Hydroxyl-Functionalized Poly(triphenylimidazolinium bromide) as an Efficient Catalyst for the Cycloaddition of CO2 to Epoxides

Abstract

In the present study, dihydroxyethylated amarine and iso-amarine ionic salts (namely AM-OH and IAM-OH) along with diethylated amarine and iso-amarine ionic salts (namely AM-Et and IAM-Et) are firstly synthesized. Subsequently, their catalytic performance for the cycloaddition of CO₂ to epichlorohydrin is quantitatively compared under identical reaction conditions. As expected, dihydroxyethylated imidazolinium bromide salts exhibit notably higher catalytic activity than that of their diethylated counterparts, whereas the catalytic activity of AM-OH and IAM-OH is almost identical, highlighting the importance of H-bonding interactions. Inspired by this finding, a dihydroxyethyl-containing poly(triphenylimidazolinium) network (namely fPPIm-OH) is synthesized by one-pot reaction of dihydroxyethylated cis-(±)-2,4,5-tris(p-formylphenyl)imidazolinium bromide (PIm) with hexamethyldisilazane (HMDS) in N,N-dimethylformamide followed by treatment with excess 2-bromoethanol in CH₃CN in the presence of K₂CO₃. The as-synthesized fPPIm-OH network with a Br⁻ content of 2.87 mmol g⁻¹ shows high catalytic activity for the cycloaddition of CO₂ to epoxides, yielding almost quantitative conversion (99%) and selectivity (99%) for a wide range of epoxides. Remarkably, the fPPIm-OH also shows relatively high activity for the cycloaddition of simulated flue gas (0.15 bar CO₂ + 0.85 bar N₂) to epoxides, in which quantitative selectivity (99%) and high conversion (92 ~ 99%) are observed under relatively harsh conditions (80–120 °C). This study presents a facile approach for the synthesis of novel imidazolinium-containing polymeric networks and may inspire more researches aiming at extending the aromatic multialdehydes and simultaneously enhancing the catalytic activity of the resultant polymeric networks.

Graphical Abstract