Highlights
Adding water to IL reduces the viscosity and facilitates high biomass loading.
Acidified [Bmim]Cl pretreatment efficiently removed hemicellulose.
Cellulose conversion of pretreated biomass was 93.56–99.81%.
Fermentation produced 21.86–29.56 g/L ethanol and ethanol yield was 0.49–0.51 g/g.
Produced bioethanol was concentrated up to 79.8% by consecutive pervaporation.
Abstract
Acidified [Bmim]Cl solution showed a great efficacy for the pretreatment of softwood, hardwood, rice straw, and sugarcane bagasse. The pretreatment resulted in 29 and 20-fold increase in cellulose digestibility of hardwood and softwood, respectively, and most cellulose in rice straw and sugarcane bagasse was converted into glucose. Fermentation of biomass hydrolysates produced 21.86–29.56 g/L ethanol, thereby achieving ethanol yields of 0.49–0.51 g/g. Ethanol was concentrated up to 79.8% from a fermentation broth containing 2.4% ethanol using four consecutive pervaporations. This work demonstrates the viability of biofuel production from diverse feedstocks and the feasibility of ethanol recovery using pervaporative separation.
Introduction
Bioethanol has long been considered as the most promising biofuel in response to dwindling petroleum resources and raising environmental concerns. Global ethanol production is estimated to increase during the outlook period from about 114 billion liters in 2014 to nearly 134.5 billion liters by 2024 [1]. Lignocellulosic biomass is the most relevant feedstock for bioethanol production because it is abundantly available, renewable, and inexpensive. Transition from using edible feedstocks to lignocellulosic biomass for bioethanol is being a significant trend in order to reduce the production cost and avoid the competition with global food and feed supplies [2]. Agricultural activities dispose of the most abundant lignocellulosic biomass, such as rice straw, wheat straw, corn straw, and sugarcane bagasse [3]. Forestry harvesting and processing also generate a large amount of lignocellulosic waste materials, such as wood chip and sawdust. The advantages of woody biomass compared with agricultural residues relate to the harvesting, storage, and transportation. Woody material has higher density, thus its transport cost is lower than agricultural biomass [4]. However, woody biomass, which is structurally denser and has higher lignin content than agricultural feedstock, is highly recalcitrant to enzymatic accessibility [5].
Ionic liquids (ILs) have been the focus of research on pretreatment of lignocellulosic biomass because of their interesting properties, such as chemical and thermal stability, non-flammability, and the ability to dissolve a wide range of materials [6], [7]. It was reported that ILs can effectively dissolve the plant cell walls and/or selectively remove hemicellulose and lignin under mild reaction conditions in many biomass-involved applications [8]. In addition, ILs disrupt the hydrogen bonding network in cellulose: their anions and cations interact with the hydroxyl protons and oxygen in cellulose, respectively, thereby resulting in the dissolution of cellulose [9], [10], [11]. ILs are being classed as “green solvents” because most of them have low vapor pressure at ambient temperatures and hence do not emit volatile organic compounds as conventional solvents. In addition, they can be designed to have high thermal stability, high conductivity, and low toxicity by selecting an appropriate cation and anion combination [12]. ILs can also be recycled and reused for several times in the biomass processing [13]. The use of ILs for bioethanol production could meet the main goals of green chemistry regarding the prevention of wastes and reduction of environmental pollutions and human health risks [14].
However, the high cost and viscous behavior of ILs limit their potential applications in biomass pretreatment, which has been conducted at low biomass loadings of 3–5% in the previous studies [5], [15]. The viscosity of ILs can be reduced by adding an appropriate amount of water during pretreatment, which allows for high biomass loading and reduced IL usage [16]. A previous study pretreated straw using aqueous 1-ethyl-3-methylimidazolium acetate ([Emim]Ac) containing up to 50% water, which resulted in a higher fermentable sugar yield (81%) than was obtained with pretreatment using pure IL (67%) [17]. Another study achieved the highest saccharification yield with a pretreatment using 1-butyl-3-methylimidazolium methyl sulfate solution containing 10–40% water [18].
Because bioethanol is produced at a low concentration in a fermentation broth, it must be separated and concentrated in the downstream process. Distillation is currently used to separate and purify bioethanol, but this method has high energy requirements and is not very efficient, particularly for low ethanol concentrations. However, the process of pervaporation is an extremely promising approach used for the recovery of alcohols produced during the fermentation process [19], [20], [21]. In the pervaporation process, molecules with a higher affinity and ability to diffuse through a membrane are preferentially separated from the feed mixture [22], [23]. The performance parameters of pervaporation, such as separation factor and flux, depend on the selective interaction between specific components in the feed mixture and on the properties of the membrane materials. The most widely used polymeric membrane for alcohol separation consists of silicone rubber-based polydimethylsiloxane (PDMS) because silicone-containing polymers have been reported to exhibit good organophilicity for separating organic aqueous mixtures [24], [25].
In this study, we developed an effective pretreatment process using 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) solution containing a suitable amount of water and an acid catalyst. Acids are able to hydrolyze hemicellulose fraction in lignocellulosic biomass, thereby exposing cellulose core and leading an improvement in enzymatic conversion of cellulose to glucose [26]. The process was performed under mild conditions and elevated biomass loadings, and we examined the effects of acid types and concentrations, duration times, water contents, and biomass loadings on glucose production. The pretreatment process was applied for various feedstocks including softwood, hardwood, rice straw, and sugarcane bagasse. The pretreated materials were then subjected to separate hydrolysis and fermentation to produce bioethanol. Separation and concentration of bioethanol were subsequently conducted using consecutive pervaporation with a composite hollow-fiber membrane module. Permeation performance was observed during pervaporative separation of bioethanol produced by the separate hydrolysis and fermentation of softwood pretreated with the aqueous acidified IL. This work shows the possibility of recovering and producing bioethanol using a pervaporation technique that involves the efficient pretreatment of lignocellulosic biomass.
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