Theoretical Elucidation of β-O-4 Bond Cleavage of Lignin Model Compound Promoted by Sulfonic Acid-Functionalized Ionic Liquid

Abstract

While the depolymerization of lignin to chemicals catalyzed by ionic liquids has attracted significant attention, the relevant molecular mechanism, especially the cleavage of specific bonds related to efficient depolymerization, still needs to be deeply understood for the complexity of this natural aromatic polymer. This work presents a detailed understanding of the cleavage of the most abundant β-O-4 bond in the model system, guaiacylglycerol β-guaiacyl ether, by a Brønsted acidic IL (1-methyl-3-(propyl-3-sulfonate) imidazolium bisulfate ([C 3 SO 3 Hmim][HSO 4 ]) using density functional theory calculation and molecular dynamics simulation. It has been found that [C 3 SO 3 Hmim][HSO 4 ] generates zwitterion/H 2 SO 4 via proton transfer with an energy barrier of 0.38 kcal/mol, which plays a dominant role in the lignin depolymerization process. Subsequently, the reaction can be carried out via three potential pathways, including (1) the dehydration of α-C-OH, (2) dehydration of γ-C-OH, and (3) the protonation of β-O. The electrophilic attack of H 2 SO 4 and the hydrogen-bonding interaction between GG and zwitterion are the two most important factors to promote the depolymerization reaction. In all steps, the dehydration of α-C-OH route is computed to be favored for the experiment. The relatively higher energy barrier for β-O-4 bond dissociation among these reaction steps is attributed to the hindrance of the self-assembled clusters of GG in the mixed system. Further, the dense distribution of H13([C 3 SO 3 Hmim]) surrounding O21(GG), indicated by sharp peaks in RDFs, reveals that -SO 3 H in cations plays a substantial role in solvating lignin. Hopefully, this work will demonstrate new insights into lignin depolymerization by functionalized ILs in biomass conversion chemistry.

Theoretical Insights Into the Depolymerization Mechanism of Lignin to Methyl p-hydroxycinnamate by [Bmim][FeCl4] Ionic Liquid

“…The above calculations have provided a mechanism understanding of the depolymerization of lignin model compound PPC catalyzed by [Bmim][FeCl 4 ], and the catalytic properties of MBILs were explained to a certain extent. According to previous studies (Sánchez-Sanz and Trujillo, 2018; Zhang et al, 2019), the solvent effect is of great importance for biomass conversion due to their impacts on the whole reaction. Figure 7 shows the relative Gibbs free energies of reactants, transition states, intermediates, and products corresponding to the reaction pathway c , which is the most energetically favorable path, in the gas phase and with solvation model.…”

“…Our previous work illustrated that the geometric and interactions between 16 ILs and lignin model compound GG, and then identified sulfonic ILs having the strongest interaction with GG (Zhang et al, 2017). Further, we investigated the reaction mechanism of the cleavage of β-O-4 bond in GG to guaiacol by -SO 3 H functionalized ILs (Zhang et al, 2019). Given the versatility of IL catalysts (He et al, 2015; Wang et al, 2015; Meng et al, 2017; Qian et al, 2018; Zhang et al, 2018) and our keen interest in the research of biomass (Cai et al, 2015; Suresh et al, 2017), an elegant approach for catalytic transform of lignin to structurally defined aromatic esters is put forward here.…”

Selective conversion of biomass catalyzed by ionic liquids

“…优 化 的 反 应 条 件 下 (160 ℃, 5 h)马来酸二乙酯收率为 40%, 选择性为 73% [51] . 催化剂表征结果表明, 杂多酸离子液体中的 [55,56] . 课题组以磷钨酸盐离子液体为催化剂, 研究了温和条件下催化浮萍水解制备还原糖的过程 [57] .…”

Stimuli‐Responsive Ionic Liquids and the Regulation of Aggregation Structure and Phase Behavior†

“…Since the 21 st century, a large number of novel ILs with cations such as quaternary ammonium, quaternary phosphonium, pyrrole, diimidazole, pyridine, naphthenate, and sulfur salts have been reported. [ 3‐5 ] Due to the designability of ILs, the third‐generation ILs, namely functionalized ILs, are prepared carefully by combining different cations and anions of functional groups based on specific applications, such as gas absorption, [ 6‐11 ] extraction and separation, [ 12‐17 ] catalytic reaction, [ 18‐24 ] organic synthesis, [ 25‐27 ] biomass utilization, [ 28‐36 ] and electrochemistry. [ 36‐39 ] These third‐generation ILs have attracted increasing attention from academia to industrial communities with the rise of green chemistry.…”