Room-Temperature and Atmospheric Pressure Coupling of Carbon Dioxide with Epoxides Catalyzed by Iodide Ions Confined in Nanopores of Periodic Mesoporous Organosilica

Abstract

This study explored the influence of the pore size and channel length of mesoporous organosilicas containing pyridine-bis-imidazolium units toward the cycloaddition of CO₂ to epoxides. Utilizing the same organosilica precursor, we synthesized two distinct types of materials: mesoporous organosilica nanoparticles (BIm-MON) with smaller channel sizes and periodic mesoporous organosilica (BIm-PMO) with larger channel sizes. Following the modification of the parent materials with iodide or bromide ions, we prepared a library of catalysts denoted as the X-BIm-PMO and X-BIm-MON series, where X is Cl, Br, or I. It was observed that after modifying BIm-PMO with iodide ions, the entrance pore size was 5.4 nm, whereas the pore sizes for chloride and bromide ions were 8.1 nm. We then compared their catalytic activities in the coupling of CO₂ with styrene oxide as a substrate under two reaction conditions A (5 bar CO₂ at 80 °C) and B (1 bar at RT). Under both reaction conditions, the I-BIm-PMO catalyst demonstrated a higher activity than the other catalysts. The enhanced performance of the I-BIm-PMO catalyst, when compared to its chloride and bromide equivalents or I-BIm-MON, can be explained by the fact that it not only still has good mass transfer but also provides enrichment of CO₂ molecules within the channels through a confinement effect. This confinement effect may be caused by the coexistence of iodide ions and bis-imidazolium groups, leading to increased catalytic activity under ambient conditions. To elucidate the role of the bis-imidazolium groups in the observed activity, we also synthesized a monoimidazolium catalyst (I-MIm-PMO) and evaluated its performance under the same reaction conditions. Due to its effective confinement effect, we found that the I-BIm-PMO catalyst can adsorb CO₂ three times more than I-MIM-PMO. Furthermore, various terminal epoxides were selectively converted into their corresponding cyclic carbonates. The catalyst was also reused four times.