(350f) Understanding Solvation Effects On Biomass Derived Platform Chemicals: A Combined Spectroscopic and Theoretical Approach

Understanding solvation effects on biomass derived platform chemicals: A combined spectroscopic and theoretical approach

G. Tsilomelekis, C. Bagia, T.R. Josephson, S. Caratzoulas, V. Nikolakis, D.G. Vlachos

Catalysis Center for Energy Innovation & Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716

The dehydration of carbohydrates, and especially fructose to 5- hydroxymethylfurfural (HMF), has attracted increasing interest since HMF is considered as a “top value-added chemical” because it can be transformed to non-petroleum based building blocks for chemicals and fuels [1,2]. Even though it is well established that the HMF selectivity of fructose dehydration reactions can be significantly affected by using organic co-solvents or by carrying out the reaction in organic solvents or in ionic liquids, the reasons behind these observations remain elusive[3,4]. Furthermore, studies correlating the effect of different solvents in HMF molecular structure in solution with HMF stability are lacking.

With the ultimate goal to unravel structure-activity relationships using insights from solvation, the present work focuses on the understanding the structural as well as the vibrational properties of dissolved HMF in DMSO-D₂O mixtures in a wide range of binary and ternary compositions by employing ATR/FTIR spectroscopy and ab-initio DFT calculations. Solvent effects, such as the formation of hydrogen bonds (HBs), are investigated by probing both HMF as well as DMSO and D₂O vibrational modes of specific functional groups under different conditions. The HMF conformational equilibrium is considered too, for the first time. HMF rehydration experiments are also carried out starting with HMF/DMSO/D₂O ternary mixtures of different compositions.

The analysis of the HMF ATR/IR spectra focused mainly on the change of the vibrational behavior of HMF functional groups, C=O and OH, located at ~1670 and ~3300-3450 cm^-1 respectively. The position, the width as well as the shape of the bands are analysed by exploring all the plausible reasons such, vibrational coupling, formation of hydrogen bonds and –cis/-trans HMF conformational equilibrium. The latter is also studied for the first time using ab-initio calculations. Calculations indicate that the trans-HMF is the more stable conformer. The predicted IR spectra are in very good agreement with our experiments. The effect of solvent composition on the C=O and OH vibrational modes of HMF, reveals significant differences which are ascribed to intermolecular interactions between HMF and DMSO or/and water. The C=O stretching vibration appears clearly in higher frequency in the case of HMF/DMSO mixtures than in HMF/water. In contrary, the OH stretching vibration of HMF appears in lower wavenumbers in DMSO compared to water, indicating that the hydrogen bonds between the H of the HMF OH and the oxygen of DMSO to be stronger. The experimental observations are confirmed with our ab-initio calculations. In the case of HMF/DMSO/D₂O ternary mixtures, the addition of D₂O molecules around HMF seems to affect significantly the frequency of the HMF OH group while, the C=O stretching band is affected only when the D₂O molar fraction is higher than 0.3. More specifically, as the D₂O content increases a shift to lower wavenumbers is observed. This effect is ascribed to a perturbation in bond length (elongation) and strength (weakening) of the C=O band caused by the formation of C=O…D hydrogen bonds between HMF and D₂O molecules. The implications of spectroscopic analysis for solvent selection and improved biomass processes will also be discussed.

References

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