Both softwood (southern yellow pine) and hardwood (red oak) can be completely dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate ([C 2 mim]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 [C 2 mim]OAc is a better solvent for wood than 1-butyl-3-methylimidazolium chloride ([C 4 mim]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.

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Certain ionic liquids have been shown to dissolve cellulose, other biopolymers, and even raw biomass under relatively mild conditions. This particular ability of some ionic liquids, accompanied by a series of concurrent advantages, enables the development of improved processing strategies for the manufacturing of a plethora of biopolymer-based advanced materials. The more recent discoveries of dissolution of lignocellulosic materials (e.g., wood) in ionic liquids, with at least partial separation of the major constituent biopolymers, suggest further paths towards the achievement of a truly sustainable chemical and energy economy based on the concept of a biorefinery which provides chemicals, materials, and energy. Nonetheless, questions remain about the use of ionic liquids and the advisability of introducing any new process which utilizes bulk synthetic chemicals which have to be made, disposed of, and prevented from entering the environment. In this article, we discuss our own journey from the discovery of the dissolution of cellulose in ionic liquids to the cusp of an enabling technology for a true biorefinery and consider some of the key questions which remain.

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We have found that 1-alkyl-3-methylimidazolium chloride ionic liquids (ILs) can form immiscible liquid mixtures with some polyethylene glycols (PEGs). Binary mixtures of 1-ethyl-3-methylimidazolium chloride with PEG of molecular weight 1500, 2000, or 3400 g mol(-1), or of 1-butyl-3-methylimidazolium chloride with PEG of molecular weight 2000 or 3400 g mol(-1), have been found to give rise to entirely liquid, stable biphasic systems over a significant temperature range (from 333.15 K to 413.15 K), while mixtures of 1-ethyl-3-methylimidazolium chloride with PEG-1000 and 1-butyl-3-methylimidazolium chloride with PEG-1000 and PEG-1500 are miscible. The mutual immiscibility of the IL and the PEG increases as the temperature increases. The evolution of the composition of the phases in equilibrium with the molecular weight of the PEG, or with the variation of the length of the alkyl substituent chain of the imidazolium cation of the IL, has been explored. The trends observed are explained through the complexity of interactions present within the binary system. A thermodynamic analysis of the liquid-liquid equilibrium data indicates negative values for the change of enthalpy and entropy of mixing. The potential application of these biphasic, entirely liquid systems, with low volatility and good solvation properties, for the dissolution and separation of cellulose and lignin at elevated temperature has been preliminarily explored, although only modest results have been achieved to date.

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