Catalytic performance of a series of guanidinium-based ionic liquids in the coupling reaction of carbon dioxide with epoxides

Abstract

In order to gain more insight into the high catalytic activity of N′′-(2-aminoethyl)-N,N,N′,N′-tetramethylguanidine bromide ([TMGC₂H₄NH₂]Br) and to select a more suitable catalyst for the synthesis of propylene carbonate (PC), the cycloaddition reaction of carbon dioxide (CO₂) into the epoxide (EO), catalyzed by a series of functional guanidinium-based ionic liquids (FGBILs), is schematically studied by the Density Functional Theory (DFT). The calculated results indicate that the formation of carbamic acid is a common pathway with a lower barrier height when the NH₂-functionalized IL encounters CO₂. What is more, the formation of carbamic acid is helpful in decreasing the barrier height of the ring-opening step. Two ILs are involved in the process of forming carbamic acid. Sparked by this, the mechanism catalyzed by two ILs is explored for comparison with the model catalyzed by one IL. In addition, the catalytic activities of other functionalized guanidinium-based ionic liquids are investigated, the results indicate that the task-specified ILs have better catalytic activity than those without functional groups because of the increased acidity. Besides the cation, the influence of different anions and substrates is also investigated.