Controlled multiphase oxidations for continuous manufacturing of fine chemicals

The feasibility of an integrated continuous biphasic oxidation process was studied, incorporating (i) electrochemical generation of an oxidant, (ii) membrane emulsification and an Oscillatory Flow Reactor (OFR) to facilitate mass-transfer in a biphasic reaction system and (iii) product extraction to enable regeneration of the oxidant. The biphasic, organic solvent-free dihydroxylation of styrene by ammonium peroxodisulfate solutions (including electrochemically generated peroxodisulfate) was investigated as a model reaction, both in batch and in an OFR. Heating of peroxodisulfate in a strongly acidic solution was demonstrated to be essential to generate the active oxidant (Caro’s acid). Membrane emulsification allowed mass-transfer limitations to be overcome, reducing the time scale of styrene oxidation from several hours in a conventional stirred tank reactor to less than 50 min in a dispersion cell. The influence of droplet size on overall reaction rate in emulsions was studied in detail using fast image capturing technology. Generation of unstable emulsions was also demonstrated during the oxidation in OFR and product yields >70% were obtained. However, the high-frequency/high-displacement oscillations necessary for generation of fine droplets violated the plug flow regime. Membrane emulsification was successfully integrated with the OFR to perform biphasic oxidations. It was possible to operate the OFR/cross-flow membrane assembly in plug flow regime at some oscillatory conditions with comparable overall oxidation rates. No mass-transfer limitations were observed for droplets <60 lm. Finally, the continuous post-reaction separation was demonstrated in a single OFR extraction unit to enable continuous regeneration of the oxidant.