Research PaperCharacterization of ionic liquid pretreatment and the bioconversion of pretreated mixed softwood biomass
Introduction
Bioethanol, a renewable biofuel to replace fossil fuels, has been approved to reduce air pollution and greenhouse gas emissions. It has produced from starch-containing sources such as sugar cane, corn, cassava, which subsequently affects food security worldwide [1], [2]. As preferentially sustainable sources to counteract the challenge of bioethanol production, lignocellulosic biomass from woods and agricultural residues have been considered for the bioethanol production. Compared with agricultural biomass, woody biomass has several advantages such as higher density property and lower pentose content in particular softwood species, which significantly lower transportation cost and facilitate bioconversion [3] However, woody biomass is structurally stronger and has higher lignin content than agricultural biomass, which is strongly recalcitrant to microbes and enzymatic accessibility.
Until now, many pretreatment technologies have been developed to overcome the recalcitrance of woody biomass and subsequently to improve biomass digestibility, such as acid, alkali, steam explosion, organosolv, biological pretreatment, and others. Recently, ionic liquids (ILs), which are regarded as green solvents, have attracted much attention due to their interesting properties such as low vapor pressure, chemical and thermal stability, non-flammability, and phase behavior [4], [5]. Furthermore, ILs can be easily prepared using different cations and anions, resulting in hydrophobic or hydrophilic types [6], [7], [8]. Ionic liquids have been utilized in the dissolution of both softwood and hardwood [9], depolymerization of hemicelluloses for yellow pine [10], and also delignification of eucalyptus, switchgrass and bagasse [11], [12].
IL is accepted as a promising solvent to disrupt the inter- and intra-molecular hydrogen bonds in the native cellulose crystalline structure, breaking the major chemical linkages in matrix polymers of various biomass feedstocks [13], [14], [15]. It is known that IL pretreatment typically decreases cellulose crystallinity, partial removal of lignin and hemicellulose or increase specific surface area, thus enhancing the digestibility and the fermentability of sugars at somewhat low temperatures in the room conditions [11], [16], [17], [18].
The previous results showed that pretreatment temperature and time are crucial factors, which intensively affect saccharification performance [19], [20], [21]. Therefore, the selection of the suitable operation conditions in the biomass pretreatment process is necessary to increase the pretreatment efficiency and minimize the energy consumption [21], [22], [23].
As far as we know, only a few studies were reported for ionic liquid pretreatment of woody biomass, which has tough and strong physical structure and high degree of lignifications [3], [9]. It is of importance to characterize the pretreated biomass properties of crystalline structure and chemical features since the pretreatment behaviors are different according to different pretreatment conditions in ionic liquid pretreatment. In this study, mixed softwoods were pretreated with an ionic liquid, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), under a wide range of temperatures and reaction times. Pretreated biomass was characterized in terms of the crystalline cellulose, surface morphology, structural and chemical features using proper methods. Moreover, the enzymatic hydrolysis experiments were performed to estimate the effect of ionic liquid pretreatment on the bioconversion efficiency.
Section snippets
Materials
Mixed softwood (Pinus rigida and Pinus densiflora) chips were purchased from Poong Lim Inc (Daejeon, Korea) and ground into powder (40–60 mesh). Ionic liquid, (1-butyl-3-methylimidazolium chloride, [Bmim]Cl) (>95% purity), was purchased from Sigma–Aldrich. For enzyme hydrolysis, cellulase (Celluclast 1.5L), β-glucosidase (NS 50013) and xylanase (NS 22036) were kindly provided by Novozyme.
Pretreatment of softwood by ionic liquid
Pretreatment was conducted by dissolution of about 0.2 g of mixed softwood (dry weight basis) with 4 g
Characterizations of ionic liquid pretreated softwoods
After [Bmim]Cl pretreatment, changes in the crystalline cellulose of pretreated biomass were observed through XRD analyses and the results were shown in Fig. 1. In the untreated softwood, two sharp peaks were found at angles of 15° and 22°, corresponding to (101) and (002) lattice planes of crystalline cellulose I polymorph, respectively [16], [27]. As shown in the figure, a decrease in intensity of two main peaks was clearly observed in the XRD patterns of pretreated softwood biomass. It is
Conclusion
In this study, softwood biomass was pretreated with an ionic liquid, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), and enzymatic saccharification was carried out to investigate the influence of temperature and time on the pretreatment performance. Softwood pretreatment with [Bmim]Cl at high temperatures significantly changed the crystalline cellulose, surface morphology, and compositions, resulting in extensive improvements to the digestibility of regenerated biomass. It was observed that
Acknowledgment
This study was carried out with the support of ‘Forest Science and Technology Projects (Project No. S121313L030120)’ provided by Korea Forest Service.
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2021, Process BiochemistryCitation Excerpt :And the disolved cellulose is subsequently precipitated with an anti-solvent, such as acetone and water [15,16], for further enzymatic hydrolysis. During this process, lignin and hemicellulose in the biomass are partially removed; the crystallinity of cellulose is significantly decreased; and the surface area of the RS residue is increased [17–19]. To realize the dissolution of biomass and production of the slurries with rheological properties [20], considerable amount of ILs has to be used.