Continuous Heterogeneously Catalyzed Oxidation of Benzyl Alcohol Using a Tube-in-Tube Membrane Microreactor

Electroreduction of CO2 toward High Current Density

Carbon dioxide (CO2) electroreduction offers an attractive pathway for converting CO2 to valuable fuels and chemicals. Despite the existence of some excellent electrocatalysts with superior selectivity for specific products, these reactions are conducted at low current densities ranging from several mA cm−2 to tens of mA cm−2, which are far from commercially desirable values. To extend the applications of CO2 electroreduction technology to an industrial scale, long-term operations under high current densities (over 200 mA cm−2) are desirable. In this paper, we review recent major advances toward higher current density in CO2 reduction, including: (1) innovations in electrocatalysts (engineering the morphology, modulating the electronic structure, increasing the active sites, etc.); (2) the design of electrolyzers (membrane electrode assemblies, flow cells, microchannel reactors, high-pressure cells, etc.); and (3) the influence of electrolytes (concentration, pH, anion and cation effects). Finally, we discuss the current challenges and perspectives for future development toward high current densities.

Insight into Au/ZnO catalyzed aerobic benzyl alcohol oxidation by modulation–excitation attenuated total reflection IR spectroscopy

The chemisorbed or dissociated oxygen species is associated with the catalytic activity in alcohol oxidation catalyzed by Au/ZnO catalysts.

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Modelling of packed bed and coated wall microreactors for methanol steam reforming for hydrogen production

A Computational Fluid Dynamics (CFD) study has been conducted to assess the performance of packed bed and coated wall microreactors for the steam reforming of methanol with a CuO/ZnO/Al₂O₃ based catalyst (BASF F3-01). The results obtained were compared to experimental data from the literature to assess the validity and robustness of the models, and a good validation has been obtained. The performance of the packed bed and coated wall microreactors is similar at a constant reforming temperature. It was found that methanol conversion is enhanced with increasing temperature, residence time, steam to methanol ratio, and catalyst coating thickness. Furthermore, internal and external mass transfer phenomena were investigated using the models, and it was found that there were no internal and external mass transfer resistances for this reactor configuration. Further studies demonstrated that larger catalyst pellet sizes led to the presence of internal mass transfer resistance, which in turn causes lower methanol conversions. The CFD models have exhibited a sound agreement with the experimental data, hence they can be used to predict the steam reforming of methanol in microreactors.

A Computational Fluid Dynamics (CFD) study has been conducted to assess the performance of packed bed and coated wall microreactors for the steam reforming of methanol with a CuO/ZnO/Al₂O₃ based catalyst (BASF F3-01).

Accelerating gas-liquid chemical reactions in flow

Over the past decade, continuous flow reactors have emerged as a powerful tool for accelerated fundamental and applied studies of gas-liquid reactions, offering facile gas delivery and process intensification. In particular, unique features of highly gas-permeable tubular membranes in flow reactors (i.e., tube-in-tube flow reactor configuration) have been exploited as (i) an efficient analytic tool for gas-liquid solubility and diffusivity measurements and (ii) reliable gas delivery/generation strategy, providing versatile adaptability for a wide range of gas-liquid processes. The tube-in-tube flow reactors have been successfully adopted for rapid exploration of a wide range of gas-liquid reactions (e.g., amination, carboxylation, carbonylation, hydrogenation, ethylenation, oxygenation) using gaseous species both as the reactant and the product, safely handling toxic and flammable gases or unstable intermediate compounds. In this highlight, we present an overview of recent developments in the utilization of such intensified flow reactors within modular flow chemistry platforms for different gas-liquid processes involving carbon dioxide, oxygen, and other gases. We provide a detailed step-by-step guideline for robust assembly and safe operation of tube-in-tube flow reactors. We also discuss the current challenges and potential future directions for further development and utilization of tubular membrane-based flow reactors for gas-liquid processes.

Determination of fast gas–liquid reaction kinetics in flow

A flow concept is developed to measure fast gas–liquid reaction kinetics based on a tube-in-tube reactor.

