Application of alkaline ionic liquid/Brønsted acid synergistic catalysis for the synthesis of cyclic carbonates from aliphatic diols and CO2†

The utilization of aliphatic diols as a substitute for epoxides in the reaction with CO₂ to produce cyclic carbonates offers a cost-effective and less hazardous synthesis route. This approach aligns with green chemistry principles and presents new strategies to combat climate change. This study primarily focuses on the innovative application of an alkaline ionic liquid in conjunction with a Brønsted acid for the catalytic synthesis of cyclic carbonates from CO₂ and aliphatic diols. Three alkaline ionic liquid catalysts [DBUH]PHY, [TBDH]PHY, and [DBUH]TBD were successfully synthesized using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), and phenol as precursors. Their chemical structures and thermal properties were systematically characterized using Fourier-transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR), carbon nuclear magnetic resonance (¹³C-NMR), and thermogravimetric (TG) analysis. The catalytic performance of the three alkaline ionic liquids combined with various Brønsted acids (including sulfuric acid, phosphoric acid, and acetic acid) for the synthesis of ethylene carbonate (EC) from ethylene glycol (EG) and CO₂ was investigated. The results demonstrated that the combination of [DBUH]PHY with sulfuric acid (H₂SO₄) exhibited the best synergetic catalytic effect. Subsequently, the process parameters were optimized to study the effects of reaction temperature, pressure, time, and catalyst loading on catalytic performance. Under optimized conditions, the catalytic efficiency of [DBUH]PHY and sulfuric acid in the synthesis of cyclic carbonates from different aliphatic diols, including ethylene glycol, propylene glycol (1,2- and 1,3-), and butanediol (1,4-), was explored. This study successfully achieved the activation of aliphatic diols and CO₂ through cooperative catalysis of alkaline ionic liquids and Brønsted acids, providing an innovative catalytic system for research in this field.