Characterization of Microstructured Reactors for Photochemical Transformations

Photochemical reactions are found in a large variety of applications such as synthesis of pharmaceuticals, polymers, and water treatment. In the last two decades, photochemistry was coupled successfully with micro‑scale reactors accelerating the research of light‑driven organic synthesis. Nowadays, the photochemical reactions are carried out in both customized and commercial flow reactors which are predominantly illuminated by Light-Emitting Diodes (LEDs). The current challenge is the lack of information about the properties of light source, reactor and reaction medium and how they influence the reported conversions and yields. This results in reproducibility problems and limited understanding which leads further to extensive experimental efforts required for the optimization and scale‑up of photochemical processes. While this information can be obtained by characterization, the characterization studies are rare. The characterization methods are either missing or not convenient for flow reactors analysis, especially when they are illuminated by visible light sources. This thesis focusses on developing new accessible tools to characterize flow reactors and their application for understanding the performance of visible-light flow reactors in conditions that are mostly encountered in organic synthesis. Firstly, a new visible-light chemical actinometer was developed for quantifying the number of photons with wavelengths ranging between 480 and 620 nm reaching the channels of flow reactors. An experimental methodology was established to determine the photon flux and optical pathlength with high reproducibility, using commercially available compounds, a simple experimental procedure, and on‑line absorbance detection. Secondly, a characterization tool complementary to actinometry was developed to quantify the irradiance and light distribution on the reactor surface when the LED light source was placed at distances smaller than 2 cm. The existing far-field model was improved using angular irradiance distributions extracted from near-field goniophotometer measurements using a commercial ray tracing software. The irradiance model together with the chemical actinometer was applied to quantify the uniformity and the energy efficiency of different LEDs arrays, designed as light sources for a microstructured glass reactor. The LED arrays differ by layout and LED number. During the light source design, the electrical and thermal properties of LEDs were also addressed as they affect the optical output, stability and lifetime of the designed light sources. From the energy efficiency analysis, it was found that 1% of the electrical energy reached the reactor channel in the form of photons. The low energy efficiency was caused mainly by the high power consumption of the driving board and the small surface fraction of the glass plate occupied by the reactor channels. Next, the chemical actinometer was combined with image analysis and residence time distribution experiments to investigate the photon transport and hydrodynamics in liquid and gas‑liquid flows in the same microreactor. The microreactor promoted a Taylor flow at all studied gas fractions. The obtained gas bubble and slug lengths were measured by image processing. The RTD measurements showed that the liquid residence time in gas‑liquid flows was similar to the value found in single-phase flow for the same overall flow rate, apart from the highest gas fraction. The photon flux per liquid volume increased exponentially with the gas content up to double the value measured in single‑phase flow. This observation was correlated to the volume of the liquid present in the film, region around the bubble caps and slug. A three-zone model was developed to predict the photon flux obtained from chemical actinometry. The new model indicated that the photon flux per liquid volume in the film and the region around the bubble caps were three times larger than the one in liquid slug. Furthermore, the optical pathlength decreased with the gas fraction. This variation can be predicted by a correlation that includes single-phase optical pathlength, gas fraction and an empirical factor. A similar study was performed in a commercial milli‑scale reactor, Corning® G1 Advanced-Flow™ Reactor. This reactor promoted bubbly flow at all investigated gas fractions. From RTD experiments, it was found that the liquid residence time in all gas‑liquid flows was higher in comparison with the value found in single‑phase flow measured at the same total flow rate. In contrast with the increasing photon flux per liquid volume observed in Taylor flow, only a moderate rise was found in bubbly flow. In the absence of a thin liquid film, the photoreactor performance was independent of the gas fraction. Furthermore, the optical pathlength also decreased with the gas fraction. As in the case of Taylor flow, the optical pathlength in bubbly flow was a function of the single-phase optical pathlength, gas fraction and an empirical factor. This thesis illustrates that the characterization of flow reactors provides a wide range of quantitative information regarding the light source, photon transport, and hydrodynamics. This information could be used as input in models to study photon transport in new flow patterns or reactor geometries. Moreover, the presented findings contributed to gain new insights regarding the parameters important for photoreactor performance. The acquired knowledge could support a rational photoreactor and process design.status: publishe

Photon Transport and Hydrodynamics in Gas-Liquid Flows Part 1: Characterization of Taylor Flow in a Photo Microreactor

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Microfluidic platform for screening the activity of immobilized photocatalysts for degradation of organic water pollutants

