Revisit coupling of CO2 to ethylene carbonate with an integrated imidazolium and zinc halides catalyst by a study on its decomposition: Active center and mechanism

Coupling of carbon dioxide (CO) to cyclic carbonates with an epoxide and further into linear carbonates as the indispensable solvents for the lithium-ion batteries has been being one of the hottest topics in transforming CO into high value-added chemicals. Nevertheless, the extremely ultrahigh purity requirement for them causes a low efficiency and a modest yield due to the undesired reactions and byproducts occurring in the separation and purification sections. In this work, a novel imidazolium based ionic liquid catalyst system with different anions and cations was studied by a thorough insight into the decomposition reaction of ethylene carbonate (EC), which reveals the synergistic mechanism of the composite catalysts both in section of cyclic carbonate separation from the catalyst for recirculated utilization and purification from the byproducts, and in section of synthesis with high efficiency. Effects of imidazolium cations and halogen anions in the ionic liquid molecule on EC decomposition were investigated by the elaborately designed experiments, thermodynamic estimations, molecular dynamics simulation and DFT assessment. It was found that combining imidazole salts and zinc halides can greatly boost the activity of EC synthesis and decomposition with the effective structure of catalytic center being [EMIm]ZnX. Furthermore, it was indicated that Br has a higher activity for EC synthesis and its reversed pyrolysis, while Cl will promote a side reaction of ring-opening polymerization of EC. Finally, a most favorable catalyst composition with ZnBr and [EMIm]Br for EC synthesis was achieved with a high efficiency in synthesis and a low tendency to initiate the side reactions in separation section.