Flow Toolkit for Measuring Gas Diffusivity in Liquids

Precise knowledge of gas diffusivity in liquids is critical for describing complex multiphase reaction systems. Here we present a high-throughput flow concept to measure gas diffusivity in liquids. This strategy takes advantage of the tube-in-tube reactor design whereby semipermeable Teflon AF-2400 tubes facilitate fast mass transfer between gas and liquid without directly contacting the two fluids. Coupled pseudosteady-state flux balances over the gas and liquid describe the gas dissolution rate and corresponding diffusivity with the aid of a single gas flow meter and a continuously ramped liquid flow rate. This in situ method demonstrates excellent accuracy in diffusion coefficient measurements, with less than 5% deviation from established techniques.

Improving Productivity of Multiphase Flow Aerobic Oxidation Using a Tube-in-Tube Membrane Contactor

The application of flow reactors in multiphase catalytic reactions represents a promising approach for enhancing the efficiency of this important class of chemical reactions. We developed a simple approach to improve the reactor productivity of multiphase catalytic reactions performed using a flow chemistry unit with a packed bed reactor. Specifically, a tube-in-tube membrane contactor (sparger) integrated in-line with the flow reactor has been successfully applied to the aerobic oxidation of benzyl alcohol to benzaldehyde utilizing a heterogeneous palladium catalyst in the packed bed. We examined the effect of sparger hydrodynamics on reactor productivity quantified by space time yield (STY). Implementation of the sparger, versus segmented flow achieved with the built in gas dosing module (1) increased reactor productivity 4-fold quantified by space time yield while maintaining high selectivity and (2) improved process safety as demonstrated by lower effective operating pressures.

The Use of Molecular Oxygen for Liquid Phase Aerobic Oxidations in Continuous Flow

Molecular oxygen (O₂) is the ultimate “green” oxidant for organic synthesis. There has been recent intensive research within the synthetic community to develop new selective liquid phase aerobic oxidation methodologies as a response to the necessity to reduce the environmental impact of chemical synthesis and manufacture. Green and sustainable chemical processes rely not only on effective chemistry but also on the implementation of reactor technologies that enhance reaction performance and overall safety. Continuous flow reactors have facilitated safer and more efficient utilization of O₂, whilst enabling protocols to be scalable. In this article, we discuss recent advancements in the utilization of continuous processing for aerobic oxidations. The translation of aerobic oxidation from batch protocols to continuous flow processes, including process intensification (high T/p), is examined. The use of “synthetic air”, typically consisting of less than 10% O₂ in N₂, is compared to pure O₂ (100% O₂) as an oxidant source in terms of process efficiency and safety. Examples of homogeneous catalysis and heterogeneous (packed bed) catalysis are provided. The application of flow photoreactors for the in situ formation of singlet oxygen (¹O₂) for use in organic reactions, as well as the implementation of membrane technologies, green solvents and recent reactor solutions for handling O₂ are covered.

Automated in Situ Measurement of Gas Solubility in Liquids with a Simple Tube-in-Tube Reactor

Data on the solubilities of gases in liquids are foundational for assessing a variety of multiphase separations and gas-liquid reactions. Taking advantage of the tube-in-tube reactor design built with semipermeable Teflon AF-2400 tubes, liquids can be rapidly saturated without direct contacting of gas and liquid. The gas solubility can be determined by performing steady-state flux balances of both the gas and liquid flowing into the reactor system. Using this type of reactor, a fully automated strategy has been developed for the rapid in situ measurement of gas solubilities in liquids. The developed strategy enables precise gas solubility measurements within 2-5 min compared with 4-5 h using conventional methods. This technique can be extended to the discrete multipoint steady-state and continuous ramped-multipoint data acquisition methods. The accuracy of this method has been validated against several gas-liquid systems, showing less than 2% deviation from known values. Finally, this strategy has been extended to measure the temperature dependence of gas solubilities in situ and to estimate the local enthalpy of dissolution across a defined temperature range.

Catalysis in flow: O2 effect on the catalytic activity of Ru(OH)x/γ-Al2O3 during the aerobic oxidation of an alcohol

Different roles of O₂ in the Ru-catalysed aerobic oxidation of alcohols have been delineated.

Aerobic oxidations in flow: opportunities for the fine chemicals and pharmaceuticals industries

This collaborative review (between teams of chemists and chemical engineers) describes the current scientific and operational hurdles that prevent the utilisation of aerobic oxidation reactions for the production of speciality chemicals and active pharmaceutical ingredients (APIs).

Liquid phase oxidation chemistry in continuous-flow microreactors

This review gives an exhaustive overview of the engineering principles, safety aspects and chemistry associated with liquid phase oxidation in continuous-flow microreactors.

Characterisation of gold catalysts

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions.