CO2-controlled reactors: epoxidation in emulsions with droplet size from micron to nanometre scale

The epoxidation of styrene in the cetyltrimethylammonium bromide (CTAB)/H2O/heptane/styrene/H2O2 emulsion system was investigated in the presence of compressed CO2. It was found that by controlling the CO2 pressure, the emulsion droplets can be changed in a wide range from micron to nanometre size; accordingly, the conversion was enhanced significantly with the reduced droplet size. At 5.27 MPa, the conversion in the emulsion (with droplet size of 39.5 nm) could be about 10 times higher than that in a surfactant-free system at the same pressure. The effects of other experimental conditions of CTAB concentration, reaction time, styrene concentration, amounts of H2O2, and alkalinity on the efficiency of the epoxidation were also studied, and a possible mechanism for the enhanced reaction efficiency with reduced droplet size was presented. This process has some unique advantages. For example, the reactant conversion can be easily tuned by controlling the pressure of CO2; CO2 can be used as bicarbonate source and no additional catalyst was used; CO2 can be easily removed by depressurization; the separation for the product is simple. It is believed that the CO2-tuned emulsions can be easily applied to many other chemical reactions.