Catalysis Centre for Energy Innovation and the Department of Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, USA.
Phys Chem Chem Phys. 2012 Feb 28;14(8):2637-44. doi: 10.1039/c2cp22694d. Epub 2012 Jan 25.
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.
选择将果糖转化为 5-羟甲基糠醛（HMF）需要使用高沸点溶剂，如二甲基亚砜（DMSO）。为了用低沸点溶剂替代 DMSO，我们必须了解 DMSO 溶剂化在保护（i）HMF 免于再水合和生成类黑精反应，以及（ii）果糖免于除脱水生成 HMF 之外的副反应中的作用。在目前的工作中，使用 OPLS-AA 力场对 HMF 和果糖在水中和水-DMSO 混合物中的分子动力学模拟进行了研究。分析了 HMF/果糖分子与溶剂分子之间的径向对分布函数、配位数和氢键网络。还计算了无法从对分布函数获得的围绕溶质的溶剂分子的局部 3 维排列的图片。我们展示了 DMSO 优先围绕 HMF 的配位，并解释了这如何为 HMF 分子提供屏蔽效应，从而保护其免受进一步再水合生成乙酰丙酸和甲酸以及生成类黑精。对于果糖，DMSO 的存在还减少了果糖附近的水分子数量。尽管围绕果糖的水分子数量较少，但它们与果糖结合得很强。对 DMSO 分子局部 3 维排列的分析表明，它保护果糖分子免受可能导致缩合或反转产物的副反应。然而，DMSO 分子的存在并不妨碍水分子与果糖的羟基氧原子接触，这是水向果糖转移质子以启动脱水生成 HMF 反应所必需的。
