Aqueous Choline Chloride: A Novel Solvent for Switchgrass Fractionation and Subsequent Hemicellulose Conversion into Furfural

The performance of aqueous choline chloride (ChCl) solution on switchgrass fractionation and further utilization was evaluated. Under a relatively mild pretreatment condition (120 °C, 25 min), acidified ChCl solution efficiently removed 76% xylan and 51% lignin from switchgrass. The solvent was recycled and reused with similar performance for at least four pretreatment cycles when an acidic condition was maintained. The solvent recycling and reuse led to an increase in cumulative lignin yield with increased pretreatment cycles, reaching 93% at the fifth cycle. Two-dimensional heteronuclear single quantum coherence (HSQC) NMR analysis revealed that lignin recovered from the fifth cycle had a similar β-O-4 linkage abundance to native lignin. Such lignin would be well-suited for aromatic monomer production via depolymerization. The solvent recycling also resulted in xylose-rich pretreatment liquor, which was a good substrate and reaction medium for furfural production. The pretreated switchgrass showed good cellulose digestibility which was 5-fold that of the untreated one. This work demonstrated that ChCl solution was a new and green solvent for fractionating lignocellulose complex and extracting lignin as well as upgrading hemicellulose-derived stream.