Scale-Up of a Heterogeneous Photocatalytic Degradation Using a Photochemical Rotor-Stator Spinning Disk Reactor

Interaction of light with gas-liquid interfaces: influence on photon absorption in continuous-flow photoreactors

Light interacts with gas bubbles in various ways, potentially leading to photon losses in gas-liquid photochemical applications. Given that light is a valuable 'reagent', understanding these losses is crucial for optimizing reactor efficiency. In this study, we address the challenge of quantifying these interactions by implementing a method that separately determines the photon flux and utilizes actinometric experiments to determine the effective optical path length, a key descriptor of photon absorption. The results reveal the unexpected impact of gas phase introduction in continuous-flow photoreactors. Notably, photon absorption, and consequently the throughput of a photoreactor, can be increased by the introduction of a gas phase. This enhancement arises from the reflection and refraction effects of gas bubbles, which can intensify light intensity in the liquid volume and thereby offset any loss in residence time. The photon absorption losses that were observed were associated with large bubbles and were less significant than anticipated. In contrast, the introduction of small bubbles was found to increase photon absorption, suggesting it is a potential strategy to optimize photoreactor performance.

Scaling Up Gas-Liquid Photo-Oxidations in Flow Using Rotor-Stator Spinning Disc Reactors and a High-Intensity Light Source

Photochemical transformations have garnered renewed interest over the past decade for their ability to enable unique reactions under mild conditions. However, scaling up such processes, particularly in multiphase systems (e.g., gas-liquid), remains challenging. Previously, we demonstrated the potential of the photochemical rotor-stator spinning disc reactor (pRS-SDR) for scaling the photooxidation of α-terpinene to ascaridole, though the system was limited by the light source, resulting in suboptimal operation in a photon-limited regime. In this work, we unlock the full potential of the pRS-SDR by integrating a high-powered light source (up to 652 W optical output) specifically designed for the reactor. The results show that the high gas-liquid mass transfer rates achievable in the pRS-SDR allow for significant productivity improvements under high irradiance (16.3 kg day-1 at 92% α-terpinene conversion and 2.52 W cm-2 in a 27 mL irradiated volume), representing an order of magnitude increase compared to our previous study. However, the photooxidation of β-citronellol exhibited notable limitations, highlighting the importance of selecting appropriate model reactions when evaluating intensified photochemical reactors.

Green Synthesis of Alzheimer's Disease Probes Aftobetin and Analogues Enabled by Flow Technology and Heterogeneous Photocatalysis

Aftobetin is a non-invasive diagnosis of Alzheimer's disease, that can bind with aggregated β-amyloid peptide in eye's lenses, used for early diagnosis of Alzheimer's disease in a rapid and painless mode. The reported synthesis of this probe fell short in the aspects of greenness and economy due to the involvement of toxic Chromium(IV) oxidant, noble palladium catalyst, elevated reaction temperature, the long reaction time as well as the cumbersome workup. Herein, a holistic optimization of the synthetic process was achieved via the employment of flow technology and heterogenous photocatalysis. Firstly, the integration of heterogenous carbon nitrides photocatalysis and circulation flow technology furnished the air oxidation of alcohol and nickel catalyzed C-N coupling at 20 g scale, thus avoiding the use of toxic Chromium and precious palladium species respectively. Flow-intensified esterification between acyl chloride and alcohol, just taking 30 seconds replaced the Steglich esterification of 6 hours, also avoiding the generation of difficult-to-remove dicyclohexylurea. Finally, C-N coupling, esterification and Knoevenagel condensation were telescoped together, thus simplifying the reaction workup. This fully-flow protocol led to the on-demand synthesis of eight probes.

A comprehensive review on TiO2-based heterogeneous photocatalytic technologies for emerging pollutants removal from water and wastewater: From engineering aspects to modeling approaches

