Efficient solvent-free fixation of CO2 catalyzed by new recyclable bifunctional metal complexes

Abstract

A series of metal complexes containing a metal ion (Zn, Cu, Pb, Ni, Co) and two quaternary phosphonium salt units anchored on the ligands were designed and synthesized as efficient single-component bifunctional catalyst for the solvent-free coupling reaction of CO2 and epoxides without any co-catalysts. The effects of various reaction variables on the catalytic activity were investigated systematically and the optimized reaction conditions were screened as (130°C, 1MPa and 5h). The order of the catalytic activity for these single-component bifunctional metal complexes was found to be Zn-PPBCl (96%)>Co-PPBCl (90%)≈Ni-PPBCl (90%)>Pb-PPBCl (86%)>Cu-PPBCl (15%). Notably, a high turnover frequency (TOF) value (1230h−1) for bifunctional catalyst Zn-PPBCl was achieved via adjusting reaction variables. Moreover, this series of catalysts can also catalyze the coupling reaction at atmospheric pressure, and most of them showed high conversion of epoxide (propylene carbonate (PC) yield>90%) within 7h. The catalysts are also applicable to a variety of epoxides and good catalytic performances were achieved for producing the corresponding cyclic carbonates in most cases. Furthermore, the catalyst can be easily recycled and reused for at least five times without dramatic activity loss after simple filtration and drying. Finally, kinetic studies were carried out preliminarily for four metal (Zn, Co, Ni, Pb) catalysts and the formation activation energies (Ea) of cyclic carbonate were obtained. The apparent activation energy Ea catalyzed by Zn-PPBCl is only 37.6kJ/mol, while the Ea (Pb-PPBCl) is 70.0kJ/mol, Ea (Ni-PPBCl) is 65.6kJ/mol, and Ea (Co-PPBCl) is 43.1kJ/mol. The sequence of Ea agrees well with the catalytic activity. Based on the kinetic studies and experimental results, an inferred mechanism was deduced.