Application of CO2-expanded solvents in heterogeneous catalysis: a case study

CO2-expanded liquid (CXL) solvents are known to provide optimum reaction and environmental benefits for certain homogeneously catalyzed oxidations. In this work, CXL solvent media are applied to heterogeneously catalyzed oxidations. The oxidation of cyclohexene is investigated as a model reaction. Conversion/selectivity measurements were made in a stirred batch reactor on a MCM-41 encapsulated iron porphyrin chloride complex (p=1–127 bar, T=25–50 °C, t=4–12 h), using either dioxygen or iodosylbenzene as oxidant. Neat acetonitrile and CO2-expanded acetonitrile are used as reaction media. The pressures required for reaction studies in CO2-based media were determined by expansion studies as well as cloud point measurements. For the oxidation with dioxygen, conversion and product yields in CO2-expanded acetonitrile (∼30% CO2) almost doubled compared to the neat organic solvent. In contrast, CO2-replacement of the organic solvent had a negligible effect on conversion and selectivity values when using iodosylbenzene as oxidant. Our results indicate that CXLs significantly reduce the use of conventional organic solvents as reaction media and also enhance catalyst stability against leaching. These advantages suggest that CXLs are promising media for developing green heterogeneous catalytic processes.