The increasing presence of emerging pollutants (EPs) in water poses significant environmental and health risks, necessitating effective treatment solutions. Originating from industrial, agricultural, and domestic sources, these contaminants threaten ecological and public health, underscoring the urgent need for innovative and efficient treatment methods. TiO2-based semiconductor photocatalysts have emerged as a promising approach for the degradation of EPs, leveraging their unique band structures and heterojunction schemes. However, few studies have examined the synergistic effects of operating conditions on these contaminants, representing a key knowledge gap in the field. This review addresses this gap by exploring recent trends in TiO2-driven heterogeneous photocatalysis for water and wastewater treatment, with an emphasis on photoreactor setups and configurations. Challenges in scaling up these photoreactors are also discussed. Furthermore, Machine Learning (ML) models play a crucial role in developing predictive frameworks for complex processes, highlighting intricate temporal dynamics essential for understanding EPs behavior. This capability integrates seamlessly with Computational Fluid Dynamics (CFD) modeling, which is also addressed in this review. Together, these approaches illustrate how CFD can simulate the degradation of EPs by effectively coupling chemical kinetics, radiative transfer, and hydrodynamics in both suspended and immobilized photocatalysts. By elucidating the synergy between ML and CFD models, this study offers new insights into overcoming traditional limitations in photocatalytic process design and optimizing operating conditions. Finally, this review presents recommendations for future directions and insights on optimizing and modeling photocatalytic processes.

Enhancing electrochemical reactions in organic synthesis: the impact of flow chemistry

Utilizing electrons directly offers significant potential for advancing organic synthesis by facilitating novel reactivity and enhancing selectivity under mild conditions. As a result, an increasing number of organic chemists are exploring electrosynthesis. However, the efficacy of electrochemical transformations depends critically on the design of the electrochemical cell. Batch cells often suffer from limitations such as large inter-electrode distances and poor mass transfer, making flow cells a promising alternative. Implementing flow cells, however, requires a foundational understanding of microreactor technology. In this review, we briefly outline the applications of flow electrosynthesis before providing a comprehensive examination of existing flow reactor technologies. Our goal is to equip organic chemists with the insights needed to tailor their electrochemical flow cells to meet specific reactivity requirements effectively. We also highlight the application of reactor designs in scaling up electrochemical processes and integrating high-throughput experimentation and automation. These advancements not only enhance the potential of flow electrosynthesis for the synthetic community but also hold promise for both academia and industry.

PDI-Functionalized Glass Beads: Efficient, Metal-Free Heterogeneous Photocatalysts Suitable for Flow Photochemistry

Perylene diimides (PDI) have an extraordinary ability to activate both energy and electron transfer processes upon light excitation; however, their extremely low solubility has hindered their wide use as photocatalysts. Here, we show that the combination of solid-supported PDIs with continuous flow photochemistry offers a promising strategy for process intensification and a scalable platform for heterogeneous photocatalysis. The photocatalyst immobilized onto glass beads is highly efficient, easy to separate, and extremely reusable, with a broad synthetic application range. Using the photo-oxidation of n-butyl sulfide as a benchmark reaction, we demonstrate that immobilized PDI are highly active, outperforming reported homogeneous photosensitizers, and capable of extensive reuse (turnover number (TON) >57,000 over 2 months). Transferring the process from batch to flow results in a 10-fold reduction in irradiation time and an increase in the space-time yield by a factor of 33 (40 vs 1338 mmol-1 h-1 L-1 batch vs flow). What is more, the same catalyst sample can be used for the preparation of a range of sulfoxides, the aza-Henry reaction between nitromethane and N-Ar tetrahydroisoquinolines, and the photo-oxidation of furfural with high catalytic activity. Overall, our work combines the remarkable photocatalytic properties of PDI with inert, easy-to-handle glass beads, producing hybrid materials that are reusable and can be adapted for performing heterogeneous photocatalysis in a range of scalable photochemical reactors.

Synthetic Chemistry in Flow: From Photolysis & Homogeneous Photocatalysis to Heterogeneous Photocatalysis

Photocatalytic synthesis of value-added chemicals has gained increasing attention in recent years owing to its versatility in driving many important reactions under ambient conditions. Selective hydrogenation, oxidation, coupling, and halogenation with a high conversion of the reactants have been realized using designed photocatalysts in batch reactors with small volumes at a laboratory scale; however, scaling-up remains a critical challenge due to inefficient utilization of incident light and active sites of the photocatalysts, resulting in poor catalytic performance that hinders its practical applications. Flow systems are considered one of the solutions for practical applications of light-driven reactions and have experienced great success in photolytic and homogeneous photocatalysis, yet their applications in heterogeneous photocatalysis are still under development. In this perspective, we have summarized recent progress in photolytic and photocatalytic synthetic chemistry performed in flow systems from the view of reactor design with a special focus on heterogeneous photocatalysis. The advantages and limitations of different flow systems, as well as some practical considerations of design strategies are discussed.

