A series of imidazolium-based compounds was synthesized and tested for the cycloaddition of carbon dioxide and propylene oxide (PO) to propylene carbonate (PC). The catalyst screening shows the influence of different substituents at the imidazolium ring on the reactivity. Thereby it could be shown that on the one hand, the acidity has a great influence on the catalytic activity due to activation of the epoxide through hydrogen-bonding. On the other hand, the degree of ion pairing has a high impact on the halide nucleophilicity, thus the activity. Based upon these results a tailor-made ionic liquid was used as recyclable catalyst with excellent conversions and selectivites under mild reaction conditions. Further investigations focused on the influence of cocatalysts on the catalytic performance of imidazolium compounds. It is known that niobium(V)chloride (NbCl5), together with nucleophiles is an exceptional reactive system for the conversion of CO2 to cyclic carbonates. When NbCl5 is combined with a series of imidazolium bromides, the acidity of the imidazolium cation and the resulting hydrogen bonding has no influence on the catalytic activity, because of the strong interaction and activation of the epoxide with the metal center. However, the solubility in the substrate and the impact of the ion pairing strongly increases within these systems. The optimization of the cation structure leads to a highly reactive tandem catalyst with NbCl5 for the production of PC at very mild reaction conditions. The combination of pentaerythritol with ionic liquids illustrates an alternative synergistic catalyst system. The epoxide is activated through hydrogen-bonding to the alcohol, which facilitates the ring-opening by the halide. Therefore, the system demonstrates an organocatalytic approach comparable to NbCl5.  As the catalyst system is easily recyclable, widely applicable and shows excellent yields, it enables the fixation of CO2 to a variety of cyclic carbonates in an especially sustainable manner.

Imidazolium based catalyst systems for the chemical fixation of CO<sub>2 </sub>?with epoxides to cyclic carbonates

248th National Meeting (2014)

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Imidazolium based catalyst systems for the chemical fixation of CO2 ?with epoxides to cyclic carbonates

Imidazolium based catalyst systems for the chemical fixation of CO₂?with epoxides to cyclic carbonates