Understanding solvent effects in the selective conversion of fructose to 5-hydroxymethyl-furfural: a molecular dynamics investigation

Abstract

Selective conversion of fructose to 5-hydroxymethyl-furfural (HMF) involves the participation of high-boiling solvents like dimethyl sulfoxide (DMSO). In order to replace DMSO with low-boiling solvents, it is imperative that we understand the effect of DMSO solvation in protecting (i) HMF from rehydration and humins formation reactions and (ii) fructose from side reactions, other than its dehydration to HMF. In the present work, molecular dynamics simulations of HMF and fructose in water and in water-DMSO mixtures are carried out using the OPLS-AA force field. Radial pair distribution functions, coordination numbers and the hydrogen-bond network between the HMF/fructose molecule and the solvent molecules are analysed. The local 3-dimensional picture of the arrangement of solvent molecules around the solute, which cannot be accessed from pair distribution functions, is also computed. We show preferential coordination of DMSO around HMF and explain how this could provide a shielding effect to the HMF molecule, thus protecting it from further rehydration to levulinic acid and formic acid and from humins formation. In the case of fructose, the presence of DMSO also reduces the number of water molecules in the immediate vicinity of fructose. Though fewer water molecules coordinate around fructose, they are bound strongly to it. Analysis of the local 3-dimensional arrangement of DMSO molecules suggests that it protects the fructose molecule from side reactions that would lead to condensation or reversion products. However, the presence of DMSO molecules does not hamper the water molecules coming into contact with the oxygen atom of the hydroxyl groups of fructose, which is required for a proton transfer from water to fructose, to initiate the dehydration reaction to HMF.

The Chemical Conversion of Biomass-Derived Saccharides: an Overview

“…74,86,89,90 Molecular dynamic investigation of fructose conversion into HMF in a DMSO/H 2 O solvent system has shown that DMSO competes with water in the solvation of both reactant and product. 78 DMSO acts as a hydrogen acceptor, interacting more effectively than water with hydrogen from hydroxyl groups in both fructose and HMF, while water can interact with the oxygen atom of the same hydroxyl groups. Therefore, the product yields tend to be higher in the presence of the aprotic solvent DMSO because: (i) its interaction with the fructose hydroxyl groups prevents deprotonation, and consequently, polymerization to undesirable products and (ii) its interaction with the HMF hydroxyl groups prevents rehydration, which could lead to the undesirable humins.…”

“…Therefore, the product yields tend to be higher in the presence of the aprotic solvent DMSO because: (i) its interaction with the fructose hydroxyl groups prevents deprotonation, and consequently, polymerization to undesirable products and (ii) its interaction with the HMF hydroxyl groups prevents rehydration, which could lead to the undesirable humins. 78 Another important effect of DMSO is increasing the fructose dehydration reaction rate. It has been found that the slowest steps of the reaction involve intramolecular hydride transfer, requiring a reorganization of the polar solvent environment and the solvation of asymmetrically distributed electronic charges.…”

“…Taking as an example the synthesis of HMF from the dehydration of fructose, a typical HMF yield in water is lower than 30%. [71][72][73][74] However, using aprotic solvents, such as dimethylsulfoxide (DMSO) [75][76][77][78] and GVL, 73,74 side reactions are suppressed, and HMF yields higher than 90% are expected using mineral acids as catalysts. The separation of the product from these high boiling point solvents is a challenge, mainly, because HMF is prone to decomposition and polymerization at high temperature.…”

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“…It is known that DMSO can promote dehydration efficiently, which could be attributed to the formation of strong acids from the thermolysis of DMSO in air such as sulfuric and methanesulfonic acids during the dehydration of fructose to 5-HMF, especially in high temperatures [25]. Moreover, DMSO would stabilize 5-HMF and suppress both the 5-HMF rehydration and the formation of condensation by-products because DMSO could prevent 5-HMF from rehydration to levulinic acid, formic acid, and humins by a preferential coordination of DMSO around the 5-HMF molecule, and protect the fructose molecule from reactions other than its dehydration to 5-HMF by a specific coordination of DMSO around the fructose molecule [26,27].…”

“…Moreover, DMSO would stabilize 5-HMF and suppress both the 5-HMF rehydration and the formation of condensation by-products because DMSO could prevent 5-HMF from rehydration to levulinic acid, formic acid, and humins by a preferential coordination of DMSO around the 5-HMF molecule, and protect the fructose molecule from reactions other than its dehydration to 5-HMF by a specific coordination of DMSO around the fructose molecule [26,27]. It was obvious that most of the fructose was converted after 1 h, and the reaction with SiO2-HNO3 was always able to obtain a higher final conversion and 5-HMF yield than it was without SiO2-HNO3, demonstrating that SiO2-HNO3 actually accelerated the reaction and promoted the production of 5-HMF, especially at low temperatures.…”