Microreactor microstructured

The application of zeolite membranes in microreactors is still in an early stage of development, and suffers sometimes from unexpected problems arising from template removal [70]. However, several application examples of zeolite membranes in microstructured devices have been demonstrated yielding similar advantages as were to be expected from experiences on the macroscale. Because of the high surface to volume ratio of microreactors, the application of zeolite membranes in these systems has great potential. 

Recently, microstructured reactors have stepped into chemical production [4] and thus microreactor process and plant design, including economic incentives, is the issue at this time. For this purpose, large-capacity microstructured apparatus is needed ( micro inside, fist- to shoebox size outside ) and plant concepts have to be proposed which include all process steps. 

Jahnisch K, Baerns M, Hessel V, Ehrfeld W, Haverkamp V, Lowe H, Wille C, Guber A (2000) Direct Fluorination of Toluene Using Elemental Fluorine in Gas/Liquid Microreactors. J Fluor Chem 105 117-128 Jahnisch K, Hessel V, Lowe H, Baerns M (2004) Chemistry in Microstructured Reactors. Angew Chem Int Ed 43 406 -46 Jensen KF (2001) Microreaction Engineering - is Small Better Chem Eng Sci 56 293-303... 

Because of the small amounts of solid reactant in the microscope sample, analyses of reaction products are performed with larger samples in a microreactor operating under similar conditions, and these are used for microstructural correlations. 

Several examples of oxidation reactions, both in the liquid and in the gas phase, have been investigated in microreactors. Often the use of the microstructured device allows a better selectivity to the product of partial oxidation, because of a better temperature control on the catalyst surface (see, for instance, several examples in reviews [61a,b]). Indeed, several gas-phase oxidations can be completed in milliseconds, at significantly high temperatures. 

Microstructures which are used in this context will be termed microreactors in the following (a more precise definition is given below and in the next chapters). It should be emphasized that microreactors are not constrained to microscopic sizes (nor to minuscule processing rates) as first outlined by Wegeng [8]. His precise definition of microreactors is repeated here ... 

The device consisted of a 75 -pm thick polyimide foil containing microstructured slits (250 pm wide and 45 mm long) sandwiched between the working and counter electrodes (Scheme 4.34). The reaction took place inside the microchannels that were in contact with both electrodes. To maintain a constant reaction temperature, a heat exchanger block was also mounted above the working electrode. This microreactor-based electrolysis afforded 98% product selectivity, which was higher than that for common industrial processes (about 85%). 

The issues to be solved for direct fluorinations are heat release and mass transfer via the gas-liquid interface. Multiphase microstructured reactors enable process intensification [230,248-250,304—306]. Often geometrically well-defined interfaces are formed with large specific values, for example, up to 20 000 m2/m3 and even more. These areas can be easily accessible, as flow conditions are often highly periodic and transparent microreactors are available. For the nondispersing... 

Similar studies were made on a mesh microreactor comprising Pd/Al203 and Pt/Al203 catalysts coated on a microstructured mesh [270,271]. The global rate constants were 56 and 1.41/s for the Pd and Pt catalysts, respectively [271]. An activation energy of 46 5 kj/mol was found for the Pt catalyst in the mesh reactor, which corresponds to the value of 39 kj/mol determined for a commercial Pt/ A1203 powder catalyst in a well-behaved batch reactor (see Figure 4.54). 

Figure 3.1 Microreactor featuring a multichannel microfluidic element fabricated from PDMS [21]. Panel a the fully assembled microreactor. Panel b microstructured multichannel plate. Panel c electron micrograph of a segment of a microfluidic channel that shows a passive mixing element.

Figure 3.2 GPMR used for biocatalytic transformations with immobilized enzymes [22]. (a) the fully assembled microreactor, (b) microstructured multichannel plate, and (c) electron micrograph of the wash-coat layer of y-aluminum oxide covering the microchannel walls.

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