Microreactors catalytic reactions

Microreactors Low conversion, catalytic reactions Simple design, transport rates can be increased by external recycling Limited ease of variation of parameters, maldistribution of flow can be prohibitive... 

Multiphase catalytic reactions, such as catalytic hydrogenations and oxidations are important in academic research laboratories and chemical and pharmaceutical industries alike. The reaction times are often long because of poor mixing and interactions between the different phases. The use of gaseous reagents itself may cause various additional problems (see above). As mentioned previously, continuous-flow microreactors ensure higher reaction rates due to an increased surface-to-volume ratio and allow for the careful control of temperature and residence time. 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

Several reactors are presently used for studying gas-solid reactions. These reactors should, in principle, be useful for studying gas-liquid-solid catalytic reactions. The reactors are the ball-mill reactor (Fig. 5-10), a fluidized-bed reactor with an agitator (Fig. 5-11), a stirred reactor with catalyst impregnated on the reactor walls or placed in an annular basket (Fig. 5-12), a reactor with catalyst placed in a stationary cylindrical basket (Fig. 5-13), an internal recirculation reactor (Fig. 5-14), microreactors (Fig. 5-16), a single-pellet pulse reactor (Fig. 5-17), and a chromatographic-column pulse reactor (Fig. 5-18). The key features of these reactors are listed in Tables 5-3 through 5-9. The pertinent references for these reactors are listed at the end of the chapter. 

SAMs on microchannel walls have been studied for surface properties in microreactors,59 to control surface wetting,60 to create zones for specific immobilization of proteins and biomolecules,61 and to conduct catalytic reactions.62 And a pH sensing monolayer confined to a glass microchannel has been reported by our group.32... 

Historically, the use of microreactors dates back to the 1940s when they were developed to measure kinetics of catalytic reactions.One of the key early findings was Denbigh s 1965 observation that if a reactor were made small enough, temperature and concentration gradients with the reactor would be negligible, so that differential (i.e., gradientless) behavior would be observed. This allowed much more accurate kinetic... 

For 2-butanol dehydrogenation, catalytic reactions were carried out at atmospheric pressure in a fixed bed flow-microreactor. The feed was 4.2 Nl/h of helium with a partial pressure of 8.5 kPa of 2-butanol. Reaction temperatures were from 373 K to 573 K. Concentrations of reactants and products were measured at the reactor outlet by on-line gas chromatography. 

A catalyst can be loaded on the surface of the channel wall of a glass microchip reactor. Such a microreactor has been used for gas/liquid/solid three-phase catalytic reactions, as shown in Figure 7.27. " ... 

The benefits of microreactors are many more ideal temperature exchange for gas phase reactions they enable the reaction process to take place in an electric field and the performance of complex catalytic reactions can be simplified [18]. These benefits will also provide access to novel structures. Additionally, the bottleneck caused by limited availability of unusual building blocks and scaffolds will be eased, or altogether eliminated, by microreactor operations and miniaturized screening formats. The advent of microreactor systems for chemical synthesis not only opens the door for direct integration of synthesis and screening, but also provides better access to novel compounds. 

In general, the geometric surface area of the microchannels in a typical microreactor is insufficient to carry out catalytic reactions at high performance. Consequently, the specific surface area must be increased, either by chemical treatment of the channel walls or by coating them with a porous layer. The porous layer may serve directly as a catalyst or as a support for the catalytically active components. Various techniques to introduce the catalyst have been developed and are summarized in the following sections [147,148]. 

The catalytic reactions were carried out in a catalytic flow microreactor at atmosheric pressure and various temperatures. The catal ic bed (Ig) was covered by silica. TTie reaction conditions were the following the oil (40(wt%) in cyclohexane) was introduced with a flow of 0.12 mkmin l simultaneoulsy with hydrogen (flow = 20 mIxmin H. After evaporation of the solvant, the products were successively treated by sodium methoxide, methanol and sulfuric acid to obtain the free-esters before analysis. The final products were analysed by gas chromatography with a flame ionization detector and AT-FILAR (Altech) capillary column (30m, I.d = 0.32 pm, film thickness = 0.25 pm) at 140 C. 

Infrared imaging was utilized in several studies of spatial effects in exothermic catalytic reactions over model catalysts, such as isolated particles, wafers, plates, discs [2]. Our approach has been to characterize the catalysts directly in a packed-bed microreactor, under realistic reaction conditions. In-situ measurements by infrared thermography of the adsorption properties of catalytic materials have been previously reported [6]. In the present study, the catalytic oxidation of compounds having different chemical properties was investigated by the same technique, with the aim of obtaining comparative data useful to better understand the factors governing the complex phenomena associated with catalytic combustion. 

Integration of various components is an important issue for the DCF systems. The simple scale-up of microreactors is not enough as the DCF system. A DCF system should consist of not only reactors but also other factory parts like a mixer, separator, and temperature controller. Many integrated microreaction systems have been reported and some of these are commercially available. For example, K. F. Jensen s group has reported an integrated microreactor system for gas-phase catalytic reactions using microstructured reactors and other devices on a computer board [13]. They have achieved computer control over the reaction system through this device as shown in Fig. 6. 

Desktop Chemical Factory, Fig. 6 Integrated microreactor system. Integrated microreactor system for gas-phase catalytic reactions [13]... 

J.F. (2007) Microreactor technology and process miniaturization for catalytic reactions - A perspective on recent developments and emerging technologies. Chem. Eng. Sci., 62 (24), 6992-7010. 

In the previous sections, the mass transfer and the pressure drop properties of three different microstructured devices for fast catalytic reactions have been assessed. In the present chapter, we compare their mass transfer performance while considering the energy demand in order to choose an appropriate design of microreactor for an eventual catalytic reaction. 

Up to the boiling point of liquid reactants, two- and three-phase heterogeneous catalytic reactions are carried out in microreactors using the block-heating concept... 

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