A meso-scale ultrasonic milli-reactor enables gas–liquid-solid photocatalytic reactions in flow

Zhengya Dong,Stefan D.A Zondag,Matthias Schmid,Zhenghui Wen,Timothy Noël

doi:10.1016/j.cej.2021.130968

Zhengya Dong, Stefan D.A Zondag + Show 3 more

Open Access

https://doi.org/10.1016/j.cej.2021.130968

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Abstract

The handling of solid reagents, catalysts and by-products is a daunting challenge in continuous-flow micro- and milli-reactors. Suspensions tend to settle over time leading to irrevocable clogging of the reaction channels. Herein, we describe our efforts to develop an ultrasonic milli-reactor which can handle such challenging solid-containing transformations. The reactor consists of a Langevin-type transducer, a sonotrode and an irradiating cylinder, on which a coiled glass capillary (12.88 mL) was attached. The ultrasonic milli-reactor was combined with an LED illuminating box and its efficacy was showcased in the photocatalytic aerobic oxidation of benzyl alcohol enabled by TiO2 particles exposed to UV-A irradiation. Ultrasound irradiation generates cavitation bubbles and causes a vigorous oscillation of both the cavitation and the Taylor bubbles. This improves the liquid mixing, the gas–liquid mass transfer and ensures resuspension of the settled particles. Moreover, these effects enhance the photon absorption by the semiconductor catalyst, which has an overall positive effect on the photocatalytic transformation.

Highlights

In the field of continuous-flow chemistry, micro- and milli-reactors display major advantages for the production of pharmaceuticals and other biologically active molecules [1,2,3]
The ultrasonic milli-reactor was combined with an LED illuminating box and its efficacy was showcased in the photocatalytic aerobic oxidation of benzyl alcohol enabled by TiO2 particles exposed to UV-A irradiation
It should be noted that a gradual decline of the amount of TiO2 over the reactor length will lead to a suboptimal photocatalytic process and should eventually lead to reactor clogging in those first two reactor loops

Summary

Introduction

In the field of continuous-flow chemistry, micro- and milli-reactors display major advantages for the production of pharmaceuticals and other biologically active molecules [1,2,3] These advantages include the occurrence of well-controlled flow patterns and increased surface-tovolume ratios leading to excellent mass and heat transfer characteris tics. Due to the contin uous nature of the reactors, chemical processes can be made much safer and can be readily scaled [8], making this type of reactors an attractive option for continuous manufacturing in the chemical and pharmaceu tical industry [9,10,11,12,13,14] Despite these apparent benefits, this reactor type lacks widespread application in industry due to the encountered weak convective mixing and commonly found problems associated with the handling of solid reagents and products in flow, which often lead to microreactor clogging [15,16,17,18]

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