Separation of polysaccharides from rice husk and wheat bran using solvent system consisting of BMIMOAc and DMI

Highlights

• Adding aprotic polar solvent to ionic liquid can promote the dissolution of biomass.

• The extraction rate of polysaccharides from wheat bran can reach as high as 71.8%.

• The regenerated material has a polysaccharides content of 94.4%.

• The recycled solvent system exhibits constant ability to separate cellulose.

Abstract

A solvent system consisting of 1,3-dimethyl-2-imidazolidinone (DMI), and ionic liquid 1-butyl-3-methylimidazolium acetate (BMIMOAc) was used to separate polysaccharides from rice husk and wheat bran. The effects of the DMI/BMIMOAc ratios, temperature, and time on the dissolution of rice husk and wheat bran were investigated, and the influence of anti-solvents on the regeneration of polysaccharides-rich material was evaluated. We found that the solvent system is more powerful to dissolve rice husk and wheat bran than pure BMIMOAc, and that polysaccharides-rich material can be effectively separated from the biomass solution. The polysaccharides content of regenerated material from wheat bran can reach as high as 94.4% when ethanol was used as anti-solvents. Under optimized conditions, the extraction rate of polysaccharides for wheat bran can reach as high as 71.8% at merely 50 °C. The recycled solvent system exhibited constant ability to separate polysaccharides from rice husk and wheat bran.

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.