Towards a combined use of Mn(Salen) and quaternary ammonium salts as catalysts for the direct conversion of styrene to styrene carbonate in the presence of dioxygen and carbon dioxide

The use of oxygen in combination with carbon dioxide to afford the direct conversion of alkenes into cyclic carbonates could help to promote the greenhouse gas while minimizing the impact of the oxidation reaction on the environment. In this work, we focused, for the first time, on the association of two catalytic systems individually efficient for the epoxidation of styrene (Mn(salen)/O2 bubbling/isobutyraldehyde at 80 °C) and the cyclocarbonatation of styrene oxide (choline chloride/CO2 at 15 bar and 120 °C). First, the feasibility of the cyclocarbonatation reaction, starting from the non-isolated epoxide, has been proven as styrene carbonate was formed with a 24% yield. The objective was, then, to determine the best conditions allowing the overall transformation in a common solvent. Taking into account the differences in optimal temperatures and kinetics of the two individual steps, it was decided to vary the temperature during the reaction [first 80 °C (3 h) and 120 °C (23 h)]. Under these conditions, styrene was converted into the epoxide but, unfortunately, styrene carbonate formation could not be demonstrated. Blank experiments have clearly shown that isobutyraldehyde, which is essential to the first step, must be completely consumed before the temperature rise. Otherwise, autoxidation of the aldehyde in the presence of styrene oxide at 120 °C leads to other products than styrene carbonate.