Purchasable Microreactors

This section starts with a classification of phase-contacting principles according to the type of catalytic bed. Advantages and disadvantages of the reactor types are explained, followed by a discussion of criteria for reactor selection and an overview of purchasable microreactors for catalytic gas-phase reactions. 

Catalytic Gas-phase Reactions 59 Table 3.3 Non-purchasable microreactors for gas-liquid contacting. 

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. 

A microreactor was also applied to this reaction. The slit interdigital micromixer was purchased from IMM (Mainz, Germany). The width of the interdigital channels is 25 pm. HPLC pumps were used to feed the two reaction solutions. One is a mixture of Boc-AMP and 1.2 molar equivalents of r-BocaO. The other is a 50% aqueous KOH solution. The microreactor was immersed in a temperature controlled cooling bath at 15 °C. The product was quenched with an acid, and samples were taken for HPLC analysis. 

How this works in practice is detailed as follows. After a compound has been identified for which several hundred to several thousand derivatives would be of value, a synthetic route is chosen that (i) permits linkage to a solid-phase support (ii) utilizes reaction steps that appear possible to optimize to > 90% yield and (iii) affords reagents in each step for which desirable variants can be purchased (or, less optimally, can be made trivially). In the synthesis itself, one of the significant advantages of the microreactor approach becomes evident one can use standard laboratory glassware and equipment to accomplish the library synthesis. There is no need for the automation of liquid-handling steps, and indeed no need for automation at all until rather large libraries are desired (vide supra). 

Tables 3.2 and 3.3 summarize microreactors from the literature and purchasable ones ordered by the classification introduced above (see Section 3.3.2). There are additional comments to the reactors to give an idea of the mode of operation.

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