The reduction is initiated by irradiation of TiO2 particles with UV-light. In a sequence of 3 consecutive two electron transfers, adsorbed nitrobenzene (1) is reduced to nitrosobenzene (2), phenylhydroxylamine (3) and finally aniline (4). Simultaneously, ethanol (5) is oxidized by photogenerated holes to acetaldehyde (6). The reaction between aniline and acetaldehyde yields ethylideneaniline (10). In a sequence of ringclosing condensations ethylvinylether (11) reacts with ethylideneaniline to 4-ethoxy-1,2,3,4-tetrahydroquinaldine (12) and finally 1,2-dihydroquinaldine (13). Finally 1,2-dihydroquinaldine disproportionates to quinaldine (14) (oxidation) and 1,2,3,4tetrahydroquinaldine (15) (reduction). Even though the photocatalytic route seems to be the perfect alternative from a preparative lab perspective, heterogeneous photocatalytic reactions raise a number of challenges from a reaction engineering point of view. Previous studies used TiO2 P25 nanoparticles suspended in the solvent with catalyst loadings of typically 0.5-5.0 g L -1 . While the use of suspensions is convenient at lab scale, several challenging aspects have to be considered from the reaction engineering perspective: TiO2 suspensions are opaque, optically dense media. Almost all photons entering the reaction mixture are absorbed in a thin layer close to the light source. Additionally, due to the presence of particles, a significant portion of photons is scattered, increasing the complexity of the radiation field in such reactors. Although suspensions benefit from a large accessible catalyst surface, adequate mixing is crucial to prevent precipitation and consequently shadowing inside the reactor. Furthermore, downstream processing, especially tedious separation of nanoparticle catalysts, are a major obstacle for a possible industrial application.