Catalysis Center for Energy Innovation and Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA.
Carbohydr Res. 2011 Apr 1;346(5):664-72. doi: 10.1016/j.carres.2011.01.029. Epub 2011 Feb 4.
We studied the energetics of the closed-ring mechanism of the acid-catalysed dehydration of d-fructose to 5-hydroxymethylfurfural (HMF) by carrying out canonical ensemble free-energy calculations using bias-sampling, hybrid Quantum Mechanics/Molecular Mechanics Molecular Dynamics simulations with explicit water solvent at 363 K. The quantum mechanical calculations are performed at the PM3 theory level. We find that the reaction proceeds via intramolecular proton and hydride transfers. Solvent dynamics effects are analysed, and we show that the activation energy for the hydride transfers is due to re-organization of the polar solvent environment. We also find that in some instances intramolecular proton transfer is facilitated by mediating water, whereas in others the presence of quantum mechanical water has no effect. From a micro-kinetic point of view, we find that the rate-determining step of the reaction involves a hydride transfer prior to the third dehydration step, requiring an activation free energy of 31.8 kcal/mol, and the respective rate is found in good agreement with reported experimental values in zeolites. Thermodynamically, the reaction is exothermic by ΔF=20.5 kcal/mol.
我们通过使用偏置采样、在 363 K 下使用显式水溶剂的混合量子力学/分子力学分子动力学模拟，对 d-果糖酸催化脱水生成 5-羟甲基糠醛（HMF）的闭环机制的能量进行了研究。量子力学计算在 PM3 理论水平上进行。我们发现反应通过分子内质子和氢化物转移进行。分析了溶剂动力学效应，我们表明氢化物转移的活化能归因于极性溶剂环境的重新组织。我们还发现，在某些情况下，分子内质子转移通过介导水而得到促进，而在其他情况下，量子力学水的存在没有影响。从微观动力学的角度来看，我们发现反应的速率决定步骤涉及第三个脱水步骤之前的氢化物转移，需要 31.8 kcal/mol 的活化自由能，并且相应的速率与沸石中报道的实验值非常吻合。热力学上，反应是放热的，ΔF=20.5 kcal/mol。
