Abstract
The synergy between photocatalysis and continuous flow chemical reactors has shifted the paradigms of photochemistry, opening new avenues of research with safer and scalable processes that can be readily implemented in academia and industry. Current state-of-the-art photocatalysts are homogeneous transition metal complexes that have favourable photophysical properties, wide electrochemical redox potentials, and photostability. However, these photocatalysts present serious drawbacks, such as toxicity, limited availability, and the overall cost of rare transition metal elements. This reduces their long-term viability, especially at an industrial scale. Heterogeneous photocatalysts (HPCats) are an attractive alternative, as the requirement for the separation and purification is largely removed, but typically at the cost of efficiency. Flow chemical reactors can, to a large extent, mitigate the loss in efficiency through reactor designs that enhance mass transport and irradiation. Herein, we review some important developments of heterogeneous photocatalytic materials and their application in flow reactors for sustainable organic synthesis. Further, the application of continuous flow heterogeneous photocatalysis in environmental remediation is briefly discussed to present some interesting reactor designs that could be implemented to enhance organic synthesis.
Highlights
1.1 Scope of the review This review aims to be of interest to synthetic organic chemists who are considering applying heterogeneous photocatalysis (HPC) and flow chemistry in their research, and especially those who are already involved in one of the two areas
We have covered the different types of Heterogeneous photocatalysts (HPCats) and how they function as photocatalysts
Enhancing charge transport and separation in semiconductor HPCats is a key factor to improve photocatalytic activity, which can be achieved through materials design and band gap engineering
Summary
1.1 Scope of the review This review aims to be of interest to synthetic organic chemists who are considering applying heterogeneous photocatalysis (HPC) and flow chemistry in their research, and especially those who are already involved in one of the two areas. Analysing the same data set of photocatalysis publications by discipline reveals that significant contributions came from engineering, materials science, and physics, in addition to chemistry (Figure 1B) This reflects the multiplex nature of photocatalysis as it requires an advanced chemical and photophysical theory to rationalise its complex mechanisms, as well as skilled engineering and reactor design to overcome the limitations of photon and mass transport. The penetration of light through the bulk of the HPCat is difficult and can render large quantities of the material redundant Overcoming these issues and producing efficient HPCats and reactors that can compete with transition metal complex photocatalysts has been described as one of the greatest challenges and opportunities in the field of photocatalysis [43,45,46]. We will cover the fundamentals and recent advances in material design of the main categories of HPCats, beginning with TiO2 as a model system for metal oxide and inorganic semiconductor HPCats
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