Microreactors material used

As Hickman and Sobeck point out, it is important to characterize the microreactor before using a system for reaction characterization. As the current generation of microreactors were not designed for use as reaction characterization systems, the performance may not be usable for this purpose. As they describe, care must be taken to understand how the reactor works so that it can be modeled to extract the intrinsic chemistry rates of the reactions. In addition, they point out that given the high surface-to-volume ratio of the reactors care must also be taken to demonstrate that there are not unexpected interactions of the materials with the reactor walls. 

There are a number of materials used for the fabrication of pTAS devices. Perhaps the most common is glass due to its low cost, ease of machining, and suitability for electrophoresis and electroosmotic flow (EOF) applications without requiring surface modifications. It is also chemically inert to most reagents (apart from hydrofluoric acid and concentrated alkali). Silicon is also a valuable material that has similar chemical inermess and can easily be machined by chemical etching. While it is more expensive, it can be easily chemically etched to yield far higher aspect ratios than are possible with glass. Silicon is not suitable for electrophoresis or EOF applications without surface pretreatment. Devices fabricated from polymers such as polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS) are also frequently used due to the low cost of the material (especially important for disposable devices) and the ease of fabrication. Perhaps one drawback with polymers is their incompatibility with solvents. They are suited to electrophoretic applications but frequently require surface modification to support EOF. Occasionally, metals are used however, these are far more frequently encountered in chemical microreactors. 

McMullen and Jensen (2010) have made a review of the automation and construction materials used in integrated microreactors, highlighting various materials and fabrication methods. Generally, five types of materials were used ceramic, glass, plastic, silicon and stainless steel (SS). The most... 

Purchasable microreactors are limited in their temperature and pressure resistance, depending mainly on the reactor material used and the fabrication of the reactor. Most metallic microreactors operate at a maximum temperature of 500 °C, whereas ceramic microreactors offer temperature resistance up to 1100°C at ambient pressure and high chemical resistance. Metallic microreactors, in contrast, can withstand higher pressures generally. Further, it has to be considered whether the reactor material has any influence on the performance of the reaction, for instance unwanted catalytic activity. Normally most metallic microreactors can be provided in several materials. 

Gillies and coworkers [48,49] subsequently used a polycarbonate microreactor, in which a chamber was designed to effect very rapid turbulent mixing. The microreactor was used to conduct the fluorination of mannose triflate (44), followed by acid hydrolysis of the intermediate (45) to synthesize [ F]fluorodeoxyglucose ([ F] FDG) (46) (Scheme 6.15), whereby 50% overall incorporation of the radiolabel was achieved with a residence time of just 4 s. However, the polymeric material did limit what solvents could be used within the system. 

TAC-101 (4-[3,5-bis(trimethylsilyl)benzamido] benzoic acid) has attracted attention as a compound with antitumor activity. An ester of this compound can be readily synthesized by using 1,3,5-tribromobenzene as a starting material, and repeating three times the Br-Li exchange followed by the reaction of the generated aryllithium species with an electrophile (Fig. 9.4). For these reactions, a system integrating six micromixers and six flow microreactors is used. When each step uses an optimum residence time, the steps together require a total residence time of 13 s. 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

Chemical transformations requiring solid starting materials, intermediates, or products are difficult to carry out in microreactors since solids may clog the channel network and hamper the continuous flow. In order to carry out reactions that use solid catalysts, several different ap-... 

Lu H, Schmidt MA, Jensen KF (2001) Photochemical Reactions and On-Line UV Detection in Microfabricated Reactors. Lab Chip 1 22-28 Manz A, Harrison DJ, Verpoorte EMJ, Fettinger JC, Ludi H, Widmer HM (1991) Miniaturization of Chemical-Analysis Systems - A Look into next Century Technology or just a Fashionable Craze. Chimia 45 103-105 McCreedy T (1999) Reducing the Risks of Synthesis. Chem Ind 15 588-590 McCreedy T (2000) Fabrication Techniques and Materials Commonly Used for the Production of Microreactors and Micro Total Analytical Systems. Trac Trends Anal Chem 19 396-401... 

Examples of using metal, polymer, and glass microreactors appear in other chapters of this volume. The present chapter focuses on microreactors created in silicon, a material that has high mechanical strength,... 

For application in flow reactors the nanocarbons need to be immobilized to ensure ideal flow conditions and to prevent material discharge. Similar to activated carbon, the material can be pelletized or extruded into millimeter-sized mechanically stable and abrasion-resistant particles. Such a material based on CNTs or CNFs is already commercially available [17]. Adversely, besides a substantial loss of macroporosity, the use of an (organic) binder is often required. This material inevitably leaves an amorphous carbon overlayer on the outer nanocarbon surface after calcination, which can block the intended nanocarbon surface properties from being fully exploited. Here, the more elegant strategy is the growth of nanocarbon structures on a mechanically stable porous support such as carbon felt [15] or directly within the channels of a microreactor [14,18] (Fig. 15.3(a),(b)), which could find application in the continuous production of fine chemicals. Pre-shaped bodies and surfaces can be... 

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