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 Zn-SBA-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 rate-determining 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.