Highly Efficient Continuous Microreactors for Controlled Synthesis of Nanoscale Particles of Functional Materials (Review)

The current state and prospects of microreactor synthesis of functional materials in liquid phase – in single- and two-phase flows are analyzed. Microreactors allow fine control over the size, composition, as well as the structure and properties of synthesized particles in co-precipitation processes. In addition, high-quality homogenization of solutions at the molecular/ionic level is also important in the preparation of organometallic compounds, for example, in hydrolysis reactions. The results obtained by various teams give grounds to expect quite extensive possibilities of controlling the processes of nucleation and particle growth in microreactors by controlling pH, concentrations of reagents, quality of micromixing, and residence time in each of the reactor zones – nucleation zone and growth zone. The advantages of microreactor synthesis are demonstrated: high quality of micromixing in the volume of 0.2–0.5 ml, which provides obtaining nanoparticles without impurities (in terms of composition), stoichiometric ratio of atoms in the product, limitation of agglomerates growth due to short residence time (of the order of several milliseconds). The transition to industrial scale is caused by rather high productivity of a single microreactor (up to 10 m³/day for suspension, up to 200–300 kg/d for solid phase). Intensive mixing in microreactors with diameters of 2–4 mm and less, caused by Taylor vortices, contributed to the application of two-phase microreactors for the synthesis of both organic and inorganic substances. For organic compounds, obtaining of which is associated with the formation of explosive products, the use of microreactors has another advantage – diameter of channels can be taken less than critical for this group of substances. It should be noted that in an industrial microreactor several tens of thousands of parallel microchannels can be involved, i.e. the name here is conditioned by the transverse size of channels (about 2–4 mm or less), and the productivity is comparable to that for standard types of chemical equipment.