Heterogeneous catalytic conversion of CO2 and epoxides to cyclic carbonates over multifunctional tri-s-triazine terminal-linked ionic liquids

Novel metal-free tri-s-triazine terminal-linked ionic liquids have been developed from various precursors of urea-derivative-based ionic liquid/urea by a co-condensation method. These were successfully used for the chemical fixation of CO2 to epoxides producing cyclic carbonates under mild and green conditions in which no co-catalyst and solvent were needed. The tri-s-triazine terminal-linked ionic liquids so prepared possess multiple functionalities of hydrogen bond donor ability, nucleophilicity, and Lewis base property, which are vital to the ring-opening of epoxide and the activation of CO2. Reaction parameters (temperature, CO2 pressure, catalyst loading, time, water content) for cycloaddition of CO2 with propylene oxide (PO) to propylene carbonate (PC) over tri-s-triazine terminal-linked ionic liquids were optimized. It has been shown that, under the optimal conditions, the catalyst is also versatile to CO2 cycloaddition with other epoxides, can be reused in up to five consecutive recycles without significant loss of activity, and can still remain active in the presence of moderate amounts of water. A possible reaction mechanism has been proposed. The single-component, metal-free, multifunctional, and easily recyclable tri-s-triazine terminal-linked ionic liquids reported here can be an effective and ecofriendly catalyst for highly efficient CO2 fixation. (C) 2016 Elsevier Inc. All rights reserved.