Swelling, viscoelastic, and anatomical studies on ionic liquid-swollen Norway spruce as a screening tool toward ionosolv pulping

Abstract

To screen ionic liquid (IL) candidates for wood ionosolv pulping, swelling and viscoelastic studies on Norway spruce wood treated with ionic liquids have been conducted. In particular, the temperature dependency and kinetics of swelling in 1-butyl-3-methylimidazolium acesulfamate ([BMIM]Ace) and in 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc) were in focus. The kinetics of volumetric swelling in [BMIM]Ace yielded an Ea of 54 kJ.mol-1 compared to 23 kJ.mol-1 in [EMIM]OAc. The viscoelastic behavior of IL-swollen wood was studied by dynamic mechanical analysis, and the lowest in situ glass transition temperature (Tg) of lignin was ∼58°C and ∼65°C in [BMIM]Ace and in [EMIM]OAc, respectively. These Tg values are lower than that found in water. Higher Tg was calculated by the Gordon-Taylor equation, which means that swelling is not only due to the plasticizing effect of the ILs. SEM images showed that [EMIM]OAc has a stronger plasticizing effect than [BMIM]Ace. Overall, [EMIM]OAc is the most efficient agent for wood swelling, softening, and disintegration and, thus, it is the most promising candidate for ionosolv pulping.