Insights into bamboo delignification with acidic deep eutectic solvents pretreatment for enhanced lignin fractionation and valorization
Graphical abstract
Introduction
With the endless exploitation of fossil resources and the increasing of energy consumption, preparation of bio-based materials, energy and chemicals with the renewable lignocellulosic biomass as raw materials has attracted increasing attention (Agbor et al., 2011; Raghavi et al., 2016). However, this renewable biomass has a high degree of chemical and biodegradation resistance due to its complex and crosslinked heterogeneous matrix structure thus hindered its high-value utilization (McClelland et al., 2017; Xiao et al., 2017). Therefore, it is usually necessary to deconstruct the recalcitrance of lignocellulosic biomass by pretreatment (Chundawat et al., 2011; Hendriks and Zeeman, 2009; Petridis and Smith, 2018). In order to meet the increasing demands of low cost, high efficiency and environmental friendliness, various pretreatment methods, such as physical, chemical, chemicophysieal, and biological methods have been developed (Li et al., 2010; Petridis and Smith, 2018). Among these, the present of lignin is the main challenge to reduce the recalcitrance of the cell walls (Ragauskas et al., 2014). Nevertheless, the traditional pre-processing method still has some drawbacks, such as harsh reaction conditions, corrosion equipment, and unsatisfactory separation effects etc (Satlewal et al., 2018).
Recently, a new type of deep eutectic solvent (DES), which is a liquid mixture formed by a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), has received much attention owing to its exceptional ability to remove lignin and hemicelluloses (Abbott et al., 2004; Smith et al., 2014; Zhang et al., 2012). Moreover, DES offers several advantages over ionic liquids as they are inexpensive, non-flammable, nontoxic, thermochemical stable, and biodegradable (Liu et al., 2017; Wang et al., 2020a, c). More importantly, it is reported that DES pretreatment of lignocellulosic biomass facilitates lignin recovery but keep cellulose intact, which is propitious to yield tailored chemicals for the production of value-added products (Alvarez-Vasco et al., 2016; Das et al., 2018; Francisco et al., 2012; Tang et al., 2017; Wang et al., 2020c; Xia et al., 2018). Lee et al. (Thi and Lee, 2019) have reported that compared with the conventional dilute sulfuric acid (H2SO4), the mild organic acid is insufficient to hydrolyses the hemicellulose during pretreatment. Subsequently, the outstanding ability of ChCl/lactic acid (LA) in cellulose, hemicellulose and lignin disruption was confirmed by FT-IR and SEM. It is reported that a powerful DES (ChCl/oxalic acid dehydrate) with a hydrogen-bond acidity has a significant effect on efficient cleavage of lignin-carbohydrate complex (LCC) and could accelerate fractionation of lignin with a high purity (93.7–96.3 %) (Liu et al., 2017). Interestingly, it has been reported that lignin nanoparticles (LNPs) in the range of 50−150 nm could be obtained when treating wheat straw with ChCl/LA DES (Lou et al., 2019). Similarly, it was found that the recovered LNPs were consisted with homogeneous and interconnected spherical particles and had a sponge-like carbonaceous nanoarchitecture (Shen et al., 2019a). DES as novel and green solvents can efficiently promote the fractionation of high-purity lignin from lignocellulosic biomass pretreatment. Revealing the fundamental chemistry of lignin during DES pretreatment would facilitate improved methodology to optimize the pretreatment in an integrated biorefinery.
Herein, an acidic biomass-derived DES (choline chloride/oxalic acid) pretreatment was developed to deconstruct the recalcitrance of bamboo for enhanced lignin fractionation and valorization. This acidic DES consisted of biomass-derived chemicals of choline chloride and oxalic acid. The structure transformation of the regenerated lignin samples obtained at different pretreatment temperatures has been investigated in comparison with enzymatic mild acidolysis lignin (EMAL) by state-of-the-art NMR (2D HSQC, quantitative 13C, 31P), GPC, and TGA analysis. Subsequently, the recycling and reusability performance of DES after the extraction of lignin were also evaluated. Furthermore, in order to illustrate the relationship between its structural characteristics and properties, the antioxidant capacity of regenerated DES lignin samples was estimated by DPPH radical scavenging test.
Section snippets
Material
Bamboo Dendrocalamus yunnanicus (40–60 mesh) were obtained from Yunan Province, China. The powder was extracted with a Soxhlet extractor with toluene/ethanol (2:1, v/v) for 8 h. The commercial enzyme (Cellic@CTec2, 100 FPU/mL; shearzyme 500 L, 100,000 IU/mL) were kindly provided from Novozymes (Beijing, China). All of the chemicals and reagents were analytical grade and used as received unless stated otherwise.
DES preparation
DES preparation was carried out by mixing choline chloride-oxalic acid (ChCl/OA) with
Delignification ratios and lignin yields
In this work, bamboo Dendrocalamus yunnanicus was pretreated with choline chloride and oxalic acid (ChCl/OA) at 80−120 °C for 4 h. After fractionation treatment, the lignin samples resulting from the DES solutions were regenerated and analysed in detail with EMAL as the control to illustrate the structural changes of lignins during the acidic DES delignification process.
The delignification efficiency and yield of lignin fractions are calculated based on the initial lignin contents, and the
Conclusions
In summary, this work demonstrate that acidic DES assisted biomass delignification and promoted the cleavage of β-O-4′ linkages at elevated reaction temperature, which led to an increase in the sustained rise of phenolic OH groups and lower molecular weights. Those lignin samples exhibited higher thermal stability and excellent antioxidant activity, thus showing high potentials for lignin valorization, such as resins synthesis, ultraviolet curable coatings, and natural additives in cosmetics.
CRediT authorship contribution statement
W.-X.L carried out all experiments. W.-X.L and L.-P.X wrote the manuscript, L.-P.X and R.-C.S designed the work and revised the manuscript, W.-Z.X, Y.-Q.Y, Q.W, X.C analyzed the data. All authors discussed the results.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51961125207), the Natural Science Foundation of Liaoning Province (2019-MS-019), the Opening Project of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (2019KF14), Dalian Support Plan for Innovation of High-level Talents (2019RQ034 and 2019RD13), Liaoning Revitalization Talents Program, and Liaoning Baiqianwan Talents Program.
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