Multiphase photochemistry in flow mode via an integrated continuous stirred tank reactor (CSTR) approach

A new photochemical CSTR system capable of handling solids in scaled continuous processes is presented. High-power UV-LEDs are integrated in these CSTRs containing an insoluble base that aids in generating pyrazolines via cycloaddition between alkenes and in situ generated diazo species. Contrary to reported batch methods product degradation via ring contraction is suppressed whilst generating gram quantities of spirocyclic pyrazolines.

Spinning Reactors for Process Intensification of Flow Photochemistry

The design of new and more sustainable synthetic protocols to access new materials or valuable compounds will have a high impact on the broader chemistry community. In this sense, continuous-flow photochemistry has emerged as a powerful technique which has been employed successfully in various areas such as biopharma, organic chemistry, as well as materials science. However, it is important to note that chemical processes must not only advance towards new or improved chemical transformations, but also implement new technologies that enable new process opportunities. For this reason, the design of novel photoreactors is key to advancing photochemical strategies. In this sense, the use of equipment and techniques embracing processes intensification is important in developing more sustainable protocols. Among the most recent applications, spinning continuous flow reactors, such as rotor reactors or vortex reactors, have shown promising performance as new synthetic tools. Nevertheless, there is currently no review in the literature that effectively summarizes and showcases the most recent applications of such type of photoreactors. Herein, we highlight fundamental aspects and applications of two categories of spinning reactors, the Spinning Disc Reactors (SDRs) and Thin Film Vortex reactors, critiquing the scope and limitations of these advanced processing technologies. Further, we take a view on the future of spinning reactors in flow as a synthetic toolbox to explore new photochemical transformations.

The Application of Porous Organic Polymers as Metal Free Photocatalysts in Organic Synthesis

Concerns about increasing greenhouse gas emissions and their effect on our environment highlight the urgent need for new sustainable technologies. Visible light photocatalysis allows the clean and selective generation of reactive intermediates under mild conditions. The more widespread adoption of the current generation of photocatalysts, particularly those using precious metals, is hampered by drawbacks such as their cost, toxicity, difficult separation, and limited recyclability. This is driving the search for alternatives, such as porous organic polymers (POPs). This new class of materials is made entirely from organic building blocks, can possess high surface area and stability, and has a controllable composition and functionality. This review focuses on the application of POPs as photocatalysts in organic synthesis. For each reaction type, a representative material is discussed, with special attention to the mechanism of the reaction. Additionally, an overview is given, comparing POPs with other classes of photocatalysts, and critical conclusions and future perspectives are provided on this important field.

The Kornblum DeLaMare rearrangement in natural product synthesis: 25 years of innovation

Covering: 1998 up to the end of 2023Since its initial disclosure in 1951, the Kornblum DeLaMare rearrangement has proved an important synthetic transformation and has been widely adopted as a biomimetic step in natural product synthesis. Utilising the base catalysed decomposition of alkyl peroxides to yield a ketone and alcohol has found use in many syntheses as well as a key strategic step, including the unmasking of furans, as a biomimetic synthetic tool, and the use of the rearrangement to install oxygen enantioselectively. Since ca. 1998, its impact as a synthetic transformation has grown significantly, especially given the frequency of use in natural product syntheses, therefore this 25 year time period will be the focus of the review.

Process Intensification of the Continuous Synthesis of Bio-Derived Monomers for Sustainable Coatings Using a Taylor Vortex Flow Reactor

