The incorporation of catalysts into microreactors

Having demonstrated the principle using small quantities of catalytic material, typically 5 mg, the authors later demonstrated the ability to generate the aforementioned materials with a throughput ranging from 0.57 to 0.94 gh-1 by simply increasing the size of the packed bed employed [3,000 pm (i.d.) x 3 cm (long)]. 

Employing silica-supported piperazine 113 (0.10 g, 1.70 mmol Ng1), the authors demonstrated the use of EOF for the semipreparative scale synthesis of a series of a,p-u nsa tura ted compounds and as previously observed in the capillary-based reactor, the compounds were obtained in excellent isolated yield and product purity (Table 13). 

Initial investigations employed a reaction temperature of 55 °C and a flow rate of 100 pi min 1 with ethyl isocyanoacetate 123 (0.75 M) 

R1 R2 Isolated yield Thiazole (%) Imidazole (%) Combined yield (%) 

Using the same reactor, a premixed solution of aldehyde or ketone and dithiol (1.0 M, 1 1) in anhydrous MeCN was passed through the Amberlyst-15 126 containing packed bed. The reaction products were again collected in MeCN and analyzed every 10 min by GC-MS. Once optimized, the reactions were conducted for 1 h, to produce the required quantity of each 1,3-dithiane or 1,3-dithiolane, the reaction products were then concentrated in vacuo and analyzed by 1H NMR spectroscopy (Tables 16 and 17) (Wiles et al., 2007b). 

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Several methods for the incorporation of catalysts into microreactors exist, which differ in the phase-contacting principle. The easiest way is to fill in the catalyst and create a packed-bed microreactor. If catalytic bed or catalytic wall microreactors are used, several techniques for catalyst deposition are possible. These techniques are divided into the following parts. For catalysts based on oxide supports, pretreatment of the substrate by anodic or thermal oxidation [93, 94] and chemical treatment is necessary. Subsequently, coating methods based on a Uquid phase such as a suspension, sol-gel [95], hybrid techniques between suspension and sol-gel [96], impregnation and electrochemical deposition methods can be used for catalyst deposition [97], in addition to chemical or physical vapor deposition [98] and flame spray deposition techniques [99]. A further method is the synthesis of zeoUtes on microstructures [100, 101]. Catalysts based on a carbon support can be deposited either on ceramic or on metallic surfaces, whereas carbon supports on metals have been little investigated so far [102]. 

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