Experimental and theoretical insights into binary Zn-SBA-15/KI catalysts for the selective coupling of CO2 and epoxides into cyclic carbonates under mild conditions

The combination of a metal-modified SBA-15 catalyst with potassium iodide was developed as heterogeneous dual catalysts for chemical fixation of CO2 to cyclic carbonates. It was observed that the binary ZnSBA- 15/KI catalysts were the most efficient among various metal-modified SBA-15/KI catalysts and showed an excellent synergetic effect in promoting the reaction under mild conditions. Moreover, the effects of reaction parameters on cycloaddition of CO2 with propylene oxide (PO) to propylene carbonate (PC) were optimized. Under the optimal conditions determined, the Zn-SBA-15/KI catalytic system was also versatile in CO2 cycloaddition with other epoxides. Additionally, the mechanistic details for the fixation of CO2 into a cyclic carbonate catalyzed by SBA-15/KI and Zn-SBA-15/KI were also contrastively elucidated using the density functional theory (DFT) method. The DFT results suggested that the zinc-modified and unmodified catalysts showed different coupling modes of CO2, and the ring-opening reaction was the ratedetermining step in the SBA-15/KI catalyzed cycloaddition reaction, but the zinc-modified SBA-15/KI catalysts could enhance the CO2 cycloaddition, as the formation of a stable complex was beneficial to CO2 trapping. As a result, the ring-closing reaction became the rate-determining step in the Zn-SBA-15/KI catalyzed cycloaddition reaction, which was a promising result to guide the catalyst design for CO2 conversion.