Facile synthesis of DBU-based ionic liquids cooperated with ZnI₂ as catalysts for efficient cycloaddition of CO₂ to epoxides under mild and solvent-free conditions

The cycloaddition of CO₂ to epoxides is an important reaction and has got increasing attention. In this work, a series of DBU-based ionic liquids (DILs) were synthesized via facile neutralization of superbase 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) with corresponding hydrogen bond donor acids, such as D(−)-tartaric acid (TA), malic acid (MA) and succinic acid (SA). The DILs cooperated with ZnI₂ were first evaluated in detail for catalyzing the coupling reaction of CO₂ with propylene oxide (PO), and DIL-1 exhibited obviously enhanced catalytic activity under 80 °C, 3 MPa and solvent-free conditions. The effects of reaction parameters on the activity for propylene carbonate (PC) synthesis were thoroughly investigated, and a possible synergistic catalytic mechanism was proposed. The catalytic system reported here could be separated by simple centrifugation from the products after reaction and then reused efficiently. Moreover, the catalytic system showed the advantages of simple synthesis, high catalytic activity, low cost, and substrate versatility for CO₂ conversion to cyclic carbonates.