(266d) Understanding the Solvation of Fructose and 5-Hydroxymethylfurfural in Water - Dimethyl Sulfoxide Mixtures Using Molecular Dynamics and Vibrational Spectroscopy

Fructose dehydration to 5-hydroxymethylfurfural (HMF) is a well-studied reaction and it is considered a characteristic example of converting carbohydrates to useful chemical intermediates [1]. The choice of solvent (DMSO, water or DMSO – water mixture) significantly affects its selectivity and yield [1,2]. For example, the acid catalyzed dehydration in an aqueous environment is accompanied by side reactions which lead to the formation of acyclic dehydration and fragmentation products, insoluble humins and HMF rehydration products. On the other hand, HMF yields higher than 90% can be achieved in DMSO [3]. Unfortunately, issues related to the separation of HMF from DMSO render this solvent unattractive for commercial applications. The goal of this work is to understand the solvation of fructose and HMF in DMSO and to provide insights about the effect of solvation on the selectivity of fructose dehydration. To achieve this goal we used a combination of vibrational spectroscopy (ATR/FTIR, Raman) and molecular simulations (Molecular Dynamics).

The analysis of the fructose hydroxyl hydrogen – DMSO oxygen radial distribution function showed that the coordination number of DMSO around fructofuranose is ~3.5. This number is smaller than the number of hydroxyl groups of fructose because one DMSO is shared between two hydroxyl groups and because of the formation of intramolecular hydrogen bonds. In the case of fructose - DMSO mixtures, a red shift of the Raman S=O asymmetric stretching is observed which indicates that fructose breaks the DMSO clusters owing to strong hydrogen bonding between the hydrogen of its hydroxyl groups and the oxygen of DMSO.  The Raman scattering cross-sections of the DMSO S=O stretching when a DMSO molecule interacts with another DMSO, fructose, water or HMF were estimated from the spectra of the binary mixtures using the coordination numbers from the MD simulations. It was also possible to use these values together with the MD predicted coordination numbers to  satisfactorily predict the effect of water fraction on the Raman scattering intensity of the S=O stretching band in ternary mixtures when the H₂O/DMSO molar ratio was less than ~0.3. MD simulations also showed that with increasing water content the DMSO orientation around fructose changes with the sulfur atom moving away from the carbohydrate. The analysis of the ATR/FTIR spectra reveal that with increasing water content the average hydrogen bond enthalpy of the fructose hydroxyls decreases by ~2.5 kJ/mol. The MD simulations indicate that this decrease is due to the reduction of the strength of DMSO – fructose hydrogen bond, and due to the replacement of a fraction of the stronger DMSO – fructose hydrogen bonds by the weaker fructose – water ones. Finally, MD simulations also show the preferential coordination of DMSO around HMF carbonyl group could shield HMF from further rehydration to levulinic acid and formic acid and from humins formation.

1. Kuster, B. F. M., Starch 1990, 42 (8), 314-321.

2. Chheda, J. N.; Roman-Leshkov, Y.; Dumesic, J. A. Green Chemistry 2007, 9 (4), 342-350

3. Musau, R. M.; Munavu, R. M., J of Chem Tech. and Biotechnology 1987, 32, 920-924

4. Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E., J. Chem Theory and Comp. 2008, 4 (3), 435-447.