Abstract
Photochemical transformations have witnessed a remarkable growth of attention in the past decade due to the popularity of photoredox catalysis and the technological progress in energy-efficient light sources. However, the scaling of these photon-induced processes remains a daunting challenge for chemists and engineers in the industry, often discouraging their implementation in the production of fine chemicals and pharmaceuticals. To address this key issue, we report herein the development of a conceptually new photochemical reactor, called photo Rotor-Stator Spinning Disk Reactor. The photocatalyzed gas–liquid oxidation of α-terpinene to the drug ascaridole with Rose Bengal as photocatalyst was achieved with throughputs of over 1 kg·day−1 (270 mmol·h−1) under visible light irradiation. The performance of the reactor is correlated to various process parameters such as rotation speed, liquid flowrate, and catalyst concentration, among others. The conversion and selectivity increase from 37% to 97% and 75% to 90% respectively with an increase of rotation speed from 100 to 2000 RPM. The reactor operates with negligible pressure drop and allows for facile fine-tuning of the mixing efficiency. In this paper, we have also compared the performance of this reactor to other reactors such as the batch, micro, thin-film, and the vortex reactor, among others, and have found the photo-Rotor-Stator Spinning Disk Reactor to have superior productivity as well as higher productivity per volume (2.8 mol ·s−1 ·m−3). The findings of this study can be used to study, design, optimize and scale photochemical processes using the rotor–stator spinning disk reactor.
Highlights
The use of light as a source for the activation of reactants can be a sustainable alternative to the conventional use of thermochemical activation[1], for instance for crosscoupling reactions[2]
The microreactor results indicate that residence times in the range of 20-40 s are interesting to investigate in the photo-rotor-stator spinning disk reactor (pRS-SDR), with a 3:1 mole-flow ratio of oxygen: substrate
The results of this study illustrate that photochemical oxidations can be carried out effectively with high productivity in a pRS-SDR
Summary
The use of light as a source for the activation of reactants can be a sustainable alternative to the conventional use of thermochemical activation[1], for instance for crosscoupling reactions[2]. The reactor described in this work, the rotor-stator spinning disk reactor relies on generating intense turbulence in an enclosed housing to intensify mass and heat transfer rates Scaling up of such reactors have mostly focused on single phase liquid reactions as gas-liquid systems are traditionally more challenging. Researchers at AbbVie were able to reach kg∙day-1 scale for a C-N coupling reaction in a CSTR using a catalyst with a low absorption coefficient, making it possible to irradiate the reactor efficiently[29] Even though these studies show that an effective reactor design significantly improves the performance of photochemical systems by increasing interfacial area and/or light penetration, only a few examples of industrial scale photochemical production are known[29,30,31], due to the absence of effective photochemical reactors with high productivity. When studying the mechanism (Fig 1) of the photooxygenation of α-terpinene to ascardidole we identified multiple steps in the process that might be susceptible for improvement by more efficient mixing; e.g., the formation of singlet oxygen via excited RB and the subsequent oxidation of α-terpinene to ascardidole due to the short life time of singlet oxygen (16 μs in ethanol)
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