Methylene blue was efficiently immobilized on silica micro- and nanoparticles by electrostatic interactions and the performances of the heterogenized photocatalysts were compared against the homogeneous conditions using the photooxidation of citronellol as a model reaction under red light, in a batch and a continuous flow photochemical reactor. In batch, the heterogeneous photocatalyst outperforms the homogeneous one, presumably due to kinetic and stability effects. The two catalytic systems are also compared in a flow reactor displaying improved mass transfer properties. We demonstrate that this results in a dramatic enhancement in photocatalyst stability, reactivity and productivity. This study highlights the importance of photocatalyst stability under homogeneous versus heterogenized conditions and in batch versus flow photochemistry.

Visible light photooxidation of naphthols to produce naphthoquinones, such as the natural product juglone, has been known for decades and has been widely utilized to benchmark the performances of a variety of photocatalytic systems. We discovered that these transformations can occur without the help of a photocatalyst and, even more intriguingly, that the photocatatyst-free process provides higher yields compared to control experiments utilizing state-of-the-art photocatalysts. In addition, we demonstrate that naphthoquinones and their corresponding naphthol precursors can act as alternatives to commonly used organic and organometallic photocatalysts with applications to challenging targets, such as the antimalarial drug artemisinin. This approach was finally transposed in continuous flow reactors where high photocatalyst stability and process efficiency are demonstrated with a 23× improvement in the space-time yield.