Scale-up of Solar Photocatalytic Reactors for the Synthesis of Fine Chemicals Using Process Intensification

Abstract

The design of new reactor configurations and the use of solar photocatalysis are some promising tools to push forward sustainable pathways to synthesise fine organic chemicals. Many advances have been done in solar photocatalytic synthesis with the development of lamps and the use of cheaper and non-toxic photocatalysts. However, there are still limitations to scale up and perform these reactions at industrial scale. The aim of this research is to develop ways to optimise and scale up solar photocatalytic synthesis reactions, by evaluating the photocatalyst, and by investigating what factors affect the scale-up and how different reactor configurations (batch stirred reactors, spinning disc reactors (SDRs), and microchannel reactors (MCRs)), and light sources (natural and artificial light) affect the reaction rates and productivities.

Firstly, the absorbance spectrum of eosin Y (EY) was extended by the addition of guanidine nitrate (GN), improving the reaction rate of the oxidative homocoupling of benzylamine by 58 % with a reduction in the catalyst loading. EY and GN form a non-fluorescent complex without modifying the reaction mechanism. Secondly, the oxidative coupling of benzylamine (non-mass-transfer-limited) and the photooxidation of L-methionine (mass-transfer-limited) were carried out and compared in batch and in the SDR. The productivity of the non-mass-transfer-limited reaction was affected by opacity in batch (3.23 mmol h-1), which was overcome by using the SDR (5.54 mmol h-1) and the MCR (5.61 mmol h-1). The mass-transfer-limited reaction exhibited a great performance in the SDR over the batch stirred reactor with high spinning speeds and surface areas. Thirdly, EY was immobilised on natural wool to allow the catalyst to be exposed to the light at all times. Different immobilisation and pre-treatment methods were tested and the presence of EY was confirmed by different characterisation techniques. Both reaction systems achieved conversions above 50 % in batch when the wool was pre-treated with alum or a cationic reagent. Finally, the reactions were carried out under natural sunlight in batch and in a new SDR, which can follow the sun’s trajectory, showing comparable results with the experiments under the solar light simulator despite the variable ambient conditions.

The results presented in this thesis demonstrated that GN acts as a photosensitiser and quencher of EY, and its applicability can be extended to the synthesis of more complex imines. Moreover, it was exhibited the importance of knowing the most significant factors to achieve higher productivities and reactions rates, and that there is no best reactor, as it all depends on the reaction conditions. Nevertheless, the SDR has shown promising results as a solar photoreactor, which can be carried out under natural conditions and can be further scaled up without the limitations of the batch stirred reactors. This reactor configuration also allows placing supported catalysts, such as the EY immobilised on wool, to perform photocatalytic reactions. Thus, the catalyst can be illuminated the entire reaction time and it can be reused, making the process more sustainable. Overall, to the best of the author’s knowledge, this is the first study where several approaches have been investigated with the aim to optimise and scale up solar photocatalytic synthesis reactions using new solar reactor configurations, which fulfil the fundamental domains of process intensification.

Date of Award	18 Jul 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Emma Emanuelsson Patterson (Supervisor) & David Carbery (Supervisor)