Microchannel zeolite-coated

To avoid high-pressure drop and clogging problems in randomly packed micro-structured reactors, multichannel reactors with catalytically active walls were proposed. The main problem is how to deposit a uniform catalyst layer in the microchannels. The thickness and porosity of the catalyst layer should also be enough to guarantee an adequate surface area. It is also possible to use methods of in situ growth of an oxide layer (e.g., by anodic oxidation of a metal substrate [169]) to form a washcoat of sufficient thickness to deposit an active component (metal particles). Suzuki et al. [170] have used this method to prepare Pt supported on nanoporous alumina obtained by anodic oxidation and integrate it into a microcatalytic combustor. Zeolite-coated microchannel reactors could be also prepared and they demonstrate higher productivity per mass of catalyst than conventional packed beds [171]. Also, a MSR where the microchannels are coated by a carbon layer, could be prepared [172]. 

Zeolite-coated microchannel reactors with a layer thickness of 1-2 pm demonstrate higher productivity per mass of catalyst than conventional packed beds [239]. The higher effectiveness is a result of the large surface-to-volume ratio of the zeolite film, which makes the entire catalyst surface easily accessible to the reactants. A stainless-steel MSR was coated with FeZSM-5 and found effective for the hydroxylation of benzene with N20 as a reagent [240]. 

Shorter reaction times require for catalyst inside a microreactor to be more active than in traditional reactors. Microreactors employ less catalyst than their batch counterparts, because of their small size. The requirement for readily separable and recyclable catalysts leads often to the application of heterogeneous catalysts. Active catalysts have been coated on a microchannel wall supported by inorganic, polymer, or zeolite coatings [144,145]. 

A. Eleta, P. Navarro, L. Costa, M. Montes, 2009, Deposition of zeolitic coatings onto FeCrAlloy microchannels washcoating vs. in situ growing, Micropor. Mesopor. Mater. 123, 113-122. 

The application of microflow reactors to the Knoevenagd condensation has been reported [28]. A layer of polydiaUyldimethylammonium chloride (PDAMAC) is first coated on the surface of the microchannel. Then, a solution of negatively charged zeolite particles is passed through the channel, which absorbed onto the positively charged PDAMAC. These zeoUte-coated microreactors can catalyze the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate and the corresponding products are produced in yields up to 80% (Scheme 5.14). 

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