CO2 adsorption and conversion into cyclic carbonates over a porous ZnBr2-grafted N-heterocyclic carbene-based aromatic polymer

Abstract

A new nanoporous N-heterocyclic carbene-based cross-linked aromatic polymer (NHC-CAP-1) incorporated with highly nucleophilic bromide anions and with a large surface area was synthesized by a simple Friedel-Crafts reaction of imidazolium salt, triphenylbenzene, and formaldehyde dimethyl acetal. Subsequently, ZnBr2 was grafted onto the NHC-CAP-1 to obtain NHC-CAP-1(Zn2+) with enhanced Lewis acidity. After systematic evaluation of the structural and chemical properties using different analytical techniques, these were explored for CO2 adsorption and CO2 chemical fixation to cyclic carbonates. Whilst NHC-CAP-1 showed a high CO2 capture capacity (188.2 mg g−1 at 273 K/1 bar) with a moderate CO2/N2 selectivity, NHC-CAP-1(Zn2+) displayed significantly enhanced CO2/N2 selectivity (100/80 at 273/298 K) at the expense of diminished CO2 capture (123.0 mg g−1 at 273 K/1 bar). These values are among the highest reported so far for porous cross-linked organic polymers. As a catalyst, NHC-CAP-1(Zn2+) showed high catalytic activities for CO2 cycloaddition to a series of epoxides to form cyclic carbonates in the absence of co-catalyst and solvent, producing a high turnover frequency (TOF) of 2202 h−1 at 100 °C. The effect of reaction parameters including temperature, reaction time, and catalyst loading was examined. NHC-CAP-1(Zn2+) could be separated readily and reuse for a minimum of 10 runs while maintaining high activity and stability owing to the strong covalent bonding of Lewis acidic Zn2+ to the NHC-CAP-1 backbones. This work demonstrated that NHC-CAP-1 and NHC-CAP-1(Zn2+) are viable porous materials that are highly efficient for both CO2 capture and catalytic conversion of CO2 to cyclic carbonates.