The developed photoreactor was designed to be operated with suspensions as well as immobilized catalytically active materials. The cyclone design ensures intense and guided convection, by which a sedimentation of suspensions can be avoided. 3D-printed inserts, manufactured with and without catalytic material, could be installed in the top part of the reactor (see Figure ). The reactor was equipped with a 365 nm UV LED (𝑞 𝑒𝑚,𝑚𝑎𝑥 ≈ 1 µmol s -1 ) and operated in recycle-mode to achieve high conversions. Different insert designs with respect to the flow guidance and the irradiated area were developed and evaluated for an optimal reactive performance. The different insert designs are shown in Figure . Two categories of inserts were studied, inserts with and without explicit flow guidance. Inserts of the first category use grids as basic design structures and differ by the grid design and the number of installed grids. The second category uses only a single outlet, forcing the reaction solution to follow distinct flow patterns. The columns insert was developed as an alternative to the grid design, in principle providing a longer contact time of the reactants with the structure. The hollow sphere inserts uses the cyclone concept for flow guidance, while the spiral insert explicitly defines the flow paths by channels. The geometrical properties of the different inserts are summarized in Table .