(656c) Photocatalytic Synthesis of Benzylic Amines Using Metal-Free Dyes: Investigating the Reaction Performance with a Spinning Disc Reactor and a Microchannel Reactor

Solar
photocatalysis has grown as a green alternative for the synthesis of fine
organic compounds. Recently, metal-free dyes have gained
interest as visible light photocatalysts as they are more environmentally
benign than transition metals, they are cheap and they have long excited-state
lifetimes.¹ Eosin Y (characteristic peak 539 nm) is an anionic dye,
which has efficiently catalysed a range of reactions including cyclisation, halogenation
and redox reactions.^2,3 Despite the advances in the lab-scale
synthesis, there is a lack of knowledge on how to scale up these visible light photocatalytic
reactions for industrial implementation. To absorb light efficiently,
thin films and large surface areas are needed. To date, most photocatalytic
reactors have been designed based on enclosed UV-light and are therefore not
suitable for visible light. Neither are traditional reactors, such as batch or
packed reactors, and thus new process intensification (PI) reactors need to be designed, optimised and evaluated to maximise the light
efficiency and to overcome the mass transfer limitations.

The
aim of this study is to evaluate two different types of thin film/large surface
area reactors (Fig 1) for the intensification of visible light photocatalysis: (i) Spinning Disc Reactor (SDR) and (ii) Microchannel
reactor (MCR). A specially designed solar light simulator (40cm*40cm) was used to carry out the oxidative homocoupling of
benzylamine, chosen as a model reaction.

                                             a)                                                                                  b)

Figure 1: Schematic representation of a) Spinning
Disc Reactor (SDR); b) Microchannel reactor (MCR).

A
range of conditions with low and high substrate concentrations and catalyst
loadings were tested with both configurations. As
opposed to batch reactors, the photon-transfer limitation was overcome due to
the creation of thin films by the centrifugal acceleration in the SDR, and the
large surface area per volume ratio in the MCR. Moreover, thin films and high
surface areas reduce the effect of the opacity of the solutions, as the light
is able to penetrate into the mixture. Therefore, high catalyst loadings and
amine concentrations can be used to improve the
productivity of the reaction. In Fig 2a, it can be observed
that for the MCR, at a high amine concentration, higher catalyst loadings increased
the conversion. The productivity also increased as, for example, with 6 mol% Eosin Y and 5 mM initial
concentration of amine, 0.41 mmol of amine reacted to
form the product whereas with 15 mM, 1.21 mmol of
amine reacted.

a)                                                                                          b)

Figure 2: a) Effect of catalyst loading and
amine concentration using the MCR; b) Effect of flow rate and spinning speed using
the SDR.

One
of the most significant factors in the performance of the SDR, similarly to the
MCR, is the interaction between the flow rate and the light intensity (Fig 2b).
This will determine the time that the catalyst spends under the light, in the
catalyst-excited state. High flow rates permit a faster recirculation of the
solution and therefore, the reaction mixture is on top of the disc more times
during a specific reaction time. Due to the thin films created by the SDR (film
thickness of 0.09 – 0.45 mm), the light is able to penetrate into the solution
and the spinning speed does not greatly influence the conversion.

Figure 3. Setup of the solar light simulator
and the SDR.

These
studies have demonstrated that the MCR and the SDR have the potential to enable
intensification of reactions with commercial importance. Further investigations
are being carried out with these reactor
configurations and with a batch reactor. Moreover, the performance of the SDR (Fig
3) is being investigated with an immobilised catalyst,
which creates a process where the catalyst can be simply recovered and reused. In
summary, the combination of metal-free dyes and novel visible light
photocatalytic reactors have shown to be a solution to maximise the light
efficiency, overcome the mass transfer limitations and create a more
sustainable process to synthesise fine organic chemicals.  

REFERENCES

1                   
J. R. Xin, J. T. Guo,
D. Vigliaturo, Y. H. He and Z. Guan, Tetrahedron, 2017, 73, 4627-4633.

2                   
D. Prasad Hari and B. König, Chem. Commun, 2014, 50, 6688-6699.

3                   
J. D. Tibbetts, D. R. Carbery and E. A. C.
Emanuelsson, ACS Sustain. Chem. Eng., 2017,
5, 9826-9835.