Abstract
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 50min 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 <60μm. Finally, the continuous post-reaction separation was demonstrated in a single OFR extraction unit to enable continuous regeneration of the oxidant.
Highlights
In recent years, greater awareness of environmental and safety issues has led to increasingly stringent regulatory controls on manufacturing processes, fuelling demand for ‘green’ chemical technologies that can deliver greater atom, process, and economic efficiency [1]
Continuous extraction of the diol by ethyl acetate Following the oxidation, it is necessary to separate the product from the aqueous phase, prior to regeneration of peroxodisulfate in the electrochemical cell
This can be done either by natural coalescence of the emulsion into two phases or by extraction of the product from the aqueous phase
Summary
Greater awareness of environmental and safety issues has led to increasingly stringent regulatory controls on manufacturing processes, fuelling demand for ‘green’ chemical technologies that can deliver greater atom, process, and economic efficiency [1]. Low space-time yields, insufficient temperature control leading to poor selectivity and difficulties in process control (scale-up in particular) are amongst other disadvantages of batch manufacturing processes. A particular challenge for the pharmaceutical industry is the development of environmentally benign oxidation methods. Oxidation chemistry is currently under-utilised in the pharma manufacturing industry (3.9% of reactions cf 14% for reductions) due to the difficulties of process implementation at scale, and manufacturing routes avoid oxidation if possible [3]. Benign oxidation methods are listed in the top ten of the most important research areas by the ACS Green Chemistry Institute’s Pharma Round Table [1,3]
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