Synergistic catalysis of sulfate ionic liquids and FeCl₃ for the dehydration of 1,4-butanediol to tetrahydrofuran

Tetrahydrofuran is a widely used bulk chemical in both laboratory and industry. In this study, a homogeneous catalytic system composed of sulfate ionic liquids and FeCl₃ was applied for the synthesis of tetrahydrofuran by the dehydration of 1,4-butanediol with a high yield of 89.2%. Kinetic studies showed that the reaction rate constant (k) of the dehydration of 1,4-butanediol catalyzed by the combination of 1-ethyl-3-methylimidazolium ethyl sulfate ionic liquid ([EMIm][EtSO₄]) and FeCl₃ was significantly higher than that of [EMIm][EtSO₄] or FeCl₃ as catalyst alone. Besides, the activation energy (Ea) of this reaction over [EMIm][EtSO₄] and FeCl₃ was lower than either of them as catalyst. Higher reaction rate constant and lower Ea indicated that [EMIm][EtSO₄] and FeCl₃ had a synergistic catalytic effect. Mechanism studies showed that 1-ethyl-3-methylimidazolium hydroxybutyl sulfate ionic liquid [EMIm][HO-BuSO₄], generated by the transesterification of [EMIm][EtSO₄] with 1,4-butanediol over FeCl₃, acted as an active species to synergistically catalyze the dehydration reaction of 1,4-butanediol with FeCl₃. Density functional theory (DFT) calculations demonstrated the synergistic catalytic mechanism of [EMIm][HO-BuSO₄] and FeCl₃, and revealed that the intramolecular nucleophilic attack was the rate-determining step of the dehydration reaction. Continuous reactive distillation of 1,4-butanediol showed [EMIm][EtSO₄] and FeCl₃ could be used continuously for 54 h with the TON of 182 without any loss in activity. The catalysts also exhibited good substrate generality in the dehydration of various diols to cyclic ethers.