We describe the optimization and scale-up of two consecutive reaction steps in the synthesis of bio-derived alkoxybutenolide monomers that have been reported as potential replacements for acrylate-based coatings (Sci. Adv.2020, 6, eabe0026). These monomers are synthesized by (i) oxidation of furfural with photogenerated singlet oxygen followed by (ii) thermal condensation of the desired 5-hydroxyfuranone intermediate product with an alcohol, a step which until now has involved a lengthy batch reaction. The two steps have been successfully telescoped into a single kilogram-scale process without any need to isolate the 5-hydroxyfuranone between the steps. Our process development involved FTIR reaction monitoring, FTIR data analysis via 2D visualization, and two different photoreactors: (i) a semicontinuous photoreactor based on a modified rotary evaporator, where FTIR and 2D correlation spectroscopy (2D-COS) revealed the loss of the methyl formate coproduct, and (ii) our fully continuous Taylor Vortex photoreactor, which enhanced the mass transfer and permitted the use of near-stoichiometric equivalents of O2. The use of in-line FTIR monitoring and modeling greatly accelerated process optimization in the Vortex reactor. This led to scale-up of the photo-oxidation in 85% yield with a projected productivity of 1.3 kg day-1 and a space-time yield of 0.06 mol day-1 mL-1. Higher productivities could be achieved while sacrificing yield (e.g., 4 kg day-1 at 40% yield). The use of superheated methanol at 200 °C in a pressurized thermal flow reactor accelerated the second step, the thermal condensation of 5-hydroxyfuranone, from a 20 h batch reflux reaction (0.5 L, 85 g) to a space time of <1 min in a reactor only 3 mL in volume operating with projected productivities of >700 g day-1. Proof of concept for telescoping the two steps was established with an overall two-step yield of 67%, producing a process with a projected productivity of 1.1 kg day-1 for the methoxybutenolide monomer without any purification of the 5-hydroxyfuranone intermediate.

Multiphasic Continuous-Flow Reactors for Handling Gaseous Reagents in Organic Synthesis: Enhancing Efficiency and Safety in Chemical Processes

The use of reactive gaseous reagents for the production of active pharmaceutical ingredients (APIs) remains a scientific challenge due to safety and efficiency limitations. The implementation of continuous-flow reactors has resulted in rapid development of gas-handling technology because of several advantages such as increased interfacial area, improved mass- and heat transfer, and seamless scale-up. This technology enables shorter and more atom-economic synthesis routes for the production of pharmaceutical compounds. Herein, we provide an overview of literature from 2016 onwards in the development of gas-handling continuous-flow technology as well as the use of gases in functionalization of APIs.

Synergistic photocatalysis of a hydrochar/CeO2 composite for dye degradation under visible light

The synthesis and characterization of a hydrochar/CeO2 composite along with its evaluation in methylene blue degradation under visible light are presented. The methodology consisted of a single-pass hydrothermal method, having as synthesis conditions 9 h of reaction time, 210 °C, autogenous pressure, and a biomass/CeO2 ratio of 100:1. The composite characterization revealed good dispersion of CeO2 in the carbonaceous matrix and significant synergy in the composite activation using visible irradiation. The photodegradation experiments showed an efficiency of 98% for white LED light, 91% for UV light, 96% for solar irradiation, and 85% for blue LED light using as conditions pH 7.0, 50 mg of composite, 50 mL of solution, 10 mg/L of dye initial concentration, and 120 min of contact time. Meanwhile, the reusability experiments evidenced a reuse capacity of up to five times with a constant photodegradation efficiency (99%); moreover, it was determined that the presence of electrolytes at pH below 7.0 during degradation negatively affected methylene blue degradation. Finally, the results of this work demonstrate that the hydrochar/CeO2 composite can be synthesized by a green method and used for the efficient treatment of water contaminated with methylene blue.

Solar-Driven Continuous CO2 Reduction to CO and CH4 using Heterogeneous Photothermal Catalysts: Recent Progress and Remaining Challenges

The urgent need to reduce the carbon dioxide level in the atmosphere and keep the effects of climate change manageable has brought the concept of carbon capture and utilization to the forefront of scientific research. Amongst the promising pathways for this conversion, sunlight-powered photothermal processes, synergistically using both thermal and non-thermal effects of light, have gained significant attention. Research in this field focuses both on the development of catalysts and continuous-flow photoreactors, which offer significant advantages over batch reactors, particularly for scale-up. Here, we focus on sunlight-driven photothermal conversion of CO2 to chemical feedstock CO and CH4 as synthetic fuel. This review provides an overview of the recent progress in the development of photothermal catalysts and continuous-flow photoreactors and outlines the remaining challenges in these areas. Furthermore, it provides insight in additional components required to complete photothermal reaction systems for continuous production (e. g., solar concentrators, sensors and artificial light sources). In addition, our review emphasizes the necessity of integrated collaboration between different research areas, like chemistry, material science, chemical engineering, and optics, to establish optimized systems and reach the full potential of this technology.

