Continuous-flow liquid phase oxidation chemistry in microreactors receives a lot of attention as the reactor provides enhanced heat and mass transfer characteristics, safe use of hazardous oxidants, high interfacial areas, and scale-up potential. In this review, an up-to-date overview of both technological and chemical aspects of liquid phase oxidation chemistry in continuous-flow microreactors is given. A description of mass and heat transfer phenomena is provided and fundamental principles are deduced which can be used to make a judicious choice for a suitable reactor. In addition, the safety aspects of continuous-flow technology are discussed. Next, oxidation chemistry in flow is discussed, including the use of oxygen, hydrogen peroxide, ozone and other oxidants in flow. Finally, the scale-up potential for continuous-flow reactors is described.

Photocatalytic radical trifluoromethylation strategies have impacted the synthesis of trifluoromethyl-containing molecules. However, mechanistic aspects concerning such transformations remain poorly understood. Here, we describe in detail the mechanism of the visible-light photocatalytic trifluoromethylation of N-methylpyrrole with gaseous CF3 I in flow. The use of continuous-flow microreactor technology allowed for the determination of different important parameters with high precision (e.g., photon flux, quantum yield, reaction rate constants) and for the handling of CF3 I in a convenient manner. Our data indicates that the reaction occurs through a reductive quenching mechanism and that there is no radical chain process present.

The synthesis of adipic acid by means of a direct oxidation of cyclohexene by hydrogen peroxide was carried out in a continuous-flow packed-bed microreactor. The reaction uses Na 2 WO 4 ·2H 2 O as a catalyst and [CH 3 (n-C 8 H 17 ) 3 N]HSO 4 as a phase transfer catalyst without additional solvent. A parametric process optimization, such as glass beads size, residence time, concentration of hydrogen peroxide, addition of acid, reaction temperature, and molar ratios of reactants and catalysts, was performed. It is demonstrated that smaller glass beads result in a better yield of adipic acid and that the addition of acid plays an important role in the oxidation of cyclohexene to adipic acid. Based on the concept of Novel Process Windows, the influence of elevated temperatures is investigated.

Hydrodynamics characteristics of a fast and highly exothermic liquid–liquid oxidation process with in situ gas production in microreactors were studied using a newly developed experimental method. In the adipic acid synthesis through the K/A oil (the mixture of cyclohexanol and cyclohexanone) oxidation with nitric acid, bubble generation modes were divided into four categories. The gas production became more intensive, unstable, even explosive with increasing the oil phase feed rate and the temperature. A novel automatic image processing method was developed to monitor the instantaneous velocity online by tracking the gas–liquid interface. The axial velocity at the same location was unstable due to the changing gas production rate. Furthermore, the actual residence time was obtained easily with being only 36% of the space–time minimally, beneficial for establishing accurate kinetics and mass transfer models with time participation. Finally, an empirical correlation was developed to predict the actual residence time under different conditions.

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