Development and mechanism elucidation of novel photoreaction processes in microreactors

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Photochemical reactions carried out in microreactors can be expected to exhibit some advantages, such as higher spatial illumination homogeneity and better light penetration through the entire reactor depth, and larger illuminated surface area per unit volume in comparison with those conducted in large-scale reactors. Therefore, we have developed microreactors optimized for photoreactions.Photodimerization and asymmetric photocycloaddition reactions were examined as model reactions. We observed not only the drastic increase of the quantum efficiency of the reaction, but also the enhancement of stereoselectivity in the microreaction system. In the micro-flow system, the residence time of the substrate is very short and the reaction vessel does not retain the reaction products. These facts may prevent sequential side reactions and increase the yield and selectivity of the asymmetric reaction.Photocatalytic oxidation and reduction of organic compounds, amine N-alkylation, and carbon dioxide fixation processes were investigated in the photocatalytic microreaction system. These model reactions proceeded quite rapidly with considerably large photonic efficiencies.Photocatalytic reduction of CO 2 was examined by using several sort of metal co-catalyst loaded on the TiO 2 surface. Though it has been known that CO 2 in aqueous solution can be photocatalytically reduced, only very low reaction yields were reported by using large-scale batch reaction systems. The reaction proceeded very quickly to form methanol as the main product in our microreaction system. The yield of methanol is more than 1000 times larger than that obtained in conventional batch reaction systems and further increased under multiphase flow condition.