Separation of polysaccharides from rice husk and wheat bran using solvent system consisting of BMIMOAc and DMI
Introduction
Biorefinery, which produces value-added fuels, chemicals, and materials from renewable and non-edible lignocellulosic biomass materials, has attracted much attention in recent years. Lignocellulosic biomass is a cheap and abundant natural resource mainly composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are promising starting materials for platform chemicals and biofuels, while lignin is a potential raw material for the production of aromatic compounds (Tuck, Pérez, Horváth, Sheldon, & Poliakoff, 2012). However, it is difficult to fractionate natural lignocellulosic biomass because of the three-dimensional cross-linked lignin network and strong hydrogen bonds within the polymer matrix (Lee, Doherty, Linhardt, & Dordick, 2009). Currently, Kraft pulping is the most dominant pretreatment technology to separate cellulose from lignocellulosic biomass (Tan et al., 2009). However, this process has resulted in serious environmental pollution (Andanson et al., 2014). Although numerous approaches, including organosolv pulping, steam explosion and ammonia fiber explosion have been developed, they are not as effective as the conventional method in view of economics. Besides, these methods do not meet the principle of green chemistry (Pinkert, Goeke, Marsh, & Pang, 2011). Thus, there is an urgent need to develop environmental-friendly solvents and process technologies to fractionate the major components of lignocellulosic biomass.
Recent years, ionic liquids (ILs) have been shown to be excellent solvents to process lignocellulosic biomass (Sun, Rodriguez, Rahman, & Rogers, 2011). Numerous ILs have been reported to dissolve high amounts of cellulose (Zavrel, Bross, Funke, Buechs, & Spiess, 2009). However, there are several disadvantages associated with the pretreatment of biomass using ILs, such as the high cost of ILs, the high viscosity of the solutions obtained, and the slow rate of dissolution (Rinaldi, 2011, Wu et al., 2013). One way to solve these problems is adding co-solvents to ILs. It has been reported that a class of solvent systems consisting of ILs and co-solvents can dissolve large amounts of cellulose instantaneously at room temperature (Rinaldi, 2011). In addition, a novel solvent system consisting of 1-butyl-3-methylimidazolium acetate (BMIMOAc) and DMSO was found to readily dissolve cellulose at room temperature (Xu, Zhang, Zhao, & Wang, 2013). These findings open up new horizons in the design and applications of cellulose solvents.
Based on the solvation capability of ILs, some methods have been developed to directly separate biopolymers such as cellulose, lignin, and chitin from raw biomass. It has been demonstrated that 1-butyl-3-methylimidazo-lium chloride (BMIMCl) and 1-allyl-3-methylimidazolium chloride (AMIMCl) are capable to separate cellulose from biomass (Fort et al., 2007; Wang, Li, Cao, & Tang, 2011). In these processes DMSO was used as co-solvent to reduce the viscosity of ILs. It has also been reported that the solvent system consisting of ILs and DMSO outperform pure ILs in the pretreatment of biomass (Pinkert et al., 2011, Wu et al., 2013). These results suggest that co-solvents may facilitate the direct extraction of cellulose from biomass. However, DMSO is a toxic organic solvent, which could cause serious environmental problems. One possible solution for mitigating the undesirable impact of DMSO is substituting DMSO with more green co-solvents, such as 1,3-dimethyl-2-imidazolidinone (DMI). Compared with DMSO, DMI has a lower viscosity (1.944 Pa s vs. 1.996 Pa s), higher boiling point (225 °C vs. 189 °C) and lower toxicity (Rinaldi, 2011).
In the present study, solvent systems consisting of DMI, and ionic liquid 1-butyl-3-methylimidazolium acetate (BMIMOAc) were employed to separate polysaccharides from rice husk and wheat bran, which are representative agricultural biomass waste. As the co-product of agricultural production, rice husk and wheat bran are more promising resource because they are more collectable than crops stalk and cheaper than grain. The effects of DMI load, dissolution time, and temperature on the dissolution of rice husk and wheat bran were investigated. The reuse of the solvent systems was also investigated.
Section snippets
Materials
Rice husk and wheat bran, supplied by Tianjin Agricultural University, were milled using a mini Mill with a 100-mesh screen (Model FZ102, Tianjin, China). The milled biomass was then extracted with benzene/ethanol (2:1 v/v) according to the standard NREL procedures, to produce extractive-free biomass sample (Sluiter, Ruiz, Scarlata, Sluiter, & Templeton, 2005). The extractive-free biomass was finally dried overnight in an oven at 105 °C before using. Microcrystalline cellulose with a degree of
Effect of co-solvent on the viscosity of the solvent system
Previous studies have shown the effectiveness of co-solvents in modifying the physicochemical properties of ionic liquids, including viscosity, polarity, and dissolution capability (Fort et al., 2007). For example, the addition of co-solvent DMSO to EMIMOAc remarkably reduced the viscosity of the mixtures (Wu et al., 2013). In the present study, DMI was employed to substitute DMSO as co-solvent. As shown in Fig. 1, DMI dramatically decreases the viscosity of the solvent systems. It was also
Conclusion
The present study reports the separation of polysaccharides from rice husk and wheat bran using the solvent system based on the IL BMIMOAc and co-solvent DMI. It was demonstrated that the solvent system could partially dissolve rice husk and wheat bran, and that the regenerated materials are mainly consisting of polysaccharides. The co-solvent DMI could not only reduce the viscosity of ILs, but could also promote the dissolution of the biomass and separation of polysaccharides. Future studies
Acknowledgements
This work was supported by the Key Projects in the Science & Technology Pillar Program of Tianjin, China (12ZCZDSF01700), International Joint Research Projects in the Science & Technology Pillar Program of Tianjin, China (13RCGFSF14300), and Projects in the Science & Technology Pillar Program of Inner Mongolia, China (12JH031400).
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