Effects of dissolution of some lignocellulosic materials with ionic liquids as green solvents on mechanical and physical properties of composite films
Highlights
► The present study realized an environmentally friendly method of cleanly extracting cellulose from woody biomass. ► Cellulose can greatly enhance the flexibility of films especially at low lignin contents. ► The treatment with [DiMIM][MeSO4] had no obvious effect on the solubility of the used materials. ► The solubilization efficiency of woody materials in ILs was found in an order of CEL > CMP ≥ PW. ► CEL film showed superior physical and mechanical properties compared with PW and CMP films.
Introduction
The use of lignocellulosic biomass based materials in various sectors (e.g. automotive and aerospace) instead of petro-materials has received increased attentions due to the growing global environmental awareness, and concepts of sustainability and industrial ecology (Hamzeh et al., 2011, Moniruzzaman and Ono, 2012). Wood is a renewable source for cellulose, lignin, hemicelluloses and extractives (Mäki-Arvela, Anugwom, Virtanen, Sjöholm, & Mikkola, 2010). A lignocellulosic material contains 35–50% cellulose, up to 35% hemicelluloses, 5–30% lignin as well as a few percent of extractives (Lynd, Weimer, Van Zyl, & Pretorius, 2002). Cellulose is the most abundant biopolymer on the Earth and, thus, it is a valuable source of raw materials (Mäki-Arvela et al., 2010). It is formed via 1,4-β-d-glucose linkage of anhydroglucose units, and contains several inter- and intra-molecular hydrogen bonds (Zhang, Wu, Zhang, & He, 2005), which should be broken during the dissolution of cellulose. Furthermore, native cellulose is considered as a rigid semi-crystalline material (Pinkert, Marsh, Pang, & Staiger, 2009). Lignin is a complex polymer composed of phenylpropanoid units consisting primarily of coniferyl, sinapyl and p-coumaryl alcohols. These components are assembled in a complex three-dimensional structure, remarkably resistant against chemicals and microbial attack that makes it very difficult to hydrolyze. The insolubility of wood in common solvents has severely hampered the development of new methods for the efficient utilization of wood and its components (Kilpeläinen et al., 2007). An effective dissociation of these components and their subsequent separation is needed for the production of high value products from lignocellulosic biomass (Hamzeh, Ashori, Khorasani, Abdulkani, & Abyaz, 2013). On the other side, if the biopolymers could be cleanly and easily separated from any lignocellulosic biomass source, they could serve as ready feedstock for not only polymeric composite materials, but also for base chemicals and fuels that are now obtained primarily from oil. The true biorefinery would not be based on production of only a fuel, but a rich, nearly limitless variety of chemicals (Fig. 1).
Traditionally, drastic conditions, such as strong base or mineral acids, are used to exploit the lignocellulosic materials. In these processes, the extracted fractions are severely and irreversibly altered, and environmental pollution is also of great concern. Moreover, conventional processes cannot make full use of non-cellulose biopolymers, i.e. lignin and hemicelluloses. In order to overcome these obstacles, some mild processes and pretreatment are proposed, e.g. enzymatic hydrolysis, steam explosion, organic solvent, and ammonia fiber explosion (Öhgren et al., 2007, Yang et al., 2013). However, this issue is still pending due to the high crystallinity of cellulose, relatively high reactivity of carbohydrates, insolubility of cellulose in conventional solvents and heterogeneity of lignocelluloses (Remsing, Swatloski, Rogers, & Moyna, 2006). Therefore, new technologies should be developed for the green bio-refinery of biomass resources.
In 2002, Swatloski et al. were the first to demonstrate that ionic liquids (ILs) are able to dissolve cellulose. However, in recent years the dissolution of lignocellulosic biomass in ILs has drawn a great deal of attention. ILs are organic salts entirely composed of cations (usually organic) and anions (usually inorganic) with generally low melting temperatures (≤100 °C). They are green solvents, which are characterized by negligible vapor pressure as well as excellent thermal and chemical stability. The use of ILs as solvents has increased recently as a result of a globally growing interest toward green chemistry. Their unique properties make them attractive alternatives to conventional organic solvents in many chemical processes. Their use can provide a concrete solution to the problem of formation of volatile organic carbons (VOCs) in conventional solvents, which are detrimental to the environment (Anderson, Ding, Welton, & Armstrong, 2002). Many factors, e.g. species of biomass, particle size, water content, affect the dissolution of lignocellulosic materials in ILs (Li et al., 2010, Sun, 2010). In addition, when the dissolving temperature is higher than the glass transition of lignin, the dissolution and fractionation of lignocelluloses can be more efficient (Li, Wang, & Zhao, 2008). The ILs have attracted increasing interest as media for dissolving cellulose from lignocellulosic biomass in the pretreatment step (Fukaya et al., 2008, Zavrel et al., 2009). The dissolved cellulose in IL can be easily regenerated with the addition of an anti-solvent, such as water, ethanol or acetone (Zhu et al., 2006). As reported in the literature, apart from extracting cellulose from the primitive resources, regenerated cellulose exhibits significantly reduced crystallinity and increased porosity, which enhance the digestibility of the material and subsequently result in a higher yield for the overall conversion process (Li et al., 2011).
The aim of this work was to explore the influences of IL treatments on the fractionation process under mild conditions. The ILs [BMIM]Cl and [DiMIM][MeSO4], which have been proved to be good solvents for lignocellulosic materials (Huddleston et al., 2001, Zhang et al., 2008), were used to directly dissolve the ball milling fibrous materials. Consequently, the resulting mixtures were used to make composite films, where they were characterized in terms of physical and mechanical properties.
Section snippets
Raw materials and chemicals
The lignocellulosic materials (LMs) used for this study were poplar wood (Popolus deltiodes) (PW), chemi-mechanical pulp (CMP) of poplar, and cotton linter (CEL) containing 99% α-cellulose. The air-dried wood and pulp samples were ground in a 1 gal porcelain jar (rotary ball mill) using alumina balls under a nitrogen atmosphere. Consequently, they were oven-dried at 105 °C for 8 h and kept inside a sealed plastic bag. All the above-mentioned materials were procured from the local market.
Two
Dissolution
The dissolution of PW, CMP and CEL materials in ILs was studied. The treatment with [DiMIM][MeSO4] had no obvious effect on the solubility of the PW films. The wood chips swelled shortly after they were added to the [BMIM]Cl solvent. After heating for 2–3 h at 90 °C under vigorous mechanical stirring, color of the solutions turned dark and their viscosities distinctively increased, indicating that the dissolution of wood had occurred. A clear, dark brown solution was obtained after filtration,
Conclusions
The present study not only realized an environmentally friendly method of cleanly extracting cellulose from woody materials under mild conditions, but also provided a new approach for utilizing biomass resources. Some conclusions of IL treatment are as follows:
- (a)
Different types of woody materials result in various dissolution profiles in ILs. In this work, CEL was found to be more suitable for dissolution and regeneration in both of the ILs used than the other two types of lignocellulosic
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2020, Journal of Molecular LiquidsCitation Excerpt :Certain ionic liquids have also been shown to be capable of selectively or directly dissolving particular biopolymers. On this basis one may distinguish ILs which dissolve cellulose well [6,10,12,50,70,72,73,188,192–198]. In further studies by Rogers et al. it was determined that ionic liquids can also act as modifiers of cellulose [6,199].