Highlights
Ionic liquid pretreatment increased significantly the biomass digestibility.
Cellulose I structure was significantly changed to amorphous structure.
Conversion to glucose was higher than 90% and the glucose yield about 78%.
Ionic liquid pretreatment showed good potential for lignocellulosic biomass.
Abstract
In this study, mixed softwoods were pretreated with an ionic liquid, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), and the bioconversion efficiencies to fermentable sugars were estimated through the enzymatic hydrolysis. The cellulose crystallinity, surface morphology, structures and compositions of softwood were significantly changed after the ionic liquid pretreatment was carried out under a wide range of temperatures and reaction times. And, biomass digestibility significantly increased with increasing pretreatment temperature and reaction time. The enzymatic degradation of pretreated softwoods was remarkably improved at the pretreatment of high temperatures via the modification of crystalline cellulose I to a mixture of easily digestable cellulose II and amorphous structure, and partial removal of hemicellulose. The conversion of cellulose to glucose reached more than 90% at relevant conditions and the highest glucose yield was measured to about 78%. Through the study, it was clearly shown that ionic liquid pretreatment is one of the effective methods to produce high fermentable sugars without lignin dissolution from lignocellulosic 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.
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