Enhanced hydrolysis of bamboo biomass by chitosan based solid acid catalyst with surfactant addition in ionic liquid
Introduction
Lignocellulosic biomass demonstrates promise as the most economical, highly renewable natural resource in the world. The development of renewable energy obtained from lignocellulosic biomass as an alternative to fossil fuel is ultimately essential for the survival of the human race (Ragauskas et al., 2006). Considerable research and development programs have been initiated worldwide to convert lignocellulosic biomass into various valuable products in biorefineries, where the liberation of reducing sugars from lignocellulose is a crucial step (Jäger & Büchs, 2012). However, lignocellulosic biomass is resistant to hydrolysis because of its complex hetero-matrix structure (Himmel et al., 2007), and cellulose fibers in the lignocellulose matrix are naturally protected by lignin and hemicellulose to resist microbial and enzymatic attack (Goshadroua & Lefsrud, 2017).
Ionic liquids (ILs) exhibit attractive properties such as chemical and thermal stability, high ionic conductivity, non-volatility, recyclability, and good dissolving capacity (Welton, 1999; Zhao, Xia, & Ma, 2005). Previous studies have indicated that ILs can decrease the crystallinity of cellulose, where hemicelluloses and lignin are partially removed; this partial removal leads to the chemical transformation of cellulose chains (Shafiei, Zilouei, Zamani, Taherzadeh, & Karimi, 2013; Swatloski, Spear, Holbrey, & Rogers, 2002). The dissolution of lignocellulosic substrates in various ILs as an effective pretreatment method has attracted significant attention worldwide (Bian et al., 2014; Silva, Lopes, Roseiro, & Bogel-Lukasik, 2013; Zavrel, Bross, Funke, Büchs, & Spiess, 2009).
From the viewpoint of green chemistry and industrialization, solid acid catalysts have become one of the excellent choices for the hydrolysis of lignocellulose into glucose. As these catalysts demonstrate tremendous potential to overcome limitations such as reactor corrosion, waste treatment, poor recyclability, isolating difficulty (Shen, Wang, Han, Cai, & Li, 2014), and energy- and cost-intensive pretreatment for enzymatic hydrolysis (Hu, Lin, Wu, Zhou, & Liu, 2015). Hara et al. (2004) and Suganuma et al. (2008) have independently reported that a carbon-based solid acid consisting of polycyclic aromatic carbon and functional groups, e.g., sulfonic acid, can act as strong solid acid catalyst for various acid-catalyzed reactions. Nata, Irawan, Mardina, and Lee (2015) have obtained highly sulfonated carbonaceous spheres in the presence of hydroxyethyl sulfonic acid and acrylic acid, which can be used as a solid acid catalyst for the hydrolysis of cornstarch. Zhong et al. (2015) have prepared a nanoscale catalyst, which can effectively hydrolyze hemicellulose while retaining cellulose and lignin. Chitosan is useful in many different applications (Deng et al., 2011, Huang et al., 2015; Xiao & Zhou, 2008; Xin et al., 2013), one of which is to synthesize resin and catalyst. In our laboratory, preliminary studies have been conducted to synthesize a novel sulfonated cross-linked chitosan solid acid catalyst (SCCAC), and examine the effectiveness of it in ILs for the hydrolysis of lignocellulose, with a comparatively ideal yield obtained for TRS (Cheng et al., 2016). Nevertheless, the yield is predominantly attributed to the large amount of the added catalyst and the consumption of the IL. Consequently, the development of an auxiliary strategy is urgently required for the economical, efficient hydrolysis of lignocellulose using solid acid catalysts in ILs.
Surfactants exhibit hydrophobic and hydrophilic properties and enhance the removal of hydrophobic substances by decreasing the surface tension between the two liquid phases (Qing, Yang, & Wyman, 2010). Furthermore, surfactants can serve as emulsifiers and dissolve the extractives present in the wood structure. These properties make surfactants prospective additives for the pretreatment of lignocellulose (Kim, Kim, & Kim, 2007; Kurakake, Ooshima, Kato, & Harano, 1994). Recent studies have investigated the synergistic effect of surfactants and IL for the hydrolysis of lignocellulose. Chang et al. (2016) have indicated that surfactant-assisted IL ([BMIM]Cl) pretreatment enhances the removal of lignin during pretreatment and cellulose conversion during subsequent enzymatic hydrolysis as compared with pretreatment without surfactants. Nasirpour, Mousavi, and Shojaosadati (2014) have assessed the effect of Tween 80 and polyethylene glycol 4000 (PEG 4000) on the pretreatment of sugarcane bagasse using [BMIM]Cl, and the removal of lignin is enhanced by 12.5% as compared to the IL-only pretreated sample. Pandey and Negi (2015) have assessed the impact of surfactant assisted acid and alkali pretreatment on pine foliage, it was proved to be efficient for removal of lignin. However, to our best of knowledge, the combination of surfactant-assisted solid acid catalysis and ILs as a novel strategy for hydrolysis has not been reported.
In this study, surfactants (e.g., Tween 80, PEG 4000, and SDS) were added to improve the hydrolysis of bamboo with SCCAC in IL (i.e., [BMIM]Cl). Reaction conditions were optimized to choose the best surfactants suited for the hydrolysis of bamboo. Furthermore, Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses were conducted to discuss the mechanism of the addition of surfactants with SCCAC and IL on hydrolysis.
Section snippets
Materials
Bamboo was readily obtained from a bamboo grove in Chongqing (China), sifted using an 80 mesh sieve. The main chemical composition of bamboo was determined by a two-step acid hydrolysis method developed by the National Renewable Energy Laboratory (NRTL) (Sluiter et al., 2008), with dry matter containing 20.3% of xylan, 22.3% of lignin, and 40.1% of glucan (Wang et al., 2014).
[BMIM]Cl used as a solvent in the system was purchased from Ke Neng Material Technology Co. Ltd (Linzhou, China). Table S1
Effect of the solid acid catalyst without surfactants on bamboo hydrolysis
Table 1 summarizes the yield of TRS obtained from the hydrolysis of bamboo biomass in [BMIM]Cl catalyzed by SCCAC without the surfactant. Results indicated that the novel solid acid catalyst is effective for the hydrolysis of bamboo biomass. The TRS yield obtained by the hydrolysis of bamboo powder by SCCAC was relatively greater than that observed for the control without SCCAC. When the mass ratios of SCCAC and bamboo were set to 1:1, 1:2, 1:5, and 1:20, the TRS yield increased to 50.73%,
Conclusions
This study has shown the enhancement of the addition of surfactants in bamboo hydrolysis with a novel sulfonated cross-linked chitosan solid acid catalyst in IL. Tween 80 was the most effective surfactant, and a maximum TRS yield of 68.01% was achieved at 120 °C after 24 h at 20 rpm. The addition of 0.08 g Tween 80, 0.04 g PEG 4000, and 0.03 g SDS increased the TRS yield by 34.06%, 15.28%, and 17.11%, respectively, compared with SCCAC alone. The addition of surfactants could break the lignocellulose
Acknowledgements
We would like to thank the financial support from the National Natural Science Foundation of China (Nos. 21106191, 21206175), the State Key Laboratory of Materials-Oriented Chemical Engineering (KL14-11), Open foundation of Hubei Key Laboratory of Edible Wild Plants Conservation and Utilization (EWPL201507), Natural Science Foundation Project of CQ CSTC (cstc2015jcyjA90008), and Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China. This work is also
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