Heterogeneous photocatalysis in flow chemical reactors

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.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

“…[118] It guarantees unique control of key reaction parameters, ensuring efficient mixing and heat transfer, unmatched by batch technology from this point of view. [119] It is also an opportunity to scale-up reactions easily, increasing productivity by lowering the reaction times. [120] Flow technology can also be coupled with other techniques, such as organic synthesis under microwave irradiation, as has been studied in depth by Barham et al [121] Despite the enormous amount of literature published in the field of photocatalysis and photoredox catalysis, industrial applications, except few examples, are scarce.…”

“…Flow chemistry is a chemical technology involving the employment of channels or tubing to conduct a reaction in a continuous stream, and is quickly becoming an indispensable tool and widespread technology among laboratories [118] . It guarantees unique control of key reaction parameters, ensuring efficient mixing and heat transfer, unmatched by batch technology from this point of view [119] . It is also an opportunity to scale‐up reactions easily, increasing productivity by lowering the reaction times [120] .…”

Towards Scaling-up Photocatalytic Process for Multiphase Environmental Applications: A Review

“…Mass transfer and contact time are key parameters in photocatalytic air processing, since in air systems, only adsorbed pollutants can undergo degradation by direct oxidation by positive holes or surface photoproduced ROSs. While this issue is generally less felt in lab scale, batch reactors, where contact times range from seconds to minutes [113], the design of photocatalytic reactors for treating large air volumes requires a careful optimization of mass transfer issues [114] in order to achieve a fast degradation of pollutants without the accumulation of undesired by-products. In real applications, high flow rates are needed to maximize mass transfer but this comes at the cost of decreasing the contact time between the pollutant and the photocatalyst surface to less than 1 s, which often results in an incomplete degradation of the pollutant.…”

Bodipy-Containing Porous Microcapsules for Flow Heterogeneous Photocatalysis

“…Compared with stirred batch reactors, fixed-bed flow reactors show the advantage of avoiding catalyst separation after the reaction. The flow reaction is a promising strategy to improve the reaction rate to a satisfactory level. , Columns are used to construct fixed-bed flow reactors in which glass beads are usually introduced to reduce the pressure drop. , This strategy can also avoid the waste of catalyst in the reactor center caused by the nonpenetration of light, but the reduction of catalyst loading will also affect the reaction efficiency. The nanosized photocatalysts also appear in the flow photochemistry but directly go through the reaction mixture without loading in the fixed-bed reactor .…”