Understanding Microwave-Assisted Lignin Solubilization in Protic Ionic Liquids with Multiaromatic Imidazolium Cations

Abstract

The low solubility of lignin in most common solvents remains a significant challenge to be solved for its use and recovery. In this work, 18 novel protic ionic liquids (ILs) containing aromatic-substituted imidazolium cations and methanesulfonate and p-toluenesulfonate anions were synthesized and evaluated for lignin dissolution under microwave irradiation. ILs containing methanesulfonate demonstrated good lignin dissolving capacities, which can facilitate its separation and structural modifications to carry out their recovery under relatively mild and environmentally friendly conditions (few minutes, 90 °C). DFT theoretical calculations provided additional insights into the high lignin dissolution observed for the best performing ILs.

Investigation of Kraft Lignin Solubility in Protic Ionic Liquids and Their Aqueous Solutions

“…THEAL demonstrated to be an excellent solvent for lignin reaching a maximum of 47.09 wt % lignin solubility at 353.15 K (Figure ). This is a noteworthy value achieved with a lower temperature than that reported in similar studies using other solvents. ,,,− …”

“…39−45 Recently, PILs have been proposed for lignin dissolution. 20,28 The ability of 18 novel multiaromatic PILs for Kraft lignin dissolution was visually demonstrated by Merino et al using microwave-assisted irradiation at 363.15 K. The results The authors also reported, in general, a negative effect of water on lignin solubility; however, a hydrotropic behavior was observed for 2-hydroxyethylammonium octanoate (HEAO) that enhanced lignin solubility in aqueous solutions to a maximum value at 40 wt % water content. 46,47 A wide range of cation and anion combinations could be used to synthesize PILs, but only a few of these possibilities are reported in the literature for lignin dissolution.…”

“…Although good results with organic solvents have been achieved, alternative solvents have been tested to achieve even higher lignin solubility values. Among these alternative solvents, ionic liquids (ILs) emerged as a promising class of solvents to promote lignin dissolution and its extraction from lignocellulosic biomass. ,− They have been considered as a more sustainable and efficient alternative to traditional organic solvents, which are usually hazardous and present low selectivity for lignin dissolution …”

“…Recently, PILs have been proposed for lignin dissolution. , The ability of 18 novel multiaromatic PILs for Kraft lignin dissolution was visually demonstrated by Merino et al using microwave-assisted irradiation at 363.15 K. The results showed that 42 wt % lignin can be dissolved in 2,3,4,5-tetraphenyl-1 H -imidazolium methanesulfonate ([TPIM]­[MeSO 3 ]) at 363.15 K in a few minutes . Lignin solubility was also determined in pyridinium-based PIL aqueous solutions by Rashid et al Pyridinium formate ([Py]­[For]) was the best PIL, achieving 70 wt % lignin solubility at a low temperature (348.15 K) within 1 h .…”

The Role of Ionic Liquids in the Lignin Separation from Lignocellulosic Biomass

“…Several studies have presented that different solvents like ethylene glycol, DMSO [80], ILs [81], g-valerolactone [34] and Tween-80/H 2 O are able to dissolve up to 70 wt% lignin [82]. Among these solvents, ILs have acquired especial levels of interest due to their particular chemo-physical properties when compared to traditional molecular solvents.…”