Comparison of [HSO4]-, [Cl]- and [MeCO2]- as anions in pretreatment of aspen and spruce with imidazolium-based ionic liquids

Ionic liquids (ILs) draw attention as green solvents for pretreatment of lignocellulose before enzymatic saccharification. Imidazolium-based ILs with different anionic constituents ([HSO4]-, [Cl]-, [MeCO2]-) were compared with regard to pretreatment of wood from aspen and spruce. The

The acidity of [HSO4]- caused extensive hydrolysis of xylan, which facilitated pretreatment of xylan-rich hardwood. Apart from that, the degree of removal of hemicelluloses and lignin did not correspond well with the improvement of the enzymatic saccharification. Taken together, the saccharification results were found to mainly reflect (i) the different capacities of the ILs to disorder the cell wall structure, (ii) the recalcitrance caused by high xylan content, and (iii) the capacity of the [HSO4]--based IL to hydrolyze xylan.

The glucose yield after IL pretreatment increased in the order [Cl]- < [HSO4]- < [MeCO2]- for aspen, but in the order [HSO4]- < [Cl]- < [MeCO2]- for spruce. For both aspen and spruce, removal of hemicelluloses and lignin increased in the order [Cl]- < [MeCO2]- < [HSO4]-. Fluorescence microscopy indicated increasingly disordered cell wall structure following the order [HSO4]- < [Cl]- < [MeCO2]-. Torrefaction of aspen converted xylan to pseudo-lignin and changed the glucose yield order to [HSO4]- < [Cl]- < [MeCO2]-. Conclusions: The acidity of [HSO4]- caused extensive hydrolysis of xylan, which facilitated pretreatment of xylan-rich hardwood. Apart from that, the degree of removal of hemicelluloses and lignin did not correspond well with the improvement of the enzymatic saccharification. Taken together, the saccharification results were found to mainly reflect (i) the different capacities of the ILs to disorder the cell wall structure, (ii) the recalcitrance caused by high xylan content, and (iii) the capacity of the [HSO4]--based IL to hydrolyze xylan.