Utilization of walnut shell by deep eutectic solvents: enzymatic digestion of cellulose and preparation of lignin nanoparticles

Highlights

• DES pretreatment and anti-solvent precipitation were combined to prepare LNPs.

• LNPs with particle size of 212.63 nm and ζ-potential of − 31.97 mV were produced.

• The enzymatic efficiency after pretreatment increased to 2.6 times.

• The elemental composition and molecular changes of LNPs was revealed.

• Mechanism of DES fractionation of lignocellulose was explained.

Abstract

Studies have been devoted to deep eutectic solvent (DES) pretreatment of biomass to improve its enzymatic efficiency, while subsequent processing of regenerated lignin was absent. Nanosizing is one of the common approaches to the value-addition of lignin, based on this, we reported a facile strategy to prepare lignin nanoparticles (LNPs) from walnut shells (WS), combining DES pretreatment and antisolvent precipitation. Three DESs (choline chloride (ChCl): oxalic acid (OA): ethylene glycol (EG) = 1:1:1, ChCl: p-toluenesulfonic acid (TsOH): EG (1:1:1), and ChCl: TsOH: EG (1:1:2)) were assessed for enzymatic hydrolysis efficiency of cellulose solids (CSs) and evaluated the yield and functional properties of LNPs. After pretreatment of ChCl: TsOH: EG (1:1: 2) at 90 ℃ for 2 h, 42.72% of lignin in WS was converted into LNPs with a particle size of 157.71 nm and ζ-potential of − 24.85 mV under dropping speed of 0.15 mL/min and reaction pH of 6, meanwhile, the pretreatment resulted in 2.6 times enzymatic efficiency relative to untreated WS. LNPs prepared from ChCl: OA: EG (1:1:1) exhibited excellent stability (ζ-potential of −31.97 mV) and antioxidant properties (radical scavenging rate of 61.43%), and the lignin molecules possessed high content of functional groups. Our study is expected to provide implications for the chemical deep processing of biomass and the preparation of LNPs.

Introduction

In recent years, studies have focused on refining biomass resources, aiming to obtain renewable chemicals and materials to reduce dependence on petroleum. As the readily available source of biomass on earth, lignocellulose is a promising material for refining platforms, containing cellulose, hemicellulose, and lignin, which can be extracted into precursors for high-value products (Zhang et al., 2020). Removal of lignin is the critical step, as it purifies cellulose and increases the efficiency of enzymatic digestion of cellulose to glucose. Dilute acids, alkali, organic solvents, and ionic liquids are traditionally applied to break biomass cell walls' recalcitrance (Lopes et al., 2018, Sun et al., 2019), while DES is a novel type of solvent consisting of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA). Due to its cost-effectiveness, low toxicity, and degradability, DES is considered an alternative to ionic liquids with promising applications to allow the selective separation of lignin and hemicellulose by cleaving the ether and hydrogen bonds in the coordination bonds between lignin-carbohydrates and lignin macromolecules (Hong et al., 2020a). Improvement of the enzymatic efficiency by removing lignin from biomass resources with DES pretreatment has been reported: Ji et al. (2021a) combined ultrasound and DES pretreatment to achieve 6.8 times more glucose release than the raw bagasse; Ma et al. (2022) reported a ternary DES pretreatment on corncob, which yielded 61.58% glucose and 26.90% xylose higher than the untreated corncob after 72 h of enzymatic hydrolysis. However, most of these studies focused on the disposal of cellulose solids (CS) and neglected the secondary processing of lignin.

LNPs represent a research hotspot in biomass, with characteristics superior to lignin, such as high specific surface area, high specific strength, and small size effect (Norgren and Edlund, 2014). LNPs have been applied to polymer-based nanocomposites, sustained-release drug carriers (Figueiredo et al., 2017), antioxidants (Yang et al., 2016), and antibacterial agents (Yang et al., 2021), and methods for the preparation of LNPs are widely reported (e.g., anti-solvent precipitation, mechanical treatment, chemical modification, and microbiological treatment) (Liu et al., 2019, Luo et al., 2021). Anti-solvent precipitation has become the most promising pathway due to its versatility and controllability, while organic solvents were usually used (e.g., tetrahydrofuran, dimethyl sulfoxide, ethanol, and acetone) in previous reports (Huang et al., 2019, Qian et al., 2017), and the volatility, flammability, low recycling, and limited solubility to lignin make it challenging to apply in large-scale production. In contrast, the no toxicity and high solubility to lignin enable DES to replace traditional organic solvents. Luo et al. (2021) obtained LNPs with 20–200 nm particle sizes by anti-solvent precipitation or dialysis, using DES as solvent. Yue et al. (2022) prepared LNPs with rich plural hydroxyl and carboxyl groups by treating wheat straw with alkaline DES.

To improve the application value of DES-regenerated lignin and reduce the consumption of organic solvents. In this study, we nanosized regenerated lignin by anti-solvent precipitation with DES-lignin solution after enhancing the efficiency of enzymatic digestion of WS by DES pretreatment (Fig. 1). Composition, crystallinity, and changes of functional groups of CSs were determined. The average particle size and ζ-potential of the LNPs suspension were measured to confirm the feasibility of the continuous method, and the linkage bonds and chemical composition were characterized to reveal the alteration of the molecular structure of lignin. Combining this study with existing reports, we explained the possible mechanism of DES pretreatment and the forming pathway of LNPs.