Continuous-flow microreactors oxidations

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. 

Catalytic activity data for both CO and propane oxidation were obtained using a conventional continuous flow microreactor. The catalyst sample (0.5g) is situated in a pyrex glass tube located within a stainless steel heated block. Catalyst samples were activated by in situ preheating in the reactor for 2 hours under a flow of air. The catalysts were then allowed to cool to ambient temperature still under the air flow before acquiring %conversion versus temperature data. Input gas mixture compositions, which were controlled by mass flow controllers, and flow rates are shown in Table 10. 

The synthesis requires TBHP as the oxidant at high temperatures thus, combination of the oxidant with ether would result in obvious safety concerns. The group overcame this issue by successfully translating to a continuous-flow/microreactor protocol, thereby representing the first application of continuous-flow processing to C—H activation chemistry (Scheme 7.26). 

The Swem-Moffatt oxidation in a continuous-flow microreactor system was investigated by Organon N.V., The Netherlands [49]. 

Choi et al. [104] synthesized ZnO NCs using a continuous-flow microreactor system. The growth mechanism and stability of ZnO NCs were studied by varying the pH value of the aqueous solution and flow conditions. It was found that external forces from convective fluid flow could affect the assembly of ZnO NCs and result in different shapes at pH 13. The ZnO NC assemblies formed particular structures such as a tactoid structure or a semispherical structure via an attachment growth mechanism. The assembly results from a competing interaction between electrostatic force caused by surface charge of NCs and external force from convective fluid flow. This study shows that the external forces from convective fluid flow could be applied to fabricate assembly of functional metal oxides with complex architectures in a continuous-flow microreactor system. 

The reports mentioned above provide a systematic coverage of the nonimmobi-lized enzymatic reactors used in biocatalytic reactions under continuous flow operation. Results from microreactor experiments were comparatively higher than conventionally mixed batch reactors in terms of conversion rate and improvement of product yield as demonstrated for hydrolysis [140], dehalogenation [141], oxidation [142], esteriflcation [143], synthesis of isoamyl acetate [144,145], synthesis of cyanohydrins [147,148], synthesis of chiral metabolites [153], reduction [151], and bioluminescent reaction [149]. The small volumes involved and the favorable mass transfer inherent to these devices make them particularly useful for the screening of biocatalysts and rapid characterization of bioconversion systems. The remarkable results of such studies revealed that the product yield could be enhanced significantly in comparison with the conventional batch runs. 

Catalytic experiments were performed in a continuous flow fixed bed microreactor using SiC diluted catalyst to obtain predetermined total volume of catalyst bed (i.e., GHSV, h ). The feed composition comprises propane, oxygen, and, in some cases, steam or ammonia. In the case of mixed oxide catalyst and Ga-ZSM-5, the condensable products (acrylic and acetic acids, acrolein, acetone, acrylonitrile,... 

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