Center for Catalytic Science and Technology, Department of Chemical & Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA.
J Am Chem Soc. 2013 Mar 13;135(10):3997-4006. doi: 10.1021/ja3122763. Epub 2013 Mar 1.
5-(Hydroxymethyl)furfural (HMF) and levulinic acid production from glucose in a cascade of reactions using a Lewis acid (CrCl3) catalyst together with a Brønsted acid (HCl) catalyst in aqueous media is investigated. It is shown that CrCl3 is an active Lewis acid catalyst in glucose isomerization to fructose, and the combined Lewis and Brønsted acid catalysts perform the isomerization and dehydration/rehydration reactions. A CrCl3 speciation model in conjunction with kinetics results indicates that the hydrolyzed Cr(III) complex Cr(H2O)5OH is the most active Cr species in glucose isomerization and probably acts as a Lewis acid-Brønsted base bifunctional site. Extended X-ray absorption fine structure spectroscopy and Car-Parrinello molecular dynamics simulations indicate a strong interaction between the Cr cation and the glucose molecule whereby some water molecules are displaced from the first coordination sphere of Cr by the glucose to enable ring-opening and isomerization of glucose. Additionally, complex interactions between the two catalysts are revealed: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of Cr(H2O)5OH. In contrast, Lewis acidity increases the overall rate of consumption of fructose and HMF compared to Brønsted acid catalysis by promoting side reactions. Even in the absence of HCl, hydrolysis of Cr(III) decreases the solution pH, and this intrinsic Brønsted acidity drives the dehydration and rehydration reactions. Yields of 46% levulinic acid in a single phase and 59% HMF in a biphasic system have been achieved at moderate temperatures by combining CrCl3 and HCl.
在水相介质中使用路易斯酸（CrCl3）催化剂和布朗斯特酸（HCl）催化剂，对葡萄糖进行级联反应生成 5-（羟甲基）糠醛（HMF）和乙酰丙酸。结果表明，CrCl3 是葡萄糖异构化为果糖的活性路易斯酸催化剂，而路易斯酸和布朗斯特酸的组合催化剂则进行异构化和脱水/再水化反应。CrCl3 形态模型与动力学结果表明，水解的 Cr(III)络合物Cr(H2O)5OH是葡萄糖异构化中最活跃的 Cr 物种，可能作为路易斯酸-布朗斯特碱双功能位。扩展 X 射线吸收精细结构光谱和 Car-Parrinello 分子动力学模拟表明，Cr 阳离子与葡萄糖分子之间存在强烈相互作用，其中一些水分子被葡萄糖取代，从而使葡萄糖开环并发生异构化。此外，还揭示了两种催化剂之间的复杂相互作用：布朗斯特酸度通过降低Cr(H2O)5OH的平衡浓度来阻碍醛糖到酮糖的异构化。相比之下，路易斯酸度通过促进副反应，提高了与布朗斯特酸催化相比果糖和 HMF 的总消耗速率。即使没有 HCl，Cr(III)的水解也会降低溶液的 pH 值，这种内在的布朗斯特酸度驱动脱水和再水化反应。通过组合使用 CrCl3 和 HCl，在温和的温度下，在单相中实现了 46%的乙酰丙酸产率，在两相体系中实现了 59%的 HMF 产率。
