Theoretical simulations and experiments were taken to explore the mechanism of cellulose conversion into 5-ethoxymethylfurfural (EMF) and ethyl levulinate (EL) catalyzed by a mixed acid of Lewis acid and Brønsted acid. Up to 64.8 % total yield of EMF and EL can be obtained when the ratios of Lewis/Brønsted acid and water/ethanol (v/v) were 1:2 and 3:7 respectively. The results showed that Lewis acid and water could reduce the activation energy of EMF formation from 75.6 to 47.9 kJ mol−1, and mixed acids and water/ethanol system were beneficial to the accumulation of EMF. Moreover, the results of molecular dynamics (MD) simulation indicated that Lewis acid could promote the distribution of Bronsted acid around the β-1,4 glycosidic bond of cellobiose. The combination of DFT and frontier molecular orbital (FMO) calculation demonstrated that the breakage of the β-1,4 glycosidic bond was the main rate-limiting step of cellulose hydrolysis. Meanwhile, the mixed acid and water/ethanol system could accelerate the hydrolysis due to the reducing energy barrier and HOMO-LUMO gap. This study gave a deep insight into the mechanism of Lewis acid with Brønsted acid on cellulose conversion into EMF and EL.

中文翻译：

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纤维素生产5-乙氧基甲基糠醛和乙酰丙酸乙酯的理论与实验研究


通过理论模拟和实验探讨了路易斯酸和布朗斯台德酸混合酸催化纤维素转化为5-乙氧基甲基糠醛（EMF）和乙酰丙酸乙酯（EL）的机理。当路易斯/布朗斯台德酸和水/乙醇(v/v)的比例分别为1:2和3:7时，EMF和EL的总产率高达64.8%。结果表明，Lewis酸和水可以将EMF形成的活化能从75.6 kJ mol−1降低到47.9 kJ mol−1，混合酸和水/乙醇体系有利于EMF的积累。此外，分子动力学（MD）模拟结果表明，路易斯酸可以促进布朗斯台德酸在纤维二糖的β-1,4糖苷键周围的分布。 DFT与前沿分子轨道（FMO）计算相结合表明，β-1,4糖苷键的断裂是纤维素水解的主要限速步骤。同时，混合酸和水/乙醇体系由于降低了能垒和HOMO-LUMO间隙而加速了水解。这项研究深入了解了路易斯酸与布朗斯台德酸将纤维素转化为 EMF 和 EL 的机制。