Formation and application of LMCT complexes as visible light photocatalysts. The first part of the project consisted in the enhancement of the visible light response of metal oxide semiconductors (MOS) by the adsorption of organic molecules (adsorbates) on their surface (called ligand to metal charge transfer: LMCT-sensitization). For that, a variety of potential adsorbates were combined with MOS under different reaction conditions. After the modification of the MOS with the selected adsorbates, a perceptible change in color was observed indicating that these ligands had a significant influence in the optical absorption of the MOS. Next, we examined the photocatalytic activity of the sensitized-MOS to carry out organic transformations induced by light. Although any improvement of the organic reactions was observed, these hybrid complex still has a great potential to be used as visible light photocatalysts to initiate valuable organic reactions.
Application of LMCT complexes as visible light photocatalysts for organic reactions in a continuous flow. Taylor flow is a gas-liquid flow pattern that consists of elongated bubbles separated by liquid slugs. The solid photocatalyst is suspended in a liquid slug and is transported through the photomicroreactor. In line with the use of MOS in photocatalysis, we exploited its application in continuous flow chemistry by using Taylor flow technology. The atom transfer radical addition (ATRA) was chosen as a model reaction. This classic reaction is an atom economic and effective methodology for the direct functionalization of olefins. The hybrid complex composed by a MOS and an adsorbate was used as the visible light photocatalyst of the chosen reaction. Although a good dispersion of the hybrid complex (LMCT-sensitized MOS) and no clogging phenomena was observed, no conversion in the desired products was obtained.
Energy-Efficient Solar Photochemistry. Luminescent solar concentrators (LSCs) are inexpensive slabs of luminophore-doped polymeric materials that harvest and concentrate solar photons. LSCs can be used as photon harvesters for photochemical reactions in so-called luminescent solar concentrator-photomicroreactors (LSC-PM). This device based on fluorescent dye‐doped polymer collects sunlight, focuses the energy to a narrow wavelength region, and then transports that energy to embedded microchannels where the flowing reactants are converted. To demonstrate the efficiency of the device, several different photochemical reactions were carried out. A medicinally relevant molecule was synthesized using this technique (artemisinin-drug against malaria). Also, with the adoption of a reaction control system, stable product quality could be obtained even during fluctuating irradiance conditions.
Unveiling the catalytic species involved in photocatalytic processes. Identifying the truly catalytically active species in heterogeneous processes is a key step towards the development of a reproducible, scalable, and efficient heterogeneous photocatalytic approach. In this regard, the second part of the project aimed to investigate the species involved in the photocatalytic processes carried out by a MOS. For that, the behavior of the Bi2O3 was studied using the ATRA reaction. By combining theoretical and experimental studies we disclosed that Bi2O3 is not the true photocatalyst of this reaction, but it is the precursor of a soluble photocatalytic species triggering the photoredox process. Moreover, we observed that the presence of certain amines accelerated the reaction fourfold by stabilization of the species formed in the reaction medium.
The exploitation and dissemination of the works mentioned above were conducted in monthly progressing meetings realized by the host group, and an exhibition for high school students in the Open Day organized by the Eindhoven University of Technology (TU/e), social media (Twitter, LinkedIn), non-specialized magazine (National Geographic-Dutch version) national and international conferences (Flow Chemistry Europe international congress, RSCPoster Twitter, Photo4future), and open access research publications (Catal. Sci. Technol. 2019, 9, 5186; ACIE 2019, 131, 14512).
