Synthetic toolbox: Flow chemistry

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Flow chemistry

The aims of green chemistry and industry support one another when a process is considered for upscaling; it needs to be safe, generate minimal waste and be energy efficient. A fundamental approach to green chemistry for industrial application is to address such issues at the outset of the project, ideally at the reaction discovery stage. Flow chemistry is an enabling technology that fits well at the interface between reaction discovery and scale up; such reactors offer substantial improvements in the management of heat and mixing as well as scalability, energy efficiency, waste generation, operational safety, offer a wide range of reaction conditions and unique opportunities for catalyst supports, telescoping reactions among others.[1][2][3]

The advantages in efficiency and sustainability that continuous flow offers are that it allows for efficient use of energy and time. Both aspects being directly related to reaction rate as a slow reaction would require more time for completion. The standard approach for speeding up a reaction is to increase its temperature, however batch reactors are limited to the boiling point of the reaction solvent/reagents. By comparison flow reactors lend themselves well to safe temperature and pressure manipulation beyond that of standard atmospheric conditions. Thus reactions carried out in flow are often faster than the corresponding batch reactions and offer improved energy, time and even space efficiency, as faster reactions will require smaller reactors.[4] Many green solvents such as acetone or methanol have low boiling points making them inapplicable for use in certain reactions; performing reactions at high pressure in a flow reactor allows for their safe use at high temperatures.[4]  Supercritical fluids add an extra facet, as they are inaccessible in the absence of high temperature conditions, thus their applicability in flow reactors offers further advantages over batch reactors.[4]

  1. C. Wiles and P. Watts, Continuous flow reactors: a perspective, Green Chem., 2012, 14, 38-54.
  2. S. V. Ley, On Being Green: Can Flow Chemistry Help?, Chem. Rec., 2012, 12, 378-390.
  3. J. -ichi Yoshida, H. Kim and A. Nagaki, Green and Sustainable Chemical Synthesis Using Flow Microreactors, ChemSusChem, 2011, 4, 331-340.
  4. S. G. Newman and K. F. Jensen, The role of flow in green chemistry and engineering, Green Chem., 2013, 15, 1456-1472.