Promotion catalytic role of ethanol on Brønsted acid for the sequential dehydration-etherification of fructose to 5-ethoxymethylfurfural
Promotion catalytic role of ethanol on Brønsted acid for the sequential dehydration-etherification of fructose to 5-ethoxymethylfurfural
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DOI：
10.1016/j.jcat.2017.06.031
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发表时间：
2017-08
影响因子：
7.3
通讯作者：
Bo Xiang;Yue Wang;Ting Qi;Huaqing Yang;Changwei Hu
中科院分区：
文献类型：
--
作者：
Bo Xiang;Yue Wang;Ting Qi;Huaqing Yang;Changwei Hu
在G4水平上研究了乙醇和Brnsted酸在乙醇溶液中对果糖脱水醚化合成5-乙氧甲基呋喃甲醛(EMF)的作用。最初，果糖倾向于脱水，而不是在Brnsted酸存在下与乙醇进行醚化反应。质子H+应与果糖以外的乙醇反应，生成[C2H5OH2]+作为催化活性物种。质子化和[C2H5OH2]+都表现出良好的催化性能，而乙醇则不是。此外，[C2H5OH2]+表现出比质子化更好的催化性能，这反映了C2H5OH对质子H+的促进催化作用。转化率分析表明，在[C2H5OH2]+催化下，5-羟甲基呋喃甲醛(HMF)醚化反应中的SN2亲核取代反应是整个反应的速度控制步骤。[C2H5OH2]+的催化性能来源于单键OH2基团的正电荷，它既有助于果糖向HMF释放H2O，又有助于HMF与EMF的醚化反应。对于果糖脱水反应，[C2H5OH2]+对质子化反应的催化优势来自于[C2H5OH2]+质子化催化的分子内氢转移到分子间氢转移，从而降低了H2O释放的活化能垒。本研究有助于了解乙醇和Brnsted酸在乙醇溶液中碳水化合物的酸催化脱水-醚化反应中的作用。
The function of ethanol and Brønsted acid for the dehydration-etherification of fructose to 5-ethoxymethylfurfural (EMF) has been theoretically investigated at G4 level in ethanol solution. Initially, fructose prefers to dehydration other than etherification with ethanol in the presence of Brønsted acid. The proton H+should be solvated on ethanol other than fructose, resulting in [C2H5OH2]+as the catalytically active species. Both protonation and [C2H5OH2]+exhibit good catalytic performance, but ethanol does not. Furthermore, [C2H5OH2]+displays better catalytic performance than protonation, which reflects the promotion catalytic role of C2H5OH on the proton H+. The turnover frequency analysis shows that the SN2 nucleophilic substitution for the etherification of 5-hydroxymethylfurfural (HMF) to EMF is the rate-controlling step in the whole reaction catalyzed by [C2H5OH2]+. The catalytic performance of [C2H5OH2]+stems from the positive charge of single bondOH2group, which assists both the H2O release of fructose to HMF and the etherification of HMF to EMF. For the dehydration of fructose, the catalytic superiority of [C2H5OH2]+to protonation comes from the shift of the intramolecular H-shift catalyzed by protonation to the intermolecular H-shift catalyzed by [C2H5OH2]+, which lowers the activation energy barrier for the H2O release. The present study is useful for understanding the roles of ethanol and Brønsted acid in acid-catalyzed dehydration-etherification of carbohydrates to biofuel in ethanol solution.