Chromium-Salophen as a Soluble or Silica-Supported Co-Catalyst for the Fixation of CO2 Onto Styrene Oxide at Low Temperatures

Abstract

Addition of a soluble or a supported Cr^III-salophen complex as a co-catalyst greatly enhances the catalytic activity of Bu₄NBr for the formation of styrene carbonate from styrene epoxide and CO₂. Their combination with a very low co-catalyst:Bu₄NBr:styrene oxide molar ratio = 1:2:112 (corresponding to 0.9 mol% of Cr^III co-catalyst) led to an almost complete conversion of styrene oxide after 7 h at 80°C under an initial pressure of CO₂ of 11 bar and to a selectivity in styrene carbonate of 100%. The covalent heterogenization of the complex was achieved through the formation of an amide bond with a functionalized {NH₂}-SBA-15 silica support. In both conditions, the use of these Cr^III catalysts allowed excellent conversion of styrene already at 50°C (69 and 47% after 24 h, respectively, in homogeneous and heterogeneous conditions). Comparison with our previous work using other metal cations from the transition metals particularly highlights the preponderant effect of the nature of the metal cation as a co-catalyst in this reaction, that may be linked to its calculated binding energy to the epoxides. Both co-catalysts were successfully reused four times without any appreciable loss of performance.

Keywords: CO2 utilization; chromium salophen complexes; cycloaddition reaction; heterogeneous catalysis; styrene oxide.