Philipp MIKSOVSKY1, Elias Nino HORN1, Michael SCHNÜRCH1, Katharina BICA-SCHRÖDER1
1TU Wien - Institute of Applied Synthetic Chemistry, Vienna, Austria
Several catalysts for the conversion of epoxides and CO2 to cyclic carbonates are reported in literature. However, the combination of supercritical CO2 (scCO2, Tc: 31.0 °C, pc: 7.38 MPa) and ionic liquids as catalysts shows a particular property of high value: The high solubility of scCO2 in ionic liquids makes them ideal candidates as catalytically active species in combination with scCO2. In contrast, ionic liquids show extremely low solubility in scCO2, thus rendering them attractive for immobilized catalytic phases in heterogeneous catalysis being commonly used in continuous flow processes.
Our studies on bioderived cyclic carbonates were motivated by an increasing annual production provoked by applications as apolar protic solvents, electrolytes in lithium ion batteries as well as monomeric building blocks for polyurethanes. Limonene as a renewable feedstock attracted our attention due to its global market of 14 million US$ in 2020 and a global annual production of 43 Mt.
Herein, we present the use of supported ionic liquid phases (SILP) for the synthesis of limonene carbonates in scCO2 continuous flow starting from diastereomeric mixtures of limonene oxide and limonene dioxide. The initial evaluation of ammonium and imidazolium halides as catalytically active species in batch mode showed promising yields and selectivities for ammonium halides. The immobilization of ammonium halides via physisorption on mesoporous silica gave the corresponding heterogeneous SILP material, used in the continuous flow process.
The continuous flow reactions were performed with a scCO2 flow device, where mixtures of substrate and scCO2, get pumped with HPLC pumps through a catalyst cartridge which is filled with the heterogeneous catalyst and placed in a HPLC oven. After CO2 release via a back-pressure regulator and a gas/liquid separator, product is collected in different fractions.
Upon optimization of the continuous process, including a screening of temperature, pressure, catalyst loading and flow rates of substrate and CO2, a constant production of different bioderived carbonates was achieved and was finalized by the evaluation of the long-term stability of the catalyst.
Acknowledgements
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864991).
References
P. Mikšovsky, E. N. Horn, M. Schnürch, K. Bica-Schröder, Org. Process Res. Dev. 2022, 26, 2799−2810