Elsevier

Bioresource Technology

Volume 238, August 2017, Pages 139-146
Bioresource Technology

Significantly enhanced enzymatic hydrolysis of rice straw via a high-performance two-stage deep eutectic solvents synergistic pretreatment

https://doi.org/10.1016/j.biortech.2017.04.027Get rights and content

Highlights

  • Enhanced enzymatic digestion of biomass after two DESs treatment in specific order.

  • A good balance of components loss achieved by the synergistic action of the two DESs.

  • Water presence facilitated the sugar recovery by tuning the severity.

  • Significant xylan removal and degradation during CO participated processes.

  • A simple and mild pretreatment process was established with a glucose yield of 90.2%.

Abstract

A two-stage deep eutectic solvents (DESs) treatment was shown to be an effective method for improving the utilization of certain DESs, and the specific order of pretreatment, such as malic acid/proline (MP) or choline chloride/oxalic acid (CO) during the first stage and choline chloride/urea (CU) during the second stage, resulted in better performance for enhancing the sugar yield due to the synergistic effect of the two DESs on biomass fractionation. Moreover, the presence of water during these processes could balance the loss of components by tuning the pretreatment severity, thus ensuring higher sugar yields. By eliminating the washing step after the first stage treatment, enhanced cellulose recovery and glucose yield were achieved for the CO-CU pretreatment in the presence of 5% water, and a simpler process was established with a glucose yield of 90.2% after a 3-h treatment at 100 °C.

Introduction

Lignocellulosic biomass is considered as a promising, renewable and low-cost energy source for reducing the dependence of humans on conventional fossil fuels (Binod et al., 2010, Procentese et al., 2016). However, lignocellulosic biomass is highly recalcitrant to chemical and biological degradation due to its complex heteromatrix structure in which the cellulose is highly crystalline, the hemicellulose circulates around the cellulose, and the lignin occupies the remaining space and prevents exposure of the polysaccharides to water, other chemicals or biocatalysts (Zhao et al., 2012). Thus, to enhance the conversion of lignocellulosic biomass to fermentable sugars or other high-value chemicals, pretreatment is always required (Agbor et al., 2011, Alvira et al., 2010). In the past decade, ionic liquids (ILs) have received considerable attention and have been extensively exploited for biomass pretreatment due to their excellent ability to destroy the crystalline structure of cellulose or remove lignin or hemicellulose, which significantly increases polysaccharide accessibility to enzymes and improves the subsequent enzymatic hydrolysis (Tadesse and Luque, 2011). However, the shortcomings of ILs, such as toxicity, poor biodegradability and high cost, restrict their use in industrial applications, even though efforts have been made to obtain new ILs that are greener and cheaper than the traditional ones (Hou et al., 2012, Pinkert et al., 2011). Recently, deep eutectic solvents (DESs), a class of interesting solvents, have been used as a sustainable alternative to ILs with similar properties but lower cost and high atom economy (Abbott et al., 2004, Zhang et al., 2012). Moreover, some DESs have been shown to be biocompatible towards biocatalysts, such as enzymes, and show promising potential for biocatalysis and biotransformation (Sheldon, 2016, Vigier et al., 2015). Furthermore, some DESs have also displayed the unique ability to dissolve biomass components and pretreat biomass, which is an application that is still in its infancy (Kumar et al., 2016, Procentese et al., 2015, Xu et al., 2016). Since Francisco et al., tested the solubility of cellulose, lignin and starch in DESs based on natural materials (Francisco et al., 2012), this novel solvent system has attracted increasing attention in the field of biomass pretreatment. The results from recent studies suggest that certain DESs have an excellent lignin extraction ability to enhance cellulose enzymatic hydrolysis, particularly for the acidic DESs, such as choline chloride (ChCl)/lactic acid. However, for DES-based biomass pretreatment, the number of DESs with excellent pretreatment capacity is limited, and no effort has been exerted to develop better uses of the mild DESs. Generally, organic-acid-based DESs, such as ChCl/lactic acid (Alvarez-Vasco et al., 2016, Kumar et al., 2016, Zhang et al., 2016) and ChCl/formic acid (Xu et al., 2016), perform well with acid/ChCl molar ratios of 2:1 or more for the processing of corncob, rice straw and wood. However, other DESs, such as weakly basic ChCl/urea (CU) and neutral ChCl/glycerol pretreatments, produce only moderate sugar yields (Procentese et al., 2015) or even similar saccharification efficiencies with the untreated sources (Xu et al., 2016). The combination between two methods was reported to achieve a higher cellulose content in the recovered solid fraction and a higher reducing sugar yield than those when employing single method processes (Chen et al., 2012, Qing et al., 2014), and pretreatments combining two methods, or two-stage chemical pretreatments, that always use dilute acid in the first stage have been shown capable of creating synergistic effects for high cellulose enzymatic degradability (Auxenfans et al., 2014, Kim et al., 2011). For example, the saccharification rate was 2-fold higher than that of the single pretreatment observed for the combination pretreatment with ILs and dilute acid in which the dilute acid selectively extracted hemicelluloses and the IL removed lignin (Auxenfans et al., 2014). Therefore, a combination pretreatment is highly likely to serve as a feasible strategy to exploit and reinforce the utilization of the defective DESs in biomass pretreatment.

Thus, in this work, three typical DESs, namely, CU, malic acid/proline (MP) and ChCl/oxalic acid (CO), were selected for rice straw pretreatment. According to the specific properties of these DESs, two-DESs combination pretreatments were used to explore the enhanced use of each DES and to ensure the maximal improvement in biomass fractionation, cellulose recovery and saccharification. In addition, to better understand the pretreatment mechanism, the composition change and hemicellulose degradation were thoroughly investigated. Finally, based on the combination pretreatment, a facile and efficient method was established to fully exploit DESs with different properties to achieve excellent cellulose recovery and glucose yield.

Section snippets

Reagents

Cellulase/xylanase from Trichoderma reesei, xylan from bench wood and Kraft lignin were purchased from Sigma-Aldrich (St. Louis, MO) and were used as received. Microcrystalline cellulose (MCC) was purchased from FMC (Philadelphia, PA). Rice straw, obtained locally, was mechanically powdered to particle sizes of 250–400 μm. The other chemicals used were of the highest purity commercially available.

DES preparation

ChCl, oxalic acid, malic acid and proline were dried under vacuum at 80 °C for 5 h before use. Three

Solubility of MCC, xylan and lignin in DESs

Three typical DESs were selected to test their capability to dissolve the main biomass components, namely, cellulose, xylan and lignin (Table 1). The solubility test was performed by the consecutive addition of a small amount of the samples to the solvents until the particulates did not disappear within 24 h because of the great difference in solubility between the solutes. CU, a weak base (Li et al., 2008), displayed excellent solubility towards the three types of biomasses without further

Conclusions

Two-stage DESs combination pretreatment was shown to be an effective method to exploit the utilization of certain defective DESs for biomass pretreatment. By carefully choosing DESs and the pretreatment order, appropriate xylan and lignin removal was achieved due to the synergistic effect of the DESs, affording readily accessible cellulose and a favourable glucose yield. Water introduction into the process and the elimination of the washing procedure after the first stage treatment could

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21406038) and Undergraduate Training Programs for Innovation of Guangdong province, China (201611845139).

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