Water addition enhanced thermal stability of alkylimidazolium acetate in Ionosolv treatment of lignin

Enhanced oxidative depolymerization of lignin in cooperative imidazolium-based ionic liquid binary mixtures

The aerobic oxidation of lignin model 2-phenoxyacetophenone (2-PAP) in cooperative ionic liquid mixtures (CoILs) with 1-ethyl-3-methylimidazolium acetate ([C₂C₁im]OAc) and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([B_ZC₁im]NTf₂) was investigated. Complete degradation of 2-PAP was achieved with [C₂C₁im]OAc/[B_ZC₁im]NTf₂ molar ratio (R_IL) of 1/1 and 1/2 at 100 °C for 2 h. The conversion and product yields from CoILs were higher than those in pure ILs, indicating the cooperative effects of [C₂C₁im]OAc/[B_ZC₁im]NTf₂ on cleaving aryl-ether bonds. [C₂C₁im]OAc promoted the catalytic cleavage of aryl-ether bonds and solvation, and [B_ZC₁im]NTf₂ induced the formation of alkyl radicals and enhanced the product selectivity. Accordingly, the highest conversion of alkali lignin (79.8%) was obtained with R_IL of 5/1 at 100 °C for 2 h, and phenol monomers (306 mg/g) were selectively produced. The CoILs exhibited good catalytic capacities for oxidative depolymerization of lignin, which strongly depends on the changes in intermolecular interactions and structural organization with varying R_IL.

Characterization of lignin streams during ionic liquid/hydrochloric acid/formaldehyde pretreatment of corn stalk

This work investigated the role of formaldehyde (FA) in lignin anti-condensation during corn stalk pretreatment based on 1-butyl-3-methylimidazolium chloride ([C₄C₁im]Cl)/hydrochloric acid (HCl). As a result of the aldolization reactions between FA and lignin, the condensation of lignin fragments was inhibited, and lignin remained in soluble fragmental molecules. Characterizations on the compositional and structural changes of lignin and its degraded products during pretreatment (80 °C-100 °C, 2-5 h) with FA addition in comparison with those in DO/HCl/FA or [C₄C₁im]Cl/HCl were conducted. Results revealed that the structural features of lignin were affected by FA addition and solvent type. In the [C₄C₁im]Cl/HCl/FA system, FA stabilization was unfavorable for the cleavage of β-O-4' bonds and lignin with low S/G ratio (3.4) and high molecular weight (Mw = 9920 g·mol^-1) was extracted. The compositions of degraded products were considerably affected by FA addition.

1-Ethyl-3-methylimidazolium acetate ionic liquid as simple and efficient catalytic system for the oxidative depolymerization of alkali lignin

The oxidative depolymerization of alkali lignin (AL) in 1-ethyl-3-methylimidazolium acetate ([C₂C₁im]OAc) system without additional catalyst was investigated under mild conditions (initial O₂ pressure of 1.5 MPa, 80 °C-100 °C). Compared with other ionic liquids (ILs), the cooperation of imidazolium cation and acetate anion successfully enhanced AL conversion. Among the investigated imidazolium acetate ILs with ethyl- to octyl-side chains, [C₂C₁im]OAc presented the best catalytic capacity for AL oxidative depolymerization. Adding an appropriate amount of water to [C₂C₁im]OAc can further improve the reaction efficiency. In the [C₂C₁im]OAc system with the addition of 0.10-0.25 mL of water, approximately 77 wt% AL was depolymerized into small molecule soluble products at 100 °C for 2 h. The extracted oil was composed mainly of phenolic derived compounds. With the use of the [C₂C₁im]OAc-based system, the specific inter-unit linkages of lignin were broken down, and residual lignin with low molecular weight and narrow polydispersity index (1.88-1.96) was obtained. Compared with that in AL conversion with fresh [C₂C₁im]OAc, only a minimal decrease (~3.2%) was observed with the recovered IL until the fifth cycle. These findings revealed that [C₂C₁im]OAc-based system is a simple and efficient catalytic system for lignin oxidative depolymerization.

Thermal Stability of Ionic Liquids: Current Status and Prospects for Future Development

Ionic liquids (ILs) are the safest solvent in various high-temperature applications due to their non-flammable properties. In order to obtain their thermal stability properties, thermogravimetric analysis (TGA) is extensively used to analyze the kinetics of the thermal decomposition process. This review summarizes the different kinetics analysis methods and finds the isoconversional methods are superior to the Arrhenius methods in calculating the activation energy, and two tools—the compensation effect and master plots—are suggested for the calculation of the pre-exponential factor. With both parameters, the maximum operating temperature (MOT) can be calculated to predict the thermal stability in long-term runnings. The collection of thermal stability data of ILs with divergent cations and anions shows the structure of cations such as alkyl side chains, functional groups, and alkyl substituents will affect the thermal stability, but their influence is less than that of anions. To develop ILs with superior thermal stability, dicationic ILs (DILs) are recommended, and typically, [C4(MIM)2][NTf2]2 has a decomposition temperature as high as 468.1 °C. For the convenience of application, thermal stability on the decomposition temperature and thermal decomposition activation energy of 130 ILs are summarized at the end of this manuscript.

Accurately-controlled recovery and regeneration of protic ionic liquid after Ionosolv pretreatment via bipolar membrane electrodialysis with ultrafiltration

Efficient recovery and regeneration of ionic liquid is significant for industrial Ionosolv pretreatment. Complicated electrolyte composition restricts the scale-up recovery and application of protic ionic liquid such as triethylammonium hydrogen sulfate [TEA][HSO₄] in biomass-related research. Recovery of [TEA][HSO₄] after Ionosolv pretreatment for miscanthus powder was studied using bipolar membrane electrodialysis (BMED) assisted with ultrafiltration (UF) by the divisional recovery of TEA⁺ as TEA and recovery of SO₄^2- as H₂SO₄ in different BMED compartments. Hence accurately-controlled regeneration of [TEA][HSO₄] could be realized. Influence of current density and feed concentration of BMED module was studied in detail. In this study, the highest recovery ratio for TEA⁺ and SO₄^2- reached 93.7% and 96.4%. The lowest energy consumption of specific [TEA][HSO₄] recovery was about 6.2 kwh/kg. Insight gained from this study suggests a potentially industrial methodology for complicated protic ionic liquid recovery after biomass processing.