Cellulose Processing in Ionic Liquids

The capacity of the ionic liquids of dissolving high concentrations of cellulose and/or lignin at relatively low temperatures makes them promising solvents for the processing of lignocellullosic biomass waste. For several years these processes have been studied and patented: pre-treatment for fermentation processes, chemical transformations for obtaining chemical or biofuels and substitution reaction for obtaining cellulose derived polymers among others. The use of ionic liquids has a number of advantages determined by the unique combination of their properties. Ionic liquids are a group of salts that exist as liquids at relatively low temperatures (<100 °C). Their properties can be tunable by appropriate selection of cation and anion, and they have an immeasurably low vapor pressure that makes them to be considered as green solvents. They present remarkable chemical properties such as super-acidity or Lewis acid/base behaviour that allows them to act as catalyst or to stabilize different catalysts improving their performance.

Ionic liquids (ILs) have recently been demonstrated to be highly effective solvents for the dissolution of cellulose in amounts as high as 25% in mass, sometimes even at room temperature. Nevertheless the high viscosity of ionic liquids that is highly increased when they dissolve cellulose is the main limitation of these processes.

It is known that when the ionic liquids dissolve small amounts of molecular solvents their viscosity is drastically decreased. Nevertheless, the use of these solvents is limited by the fact that they do not cause cellulose precipitation (such as water or other protic solvents) and that can be easily recovered. Carbon dioxide presents as a promising co-solvent for the ionic liquid processing of lignocellulose as it is an inert gas without environmental limitation that presents high solubilities in ionic liquids even at low pressures, and can be easily separated of the mixture by depressurization.

References:

Swatloski, R. P., Spear, S. K., Holbrey, J. D., Rogers, R. D., “Dissolution of Cellulose with Ionic Liquids”, Journal of American Chemical Society, vol. 124, pp. 4974-4975, 2002.

Pinkert, K. N. Marsh, S. Pang, M. P. Staiger, “Ionic Liquids and their interaction with Cellulose,” Chemical Reviews, vol.109, pp. 6712-6728, 2009

FitzPatrick M., Champagne P., Cunningham M. F., “The effect of subcritical carbon dioxide on the dissolution of cellulose in the ionic liquid 1-ethyl-3-methylimidazolium acetate”, Cellulose, vol. 19, pp. 37-44, 2012