It could be argued that the popularity of photochemistry amongst synthetic organic chemists in industry has suffered from its limited scale-up potential. This can be attributed to the attenuation effect of photon transport. The intensity of the energy rapidly decreases the further it is away from the reaction, as described by the Bouguer-Lambert-Beer law. With the increase in reactor volume, this effect is enhanced leading to poor light transmission to the reaction mixture. This limits the scalability of photochemistry even on a laboratory scale under batch conditions. This effect also limits the classical way in which flow chemistry is often scaled, by increasing reactor volumes and flow rates. Reactor volumes are often increase in a dimension-enlarging fashion, where the attenuation of light also restricts this scale-up method. Flow photochemistry has a couple of strategies that can help in the scale up of photocatalytic reactions.
We can increase the throughput by running the reactions for longer:
- We can increase the number of reactors
- We can increase the photon flux into the reaction
The first method is well known as a benefit of traditional flow chemistry. We can use the same reactor to produce more material by increasing the reaction time. The second point is to number-up reactors and can be achieved in several ways. Linear numbering-up is where several reactors are placed along the fluidic pathway to increase the volume, and subsequent flow rate and through put. External numbering-up is where the entire flow system is duplicated and internal numbering-up is where multiple reactors are fed from the same pump source with the fluid channels split into multiple streams. External numbering-up is clearly a more expensive option. Internal numbering-up can lead to irregular residence times across the reactors. Internal numbering up is the better option as we can maintain the same light conditions within each reactor, but this is also a costly approach.
It is useful to consider here a light source as emitting a flux of photons. An equation to calculate the number of moles of photons (einsteins) per hour at a given wavelength (λ) (if the total power of emissions at that wavelength is known), is shown in Equation 2.
If we compare batch and flow photochemistry techniques, flow photochemistry reactors have been shown to deliver around a 150-fold higher absorbed photon flux density compared to their batch equivalent. Photon flux is calculated by dividing the photon flux (einsteins/s) by the reactor volume. The photon flux strongly affects the reaction rate of photochemical processes; the higher the photon flux, the faster the reaction will be completed.1 This explains clearly why photochemical reactions can be substantially accelerated in microreactors.
As we consider the photon as a reagent in photochemistry it makes sense that by increasing the number of photons into a photo mediated reaction could increase the reaction rate. If we can increase the light intensity into the same photochemistry reactor then we can use the same platform to scale-up our reactions.
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1 Cambié, D.; Bottecchia, C.; Straathof, N. J. W.; Hessel, V.; Noël, T. Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment. Chem. Rev., 2016, 116 (17), 10276–10341. https://doi.org/10.1021/acs.chemrev.5b00707
2 Knowles, J. P.; Elliott, L. D.; Booker-Milburn, K. I. Flow Photochemistry: Old Light through New Windows. Beilstein J. Org. Chem., 2012, 8, 2025–2052. https://doi.org/10.3762/bjoc.8.229