Journal of Flow Chemistry最新文献

Catalytic enantioselective Strecker reactions on an achiral substrate using sub-stoichiometric amounts of a chiral catalyst represent an evolving key strategy for the effective synthesis of α-amino nitriles. We herein disclosed the set-up of a flow-based methodology for enantioselective Strecker, employing ethyl cyanoformate as a relatively safe cyanide source, a cinchona-based catalyst, and methanol as additive. A thorough exploration of key parameters allowed the identification of the most efficient reagent mixing mode, the optimum solvent for the flow synthesis, minimum catalyst loading, additive, temperature, and residence time. The newly developed method allows straightforward reaction channeling towards the fast and complete formation of the α-amino nitrile products, thus reducing the yield drop due to indolenine degradation during long-lasting batch-wise reactions. Moreover, we herein provide preliminary hints for sustainability, by proposing a simple procedure for catalyst recycling, thus opening the way for further optimization of the proposed methodology.

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A high-temperature continuous flow protocol is reported for the intensified synthesis of an important industrial raw material via aromatic Claisen rearrangement of the corresponding diallyl ether precursor. The process takes advantage of solvent-free conditions, thereby maximizing productivity whilst reducing cost and environmental impact. By precise control over reaction temperature and residence times, a high-yielding and selective synthesis is achieved that ensures improved safety and scalability of the exothermic transformation compared with earlier batch methodologies.

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A low-cost 3D printed standardized flow-photochemistry setup has been designed and developed for use with a pressure-driven flow system using photochemistry lamps available in most laboratories. In this research, photochemical reactors were 3D printed from polypropylene which facilitated rapid optimization of both reactor geometry and experimental setup of the lamp housing system. To exemplify the rapidity of this approach to optimization, a Kessil LED lamp was used in the bromination of a range of toluenes in the 3D printed reactors in good yields with residence times as low as 27 s. The reaction compared favorably with the batch photochemical procedure and was able to be scaled up to a productivity of 75 mmol h⁻¹.

A flow chemistry based continuous morphology-controllable precipitation strategy was successfully developed for synthesis of europium oxalate hydrate microparticles. The effects of flow ratio between raw materials within microchannels on the crystal structure, morphology and particle size distribution of the precipitated products were firstly studied. The results shown that both high yield and controllable morphology were achieved for Eu³⁺ precipitation reactions under its low concentration condition. The effects of supersaturation, mixing intensity, and reaction temperatures were also investigated in detail, which proved the continuous preparation of layered microparticles with concentrated size distribution can be achieved by this strategy. Multiple characterizations and comparison experiment synergistically reveal that the feed flow ratios of nitric acid and oxalic acid determines the morphology and particle size distribution due to affecting the mixing degree and phase composition of the precipitation reaction. In addition, the phase and morphology conversion of precipitates after calcination treatment were also studied, the as-calcined metal oxide powder exhibited a decent photoluminescence characteristic. In summary, this work demonstrates a promising precipitation strategy within micro-channels for mass controllable production of high-quality metal oxide materials.

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Conventional batch synthesis of hexafluoroacetone (HFA), an important pharmaceutical intermediate, suffers from complex catalyst preparation, harsh reaction conditions (up to 200 °C), and low selectivity. In this study, we developed a continuous flow system that employs a micro packed-bed reactor (MPBR) filled with Lewis acid catalysts. After an initial screening of reaction conditions and catalysts in the batch reactor, a Bayesian Optimization model and the multi-objective optimization algorithm qNEHVI were used to find a compromise between conversion and energy efficiency for the reaction in the continuous flow system. After 14 rounds of experiments, BO found the best results with conversion of 98.6%, selectivity of 99.9%, and an energy cost of 0.121 kWh per kg of product at 25.1 °C, atmospheric pressure, and a GHSV of 931.5 h^− 1 reaction conditions. The study demonstrates that BO can be used as an efficient tool for multi-objective optimization of heterogeneous catalysis in continuous flow.

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Abstract

The nitration of toluene with mixed acid is one of the most representative nitration reactions. Accurate kinetic study is essential for controlling the reaction and designing reactors. Due to the characteristics of fast rate, high exothermicity and heterogeneity in toluene nitration, the effects of mass and heat transfer may result in inaccurate determination of kinetics. In this work, the adiabatic temperature rises of the reaction system were studied to provide precise ranges of experimental conditions for accurately controlling the reaction rate and heat release rate in a liquid–liquid microflow system. The adiabatic temperature rise was successfully controlled to below 0.3 °C. The effects of mass and heat transfer on the reaction rate were completely eliminated, so that the kinetic study was carried out under the control of intrinsic kinetics only. The activation energy for toluene nitration was determined to be 28.00 kJ/mol. The activation energies for the formation of o-nitrotoluene and p-nitrotoluene were obtained for the first time, which were 25.71 and 31.91 kJ/mol, respectively. The obtained kinetic models can predict the reaction performance of toluene nitration very well.

An expedient and efficient synthesis of acetyl phloroglucinol from phloroglucinol under continuous flow is reported. This compound is an important building block to access numerous flavonoids. The reported acetylation reaction of phloroglucinol is more efficient than under batch conditions (residence time of 1 min in flow vs 10–15 h in batch; ≥ 98% yield in flow vs 65–86% yield in batch), and proceeds without any formation of the diacetylated product. The desired product was produced with a throughput of ≥ 98 g/h using a simple set-up. The conditions that highlight the use of ethyl acetate, a biomass-derived solvent and acetic anhydride or acetyl chloride were tested in the Friedel–Crafts acetylation of other electron-rich phenols and heterocycles.

Photocatalytic generation of H₂O₂ with heterogeneous catalysts has attracted much attentions and impressive strategies have been used to increase the photocatalytic efficiency. However, applications of these strategies to large scale are still underdeveloped. For this reason, development of flow photocatalytic strategy is highly necessary. Considering this, we have developed a serial micro-batch flow reactor for the generation of H₂O₂ which could easily scale up the reaction to1L scale with high reproducibility with the modified g-C₃N₄. With this flow reactor, the generated concentration of H₂O₂ has been reached to 6.89 mM and 5.89 mM in pure water and seawater, respectively.

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