Recent advancements in process intensification for singlet oxygen-mediated photooxidative transformations using flow photoreactors and photocatalytic materials

Abstract

Singlet oxygen-mediated photooxidation processes fall under the domain of green and sustainable chemistry, as they are driven through traceless photons of visible light under mild reaction conditions. The state-of-the-art designs of flow photoreactors have revolutionized these photooxidative transformations due to their high specific interfacial area, fast mass transport and kinetic rates, inherently safe design, green credentials, and high scalability potential. The developments in heterogeneous photocatalysts and porous organic frameworks further pave the way towards photostability and long-term reliability of these photooxidation processes. In this contribution, we prudently describe the process intensification strategies for these photooxidative processes using capillary-based, packed-bed, spinning-disk, falling-film, and miscellaneous flow photoreactors. In addition, we meticulously orchestrate organic dye-based photosensitizers, support-based photocatalysts, and porous frameworks used for the singlet oxygen-mediated transformations. The reviewed literature on this subject indicates that efficient singlet-oxygen photooxidation requires an intensified flow photoreactor, an effective photocatalytic material, a reliable LED light source, and a green solvent with a high lifetime of singlet oxygen. An intensified flow photoreactor should possess a significant ability to expedite mass transport and operate in shaded-free zones, offering high scalability potential. Similarly, photocatalytic materials should exhibit considerable singlet-oxygen quantum yield, remarkable photostability under prolonged irradiation, and effective singlet-oxygen storage and controlled-release abilities. The visible LED light source should be wavelength-selective and have a considerable penetration depth inside the reaction medium. Similarly, the selected solvent should possess substantial green credentials and an inability to quench singlet oxygen. We critically analyze these factors and present a systematic roadmap with a sound comparative analysis of various flow photoreactors and photocatalytic materials used for the singlet oxygen-mediated process. Finally, insightful outlooks and challenges are presented to develop a green, autonomous, and scalable singlet-oxygen photooxidative platform. This review is expected to be a useful reference for photocatalytic researchers to gain a thorough understanding of intensified flow photoreactors and efficient photocatalytic materials used for singlet oxygen-mediated photooxidative transformations.