Reaction characteristics of metal-salt coordinated deep eutectic solvents during lignocellulosic pretreatment. Issue 2 (April 2023)

• Record Type: Journal Article
• Authors: Chen, Huan; Sun, Chihe; Hu, Yun; Xia, Changlei; Sun, Fubao; Zhang, Zhanying
• Is Part Of: Journal of environmental chemical engineering. Volume 11:Issue 2(2023)
• Publication Date: 2023-04
• Subjects: Deep eutectic solvent -- Lignocellulose -- Pretreatment -- Enzymatic hydrolysis -- Reaction characteristics; Chemical engineering -- Environmental aspects -- Periodicals; Environmental engineering -- Periodicals; Chemical engineering -- Environmental aspects; Environmental engineering; Periodicals; 660.0286
• DOI: 10.1016/j.jece.2023.109531
• Languages: English
• ISSNs: 2213-2929

Abstract

Abstract: A binary deep eutectic solvent consisting of choline chloride and glycerol (ChCl/Gly) is proven to be ineffective in lignocellulosic pretreatment, and arbitrarily screening coordination agents makes it difficult to directionally construct a valid solvent. This study investigated the designable diversity of ternary ChCl/Gly coordinated with various metal-salts by elucidating solvent properties and reaction characteristics. Alkaline/acidic strengths and hydrogen-bonding forces were key factors for component fractionation. Pretreatment performance was typically poor for solvents with few acidic sites and active protons ( i.e., ChCl/Gly-KCl and MgCl2 ), as well as those with close proximity of H-bond donor and acceptor abilities (ChCl/Gly-ZnCl2 ). By contrast, strong alkaline ChCl/Gly-K2 CO3 could achieved 72 % of delignification with a remarkable holo-cellulose preservation (70.8–80.9 %). Delignification underwent an independent rate-control of solvent diffusion and surface reaction with an endothermic nature (17.1 kJ/mol of Δ H and 20.3 kJ/mol of apparent activation energy). High temperature was beneficial for mitigating diffusion constraint at the initial pretreatment stage, maximally increasing the reaction rate by 7.7-fold. Moreover, Lewis acidic ChCl/Gly-FeCl3 (or AlCl3 ) removed 76–84.4 % of hemicellulose and 47.6–61.9 % of lignin while significantly deconstructing the aryl-ether bonds and β-O-4 linkages. But for acidic ChCl/Gly solvents, biomass dissolution was inconformity with the independent diffusion/reaction-controlling process, owing to the excessive hemicellulose degradation and hydrolytic by-product formation (maximally 91 g/kg xylose and 4 g/kg furans). Based on the lignin structural characterization, the potential reaction mechanism related to solvent properties was proposed to develop efficient ternary coordinated deep eutectic solvents. Graphical Abstract: ga1 Highlights: Acidic/alkaline strengths were critical for metal-salt coordinated DES pretreatment. Alkaline ChCl/Gly had high delignification while preserving holo-cellulosic sugars. Alkaline ChCl/Gly had independent rate controls of diffusion and surface reaction. Acidic ChCl/Gly contributed to the excessive fracture of lignin β-O-4 linkages. Thermodynamic parameters and energy distribution during pretreatment were analyzed.