Application of semiconductor photocatalysis for organic synthesis: From photons to the process

In photocatalysis light drives the reactions. When a semiconductor, such as titania, is exposed to light of a specific wavelength range, several physical processes occur and chemical reactions can take place at its surface in the presence of a reactant. This technology works well for wastewater and air cleaning applications. In these, trace hazardous compounds present in water and air are converted (i.e. completely oxidized) to harmless ones such as carbon dioxide and water. A novel area where photocatalysis might be applied is in organic synthesis as a possible alternative for the current energy intensive processes. In this case the objective is to selectively steer the reaction to a certain product, valuable for the bulk chemistry industry. For the case of the selective oxidation of cyclohexane using air as oxidant, the photocatalytic route has shown great potential for cyclohexanone production, which is a very valuable compound in the Nylon industry. The process is very selective but catalyst stability and activity are the major challenges. This work shows how challenging this field is in terms of improving cyclohexanone production and it gathers, in a unique way, several disciplines (e.g. photo-physics, catalytic surface chemistry) otherwise seen apart. In this way not only the catalytic but also the photo-physical events occurring in such a complex process can be better understood and correlated with the overall performance. An optimized catalyst morphology together with the usage of humid air as oxidant resulted in more stable catalysts, providing a breakthrough in this field and bringing the practical application of semiconductor photocatalysis for liquid phase organic synthesis a step closer to reality.