011). The regeneration mechanism of cellulose in cellulose/IL mixture has important theoretical guiding significance for the selection of suitable anti-solvents. As one kind of efficient IL for cellulose dissolution and regeneration, 1-butyl-3-methylimidazolium acetate (BmimOAc) has been successfully used in the utilization of lignocellulose(Andanson J-M et al., 2014; Xu A et al., 2015). However, the regeneration mechanism of cellulose in the BminOAc and water mixed system remains ambiguous.
In this work, as a common IL in the dissolution of cellulose, BmimOAc is used to study the effects of water on the regeneration of cellulose. Due to the limited computational capacity by using DFT calculations, cellulose is modeled by cellobiose which consists of glucopyranose units by covalent β-1,4-glycosidic bonds.(Cao B et al., 2016; Yao Y et al., 2015) We mainly study the roles of water in the BmimOAc and cellulose-ILs system. A series of BmimOAc-nH2O (0 ≤ n ≤ 6) clusters were optimized to reveal the roles of water in ILs. The geometries, interaction energies, and H-bonds of ILs/water clusters had been analyzed. To further reveal the regeneration mechanism of cellulose, a series of cellobiose-OAc-nH2O and cellobiose-Bmim-nH2O (0 ≤ n ≤ 6) complexes were analyzed by the geometries, interaction energies. In addition, the quantum theory of atom in molecules (AIM) and Independent Gradient Model (IGM) analysis were used to distinguish the bonding properties of the cellobiose-ILs-nH2O configurations. The regeneration mechanism of cellulose in BmimOAc/water mixtures was preliminarily proposed by DFT calculations, and the microscopic mechanism of cellulose regeneration can provide a certain theoretical basis for the utilization of cellulose.