Scale-Up of Photochemical Reactions: Transitioning from Lab Scale to Industrial Production

In the past two decades, we have witnessed a rapid emergence of new and powerful photochemical and photocatalytic synthetic methods. Although these methods have been used mostly on a small scale, there is a growing need for efficient scale-up of photochemistry in the chemical industry. This review summarizes and contextualizes the advancements made in the past decade regarding the scale-up of photo-mediated synthetic transformations. Simple scale-up concepts and important fundamental photochemical laws have been provided along with a discussion concerning suitable reactor designs that should facilitate scale-up of this challenging class of organic reactions.

A field guide to flow chemistry for synthetic organic chemists

Flow chemistry has unlocked a world of possibilities for the synthetic community, but the idea that it is a mysterious "black box" needs to go. In this review, we show that several of the benefits of microreactor technology can be exploited to push the boundaries in organic synthesis and to unleash unique reactivity and selectivity. By "lifting the veil" on some of the governing principles behind the observed trends, we hope that this review will serve as a useful field guide for those interested in diving into flow chemistry.

Dual role of benzophenone enables a fast and scalable C-4 selective alkylation of pyridines in flow

The efficient C-4 selective modification of pyridines is a major challenge for the synthetic community. Current strategies are plagued with at least one drawback regarding functional group-tolerant electronic activation of the heteroarene, mild generation of the required alkyl radicals, regioselectivity, safety and/or scalability. Herein, we describe a fast, safe and scalable flow process which allows preparation of said C-4 alkylated pyridines. The process involves a photochemical hydrogen atom transfer (HAT) event to generate the carbon-centered radicals needed to alkylate the C-2 blocked pyridine. In a two-step streamlined flow process, this light-mediated alkylation step is combined with a nearly instantaneous inline removal of the blocking group. Notably, cheap benzophenone plays a dual role in the pyridine alkylation mechanism by activating the hydrocarbon feedstock reagents via a HAT mechanism, and by acting as a benign, terminal oxidant. The key role of benzophenone in the operative reaction mechanism has also been revealed through a combination of experimental and computational studies.

Heterogeneous metallaphotoredox catalysis in a continuous-flow packed-bed reactor

Metallaphotoredox catalysis is a powerful and versatile synthetic platform that enables cross-couplings under mild conditions without the need for noble metals. Its growing adoption in drug discovery has translated into an increased interest in sustainable and scalable reaction conditions. Here, we report a continuous-flow approach to metallaphotoredox catalysis using a heterogeneous catalyst that combines the function of a photo- and a nickel catalyst in a single material. The catalyst is embedded in a packed-bed reactor to combine reaction and (catalyst) separation in one step. The use of a packed bed simplifies the translation of optimized batch reaction conditions to continuous flow, as the only components present in the reaction mixture are the substrate and a base. The metallaphotoredox cross-coupling of sulfinates with aryl halides was used as a model system. The catalyst was shown to be stable, with a very low decrease of the yield (≈1% per day) during a continuous experiment over seven days, and to be effective for C–O arylations when carboxylic acids are used as nucleophile instead of sulfinates.

Variable Temperature LED-NMR: Rapid Insights into a Photocatalytic Mechanism from Reaction Progress Kinetic Analysis

A multitude of techniques are available to obtain a useful understanding of photocatalytic mechanisms. The combination of LED illumination with nuclear magnetic resonance spectroscopy (LED-NMR) provides a rapid, convenient means to directly monitor a photocatalytic reaction in situ. Herein, we describe a study of the mechanism of an enantioselective intermolecular [2 + 2] photocycloaddition catalyzed by a chiral Ir photocatalyst using LED-NMR. The data-rich output of this experiment is suitable for same-excess and variable time normalization analyses (VTNA). Together, these identified an unexpected change in mechanism between reactions conducted at ambient and cryogenic temperatures. At -78 °C, the kinetic data are consistent with the triplet rebound mechanism we previously proposed for this reaction, involving sensitization of maleimide and rapid reaction with a hydrogen-bound quinoline within the solvent cage. At room temperature, the cycloaddition instead proceeds through intracomplex energy transfer to the hydrogen-bound quinolone. These results highlight the potential sensitivity of photocatalytic reaction mechanisms to the precise reaction conditions and the further utility of LED-NMR as a fast, data-rich tool for their interrogation that compares favorably to conventional ex situ kinetic analyses.