Selective extraction of hemicelluloses from spruce using switchable ionic liquids

► Switchable ionic liquids made from, either hexanol or butanol, and CO2 together with an amidine (DBU) were studied as dissolution/fractionation solvents for spruce wood. ► After the treatment undissolved fraction contained 38wt.% less hemicelluloses. ► We conclude that switchable ionic liquids are effective solvent for fractionation of wood material, and they are easily recycled and can be reused. Switchable ionic liquids (SILs) made from alcohols, either hexanol or butanol, and CO2 together with an amidine (1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)) were investigated as dissolution/fractionation solvents for wood material. Both native spruce (Picea abies), and pre-extracted spruce were treated with either butanol SIL (SIL1) or hexanol SIL (SIL2) for 5 days at 55°C under normal pressure. The SILs were formed by bubbling CO2 through an equimolar mixture of either 1-hexanol or 1-butanol and DBU. The viscosity of the mixture increased from 7.1mPas to 2980mPas for SIL2 and 5.1 to 1600mPas for SIL1. Melting points of the SILs 1 and 2 were at 8 and 14°C, respectively. After the treatment time (5 days), the undissolved fraction contained 38wt.% less hemicelluloses compared to native spruce. There was an increase in the glucose content of the milled spruce treated with both SILs, since the milling step reduced the cellulose crystallinity of the wood and facilitated an easier SIL access into the wood. The solvents were very neutral in terms of lignin removal. Consequently, only about 2% of the lignin was removed from native wood. Moreover, a priori removal of the wood extractives did not influence the lignin removal.