18958 | Phys. Chem. Chem. Phys., 2016, 18, 18958--18970 This journal is © the Owner Societies 2016 Cite this: Phys. Chem. Chem. Phys., 2016, 18, 18958 Why are some cyano-based ionic liquids better glucose solvents than water?† Marta L. S. Batista, a Helena Passos, a Bruno J. M. Henriques, a Edward J. Maginn, b Sima ˜ o P. Pinho, c Mara G. Freire,* a Jose ´ R. B. Gomes* a and Joa ˜ o A. P. Coutinho a Among different classes of ionic liquids (ILs), those with cyano-based anions have been of special interest due to their low viscosity and enhanced solvation ability for a large variety of compounds. Experimental results from this work reveal that the solubility of glucose in some of these ionic liquids may be higher than in water – a well-known solvent with enhanced capacity to dissolve mono- and disaccharides. This raises questions on the ability of cyano groups to establish strong hydrogen bonds with carbohydrates and on the optimal number of cyano groups at the IL anion that maximizes the solubility of glucose. In addition to experimental solubility data, these questions are addressed in this study using a combination of density functional theory (DFT) and molecular dynamics (MD) simulations. Through the calculation of the number of hydrogen bonds, coordination numbers, energies of interaction and radial and spatial distribution functions, it was possible to explain the experimental results and to show that the ability to favorably interact with glucose is driven by the polarity of each IL anion, with the optimal anion being dicyanamide. Introduction The carbohydrate D-glucose, a hexopyranose with two stereo- isomers ( a- D-glucopyranose and b- D-glucopyranose), is the monomer of cellulose, the most abundant biopolymer found in nature and a major component of wood. 1 D-Glucose is an essential compound involved in many biological functions and an important raw material for many bio/chemical industrial processes, foremost among these is its role as a renewable feedstock in the production of biofuel. 2–4 The major goal of second generation biofuel production is to recover cellulose from wood, and to proceed to its hydrolysis in order to obtain glucose, upon which fermented bioethanol is produced. 5–8 Several alternatives for the separation of cellulose from wood are available. 6,9 It is known that common solvents, e.g. water, are not able to dissolve such a recalcitrant polymer, such as cellulose, which has led to the use of other solvents. However, common solvents able to dissolve cellulose (carbon disulfide, LiCl-based solvents, 10 dimethylsulfoxide (DMSO)/paraformal- dehyde 11 and N-methylmorpholine-N-oxide (NNMO) 12 ) are either volatile, toxic, expensive or difficult to recover. 13 Among the several alternatives being investigated as solvents for cellulose, ionic liquids (ILs) have been the object of extensive studies. During the past decade, the use of ILs for the pre-treatment and dissolution of wood and cellulose has been reported. 4,13,14 The difficulty is determining the suitable IL solvent with optimal properties. A good solvent should have a low melting tempera- ture, low viscosity, should be non-toxic and non-corrosive, easy to recover and store, should have high stability, and should not decompose lignocellulosic monomers. If an IL is found, which possesses all of these properties, it would be the key to the development of a ‘‘greener’’ and sustainable process of biofuel production. 4,13 Recent reviews published by Ma ¨ki-Arvela et al. 13 and by Pinkert et al. 14 describe extensive experimental work carried out hitherto by applying different ILs to dissolve cellulose, lignin and wood. Complementing these studies, Holm and Lassi 4 published their perspective on the application and perfor- mance of different ILs as enhanced solvents for cellulose. These authors 4 showed that dissolution proceeds by the establish- ment of hydrogen bonds, similar to those existing between glucose molecules that are responsible for the crystalline structure of cellulose. Accordingly, the authors 4 stated that small cations with functionalized groups, and anions with a high ability to establish hydrogen bonds, are the best ions to interact with cellulose, and thus able to dissolve the bio- polymer. Furthermore, for the a priori screening of the best IL anions, the hydrogen bond basicity parameter, b, has been proposed. 13,15,16 The chloride, acetate, formate and phosphate anions were highlighted as promoters of cellulose dissolution. In summary, the right combination between cations and anions a CICECO – Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, Campus Universita ´rio de Santiago, 3810-193 Aveiro, Portugal. E-mail: maragfreire@ua.pt, jrgomes@ua.pt; Tel: +351 234 370 200 b Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, Indiana 46556, USA c Associate Laboratory LSRE-LCM, Departamento de Tecnologia Quı ´micaeBiolo´gica, Instituto Polite ´cnico de Bragança, 5301-857 Bragança, Portugal † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c6cp02538b Received 15th April 2016, Accepted 14th June 2016 DOI: 10.1039/c6cp02538b www.rsc.org/pccp PCCP PAPER Published on 15 June 2016. Downloaded by Universidade de Aveiro (UAveiro) on 14/07/2016 10:37:51. View Article Online View Journal | View Issue