Influence of ionic liquid type on porous carbon formation during the ionothermal pyrolysis of cellulose

Ionothermal strategy using ionic liquid (IL) enables production of functional porous carbons at a low temperature from biomass and its components in one-step. However, despite diverse physicochemical properties of ionic liquids, there has been little understanding of the influence of IL type on reactions occurring in the pyrolytic conversion to carbon. In this study, 18 types of typical imidazolium-based ILs were screened in ionothermal pyrolysis of cellulose, where cellulose was mixed with IL, pyrolyzed, and then washed by a solvent for the removal and recovering of IL. The capability of IL for the porous carbon formation depended significantly on the type of IL and was reasonably explained by the hydrogen bond basicity. Among ILs tested, ILs with sulfonate anions showed the best performance in terms of the surface area of resulting carbon and IL recovery. The chemical interaction of IL with cellulose, caused by the moderate hydrogen bond basicity, facilitated the dehydration at low temperatures to form microporous carbon domains, and the presence of IL prevented the shrinkage of carbon framework. The surface area reached 1067 m²/g in the pyrolysis at 350 °C, which was the highest among ionothermally-synthesized carbons from biomass, reported so far. The function of IL, working as a pore template, enabled the control of pore size distribution over a wide range, e.g., carbons with a variety of total pore volume (0.33–1.13 cm³/g) and high surface area at a similar level (719–886 m²/g) were obtained by changing IL fraction in the mixture with cellulose within the range of 9–50 wt%. Furthermore, more than 90% of the IL could be recovered after pyrolysis, and the reuse led to a reproducible yield of carbon with similar pore sizes.