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

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 nano-particles (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 celcius for 2 h, 42.72% of lignin in WS was converted into LNPs with a particle size of 157.71 nm and zeta-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 (zeta-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 impli-cations for the chemical deep processing of biomass and the preparation of LNPs.

Automated summary

Studies have focused on the use of deep eutectic solvents (DES) for pretreating biomass to enhance enzymatic efficiency, but little attention has been given to the subsequent processing of regenerated lignin. In this study, lignin nanoparticles (LNPs) were prepared from walnut shells through DES pretreatment and antisolvent precipitation. Three DESs were utilized and evaluated for enzymatic hydrolysis efficiency of cellulose solids and the yield and functional properties of LNPs. The results showed that the ChCl:TsOH:EG (1:1:2) DES pretreatment resulted in the conversion of 42.72% of lignin in walnut shells into LNPs with a particle size of 157.71 nm and zeta-potential of -24.85 mV. The pretreatment also improved the enzymatic efficiency by 2.6 times compared to untreated walnut shells. The LNPs prepared from ChCl:OA:EG (1:1:1) exhibited excellent stability (zeta-potential of -31.97 mV), antioxidant properties (radical scavenging rate of 61.43%), and a high content of functional groups. This study has important implications for the chemical processing of biomass and the preparation of LNPs.