Mn(III)(salen)-catalyzed synthesis of cyclic organic carbonates from propylene and styrene oxide in “supercritical” CO 2

Abstract

Various homogeneous and immobilized manganese-salen complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from two liquid epoxides (propylene oxide and styrene oxide) and CO 2, which served as reactant and solvent. Reaction rates in terms of turnover frequencies up to 255 mol product mol Mn −1 h −1 at 98% selectivity were achieved by optimizing the salen ligand as well as the reaction temperature and CO 2 pressure. The reaction rates did not only strongly depend on the kind of salen ligand, the phase behaviour, and the reaction conditions but significant differences were also observed between the epoxide reactants. In addition, two different and simple ways for the immobilization of Mn-salen complexes were examined, among which one led to a highly active, stable and reusable heterogeneized catalyst. In this heterogeneous catalyst the salen ligand was covalently bound to the silica surface and showed low deactivation and almost no leaching of Mn during repetitive use. A coordinatively bound Mn-salen complex resulted in a high loading of the Mn-salen complex on a specially modified silica surface, but was not stable enough. Furthermore, phase behaviour studies and ATR-IR spectroscopic investigations on the product formation are reported that gave further insight into this catalytic reaction taking place at elevated pressure.