Mechanism of hexaalkylguanidinium salt/zinc bromide binary catalysts for the fixation of CO2 with epoxide: A DFT investigation

• Elucidate the mechanism of guanidinium bromide/ZnBr2-catalyzed reaction.
• Investigate the influence of cations on the performance of ionic liquids/ZnBr2.
• Identify the effect of the bulk of guanidinium salt.
• Compare the activity of different binary catalysts.

Numerous binary catalysts of IL/Lewis acid have been developed for the coupling reaction of carbon dioxide and epoxides to form cyclic carbonates with high catalytic activity under benign environment. However, the mechanism is still obscure for most of catalysts. The catalytic mechanism of the binary catalysts hexaalkylguanidinium bromide/ZnBr2 is elucidated by theoretical method in this work to obtain the reason of their high catalytic activity. Owing to the complicated of the binary catalysts, there are lots of possible attack forms. Finally, it is confirmed that the electrophilic attack from the Zn complex and the nucleophilic attack from the Br− anion are the essential factors to promote the ring opening of PO. Following the most favorable route, the catalytic activity of different binary catalysts, including the ILs/ZnBr2 and NBu4Br/Zn(salphen), is compared. Moreover, the influence of the bulk of hexaalkylguanidinium salt on the catalytic activity is studied. The catalytic activity is enhanced with the increased bulk of the hexaalkylguanidinium salt. It is expected that our theoretical study would provide valuable clues to further refine the binary catalysts.

Figure optionsDownload as PowerPoint slide