FeCl₃-catalyzed ethanol pretreatment of sugarcane bagasse boosts sugar yields with low enzyme loadings and short hydrolysis time

Highlights

• FeCl₃-catalyzed ethanol pretreatment was developed to generated highly digestible solids.

• FeCl₃-catalyzed ethanol pretreatment selectively extract up to ∼100% of the hemicellulose.

• The Tween 80 additive presented more efficient enhancement than other additives.

• Tween 80 showed great potential for reducing cellulase dosage and shorten hydrolysis time.

Abstract

An organosolv pretreatment system consisting of 60% ethanol and 0.025 mol·L⁻¹ FeCl₃ under various temperatures was developed in this study. During the pretreatment, the highest xylose yield was 11.4 g/100 g raw material, representing 49.8% of xylose in sugarcane bagasse. Structural features of raw material and pretreated substrates were characterized to better understand how hemicellulose removal and delignification affected subsequent enzymatic hydrolysis. The 160 °C pretreated solid presented a remarkable glucose yield of 93.8% for 72 h. Furthermore, the influence of different additives on the enzymatic hydrolysis of pretreated solid was investigated. The results indicated that the addition of Tween 80 shortened hydrolysis time to 6 h and allowed a 50% reduction of enzyme loading to achieve the same level of glucose yield. This work suggested that FeCl₃-catalyzed organosolv pretreatment could improve the enzymatic hydrolysis significantly and reduce the hydrolysis time and enzyme dosage with the addition of Tween 80.

Introduction

The current high depletion of fossil fuels and environment concerns had significantly facilitated the bio-refinery of lignocellulosic biomass. Lignocellulosic biomass, consisting of cellulose, hemicellulose, and lignin, could be used to produce value-added products, such as energy, chemicals, and fuels (Khazraie et al., 2017, Lancefield et al., 2017). Sugarcane bagasse, as one of the most abundant lignocellulosic biomass in southern China, was usually discarded away, burned up or used for the production of pulp, which was unsustainable and unfriendly to the environment (Biswas et al., 2014, Batalha et al., 2015, Tang et al., 2017). Therefore, a high-efficiency bio-refinery of sugarcane bagasse producing renewable products was required. Currently, the bio-refinery producing cellulosic ethanol from lignocellulosic biomass received a lot of attention. However, the intact structure of lignocellulosic biomass impeded the efficiency of enzymatic hydrolysis. Hence, a pretreatment was needed to destroy the recalcitrant structure (Khazraie et al., 2017, Bansal et al., 2016, Meng et al., 2015).

Recently, various pretreatment methods had been proposed, such as using liquid hot water, dilute acid, alkali, ammonia, ionic liquid, and organosolv strategies (Pu et al., 2013, Zhu et al., 2015, Chen et al., 2013, Jin et al., 2016a, Shafiei et al., 2013, Amiri et al., 2014). Among them, ethanol-based organosolv process is favored by its advantages, such as recycling, low toxicity, and yielding digestible pretreated solid (Zhang et al., 2016). Generally, the ethanol-based organosolv pretreatment was operated under the acidic condition provided by the degradation of acetyl group in hemicellulose to acetic acid or by adding a strong acid catalyst (Chen et al., 2015, Wildschut et al., 2013). The weak acidic condition generated by the former was not enough for the degradation of lignin and hemicellulose and also required a high pretreatment temperature to obtain digestible substrate (Zhang and Wu, 2014, Wei et al., 2017). The harshly acidic condition provided by the latter could accomplish the intensive removal of hemicellulose and lignin, but also lead to the degradation of cellulose or generated xylose to furfural (Wildschut et al., 2013). To overcome these problems, ethanol-based organosolv pretreatment conducted using Lewis acids had been proposed to degrade hemicellulose and lignin, retain cellulose, and improve subsequent enzymatic hydrolysis of substrate (Schwiderski et al., 2014, Constant et al., 2016).

Constant et al. investigated organosolv pretreatment of wheat straw catalyzed by different Lewis acids on the structural and chemical properties of lignin, suggesting that the harder Lewis acids lead to a greater degree of delignification (Constant et al., 2015). The catalytic influence of MgCl₂- or AlCl₃-assisted organosolv pretreatment of various lignocellulosic biomass on the production of lignin and degradation of hemicellulose were also determined (Huijgen et al., 2011, Wang et al., 2016, Schwiderski et al., 2014). Most of these literatures focused on the influence of Lewis acid on the properties of organosolv lignin or hemicellulose degradation, paid less attention to determining the enzymatic hydrolysis of pretreated substrate (Constant et al., 2015). Hence, a pretreatment using Lewis acids catalyzed ethanol pretreatment was proposed in this study to investigate the influence of it on the delignification, hemicellulose removal, and the digestibility of pretreated solids. As the commonly used Lewis acids, FeCl₃ was involved in various biomass conversion processes due to its unique properties such as low-cost, nontoxicity, and abundance (Zhang et al., 2017). Hence, it is necessary and interesting to investigate the FeCl₃-catalyzed ethanol pretreatment on the bioconversion of sugarcane bagasse.

As mentioned above, the digestibility of enzymatic hydrolysis could be enhanced by using various pretreatment systems (Pihlajaniemi et al., 2016). Clearly, the loading of enzyme played an important role in affecting the enzymatic hydrolysis. However, high cost of enzyme has prompted research to explore techniques that can activate the enzyme sufficiently or decrease the enzyme loading while maintaining the same level of enzymatic digestibility. It is reported that the addition of additives such as surfactant or sulfonated lignin could enhance the efficiency of enzymatic hydrolysis of pretreated substrates by increasing cellulase activity and stability, reducing unproductive enzyme binding, or intensifying positive interactions between substrate and enzyme (Kumar et al., 2012, Harrison et al., 2014, Zhang et al., 2017). Alkasrawi et al. found that with the addition of Tween 20, the amount of enzyme loading could be reduced by 50% during SSF process without a reduction in ethanol yield (Alkasrawi et al., 2003).

Though previous reports presented the positive influence on the enzymatic hydrolysis and decreasing enzyme loading with addition of additives, systematic studies accounting for the additives with decreased enzyme loading and hydrolysis time at the same time were scarce. In this study, a FeCl₃-catalyzed ethanol pretreatment consisting of 60% ethanol and 0.025 mol·L⁻¹ FeCl₃ was proposed. The influence of pretreatment temperatures (140–190 °C) on the degradation of hemicellulose and lignin, and the subsequent enzymatic hydrolysis was investigated. Structural features of raw material and pretreated substrates were characterized by XRD, SEM, and TG analysis. Further experiments were carried out to determine the influence of different additives (BSA, Tween 80, organosolv lignin, sulfonated lignin, and PEG 4000) on the enzymatic hydrolysis of pretreated substrate with different hydrolysis time. In addition, the enzymatic hydrolysis of pretreated substrate in the presence of different loadings of Tween 80 and enzyme were also determined.