Ionic liquid-assisted process: biomass-derived carbonaceous materials
Creators: name: Anuchi, Samson Oyemechi
Publisher: name: Imperial College London
Year: 2023

description: As a sustainable alternative, biomass from lignocellulose has the potential to partially displace fossil fuels for energy, chemicals, and materials. The realisation of this potential requires effective lignin valorisation for a sustainable and competitive biorefinery industry. This work uses ionic liquids to fractionate high-lignin biomass feedstocks and synthesize biomass-derived carbonaceous materials. It also aims to develop advanced lignin-derived porous carbon materials using ionic liquids for supercapacitor applications. Firstly, a low-cost protic ionic liquid, N,N -dimethyl-N-butylammonium hydrogen sulfate was utilised to fractionate lignin-rich coconut wastes (i.e., shells and husks) into separate streams of a cellulose-rich pulp and high-quality lignin. After pretreatment, the shells released nearly 90 % of the theoretical yield of glucose, while husks released less than 70 %. The glucose release was directly linked to the amount of lignin removed from the biomass feedstocks, as higher lignin removal occurred in the shell (82 wt.%) than in the husk (77 wt.%). It was concluded that biomass fractionation was less energy intensive with the shells. Lignins isolated from both feedstocks under their optimum pretreatment conditions were condensed and of the SGH type. The shell lignin had a higher level of S-units. The isolated shell lignin was used to produce advanced carbon materials for supercapacitors. Some imidazolium and pyrrolidinium ionic liquids containing anions ranging from [PF6], [NTF2], [OTF], [BF4], …….to [Cl], were investigated on their ability to induce porosity in lignin-derived carbon by using key ionic liquid properties (solvent, thermal, and size) at low temperature (300 – 400 °C). In the study, only ionic liquid size played a major role in determining the ability of ionic liquids to generate pores, and ionic liquids with a bulky anion (≥ 0.76 nm) and a small cation (≤ 0.72 nm) were most effective. Consequently, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2MIm][NTF2]) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([C4MIm][OTF]) were selected based on their ability to generate higher-surface area lignin-derived carbons ([C2MIm][NTF2]: 528 m2 g-1 &gt; [C4MIm][OTF]: 374 m2 g-1 &gt; no IL: ∽ 1 m2 g-1). The surface areas and porosity of the lignin-derived carbons produced by the use of (1) no IL, (2) [C4MIm][OTF], and (3) [C2MIm][NTF2], respectively were enhanced through CO2 activation at 900 – 950 °C for supercapacitor application. It was found that the use of no IL or [C4MIm][OTF] produced lignin-derived activated carbons that are purely microporous, whereas [C2MIm][NTF2] produced lignin-derived activated carbons with a hierarchical structure. The hierarchical porous [C2MIm][NTF2] carbons displayed superior supercapacitive performance in both aqueous and ionic liquid-based electrolytes with the highest specific capacitance exceeding 180 F g-1 and 110 F g-1, respectively. Furthermore, the study compares the electrochemical performance of neat [C4MIm][OTF] and [C2MIm][NTF2] with their recycled counterparts obtained from co-pyrolysis experiments at 400 °C. It was found that the cell containing recycled [C2MIm][NTF2] produced a higher energy density (∽ 37 Wh kg-1). This is contrary to recycled [C4MIm][OTF], which produced a lower energy density (∽ 13 Wh kg-1) due to lower electrochemical stability.