A μ2-process in the Ullmann-type CO coupling of potassium phenolate and 4-chloropyridine was successfully performed in a combined microwave (MW) and microflow process. Selective MW absorption in a micro-fixed-bed reactor (μ-FBR) by using a supported Cu nanocatalyst resulted in an increased activity compared to an oil-bath heated process. Yields of up to 80 % were attained by using a multisegmented μ-FBR without significant catalyst deactivation. The μ-FBR was packed with beads coated with Cu/TiO2 and CuZn/TiO2 catalysts. Temperature measurements along axial positions of the reactor were performed by using a fiber-optic probe in the catalyst bed. The simultaneous application of MW power and temperature sensors resulted in an isothermal reactor at 20 W. Initially, only solvent was used to adjust the MW field density in the cavity and optimize the power utility. Subsequently, the reaction mixture was added to ensure the maximum MW power transfer by adjusting the waveguide stub tuners to steady-state operations as a result of the changed reaction mixture composition and, therefore, the dielectric properties. Finally, the beneficial influence of the Cu/TiO2- and CuZn/TiO2-coated glass beads (200 μm) on the MW absorption as a result of the additional absorbing effect of the metallic Cu nanoparticles was optimized in a fine-tuning step. For the catalyst synthesis, various sol–gel, deposition, and impregnation methods provided Cu catalyst loadings of around 1 wt %. The addition of Zn to the Cu nanocatalyst revealed an increased catalyst activity owing to the presence of stable Cu0. Multilaminar mixing was necessary because of the large difference in fluid viscosities. To the best of our knowledge, this work is the first extended experimental survey of the decisive parameters to combine microprocess and single-mode MW technology following the concepts of “novel process windows” for organic syntheses.
If you can't find a tool you're looking for, please click the link at the top of the page to "Go to old article view". Alternatively, view our Knowledge Base articles for additional help. Your feedback is important to us, so please let us know if you have comments or ideas for improvement.