Photochemistry screening platforms have the potential to accelerate the discovery and development of new photocatalysts. This study presents the design and characterization of a novel activity screening platform of immobilized photocatalytic films for degrading water pollutants. The compact testing system is engineered with four 3D-printed microreactors and a rotative multi-wavelength LED light source, which is capable of emitting at the 395, 409, 413, and 443 nm. Despite the different LEDs being placed in a compact space, 95% of the light that reaches the photocatalytic films is emitted by the LEDs directly opposite them. Therefore, the design allows for a minimum of 16 distinct testing conditions by simply rotating the light source. The performance of the microfluidic platform was characterized using the photocatalytic degradation of a pesticide, imidacloprid, in the presence of P25 TiO2, immobilized as thin film on glass plates. The results demonstrated a consistent degradation efficiency of around 35 % at 395 nm, with negligible variation across the four microreactors and no influence of the testing order at 395, 409, 413 and 443 nm. Notably, the photocatalytic film activity did not decrease after 6 hours of operation and under five successive illumination conditions. The screening conditions were optimized using the dynamic water infusion which increased the degradation efficiency of the imidacloprid to 71 %. In addition, the dynamic illumination allowed the sequential operation of the 4 types of LEDs, and led to a halved degradation efficiency despite the LEDs were lighted up for only a quarter of the time. This microfluidic platform diminishes the manual labor and the quantities of photocatalyst and polluted water required per test compared to the batch screening, consequently, it emerging as an efficient and sustainable tool that is suitable for the automated screening of immobilized photocatalysts

Photon Transport and Hydrodynamics in Gas-Liquid Flow Part 2: Characterization of Bubbly Flow in an Advanced-Flow Reactor

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Wavelength-dependent activity screening of reduced titania for photocatalytic degradation of imidacloprid in batch and flow-mode

Water reuse is an emerging solution to decrease pressure on freshwater supplies and meet the increasing demand. This study explores the use of semiconductor photocatalysis for pesticide removal, focusing on extending TiO2 absorption to visible light and accelerating the screening of its wavelength-dependent photocatalytic activity. Grey to black TiO2 photocatalysts with lower direct and indirect band gap energies, up to 1.56 eV and 2.16 eV, respectively, were prepared by the chemical reduction of titania P25. The XPS analysis showed considerable oxygen vacancies, especially at the highest reduction temperature of 400 °C. The fraction of oxygen in the TiO2 lattice decreased from 90 % in the case of P25 to 53% for the photocatalyst obtained at 400 °C. The wavelength-dependent photocatalytic activity for the degradation of imidacloprid was screened in a batch photoreactor. TiO2 P25 presented higher photocatalytic activity than the reduced materials at 400 and 413 nm. At 443 nm, the material reduced at 400 °C exhibited the highest degradation efficiency of 16.8 % compared to 4.2 % as found for P25. Selected photocatalysts were then immobilized as thin films and tested in a 3D-printed flow photoreactor. Wavelength and photocatalyst’s impact on imidacloprid degradation in flow mode aligned with batch mode observations. The film activity remained stable after multiple reaction conditions and at least 150 min of operation. The proposed in-flow screening strategy is a promising approach to rapidly identify visible-light active catalysts, while minimizing the consumption of photocatalytic material and water contaminated with model pollutants.This work was supported by the Romanian Ministry of Research, Innovation and Digitization, CNCS−UEFISCDI (grant PN-III-P1–1.1-PD-2021–0387), MINECO (PID2019–108453GB-C21) and MCIN/AEI/10.13039/501100011033 and EU “NextGeneration”/PRTR (Project PCI2020–111968/ERANET-M/3D-Photocat)

An accessible visible-light actinometer for the determination of photon flux and optical pathlength in flow photo microreactors

Coupling photochemistry with flow microreactors enables novel synthesis strategies with higher efficiencies compared to batch systems. Improving the reproducibility and understanding of the photochemical reaction mechanisms requires quantitative tools such as chemical actinometry. However, the choice of actinometric systems which can be applied in microreactors is limited, due to their short optical pathlength in combination with a large received photon flux. Furthermore, actinometers for the characterization of reactions driven by visible light between 500 and 600 nm (e.g. photosensitized oxidations) are largely missing. In this paper, we propose a new visible-light actinometer which can be applied in flow microreactors between 480 and 620 nm. This actinometric system is based on the photoisomerization reaction of a diarylethene derivative from its closed to the open form. The experimental protocol for actinometric measurements is facile and characterized by excellent reproducibility and we also present an analytical estimation to calculate the photon flux. Furthermore, we propose an experimental methodology to determine the average pathlength in microreactors using actinometric measurements. In the context of a growing research interest on using flow microreactors for photochemical reactions, the proposed visible-light actinometer facilitates the determination of the received photon flux and average pathlength in confined geometries.status: publishe