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.
Experimental and modeling toolbox to quantify the light uniformity, photon flux and energy efficiency of microstructured photoreactors.
Gas-liquid photoreactions are increasingly implemented in microreactors. Taylor flows containing an inert dispersed phase were previously used to increase the conversion of photochemical reactions in comparison to using a single liquid phase. However, identifying the optimal flow conditions requires an extensive experimental effort. This work aims to understand the photon transport and hydrodynamics in a Taylor flow photo microreactor so that the reactor behavior can be understood and predicted. Chemical actinometry, flow imaging and residence time distribution experiments were used to develop a multi-region photochemical reaction model. This model shows that the conversion is significantly affected by the liquid distribution, and not by the light scattering or liquid mixing. Moreover, an empirical relation was proposed to predict the optical pathlength in gas-liquid flows. The knowledge gained in this study helps to optimize the performance of Taylor flow photo microreactors, but also to design improved multiphase flow photochemical systems.
The preparation of supported Pd nanoparticles on porous materials was successfully achieved using a simple, fast and efficient reactive milling (mechanochemical) method. The catalytic activity of different Pd/Al-SBA-15 and Pd/Al-MCF materials was investigated in the hydrogenation of p-nitrophenol (PNP) to p-aminophenol (PAP) using NaBH4 as a reducing agent.
The Corning® G1 Advanced-Flow™ Reactor (G1 AFR) is a commercially available meso-scale reactor which promotes a bubbly flow regime and was previously used to scale-up gasliquid photoreactions. In this paper, we study how the photon transport and hydrodynamics affects the performance of G1 AFR in gas-liquid flow. This was realized by analyzing the flow pattern, liquid residence time, photon flux per liquid volume and optical pathlength using image analysis, residence time distribution (RTD) experiments and chemical actinometry. While the gas content did not significantly influence the RTD responses and photon flux per liquid volume, it affected the liquid residence time and optical pathlength. An empirical correlation was proposed to predict the optical pathlength in gas-liquid flow. The constant photon flux per liquid volume and hydrodynamics found for bubbly flow in G1 AFR translate into high flexibility to choose the flow conditions without affecting the performance of this photoreactor.
Microreactors have emerged as a promising platform for conducting photo-oxidations, offering green routes to facilitate significant chemical transformations. In this study, the sustainability of photooxidations is improved by a three-phase gas/liquid/liquid flow reaction in a photo-microreactor with continuous recycling, which is applied to a model reaction. Recycling is enabled by the photosensitizer and the substrate, which are in immiscible phases. The effect of various operational parameters on the reaction and the contact pattern among phases is explored, and the interplay between the flow pattern and photo-oxidation is discussed. The highest conversion is detected for flow conditions that display significantly lower diffusion distance between the dispersed photosensitizer slug and the oxygen bubble, highlighting the link between the flow pattern and mass transfer. Furthermore, phase separation was automated using a liquid−liquid separator where the photosensitizer and organic substrate were recycled continuously to achieve full conversion. The proposed approach eliminates the downstream purification step and provides a sustainable pathway toward photooxidations in microreactors.
Despite the progress in the past decades in the area of light-driven reactions, only a limited number of photochemical reactions is implemented at large scale. [1,2] The major hurdle which needs to be overcome is linked to the scale-up strategy. [3] The limited light penetration (described by Lambert-Beer's law) rapidly decreases the transformation efficiency in large-scale batch reactors as only non-uniform light intensities are achieved in the vessel. However, using continuous flow reactors provides an avenue for efficient scale-up. Moreover, the characteristic reactor sizes on the micro-or milli-scale allow for a homogeneous light distribution in the reacting volume and advanced designs ensure sufficient productivity at the desired scale. [4] Another reason is the difficulty of modeling photochemical reactors. [3] Scaling-up can be realized by starting working with a laboratory-scale reactor and continue by gradually increasing the reactor size until reaching the desired productivity or by predicting the performance of the large scale reactor with mathematical modeling. Modeling is more precise and cost efficient, but is characterized by a higher complexity. [5,6] Consequently, most pilot-scale photoreactors are still designed using empirical or semiempirical methods. [7] Predicting the performance of a large-scale reactor of a different geometry than the one investigated in the laboratory requires knowing the intrinsic kinetic parameters of the reaction. [6,8] Alfano and Cassano described a methodology for scaling-up photoreactors which uses modeling at small and large scale: firstly for determination of the kinetic parameters and then for predicting the pilot-scale reactor performance. [8] This chapter aims to introduce the main concepts of a holistic scale-up strategy based on combined modeling and experiments. We will highlight the important steps of photoreactor modeling such as the kinetic model, radiance, mass and momentum balance etc., and which are the variables to be exchanged between them to ensure efficient and robust scaling. Furthermore, examples from various application fields of photochemistry, e.g. water treatment and synthetic chemistry, will be offered to illustrate the implementation of the described concepts. This chapter will appeal to both chemical engineers and chemists aiming to develop modeling as a tool for scaling-up photochemical reactors. Modeling continuous photochemical processes for photoreactor scaling-upAlfano and Cassano reported a scale-up methodology based on photoreactor modeling by coupling the radiation, mass and momentum balances with the kinetic model. [8] Figure 1.1 presents the main steps of this method and was adapted from the flow scheme reported by Ghafoori. [6]
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