Novel Ionic Liquid-Mediated Technologies for the Extraction of Nanocellulose Directly from Wood

The current environmental concerns generate an increasing interest for the extraction and processing of bio-based polymers and materials. Among these is Nanocellulose. Cellulose Nanocrystals (CNCs), one form of nanocellulose, is a high-value nanomaterial with very interesting properties: light weight (density = 1.6 g/cm) with an elastic modulus higher than that for Kevlar fibers (110-220 GPa for CNCs, 125-130 GPa for Kevlar) (Moon, Martini et al. 2011). It also lends itself for a wide range of functional modifications and organizational patterns. Therefore, CNCs has been explored for many applications from automotive industry, membranes, to tissue engineering and pharmaceutical formulation. The production methods of CNCs have varied a lot with the sulfuric acid hydrolysis developed by Ranby in 1949 being the most famous till now. However, all these methods share one main disadvantage; they all require pure forms of cellulose, such as pulp and microcrystalline cellulose, as a starting material. In other words, no reports exist for the direct extraction of CNCs from lignocelluloses such as wood. Such approach, we believe, would foster the production and use of CNCs. In this work, ionic liquids (ILs) are proposed to ensure the multiple functions of delignifying wood and hydrolyzing wood amorphous holocellulose while at the same time preserving cellulose native crystallinity to liberate the cellulose crystallites in the form of nanocrystals. To screen ionic liquid candidates for this objective, 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 compared to 23 kJ.mol 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 towards the extraction of CNCs from wood. Although the previous swelling and softening studies didn’t recommend using [BMIM]Ace, a systematic kinetic and structural study of [BMIM]Ace-mediated delignification of spruce wood at 120 °C allowed shedding light on the possible interactions between wood components and this ionic liquid. Upon the treatment, wood residues and extracts were collected and thoroughly characterized with a wide range of analytical techniques. This extensive structural study evidenced prominent degradation of the acesulfamate anion into sulfate salts in lieu of the expected delignification. This unexpected behavior is explained by extensive derivatization of minute lignin extracts with the sulfate salts, which can lead to an overestimation of delignification