Melamine–ZnI2 as heterogeneous catalysts for efficient chemical fixation of carbon dioxide to cyclic carbonates

Abstract

In this contribution, the combination of melamine with Lewis acid ZnI₂ was developed as heterogeneous dual catalysts for the cycloaddition of carbon dioxide with epoxides yielding the corresponding cyclic carbonates. With molar ratio of 1 : 3.3 of ZnI₂ to melamine, high yield (96%) and selectivity (99%) of propylene carbonate was obtained at 150 °C and 3.0 MPa for 4.0 h. The binary catalysts were also effectively versatile for CO₂ cycloaddition to other epoxides, especially the less active epoxides such as styrene oxide and cyclohexene oxide. Additionally, the catalysts could be separated easily from the products after reaction and then reused efficiently. Furthermore, a possible synergistic catalytic mechanism was proposed, wherein, melamine played the dual roles to activate CO₂ and epoxide simultaneously, ZnI₂ activated epoxide and subsequently attacked the activated epoxide, the synergetic effects from melamine and ZnI₂ promoted the reaction smoothly. The binary catalysts showed the advantages of simple preparation, low cost, abundant availability and high catalytic activity for CO₂ chemical fixation into valuable chemicals.