Guanidine Hydrochloride/ZnI₂ as Heterogeneous Catalyst for Conversion of CO₂ and Epoxides to Cyclic Carbonates under Mild Conditions

摘要

In this article, the combination of guanidine hydrochloride with co-catalyst ZnI₂ proved to be a highly efficient heterogeneous catalyst for the environmentally benign, solvent-free synthesis of cyclic carbonates under mild reaction conditions. The effects of different co-catalysts as well as reaction parameters including catalyst loadings, CO₂ pressure, reaction temperature, and reaction time on the coupling reaction of CO₂ to propylene oxide were thoroughly investigated. With the molar ratio of guanidine hydrochloride to ZnI₂ at 5:1, excellent yield (94%) and selectivity (≥99%) of propylene carbonate were obtained under 100 °C and at 1 MPa for 1.5 h. Additionally, ZnI₂ could be recycled, but because of the washing loss of guanidine hydrochloride, there was a slight decrease in the yield of propylene carbonate. Gratifyingly, the activity of the catalytic system could be restored by adding additional 20 mol% of fresh guanidine hydrochloride, thus exhibiting excellent recyclability of the ZnI₂ catalyst. Moreover, the binary catalysts were also versatile when using other epoxides for CO₂ cycloaddition. A possible reaction mechanism was proposed wherein guanidine hydrochloride plays a dual role in activating CO₂ and epoxide, and ZnI₂ activated epoxide, simultaneously. The synergistic effect of guanidine hydrochloride and ZnI₂ ensure the reaction proceeds effectively.