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The synthesis of commercially relevant organic carbonates from CO2 can contribute to a sustainable utilization of this greenhouse gas. The catalytically controlled reaction with epoxides leads to the production of cyclic carbonates and aliphatic polycarbonates. In this work, we succeeded in finding innovative homogeneous catalysts for this task. The foundation was a very variable ligand system closely related to the salen compounds with an N2O2 framework, that was converted with the metals iron, zinc and aluminum to the corresponding metal complexes. The various combinations of ligand and metal yielded 66 different potential catalysts, of which some structures were investigated via x-ray structure analysis. High-pressure experiments were carried out to test the catalysts’ performance, allowing some insights in the relationship between structure and catalytic activity. The conversions were documented by means of the test substrates propylene oxide and cyclohexene oxide. Additionally a wider range of epoxides has been tested in some promising cases. One result is, that the control of the product spectrum via selection of the optimal epoxide-catalyst-combination is possible. The most active iron(III) catalyst was able to produce quantitative yields of propylene carbonate from propylene oxide without the addition of a cocatalyst. With the zinc catalysts the same result was possible, also under mild reaction conditions (40 ◦C, 2 bar CO2, cocatalyst Bu4NI). However, the aluminum catalysts were suitable for the quantitative conversion of cyclohexene oxide to fully alternating polycarbonates. Another important finding is, that the contributing nucleophiles in the reaction have a decisive influence on the formation of the product, regardless if they came from the catalyst itself or from a cocatalyst. While the more nucleophile and according to the HSAB-concept softer iodide anion ismore suitable for the synthesis of cyclic carbonates, the anions bromide and chloride are rather successful in the synthesis of polycarbonates. By the analysis of the many catalytic test results and additional kinetic measurements via in situ FT-IR spectroscopy, it was possible to postulate mechanisms for the different new catalysts of this work. |
