Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3-methylimidazolium acetate

TLDR

The ionic liquid 1‑ethyl‑3‑methylimidazolium acetate is a superior wood solvent compared to 1‑butyl‑3‑methylimidazolium chloride, with dissolution influenced by wood type, load, and particle size. Wood is fully dissolved by heating in an oil bath (microwave or ultrasound can accelerate the process), then reconstituted with acetone/water to produce lignin‑free cellulose and reduced lignin fractions that are characterized against the original material. Both southern yellow pine and red oak dissolve completely, with red oak dissolving faster; reconstitution yields cellulose‑rich material containing 59 % of pine holocellulose and 31–38 % of lignin, while lignin content is reduced by 26.1 % (pine) and 34.9 % (oak).

Abstract

Both softwood (southern yellow pine) and hardwood (red oak) can be completely dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim]OAc) after mild grinding. Complete dissolution was achieved by heating the sample in an oil bath, although wood dissolution can be accelerated by microwave pulses or ultrasound irradiation. It has been shown that [C2mim]OAc is a better solvent for wood than 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) and that variables such as type of wood, initial wood load, particle size, etc. affect dissolution and dissolution rates; for example, red oak dissolves better and faster than southern yellow pine. Carbohydrate-free lignin and cellulose-rich materials can be obtained by using the proper reconstitution solvents (e.g., acetone/water 1 : 1 v/v) and approximately 26.1% and 34.9% reductions of lignin content in the reconstituted cellulose-rich materials (from pine and oak, respectively) have been achieved in one dissolution/reconstitution cycle. The regenerated cellulose-rich materials and lignin fractions were characterized and compared with the original wood samples and biopolymer standards. For pine, 59% of the holocellulose (i.e., the sum of cellulose and hemicellulose) in the original wood can be recovered in the cellulose-rich reconstituted material; whereas 31% and 38% of the original lignin is recovered, respectively, as carbohydrate-free lignin and as carbohydrate-bonded lignin in the cellulose-rich material.