Carbohydrate free lignin: a dissolution–recovery cycle of sodium lignosulfonate in a switchable ionic liquid system

In this work, carbohydrate free lignin was obtained by treating a technical lignin such as sodium lignosulfonate (SLS) with a switchable ionic liquid (SIL). The SIL was synthesized from molecular moieties such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), monoethanol amine (MEA), and carbon dioxide gas and characterized by H-1, C-13, and H-1-C-13 two-dimensional heteronuclear multiple-bond correlation nuclear magnetic resonance (NMR) spectroscopy. The as-synthesized SIL was used as a solvent medium for dissolution of SLS-containing polysaccharide (e.g., glucan and xylan) impurities under various reaction conditions. Parameters such as dissolution time and temperature as well as the SLS concentration were varied. In a manner independent of the dissolution time and temperature, 2 g of SIL was able to completely dissolve 0.3 g of SLS and approximate to 60 SLS was recovered upon precipitation with an ethanol/hexane antisolvent system. The nonrecovered SLS remained in the viscous SIL phase. However, the dissolution ability of the SIL steadily decreased with increasing amounts of accumulated SLS. The recovered solids were analyzed by H-1-C-13 two-dimensional heteronuclear single-quantum coherence NMR spectroscopy to elucidate the potential structural changes occurring in the SLS structure after dissolution- recovery treatment in the SIL. It was observed that in all experiments, SIL demonstrated its ability to extract the interlinked polysaccharide impurities from the SLS while the linkages and aromatic subunits remain unaffected during the dissolution-recovery cycle. Most importantly, here we describe that the SIL can be used as an affordable solvent medium (in comparison to typical commercially available ionic liquids) to obtain carbohydrate free lignin from an impure lignin source.