Microreactors offer excellent mass and heat transfer performance for extraction and multiphase reactions. They provide a powerful tool for process intensification and micro scale processing. This paper reviews the structures of microreactors and units, and their applications on the synthesis of nanoparticles, organics, polymers and biosubstances. The structural evolution and properties of the commercialized and lab-made microreactors are introduced in detail. Recent developments of the fabrication, structures and applications of micro-structured reactors are highlighted. The promising direction in science and technology for future microreaction technology is also discussed.
Micro-synthesis technique in both interdisciplinary engineering and sciences connects physics, chemistry, biology, and engineering arts for various applications. A microfluid segment in microreactor is defined as a minimum unit having microproperties that can be used to improve various unit operations and reactions in microspace. Chemist George Whitesides initially created inexpensive microfluidic devices using poly dimethylsiloxane (PDMS), and through the microreactor community led by the Institute for Molecular Manufacturing (IMM) in Germany and Yoshida's Microreactor Initiatives in Japan, considerable interest in the microreactor area has been built up. High throughput screening in microanalytical chemistry , biological analysis of cells and proteins , reaction kinetics and mechanisms studies  were the initial uses of microreactors. Microreactors have shown superior heat and mass transfer rates, and the contact time, shape and size of the interface between fluids can be easily and precisely controlled . These attributes make microreactors ideal for fast reactions , highly exothermic reactions , and even explosive reactions [7,8]. The small volume capacity of microreactors has also allowed the efficient development of more sophisticated continuous flow reactions on increasingly complex molecular targets since they greatly reduce the quantities of materials needed to optimize reaction conditions.
Microreactors are fabricated in a range of materials, including ceramics, polymers, stainless steel, and silicon. The microreaction devices can be classified into two groups: chip-type microreactors and microcapillary devices. Chip-type microreactors offer several advantages including easy control of microfluidics, and integration of many processes into one reaction device. Manufacturing processes of such devices are mainly adaptations from the microelectronics industry. Dry- or wet-etching processes have been used for creating channels on a silicone or glass plates. Glass microreactors offer the benefit of visualizing the reaction progress, but are limited in reactor designs due to the difficulty of creating high aspect ratio structures. Polymer-based materials (e.g. Poly-dimethylsiloxane (PDMS), polymethylmarthacrylate (PMMA), polycarbonate, and Teflon) can be used for preparation of enzyme microreactors because most enzyme reactions have been performed in aqueous solution, especially for bio-analytical use. Stainless steel microreactor networks range from simple systems comprised of T-shape micromixers and narrow tubing to commercial systems with micro-fabricated components . They can be operated at high pressure and temperature. These plates can be processed by photolithography, soft lithography, injection molding, embossing, and micromachining with laser or microdrilling. The LIGA (Lithographie Garbanoforming Abforming) process that combines lithography, electrochemical technology and molding, can also be used for the production of microreactors.
Microreactors have been primarily used as tools for analytical chemistry. Owing to their unique physical and chemical properties, microreactors have exhibited excellent processibility in a range of processes including the synthesis of inorganic, metal nanoparticles and organic, which have been applied in pharmaceutical and chemical engineering in a flexible and controllable manner. Up to now, these microstructured reactors also have been extensively studied as micromixers, and microseparators. Therefore, with small structures that combine different levels of porosities, channels could significantly facilitate diffusion of species. More research efforts is needed to design and fabricate microstructured reactors with optimal channel structure and interconnection, channel size distribution, and channel volume for better yield and selectivity in different systems. Since high performance microreactor structures are readily obtained from glass, ceramic, plastic, silicon or prepolymer, and metal/metal oxide nanoparticles, they have been successfully used in vehicles and aircraft. It would be interesting to incorporate multifunctional nanomaterials into microreactors, which could achieve more functionality and improve their performances in uses such as high-efficiency hydrogenation. Inspired by direct fabrication of microreactors from various materials, it would be worthwhile to devote much effort in the wallcoated microreactor by selecting suitable surfactants and manipulating polymerization conditions . Again, high surface reactivity would enable us to attach functional groups to freshly synthesized microreactors inside nanocomposites or microchannel. The progress of the research into composites-based microreactors has been quite encouraging .Successful direct synthesis of microreactors with ordered porous zeolite or ceramic foams with additional functionalities has been achieved. In particular, pilot plant microreactors are valuable when fast reactions are needed, where the outcome will be highly dependent on the mixing quality, and when exploring reactions at high temperature and pressures. Many technical barriers have to be overcome for such connection, parallel control of fluid and reaction conditions, integration, monitoring and critical cost analysis should be conducted, but microreaction technology is sure to evolve and become a new environmentally benign processing technology, which will be widely used in the near future.