(636g) Development of Laser Driven Light Diffusing Fibre Photoreactors for Organic Synthesis

The importance and utility of photoreactions and photocatalysed reactions has been increasing, notably for applications in organic synthesis, and in particular photoredox reactions. The increased interest in photocatalysis is because (1) the reactivity of some species would be very difficult or impossible to generate by other means, (2) photons are more sustainable and are essentially traceless reagents, (3) due to selective generation of highly reactive intermediates, (4) the development of photocatalysts capable of harnessing visible light, and (5) the simultaneous development of robust and inexpensive Light-Emitting Diodes (LEDs).

Different radiation sources have been used in photoreactors, such as: Mercury, Halogen and Excimer lamps, LEDs, Lasers and even sunlight. The geometrical constrains of the different irradiation sources have given rise to a plethora of reactor designs. For example, most lamps are cylindrical and irradiate light radially, thereby these are commonly placed in the centre of the reactors; while LEDs are a directional photon source and are commonly placed in panels that irradiate transparent reactors from its edges. LEDS have a relatively narrow emission band, are available for UV and in the visible spectra but also have major disadvantages: the reactors’ construction materials have to be transparent (e.g. polymers like FEP or PFA, or quartz), the photoreactors need a secondary barrier or shield to protect the operators from the light, high-power LEDs have a broader wavelengths that extends to the IR band which in turn heats up the reactor, so the LED panels need some cooling media and the divergence of the light emitted inherently reduces the reactor’s efficiency.

In an attempt to overcome some of the limitation of LED and other photon sources, in this work radial light diffusing fibres (LDF) were coupled with laser diodes via a total internal reflectance (TIF) based transmission fibre which brings the photons generated by the laser source to the reactor for internal distribution by optical scattering using the LDF (Figure 1a). This enables decoupling of photon generation and associated heat from distribution of photons within the reactor volume. Furthermore photons can be generated outside of production suite and supplied to the photoreactor as a utility negating the negating the need for specialist ATEX equipment and powerful light sources to be present within a chemical production environment.

The LDFs irradiate photons radially from within the reactor which is preferred due to the primary challenge in scaling up photochemical reactions: the Beer-Lambert Law. Radial emission of photons ensures that all photons are captured and allows for a better photon penetration as contact area in-between the irradiating source and the reagents is increased.

The modular annular photoreactor, used within this study, is fabricated using standard fluidic components with the LDF placed inside and protected by transparent FEP tubing that crosses a stainless steel cylindrical housing creating and Annular Photoreactor (APhR). Additionally, the APhR has two fluid connectors and thus, can be operated continuously. An external heating/cooling jacket was also installed to aid control the temperature. The appearance of the APhR and a schematics of its cross sectional area is shown in Figure 1b and Figure 1c . The arrangement of equipment and accessories used, along with the vertically positioned APhR is shown in Figure 1a. Multiple APhRs of different housing diameter were constructed to study the effect of the annular gap size on conversion.

The photon efficiency of the APhR with LDF was then determined by ferrioxalate actinometry, which allows to measure the photon flux by tracking the reduction of photosensitive ferric ions into ferrous one[1]. To compare the effectiveness of the APhR with the literature, the C-N photoredox coupling of 4-Bromobenzotrifluoride (0.4 M) and Pyrrolidine (0.6 M) (Eq. 1) was used. The pre-photocatalyst, photocatalysts, photocatalytic quencher and solvent used were NiBr₂•3H₂O, Ru(bpy)₃(PF₆)₂, DABCO (1,4-diazabicyclo[2.2.2]octane) and DMSO (dimethyl sulfoxide) respectively.

The current work has proven that lasers coupled through TIF fibres to a photoreactor that diffuse scattering based optical distribution using LDFs (1) are more efficient in distributing photons (proven by actinometry), (2) can be operated in a safer way because the emitted light can be restrained inside a stainless-steel casing, in addition to improved heat transfer, (3) photon-emission and heat generation can be decoupled allowing for better temperature control and (4) the well-established regular geometry should allow for more accurate modelling, with associated benefits in process understanding, scale-up and control. (5) enables photo Chemistry can be conducted in modular stainless steel equipment facilitating deployment and scale-up.

Acknowledgement

This publication is supported by Science Foundation Ireland (SFI) through I-Form, the Science Foundation Ireland Centre For Advanced manufacturing co-funded under the European Regional Development Fund under grant number (16/RC/3872).

[1] Hatchard, C. G., and C. A. Parker. 1956. “A New Sensitive Chemical Actinometer. II. Potassium Ferrioxalate as a Standard Chemical Actinometer.” Proceedings of the Royal Society of London Series A 235 (June): 518–36.