Insights into bamboo delignification with acidic deep eutectic solvents pretreatment for enhanced lignin fractionation and valorization

Highlights

• Acidic DES assisted bamboo delignification and promoted the cleavage of β-O-4′ linkages.

• Elevated reaction temperature lead to increased phenolic OH groups but lower molecular weights.

• The regenerated lignins exhibited higher thermal stability and excellent antioxidant activity.

• Lignin condensation occurs with depolymerization process.

Abstract

Deep eutectic solvent (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 will improve 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. 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. The results revealed that acidic DES assisted bamboo delignification and promoted the cleavage of β-O-4′ linkages at elevated reaction temperature, leading 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, suggesting great potential applications in lignin-based chemicals and material. Lignin condensation with unsaturated reactive intermediates which is accompanied by the depolymerization process during this acidic DES system. The results also highlight the utilization of acidic DES as a low-cost, high performance with a long delignification durability and thermal stability. Overall, the insights gained here could shed new light on the design and fabrication of efficient and durable DES-based pretreatment for the production of bio-fuel, bio-based materials and bio-chemicals.

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.